GB2530482A

GB2530482A - Method of removing metal ions from aqueous solutions

Info

Publication number: GB2530482A
Application number: GB1415456.1A
Authority: GB
Inventors: Philip Cheung
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-09-02
Filing date: 2014-09-02
Publication date: 2016-03-30
Anticipated expiration: 2034-09-02
Also published as: GB2530482B; GB201415456D0

Abstract

A method of extracting metal ions from an aqueous solution which comprises a chelated metal-ligand complex, the method comprises the steps of: (a) reacting the metal-ligand complex at a temperature of at least 60 Â°C with a persulfate oxidant to decompose the chelating agent and thereby release the metal ions from the metal-ligand complex; (b) raising the pH of the solution to precipitate the metal ions in the form of a metal oxide or metal oxyhydroxide; and (c) separating the metal oxide or metal oxyhydroxide precipitate from the solution. Preferably, the persulfate oxidant is peroxomonosulfuric acid, peroxodisulfuric acid, a peroxomonosulfate salt, a peroxodisulfate salt or a mixture of two or more thereof. The chelating agent can be a citrate, gluconate, tartrate, malonate or ethylenediaminetetraacetic acid (EDTA). The metal ions may comprise cobalt, copper, nickel, chromium, manganese, zinc, cadmium, mercury or lead. The aqueous solution can comprise wastewater generated from electroplating or electronic circuit board manufacturing. The separation step (c) may be carried out by sedimentation and/or filtration.

Description

Method of Removing Metal Ions from Aqueous Solutions The present invention relates to a method of extracting metal ions from aqueous solutions in which the metal ions are chelated by organic chelating agents.

in one aspect, this invention relates to a method for removing metal ions from wastewater generated from ndustria manufacturing processes that empoy chelating agents. Chelating or sequestering agents are used in a number of industrial processes that also employ metal ions. For example, citrate ions are present in surface4inishing baths, e.g., in electroless nickel plating, and ethylenediaminetetraacetic acid (EDTA) is used in the electroplating of copper onto electronic circuit boards.

With an increased emphasis on environmental safety and pollution control, the efficient removal of contaminants from wastewater has become of vital importance to a number of manufacturing industries. Furthermore, the continual lowering of the discharge limits of various metals into public sewer systems has important consequences for manufacturing plants, and many types of waste effluent cannot therefore be directly and conveniently disposed of in this manner. This means expensive removal of the untreated wastewater by tanker trucks.

it is generafly known that non-ch&ated metals ions can be removed from wastewater by increasing the alkalinity of the solutbn and precipitating the metaffic ions from the solution as metal hydroxides.

The presence of organic chelating agents in wastewater however poses problems for the removal of metals from aqueous solutions in this way. This is because the chelating agents will shield the metal ion from the approach of, and therefore substitution by, hydroxyl ions (OH-). The presence of the chelating agents therefore means that the metal species are less amenable to removal by methods dependent upon the formation of precipitates when the pH is raised.

A further problem with this method is that the metal hydroxides that precipitate on raising the pH of the solution are often produced as hydrophilic gels (e.g. nickel (II) hydroxide) rather than filterable solids. These hydrophilic gels can be difficult to dehydrate and isolate. It would be desirable therefore to be able to recover the metal ions from the aqueous solution as a solid powder form, which can be easily isolated by filtration and therefore recycled or reused.

There is therefore is an ongoing need for the development of new methods for the removal of metals from aqueous solutions, and in particular those which also contain chelated complexes of metal ions with organic chelating agents.

According to a first aspect of the present invention, there is provided a method of extracting metal ions from an aqueous solution comprising the metal ions chelated by a chelating agent to form a chelated metal-ligand complex. The method comprises the steps of (a) reacting the metal-ligand complex at a temperature of at least 60 °C with a persulfate oxidant in an amount sufficient to decompose the chelating agent and thereby release the metal ions from the metal ligand complex, (b) raising the pH of the mixture to precipitate the metal ions from the mixture, in the presence of the persulfate oxidant, in the form of a metal oxide or metal oxyhydroxide; (c) separating the metal oxide or metal oxyhydroxide precipitate from the mixture.

