Method for decontaminating heavy metals in an aqueous solution

Abstract

The present invention concerns a method for electrochemically depolluting an aqueous solution containing at least one heavy metal, said method comprising the following steps: a) a step of measuring the pH of the aqueous solution, optionally followed by a step of adapting said pH by adding a strong acid or a strong base, b) bringing said aqueous solution into contact with a reference electrode, a counter-electrode and a working electrode comprising a conductive substrate, c) applying a constant potential to the arrangement, as a result of which a film of at least one heavy metal oxide is formed on said working electrode, this step being capable of being repeated when the aqueous solution contains several heavy metals, and d) recovering a depolluted aqueous solution and said film.

Claims

1. Method for electrochemically depolluting an aqueous solution containing at least one heavy metal including lead, said method comprising the following steps: a) a step to measure the pH of the aqueous solution, optionally followed by a step to adapt said pH through the addition of a strong acid or strong base; b) contacting said aqueous solution with a reference electrode, a counter-electrode and a working electrode comprising a conductive substrate; c) applying a constant potential to the assembly, after which a film is formed of at least one heavy metal oxide on said working electrode; and d) recovering a depolluted aqueous solution and said oxide film; wherein steps a), b), c) and d) can be repeated when the aqueous solution contains several heavy metals, wherein the method is conducted at a temperature of between 10 C. and 50 C., and wherein step c) is applied until the current becomes zero and constant, such that there is no more lead in the aqueous solution.

2. The method according to claim 1, wherein the conductive substrate is selected from the group composed of stainless steel, semiconductors, noble metals and mixtures thereof.

3. The method according to claim 1, wherein the other heavy metal is selected from the group consisting of copper, nickel, mercury, aluminium, titanium, arsenic, silver, bismuth, and mixtures thereof.

4. The method according to claim 1, wherein the aqueous solution comprises nickel and/or copper in addition to lead.

5. The method according to claim 1 for depolluting an aqueous solution containing lead, having a pH lower than 8, wherein: if the pH of the aqueous solution containing lead is lower than or equal to 4, the potential at step c) is between 1.65 V and 1.95 V (relative to the standard hydrogen electrode); and if the pH of the aqueous solution containing lead is between 4 and 8, the potential at step c) is between 1.15 V and 1.85 V (relative to the standard hydrogen electrode).

6. The method according to claim 1 for depolluting an aqueous solution containing lead, having a pH higher than 8, wherein step a) is followed by a step to add a strong acid to obtain an aqueous solution having a pH of between 4 and 8, and wherein at step c) a potential is applied of between 1.15 V and 1.85 V (relative to the standard hydrogen electrode).

7. The method according to claim 1 for depolluting an aqueous solution containing copper in addition to lead, wherein: if the pH of said aqueous solution containing copper, measured at step a), is higher than 8, said step a) is followed by a step to add a strong acid, and if said pH is lower than 4, said step a) is followed by a step to add a strong base, to obtain an aqueous solution having a pH of between 4 and 8; and wherein the potential at step c), applied to said aqueous solution having a pH of between 4 and 8, is between 0.15 V and 0.25 V (relative to the standard hydrogen electrode).

8. The method according to claim 1 for depolluting an aqueous solution containing nickel in addition to lead, wherein: if the pH of said aqueous solution containing nickel, measured a step a), is higher than 8, said step a) is followed by a step to add a strong acid, and if said pH is lower than 4, said step a) is followed by a step to add a strong base, to obtain an aqueous solution having a pH of between 4 and 8; and wherein the potential at step c), applied to said aqueous solution having a pH of between 4 and 8, is between 1.35 V and 1.55 V (relative to the standard hydrogen electrode).

9. The method according to claim 1, wherein step c) comprises the performing of chronoamperometry using a potentiostat.

10. The method according to claim 1, which is conducted at a temperature of between 10 C. and 40 C.

11. The method according to claim 1, which is conducted at ambient temperature.

12. The method according to claim 1, wherein the method is conducted at atmospheric pressure.

13. The method according to claim 1, wherein the method is conducted without degassing to remove dissolved oxygen.

14. The method according to claim 1, wherein the method is conducted under stirring.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a Scanning Electron Microscopy (SEM) image of a working electrode indicating the removed lead crystals and the conductive substrate of tin dioxide; and

(2) FIG. 2 is a graph showing an Energy Dispersive Spectroscopy (EDS) of the film composed of lead and oxygen.

EXAMPLES

(3) In all the examples described below, the decontamination method was conducted in the following manner: a conventional 80 mL reactor (beaker type); and three electrodes: a reference electrode (mercury sulfate), a counter-electrode (platinum wire) and the working electrode on which the adhering film will be deposited, which is either a glass plate coated with SnO.sub.2 or it is in stainless steel (6015 mm) where the surface in contact with the solution is limited by scotch tape (3 cm.sup.2).

(4) Experiments were conducted in air without the need for degassing to remove dissolved oxygen, at ambient temperature, under average constant stirring at atmospheric pressure. For stirring, a magnetic stir bar was used (speed set at 250 rpm). The advantage of stirring is fast acceleration of the method.

(5) The decontamination method uses a potentiostat for chronoamperometry, i.e. applying a constant potential which remains the same throughout the entire duration of the experiment.

(6) The value of the applied potential varies according to the heavy metal and the pH of the solution, as specified in the examples below.

(7) The time length of the experiment is a function of the initial concentration of the dissolved heavy metal contained in solution, since when it is fully removed the current becomes practically zero and constant, indicating that the experiment is completed.

