Method for reducing arsenic concentration in aqueous solutions

Abstract

Method of reducing the arsenic concentration in an aqueous solution comprising undesired arsenic, which method comprises contacting the aqueous solution with a complex of Formula (I), (Formula (I)) wherein M1 and M2 are the same or different and are independently selected from V, Mn, Ga, Cu, Ni, Co, Fe or Zn; wherein a is 0, or 1, and b is 0, or 1, provided that a+b together must be at least 1; Q is a negatively charged counter ion; n is from 1 to 5; X1 is OH, O, SH or S; L1 is a group selected from La1-C(O)NR, La2-C(O)OR, La3-NRC(O), La4-OC(O), La5-O or La6-NRO, wherein La1, La2, La3, La4, La5 and La6 are each C1-6 alkyl, optionally substituted, R is H or C1-6 alkyl optionally substituted; Linker is a polyethylene glycol (PEG) chain with from 1 to 10 repeating units, a C1-16 polyamine chain or a C1-16 alkyl chain; Z is a solid support; L2 to L7 are independently C1-3 alkyl, optionally substituted; and Het1 to Het4 are independently 5 to 14 membered heteroaryl group having at least one N atom and optionally substituted. ##STR00001##

Claims

1. A method of reducing the arsenic concentration in an aqueous solution that comprises arsenic, which method comprises contacting the aqueous solution with a complex of Formula I, ##STR00016## wherein M.sup.1 and M.sup.2 are the same or different and are independently selected from V, Mn, Ga, Cu, Ni, Co, Fe or Zn; wherein a is 0, or 1, and b is 0, or 1, provided that a+b together must be at least 1; Q is a negatively charged counter ion, wherein the negatively charged counter ion is NO.sub.3.sup., ClO.sub.4.sup., AcO.sup., PF.sub.6.sup., BF.sub.4.sup., Cl.sup., BPh.sub.4.sup. or Br.sup.; n is from 1 to 5; X.sup.1 is OH, O, SH or S; L.sup.1 is a group selected from -L.sup.a1-C(O)NR, -L.sup.a2-C(O)OR, -L.sup.a3-NRC(O), L.sup.a4-OC(O), L.sup.a5-O or L.sup.a6-NRO, wherein L.sup.a1, L.sup.a2, L.sup.a3, L.sup.a4, L.sup.a5 and L.sup.a6 are each C.sub.1-6 alkyl, optionally substituted, R is H or C.sub.1-6 alkyl optionally substituted; Linker is a polyethylene glycol (PEG) chain with from 1 to 10 repeating units, a C.sub.1-16 polyamine chain or a C.sub.1-16 alkyl chain; Z is a solid support; L.sup.2 to L.sup.7 are independently C.sub.1-3 alkyl, optionally substituted; and Het.sup.1 to Het.sup.4 are independently 5 to 14 membered heteroaryl group having at least one N atom and optionally substituted.

2. A method according to claim 1, wherein the method reduces the concentration of arsenic in the aqueous solution to less than about 10 g L.sup.1.

3. A method according to claim 2, wherein the method reduces the concentration of arsenic in the aqueous solution to less than about 5 g L.sup.1.

4. A method according to claim 3, wherein the method reduces the concentration of arsenic in the aqueous solution to less than about 1 g L.sup.1.

5. A method according to claim 1, wherein the solid support is a polystyrene based resin.

6. A method according to claim 1, wherein the Linker is a polyethylene glycol (PEG) chain with from 1 to 10 repeating units.

7. A method according to claim 6, wherein the solid support is a polystyrene based resin and the Linker is a polyethylene glycol (PEG) chain with from 1 to 10 repeating units.

8. A method according to claim 1, wherein one of M.sup.1 or M.sup.2 is zinc or both M.sup.1 and M.sup.2 are zinc.

9. A method according to claim 1, wherein the initial pH of the aqueous solution prior to a point of contact with the complex of Formula I is from about 1 to about 10.

10. A method according to claim 9, wherein the initial pH of the aqueous solution prior to a point of contact with the complex of Formula I is from about 5 to about 10.

11. A method according to claim 10, wherein the initial pH prior to a point of contact with the complex of Formula I is about 7.