The term "persulfate oxidant" as used herein means any material which can serve as a precursor for the persulfate anion, [SO5]2. Suitable persulfate oxidants include the potassium, sodium or ammoniurn peroxomonosulfate sa'ts or peroxodisulfate salts, as w&l as corresponding triple salts, e.g. 2KHSO5KHSO4-K2S04(commerdally avaUabie as Oxone®). Also included are peroxosulfuric acids, for example, peroxomonosulfuric acid (also known as Cams acid) and peroxodisulfuric acid (also known as Marshalis acid).

Preferably, the persulfate oxidant is peroxomonosulfuric acid, peroxodisulfuric acid, a peroxomonosulfate salt, a peroxodisulfate salt, or a mixture of two or more thereof.

As persulfate oxidants are generally acidic, the addition of the oxidant decreases the pH of the solution. Preferably, the pH following the addition of the persulfate oxidant is less than or equal to 3.

In order to precipitate the metal ions from the solution as a metal oxide or metal oxyhydroxide, it is necessary to increase the alkalinity of the solution, preferably to a pH n of at least 11. In a preferred embodiment, the pH is increased by the addition of a soluble metal hydroxide, for example, sodium hydroxide.

The term "chelating agent" as used herein means any neutral molecule or charged ion having two or more moieties that each independently are capable of binding to the metal ion. In one embodiment, the chelating agent comprises at least one carboxylic acid group. Examples of such chelating agents are ethylenediaminetetraacetic acid (EDTA), a citrate, a gluconate, a tartrate, or a malonate and combinations of these agents.

Typically, the chelating agent is a citrate or EDTA.

The method of the present nvention is particidarly useful for predpitating transition metals from aqueous solutions being produced as wastewater n industrial applications such as electroplating and electronic circuit board manufacturing. The method is particularly useful for treating solutions containing one or more of copper, cobalt, nickel, zinc, and manganese. The metal may be, but is not limited to cobalt, copper, nickel, chromium, zinc, manganese, lead, cadmium, or mercury.

As indicated above, the method is particularly suitable for treating solutions in which the metal-ligand complex is a metal-citrate or rnetaEDTA. e.g. nickel ethylenediaminetetraacetic acid, copper ethylenediaminetetraacetic acid, cobalt ethylenediaminetetraacetic acid, zinc ethylenediaminetetraacetic acid, manganese ethylenediaminetetraacetic acid, copper citrate, nickel citrate or cobalt citrate.

In a preferred embodiment, the mole ratio of oxidant to chelated metal ions present in the aqueous solution is greater than 5:1, and more preferably greater than 10:1. The mole ratio may be less than 21:1, for example less than 17:1.

Advantageously, the metal oxide and metal oxyhydroxide precipitates produced by the method of the present invention are solids, which can be separated and removed from the solution by filtration.

In a preferred embodiment, the recovery of metal ions from the solution as a metal oxide or metal oxyhydroxide is at least 90%.

I

Detailed Description

The present invention is directed towards a method of extracting metal ions from an aqueous solution comprising the metal ions chelated by a chelating agent to form a metal-ligand complex, such as wastewater obtained from industrial processes. The present nventon addresses the drawbacks and shortcomings of exstnq wastewater treatment methods.

The method of the present apphcatbn uses persuffate oxdants to decompose the metal-ligand complex by denaturing the ligand to the extent that its chelating ability is lost. For example, when the ligand is chelated to the metal centre through one or more carboxylate groups, treatment with a persulfate oxidant causes detachment of the carboxylic groups from the ligand moiety, thus reducing the chelating ability of the ligand and releasing the metal ions into the aqueous solution. It is then necessary to precipitate the soluble metal ions from the aqueous solution through formation of insoluble metal oxides and oxyhydroxides.

Commonly used persulfates are the oxoacids including Caro's acid (peroxomonosulfuric acid, H2S05) and Marshall's acid (peroxodisulfuric acid, H2S208). A "peroxo" compound contains at least one pair of oxygen atoms which are covalently bonded, with each oxygen atom having an oxidation number of-i. Caro's acid is a white crystalline solid that melts and decomposes at 45°C. Marshall's Acid (peroxodisulfuric acid) H2S203 is a white crystalline solid that and melts and decomposes at 65°C (Kim & Edwards 1955).

Sodium, potassium and ammonium peroxodisulfates are available commercially, for example, Na2S2O3 (commercially available from Aldrich). Potassium peroxomonosulfate is also available as a "triple salt" under the brand name Oxone®, the formula of which is 2KHSO5KHSO4K2SO4. The peroxomonosulfate is the active component.

