Effective mercury removal from water using aromatic isocyanides as reducing agents

Abstract

The invention relates to a process for the effective, selective and rapid removal of mercury from aqueous solutions without producing harmful by-products to achieve ng / L (ppt) residual mercury levels in treated water. The process involves isocyanides in combination with air stripping, which is a very effective and highly selective method for the removal of mercury.

Description

[0001] EFFECTIVE MERCURY REMOVAL FROM WATER USING AROMATIC ISOCYANIDES AS REDUCING AGENTS

[0002] BACKGROUND

[0003] Mercury is classified as a persistent, bio -accumulative toxin, which is a serious threat to human health due to its very high toxicity and mobility. Mercury emissions to air from anthropogenic sources in 2015 quantifies global emissions from 17 key sectors at about 2220 tonnes (UN-Environment, Global Mercury Assessment 2018, UN- Environment Programme, Chemicals and Health Branch, Geneva, Switzerland, 2019, p. 59.). Globally, artisanal and small-scale gold mining is the largest source of anthropogenic mercury emissions (37.7%), followed by stationary combustion of coal (21%), non-ferrous metals production (15%) and cement production (11%). Unfortunately, both natural and human emissions are expected to further worsen as a result of Climate change (P. F. Schuster, K. M. Schaefer, G. R. Aiken, R. C. Antweiler, J. F. Dewild, J. D. Gryziec, A. Gusmeroli, G. Hugelius, E. Jafarov, D. P. Krabbenhoft, L. Liu, N. Herman-Mercer, C. Mu, D. A. Roth, T. Schaefer, R. G. Striegl, K. P. Wickland and T. Zhang, Geophysical Research Letters 2018, 45, 1463-1471) and as the capacity of coal-fueled power plants is on the rise owing to the current energy crisis.

[0004] Accordingly, mercury contamination in water sources is a significant environmental and public health concern. Mercury can enter water bodies through various industrial processes, including mining, coal combustion, and the improper disposal of mercury-containing products. Once in the water, mercury can transform into methylmercury, a highly toxic compound that bioaccumulates in aquatic organisms and can make its way up the food chain, posing risks to wildlife and humans who consume fish and shellfish.

[0005] Currently, mercury is not only one of the 13 priority pollutant metal elements (S. Sasnett, T. Murray, S. Canavan, J. Alter, K. Davey and P. Matthai in An Environmental Protection Agency Multimedia Strategy for Priority Persistent, Bioaccumulative, and Toxic Pollutants, Vol. 773 2000, pp. 114-123.) listed by the United States Environmental Protection Agency (US EP A), but also one of surface water pollutants by the European Water Framework Directive. In the US and Canada policies have been developed aimed at curtailing mercury emissions and specific actions to reduce and / or eliminate mercury were required. Many practical methods have been developed to solve water mercury pollution problem, such as precipitation, adsorption, membrane separation, biological treatment and ion exchange methods (K. Hua, X. Xu, Z. Luo, D. Fang, R. Bao and J. Yi, Current Nanoscience 2020, 16, 363- 375.). Traditional methods for removing mercury from water, often face challenges such as incomplete removal, high operational costs, and the generation of secondary pollutants. There is a pressing need for more efficient, cost- effective, and environmentally friendly methods to address mercury pollution in water. The development of processes that not only remove mercury but also recover valuable by-products could provide significant advantages in terms of sustainability or resource recovery. To be viable, these approaches must treat large volumes of water containing trace levels of mercury in the presence of other ions at a unit cost that is below conventional metals removal methods. In addition, to be truly attractive, the process must be effective at lowering mercury in solution below the limits set by the World Health Organization for drinking water (6 pg L-1inorganic mercury) (WHO Guidelines Approved by the Guidelines Review Committee, World Health Organization, Geneva, 2017). Significantly, the surface water protection standards promulgated to support these various initiatives, typically in the range of 1 to 50 ng / L total mercury. The chemical / electrical reduction of (Hg2+, Hg22+, CH3Hg+) is an emerging and promising technology. The main advantage claimed for reduction is that mercury can be recovered in the metallic state (J. W. Patterson and J. W. Patterson, Industrial wastewater treatment technology, Butterworth, Boston, 1985, p. xii, 467 p.). Until recently most reduction processes (using zinc, iron, NaBH4) could not effectively achieve mercury levels below 100 pg / L, and their use would likely require second-stage polishing. However, this issue may be eliminated by more recent reduction methods, which include the incorporation of mercury ions in a solid and stable metallic alloy, which is afterwards removed from solution. These amalgamation processes rely on selenium nanospheres (S. Ahmed, J. Brockgreitens, K. Xu and A. Abbas, Advanced Functional Materials 2017, 27), copper(brass) shavings (a) P. Huttenloch, K. E. Roehl and K. Czurda, Environmental Science & Technology 2003, 37, 4269-4273; b) J.-H. Richard and H. Biester, Water Research 2016, 99, 272-280.), mossy tin filters (H. Biester, Water Research 2000, 34, 2031-2036.) and gold nanoparticles (a) G. Chen, J. Hai, H. Wang, W. Liu, F. Chen and B. Wang, Nanoscale 2017, 9, 3315-3321; b) I. Ojea-Jimenez, X. Lopez, J. Arbiol and V. Puntes, ACS Nano 2012, 6, 2253-2260.) with excellent mercury removal efficiencies. Very recently, Cristian Tunsu et. al. reported the Effective removal of mercury from aqueous streams via electrochemical alloy formation on platinum (C. Tunsu and B. Wickman, Nat Commun 2018, 9, 4876.). Despite the low residual concentration and good removal efficiencies these methods are very costly and might not be appropriate for the treatment of large amounts of water.