12. A method according to claim 1, comprising the additional step of regenerating the complex of Formula I.

13. A method according to claim 12, wherein the complex of Formula I is regenerated by contacting a complex of Formula I to which arsenic is bound with a solution of an alkali metal salt or an alkali earth metal salt having a pH of from about 7 to about 10.

14. A method according to claim 13, wherein the solution is of a halide salt of an alkali metal.

15. A method according to claim 14, wherein the complex of Formula I is regenerated by contacting the complex of Formula I to which arsenic is bound with a NaCl or NaOAc solution having a pH from about 7 to about 10.

16. A method according to claim 1, further comprising the step of: contacting the complex of Formula I to which arsenic is bound formed in step (i) with a NaCl or NaOAc solution having a pH of from about 7 to about 10.

17. A method according to claim 1, comprising the additional step of filtering the aqueous solution comprising arsenic before contacting with a complex of Formula I.

18. A method according to claim 1, comprising the additional step of filtering the aqueous solution comprising a complex of Formula I to which arsenic is bound after the aqueous solution has been contacted with a complex of Formula I.

19. A method according to claim 1, comprising the additional step of reacting the aqueous solution comprising arsenic with an oxidising agent before contacting with a complex of Formula I, such that any arsenite present in the aqueous solution is oxidised to arsenate.

20. A method according to claim 1, wherein the complex of formula I is used in combination with an additional absorbent.

21. A method according to claim 20, wherein the additional absorbent is an iron oxide absorbent.

22. A method for providing potable water comprising a method according to claim 1.

23. A complex of Formula IV, ##STR00017## wherein Z is a solid support and the linker is a polyethylene glycol (PEG) chain with from 1 to 10 repeating units, a C.sub.1-16 polyamine chain or a C.sub.1-16 alkyl chain.

24. A complex according to claim 23, wherein the solid support is a polystyrene based resin.

25. A complex according to claim 24, wherein the solid support is a polystyrene based resin and the Linker is a polyethylene glycol (PEG) chain with from 1 to 10 repeating units.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Reference FIG. 1 shows a plot of the relative changes in UV/Vis absorbance at 444 nm upon addition of 1 equivalent (clear) and 10 equivalents (shaded) of [HAsO.sub.4].sup.2, [HPO.sub.4].sup.2 and [SO.sub.4].sup.2 anions to a solution containing 50 M of either L.sup.1-Zn.sub.2, L.sup.1-Cu.sub.2, L.sup.1-Ni.sub.2, and 50 M PV in 100 mM HEPES at pH 7.

(2) Reference FIG. 2 shows the raw Isothermal titration calorimetry (ITC) data (top) and integrated binding curve (bottom) obtained upon making: (a) 2010 L injections of 3 mM Na.sub.2HPO.sub.4 into a cell containing 0.2108 mM of L.sup.1-Zn.sub.2; (b) 2010 L injections of 3 mM Na.sub.2HAsO.sub.4.7H.sub.2O into a cell containing 0.2108 mM of L.sup.1-Zn.sub.2.

(3) FIG. 3 shows (a) arsenate adsorption isotherms with Langmuir model for the Zn-HypoGel complex (Ia) and Bayoxide carried out using a 10 mM HEPES solutions (pH 7); (b) arsenate adsorption isotherms for Zn-HypoGel and Bayoxide shown in terms of the number of moles adsorbed per mole of active sites available.

(4) FIG. 4 shows the arsenic adsorbed onto Bayoxide and the Zn-HypoGel complex (Ia) after shaking 5 mg of sorbent with 50 ml of 300 ppb arsenate solution for 24 hours.

(5) FIG. 5 shows (a) the number of moles of arsenate adsorbed onto 5 mg of Bayoxide and 5 mg of the Zn-HypoGel complex (la) after 24 hours shaking in 50 ml of Challenge Water at pH 7 and (b) the number of moles of arsenate adsorbed per mole of active sites available.

(6) FIG. 6 shows a plot showing adsorption and subsequent desorption (5 cycles) of arsenate from the Zn-HypoGel complex (Ia) resin.

(7) FIG. 7 shows the X-ray crystal structure of L.sup.1-Zn.sub.2 bound to arsenate.