When the chelating agents in the metal-ligand complexes are oxidised by the persulfate oxidant, the HSO anion is reduced to the innocuous sulfate anion SO42. This is advantageous over prior art oxidants which decompose to produce harmful by-products, for example, chlorine dioxide, which decomposes to produce chlorine gas. Furthemiore, the persulfate oxidants are generally highly water soluble and effective under both acidic and basic reaction conditions.

The solid metal oxides and oxyhydroxides extracted from the wastewater using the method of the present invention can be dried and recycled, for example, in ore smelters, to produce elemental metals. The treated effluent can be discharged into normal waste streams such as public sewers. The method of the present invention is therefore able to reduce the need for solid waste and hazardous waste management and disposal, for example incinerators, waste transfer stations and landfills.

Examples

The following examples and comparative examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof.

Examples Ito 9

Treatment of aqueous solutions of metal citrates with potassium peroxomonosulfate triple salt (Oxone®, 2KHSO5.KHSO4.K2S04) In order to simulate wastewater obtained from the industrial process of metal electroplating, aqueous solutions of the citrates of the first row transition elements from chromium to copper, zinc, mercury, cadmium and lead were prepared. These solutions were treated with the triple salt persulfate oxidant 2KHSO5.KHSO4.K2S04 (commercially available from Du Pont, sold under Oxone®), of which HS05 is the active ingredient.

Sodium hydroxide was added to the resulting solutions in order to precipitate the metal ions as metal oxides and oxyhydroxides. These were preliminary experiments to establish the general principle and therefore a large excess of oxidant was used (approximately 48:1 with respect to the chelated metal ions).

General Procedure 1 A 0.01 mol dm3 solution of the metal citrate was prepared by dissolving stoichiometric quantities of trisodium citrate in distilled water, followed by addition of the metal sulfate.

For mercury and lead, the nitrates were used instead of sulfates.

g of Oxone® (2KHSO5.KHSO4.K2S04) was added to 200 mL of the 0.01 mol dm3 aqueous solution of the metal citrate (0.002 mol). The formula mass of Oxone® is 614 and the ionic mass of the active oxidant HS05 is 113. Since 2 moles of HSO5 are available per mole of Oxone®, the mass of HS05 contained in 614 g of Oxone is given by 2 x 113 = 226. Thus, 30 g of Oxone® contains 11 g (0.097 mol) of HS05. The molarity of the oxidant with respect to the HSO5 anion was therefore 0.48 mol dm3.

The resulting mixture was heated to a temperature of 65°C over a period of 20 minutes and kept at 65°C (± 2°C) in a water bath for 1 hour and then allowed to cool to room temperature (20°C) overa period of 1 hourwhilst stirring. NaOH (109,0.25 mol) was added and the mixture was left to stand for 18 hours to allow for precipitation and sedimentation of the metal oxide or oxyhydroxide.

The solids were separated from the liquid by filtration. The filtrate was analysed by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) to determine the residual concentration of metal ions in the treated water, using a Jobin Yvon Ultima 2C ICP Optical Emission Spectrometer. A decontamination factor was also calculated for each experiment, defined as the molar ratio of metallic ions removed from the solution to metallic ions remaining in solution. The pH of the reaction mixture was monitored and recorded at multiple stages in the procedure as set out below. The reaction mixture was also monitored for colour changes and precipitate formation. The results are documented below in Tables 1 and 2.

Table 1 -pH measurements and colour changes recorded at specific stages.

Step 1: pH of the deionised water.

Step 2: pH after the addition of trisodium citrate.

Step 3: pH after the addition of the metallic salt.

Step 4: pH after the addition of 2KH505.KHSO4.K2504.

Step 5: pH after the addition of NaOH.

Ex. Ion ____ ____ Step ____ Observations __ ___ 1 2 3 45 _____________________ The slight green colour of the nickel citrate solution remained unchanged after addition 1 Ni2 8.4 8.8 7.8 <3 >11 ofoxone®.

The solution was decolourised on heating.

Addition of NaOH resulted in a colour change to black.

Sedimentation gave a clear, colourless supernatant solution and a black precipitate.

The reddish pink colour of the cobalt citrate solution changed to deep purplish pink after addition of Oxone®.

2 Co2 8.4 8.8 7.3 <3 >11 The solution was decolourised on heating.

Addition of NaOH resulted in a colour change to black.

The solution changed from a dark blue to light blue after addition of Oxone®.

