Process for the production of clay mineral composites and their use for the separation of organic, halogenated compounds from aqueous solutions

Abstract

Process for the production of clay mineral composites comprising the steps: a) providing a mineral raw material comprising at least one three-layer clay mineral, b) suspension of the mineral raw material in at least one alkanol, c) adding an aqueous solution containing at least one cationic surfactant, and d) Separation of the aqueous solution and drying.

Description

The invention relates to a method for producing clay mineral composites, to the use of the clay mineral composite for separating organic halogenated compounds from aqueous solutions, to a method for separating organic halogenated compounds from aqueous solutions and to the use of the clay mineral composite in barriers and / or protective layers for collecting organic halogenated compounds.Per- and polyfluorinated alkyl substances (PFAS, also per- and polyfluorinated compounds, PFCs) are compounds in which hydrogen atoms in the molecule have been replaced either partially (poly) or completely (perfluorinated) by fluorine atoms, such as polytetrafluoroethylene PTFE (Teflon®) or polyvinyl fluoride (PVF). Per- and polyfluorinated alkyl compounds are nowadays included in many water- and soil-repellent coatings. The compounds can separate from coatings of clothing and other objects and thus enter the environment. It is problematic here that a large number of these compounds are not biodegradable and thus remain in the environment for a long time. Many PFAS are also toxic and are suspected of promoting or triggering cancer.Activated carbon, for example, is used to remove PFAS, in particular at elevated loads of water or industrial wastewater used for drinking water production. Depending on the adsorption of the present PFAS, the use of high specific amounts of activated carbon may be necessary, as a result of which this process with about 2 ≅ / kg of activated carbon can be very cost-intensive (Janda et al. 2017).In addition to activated carbon (Zhi and Liu 2018), ion exchange resins are also tested and used for the removal of PFASs, for example for the purification of groundwater (Ross et al. 2018).A further possibility for removing pollutants including medicament residues consists in the use of clay mineral-organic composites, wherein the clay minerals have been produced by geochemical processes in the deposit and the organic components are derived from fossil residues from algae. Trobs et al. describe a process for separating endocrine active substances such as bisphenol A or 17β-estradiol from water using the rock alginite (Trobs et al. 2018).Starting from the use of natural clay mineral-organic composites, synthetic clay mineral-organic composites were synthesized for use as adsorbents. Here, three-layer clay minerals such as montmorillonite serve as the base component. This type of clay mineral has the advantage that the layer structure has negative charges in the interlayer region. The natural negative charge in the crystal lattice results from the incomplete saturation with positively charged metal ions, thus permitting easier exchange of metal ions and acid protons.Yan et al. disclose the removal of PFASs from groundwater collected on a former U.S. air weapon support point on which aqueous film-forming foams containing PFAS have been used, by smectite modified by quaternary ammonium ions (Yan et al. 2020). Further, a purification efficiency of 74 to 99% is described and the results are compared with the adsorption of the PFASs by activated carbon, biocarbon and ion exchange resins.Zhou et al. describe the binding of perfluorooctanesulfonate to organo-montmorillonite which has been modified by hexadecyltrimethylammonium bromide (Zhou et al. 2010). The modification is carried out by means of hot water at a temperature of 80° C. for 2 h with gentle stirring. The structure of mineral raw materials comprising triple-layer clay minerals is disadvantageously altered at temperatures above 50° C. or even destroyed over relatively long periods of time (Kernland et al. 2006).WO 2010 / 065996 A1 discloses modified clay mineral adsorbents for sorption of chemical compounds, in particular anionic and / or hydrophobic compounds. The modified clay mineral comprises palygorskite, a 2:1 layered silicate having the formula (Mg,Al) 2 Si 4 O 40( OH)·4(H 2 O) and a structure of continuous planes of Si tetrahedrons, wherein the apical oxygen atoms point alternately upward and downward in relation to the basal oxygen plane, such that the structure has a chain-like characteristic modified with a cationic surfactant, in particular a quaternary ammonium cation, particularly preferably octadecyltrimethylammonium cation or a salt thereof.WO 2011 / 069189 A1 describes amine-modified clay mineral adsorbents, in particular for sorption of hydrophobic compounds, in particular constituents of aqueous film formin foam (AFFFs), surfactants and / or perfluorinated or polyfluorinated compounds. The amine-modified clay mineral comprises palygorskite modified with a fatty amine, in particular a primary, secondary, tertiary