Metallic arsenic from arsenic-bearing mineral

Abstract

The present invention relates to an amorphous form of metallic arsenic (As(0)) and a method of isolating said amorphous As(0) from an arsenic bearing mineral. In particular the present invention relates to amorphous form of As(0) obtainable from a composition comprising an arsenic-bearing mineral, such as groundwater treatment sludge, mining waste, and contaminated soils.

Description

[0001] Metallic Arsenic from Arsenic-bearing Mineral

[0002] Technical field of the invention

[0003] The present invention relates to an amorphous form of metallic arsenic (As(0)) and a method of preparing said amorphous As(0). In particular, the present invention relates to amorphous form of As(0) obtainable from a composition comprising an arsenic-bearing mineral, such as groundwater treatment sludge, contaminated soil, and mining waste.

[0004] Background of the invention

[0005] Arsenic removal from groundwater via sorption to iron (Fe) (oxyhydr)oxides is the most widely deployed method to meet arsenic drinking water limits in both high- and low-income areas. Although for example Fe-based arsenic (As) treatment is effective, large quantities of As-bearing hydrous ferric oxide (HFO) sludge is generated as a treatment by-product. Due partly to negative perceptions of arsenic, these As-bearing waste, such as sludge, have traditionally been viewed as a disposal challenge rather than a resource. Consequently, unsustainable and unsafe disposal methods are applied for As-bearing waste, such as sludge, including landfilling in high-income areas and open disposal to surface waters and soils in low-income areas for As-bearing sludge, freezing the chambers, and stopes that contain As-mining waste, and stabilizing and encapsulating As-bearing contaminated soils. However, the societal value of arsenic is rapidly evolving due to the growing ubiquity of arsenic in high-value products needed for the green transition, such as high-speed electronics and batteries. Indeed, many regions, including the EU and USA, now classify arsenic as a critical raw material (CRM).

[0006] W09006820 discloses a method of removing arsenic from iron hydroxide sludge and solid waste materials. It is disclosed that a useful starting material is iron hydroxide sludge generated as a by-product during the purification of groundwater for drinking water or purification of domestic wastewater. The method involves e.g. thermal treatment and anion exchangers. Addition of a base directly to the sludge or further reduction of extracted As into amorphous metallic As(0) is not disclosed. Shan J, Saez AE, Ela WP.. J Environ Eng (New York). 2010 Feb;136(2):238-245 discloses extraction of As from arsenic sludge generated by iron-based adsorption processes. Extraction of As in different sludge preparations is exemplified. Arsenic is extracted by a 4-step sequential procedure comprising the steps of:

[0007] 1. treatment with MgCI2 solution at pH 7 to extract exchangeable arsenic phases,

[0008] 2. treatment with NaH2PO4 solution at pH of 5 to target strongly adsorbed As phase on the surface of iron oxide (ferrihydrite, Fe(OH)3),

[0009] 3. treatment with HCI solution to solubilize the amorphous iron (Fe (III) and Fe(II)) arsenate species, and

[0010] 4. treatment with Na-citrate at pH 5 to extract the crystalline arsenic via coordination with iron, extraction of arsenic from waste, including sludge, soils, and mining waste under alkaline conditions, and production of metallic amorphous As(0) are not disclosed.

[0011] Hence, an improved amorphous form of metallic arsenic would be advantageous, and in particular, a more efficient method of isolating metallic arsenic from arsenic bearing minerals which can be effectively converted into high-value materials would be advantageous.

[0012] Summary of the invention

[0013] The present invention has been made within the field of waste management and resource recovery, such as in the domain of waste produced from groundwater treatment, mining, and other industrial activities, and relates to obtaining metallic elemental arsenic (As(0)) from a composition comprising an arsenic-bearing mineral, such as arsenic-laden sludge produced from water treatment, arsenicladen mining waste, and arsenic-contaminated soils. The invention comprises separation of arsenic, such as As(V) and / or As(III) from a composition comprising an arsenic-bearing mineral and reducing As(V) and / or As(III) to form particulate metallic elemental As(0). The method further provides for a specific metallic arsenic product which is amorphous rather than crystalline leading to advantages in the fields of e.g. semiconductors and optics. An important first step to valorize arsenic is to separate arsenic from the solid waste, which theoretically facilitates subsequent conversion to valuable As compounds, such as metallic As(0). Alkali extraction, which uses base to increase pH and release arsenic, is an attractive option because it requires no advanced equipment and has a simple supply chain, which is favourable for implementation of modular sludge recovery systems on-site. The resulting mobilised (water- solubilized) arsenic ions are easily separated from the mineral which remains solid. In a second chemical reaction step the mobilised arsenic ions (As(III) and As(V)) are reduced to metallic arsenic (As(0)) which precipitates in aqueous solutions enabling separation from any remaining solutes, such as phosphates.

[0014] Thus, an object of the present invention relates to the provision of a novel amorphous metallic arsenic As(0) having low or no crystallinity for easy transformation into high-value materials, such as optoelectronic devices and / or high-speed semiconductors.

[0015] In particular, it is an object of the present invention to provide a method for obtaining metallic arsenic As(0) from a composition comprising an arsenic-bearing mineral, such as groundwater treatment sludge, mining waste, and contaminated soils that solves the above mentioned problems of the prior art with unsustainable and unsafe disposal of As-bearing waste, such as sludge. Thereby, arsenic recovery and upcycling from groundwater treatment sludge, mining waste, and contaminated soils can eliminate unacceptable disposal practices and help create local sources of arsenic compounds, which offsets conventional methods of obtaining these materials by environmentally-degrading mining activities.

[0016] Thus, one aspect of the invention relates to an amorphous form of metallic arsenic (As(0)), wherein said amorphous form displays a pair distribution function (PDF) peak at 2.5 A and wherein said amorphous form displays no X-ray diffraction (XRD) peaks.

[0017] Yet another aspect of the present invention is to provide a method for isolating metallic arsenic (As(0)) from a composition comprising an arsenic-bearing mineral, the method comprising the steps of: a) providing a composition comprising the arsenic-bearing mineral, b) adding a base to the composition of step a) to obtain a first aqueous phase comprising arsenic ions and a first solid phase, c) separating the first aqueous phase comprising arsenic ions from the first solid phase, d) contacting an aqueous solution comprising the arsenic ions of step c) with a reducing agent, to obtain a second aqueous phase and a second solid phase, wherein the second solid phase comprises the metallic arsenic (As(0)), and e) separating the second solid phase from the second aqueous phase of step d) to isolate the metallic arsenic (As(0)).

[0018] Still another aspect of the present invention is to provide use of a metallic arsenic (As(0)) according to the invention for semiconductor synthesis and / or in optoelectronic devices.

[0019] Another aspect of the invention relates to a method for removing arsenic from sludge, the method comprises the steps of a) providing a sludge comprising arsenic-bearing mineral, b) adding a base to the sludge and obtaining an aqueous phase comprising aqueous arsenic ions and a solid phase comprising mineral, and c) separating the aqueous phase from the solid phase to obtain the mineral and an aqueous phase comprising arsenic ions.

[0020] Brief description of the figures

[0021] Figure 1 shows arsenic extraction efficiency for the Ref sludge (synthetic As- bearing sludge) using 0.1 M NaOH,

[0022] Figure 2 shows time evolution of As and P extraction efficiency with different NaOH concentrations for the (a) HH (Holmehave (Fyn)), (b) Kerte, and (c) WB (West Bengal (India)) field sludges,

[0023] Figure 3 shows time evolution of arsenic (As) extraction efficiency with different NaOH concentrations for the (a) Ref (Synthetic As-bearing sludge) and (b) Ref+C (Ref sludge with added calcite) sludges, Figure 4 shows time evolution of arsenic extraction efficiency using 1 M NaOH from Ref sludge that was aged in air-dried conditions for 5 and 50 days,

[0024] Figure 5 shows (a) Fe K-edge EXAFS spectra and (b) corresponding Fourier transforms of EXAFS standards, field sludges (HH and WB) and synthetic sludges before and after extraction. Goe, Lp, 2LFh and Oxy-HFO represent goethite, lepidocrocite, 2-line ferrihydrite and oxyanion-rich hydrous ferric oxide, respectively, (c) The output of the linear combination fits (LCFs) of the samples using the Fe standard spectra is given in the bar charts,

[0025] Figure 6 shows X-ray diffraction patterns of field (HH and WB) and synthetic sludges following alkali extraction. The peaks identified by G, L, T and C represent goethite (a-FeOOH), PDF-#29-0713), lepidocrocite (y-FeOOH), PDF-#44-1415), trona (Na3H(CO3)2-2H2O, PDF-#29-1447) and calcite (CaCO3, PDF-#47-1743), respectively,

[0026] Figure 7 shows Selected Area Electron Diffraction (SAED) and High-resolution Transmission Electron Microscopy (HRTEM) images of As(0) nanoparticles produced from aqueous As(V) reduction using thiourea dioxide,

[0027] Figure 8 shows (a) Arsenic K-edge XANES, (b) EXAFS and (c) corresponding Fourier-transforms of upcycled As(0). From top to bottom, the spectra represent samples of initial aqueous As(III) and As(V), As(V) adsorbed to 2-line ferrihydrite (2LFh) and two sludge samples obtained from Holmehave (HH) and Kerte water works. Shell-by-shell fitting output (solid lines) is overlain on the data (dotted lines) in panel (C).

[0028] Figure 9 shows (a) Arsenic K-edge XANES, (b) EXAFS and (c) corresponding Fourier-transforms of upcycled As(0) compared to commercial As(0). The commercial As(0) shows a number of distinct As-As peaks at distances beyond the first As-As peak, which is not observed in the upcycled As(0) data,

[0029] Figure 10 shows Pair Distribution Function (PDF) data of the upcycled Holmehave

[0030] As(0) of the invention compared to commercial As(0) metal. The commercial As(0) PDF displays many As-As peaks extending out to at least 20 A, whereas no peaks in the upcycled As(0) data appear beyond 6-8 A, and

[0031] Figure 11 shows synchrotron XRD data of the upcycled As(0) of the present invention collected from the Kerte treatment plant, (a) XRD data of the upcycled As(0) of the present invention compared to commercial As(0) metal, and (b) upcycled As(0) of the present invention. No Bragg diffraction peaks are observed in the XRD of the upcycled As(0), which contrasts the strong Bragg diffraction peaks for the commercial As(0).