(8) FIG. 8 shows (a) the raw UV/vis data obtained upon addition of a zinc standard to a solution of 100 M PV at pH 7 and (b) the calibration line obtained upon plotting the absorbance at 605 nm vs zinc concentration with a Coefficient of Determination (R.sup.2) of 0.9893.

(9) FIG. 9 shows how the zinc loading was increased by extending the incubation time.

(10) FIG. 10 shows a diagram of a fixed bed column as used in Experiment 9.

(11) FIG. 11 shows a plot of normalised effluent As.sup.V concentration against bed volumes treated by passing a solution of 10 ppm As.sup.V through a glass column packed with Zn-HypoGel-90.

(12) FIG. 12 shows a plot of the total mmoles As.sup.V desorbed from Zn-HypoGel-90 column against the number of bed volumes of NaOAc wash solution.

(13) FIG. 13 shows the effluent arsenate concentration obtained from a column of Zn-HypoGel using an influent solution containing 100 g/L of As.sup.V and 20 mM HEPES at pH 7.5. The green line denotes the influent concentration, the red line the 10 g/L MCL and the blue line 5 g/L.

(14) FIG. 14 shows the effluent arsenate concentration after passing Challenge Water through Zn-HypoGel column where the influent A.sup.V concentration is 50 g/L.

(15) FIG. 15 shows the effluent arsenate concentration after passing Challenge Water through Zn-HypoGel column where the influent As.sup.V concentration is 300 g/L.

(16) The present invention is now illustrated, but not limited, by the following Examples.

(17) Metal complexes L.sup.1-Cu.sub.2, L.sup.1-Ni.sub.2, L.sup.1-Zn.sub.2 (as defined below) were prepared by slight modifications of previously reported procedures by Torelli et al., Inorg Chem., 2000, 39, 3526 to 3536, Adams et al., Inorganica Chimica Acta, 332, 2002, 195 to 200 and Han et al., Angew Chem., 2002, 114, 3963 to 3965.

REFERENCE EXAMPLE 1

(18) Comparative Screening Affinity of metallo-receptors L.sup.1-M2 by Indicator Displacement Assays (IDAs).

(19) The binding of arsenate, phosphate and sulphate by complexes 1-3 was investigated by indicator displacement assays (IDAs) using the following procedure.

(20) ##STR00012##

(21) A 5 mM stock solution of each complex was prepared in acetonitrile and stored at 18 C. A solution of 100 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) in Milli-Q water was adjusted to pH=7.5 with 2 M NaOH and used as buffer. A 5 mM stock solution of pyrocatechol violet (PV) indicator was prepared freshly as required using this buffer solution. A 250 M working solution of each complex and indicator was prepared via a twenty-fold dilution in buffer. 500 M working solutions of Na.sub.2HAsO.sub.4.7H.sub.2O, Na.sub.2HPO.sub.4 and (NH.sub.4)HSO.sub.4 were also prepared in 100 mM HEPES at pH 7.5 by dilution of 50 mM stocks.

(22) 25 L of PV working solution and 25 L of complex working solution were added to a series of wells on a 96 well plate. 1 and 10 equivalents of [HAsO.sub.4].sup.2, [HPO.sub.4].sup.2 and [SO.sub.4].sup.2 anions were then added to the corresponding wells, and every solution was made to a final volume of 250 L with HEPES buffer. The absorbance of each solution was then recorded at =445 nm.

(23) As shown in FIG. 1, the Zn complex (L.sup.1-Zn.sub.2) did not show any binding to sulphate under the experimental conditions.

REFERENCE EXAMPLE 2

(24) Interaction of L.sup.1-Zn.sub.2 with oxoanions by isothermal titration calorimetry.

(25) Isothermal titration calorimetry (ITC) was used to study the interaction of receptor L.sup.1-Zn.sub.2 with arsenate, phosphate and sulphate using the following procedure.

(26) A 0.2 mM solution of each complex was prepared in 100 mM HEPES buffer at pH 7.5, and stored at 18 C. 3 mM anion solutions of Na.sub.2HAsO.sub.4.7H.sub.2O, Na.sub.2H.sub.2PO.sub.4 and (NH.sub.4)HSO.sub.4 were also prepared in 100 mM HEPES at pH 7.5 by dilution of 50 mM stocks. All solutions were filtered through a 0.45 m syringe filter and degassed before use.