[0006] In contrast, chemical reduction may be more effective as was detailed in a previous study by Looney et. al., where it was demonstrated that chemical reduction coupled with air stripping is most effective for treating mercury (B. B. Looney, M. E. Denham, K. M. Vangelas and N. S. Bloom, Journal of Environmental Engineering 2003, 129, 819-825.). The technology does not produce any liquid or solid secondary wastes, and off-gas treatment may not be required for the expected air concentrations and mass release. The technology tested used stannous (Sn2+) chloride (in dosages ranging from 0 to 766 mg / L) to reduce Hg2+(120-150 ng / L, >95% Hg2+) to Hg°, which is volatile (vapor pressure of 0.0027 mg Hg / L of air), followed by collection of elemental mercury from the headspace air to remove the elemental mercury from water. Stannous chloride doses greater than 0.011 mg / L resulted in more than 94% mercury removal, with the residual total mercury reduced to levels below 10 ng / L. However, the application of Sn2+in Hg2+reduction followed by air stripping may be viable in the case of a wide range of natural and or tap waters, there may be serious drawbacks: 1. The method may not be selective, when other more electropositive metal ions (Cu2+, Fe3+, Cd2+, Ag+) are present. 2. Sn2+ / Sn4+is leaked in the treated water, where the formation of methylated tin species by the action of bacteria present in soil and marine sediments (a) E. Dopp, L. M. Hartmann, A. M. Florea, A. W. Rettenmeier and A. V. Himer, CritRev Toxicol 2004, 34, 301-333; b) A. V. Himer and A. W. Rettenmeier, Met Ions Life Sci 2010, 7, 465-521) is very likely. Organotin compounds possess high toxicity (E. Dopp and A. W. Rettenmeier in Tin, Toxicity, Eds.: R. H. Kretsinger, V. N. Uversky and E. A. Permyakov), Springer New York, New York, NY, 2013, pp. 2235-2239.), particularly in aquatic environments (T. Gajda and A. Jancso, Metal ions in life sciences 2010, 7, 111-151.), however due to accumulation they may pose a significant threat to human health, too.

[0007] Recently, Nagy et al (D. Racz, M. Nagy, A. Mandi, M. Zsuga and S. Keki, Journal of Photochemistry and Photobiology A: Chemistry 2013, 270, 19-27) demonstrated that l-amino-5-isocyanonapthalene (ICAN) and its derivatives react quickly and selectively (23 other possibly interfering ions were studied) with Hg2+in aqueous media. Therefore, this reaction can be utilized for the fluorescent ratiometric determination of Hg2+(M. Nagy, S. L. Kovacs, T. Nagy, D. Racz, M. Zsuga and S. Keki, Taianta 2019, 201, 165-173.). The specially designed chemical structure of the solvatochromic amino-isocyanonaphthalene (ICAN) dye family enables the selective detection of Hg2+and at the same time is able to indicate the presence of Ag+. In addition to its easy preparation and nontoxic nature, ICAN is the lowest molecular weight dye reported for ratiometric fluorescent Hg2+detection in water, so far. The practical applicability of the method was demonstrated on dental amalgam. The reaction between HgCL and isocyanides were first studied in the 1970s by Sawai et al, in a number of solvents (a) H. Sawai and T. Takizawa, Tetrahedron Letters 1972, 13, 4263-4266; b) H. Sawai and T. Takizawa, Journal of Organometallic Chemistry 1975, 94, 333-343; c) H. Sawai and T. Takizawa, Bulletin of the Chemical Society of Japan 1976, 49, 1906-1908.). They established and our findings supported that the isocyano group is converted into amino (NH2) group in the presence of water in a complex redox reaction. It has long been assumed that Hg2+is reduced to atomic Hg° (A. Adamoczky, L. Nagy, M. Nagy, M. Zsuga and S. Keki, International Journal of Molecular Sciences 2020, 21.), however elementary Hg has not been detected directly, until now. These investigations included a number of Hg(II) and one Hg(I) compounds and their reactivity towards isocyanides were found to be in the following order: Hg(OAc)2= Hg(NO2)2> HgBr2, > HgCl2> Hg2Cl2> Hg(SCN)2> HgSO i > Hg(CN)2. It is well established that both Hg(II) and Hg(I) ions react with isocyanides and most interestingly virtually water insolvable Hg(II) salts also react opening up the possibility of solid waste (sludge) treatment, too. This reaction was not used for more than 40 years for the treatment of Hg contaminated water. It may be explained by the fear from the notoriously putrid odor of the isocyanides (A. S. Mikherdov, A. S. Novikov, V. P. Boyarskiy and V. Y. Kukushkin, Nature Communications 2020, 77.). Among other applications, the in vitro antifungal activity of the l-amino-5-isocyanonaphthalenes (ICANs) was tested against reference strains of clinically important Candida species (Nagy M, Szeman-Nagy G, Kiss A, Nagy ZL, Talas L, Racz D, Majoros L, Toth Z, Szigeti ZM, Pocsi I, et al. Antifungal Activity of an Original Amino-Isocyanonaphthalene (ICAN) Compound Family: Promising Broad Spectrum Antifungals, Molecules, 2020; 25(4):903.). Structure -activity studies revealed that the naphthalene nucleus and the isocyano together with the amino moieties are all necessary for antifimgal activity. 1,1- N-dimethylamino-5-isocyanonaphthalene (DIMICAN), the most promising candidate, was further tested in vitro against clinical isolates of Candida species, yielding a minimum inhibitory concentration (MIC) of 0.04-1.25 pg / mL.