[0032] Figure 12 shows As(0) extraction from mining waste collected from Giant Mine site located in Yellowknife, Canada, using 0.1 M NaOH. 86% of As(0) was released to solution after 1 hour of extraction.

[0033] Figure 13 shows reductive transformation of extracted aqueous As(0) from mining waste at different TDO / As mol ratios. >95% of extracted aqueous As(0) was converted to amorphous As(0) at a TDO / As molar ratio of 15.

[0034] Figure 14 shows pair distribution function (PDF) analysis of the amorphous structure of upcycled As(0) from mining waste compared to commercial As(0) with crystalline structure.

[0035] Figure 15 shows transmission electron microscopy (TEM) image of the amorphous upcycled As(0) from mining waste and the inset image of the selected area electron diffraction pattern.

[0036] The present invention will now be described in more detail in the following.

[0037] Detailed description of the invention

[0038] Definitions

[0039] Prior to discussing the present invention in further details, the following terms and conventions will first be defined: Composition

[0040] In the present context a "composition" is to be understood as a chemical composition, formulation, or mixture of two or more substances. A composition may refer to specific elements and compounds present in a substance and how they are combined. The substances may be gases, fluids, powders or solids, solutes, etc.

[0041] Arsenic-bearing mineral

[0042] In the present context, "arsenic-bearing mineral" is to be understood as arsenic being complexed to a mineral or contained in the mineral matrix. A complex is a molecular entity formed by loose, typically non-covalent association involving two or more component molecular entities (ionic or uncharged). Arsenic may be present within the structure and / or on the surface of the mineral.

[0043] Sludge

[0044] In the present context "sludge" is to be understood as a semi-solid slurry that can be produced from a range of industrial processes, such as for example from water treatment, wastewater treatment or on-site sanitation systems. Sludge can be described as a thick, soft, wet mud or a viscous mixture of liquid and solid components. The term "sludge" is also sometimes as a generic term for solids separated from suspension in a liquid; this soupy material contains interstitial water between the solid particles. Sludge may comprise a variety of particles. Sludge wherein arsenic is present may be referred to as arsenic-bearing sludge.

[0045] Mining Waste

[0046] In the present context "mining waste" is to be understood as the materials left over after the extraction of specific valuable minerals and metals from ore during the mining process. Mining waste can take various forms and are typically classified into the following types: waste rock, tailings, slags and overburden.

[0047] Arsenic-contaminated soils

[0048] In the present context "arsenic-contaminated soils" is to be understood as soils that contain elevated levels of arsenic. Arsenic contamination in soils results from both human activities and natural sources, particularly from mining and smelting of metals, use of arsenic-based pesticides, herbicides, and wood preservatives, irrigation with arsenic-contaminated water, industrial emissions and improper waste disposal, application of contaminated fertilizers and livestock feed additives, and weathering of arsenic-rich rocks and minerals. In here "arsenic-contaminated soils" and "contaminated soils" may be used interchangeably.

[0049] Base

[0050] In the present context a "base" is a substance that can neutralize acid by reacting with hydrogen ions in an acid-base reaction. Thus, a base is a substance that reacts with acids to form a salt and which donates electrons, accepts protons, or releases hydroxide (OH-) ions in aqueous solution. In the present contexts a base is any base capable of mobilising arsenic ions in arsenic bearing minerals, i.e. making them soluble in the relevant solvent used. The following are different definitions of certain kinds of bases:

[0051] - Arrhenius Bases: These substances dissociate in aqueous solution to form hydroxide ions (OH“). Examples include sodium hydroxide (NaOH) and calcium hydroxide (Ca(OH)2). Arrhenius bases are slippery, taste bitter, and turn red litmus paper blue.

[0052] Bronsted Bases: These substances can accept hydrogen cations (H+), also known as protons. Aqueous hydroxides (OH“) are Bronsted bases because they react with H+to form water. Other examples include ammonia (NHs) and its organic derivatives (amines).

[0053] Lewis Bases: These compounds donate electron pairs. Lewis bases are more general and include species beyond hydroxides, such as molecules with lone pairs.

[0054] A base may be an inorganic base, which does not comprise organic elements. Inorganic bases are often salts of two ions, where one or both of the ionic partners is an element of the period table. Common inorganic bases may include combinations of hydroxy ions (OH-) and a positive ion (i.e. Arrhenius bases). A water soluble base is a base that may be solubilised in water in a sufficient amount to mobilise arsenic from arsenic bearing minerals. This may require a solubility of e.g. at least 0.01 mol / L.

[0055] Reducing agent

[0056] In the present context a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an electron recipient (called the oxidizing agent, oxidant, oxidizer, or electron acceptor). In the present context a reducing agent is capable of reducing the extracted arsenic ions to form metallic As(0). Thiourea oxide

[0057] In the present context thiourea dioxide or Thiox is an organosulfur compound that is used in the textile industry. It functions as a reducing agent. An aqueous solution of thiourea dioxide has a pH about 6.5. In here "Thiourea dioxide" and "Thiox" may be used interchangeably. Thiourea oxide has the formula (NH)(NH2)CSO2H.

[0058] Arsenic

[0059] In the present context arsenic is a natural component of the earth's crust and is widely distributed throughout the environment in the air, water, and land. It is highly toxic in its inorganic form. The greatest threat to public health from arsenic originates from contaminated groundwater. Inorganic arsenic is naturally present at high levels in the groundwater of a number of countries. The most common valence states of arsenic are:

[0060] • As(0) (metalloid arsenic, 0 oxidation state),

[0061] • As(III) (trivalent, 3 oxidation state, such as arsenites),

[0062] • As(V) (pentavalent, 5 oxidation state, such as arsenates), and

[0063] • Arsine Gas (-3 oxidation state).

[0064] Metallic arsenic (As( 0)

[0065] In the present context metallic As(0) is also known as elemental As(0).

[0066] Organic alkane arsenicals are of low toxicity, and elemental arsenic is virtually nontoxic. Arsenic in its pure elemental form is a silver-gray brittle, crystalline solid. Metallic arsenic is used as an alloying element in lead and copper alloys. Further, metallic arsenic is a semi-conductor with low thermal conductivity.

[0067] Amorphous As(0)

[0068] In the present context an amorphous solid is any non-crystalline solid in which the atoms and molecules are not organized in a definite lattice pattern. Typical characteristics of amorphous solids are:

[0069] - When cleaved or broken, amorphous solids produce fragments with irregular, often curved surfaces.

[0070] - They have poorly defined patterns when exposed to X-rays because their components lack a regular array.

[0071] - An amorphous, translucent solid is commonly referred to as a glass. In contrast to crystalline materials, amorphous solids lack long-range order at the atomic level. While crystalline solids have well-defined repeating patterns, amorphous solids do not exhibit such regularity.

[0072] In the present context "amorphous As(0)" is to be understood as As(0) not having a crystal lattice structure i.e. it is without long range ordered structure. While amorphous As(0) can aggregate to form solid flocs that can be separated by simple methods (filtration, gravitational settling), the As-As bonding throughout the solid is not coherent. Therefore, amorphous As(0) will lack a crystal structure and thereby have a low or no crystallinity. In general, amorphous compounds are more easily transformed than crystalline solids.

[0073] X-ray diffraction

[0074] In the present context X-ray diffraction (XRD) is a phenomenon in which the atoms of a crystal, by virtue of their uniform spacing, cause an interference pattern of the waves present in an incident beam of X-rays. The atomic planes of the crystal act on the X-rays in exactly the same manner as does a uniformly ruled diffraction grating on a beam of light. A beam of X-rays contacts a crystal with an angle of incidence 9. It is reflected off the atoms of the crystal with the same angle 9. The X-rays reflect off atomic planes in the crystal that are a distance d apart. The X-rays reflecting off two different planes must interfere constructively to form an interference pattern; otherwise, the X-rays would interfere destructively and form no pattern. To interfere constructively, the difference in path length between the beams reflecting off two atomic planes must be a whole number (n) of wavelengths (A), or nA. This leads to the Bragg law nA = 2d sin 9. By observing the interference pattern, the internal structure of the crystal can be deduced. X-ray diffraction (XRD) is the only laboratory technique that non-destructively and accurately obtains information such as chemical composition, crystal structure, crystal orientation, crystallite size, lattice strain, preferred orientation and layer thickness

[0075] XRD includes Synchrotron X-ray diffraction (SXRD), which is another complementary technique that provides more definitive information about crystalline soil minerals. SXRD offers better sensitivity and resolution of diffraction peaks than conventional laboratory XRD and can distinguish between low crystallinity and amorphous materials. Pair distribution function (PDF)

[0076] In the present context pair distribution function analysis (PDF) is an analytical technique that can provide structural information from disordered materials by using the complete powder XRD pattern. PDF is a synchrotron-based X-ray technique that yields information in the form of peaks that correspond to atomic pairs at a given radial distance. When the peaks decay or fade to zero, that corresponds to the coherent scattering domain or crystallite size (i.e., there are no more atomic pairs at longer interatomic distances). This technique is well- known for its ability to investigate highly disordered or amorphous materials. The pair distribution function technique is also known as total scattering analysis. Pair distribution function analysis is applied to the structural characterization of intrinsically disordered materials. Such materials can be amorphous, poorly crystalline, nano-crystalline or nano-structured. Pair distribution function (PDF) is an X-ray scattering technique that can be used to study the local structure of materials at the atomic scale. The technique requires scattering data to be collected to very high scattering angles using high energy powder diffraction coupled with Fourier transformation to produce the PDF data. Mathematically, the total scattering data are Fourier transformed into the Pair Distribution Function (PDF), which contains information on the distribution of atomic pairs within a sample.

[0077] Amorphous form of metallic arsenic (As(0))

[0078] The present inventors surprisingly found that using a specific method of isolating metallic arsenic from arsenic bearing minerals, as described further below, a novel amorphous form of arsenic metal (As(0)) is obtained, which displays a pair distribution function (PDF) peak at 2.5 A.

[0079] Thus, one aspect of the present invention relates to an amorphous form of metallic arsenic (As(0)), wherein said amorphous form displays a pair distribution function (PDF) peak at 2.5 A and wherein said amorphous form displays no X-ray diffraction (XRD) peaks, such as no X-ray diffraction peaks in an X-ray diffraction (XRD) measurement in the range of 1.6 to 2.4 A1in Q-space.

[0080] As can be seen in Example 3, Pair Distribution Function (PDF) data of the metallic As(0) of the invention (Figure 10) shows a single primary peak at 2.5 A, hence the metallic As(0) of the invention has single main As-As bond length than e.g. commercial As(0) metal which has a larger number of peaks between 2-20 A.