Heating the solution caused a colour change to a greyish blue.

Addition of NaOH resulted in a colour change to deep blue.

3 Cu 8.4 8.8 5.1 <3 >11. . The solution was then boiled to give a suspension of dark brown substances.

Sedimentation gave a clear, colourless supernatant solution and a dark brown precipitate.

The solution remained colourless throughout 4 Zn 8.4 8.7 7.5 <3 >11 the reaction.

Addition of NaOH followed by sedimentation gave a clear, colourless supernatant solution and a white precipitate The solution remained very slightly pink after addition of Oxone®.

Mn 8.4 8.9 7.7 <3 >11 No colour change was observed upon heating Addition of NaOH resulted in a colour change to dark brown.

Sedimentation gave a colourless supernatant solution and a dark brown precipitate.

The iron citrate solution was yellowish green.

Addition of Oxone® reduced the intensity of 6 Fe2 8.4 8.8 8.8 3 >11 the colour.

No colour change was observed upon heating.

Addition of NaOH resulted in a colour change to reddish orange.

Sedimentation gave a clear, colourless supernatant solution and a dark brown ____ ______ ____ ____ ____ _____ precipitate.

The solution remained colourless throughout.

7 Pb 8.4 8.8 6.5 <3 >11 Addition of NaOH followed by sedimentation gave a clear, colourless supernatant solution and a yellow precipitate.

The solution remained colourless throughout.

8 Cd 8.4 8.7 7.1 <3 >11 Addition of NaOH followed by sedimentation gave a clear, colourless supernatant solution and a white precipitate.

The solution remained colourless throughout.

Addition of NaOH followed by sedimentation 9 Hg 8.4 8.7 7.3 <3 >11 gave a clear, colourless supernatant solution and a yellow gum.

S

Table 2-Residual concentration of metal ions in the treated solutions.

Ex. Ion Final Conc. Decontamination Observations ____ _______ ppm mol dm Factor __________________________ 1 Ni2'3 5 8 x 125 Filtrate was clear and colourless. Crystallisation of a characteristic green nickel salt was evident.

2 Co2'3 6 lOx iO 100 Filtrate was clear and colourless.

3 Cu 1 1.5x105 667 Filtrate was clear and colourless. Crystallisation was evident.

4 Zn 20 30 x i0 33 Filtrate was clear and colourless.

Mn 2 4 x 1 O 250 Filtrate was clear and colourless.

Manganese was not further oxidised to purple Mn04.

6 Fe3 4 7 x i0 142 Filtrate was clear and colourless.

7 Pb2 <1 >100 Filtrate was clear and colourless.

8 Cd2 9 8x105 125 Filtrate was clear and colourless.

9 Hf <1 ->100 Filtrate was clear and colourless.

As can be seen from this data, the triple salt oxidant 2KHSO5.KHSO4.K2S04 is effective in decomposing the metal-citrate complexes of nickel, cobalt, copper, zinc, manganese, iron, lead, cadmium and mercury. The addition of NaOH causes precipitation of the metals as metal oxides or oxyhydroxides, which can be separated from the solutions by filtration.

Examples la, 2a, and 3a Treatment of aqueous solutions of the citrates of nickel, cobalt and copper with peroxodsulfudc add disodhim salt (Na2S2O8) Aqueous solutions of the citrates of nickel, cobalt and copper were treated with peroxodisulfuric acid disodium salt (Na2S2O8) as the oxidant using the same method as outhned above n General Procedure I (Examples la, 2a and 3a respectively). Na2S2O3 was used in an amount to give the same mole ratio of oxidant to chelated metal ions as

in Examples ito 9.

In all cases, the addition of the oxidant reduced the intensity of the colour of the solution and resulted in reduction in pH. The addition of NaOH resulted in the formation of a black precipitate and sedimentation resulted in a clear, colourless supernatant solution.

Examples lOto 14

Treatment of aqueous solutions of metal-EDTA complexes with peroxothsulfurc add dsodium salt (Na2S2O3) Determining the preferred mole ratio for the oxidation of Ni(ll)-EDTA using Na7S7O The following experiments were conducted in order to establish the optimum mole ratio for the oxidation of NiOD-EDTA complex using the peroxodisulfuric add disodiurn salt (Na2S2O3). This is the sodium salt of Marshall's acid. Solutions of Ni(ll)-EDTA were prepared to simulate the wastewater obtained from metal electroplating.