[0008] BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1. a) The spectral changes (shift of the emission maximum) of ICAN in the presence of common metal ions. The inset picture shows the aqueous solution of ICAN before (left) and after (right) the addition of Hg2+under L = 365 nm UV light, b) The experimental setup (gas liquid separator) for the reaction between Hg2+and ICAN. c) Calibration curve and Intensity versus time curves (d) recorded by MP -AES from the reaction of aqueous Hg2+and ICAN. The inset shows the repeatability of the experiment at Hg2+= 10 ppm. [ICAN]=2.28 mM, T=25 °C, [Hg2+] = (1-10 ppm), pNitrogen=100 kPa.

[0010] Figure 2. Demonstration of Hg2+removal efficiencies from water using different isocyanides as reducing agents in combination with gas stripping. Initial (blue) and remaining (orange) Hg2+concentrations in the cases of a) 1000 ppb Hg2+and b) 10,000 ppb Hg2+. c) shows the scheme of the batch removal process.

[0011] Figure 3. Schematic representation of the VGA 77 accessory coupled to the atomic absorption spectrophotometer.

[0012] Figure 4. Comparison of the Hg cold vapor formation in the VGA -77 apparatus, using SnCl2and ICAN as reducing agents. Higher absorbance means higher Hg concentration in the gas phase. Left column SnCl2; right column ICAN.

[0013] Figure 5. Comparison of the Hg cold vapor formation in the VGA-77 apparatus, using SnCl2and K-2-ICA as reducing agents. Higher absorbance means higher Hg concentration in the gas phase. Left column SnCl2; right column ICAN TECHNICAL PROBLEM TO BE SOLVED BY THE INVENTION

[0014] Mercury generated from both natural and anthropogenic sources presents a difficult environmental problem owing to its high mobility and toxicity. The development of novel materials and / or technologies for the removal of Hg from water is therefore of paramount importance. The background art describes a process that involves tin chloride in combination with air stripping, which is a very effective method for the removal of mercury, however, the remaining tin-containing residue is a polluting output and selectivity is still a question when other more electropositive metal ions like Cu2+, Fe3+, Cd2+, Ag+are present. The other serious drawback of this method is when Sn2+ / Sn4+is leaked in the treated water, the formation of methylated tin species occurs by the action of bacteria present in soil and marine sediments. There is a need for a process that ensures the effective, selective and rapid removal of mercury from aqueous solutions without producing harmful by-products or other unpleasant circumstances.

[0015] DISCOVERY ON WHICH THE INVENTION IS BASED

[0016] An investigation was conducted to ascertain whether the combination of chemical conversion using ICAN or K-2-ICA and air stripping could enhance the effectiveness of the water treatment. It was found that the combined approach not only proved more effective than either method alone, but also demonstrated the ability to remove mercury from low-concentration aqueous solutions almost instantaneously, without the generation of harmful byproducts or unpleasant odors. Furthermore, in contrast to tin and tin-containing substances, isocyanides can be bonded to a solid substrate in a manner that prevents the introduction of foreign substances into the treated water.

[0017] BRIEF DESCRIPTION OF THE INVENTION

[0018] 1. A process for the selective, rapid removal of mercury from aqueous solution without harmful by-products, characterized in that a) according to reaction scheme rl to r3 below, the water sample containing mercury in its ionic form is reduced by the addition of a solution of an amino isocyano naphthalene compound to give elemental mercury and an amino isocyanate naphthalene derivative: where R is a naphthalene compound; b) the resulting solution containing elemental mercury is purged by a flow of air or an inert gas-containing gas, preferably nitrogen or argon-containing inert gas, preferably argon containing inert gas, preferably nitrogen or argon, most preferably argon; and c) the resulting elemental mercury is isolated.