[0081] Amorphous metallic As(0) with a single primary PDF peak at around 2.5 A has a low or no crystallinity and thereby said amorphous metallic As(0) can be used to form high-value materials (such as arsenene optoelectronic devices, high-speed semiconductors) using less resources and energy as amorphous compounds having a low or no crystallinity are more easily transformed than crystalline solids.

[0082] Another aspect of the invention relates to an amorphous form of metallic arsenic (As(0)), wherein no peaks are observed in an X-ray diffraction (XRD) measurement in the range of 1.6 to 2.4 A1in Q-space. Example 3 herein indeed provides evidence that the amorphous As(0) of the present invention lacks Bragg diffraction peaks in XRD (Figure 11A-11B).

[0083] The PDF spectra may be obtained from a synchrotron-based X-ray method, preferably a synchrotron X-ray total scattering method.

[0084] In an embodiment, the amorphous form displays a single primary As-As atomic pair peak in a pair distribution function (PDF) spectrum in the range from 1 A to 20 A, such as 1 A to 10 A, preferably 1 A to 5 A. In an embodiment the amorphous As(0) of the present invention comprises no peaks above 8 A, such as above 6 A.

[0085] The PDF data (Figure 10) displays one or more minor PDF peaks around the major PDF peak at 2.5 A. The one or more minor peaks are displayed at around 2.8 A, and 3.8 A. Thus, in an embodiment, the amorphous form further displays pair distribution function (PDF) peaks at 2.8 A, and / or 3.8 A. In an embodiment, the single primary As-As atomic pair has a bond length of 2.5 A.

[0086] In an embodiment, the XRD is provided using a synchrotron-based X-ray method.

[0087] In a further embodiment, the amorphous form displays a peak at 11867 eV in an arsenic K-edge XANES spectrum. The arsenic K-edge XANES spectrum relates to the oxidation state of arsenic. Amorphous metallic As(0) having a small crystallite size are more easily transformed into high-value materials (such as arsenene optoelectronic devices, high-speed semiconductors) than crystalline solids. This is due to the fact that amorphous metallic As(0) having a small crystallite size, less than 1 nm, such as less than 0.7 nm, requires less resources and energy to be transformed into high- value materials compared to crystalline solids and / or amorphous metallic As(0) having a higher crystallite size. Thus, in a preferred embodiment, said amorphous form of arsenic metal has a crystallite size of less than 1 nm, such as less than 0.7 nm, such as around 0.3 nm.

[0088] In an embodiment, the amorphous form of metallic arsenic is obtainable by the method of the invention as described below. In another embodiment the amorphous form is obtained by said method.

[0089] Method for isolating metallic As(0)

[0090] The present inventors have surprisingly found that metallic arsenic may be isolated from arsenic-bearing minerals, such as for example groundwater treatment sludge and mining waste, using base treatment to mobilise arsenic ions in the mineral and subsequently separate and reduce the arsenic ions to metallic arsenic.

[0091] Thus, a further aspect of the invention relates to a method for isolating metallic arsenic (As(0)) from a composition comprising an arsenic-bearing mineral, the method comprising the steps of: a) providing a composition comprising the arsenic-bearing mineral, b) adding a base to the composition of step a) to obtain a first aqueous phase comprising arsenic ions and a first solid phase, c) separating the first aqueous phase comprising arsenic ions from the first solid phase, d) contacting an aqueous solution comprising the arsenic ions of step c) with a reducing agent, to obtain a second aqueous phase and a second solid phase, wherein the second solid phase comprises the metallic arsenic (As(0)), and e) separating the second solid phase from the second aqueous phase of step d) to isolate the metallic arsenic (As(0)).

[0092] Arsenic-bearing minerals are preferably in the form of a mineral where arsenic is comprised via complexation, i.e. by relatively weak non-covalent bonding to the mineral, but can also include arsenic contained in the mineral matrix. For example, arsenic oxides (arsenolite) and arsenic sulfides are considered an arsenic-bearing mineral because arsenic is contained in the mineral structure. Thus, in an embodiment, the arsenic-bearing mineral comprises arsenic ions complexed to the mineral and / or arsenic comprised in the mineral matrix.

[0093] Compositions comprising arsenic bearing minerals are particularly relevant in the fields of mining, wastewater treatment, ground water treatment, and contaminated soils. Arsenic is often removed from water using minerals, such as iron oxides, which results in a sludge comprising the mineral with bound or complexed arsenic. Thus, in an embodiment, the composition is a sludge, preferably a groundwater treatment sludge. As can be seen in Example 1, sludge or groundwater treatment sludge, either collected from field or made synthetically, comprises arsenic bound to mineral, such as arsenic complexed to mineral.

[0094] In an embodiment, the composition is a contaminated soil or a mining waste, preferably a mining waste. In an embodiment, the soil is arsenic-contaminated soil.

[0095] Sludges may contain several other components than the arsenic-bearing mineral, and thus in an embodiment, the sludge comprises one or more ions, wherein the one or more ions are selected from the group consisting of Al3+, Si4+, PO43-, Ca2+, Mn2+, Mn4+, Fe2+, Fe3+, S2-, Sb3+, Sb5+, As3+, As5+, and SC>42’.

[0096] In an embodiment, composition comprising arsenic-bearing mineral further comprises phosphate. This is of particular interest, since phosphate is also a valuable resource. In this regard the present method allows for the isolation of phosphate as well as metallic arsenic, as further described below. As can be seen in Example 1, sludge or groundwater treatment sludge, either collected from field or made synthetically, may comprise phosphate. The phosphate is preferably a derivative of a phosphoric acid, such as an anion, salt, or ester derived from phosphoric acid.

[0097] A number of minerals may be used to complex arsenic or comprises complexed arsenic naturally. In one embodiment of the present method, the mineral (i.e. the arsenic-bearing mineral) is selected from the group consisting of metal oxide, metalloid oxide, and aluminosilicate, or any combination thereof. Preferably, the mineral is selected from the group consisting of iron oxide, manganese oxide, and aluminosilicate, or any combination thereof, such as the group consisting of magnesium oxide, calcium oxide, aluminium oxide, and iron oxide. In a preferred embodiment, the metal oxide is iron oxide and / or manganese oxide.

[0098] Alkali extraction is particularly effective because groundwater treatment sludge typically consists of arsenic complexed to HFO surfaces, mining waste consists of arsenic oxides, and contaminated soils consist of arsenic oxides and arsenic sorbed to Fe oxides. In addition to favoring arsenic desorption from arsenic- bearing minerals, such as HFO sludge, alkaline conditions also promote Fe(III) phase transformation, which can enhance arsenic extraction by decreasing the abundance of reactive surface sites. For example, at alkaline pH, poorly-ordered Fe(III) precipitates, such as 2-line ferrihydrite and HFO, can transform readily to crystalline goethite (Goe) or lepidocrocite (Lp), with the transformation rate influenced by pH and co-sorbed arsenic and phosphate (P) species. Therefore, in a preferred embodiment, the metal oxide is iron oxide. In an embodiment, the metal oxide is hydrous metal oxide, such as hydrous ferric oxide (HFO). In an embodiment, the metal oxide is metal oxyhydroxide, such as ferric oxyhydroxide.

[0099] In an embodiment, the metalloid oxide is selected from the group consisting of silicon oxide and antimony oxide.

[0100] Arsenic may be in different oxidation states in its ionic forms, where As3+and / or As5+are the most common in arsenic bearing minerals such as those in sludges. Thus, in one embodiment, the arsenic ions are As3+and / or As5+. Also, the arsenic ions in the first aqueous phase are preferably in the form As3+and / or As5+. The extraction of arsenic from mineral in step b) is performed using a base. The alkali extraction, wherein pH is increased using a base for release of arsenic, is an attractive approach because it requires no advanced equipment and has a simple supply chain. By increasing the pH, the bond between the mineral, such as solid Fe oxide sorbent and surface-bound arsenic (and phosphate), is destabilized due to electrostatic repulsion. Increasing the pH also facilitates dissolution of other arsenic-bearing minerals.

[0101] The base added in step b) of the present method is thus preferably a base capable of separating the arsenic ions from the arsenic-bearing mineral. This is to form dissolved arsenic ions and solid mineral, thereby facilitating the separation of these components of the composition. Preferably the base is also capable of separating phosphate from the mineral. In an embodiment, the base has a pKa in the range 12 to 16, such as 13-16, 14-16, preferably between 15-16. In an embodiment, the base is an inorganic base, or any mixture thereof. In a further embodiment, the base is a water-soluble base, or any mixture thereof. In a preferred embodiment, the base is an alkali hydroxide, or any mixture thereof. In an embodiment, the base is selected from the group consisting of NaOH, LiOH, KOH, RbOH, CsOH, FrOH, Ba(OH)2, Sr(OH)2, or any mixture thereof. In a preferred embodiment, the base is LiOH, KOH and NaOH, preferably NaOH. NaOH is a widely available and low-cost base for increasing pH.

[0102] As the base may be a water-soluble base, the water-solubility of said base may be at least 1.5 g / L at 20°C, such as at least 2 g / L at 20°C, such in the range 1.5 g / L to 2000 g / L at 20°C, preferably in the range 15 g / L to 1900 g / L at 20°C. Preferably, the base is not Ca(OH)2 and / or NasCO2.

[0103] In an embodiment, the base is added to the composition in a solid-to-liquid ratio in the range 1 g / L to 300 g / L, preferably in the range 2 g / L to 250 g / L, more preferably 4 g / L to 200 g / L. For example, 300 g of sludge is mixed with 1 L of 1 M NaOH to yield 300 g / L solid-to-liquid ratio of NaOH (liquid) and sludge (solid). Concentrations of base used whether in g / L or molar concentration (M) may depend on the pKa, solubility and molecular weight of said base. However, in an embodiment, the base has a concentration of 0.05 to 10 M in the first aqueous phase of step b), such as 0.1-5 M, such as preferably 0.5 to 2 M. In an embodiment, the base is added to the sludge in a sludge-to-liquid (S / L) ratio in the range 10 g / L to 300 g / L, such as in the range 12 g / L to 250 g / L, such as in the range 13 g / L to 80 g / L, such as in the range 80 g / L to 200 g / L preferably in the range 13 g / L to 250 g / L, more preferably 13 g / L to 200 g / L. Alkali extraction, such as with NaOH, is highly effective at mobilizing arsenic and phosphate from treatment sludge leading to arsenic extraction efficiencies exceeding 99% and aqueous arsenic levels >250 mg / L, depending on the sludge / liquid (S / L) ratio and sludge composition. At a lower S / L ratio 98-99% of solid-phase arsenic is released for almost all sludges, resulting in minimal arsenic bound to the residual Fe(III) solids. At a higher S / L ratio, such as 200 g / L, the highest arsenic concentration is measured in the extraction solution for all sludges.