(27) A solution of the L.sup.1-Zn.sub.2 complex was stirred at constant temperature in the calorimetric cell. A solution of anion was then accurately titrated into the cell, and the heat change upon each addition was measured. This titration data was then integrated using MicroCal Origin software to produce a binding isotherm, from which K, H and the stoichiometric parameter n could all be determined.

(28) Raw ITC data and the integrated binding curves for the titration of complex L.sup.1-Zn.sub.2 with arsenate and phosphate are shown in FIG. 2. From these data, it was possible to determine the binding constants between L.sup.1-Zn.sub.2 and these anions (see Table 1).

(29) These results are consistent with those obtained during the indicator displacement assays (vide supra), showing that L.sup.1-Zn.sub.2 binds with good affinity to aqueous arsenate (at pH 7.5, hence present as HAsO.sub.4.sup.2) but not to sulphate.

(30) The ITC data was also used to determine H for the binding process for both anions, which was determined to be negative, showing that these are enthalpy-driven interactions. This conclusion was also reached by Han et al (Angew. Chem., 2002, 114, 3963-3965) during their studies of this zinc complex.

(31) TABLE-US-00001 TABLE 1 Anion K (M.sup.1) ITC K (M.sup.1) IDA H (kJ mol.sup.1) ITC HAsO.sub.4.sup.2 (1.45 0.32) 10.sup.4 (1.63 0.35) 10.sup.4 4.02 0.50 HPO.sub.4.sup.2 (2.08 0.51) 10.sup.4 (2.10 0.40) 10.sup.4 3.68 0.21 SO.sub.4.sup.2 No binding No binding n/a detected detected

EXAMPLE 3

(32) Preparation of Solid Supported Di-Zinc Complex (Zn-HypoGel)

(33) An ethyl amine derivative of L.sup.1 was prepared (see scheme 1) by following the procedure adapted by Kwon et al (Chem. A Eur. J., 2008, 14, 9613-9619).

(34) ##STR00013##

(35) The resulting amine-functionalised L1 (compound 10) was then reacted with HypoGel resin containing 0.9 mmol g-1 loading of succinic acid.

(36) ##STR00014## ##STR00015##

(37) This reaction was carried out using standard amide coupling reagents in DMF, as shown in Scheme 2. Ligand concentration in the reaction solution was monitored by HPLC analysis. Following the reaction, the resin was washed with DMF, CH.sub.2Cl.sub.2, methanol and finally diethyl ether, and then dried under reduced pressure until the weight remained constant. From the increase in dry weight of the resin, the extent of L.sup.1 functionalisation was determined to be 0.29 mmol g-1. The functionalised HypoGel beads were than loaded with zinc(II) by shaking a suspension of beads in a solution of Zn(NO.sub.3).sub.2.6H.sub.2O (10 mM HEPES at pH 7). The zinc(II) concentration in the initial and final solutions was quantified using a colorimetric dye. This showed a loading of zinc(II) on the beads of 0.30 mmol g-1, i.e. 52% of the ligand sites were filled with zinc.

EXAMPLE 4

(38) Loading to HypoGel Resin and Quantification of Loading Achieved

(39) Zn-loading to HypoGel resin: HypoGel resin and compound 10 (as defined in Example 3) were shaken in 10 mM HEPES buffer at pH 7 (5 ml) for 30 minutes to allow the resin to swell (loading: 0.254 mmol/g, 0.233 g, 0.059 mmol of compound 10).

(40) The buffer was then removed by filtration and the resin shaken in a HEPES buffered solution containing Zn(NO.sub.3).sub.2.6H.sub.2O (0.073 g, 0.244 mmol) at room temperature for 24 hours. After this time the solution was removed by filtration, and the resin washed 3 times with 3 ml of buffer (30 minutes for each wash).

(41) The zinc concentration in the starting and final solutions, as well as in the 3 buffer washes, was quantified using UV/vis spectroscopy in conjunction with a colorimetric zinc indicator (PV). Aliqouts were taken from the zinc loading solutions and mixed with 100 M PV. The absorbance at 605 nm was recorded and the zinc concentration in the solutions could be determined.