[0019] 2. A process according to Point 1, characterized in that the mercury content of the water sample containing mercury in ionic form is reduced by the addition of a l-amino-5-isocyanonaphthalene (ICAN). 3. A process for the selective, rapid removal of mercury from aqueous solution without harmful by-products, characterized in that a) according to reaction scheme rl to r3 below, the water sample containing mercury in its ionic form is reduced by the addition of a solution of potassium 2 -isocyanoacetate (K-2-ICA) to give elemental mercury and an amino isocyanate naphthalene derivative: b) the resulting solution containing elemental mercury is purged by a flow of air or an inert gas-containing gas, preferably nitrogen or argon-containing inert gas, preferably argon containing inert gas, preferably nitrogen or argon, most preferably argon; and c) the resulting elemental mercury is isolated.

[0020] 4. The process according to any one of Points 1 to 3, characterized in that the aminoisocyanonaphthalene, preferably ICAN or K-2-ICA reagent is added in stoichiometric amounts to the water sample containing mercury.

[0021] 5. The process according to any one of Points 1 to 3, characterized in that the aminoisocyanonaphthalene, preferably ICAN or K-2-ICA reagent is added in large excess to the water sample containing mercury.

[0022] 6. The process according to any one of Point 5, characterized in that the aminoisocyanonaphthalene, preferably ICAN or K-2-ICA reagent is deposited on a surface.7.

[0023] 7. A process according to any one of Points 1 to 6, characterized in that steps a) and b) are carried out simultaneously in time, preferably in a continuous flow apparatus, preferably in a continuous flow vapour generation apparatus, more preferably in a VGA (vapour generation accessory) or equivalent apparatus.

[0024] 8. A process according to any of Points 1 to 7, characterized in that in step c) the product is isolated in a gas-liquid separator.

[0025] 9. A process for the treatment of mercury -contaminated waste water, characterized in that a) optionally, the initial mercury content is determined with a known process, by which the amount of aminoisocyanonaphthalene reagent required is determined; b) elemental mercury is released by the process according to any one of Claims 1 to 8; c) elemental mercury is isolated; d) optionally, the final mercury content is determined with a known process. 10. Use of a process according to any of Points 1 to 9 for the treatment of waste water.

[0026] DETAILED DESCRIPTION OF THE INVENTION

[0027] Hereby, we report the first application of an isocyanide (ICAN) or K-2-ICA as the reducing agent in combination with air stripping for the effective removal of mercury from water down to ng / L (ppt) concentrations. Until now it was unprecedented that the same low molecular weight organic compound could be used for the fluorometric detection and treatment of mercmy(II). The mechanism of the redox reaction between ICAN and Hg2+was calculated for the first-time using quantum chemical calculations. In the majority of instances, anadvantage of ICAN and other aromatic isocyanides over aliphatics is that they are odourless, which makes them ideal for use in a variety of applications. l-amino-5-isocyanonaphthalene (ICAN)

[0028] Mechanism of the reaction between isocyanides and Hg(II) ions is the following:

[0029] Scheme 1. Reactions of l-amino-5-isocyanonaphthalene (ICAN) with Hg2+in the presence of water.

[0030] It is seen in reaction Scheme 1, elementary mercury forms in the first step (rl), as a result of a complex redoxreaction between the isocyano moiety and Hg(II) ion in the presence of water. The isocyano group is converted to an isocyanate intermediate, which can further react with water to form carbamic acid (r2) followed by its decomposition to yield the amine derivative (r3).

[0031] The reactions r2 and r3 are well-known reactions in polyurethane chemistry that are used in the process of making polyurethane foams. The overall rate of transformation of the isocyano group to amino moiety increased with the water concentration and the polarity of the co-solvent. Since water is also a reactant in rl, the formation of elementary mercury proceeds at the highest rate in purely aqueous solutions, where no organic solvent is present.

[0032] The other reagent according to the invention is potassium 2-isocyanoacetate, which has the following formula: potassium 2 -isocyanoacetate (K-2-ICA)

[0033] Its reaction mechanism is described by the following equation (Scheme 2):

[0034] Scheme 2. Reactions of potassium 2-isocyanoacetate (K-2-ICA) with Hg2+in the presence of water.

[0035] ICAN and its N-methylated derivatives can be used as fluorometric turn-off probes to detect the presence of Hg2+ions in the concentration range of 170 nM-50 pM. The selectivity of the method and the turn-off behavior of the ICAN fluorescence upon the addition of Hg2+is demonstrated in Figure la. This unique behavior of ICAN would enable to construct a continuous water treatment apparatus, where the presence of Hg2+could be detected in a flow- through fluorometric cell. Since the detection is ratiometric, i.e., independent from the environmental conditions, the Hg2+concentration could be also detected and an automatic valve could always add the required amount of ICAN to the contaminated water.

[0036] The performance of ICAN was compared to a commercially available water-soluble aliphatic isocyanide, namely K-2-ICA to prove the concept that the reduction of Hg2+ions can be realized independently from the structure of the isocyanide.

[0037] According to the process of the invention, a water sample containing mercury ions may be treated with a solution of an aminoisocyanonaphthalene compound. This treatment may involve the use of a reducing agent, such as l-amino-5 -isocyanonaphthalene (ICAN), which may be added in stoichiometric amounts to ensure the complete reduction of mercury ions to elemental mercury. The elemental mercury formed and the gaseous reactions products are swept by a flow of air or an inert gas, such as nitrogen or argon, to facilitate its isolation.