[0104] In an embodiment, the base is added to the mining waste in a sludge-to-liquid (S / L) ratio in the range 2 g / L to 300 g / L, such as in the range 10 g / L to 300 g / L, such as in the range 2 g / L to 200 g / L, such as in the range 2 g / L to 150 g / L, preferably in the range 2 g / L to 100 g / L, more preferably in the range 2 g / L to 50 g / L. Mining waste has a higher As% than groundwater treatment sludge.

[0105] The mobilisation of arsenic ions with base in step b) may typically be conducted at ambient temperature. In an embodiment, the step b) is performed at a temperature in the range 0 °C to 100 °C, such 5 °C to 50 °C, preferably 10 °C to 40 °C.

[0106] As mentioned above, sludges in particular may also comprise phosphates, which is a valuable resource. It is an advantage of the present method that phosphate is also released or mobilised from the arsenic-bearing mineral when increasing pH, and thus becomes part of the first aqueous phase along with the arsenic ions. Thus, in an embodiment, the first aqueous phase comprising arsenic ions, further comprises phosphate. The added base has also adjusted the pH of the first aqueous phase, and thus in one embodiment, the base is added to the composition in step b) to obtain a pH of the first aqueous phase in the range of pH 11 to pH 16, such as pH 11 to pH 14, preferably pH 12.0 to pH 13.5. Another advantage of the present method is that the mineral may also be isolated and re-used to e.g. capture arsenic from groundwater, because the first solid phase will mainly consist of said mineral without arsenic and phosphate complexed to it anymore. Thus, in an embodiment, the first solid phase comprises mineral. In a further embodiment, the first solid phase comprises less than 150 mg / kg arsenic ions, such as less than 100 mg / kg arsenic ions, preferably less than 50 mg / kg arsenic ions.

[0107] In principle, any method known to the skilled person for separating solids from non-solids, such as a liquid, can be used. Particularly, the separation of step c) may be by filtration, centrifugation, gravitational setting, or any combination thereof.

[0108] The reducing agent of step d) is added to convert the water-soluble arsenic ions to metallic arsenic, which precipitates and thus enables the separation of arsenic metal from the second aqueous phase formed. Hence, in an embodiment, the reducing agent is capable of reducing the arsenic ions to metallic arsenic (As(0)), preferably and amorphous form of metallic arsenic (As(0)).

[0109] Various reducing agents are applicable, but sulphur based reducing agents are particularly useful at high pH, which enables direct reduction in the first aqueous phase without any pH adjustment beforehand. Thus, in an embodiment, the reducing agent is a sulphur-based reducing agent, such as an organosulphur- based reducing agent. In an embodiment, the sulphur-based reducing agent is selected from the group consisting of thiourea oxide, sodium dithionite, sodium hydroxymethanesulfinate.

[0110] The sulphur-based reducing agent reacts to form sulphoxylate intermediates, wherein the sulphoxylate reduces arsenic (As(III) and / or As(V)) to form the metallic As(0). Sulphoxylate is very unstable, thus sulphur-based reducing agent acts as a precursor compound to form sulphoxylate. In a particular preferred preferred embodiment, the sulphur-based reducing agent is thiourea oxide ((NH)(NH2)CSO2H). As can be seen in Examples 1 and 2, the combination of a base, such as NaOH, and a reducing agent, which is active at high pH, such as thiourea oxide, is advantageous because adding a base, such as NaOH, to increase pH mobilizes As(III) and As(V) effectively from the treatment sludge.

[0111] The reduction activity of thiourea oxide increases with increasing pH, which prevents added processing steps. Furthermore, NaOH and thiourea oxide are both low-cost, have low toxicity, are easy to find and relatively stable (i.e., these reactions proceed in the presence of O2). An advantage of using e.g. NaOH and thiourea oxide is that thiourea oxide is a selective reductant that does not react with phosphate and does not reduce arsenic completely into highly-toxic arsine gas (AsHs). Therefore, co-recovery of valuable phosphate is possible. Phosphate occurs at high levels in the raw groundwater treatment sludge, thus providing additional revenue sources. In addition, the reaction of thiourea oxide with arsenic generates urea, which is a nitrogen-based fertilizer, leaving yet another potential resource to be recovered.

[0112] In an embodiment, the thiourea oxide is added to the sludge in a thiourea oxide to arsenic (TDO / As) mol ratio in the range 20 mol / mol to 600 mol / mol, such as in the range 20 mol / mol to 600 mol / mol, preferably in the range 30 mol / mol to 500 mol / mol, more preferably in the range 40 mol / mol to 300 mol / mol, most preferably in the range 50 mol / mol to 200 mol / mol.

[0113] The addition of thiourea oxide results in >99% removal of aqueous arsenic from the extraction solutions in optimum conditions, with the efficiency increasing with the TDO / As mol ratio and aqueous arsenic concentration. For example, aqueous arsenic in an extraction solution decreases 99.3% from 300 mg / L to <2 mg / L at a TDO / As mol ratio of 200.

[0114] In an embodiment, the thiourea oxide is added to the mining waste in a thiourea oxide to arsenic (TDO / As) mol ratio in the range 2 mol / mol to 200 mol / mol, such as in the range 3 mol / mol to 100 mol / mol, preferably in the range 4 mol / mol to 75 mol / mol, more preferably in the range 5 mol / mol to 50 mol / mol.

[0115] Mining waste has higher arsenic levels, thus less thiourea oxide is needed compared to sludge. In an embodiment, the temperature during reduction is in the range 25-150 °C, such as 30-120 °C, preferably 40-100 °C, more preferably 50-90 °C.

[0116] The arsenic reduction experiments are conducted at the above temperatures, such as at 80°C, and optionally under N2 atmosphere to avoid the oxidation of arsenite (As(III)) in the presence of extraction solutions. The arsenic reduction can be performed in both IXb and standard air atmospheres. In an embodiment, the reduction is performed under N2 atmosphere.

[0117] As the metallic arsenic precipitates, the second aqueous phase is obtained which comprises remaining solutes other than arsenic which were part of the first aqueous phase. Phosphate is one such solute and thus in one embodiment, the second aqueous phase of step d) comprises phosphate.

[0118] Arsenic ions are essentially removed from the second aqueous phase via reduction and precipitation. Thus, in an embodiment, the second aqueous phase of step d) comprises arsenic ions in an amount of less than 10 mg / L, such as less than 8 mg / L, preferably less than 4 mg / L, more preferably less than 1 mg / L. The arsenic ions have been reduced into metallic arsenic solids, thus leaving a small amount or no arsenic ions in the second aqueous phase.

[0119] The second solid phase essentially consists of metallic arsenic (As(0)). Thus, the second solid phase preferably comprises 90% metallic arsenic, such as 95% metallic arsenic, such as preferably 99% metallic arsenic. Impurities may be further removed by conventional methods such as washing, filtering with various solvent washes, etc.

[0120] In principle, any method known to the skilled person for separating solids from non-solids, such as a liquid, can be used. Particularly, the separation of step e) may be by filtration, centrifugation, gravitational setting, or any combination thereof.

[0121] An advantage of the present method is also that an amorphous form of metallic arsenic is obtained. Thus, in one embodiment, the metallic arsenic (As(0)) is obtained in an amorphous form, preferably an amorphous form according to the present invention, i.e. an amorphous form of metallic arsenic (As(0)), wherein said amorphous form displays a pair distribution function (PDF) peak at 2.5 A. The amorphous metallic As(0) of the present invention displays a single primary PDF peak at 2.5 A and no peaks beyond 6 A.

[0122] The composition comprising arsenic-bearing mineral, such as solid groundwater treatment sludge, can contain natural organic carbon, which is extracted by adding a base, such as NaOH. Thus, an extraction solution that comprises dissolved organic carbon (in addition to other ions such as As, P and Si) is easily recognized by the brown translucent color of the extraction solution. The dissolved organic carbon may originate from natural organic matter, such as biomass or plant matter, which degrades in groundwater and leads to a mixture of dissolved carbon compounds, such as acetate, oxylate, and citrate, that can be embedded in, attached to, and / or complexed to the groundwater treatment sludge. Dissolved organic carbon refers to a wide range of natural organic carbon compounds that are dissolved in groundwater. The presence of dissolved organic carbon renders a reducing agent, such as thiourea dioxide less effective for arsenic reduction. Dissolved organic carbon may be removed by oxidation. Therefore, an additional step whereby an oxidant, such as a H2O2 solution, is added to completely oxidize the dissolved organic carbon to CO2.

[0123] Thus in an embodiment, the method further comprises a step between step c) and step d) of c') contacting the first liquid phase of step c) with an oxidant.

[0124] Preferably the oxidant is selected from the group consisting of H2O2, ozone (O3), and HOCI. In an embodiment, the H2O2 is added in an amount to obtain a concentration in the liquid phase of step c) of 6% H2O2. In an embodiment, the added H2O2 is 30% w / v H2O2 in water. In an embodiment, the 30% w / v H2O2 is added in a I iqu id : liqu id ratio of 1:5.

[0125] As mentioned, the present method enables the isolation of phosphate, ultimately from the second aqueous phase. For example, as seen in Example 1, sludge or groundwater treatment sludge, either collected from field or made synthetically, may comprise phosphate which may be isolated. Therefore, co-recovery of valuable phosphate is possible. Phosphate occurs at high levels in the raw groundwater treatment sludge, thus providing additional revenue sources. Therefore, in an embodiment, the method further comprises: f) isolating phosphate from the second aqueous phase.

[0126] As previously mentioned, the composition comprising arsenic-bearing mineral may also comprise phosphate.

[0127] In an embodiment, the step f) of isolating phosphate comprises the steps of: i. adding a bivalent cation salt, such CaCh or MgCh, and ii. precipitating solids comprising Ca2+and phosphate and / or Mg2+and phosphate.

[0128] Amorphous arsenic metal is particularly useful in the semiconductor and optoelectronic industry. Thus, a further aspect of the invention relates to use of a metallic arsenic (As(0)) according to the invention for semiconductor synthesis and / or in optoelectronic devices.