(42) These concentration values were used to determine the amount of zinc that had been taken up by the resin after 24 hours by comparing the values obtained with the calibration line produced using the zinc quantification procedure described below.

(43) The loading procedure described above was then repeated a further two times and the loading after each subsequent 24 hour incubation was calculated with the results shown in FIG. 9.

(44) Total Zn content after 72 hours=0.46 mmol/g (91% sites filled).

(45) Zinc quantification: A 13.5 mM working solution of Zn(NO.sub.3).sub.2.6H.sub.2O was prepared in 10 mM HEPES at pH 7. Aliquots of this zinc standard were then added to a cuvette containing 100 M Pyrocatechol Violet (PVa dye commonly used for colorimetric detection of metal ions), and the absorbance spectrum recorded after each addition. This data was used to produce a calibration line of absorbance at 605 nm vs zinc concentration with a Coefficient of Determination (R.sup.2) of 0.9893 (see FIG. 8).

EXAMPLE 5

(46) Batch Equilibrium Arsenate Adsorption Studies

(47) The arsenate adsorption properties of the resin were studied by carrying out batch equilibrium adsorption experiments using the following procedure.

(48) A solution of 10 mM HEPES was prepared in Milli-Q water and adjusted to pH 7 with 1 M NaOH. A 1000 ppm arsenic stock was prepared by dissolving Na.sub.2HAsO.sub.4.7H.sub.2O in buffer, and a 100 ppm solution was prepared by subsequent dilution of the stock. 5 ml of buffer was added to each of 7 Luer lock syringes, fitted with a frit and cap. Varying volumes of the arsenic stock and working solutions were then added to the syringes, to give an arsenic concentration range from 1 ppm to 25 ppm. 5 mg of adsorbent was then added to each syringe, and the solution placed on an orbital shaker at 100 rpm for 24 hours. After this time, the shaking was stopped and each solution was removed and acidified with 0.1 M HCl. The arsenic concentration was then determined by Differential Pulse Anodic Stripping Voltammetry.

(49) Origin was used to fit the isotherm data using the Langmuir equation shown below, where qe is the amount of arsenic adsorbed at equilibrium (mg g-1), Ce is concentration of arsenic in solution at equilibrium (mg L-1), Qmax is the theoretical arsenic adsorption capacity and b is the affinity coefficient.

(50) $\mathrm{qe}=\frac{\mathrm{bQ}\max \mathrm{Ce}}{1+\mathrm{bCe}}$

(51) FIG. 3 shows the Langmuir fitted arsenate adsorption isotherms for both the Zn-HypoGel resin and a commercial iron oxide (Bayoxide). From this data, the theoretical arsenic absorption capacity (Qmax) for the Zn-HypoGel was determined to be 6.91.1 mg g-1 and the Langmuir affinity coefficient (b) was 12.27.9 (L/mg).

(52) The arsenate adsorption isotherm for the commercial iron oxide (Bayoxide) is also shown in FIG. 3 and the Qmax was determined to be 17 mg g-1. The two isotherms are compared more closely in FIG. 3 where the arsenic uptake is normalised to the number of moles of sorbent sites. Determination of the number of Bayoxide active sites present was based on calculations previously carried out by Kanematsu et al (Geochim. Cosmochim. Acta., 2013, 106, 404-428), and the number of Zn-HypoGel sites was calculated from the results of the zinc loading experiments. FIG. 3 clearly illustrates the greater arsenate affinity of Zn-HypoGel sites as compared with Bayoxide, i.e. 1 mole of Zn-Hypogel sites would adsorb 0.7 moles of arsenate, while 1 mole of Bayoxide sites would adsorb 0.2 moles of arsenate.

(53) The difference in affinity is also demonstrated when comparing the arsenate adsorption in the isotherm solutions at the lower end of the concentration range studied. For example, Zn-HypoGel reduced an initial concentration of 1000 g L-1 to 6 g L-1 after 24 hours, whereas for Bayoxide the final concentration was 60 g L-1 (99% adsorption compared to 94%). This clearly shows the superior arsenate adsorbing properties of Zn-HypoGel.