or quaternary ammonium compound having one or more hydrophobic organic substituents, particularly preferably oleylamine and / or octylamine.The object of the present invention is to provide a mild process for the production of clay mineral composites.It is a further object of the invention to provide clay mineral composites for use in the separation of organic halogenated compounds from aqueous solutions.According to the invention, the object is achieved by a method for producing clay mineral composites comprising the steps: a) providing a mineral raw material comprising at least one triple-layer clay mineral, b) suspending the mineral raw material in at least one alkanol, c) adding an aqueous solution comprising at least one cationic surfactant, and d) separating off the aqueous solution and drying.According to the invention, the process takes place in the sequence of steps a), b), c) and d) mentioned.The mineral raw materials are advantageously favorable and readily available as starting materials. It is also advantageous that the process of the invention is mild and rapid. According to the invention, the term "mild" is understood to mean a process with low temperatures, preferably below 60° C., and low mechanical actions. According to the invention, the term "fast" is understood to mean a process having a duration of less than 1 h, preferably in the range from 10 min to 30 min, particularly preferably in the range from 20 min to 30 min. The mild process reduces the change in the structure of the mineral raw material. The structure of the mineral raw material is preferably retained by the method according to the invention. Furthermore, the solvent or water requirement is reduced by the process according to the invention.The term "clay mineral composite" is understood to mean an organic, inorganic composite material based on three-layer clay minerals.In embodiments, the mineral raw material comprises a native three-layer clay mineral. A "native triple-layer clay mineral" is understood to mean a triple-layer clay mineral which has not been dried or otherwise pretreated and thus has a native structure.In embodiments, the process according to the invention, in particular steps b), c) and d), is carried out at a temperature of not more than 50° C., preferably in the range from 0° C. to 50° C. In further embodiments, steps b) and c) are carried out at a temperature in the range from 15° C. to 30° C., preferably at room temperature. In further embodiments, step c) is carried out at a temperature in the range from 40° C. to 50° C.In embodiments of the method according to the invention, the mineral raw material has a content of the at least one triple-layer clay mineral in the range from 15% (m / m) to 100% (m / m), preferably in the range from 70% (m / m) to 95% (m / m).In embodiments, the at least one trilayer clay mineral is a dioctahedral or trioctahedral trilayer clay mineral. In embodiments, the at least one trilayer clay mineral is selected from the smectite group. Preferably, the at least one triple layer clay mineral is montmorillonite. The three-layer clay mineral advantageously has a layer structure with negative charges in the interlayer region.In embodiments, the mineral raw material is bentonite. Bentonite advantageously has a high proportion of montmorillonite.In further embodiments, the mineral raw material further comprises mineral constituents, preferably selected from quartz, biotite, pyroxenes, feldspar, zirconium and anatase.In embodiments, the at least one alkanol is selected from methanol, ethanol and i-propanol. Preferably, the at least one alkanol is ethanol.In embodiments, the at least one alkanol has a content in the range from 70% (v / v) to 100% (v / v), preferably 90% (v / v) to 96% (v / v). The constituents of the alkanol expediently add up to 100% by volume. In preferred embodiments, the at least one alkanol comprises water as a further constituent.In embodiments of the method according to the invention, the mineral raw material and the at least one alkanol are suspended with a mass ratio in the range from 0.5:1 (m / m) to 2:1 (m / m), preferably in the range from 0.75:1 (m / m) to 2:1 (m / m).In embodiments, the cationic surfactant is a quaternary ammonium compound having a C10to C20radical, preferably having a C16radical. Advantageously, the quaternary ammonium compound serves as an anchor for the cationic surfactant in the interlayer region of the trilayer clay mineral.The term "C10, C16or C20radical" is understood to mean an organic group which has 10, 16 or 20 C atoms. Advantageously, hydrophobic interactions with organic halogenated compounds are possible by the hydrophobic organic radicals.In embodiments, the cationic surfactant is a quaternary alkylammonium compound having a C10to C20alkyl radical, preferably having a C16alkyl radical.In further embodiments, the cationic surfactant has at least one ester group, preferably in the α-, β-, γ-, δ- and / or ε-position relative to the quaternary ammonium group.In embodiments of the method according to the invention, the amount of substance of the cationic surfactant (mol) in step c) is added at a ratio in the range from 0.5:1 