[0129] Another aspect of the invention relates to a method for removing arsenic from sludge, the method comprises the steps of a) providing a sludge comprising arsenic-bearing mineral, b) adding a base to the sludge and obtaining an aqueous phase comprising aqueous arsenic ions and a solid phase comprising mineral, and c) separating the aqueous phase from the solid phase to obtain the mineral and an aqueous phase comprising arsenic ions.

[0130] Another aspect of the invention relates to a method for removing arsenic from soil or mining waste, the method comprises the steps of a) providing a soil or mining waste comprising arsenic-bearing mineral, b) adding a base to the soil or mining waste and obtaining an aqueous phase comprising aqueous arsenic ions and a solid phase comprising mineral, and c) separating the aqueous phase from the solid phase to obtain the mineral and an aqueous phase comprising arsenic ions.

[0131] In an embodiment, the soil is arsenic-contaminated soil. In an embodiment, the base is selected from the group consisting of NaOH, LiOH, KOH, RbOH, CsOH, FrOH, Ba(OH)2, Sr(OH)2, or any mixture thereof. In a preferred embodiment, the base is LiOH, KOH and NaOH, preferably NaOH.

[0132] It should be noted that embodiments and features described in the context of one of the aspects of the present invention also apply to the other aspects of the invention.

[0133] All patent and non-patent references cited in the present application, are hereby incorporated by reference in their entirety.

[0134] The invention will now be described in further details in the following non-limiting examples.

[0135] Examples

[0136] Example 1 - Extraction of arsenic from Arsenic-bearing mineral Aim of study

[0137] To extract metallic arsenic (As(0)) from As-bearing sludge for upcycling of arsenic.

[0138] Materials and methods

[0139] Field As-bearing sludge

[0140] Field sludges were collected from three Fe-based groundwater treatment plants, herein referred to as Holmehave, DK (HH); Kerte, DK; and West Bengal, IN (WB). These sludges were selected intentionally to span a wide range of treatment plant capacity, As removal mechanism and groundwater composition. The HH facility is a large-scale conventional aeration-filtration plant (i.e., Fe is sourced from natural Fe(II) in groundwater) with a permitted capacity of 5,500,000 m3 / year. No Fe dosing is required to remove the low influent arsenic levels (< 15 pg / L) below local drinking water limits. Kerte is a smaller scale plant (80,000 m3 / year) that doses FeCH to decrease the slightly higher influent arsenic levels (12-22 pg / L). The WB plant is a decentralized, community-scale facility (3,650 m3 / year) that uses electrochemical Fe(0) oxidation (i.e., Fe electrocoagulation) to decrease the high influent As levels (250-300 pg / L) to below 10 pg / L. The WB plant also uses aluminum sulfate (alum) as a coagulant, which results in high Al and S mass fractions in the sludge (Table 1).

[0141] The initial arsenic mass fractions (dry weight) of the HH, Kerte and WB sludges were 0.78±0.02, 1.03±0.03 and 0.69±0.04 g / kg, respectively, which corresponds to As / Fe ratios of 0.17-0.42 mol%. Consistent with the higher levels of phosphate (P) (0.1 to 1.6 mg / L) than arsenic in the raw groundwater, the initial P mass fractions of the HH (19.0±0.6 g / kg), Kerte (17.0±0.5 g / kg) and WB (3.9±0.1 g / kg) sludges greatly exceeded those of arsenic. All solids initially consisted of poorly ordered hydrous ferric oxide (HFO) with less structural order than synthetic 2-line ferrihydrite, with total Fe mass fractions of 18-32%. However, a considerable amount of calcite was also present in the WB sludge (Ca mass fraction of 10%; Table 1). The initial speciation of arsenic in all sludges was As(V) bound in the binuclear-corner sharing (2C) geometry to HFO.

[0142] All chemicals to synthesize As-bearing sludges were analytical grade and Milli-Q water (18.2 MQ-cm) was used for all experiments. Stock solutions of Fe(III) (500 mM) and As(V) (10 mM) were prepared with FeCl3-6H2O and Na2HAsO4-7H2O, respectively. Synthetic As-bearing sludge, herein referred to as the reference (Ref) sludge, was prepared by Fe and arsenic co-precipitation at room temperature by mixing 50 and 12.5 mL of the Fe and arsenic stock solutions, respectively, in 100 mL Milli-Q water. The suspension was then titrated with 1 M NaOH dropwise to pH 7.5 under vigorous stirring, adjusted to 500 mL with Milli-Q water and left to react for one day. The solids were separated by centrifugation (2466g for 5 min) and washed 6 times with Milli-Q water. The As / Fe solids ratio of the Ref sludge (0.47 mol%), which was verified by ICP-OES measurements of As and Fe in acid digestions, was consistent with that of the field sludges.

[0143] Several variations of the Ref sludge were synthesized to systematically investigate the impact of key variables on alkali arsenic extraction. First, we examined differences in synthetic sludge composition by co-precipitating Fe(III) and As(V) in single-solute systems of P, Ca, Si and Mn (i.e., P alone, Ca alone, etc) to represent major inorganic species and citrate to represent dissolved organic carbon (DOC). Fe(III) and As(V) were also co-precipitated in a synthetic groundwater matrix that contained all aforementioned species (Table 2). The concentrations of P, Ca, Si, Mn and DOC in both single-solute and synthetic groundwater experiments were selected to reflect the composition of As- contaminated groundwater in South Asia. The As / Fe, Ca / Fe, Si / Fe and Mn / Fe mol ratios of the initial synthetic groundwater sludge are reported in the SI (Table 3), alongside additional details on synthetic solution preparation. Next, since sludge at groundwater treatment facilities is often retained for extended periods in open- air dewatering basins before final disposal, the Ref sludge was aged in air-dried conditions for up to 50 days. Finally, to probe the role of co-occurring calcite, 10 mass% calcite was added to the Ref sludge prior to alkali arsenic extraction (sample herein referred to as Ref+C).

[0144] Alkali arsenic extraction

[0145] Alkali arsenic extraction was examined using a range of base concentrations (0.01, 0.1 and 1 M) to increase pH and extraction times of 1 hour, 1 day and 1 week, which was selected to span the range of timescales expected for arsenic desorption and the transformation of oxyanion-rich HFO. The alkali solutions were mixed with field or synthetic sludge at solid-to-liquid ratios of 13 g / L. Several bases were tested in initial experiments, but NaOH was focused on here (Figure 1). Unless otherwise noted, sample names are identified by the sludge type followed by concentration of NaOH and extraction time (i.e., WB sludge extracted using 1 M NaOH for 1 week is given by WB-1M-1W). The pH of the NaOH extractions was monitored for each experiment (Table 4), with little difference in pH measured in the presence of absence of the field or synthetic sludges. Following extraction, the suspension was separated using 0.22 pm nitrocellulose filters, with the filtered solids reserved and kept cold until solid-phase analysis. Total dissolved concentrations of arsenic, phosphate (P), and >25 other metal(loid)s and non-metals in the extraction solutions passing the filter were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) with a Perkin Elmer Avio 550 or Varian 720-ES instrument. The arsenic (and phosphate) extraction efficiency (in %) was calculated by dividing the arsenic mass released during the experiment by the initial arsenic mass in the solid phase. All extraction experiments were replicated, with the extraction efficiency data points and error bars representing the average and standard deviation of replicates (solid-phase characterization data were collected for one of the replicates).

[0146] Iron (Fe) and arsenic (As) K-edge XAS data were collected at the Balder beamline of MAX IV (Lund, SE) and at the P.65 beamline of DESY (Hamburg, DE). Fe K- edge XAS data were recorded in transmission mode with ionization chambers out to k of 13 A-l at room temperature. Arsenic K-edge XAS data were collected in fluorescence mode out to k of 13-14 A-l using a liquid helium cryostat. Replicate scans were merged, background-subtracted and normalized using the Athena software package.34 Fe K-edge EXAFS spectra were extracted using k3-weighting and were Fourier-transformed over the k-range of 2 to 12.5 A-l using a Kaiser- Bessel window with dk of 3 A-l. Following previous approaches, the major and minor Fe phases in post-extraction solids were quantified by linear combination fits (LCFs) of the Fe K-edge EXAFS spectra using the spectra of Fe standards. The standard spectra included goethite (a-FeOOH), lepidocrocite (y-FeOOH), 2-line ferrihydrite (2LFh) and a poorly ordered oxyanion-rich hydrous ferric oxide (Oxy- HFO) with less structural order than 2LFh. Additional details on the synthesis and characterization of the standards used in the LCFs are provided elsewhere. While the focus was largely on Fe structural transformations in this work, arsenic K-edge EXAFS shell-by-shell fits on a subset of samples was also performed to test for hypothetical differences in the arsenic bonding environment before and after extraction. Details of the shell-by-shell fitting procedure, as well as additional information on XAS data collection, processing, and analysis are provided in the SI.

[0147] XRD and BET analyses

[0148] Samples of the field and synthetic sludges (pre- and post-extraction) were airdried and ground with a mortar and pestle for XRD characterization. A Philips X'Pert Pro diffractometer equipped with a rotating sample stage and Co K-alpha radiation (1.79 A) was used to collect data over the range of 17.5 < 29 < 75°. A step size of 0.02° was used, resulting in total data collection time of ~4 h per sample. The MDI Jade 6 software was used for data analysis. The SSA of the samples was quantified by BET analysis using a Micromeritics Gemini VII instrument. Air-dried samples were lightly ground with a mortar and pestle and were degassed under vacuum at 60 °C overnight prior to BET measurements. The SSA values were obtained using the multi-point BET approach (N2 adsorption) with partial pressures of <0.3.

[0149] Results

[0150] As extraction efficiency

[0151] Field As-bearing sludge

[0152] For the HH sludge, arsenic extraction increased with NaOH concentration for all extraction times (Figure 2a). For example, 35.9±0.5% and 81.1±5.9% of solidphase arsenic was extracted after 1 hour using 0.1 M and 1 M NaOH, respectively. The arsenic extraction efficiency also tended to increase with extraction time, though to a lesser extent than with increasing NaOH concentrations. The effect of extraction time was most pronounced for the 0.1 M NaOH experiments, with arsenic extraction for the HH sludge increasing from 35.9±0.5 % at 1 hour to 66.8±8.9 % at 1 day. The highest arsenic extraction efficiency for the HH sample reached 97.2±2.7% (extraction solution contained 9.8±0.3 mg / L As), which was measured at the highest NaOH concentration of 1 M (pH = 13.2; Table 4), with little difference between 1 day and 1 week. Concurrent with arsenic release, alkali extraction of the HH sludge also mobilized phosphate, with the HH-1M-1D sample having the highest phosphate extraction efficiency of 82.6±10.2% (extraction solution contained 204±25 mg / L P). The extraction solution for the HH sludge contained no other elements at concentrations near or above arsenic and phosphate (and sodium (Na)), with the exception of Si (Table 1), highlighting the relative purity of the solution.