EXAMPLE 6

(54) pH Effect on Arsenate Adsorption

(55) The arsenate uptake over a range of pH values was investigated by batch studies using the following procedure.

(56) Solutions over a pH range of 3-10 were prepared by buffering Milli-Q water with 10 mM sodium formate (pH 3), sodium acetate (pH 4 and 5), HEPES (pH 7 and 8) and sodium tetraborate (pH 9 and 10). A 1,000 ppm arsenic stock was prepared by dissolving Na.sub.2HAsO.sub.4.7H.sub.2O in Milli-Q water. 50 ml of each buffer was added to separate reaction bottles, followed by 15 L of As stock to give an initial arsenic concentration of 300 ppb. 5 mg of Zn-HypoGel or Bayoxide was then added to the reaction solutions, and the bottles were placed on an orbital shaker at 100 rpm for 24 hours. After this time, the shaking was stopped and a sample removed and acidified with 0.1 M HCl, ready for analysis by DPASV.

(57) The arsenate uptake over pH range 3-10 is shown in FIG. 4.

(58) As can be seen from FIG. 4, Bayoxide performs better compared to Zn-HypoGel at low pH, and adsorption is inhibited above pH 8.5 (the point of zero charge). The Zn-HypoGel resin demonstrates a greater affinity for arsenate above pH 5, and the adsorption was greatest at pH 7. This is advantageous as pH adjustment of natural water samples would not be required to reach the optimum performance of the Zn-HypoGel resin.

EXAMPLE 7

(59) Adsorption in the Presence of Competitive Ions

(60) In order to investigate the adsorption performance of the Zn-HypoGel under more realistic conditions, batch experiments were also carried out using solutions whose composition was based on NSF Standard 53, so called Challenge Water, which is used as a standard in many laboratories to assess the performance of potential arsenic adsorbents using the following procedure.

(61) A solution consisting of Na.sub.2SiO.sub.3, NaHCO.sub.3, MgSO.sub.4, NaNO.sub.3, NaF, NaH.sub.2PO.sub.4H.sub.2O, CaCl.sub.2 and Na.sub.2HAsO.sub.4.7H.sub.2O was prepared according to the procedure described for NSF Standard 53. The solution was adjusted to pH 7 with 1 M HCl. 50 ml of this solution was added to a plastic bottle, followed by 5 mg of Zn-HypoGel or Bayoxide. The bottle was then placed on an orbital shaker at 100 rpm for 24 hours, after which time the shaking was stopped and a sample removed and acidified with 0.1M HCl. The arsenic concentration was then determined by Differential Pulse Anodic Stripping Voltammetry.

(62) The composition of the solutions prior to contacting with Zn-HypoGel or Bayoxide is shown in Table 2.

(63) TABLE-US-00002 TABLE 2 The concentration of each component of the competitive solutions, as well as their relative proportion to arsenate, which was present at 300 ppb. Concentration Equivalents (relative Ion (mM) to arsenate) SiO.sub.3.sup.2 0.34 82 HCO.sub.3.sup. 2.97 725 SO.sub.4.sup.2 0.50 122 NO.sub.3.sup. 0.14 34 F.sup. 0.05 13 HPO.sub.4.sup.2 0.001 0.33 Cl.sup. 1.98 482 Mg.sup.2+ 0.50 122 Ca.sup.2+ 0.99 241

(64) The resulting arsenate uptake after shaking both Zn-HypoGel and Bayoxide for 24 hours in Challenge Water is shown in FIG. 5. It can be seen clearly that 5 mg of the Zn-HypoGel sorbent was able to adsorb twice as much arsenic from this competitive solution than Bayoxide. This can be attributed to the greater selectivity of Zn-HypoGel adsorption sites towards arsenate over the majority of the species present in the challenge water.

EXAMPLE 8

(65) Regeneration of HypoGel resin.

(66) In order to demonstrate that the Zn-HypoGel sorbent could be regenerated after use, a series of adsorption/desorption cycles were carried out using the following procedure.