[0038] According to the process of the invention, the process may also include the conversion of the aminoisocyanate naphthalene derivative into carbamic acid. That is the isocyano group is converted to an isocyanate intermediate, which can further react with water to form carbamic acid (r2) followed by its decomposition to yield the amine derivative (r3). This conversion may occur in an aqueous medium during the mercury removal process. The approach may ensure effective mercury removal and resource recovery, making it suitable for environmental management and industrial applications.

[0039] According to an embodiment of the invention, the process was performed in a simple gas-liquid separator. The Hg-vapor generation from aqueous Hg(II) solutions was proven, shown in Figure lb and described in Example 2.

[0040] According to an embodiment of the invention, the process was conducted in a batch setup, see Example 3. The results are summarized in Figure 2. According to an embodiment of the invention, Hg removal efficiency in a batch setup was 90.7% and 83.1% in the case of the 1000 and 10,000 ppm samples, respectively. These values were significantly lower in the case of the aliphatic isocyanide (K-2-ICA), where 58.2% and 73.5% removal could be achieved. The reaction between ICAN and Hg2+is quick and quantitative as was demonstrated earlier, the lower than 100% removal could be explained by the poor solubility of ICAN and / or the formed diamine.

[0041] According to an embodiment of the invention, continuous process was performed in a continuous tubular reactor, see Example 4.

[0042] According to an embodiment of the invention, the detection limit in the continuous process is 0.05 pg / L Hg2+.

[0043] According to an embodiment, the process is used for the purification of wastewater.

[0044] According to an embodiment of the invention, ICAN can be physically / chemically bonded to different surfaces, and this way, elemental mercury is isolated by binding to the surface. It means that no additional substances are added to the water to be treated.

[0045] Comparison ofICAN-VAPOR with conventional Hg-removal technologies

[0046] To be viable and competitive with already existing methods, novel Hg-removal techniques must have several advantages. In their recent paper Kang Hua et al[5]summarized the characteristics of the most common methods, which is presented in Table 3.

[0047] Table 1. A summary of the advantages and defects of different methods.[5]

[0048] Methods Advantages _ Defects _ Mechanism _

[0049] Precipitation Process simple, low cost Secondary pollution, Sludge Chemical reaction treatment, poor selectivity

[0050] Adsorption Operation simple, Cost-effective, Contaminant sensitivity Chemical or physical low second pollution, high fouling and plugging, spent adsorption adsorption rate media disposal

[0051] Membrane High separation selectivity and Membrane clogged / fouling, Selective permeability

[0052] Separation efficiency, small space requirement decay faster, high energy of the membrane consumption

[0053] Biological Low cost, continuous operation, Technical complexity, preTransformation, degratreatment simple process, high selectivity, treatment requirements, timedation and adsorption environmental friendly consuming

[0054] Ion exchange Low time consumption and technical High capital cost Reversible chemical uses, simple process reaction

[0055] Air stripping Low cost, low time consumption, Chemical reaction

[0056] (reduction) high selectivity, does not produce any followed by desorption id or solid wastes

[0057] The advantages of the application of isocyanides in combination with air stripping:

[0058] The advantage of the invention is that reduction with ICAN is highly selective, as the isocyanide reacts exclusively with Hg(II) or Hg(I) ions. A comprehensive investigation was conducted on the 23 most prevalent metal cations, and no interference was observed. The high degree of selectivity is particularly advantageous when treating waters with a high concentration of other ions, such as seawater and wastewater from mining and electrolysis processes. In comparison, reduction methods involving Sn(II) or precipitation techniques are considerably less selective. Moreover, the use of isocyanides avoids the leakage of Sn ions and the subsequent formation of highly toxic organotin compounds. Besides, the technology in question may be conducted in both batch and continuous modes. Additionally, the technology may be employed for the handling of large quantities of water, with a capacity exceeding 1000 m3per day. Furthermore, the synthesis of isocyanides can be achieved from any amine in a cost-effective manner. For example, it may be possible to utilize microorganisms to produce natural isocyanides, thus integrating the new technology into biological treatment methods. Finally, it is possible to develop non-toxic isocyano derivatives into drugs for the removal of methylmercury from animals and / or humans. There is a specific application example, as in fume gases (and in other sources, as well), mercury exist in two forms: elemental (HgO) and ionic (Hg2+, Hg22+). The average speciation of mercury in flue gases is 79% ionic, predominantly in the forms of HgCL and HgO. In contrast, the speciation in sub -bituminous and lignite coals is reversed. The removal of elemental mercury from fume gases represents a significant challenge. Promising plasma technologies, however, are being developed that convert all mercury content to Hg2+, which can be effectively scrubbed, absorbed, and further treated.