Example 1: Case of Lead Removal

(8) For an acid solution of pH 4, the lead decontamination method is conducted under a potential of between 1 and 1.2 V/Mercury sulfate electrode, corresponding to 1.65 and 1.85 V/SHE (Standard Hydrogen Electrode).

(9) For a solution having a pH of between 4 and 8, the Pb decontamination method is conducted under a potential of between 0.7 and 0.9 V/Mercury sulfate electrode (1.35 and 1.55 V/SHE).

(10) All the Pb is removed in the form of a thin adhering oxide film deposited on the conductive substrate, composed solely of lead and oxygen.

(11) Example of Treating a Solution Having a Concentration of 27 mg/L Pb (II) (6.510.sup.6 Mol in 50 mL, i.e. 1.310.sup.4 mol/L) on SnO.sub.2 Electrode:

(12) Two Possible Treatments Depending on the Initial pH of the Solution: 1If pH=2, the Pb was fully removed from this solution in the form of an adhering film of lead oxide composed solely of lead and oxygen (about 70% and 30% Pb), deposited on the SnO.sub.2 substrate, by applying a potential of 1 V/Mercury sulfate electrode for 12 hours. 2If pH=4.2 the Pb was fully removed in the form of an adhering film of lead oxide composed solely of lead and oxygen (about 60% and 40% Pb), deposited on the SnO.sub.2 substrate, by applying a potential of 0.7 V/Mercury sulfate electrode for 8 hours.

(13) Decontamination of Pb (II) under these two conditions was 99.99%. After these treatments, the measured remaining concentration was approximately 10.sup.8 M (2.07 g/L) which corresponds to environmental and drinking water standards.

(14) The solid formed on the working electrode (e.g. SnO.sub.2) was characterized by Scanning Electron Microscopy (SEM) as illustrated in FIG. 1 (1 indicating removed lead crystals and 2 indicating the conductive substrate of tin dioxide).

(15) Energy dispersive spectroscopy EDS confirmed that the film was composed solely of lead and oxygen with the following percentages: about 60% O and about 40% Pb (FIG. 2).

(16) To test the impact of the presence of salts in solution, the same tests were conducted with a solution containing either NaCl, or Na.sub.2SO.sub.4, or NaNO.sub.3 or sodium acetate, at a concentration each time of 1 mol/L. In all cases, full decontamination of Pb was always obtained in the form of an adhering film of pure lead oxide solely containing oxygen and lead with equivalent percentages to the foregoing (60% O and 40% Pb), on the conductive electrode (e.g. SnO.sub.2), irrespective of the species present in solution.

Example 2: Case of Copper Removal

(17) If the pH of the solution is acid and lower than 4, Cu is not removed according to this decontamination method.

(18) For a solution having a pH between 4pH8, the Cu decontamination method is conducted under a potential of between 0.4 V and 0.5 V/Mercury sulfate electrode (0.25 and 0.15 V/SHE). Under these conditions, decontamination of Cu (II) is about 99.99%. Cu is removed in the form of a thin adhering film of copper oxide (about 50% O and 50% Cu), deposited on the conductive substrate (e.g. SnO.sub.2).

(19) Example of Decontamination for a Solution of 8.2 mg/L (6.510.sup.6 mol in 50 ml, i.e. 1.310.sup.4 mol/L) on SnO.sub.2 Electrode:

(20) For a solution having pH=7, Cu was fully removed in the form of an adhering film deposited on the SnO.sub.2 substrate, by applying a potential of 0.45 V/Mercury sulfate electrode for 48 h (0.2 V/SHE).

Example 3: Case of Nickel Removal

(21) For a solution having a pH of between 4 pH 8: the Ni decontamination method is conducted under a potential of between 0.7 and 0.9 V/Mercury sulfate electrode (1.35 and 1.55 V/SHE).

(22) Example of Decontamination for a Solution of 7.6 mg/L (6.510.sup.6 mol in 50 ml, i.e. 1.310.sup.4 mol/L)

(23) For pH=4.2, Ni was fully removed in the form of an adhering film deposited on the SnO.sub.2 substrate, by applying a potential of 0.7 V/Mercury sulfate electrode (1.35 V/SHE) for 72 h.

Example 4: Case of Cadmium Alone

(24) Cd cannot be removed with this method irrespective of pH conditions of the solution since it only has one oxidation state. This is a point of great interest for very good separation of cadmium from other heavy metals since it remains quantitatively in solution.

Example 5: Case of a Mixture of Lead, Nickel, Copper and Cadmium

(25) An experiment was conducted with a mixture of these 4 heavy metals (Pb, Cu, Ni, Cd) having the same initial amount in solution (6.510.sup.6 mol in 50 mL, i.e. a concentration of 1.310.sup.4 mol/L for each). The value of the initial pH of the solution, before any adjustment, was close to 1.9.

(26) Removal treatment of each metal contained in this mixture was conducted separately:

(27) First, to remove Pb, the pH of the solution, before any adjustment, was 1.9 (acid), therefore no need to change the pH. A potential E=1 V was applied.

(28) After 24 hours, all the Pb was removed in the form of a solid oxide film adhering to the electrode.

(29) Next, to remove Ni, the pH was adjusted to pH 4 through the addition of 400 L of 1M NaOH for a volume of 50 mL. A potential E=0.7 V was applied. After 3 days, all the Ni was removed in the form of a solid film adhering to the electrode.

(30) The pH value was then adjusted to pH 7 to remove Cu by applying a potential E=0.45 V for 2 days. After this treatment, the copper was removed in the form of a solid film adhering to the electrode.

(31) The Cd remained fully and quantitatively in solution.