(67) A 5 ml solution of HEPES buffer (pH 7) and 15 L of 1,000 ppm arsenic stock were added to a luer syringe fitted with a frit and lock, followed by 5 mg of ZnHypoGel sorbent. After 24 hours of shaking at 100 rpm, the solution was removed by filtration and the concentration of arsenic remaining in solution was determined. A solution of 1.37 M NaCl was prepared and adjusted to pH 10 with 1 M NaOH. 5 ml of the brine was added to the As-laden sorbent and the syringe returned to the shaker. The arsenic concentration in the brine was determined after 24 hoursthis process was repeated until no further arsenic was desorbed. The resin was then washed with copious Milli-Q water and buffer, before the next arsenic solution was added, and the adsorption/desorption cycle was repeated.

(68) The results of the regeneration studies is shown in FIG. 6.

(69) It can be seen from FIG. 6, that it was possible to desorb all of the arsenic from the Zn-HypoGel without effecting the adsorption of arsenic in further cycles.

EXAMPLE 9

(70) Use of Zn-HypoGel Resin in Fixed-Bed Columns.

(71) In order to determine whether Zn-HypoGel had potential to be used in a flow-through water treatment system, fixed-bed column studies were carried out. Dry Zn-HypoGel was added to a glass column (1.4 g resin, 3.5 cm bed depth, 1 cm column diameter) and packed in Milli-Q water. Glass beads were added on top of the wet resin to act as a solution distributor as shown in FIG. 10. The column was washed with HEPES buffer before adsorption experiments were carried out. The bed volume (BV) was 2.75 cm.sup.3.

(72) A solution of 10 ppm arsenate buffered at pH 7 with HEPES was then passed through the column using a Watson-Marlow 101 U peristaltic pump. The flow rate was maintained at 1 ml/min giving an empty bed contact time (EBCT) of 2.8 minutes. Fractions of the column effluent were collected and the total arsenic concentration was determined by DPASV.

(73) FIG. 11 shows that adsorption by Zn-HypoGel in the column was very efficient. The effluent concentration was <100 ppb for up to 1000 bed volumes (BV) i.e. >99% adsorption occurred in the column.

(74) Arsenic began to break through (i.e. the column becomes saturated) after 2.75 L (1000 BV) of the 10 ppm As.sup.V solution had been treated. Given that the average adsorption measured over this time was 99% this translates to total adsorption of c.a. 0.35 mmoles As.sup.V by the column. This correlates very closely with the expected number of available binding sitesfrom the zinc loading calculations it was determined that Zn-HypoGel contained 0.23 mmoles sites/g therefore a 1.4 g column contains 0.32 mmoles of adsorption sites.

EXAMPLE 10

(75) Regeneration of Zn-HypoGel in a Fixed Bed Column

(76) The regeneration of Zn-HypoGel with 1 M NaOAc at pH 10 was shown to be efficient during batch experiments. Therefore this solution was used to regenerate the fixed bed column after As.sup.V saturation. As shown in FIG. 12, near total removal of adsorbed As.sup.V was achieved after flushing with 40 BV of acetate solution. After As.sup.V desorption the column was again washed with HEPES and stored in buffer awaiting re-use.

EXAMPLE 11

(77) Use of Zn-HypoGel at Low Arsenic Concentrations

(78) Having established the possibility of using Zn-HypoGel in column format at high (ppm range) As.sup.V concentrations, experiments at lower As.sup.V concetrations were also carried out.

(79) A solution of 100 g/L As.sup.V in 20 mM HEPES (pH 7.5) was prepared. This concentration was chosen as being representative of a wide range of influent waters as well as facilitating comparison with literature. The solution was then passed through the column at a flow rate of 2 ml/min, giving a short contact time of 1.5 minutes.

(80) As shown in FIG. 13, the effluent As.sup.V concentration was reduced to below 10 g/L (red line) and in general below 5 g/L (blue line). The effluent pH was also measured and remained constant at pH 7.5.

(81) Column breakthrough was not reached even after treatment of 10 000 bed volumes of water (28 L). This is consistent with the As.sup.V capacity determined previously, see Example 9 above. It was shown that 1.5 g of Zn-HypoGel could adsorb 25 mg of As.sup.V, therefore if the adsorption was maintained at 95% this column could theoretically treat 95 000 BV of 100 g/L As.sup.V solution (i.e. 260 L).