General Procedure 2 The following procedure was carried out using various quantities of the peroxodsuIfuric acid disodium salt (Na2S2O3).

Na2S2O5was added to an 0.01 mol dm3 aqueous solution of Ni(ll)-EDTA (100 mL, 0.001 mol) and the mixture was heated to a temperature of 65°C of a period of 20 minutes and kept at 65°C (± 2°C) in a water bath for 1 hour. The amounts of Na2S2O8 used in each experiment are as shown in Table 3. The reaction mixture was allowed to cool to room temperature (20 °C), NaOH()(5 g, 0.125 mol) was added and the mixture was allowed to stand for 18 hours to allow for sedimentation of any solids formed.

Table 3 -Mass of Na2S2O3 used in Examples 10 to 14 Ex. Mass Mole ratio of Na2S2O5 (g) HS05: complexed Ni(ll) ions 1 4.2:1 11 2 6.4:1 12 3 13:1 13 4 17:1 14 5 21:1

Example 10

1 g of Na28208 was used. Three distinct phases were observed after 18 hours. The bottom phase was a black precipitate comphsed of nickel oxides and oxyhydroxides.

The middle phase was the heavily hydrated green gel of Ni(OH)2.nH2O. The blue colour of the upper aqueous phase indicated that Ni(ll)-EDTA remained in solution indicating that 1 g of Na28205 was not sufficient to completely decompose the Ni(ll)-EDTA complex. The final concentration of nickel ions in the supernatant was 242 ppm.

Example 11

2 g of Na2S2O3 was used. A two-phase mixture of a green gel precipitate of Ni(OH)2.nH2O and a transparent, colourless aqueous phase was observed. The colourless aqueous phase indicated that the Ni(ll)-EOTA complex was completely decomposed and all Ni(ll) ions were released into solution but no black solid precipitates were formed. The final concentration of nickel ions in the supernatant was 80 ppm.

Example 12

3 g of Na2S2O3was used. A two-phase mixture of a green gel precipitate of Ni(OH)2.nH2O and a transparent, colourless aqueous phase was observed. The colourless aqueous phase indicated that the Ni(ll)-EDTA complex was completely decomposed and all Ni(ll) was released into solution but no black solid precipitates were formed. The final concentration of nickel ions in the supernatant was 10 ppm.

Example 13

4 g of Na2S2O8 was used. The predominant nickel species was a permanent black precipitate of nickel oxides and oxyhydroxides with small quantities of the green gel Ni(OH)2.nH2O in the supernatant. The final concentration of nickel ions in the supernatant was 1.08 ppm.

Example 14

g of Na28208 was used. A transparent, colourless supernatant was observed with a black precipitate comprising nickel oxides and oxyhydroxides. No green gel of Ni(OH)2.nH2O was observed. The final concentration of nickel ions in the supematant was 0.08 ppm.

Therefore, in order to successfully extract the metals as insoluble, filterable oxides and oxyhydroxides, the mole ratio of HSO5 to complexed Ni(ll) ions is preferably at least 17:1, and preferably, atleast2l:1.

Examples 15 to 17

Treatment of aqueous solutions of metal-EDTA complexes with peroxodsuffuric acid dsodum saR In order to mimic wastewater obtained from the manufacture of electronic circuit boards and metal electroplating, aqueous solutions of the EDTA complexes of nickel, copper and cobalt were prepared. These solutions were treated with peroxodsuifuric acid disodum saft (Na2S2O3, the disod!urn sat of Marshafls acid). The active oxidant s the anion HS05. Sodium hydroxide was added to the resulting solutions in order to precipitate the metal ions as metal oxides and oxyhydroxides.

Example 15 -Ni(ll)-EDTA Na2S2O5(5 g, 0.021 mol) was added to a 0.01 mol dm3 aqueous solution of Ni(ll)-EDTA (100 mL, 0.001 mol). The resulting mixture was heated to 60°C over a period of 20 minutes, followed by heating in a water bath at 65°C for 2 hours. The reaction mixture was allowed to cool to room temperature (20°C), NaOH)(5 g, 0.125 mol) was added and a suspension of black colloidal particles formed immediately with no green gel of Ni(OH)2.nH2O. The reaction mixture was filtered, and the precipitate collected. The filtrate was analysed by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) using the method described above. The final concentration of nickel ions in filtrate was 0.08 ppm.