to 3:1, preferably in the range from 0.75:1 to 1:1, to the cation exchange capacity of the mineral raw material (in cmol / kg).The "cation exchange capacity" is understood to mean a measure of the exchangeable cations and thus the number of negative binding sites in raw materials. The cation exchange capacity of a mineral raw material depends on the deposit. The determination of the potential cation exchange capacity and of the exchangeable cations using a barium chloride solution buffered at pH=8.1 is carried out, for example, according to DIN ISO 13536.In embodiments of the method according to the invention, the aqueous solution is added to the mass of the mineral raw material at a ratio in the range from 1:1 to 3:1 (m / m), preferably in the range from 1:1 to 2:1 (m / m).In embodiments, the process according to the invention, in particular step c), is carried out at a pH in the range from pH 6 to pH 8.In embodiments, the method according to the invention comprises at least one further step, wherein the further step is a washing step after step c), preferably with water or ethanol. The washing step is expediently carried out after step c) if the aqueous solution comprising at least one cationic surfactant is prepared from a salt. Advantageously, the counterion of the salt is removed by the washing step.A further aspect of the invention relates to a clay mineral composite obtained by the method according to the invention.Advantageously, the clay mineral composite according to the invention has a more favorable production than activated carbon.A further aspect of the invention relates to the use of the clay mineral composite according to the invention, produced by the method according to the invention, for separating organic halogenated compounds from aqueous solutions.The term "aqueous solution" is understood to mean a homogeneous or heterogeneous mixture comprising water and at least one dissolved solid, liquid or gaseous compound.In preferred embodiments, the use is effected for the separation of per- and / or polyfluorinated alkyl compounds (PFAS), particularly preferably long-chain PFASs. Advantageously, the clay mineral composite, produced by the method according to the invention, has a high removal capacity for PFASs, wherein a separation of up to 100% (m / m) from the aqueous solution can be achieved within 4 h. It is also advantageous that the clay mineral composite does not adsorb any larger amounts of humin substances compared to activated carbon and can therefore be used with preference in humin substance-rich waters.In preferred embodiments, the organic halogenated compound is perfluorooctanesulfonic acid (PFOS), perfluorohexanesulfonic acid (PFHxS), and / or perfluorooctanoic acid (PFOA).In embodiments, the aqueous solution is selected from groundwater, surface water, well water, water from drinking water treatment and wastewater, in particular sewage plant wastewater, industrial wastewater, fire-extinguishing foam wastewater and / or textile cleaning wastewater.In embodiments, the clay mineral composite according to the invention is used for separating organic halogenated compounds in the 4th purification stage in wastewater treatment.In embodiments, the clay mineral composite according to the invention is used in the form of pellets or granules.In embodiments, the use takes place in the form of pellets or granules as a mixture with at least one further substance, in particular a carrier material.In further embodiments, the clay mineral composite according to the invention is used in the form of a packed column.The invention also relates to a method for separating organic halogenated compounds from aqueous solutions, comprising the steps: i. providing a clay mineral composite by means of the method according to the invention, ii. adding the clay mineral composite to an aqueous solution comprising at least one organic halogenated compound and immobilizing the at least one organic halogenated compound from the aqueous solution by adsorption on the clay mineral composite, and iii. separating the at least one organic halogenated compound from the clay mineral composite by thermal treatment of the clay mineral composite.In embodiments, the clay mineral composite is provided in the form of pellets or granules.In embodiments, the clay mineral composite is provided in the form of pellets or granules as a mixture with at least one further substance, in particular a carrier material.In further embodiments, the clay mineral composite is provided in the form of a packed column.In embodiments, the clay mineral composite is added to the aqueous solution comprising at least one organic halogenated compound with a concentration in the range from 5 mg to 1,000 mg clay mineral composite / l aqueous solution, preferably 10 mg to 100 mg clay mineral composite / l aqueous solution. The clay mineral composite produced by the method according to the invention is expediently used in accordance with the cation exchange capacity and the concentration of the at least one organic halogenated compound in the aqueous solution.In preferred embodiments, the organic halogenated compound is a per- and / or polyfluorinated