[0059] EXAMPLES

[0060] Example 1. The synthesis of l-amino-5-isocyanonaphthalene

[0061] The synthesis of l-amino-5-isocyanonaphthalene (ICAN), is known from the background art. We followed the method described in the following article: D. Racz, M. Nagy, A. Mandi, M. Zsuga and S. Keki, Journal of Photochemistry and Photobiology A: Chemistry 2013, 270, 19-27.

[0062] In a 250 ml three-necked flask equipped with a condenser, dropping funnel, gas inlet and magnetic stirrer, 3.00 g (19.0 mmol) of 1,5 -diamino naphthalene are dissolved in a mixture of 30 ml of chloroform, 20 ml of toluene and 10 ml of ethanol. A 50 ml aqueous solution of 12.0 g (0.214 mmol) of potassium hydroxide is carefully added dropwise to the stirred mixture, which is then boiled under argon with vigorous stirring for 12 hours. Following cooling, 100 ml of acetone is added to the reaction mixture, the organic phase is separated in a separatory funnel, which is then washed three times with water and then dried over anhydrous magnesium sulfate. The solvent was removed by rotary evaporation, and the residue was purified by column chromatography on normal phase silica gel using dichloromethane as eluent. The product was a pale yellow crystalline solid (yield: 0.32 g, 9.9%). A considerable amount of 1,5 -diisocyanonaphthalene was also formed as a by-product of the reaction, which was recovered as a white crystalline solid (0.11 g, 3.4%) after column chromatography.

[0063] Example 2. Testing the concept of Hg-vapor generation from aqueous Hg(II) solutions using isocyanides

[0064] To test the concept of Hg-vapor generation from aqueous Hg(II) solutions using isocyanides a simple gasliquid separator was constructed, which is shown in Figure lb. The apparatus is a modified 50 mL gas washer into which 5 mL ultrapure water (UPW) and different amounts of Hg(NOi)i standards (10 mg / L, 10 000 ppm) were added to set the final concentration of 1-10 ppm Hg2+. The reducing agent ICAN (0.1 mL, 114 mM in MeCN) was introduced through a syringe at the top of the gas-washer. The Hg vapor formed was purged from the glassware with 100 kPa nitrogen, which was connected to a microwave plasma atomic emission spectrometer (MP -AES) and mercury was detected at 253.7 nm. The duration of the detection was 100 s and intensity was recorded in each second. As it can be seen in Figure 1. the Hg(II) reduction was fast, all the mercury released was detected within the 100 s window. The reduction reaction is presumably much faster, however, the relatively large gas space above the Hg(II) solution may have contributed to the elongated signal (Figure Id). Quantification can either be done by integrating the intensitytime curves or registering the intensity values at 20 s. Using the latter method, a calibration curve was constructed which has a very good regression (R2>0.99) in the 0-10 ppm Hg concentration range (Figure 1c). The repeatability of the experiments was also investigated. 5 intensity-time curves were recorded at 10 ppm Hg concentration. It is evident from Figure Id inset that the 5 plots are almost identical, showing a good reproducibility of the Hg-vapor generation. These results imply that ICAN effectively reduces Hg2+ions in water and the elemental mercury can be effectively purged from the solution.

[0065] Example 3. Testing of the method in Batch setup

[0066] To test the practical applicability of the method 100 ml UPW water samples were placed in a specially designed 250 ml gas washer (Figure 2c) where the gas / liquid contact area was much higher than in the case of the MP-AES setup. Nitrogen was also exchanged to argon since it strips elemental mercury better from water than N2or air. The performance of the setup was determined on a 100 ppm Hg(NOi)’ sample using 20% (v / w) SnCL. The exiting argon containing Hg vapor was let through 2 gas washers containing 65 % HNO3. Cold Vapor Atomic Absorption measurements found 65 ppm Hg2+in the first and 35 ppm in the second gas washer proving that all the Hg2+content could be stripped from the original spiked sample. The live measurements were carried out using 100 ml UPW samples spiked with Hg(NOi)’ standards to the final concentrations of 1000 and 10,000 ppm Hg, respectively. The reaction was started with the addition of 63 pM ICAN or K-2-ICA and the argon flow was turned on immediately. Stripping lasted for 30 minutes, then the samples were digested according to (EP A) and were measured by CV-AAS technique. The results are summarized in Figure 2a and 3b and in Table 1.

[0067] Table 1. Results of batch Hg removal with ICAN reducer.

[0068] ICAN K-2-ICA

[0069] [Hg2+] Spike (pg / L) 1000 10,000 1000 10,000

[0070] Residual [Hg2+] (pg / L) 93±3 1690±57 418±15 2648±130

[0071] Removal (%) 90.7 83.1 58.2 73.5

[0072] ICAN performed well, its Hg removal efficiency was 90.7% and 83.1% in the case of the 1000 and 10,000 ppm samples, respectively. These values were significantly lower in the case of the aliphatic isocyanide (K-2-ICA), where 58.2% and 73.5% removal could be achieved. However, the reaction between ICAN and Hg2+is quick and quantitative as was demonstrated earlier, the lower than 100% removal could be explained by the poor solubility of ICAN and / or the formed diamine. Indeed, we observed the formation of a pink(purple) deposit that could bind the atomic mercury formed and this adsorbed mercury could not be purged from the solution. It should be noted here, that the ICAN concentration used during the MP-AES experiments was two orders of magnitude higher (2.28 m versus 63 pM) and the same can be said for the acetonitrile concentration (2% v / v compared to 0.05% v / v). The batch technique needs further optimization, however these issues can be overcome using the continuous method as is presented in the next chapter.