Example 16 -Cu(ll)-EDTA Na2S2O5(5 g, 0.021 mol) was added to a 0.01 mol dm3 aqueous solution of Cu(ll)-EDTA (100 mL, 0.001 mol). The resulting mixture was heated to 60°C over a period of 20 minutes, followed by heating in a water bath at 65°C for 2 hours. The reaction mixture was allowed to cool to room temperature (20°C), NaOH) (5 g, 0.125 mol) was added and a suspension of black colloidal particles formed immediately. The reaction mixture was filtered, and the precipitate collected. The filtrate was analysed by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) using the method described above. The final concentration of copper ions in the filtrate was 0.78 ppm.

Example 17-Co(ll)-EDTA Na2S2O3(5 g, 0.021 mol) was added to a 0.01 mol dm3 aqueous solution of Co(lD-EDTA (0.01 mol dm3, 100 mL). The resulting mixture was heated to 60°C over a period of 20 minutes, followed by heating in a water bath at 65°C for 2 hours, during which the formation of a black precipitate was observed. The reaction mixture was filtered and the black precipitate was recovered. The filtrate was acidified by the addition of dilute sulphuric acid. Na28203 (5 g, 0.021 mol) was added and the reaction mixture was heated in a water bath at 65°C for a further 2 hours. The reaction mixture was allowed to cool to room temperature (20°C), NaOH()(5 g, 0.125 mol) was added and a suspension of black colloidal particles formed immediately. The reaction mixture was filtered and the precipitate was collected. The filtrate was analysed by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) using the method described above. The final concentration of cobalt ions in the filtrate was 0.06 ppm.

The results are summailsed below in Table 4.

Table 4-Residual concentrations of metal ions in filtrates.

Ex. Metal-ligand complex Conc. of ions in filtrate (ppm) Ni(lD-EDTA 0.08 16 CuOD-EDTA 0.78 17 C000-EDTA 0.06 In all cases a solid precipitate was formed indicating the formation of the metal oxides and hydroxides as filterable solid. These could be easily separated from the reaction mixtures.

Examples 18 to 47

Extraction of metallic ions from hydroxycarboxylate complexes using salts of peroxosulfuric acids The following experiments demonstrate the efficiency of the method of the present invention in recovering the ions of CoOl), Ni(ll) and Cu(ll) as insoluble oxides or oxyhydroxides from their hydroxycarboxylate complexes (i.e. citrates, gluconates, tartrates, malates and malonates).

The oxidations were carried out using peroxodisulfutic acid disodiurn sait (the sodurn salt of MarshaH's acid) and the potassium peroxomonosulfate triple salt, Oxone®, of which the active oxidant is HSO5.

General Procedure 3 -Oxidation using peroxodisulfuric add dsodium salt Na2S2O8 (5 g, 0.26 mcI) was added to a 0.01 mcI dm3 aqueous solution of metal-ligand complex (100 mL, 0.001 mol). The resulting mixture was heated to 60°C over a period of 20 minutes, followed by heating in a water bath at 65°C for 2 hours. The reaction mixture was allowed to cool to room temperature (20°C). NaOH(5) (5 g, 0.125 mol) was added and the reaction mixture was allowed to stand for 18 hours to allow for precipitation of the metallic oxide or oxyhydroxide. The precipitate was separated from the reaction mixture by filtration.

General Procedure 4 -Oxidation using potassium peroxomonosulfate triple salt 2KHSO5.KHSO4.K2SO4 (6.25 g, 0.01 mcI) was added to a 0.01 mol dm3 aqueous solution of metal-ligand complex (100 mL, 0.001 mcI) and the resulting mixture was heated to 60 °C over a period of 20 minutes, followed by heating in a water bath at 65°C for 2 hours. The reaction mixture was allowed to cool to room temperature (20°C), NaOH(5) (5 g, 0.125 mcI) was added and the reaction mixture was allowed to stand for 18 hours to allow for precipitation of the metallic oxide or oxyhydroxide. The precipitate was separated from the reaction mixture by filtration.

The resulting filtrates were analysed by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) using the method described above in order to determine the residual concentration of metallic ions. The decontamination factor was also calculated for each experiment. The results are tabulated below in Tables 5 and 6.