[0153] Consistent with the arsenic extraction behavior for the HH sludge, arsenic release from the Kerte sludge reached a maximum of >97% using 1 M NaOH for both 1 day and 1 week (Figure 2b). The highest arsenic extraction efficiencies for the Kerte and HH sludges resulted in residual solid phase As contents of 48.4 mg As / kg (Kerte) and 21.8 (HH), which is below the 50 mg As / kg EU lower guideline value for ecological risk in agricultural soils. With respect to phosphate release, the Kerte sludge showed an inverse trend in phosphate extraction efficiency with time, leading to the most effective phosphate extraction at 1 hour, which might be due to the formation of P-bearing solids at longer extraction times. Similar to the HH sludge, the extraction solutions for the Kerte sludge largely consisted of As, P and Si oxyanions, with minor concentrations of S (3.8±0.5 mg / L) and Al (4.1±0.1) at the highest NaOH concentration (Table 1).

[0154] The major difference in arsenic extraction efficiency among the three field sludges was observed for the WB sludge. Although the WB sludge exhibited similar trends in arsenic release with NaOH concentration and extraction time, the As extraction efficiency never exceeded 60% for any condition (Figure 2c). For example, at 1 hour of extraction time, the arsenic extraction efficiency increased from 10.0±l.l% to 20.7±2.9% using 0.1 M and 1 M NaOH, respectively. Consistent with the HH and Kerte sludges, the highest arsenic extraction efficiency was measured for the WB-1M-1W sample, but this value was only 57.4±4.6% (extraction solution contained 5.1±0.5 mg / L As).

[0155] Similar to the arsenic release behaviour, phosphate release was lower for the WB sludge. The most effective phosphate extraction conditions for the WB sludge (WB-1M-1D) yielded only 41.9±4.9% phosphate release, which corresponded to 21.2±2.5 mg / L P in the extraction solution. Finally, due to the high Al and S mass fractions of the initial WB sludge, the extraction solutions for this sludge contained substantial Al (>400 mg / L) and S (>20 mg / L) concentrations in addition to As, P and Si (Table 1).

[0156] Table 1 : Summary of field sludge data for 1 day of extraction for a broad range of element.AMany additional elements were measured but are not included here because they were below the instrument detection limit (DL), which was calculated as 3 times the standard deviation of replicate measurements of a 2% HNO3 blank. The instrument DL for all measured elements are as follows: Ca = 30 pg / L; B, S = 15-25 pg / L; Al, Sb, P, Si, Se = 5-10 pg / L; Cu, Pb, As, Ge, Te = 1-5 pg / L; Ba, Cd, Cr, Co, Fe, Li, Mg, Mn, Mo, Ni, K, Ti, V, Zn, Sn = <1 pg / L.

[0157] Reference synthetic As-bearing sludge

[0158] Alkali arsenic extraction from the Ref sludge resembled that of the HH sludge (Figure 3a-3b), but slightly higher extraction efficiencies were observed in some conditions. For the Ref-0.1M-1H sample, 62.7±0.6 % As was released, which was higher than 35.9±0.5% released for the HH-0.1M-1H sample. Consistent with the HH sludge, arsenic release from the Ref sludge increased with NaOH concentration, reaching a maximum of >97% for the Ref-IM-ID and Ref-lM-lW samples (Figure 3a-3b).

[0159] Table 2: Major ion composition of synthetic groundwater. The single-solute experiments used the same concentration of each individual ion. The synthetic groundwater composition is in good agreement with the range of values reported for As-contaminated groundwater across South Asia.

[0160] Concentration Element

[0161] (mg / L)

[0162] Si 15.3

[0163] Mn 5.8

[0164] P 1.6

[0165] Fe 13.7

[0166] Ca 122.4

[0167] Citrate 60

[0168] Table 3: Composition of synthetic sludge generated in synthetic groundwater.

[0169] Solids Ratio

[0170] Solutes As / Fe Si / Fe Mn / Fe Ca / Fe

[0171] (mol%) (mol%) (mol%) (mol%)

[0172] Synthetic

[0173] 0.47±0.05 1.8±0.3 0.15±0.02 0.32±0.08

[0174] Groundwater Table 4: Measured pH for NaOH solutions before and after alkali extraction for selected samples.

[0175] Sample Name pH

[0176] 0.1 M NaOH 12.77

[0177] 1 M NaOH 13.15

[0178] Ref-0.1M-1W 12.69

[0179] Ref-lM-lW 13.12

[0180] HH-1M-1W 13.15

[0181] WB-1M-1W 13.11 Synthetic As-bearing sludge variations

[0182] Sludge synthesized in single-solute systems of bivalent cations (Ca, Mg), oxyanions (Si and P), and dissolved organic carbon (DOC) exhibited similar arsenic extraction efficiency as the Ref sludge (Table 5). Table 5: Arsenic extraction efficiency for synthetic sludge variations using 1 M NaOH. For comparison, the arsenic extraction efficiencies for the Ref sludge using 1 M NaOH were 86.1±2.0° / o, 99.6±0.6% and 97.8±2.3° / o for 1 hour, 1 day and 1 week, respectively.

[0183] As extraction efficiency (%) Solutes

[0184] 1 hour 1 day 1 week

[0185] Si 79.9 95.7 94.1

[0186] Mn 87.3 100 92.7

[0187] P 83.3 98.3 90.4

[0188] Ca 84.6 100 95.8

[0189] Citrate 83.3 93.5 85.5

[0190] Synthetic 86.3±6.5 97.2±4.8 94.8±9.0 groundwater Furthermore, the As-bearing sludge generated in synthetic groundwater also released arsenic with similar efficiencies as the Ref sludge, with 86.3±6.5%, 97.2±4.8% at 94.8±9.0% measured using 1 M NaOH at 1 hour, 1 day and 1 week extraction times, respectively. These results indicate that the arsenic extraction efficiency did not depend strongly on the ionic composition of the solution in which the synthetic As-bearing sludge was generated.

[0191] For the aged sludge series, the arsenic extraction efficiency was generally similar regardless of aging, but slightly lower arsenic release was measured. At 1 hour of extraction time, 1 M NaOH mobilized ~73% of the arsenic for sludge aged for both 5 and 50 days (Figure 4). Increased extraction time resulted in more effective arsenic solubilization for the aging samples, with the highest arsenic extraction efficiency of >99% occurring at 1 week of extraction time, which closely matched the arsenic extraction behavior of the Ref sludge. No major differences in arsenic extraction efficiency were observed between the 5 and 50 day aged synthetic sludge.

[0192] In contrast to the presence of co-occurring ions during sludge synthesis and the aging of sludge, which both did not considerably alter arsenic extraction efficiency, the presence of 10 mass% calcite (Ref+C sample) inhibited arsenic release. Using 0.1 M NaOH, the release of arsenic was 8.0±2.5% at 1 hour, 23.6±3.5% at 1 day, and 27.4±6.6% at 1 week, which was substantially lower than the Ref sludge free of calcite, but matched the corresponding values for the WB sludge (WB- 0.1M-1H = 10. Oil.1%, WB-0.1M-1D = 26.3i0.7%, WB-0.1M-1W = 37.5+0.3%). Furthermore, the maximum arsenic extraction efficiency of 69.0i5.6% for the Ref+C sludge (1 M NaOH for 1 week) was much lower than the maximum for the Ref sludge (>99%).

[0193] Fe K-edge EXAFS spectroscopy

[0194] Field As-bearing sludge

[0195] Since little difference in arsenic extraction was observed for the HH and Kerte sludges, the structural analyses of the field sludges focused largely on the HH and WB samples. For the HH sludge, the first EXAFS oscillation transitioned from symmetric in the initial sludge to asymmetric after 1 M NaOH extraction (Figure 5a). In addition, the peak from 6.8-8.0 A-l and the oscillations at >8 A-l in the EXAFS spectra all increased in amplitude following alkali extraction. These changes were manifest in the Fourier transforms as an increased second-shell peak amplitude arising from edge-sharing Fe-Fe bonds (Fe-FeE; vertical line in Figure 5b), suggesting the transformation of poorly-ordered solids to Lp during arsenic extraction. Indeed, the LCFs of the HH sludge indicated an increase in Lp fraction following extraction (Lp = 8.7% for HH-0.1M-1W, Lp = 29.4% for HH-1M- 1W; Table 6) at the expense of Oxy-HFO, with the initial HH solids consisting entirely of poorly-ordered Fe(III) precipitates (93.9% Oxy-HFO, 6.1% 2LFh).

[0196] Table 6: Quantification of Fe phases by Fe K-edge EXAFS LCFs- The k-range used in the fits was 2 to 12.5 A1. The sum of all Fe standards in the LCFs was constrained to 100%.