[0073] Example 4. Testing Hg-vapor generation in a continuous tubular reactor

[0074] Agilent Vapor Generation Accessory (VGA 77, Figure 3) was used in case of the continuous experiments. A vapor generation setup employs continuous flow technology where liquid reagents are pumped together and mixed. The sample capillary is placed in the solution to be measured and the VGA 77 (Agilent) pumps the sample through a reaction coil where it is mixed with a suitable reductant. It should be noted here, that the acid capillary seen in Fig. 4 is only used during hydride generation and only UPW is sipped through it during Hg-cold vapor generation. The gaseous reactions products are swept by a flow of argon gas into the spectrometer, where Hg concentration is determined based on its absorption at 253.7 nm in a quartz flow -through cell by the aid of a hollow cathode tube. Since absorbance can be expressed as: A = e * c * I , where A is the absorbance, e is the molar extinction coefficient and / is the optical pathlength. During the measurement both e and / are constant, therefore absorbance can be directly related to the Hg-concentration. The detection limit using the VGA77 is 0.05 pg / L Hg2+.

[0075] The experiments were performed using Hg / NChh calibrating solutions in the range of 1-100 ppb (pg / L) with fresly prepared ICAN solution (20 pL stock, 0.11 M in MeCN diluted to 50 mL in UPW). This aqueous solution was pumped in the reductant capillary of the VGA-77 apparatus and absorbance data was collected. The experiments were repeated 3 times. All the measurements were carried out using 46 pM ICAN concentration, since if we calculate the stoichiometry of the reaction (sample is mixed with the reductant in 7 : 1 , v / v), even at 100 pg / L Hg concentration the ICAN is at ~13x molar excess relative to Hg2+.

[0076] Increasing the ICAN concentration did not result in higher absorbance values, instead we noticed serious memory effects. ICAN in high concentration deposited on the surface of the inner reactor of the VGA-77 and 90- 100% Hg recovery was achieved without pumping reductant through the system. In this way, isocyanide-coated tubes / fillings can be used to remove mercury content while preserving the original sample. This method ensures that no additional substances are introduced to the water sample that could be preferable for use in the food industry.

[0077] Example 5: Comparison of ICAN measurements with SnCh measurements

[0078] Hg(NO3)2 calibrating solutions in the range of 1-100 ppb (pg / L) were made in 5% (v / v) nitric acid and were used to calibrate the AA spectrometer using 20% w / v SnCh in 20% v / v HC1 as the reducing agent. The absorbance data is summarized and compared with ICAN test results in Table 2, while Figure 4 illustrates the results, as well.

[0079] Table 2. Absorbance data collected using CV-AAS technique and SnCh and ICAN as the reducing agents.

[0080] [Hg2+] Absorbance Difference

[0081] (pg / L, ppb) SnCl2ICAN SD RSD (%)

[0082] 1 0.0125 0.0124 0.0012 9.9 0.5

[0083] 3 0.0350 0.0363 0.0018 4.8 3.8

[0084] 5 0.0638 0.0597 0.0011 1.8 6.5

[0085] 7 0.1049 0.1061 0.0008 0.7 1.2

[0086] 10 0.1482 0.1503 0.0017 1.1 1.4

[0087] 20 0.2868 0.2906 0.0053 1.8 1.3

[0088] 30 0.4240 0.4280 0.0058 1.4 1.0

[0089] 40 0.5548 0.5565 0.0077 1.4 0.3

[0090] 50 0.6619 0.6681 0.0084 1.2 0.9

[0091] 100 1.2439 1.2565 0.0163 1.3 1.0

[0092] Difference % was calculated using the formula:AsnCl2 A / c^w*100

[0093] ASnCl2

[0094] As can be seen from the data of Table 2, ICAN proved to be as effective in Hg-vapor generation as SnCL. The efficacy of the two methods is virtually the same, since the absorbance (and the concentration) difference of the Hg-vapor generated is close to 1% in most of the cases and only two higher differences were found 3.8% for 3 ppb Hg2+and 6.5% for 5 ppb Hg2+. The reproducibility of the ICAN method is also very good the errors of the 3 experiments never exceed 10% and are close to 1-2 % in most of the concentration range.

[0095] Based on the results, ICAN is at least as effective as SnCL in reducing Hg2+to elementary Hg (mercury vapor generation), however its application has two main advantages: 1. ICAN is highly selective for Hg2+and reduces only the Hg(II) content of the sample. 2. Since the reaction is stoichiometric and fast ICAN can be applied in very low concentrations making the Hg-removal procedure more cost effective.

[0096] Example 6: Comparison of K-2-ICA measurements with SnCh measurements

[0097] The performance of the commercially available water-soluble isocyanide: K-2-ICA was compared to SnCL in the VGA 77 apparatus. The results are summarized in Figure 5 and Table 3.