alkyl compound (PFAS), particularly preferably a long-chain PFAS.In embodiments, the aqueous solution comprises at least one organic halogenated compound having a concentration in the range of 1 ng / L to 50 mg / L.Advantageously, the method according to the invention has a short contact time. In embodiments, step b) is carried out for a duration in the range from 1 min to 6 h, preferably 1 h to 4 h.A further aspect of the invention relates to the use of a clay mineral composite produced by the method according to the invention in barriers and / or protective layers for collecting organic halogenated compounds, in particular for soils and / or walls.In embodiments, the clay mineral composite is used in bentofix mats.For the realization of the invention, it is also expedient to combine the above-described embodiments and features of the claims.The invention will be explained in more detail below with reference to some exemplary embodiments and associated figures. The exemplary embodiments are intended to describe the invention without limiting it.It shows the following: FIG. 1 shows removal rates (%) for eight different PFAS when using the clay mineral composite according to the invention.Method for Producing Clay Mineral CompositeFirst, a surfactant solution is prepared from water and a cationic surfactant, in particular hexadecyltrimethylammonium bromide (CTAB), in a concentration of 100 mM. The concentration depends on the content of the cation exchange capacity of the mineral raw material. 5 g of clay, in particular bentonite, are suspended in 5 ml of ethanol, 10 ml of the surfactant solution are added and shaken for 20 min to 6 h at 50 to 70 U / min. Excess solution is separated, the solid washed with water to remove the counterion contained in the surfactant solution. The clay mineral composite is then dried at 50° C.Process for the Separation of Organic Halogenated Compounds from Aqueous SolutionsAt a dosage of 5 mg clay mineral composite to 1 l pollutant-containing solution (in each case 50 μg / l of eight different PFAS: perfluorobutyric acid (PFBA), perfluoropentanoic acid (PFPA), perfluorohexanoic acid (PFHxA), perfluoroheptanoic acid (PFHpA), perfluorooctanoic acid (PFOA), perfluorobutanesulfonic acid (PFBS), perfluorohexanesulfonic acid (PFHxS), perfluorooctanesulfonic acid (PFOS)), removal of three PFAS with more than 50% (PFOA, PFHxS and PFOS) was achieved within one hour and, in particular, removal of 94% from the mixed solution for the compound perfluorooctanesulfonic acid (PFOS). At higher dosage and longer contact time, increased removal was achieved.FIG. 1 shows the removal of eight different PFAS by 30 mg clay mineral composite per 1 l pollutant-containing solution (in each case 50 μg / l PFBA, PFPA, PFHxA, PFHpA, PFOA, PFBS, PFHxS, PFOS) after 4 h.Comparison to Adsorption on Activated Carbon in View of Removal of PFASA drinking water sample containing 50 μg / l PFAS is contacted with powdered activated carbon (Chemviron Filtrasorb 300) for 24 hours. The residual PFAS concentration is then determined. The results of contacting with clay mineral composites according to the invention are listed for comparison. The results of the investigations of both adsorbents at 30 mg / l adsorbent and a contact time of 24 hours in Karlsruhe drinking water are shown in Tab. The comparison is compared with FIG. 1. Tab. 1 Comparison of removal rates (in %) for activated carbon and clay mineral composite of the present invention. Tab. 1 Comparison of removal rates (in %) for activated carbon and clay mineral composite of the present invention.PFBA17 %7%PFPA18 %37 %PFHxS62 %100 %PFOS96 %100 %The use of activated carbon Filtrasorb 300 showed in part a higher removal rate for PFAS. However, for PFBA, a higher distance was achieved with the clay mineral composite of the invention. For PFBA and PFPA, removal rates of 7% and 37% are achieved for activated carbon under the experimental conditions chosen. Comparably high removal rates of 95% and 100% were achieved for PFOS.According to the examination of the BET surface area, the activated carbon used has a significantly higher value (1027,55 m 2 / g) compared with the clay mineral composite according to the invention (31.84 m 2 / g). The clay mineral composite according to the invention therefore has a higher surface-related load, since the similarly low residual concentration of PFOS is based on a significantly lower BET surface area. With the high surface-related load, it can be established for the clay mineral composite according to the invention that this adsorbent interacts more specifically with the adsorbent compared to the Filtrasorb 300 activated carbon used.When activated carbon is used to remove trace substances, it is known that the maximum achievable adsorption capacity decreases with increasing concentration of natural organic water constituents. In practice, this means that, with an increasing DOC, breakthrough of the activated carbon filter is achieved significantly earlier (Sontheimer et al. 1988). The clay mineral composite according to the invention, on the other hand, showed in experiments that natural organic water constituents do not significantly influence the adsorption of the PFAS.Furthermore, experiments have shown that, in contrast to activated carbon, a shorter contact time of four hours is sufficient for setting the adsorption equilibrium in the clay mineral composite according to the invention.Cited non-patent literatureJanda J, Lange FT, Bar M (2017) Further acquisition of PFC sources in the feed range of waterworks and removal of short-chain, persistent PFC, German Agreement of the Gas and Water Compartment e. V. Technische-Wissenschaftliche Verein, DVGW conveyor characteristics W 07-03-14 star.Kranland O, Birgersson M (2006) Montmorillonites stability with special response to KBS-3 conditions. Technical Report TR-06-11 Clay Technology. 