EXAMPLE 12

(82) Effect of Influent Water Composition on Zn-HypoGel Fixed-Bed Columns.

(83) A range of studies were then carried out in order to determine the effect of the influent water conditions on the ability of Zn-HypoGel to adsorb As.sup.V in a column. Four different influent waters containing competing anions (Challenge WaterCW) were studied. The chemical composition of the Challenge Water is given in Table 3. The 4 solutions contained a variation of pH and As concentration (either pH 6.5 or 8.5, and 50 g/L or 300 g/L).

(84) TABLE-US-00003 TABLE 3 the ionic composition of Challenge Water solutions Concentration Ratio to Ion (mM) 300 g/L As.sup.V SiO.sub.3.sup.2 0.34 85 HCO.sub.3.sup. 2.97 742.5 SO.sub.4.sup.2 0.50 125 NO.sub.3.sup. 0.14 35 F.sup. 0.05 12.5 H.sub.2PO.sub.4.sup./HPO.sub.4.sup.2 0.0013 0.325 Cl.sup. 1.98 495 Mg.sup.2+ 0.50 125 Ca.sup.2+ 0.99 247.5

(85) The experiments were carried out as follows: the influent was passed through the column at 2 ml/min until 100 bed volumes were treated. 5 samples were collected in this time and analysed for As.sup.V. The column was then regenerated with sodium acetate and the next influent treated.

(86) Low Arsenate Concentrations

(87) FIG. 14 shows the concentration of As.sup.V remaining after CW containing 50 g/L at pH 6.5 and 8.5 was passed through the Zn-HypoGel column. The adsorption process was more efficient at the lower pH value. There are two likely explanations for this; firstly, pH 8.5 is greater than the second pKa of phosphate the most competitive anion (here present at 2 equivalents). Therefore phosphate is mostly present as a doubly negative anion and so is a stronger competitor for arsenate at pH 8.5. Secondly, at the more basic pH the presence of competitive hydroxide ions must be considered.

(88) High Arsenate Concentrations

(89) The concentration of As.sup.V remaining after CW containing arsenate at 300 g/L at either pH 6.5 or 8.5 is shown in FIG. 15. The adsorption was much more efficient at the lower pH value, presumably for the same reasons as outlined above. Here phosphate is present at 0.3 equivalents relative to arsenate. Interestingly the effluent concentration at pH 8.5 of around 20 g/L is very similar to that observed in FIG. 14 above, even though the influent concentrations of 50 g/L and 300 g/L are quite different.

(90) This experiment showed that Zn-HypoGel can remediate high levels of arsenate from CW to well below the current MCL at pH 6.5.

(91) Conclusions

(92) A range of metal complexes were synthesised and fully characterised. The di-zinc complex L.sup.1-Zn.sub.2 was shown, by two independent methods, to be a good arsenate receptor.

(93) An X-ray crystal structure (FIG. 7) was obtained which revealed the mode of binding to the metal centre this being the first such structure showing arsenate bound to a metallo-receptor.

(94) It was found that immobilising complex L.sup.1-Zn.sub.2 on a polystyrene resin resulted in a functional material (Zn-HypoGel) with a high affinity for arsenate. Zn-HypoGel was found to adsorb arsenate efficiently over a wide pH range.

(95) Adsorption from solutions containing high levels of competing ions demonstrates that the material has potential for use in water remediation.

(96) It was surprisingly found that arsenate uptake from competitive solutions was greater than that of Bayoxide which is currently used commercially as an arsenic adsorbent.

(97) It was also found that the material could be regenerated and re-used by simply washing with a basic brine solution. It was surprisingly and unexpectedly found that the regeneration resulted in no loss of performance even after multiple desorption cycles.

(98) Zn-HypoGel has been successfully used as resin in fixed-bed columns to remediate As.sup.V-contaminated waters. The columns could remediate highly competitive Challenge Water to very low levels of arsenate when the influent pH was 6.5. However, when the influent pH was 8.5, the effluent As.sup.V concentration remained above 20 g/L. The presence of phosphate (as HPO.sub.4.sup.2) and hydroxide ions in solution could increase the competition for adsorption sites and therefore inhibit As.sup.V uptake at higher pH.