[0098] Table 3. Absorbance data collected using CV-AAS technique and SnCT and K-2-ICA as the reducing agents.

[0099] [Hg2+] Absorbance Difference

[0100] (pg / L, ppb) SnCl2K-2-ICA RSD (%)

[0101] (20% w / v) (46 pM) (%)

[0102] 5 0.0638 0.0626 1.3 1.9

[0103] 10 0.1482 0.1035 2.1 30.2

[0104] 15 0.1935 0.1335 1.9 31.0

[0105] 20 0.2868 0.1511 3.0 47.3

[0106] 25 0.3225 0.1699 2.5 47.3

[0107] It is evident from the data, that the aliphatic isocyanide K-2-ICA, performed 30-50% worse than the aromatic ICAN and SnCL. The explanation may be the slower reaction between the aliphatic isocyanide and Hg2+ions in aqueous medium.

[0108] INDUSTRIAL APPLICABILITY

[0109] Mercury generated from both natural and anthropogenic sources presents a difficult environmental problem owing to its high mobility and toxicity. The development of novel materials and / or technologies for the removal of Hg from water is therefore of paramount importance. Hereby, we demonstrate that isocyanides in combination with air stripping could be a very effective and highly selective method to achieve ng / L (ppt) residual mercury levels in treated water. The performance of an aromatic isocyanide: l-amino-5-isocyanonaphthalene (ICAN) and an aliphatic one: potassium 2-isocyanoacetate (K-2-ICA) was tested in both batch and continuous air stripping processes in the Hg2+concentration range of 1-10,000 pg / L (ppb). In a simple gas washer ICAN removed 90% of 1000 ppb and 83% of 10,000 ppb Hg2+, while these values were 58% and 73%, respectively for K-2-ICA. Using the Agilent VGA-77 cold vapor formation unit as an optimized flow (continuous) reactor ICAN (63 pM) performed virtually the same as 20% v / w SnCL (specially applied for this task) in the 1-100 1-100 pg / L Hg2+concentration range. The results show that residual levels of Hg2+lower than 1 ppb (1000 ppt) could be reached, which is well below the limits set by the World Health Organization for drinking water (6 pg L-1inorganic mercury).

Claims

CLAIMS1. A process for the selective, rapid removal of mercury from aqueous solution without harmful by-products, characterized in that a) according to reaction scheme rl to r3 below, the water sample containing mercury in its ionic form is reduced by the addition of a solution of an amino isocyano naphthalene compound to give elemental mercury and an amino isocyanate naphthalene derivative:where R is a naphthalene compound; b) the resulting solution containing elemental mercury is purged by a flow of air or an inert gas-containing gas, preferably nitrogen or argon-containing inert gas, preferably argon containing inert gas, preferably nitrogen or argon, most preferably argon; and c) the resulting elemental mercury is isolated.

2. A process according to Claim 1, characterized in that the mercury content of the water sample containing mercury in ionic form is reduced by the addition of a l-amino-5-isocyanonaphthalene (ICAN).

3. A process for the selective, rapid removal of mercury from aqueous solution without harmful by-products, characterized in that a) according to reaction scheme rl to r3 below, the water sample containing mercury in its ionic form is reduced by the addition of a solution of potassium 2 -isocyanoacetate (K-2-ICA) to give elemental mercury and an amino isocyanate naphthalene derivative:b) the resulting solution containing elemental mercury is purged by a flow of air or an inert gas-containing gas, preferably nitrogen or argon-containing inert gas, preferably argon containing inert gas, preferably nitrogen or argon, most preferably argon; and c) the resulting elemental mercury is isolated.

4. The process according to any one of Claims 1 to 3, characterized in that the aminoisocyanonaphthalene, preferably ICAN or K-2-ICA reagent is added in stoichiometric amounts to the water sample containing mercury.

5. The process according to any one of Claims 1 to 3, characterized in that the aminoisocyanonaphthalene, preferably ICAN or K-2-ICA reagent is added in large excess to the water sample containing mercury.

6. The process according to any one of Claims 5, characterized in that the aminoisocyanonaphthalene, preferably ICAN or K-2-ICA reagent is deposited on a surface.7.

7. A process according to any one of Claims 1 to 6, characterized in that steps a) and b) are carried out simultaneously in time, preferably in a continuous flow apparatus, preferably in a continuous flow vapour generation apparatus, more preferably in a VGA (vapour generation accessory) or equivalent apparatus.

8. A process according to any of Claims 1 to 7, characterized in that in step c) the product is isolated in a gas-liquid separator.

9. A process for the treatment of mercury -contaminated waste water, characterized in that a) optionally, the initial mercury content is determined with a known process, by which the amount of aminoisocyanonaphthalene reagent required is determined; b) elemental mercury is released by the process according to any one of Claims 1 to 8; c) elemental mercury is isolated; d) optionally, the final mercury content is determined with a known process.

10. Use of a process according to any of Claims 1 to 9 for the treatment of waste water.

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