[0197] Sample Oxy-rich 2LFh Goe Lp (%) R-

[0198] Name HFO (%) (%) (%) Factor

[0199] HH 93.9 6.1 (1.7) 0.018

[0200] (2.2)

[0201] HH-0.1M-1W 85.8 5.5 (2.8) 8.7 (0.9) 0.022

[0202] (4.2)

[0203] HH-1M-1W 54.6 16.0 29.4 (0.7) 0.008

[0204] (2.9) (1.9)

[0205] WB 30.2 59.3 10.5 0.029

[0206] (3.6) (2.8) (1.0)

[0207] WB-1M-1W 16.7 72.5 10.8 0.026

[0208] (2.8) (3.6) (1.0)

[0209] Ref-O.1M-1H 58.3 22.3 19.4 0.045

[0210] (3.9) (3.3) (1.2)

[0211] Ref-O.1M-1D 44.3 26.1 29.6 0.028

[0212] (3.3) (2.6) (0.9)

[0213] Ref-0.1M-1W 32.8 23.8 43.4 0.015

[0214] (2.3) (1.9) (2.3)

[0215] Ref-IM-IH 45.2 54.8 0.024

[0216] (3.6) (1.1)

[0217] Ref-IM-ID 42.8 57.2 0.026

[0218] (2.4) (1.2)

[0219] Ref-lM-lW 29.3 70.7 0.042

[0220] (2.9) (1.4)

[0221] Ref+C 20.5 75.5 4.0 (0.8) 0.020

[0222] (3.1) (2.4) Ref+C-lM-lW 18.5 74.7 6.8 (1.3) 0.047

[0223] (4.8) (3.8)

[0224] In contrast to the EXAFS spectra of the HH samples, the EXAFS spectra of the WB sludge pre- and post-extraction were essentially identical. All characteristic EXAFS features were unchanged following alkali extraction and the shape of the second- shell peak, including the relative amplitudes of edge- and corner-sharing Fe-Fe (Fe-FeC) peaks, were similar. The EXAFS LCFs confirmed no major Fe phase transformations in the WB sludge following alkali extraction in conditions that released the most arsenic. Poorly-ordered Fe(III) precipitates made up the majority of the WB solids before (30.2% Oxy-HFO, 59.3% 2LFh) and after (16.7% Oxy-HFO, 72.5% 2LFh) extraction using 1 M NaOH for 1 week. The LCFs returned a minor fraction of Goe to make up the remainder (10.5% initially, 10.8% after extraction), though it is likely this Goe fraction reflects a small difference in Fe coordination between the WB samples and the poorly ordered EXAFS standards, rather than the presence of an actual Goe fraction.

[0225] Reference synthetic As-bearing sludge

[0226] The transformation of the Ref sludge following alkali arsenic extraction was reflected in the EXAFS spectra largely by a progressive flattening of the first EXAFS oscillation from 3 to 5 A-l and increased intensities of the peak near 7.5 A- 1 and oscillations at k >8 A-l. These changes coincided with an increase in the amplitude of the Fe-FeE and Fe-FeC peaks in the Fourier transforms, with the shape of the second-shell peaks progressively resembling Goe. The transformation of the Ref sludge to Goe was quantitatively determined by LCFs to increase with NaOH concentration and extraction time. For example, at 0.1 M NaOH, the fraction of Goe in the post-extraction samples increased from 19.4 to 29.6 and 43.4% at 1 hour, 1 day and 1 week, which was coupled to a decrease in Oxy-HFO from 58.3 to 32.8% (2LFh was consistently 22.3-26.1%). Similarly, at 1.0 M NaOH, the fraction of Goe increased from 54.8 to 57.2 and 70.7% at 1 hour, 1 day and 1 week of extraction, which was again balanced by a decrease in Oxy- HFO from 45.2 to 29.3%. Conditions that released arsenic most effectively were also found to yield the highest Goe fraction. However, the LCFs did not return a value of 100% Goe for any sample, which could also be an artifact of a difference in crystallinity between the Goe standard used in the LCFs and the Goe formed by alkali extraction. No Lp was detected in any Ref sludge extraction sample.

[0227] Reference synthetic sludge with calcite.

[0228] No major changes in the EXAFS spectra of the Ref+C sludge were apparent before and after As extraction. The EXAFS spectra of the pre- and post-extraction solids both contained a roughly symmetric first oscillation, a peak near 7.5 A-l with nearly identical amplitude, and similarly broad and low intensity oscillations at k >8 A-l. The Ref+C solids pre- and post-extraction also displayed a similar shape and relative amplitude of the second-shell peak in the Fourier transforms, which matched that of the 2LFh standard. The LCFs of the Ref+C sludge returned nearly identical fractions of Oxy-HFO (18.5-20.5%), 2LFh (74.7-75.5%) and Goe (4.0- 6.8%) before and after arsenic extraction. These LCF results indicate that Fe in the Ref+C sludge was resistant to transformation even in the most aggressive extraction conditions, consistent with the WB sludge.

[0229] X-ray diffraction

[0230] Field As-bearing si

[0231] The XRD pattern of the HH-1M-1W sample was generally characterized by diffuse scattering, with broad peaks near 38° and 72° 20, indicative of poorly-ordered Fe(III) precipitates (Figure 6). However, this sample also exhibited two characteristic Bragg peaks of Lp near 55° and 72.5° 29, in agreement with the EXAFS LCF results. In addition, the HH-1M-1W sample contained a small peak near 34.4° 29, which is consistent with the presence of a small fraction of trona, a Na-rich evaporite mineral that likely formed due to the high NaOH content of the extractions.

[0232] In contrast to the HH-1M-1W sample, the WB-1M-1W sample displayed an XRD pattern that was dominated by intense Bragg diffraction peaks of calcite. No diffraction peaks for any crystalline Fe(III) (oxyhydr)oxides (or secondary As- or P-bearing minerals) were present in the XRD pattern of the WB-1M-1W sample, consistent with the predominance of poorly-ordered Fe(III) precipitates determined in the EXAFS analysis.

[0233] Synthetic As-bearing sludge For the Ref-0.1M-1W sample, the XRD pattern was dominated by diffuse scattering and broad peaks of poorly-ordered Fe(III) precipitates, but several low intensity peaks indicative of Goe were apparent. Additional and more resolved Bragg peaks for Goe appeared in the Ref-lM-lW sample, which agrees well with the increase in Goe fraction with increasing NaOH concentration resolved by the EXAFS LCFs. In addition, the Ref-lM-lW sample also contained low-intensity peaks from trona, similar to the post-extraction field sludges.

[0234] The XRD pattern of the Ref+C-lM-lW solids was characterized by many well- resolved Bragg peaks of calcite and some low-intensity peaks of trona. Bragg diffraction peaks from Goe, Lp or any other Fe(III) (oxyhydr)oxides were absent from the XRD pattern. However, a broad and low amplitude peak centered near 38-40° was apparent (Figure 6), which suggests the presence of a high fraction of poorly-ordered Fe(III) precipitates, consistent with the EXAFS LCFs results.

[0235] Specific surface area (SSA) measurements

[0236] Field As-bearing sludge

[0237] The specific surface area (SSA) of the initial HH sludge was 224 m2 / g, which matches the typically high SSA (~300 m2 / g) of freshly precipitated nanoparticulate HFO (Table 7). A moderate reduction of the HH sludge SSA with extraction time was measured using 0.1 M NaOH, with the largest incremental decrease to 189 m2 / g observed after 1 hour of extraction (184 m2 / g at 1 day, 177 m2 / g at 1 week). By contrast, a drastic reduction in SSA to <15 m2 / g (<6% of the initial SSA) was measured using 1 M NaOH for all extraction times. This decrease in SSA agrees well with the formation of more crystalline solids identified in the EXAFS and XRD data and is consistent with poor retention of arsenic in this sample presumably due to a decrease in arsenic sorption sites.

[0238] The SSA properties of the WB sludge before and after extraction were considerably different from the HH sludge. First, the initial WB solids had a relatively low SSA of 102 m2 / g, consistent with the presence of crystalline calcite in addition to poorly-ordered Fe(III) precipitates. Second, arsenic extraction using 1 M NaOH decreased the SSA of the WB sludge to a minimum of 53.7 m2 / g, which was observed for 1 hour of extraction time (61.8 m2 / g at 1 day, 63.4 m2 / g at 1 week). This relatively low decrease in SSA of 47% contrasts the >90% reduction observed for the HH sludge in identical extraction conditions.

[0239] Synthetic As-bearing sludge

[0240] The SSA of the Ref sludge decreased following alkali extraction similarly to the HH sludge, but the decrease was slightly higher for the Ref sludge. The Ref sludge SSA was initially 329 m2 / g and decreased to 155, 121 and 93.1 m2 / g following extraction using 0.1 M NaOH for 1 hour, 1 day and 1 week, respectively. The 1 M NaOH extraction produced an even greater decrease in SSA of the Ref sludge to < 10 m2 / g, with little deviation in SSA measured using different extraction times (Table 7).

[0241] Table 7: Summary of sludge SSA measurements before and after arsenic extraction

[0242] Sample SSA (m2 / g)

[0243] Extraction Solution -

[0244] Name Initial 1 hour 1 day 1 week

[0245] 0.1 M NaOH 189 184 177

[0246] HH 224

[0247] 1 M NaOH 13.5 10.9 7.6

[0248] WB 1 M NaOH 102 53.7 61.8 63.4

[0249] 0.1 M NaOH 155 121 93.1

[0250] Ref 329

[0251] 1 M NaOH 4.9 5.4 5.1

[0252] Ref+C 1 M NaOH 165 8.4 10.7 7.8

[0253] For the Ref+C sludge, the initial SSA was 165 m2 / g, illustrating the impact of crystalline calcite. Although the X-ray characterization data did not indicate a major Fe phase transformation of the Ref+C sludge during alkali extraction, the SSA was significantly modified. After 1 hour of extraction using 1 M NaOH, the SSA decreased to 8.4 m2 / g and remained < 11 m2 / g at longer extraction times. Conclusion

[0254] NaOH concentration

[0255] The arsenic extraction efficiency increased with NaOH concentration (and thus pH) in all experiments, regardless of sludge composition and synthesis procedure, with extraction time playing a secondary role. For all sludges, arsenic K-edge EXAFS analysis indicated that arsenic was present initially as arsenate (As(V)) bound to Fe(III) precipitates. As(V)-HFO surface complexes are destabilized by NaOH addition via intense charge repulsion between the negatively charged sorbent surface and As(V) at high pH. In addition to charge repulsion, NaOH promoted Fe(III) precipitate crystallization and SSA reduction. Thus, both electrostatic repulsion and precipitate crystallization play key roles for arsenic release and extraction.

[0256] Concomitant with arsenic release, the initial Fe(III) solids tended to increase in crystallinity and decrease in SSA with increasing NaOH concentration and extraction time, which can contribute to arsenic extraction due to a decrease in arsenic sorption sites and potentially by arsenic release during the dissolution- reprecipitation pathway of crystallization.

[0257] An essential initial step to upcycle groundwater treatment sludge is to effectively separate arsenic from the HFO sorbent, with the end goal of subsequently upcycling the mobilized arsenic. The results clarified the chemical conditions and timescales required for effective arsenic extraction.