1-38.Ross I, McDonough J, Miles J, Storage P, Thelakkat Kochunarayan P, Kalve E, Hurst J, Dasgupta SS, Burdick J (2018) A review of emerging technologies for remediation of PFAS. Sem. J. 28(2), 101-126.Sontheimer H, Crittenden JC, Summers RS (1988) Activated Carbon for Water Treatment. DVGW research center, Engler-Bunte-Institute, University of Karlsruhe (TH), 512-513.Trobs R, Guhl AC, Bertau M (2018) Gestein Alginit: Novel adsorbent for hormonally active substances. Chem. Our time 52, 280-281.Yan B, Munoz G, Sauvé S, Liu J (2020) Molecular Mechanisms of Per- and Polyfluoroalkyl Substances on a Modified Clay: A Combined Experimental and Molecular Simulation Study. Water Research, doi: https: / / doi.org / 10.1016 / j.waters.2020.116166.Zhi Y, Liu JX (2018) Sorption and desorption of anionic, cationic and zwitterionic polyfluoroalkyl substances by soil organic matter and pyrogenic carbonaceous materials. Chem. Eng. J. 346, 682-691.Zhou Q, Deng S, Yu Q, Zhang Q, Yu G, Huang J, He H (2010) Sorption of perfluorooctane sulphonate on organo-montmorillonites. Chemospheres 78, 688-694.

Claims

A method for producing clay mineral composites comprising the steps: a) providing a mineral raw material comprising at least one triple-layer clay mineral, b) suspending the mineral raw material in at least one alkanol, c) adding an aqueous solution containing at least one cationic surfactant, and d) separating off the aqueous solution and drying.Method according to claim 1, characterised in that the mineral raw material has a content of the at least one triple layer clay mineral in the range from 15% (m / m) to 100% (m / m).Method according to claim 1 or 2, characterized in that the at least one trilayer clay mineral is a dioctahedral or trioctahedral trilayer clay mineral.Process according to any of Claims 1 to 3, characterized in that the at least one alkanol is selected from methanol, ethanol or i-propanol.Process according to any of Claims 1 to 4, characterized in that the at least one alkanol has a content in the range from 70% (v / v) to 100% (v / v).The method according to any one of claims 1 to 5, characterized in that the mineral raw material and the at least one alkanol are suspended with a mass ratio in the range of 0.5:1 (m / m) to 2:1 (m / m).The method according to any one of claims 1 to 6, characterized in that the cationic surfactant is a quaternary ammonium compound having a C10 to C20 radical.Method according to one of Claims 1 to 7, characterized in that the amount of substance of the cationic surfactant in step c) is added at a ratio in the range from 0.5:1 to 3:1 to the cation exchange capacity of the mineral raw materialThe method according to any one of claims 1 to 8, comprising at least one further step, wherein the further step is a washing step after step c).A clay mineral composite obtained according to any one of claims 1 to 9.Use of a clay mineral composite produced according to any one of claims 1 to 9 for separating organic halogenated compounds from aqueous solutions.Use according to claim 11, characterised in that the aqueous solution is selected from groundwater, surface water, well water, water from drinking water treatment and wastewater, in particular sewage plant wastewater, industrial wastewater, fire-extinguishing foam wastewater and / or textile cleaning wastewater.A method for separating organic halogenated compounds from aqueous solutions comprising the steps of: i. providing a clay mineral composite by means of a method according to any one of claims 1 to 9, ii. adding the clay mineral composite to an aqueous solution comprising at least one organic halogenated compound and immobilizing the at least one organic halogenated compound from the aqueous solution by adsorption to the clay mineral composite, and iii. separating the at least one organic halogenated compound from the clay mineral composite by thermal treatment of the clay mineral composite.Method according to claim 13, characterised in that the clay mineral composite is added to the aqueous solution comprising at least one organic halogenated compound with a concentration in the range from 5 mg to 1,000 mg clay mineral composite / l aqueous solution.Use of a clay mineral composite produced according to any one of claims 1 to 9 in barriers and / or protective layers for collecting organic halogenated compounds, in particular for floors and / or walls.

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