Table 5 -Residual concentrations of metallic ions in the filtrate resulting from oxidation using Na2S2O5 Ex. Metallic ion Ligand EM1 in filtrate Decontamination (ppm) factor 18 Co2 citrate 0.03 19600 19 Co2 gluconate 0.09 6500 002+ tartrate 0.03 19600 21 Co2 malate 0.01 58900 22 Co2 malonate 0.01 58900 23 Ni2 citrate 0.17 3400 24 Ni2 gluconate 0.18 3200 Ni2 tartrate 0.28 2100 26 Ni2 malate 0.15 3900 27 Ni2 malonate 0.80 700 28 Cu2 citrate 0.07 9000 29 Cu2 gluconate 0.93 600 Cu2 tartrate 0.03 21000 31 Cu2 malate 0.03 21000 32 Cu2 malonate 0.09 7000 Table 6 -Residual concentrations of metallic ions in the filtrate resulting from oxidation using Oxone®.

Ex. Metallic ion Ligand EM1 in filtrate Decontamination ppm factor 33 002+ citrate 1.60 150 34 0o2+ gluconate 12.57 19 002+ tartrate 0.06 39200 36 002+ malate <0.01 58900 37 Co2+ malonate 0.13 19600 38 Ni2 citrate 1.50 400 39 Ni2 gluconate 0.11 5300 Ni2 tartrate 0.10 5800 41 Ni2 malate 0.02 29300 42 Ni2 malonate 0.22 2600 43 Cu2 citrate 1.10 600 44 Cu2 gluconate 1.14 500 Cu2 tartrate 0.05 12700 46 Cu2 malate 0.17 3700 47 Cu2 malonate 0.35 7400 The results in Tables 5 and 6 above demonstrate that persulfate oxidants are effective for the extraction of metal ions aqueous solutions when the metals are complexed with hydroxycarboxylate ligands.

Embodiments of the present invention have been described with particular reference to @ the examples illustrated. However, it will be appreciated that variations and modifications may be made to the examples described within the scope of the present invention.

Claims

Claims 1. A method of extracting metal ions from an aqueous solution comprising the metal ions chelated by a chelating agent to form a chelated metal-ligand complex, the method comprising the steps of: a. reacting the metal-ligand complex at a temperature of at least 60°C with a persulfate oxidant in an amount sufficient to decompose the chelating agent and thereby release the metal ions from the metal-ligand complex; b. raising the pH of the solution to precipitate the metal ions from the mixture in the form of a metal oxide or metal oxyhydroxide; c. separating the metal oxide or metal oxyhydroxide precipitate from the solution.
2. The method of Claim 1, wherein the pH of the solution after the addition of the persulfate oxidant in step (a) is less than or equal to 3.
3. The method of any of the preceding claims, wherein the pH of the mixture in step (b) is raised to atleastli.
4. The method of Claim 3, wherein raising of the pH of the mixture in step (b) is achieved through the addition of a soluble metal hydroxide.
5. The method of any of the preceding claims, wherein the persulfate oxidant is peroxomonosulfuric acid, peroxodisulfuric acid, a peroxomonosulfate salt, a peroxodisulfate salt, or a mixture of two or more thereof.
6. The method of any of the preceding claims, wherein the chelating agent comprises at least one carboxylic acid group.
7. The method of any of the preceding claims, wherein the chelating agent is ethylenediaminetetraacetic acid, a citrate, a gluconate, a tartrate, or a malonate.
8. The method any of the preceding claims, wherein the metal ions comprise cobalt, copper, nickel, chromium, manganese, zinc, cadmium, mercury or lead.
9. The method of any of the preceding claims, wherein the metal-ligand complex is selected from the EDTA or citrate complexes of cobalt, copper, nickel, chromium, manganese, zinc, cadmium, mercury and lead.
10. The method of any of the preceding claims, wherein the aqueous solution comprises wastewater that is generated from an industrial process selected from metal electroless and electroplating, and electronic circuit board manufacturing.
11. The method of any of the preceding claims, wherein the mole ratio of oxidant to chelated metal ions present in the aqueous solution is less than 21:1
12. The method of any of the preceding claims, wherein the mole ratio of oxidant to chelated metal ions present in the aqueous solution is from 5:1 to 17:1.
13. The method of any of the preceding claims, wherein the step of separating the metal oxide or metal oxyhydroxide precipitate from the solution is carried out by sedimentation and/or filtration.
14. The method of any of the preceding claims wherein the recovery of metal ions from the solution as a metal oxide or metal oxyhydroxide is at least 90%.