[0258] Example 2 - Reduction of extracted arsenic to form metallic As(0) Aim of study

[0259] To convert arsenate (As(V)) or arsenite (As(III)) into metallic As(0) particles. Materials and methods

[0260] The extracted arsenic (see Example 1) is reduced to form metallic As(0) using sulfur-based chemical reductants. The most promising reductant is formamidinesulfinic acid (common name: thiourea dioxide; Thiox), a lost-cost and low-toxicity reductant used in the textile industry. The use of thiourea dioxide is strategic because its reducing reactivity increases with increasing pH, which is the opposite of most reductants that can reduce As(III) or As(V), such as divalent tin (SnII), which increases in reducing reactivity with decreasing pH. Therefore, thiourea dioxide is optimal for applying directly to the "extraction" solution created by adding NaOH to the raw groundwater treatment sludge (Example 1). The data show that the addition of 400 mM thiourea dioxide to the extraction solutions heated to 80°C results in rapid (< 10 minutes) and essentially complete transformation (>95%) of aqueous arsenic (initial As >50 mg / L) to particulate As(0) (Table 8). Table 8: Aqueous composition of NaOH extractions before and after thiourea dioxide addition

[0261] Results

[0262] After separation of the particulate As(0) by filtration, centrifugation or gravitational settling, the remaining solution contains low dissolved arsenic levels (1-3 mg / L) and high phosphate concentrations (>500 mg / L).

[0263] Conclusion

[0264] Metallic As(0) was obtained by reducing aqueous arsenic using thiourea dioxide and leaving a remaining solution containing low dissolved arsenic levels (1-3 mg / L).

[0265] Example 3 - Analysis of recovered AsfO)

[0266] Aim of study

[0267] To characterize the recovered As(0) and comparing the recovered As(0) with commercial As(0) metal.

[0268] Materials and methods

[0269] See Examples 1 and 2. Synchrotron X-ray diffraction was performed.

[0270] Results

[0271] The recovered As(0) in the process as described in Examples 1 and 3 lacks Bragg diffraction peaks in XRD (Figure lla-llb), which indicates an amorphous nature. Indeed, HRTEM (Figure 7) and synchrotron-based arsenic K-edge XAS data (Figure 8a-8c) reveal an amorphous structure that consists of only a single As-As bond positioned at 2.45 A (no coherently-scattering second-neighbor As-As bonds exist in this material).

[0272] Pair Distribution Function (PDF) data of the upcycled As(0) of the invention compared to commercial As(0) metal (Figure 10) provides evidence that the recovered As(0) has shorter As-As bonds (the first peak at lowest interatomic distance) than commercial As(0) metal. This amorphous structure is significantly different than commercial As(0) purchased from a chemical supply company that contains intense Bragg diffraction peaks (Figure lla-llb) and many As-As bonds located at different interatomic distances (Figure 9a-9c). The finding that the recovered As(0) is amorphous is important because the subsequent conversion of As(0) to form high-value materials (such as arsenene optoelectronic devices, high-speed semiconductors) is expected to require less resources and energy if the initial solid is not highly- crystalline because amorphous compounds are more easily transformed than crystalline solids.

[0273] Conclusion

[0274] The recovered As(0) lacks Bragg diffraction peaks in XRD, which indicates an amorphous nature. This amorphous structure is significantly different than commercial As(0) purchased from a chemical supply company.

[0275] Example 4 - Extraction of arsenic from arsenic mining waste

[0276] Aim of study

[0277] To extract metallic arsenic (As(0)) from arsenic mining waste for upcycling of arsenic.

[0278] Materials and methods

[0279] Arsenic mining waste

[0280] Arsenic mining waste samples were collected from the Giant Mine site located in Yellowknife, Canada, where previous gold mining produced an astonishing 237,000 tons of arsenic waste. Arsenic dominated the bulk composition of arsenic mining samples with concentrations ranging from 438 to 901 g kg-1.

[0281] Alkali arsenic extraction

[0282] Alkali arsenic extraction was examined using 0.1 NaOH solutions after 1 hour at room temperature. The initial mining waste solid to liquid ratio was set to 5 g / L. Following extraction, the suspension was separated using 0.22 pm nitrocellulose filters, with the filtered solids reserved and kept cold until solid-phase analysis. Total dissolved concentrations of arsenic in the extraction solutions passing the filter were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) with a Perkin Elmer Avio 550 or Varian 720-ES instrument. The arsenic extraction efficiency (in %) was calculated by dividing the arsenic mass released during the experiment by the initial arsenic mass in the solid phase.

[0283] Results

[0284] As extraction efficiency

[0285] After reaction for 1 hour, 86% of arsenic was extracted from arsenic mining waste and the remaining 14% of arsenic was in the form of arsenic-solid phases (Figure 12).

[0286] Conclusion

[0287] After reaction for 1 hour, 86% of arsenic was extracted from arsenic mining waste.

[0288] Example 5 - Reduction of extracted arsenic from mining waste to form metallic As(0)

[0289] Aim of study

[0290] To convert aqueous arsenate (As(V)) or arsenite (As(III)) into metallic As(0) particles.

[0291] Materials and methods

[0292] The extracted arsenic (see Example 4) was reduced to form metallic As(0) using sulfur-based chemical reductants, which was described in detail in Example 2. Arsenic reduction experiments were conducted at 80°C under N2 atmosphere to avoid the oxidation of arsenite (As(III)) in the presence of extraction solutions. After the extraction solutions were heated to 80°C, solid thiourea dioxide (TDO) was added and the molar ratio of thiourea dioxide to aqueous arsenic was set to 5 and 15. The higher ratio essentially completely transforms (>95%) aqueous arsenic to particulate As(0).

[0293] Results

[0294] After separation of the particulate As(0) by filtration, centrifugation or gravitational settling, the remaining solution contains low dissolved arsenic levels. 96% of arsenic was reduced to particulate As(0) at the higher TDO / arsenic molar ratio (15) (Figure 13).

[0295] Conclusion

[0296] 96% of arsenic was reduced to particulate As(0) at the higher TDO / arsenic molar ratio. Example 6 - Analysis of recovered As(O) from mining waste

[0297] Aim of study

[0298] To characterize the recovered As(0) from mining waste and comparing the recovered As(0) with commercial As(0) metal.

[0299] Materials and methods

[0300] Pair Distribution Function (PDF) data of the upcycled As(0) of the invention compared to commercial As(0) metal (Figure 14) provides evidence that the recovered As(0) with an amorphous structure has shorter As-As bonds (the first peak at lowest interatomic distance) than commercial As(0) metal. Additionally, transmission electron microscopy (TEM) image with the selected area electron diffraction pattern (Figure 15) reveals an amorphous structure of recovered As(0) with nano-sized spherical morphology.

[0301] Results

[0302] The recovered As(0) from mining waste lacks long-range order, which indicates an amorphous nature. This amorphous structure is significantly different than commercial As(0) purchased from a chemical supply company (Figures 14-15).

[0303] Conclusion

[0304] The recovered As(0) from mining waste have an amorphous structure that is significantly different from commercial As(0).

[0305] References

[0306] W090 / 06820

[0307] Shan J, Saez AE, Ela WP.. J Environ Eng (New York). 2010

[0308] Feb;136(2):238-245

Claims

Claims1. An amorphous form of metallic arsenic (As(0)), wherein said amorphous form displays a pair distribution function (PDF) peak at 2.5 A and wherein said amorphous form displays no X-ray diffraction (XRD) peaks.

2. The amorphous form of metallic arsenic (As(0)) according to claim 1, wherein the amorphous form further displays pair distribution function (PDF) peaks at 2.8 A and / or 3.8 A3. The amorphous form of metallic arsenic (As(0)) according to any one of the preceding claims, wherein the amorphous form displays a single As-As atomic pair peak in a pair distribution function (PDF) spectrum in the range from 1 A to 20 A, such as 1 A to 10 A, preferably 1 A to 5 A.

4. The amorphous form of metallic arsenic (As(0)) according to any one of the preceding claims, wherein no peaks are observed in an X-ray diffraction (XRD) measurement in the range of 1.6 to 2.4 A1in Q-space.

5. The amorphous form of metallic arsenic according to any one of the preceding claims, wherein said amorphous form has a crystallite size of less than 1 nm, such as less than 0.7 nm.

6. A method for isolating metallic arsenic (As(0)) from a composition comprising an arsenic-bearing mineral, the method comprising the steps of: a) providing a composition comprising the arsenic-bearing mineral, b) adding a base to the composition of step a) to obtain a first aqueous phase comprising arsenic ions and a first solid phase, c) separating the first aqueous phase comprising arsenic ions from the first solid phase, d) contacting an aqueous solution comprising the arsenic ions of step c) with a reducing agent, to obtain a second aqueous phase and a second solid phase, wherein the second solid phase comprises the metallic arsenic (As(0)), ande) separating the second solid phase from the second aqueous phase of step d) to isolate the metallic arsenic (As(0)).

7. The method according to claim 6, wherein the arsenic-bearing mineral comprises arsenic ions complexed to the mineral or arsenic comprised in the mineral matrix.

8. The method according to any one of claims 6 or 7, wherein the composition is selected from the group consisting of sludge, soil, and mining waste.

9. The method according to any one of the claims 6-8, wherein composition comprising arsenic-bearing mineral further comprises phosphate.

10. The method according to any one of the claims 6-9, wherein the base is an inorganic base, or any mixture thereof.

11. The method according to any one of the claims 6-10, wherein the base is selected from the group consisting of NaOH, LiOH, KOH, RbOH, CsOH, FrOH, Ba(OH)2, Sr(OH)2, or any mixture thereof.

12. The method according to any one of the claims 6-11, wherein the first aqueous phase comprising arsenic ions, further comprises phosphate.

13. The method according to any one of the claims 6-12, wherein the reducing agent is a sulphur-based reducing agent, such as an organosulphur-based reducing agent.

14. The method according to any one of the claims 6-13, wherein the sulphurbased reducing agent is selected from the group consisting of thiourea oxide, sodium dithionite, and sodium hydroxymethanesulfinate.

15. The method according to any one of the claims 6-14, wherein the second aqueous phase of step d) comprises phosphate.

16. The method according to any one of the claims 6-15, wherein the second solid phase comprises 90% metallic arsenic, such as 95% metallic arsenic, such as preferably 99% metallic arsenic.

17. The method according to any one of the claims 6-16, wherein the metallic arsenic (As(0)) is obtained in an amorphous form, preferably an amorphous form according to any one of claims 1-5.

18. The method according to any one of the claims 6-17, wherein the method further comprises a step between step c) and step d) of c') contacting the first liquid phase of step c) with an oxidant.

19. The method according to any one of the claims 6-18, wherein the method further comprises: f) isolating phosphate from the second aqueous phase.

20. An amorphous form of metallic arsenic according to any one of claims 1-5, wherein the amorphous form is obtainable by the method of claims 6-19, such as obtained by the method of claims 6-19.

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