JPH0763699B2 - Method for treating waste liquid containing heavy metals and organic substances - Google Patents

Description

Detailed Description of the Invention [Industrial interest field]

The present invention relates to a method for treating wastewater containing heavy metals and organic substances by using electrolysis.

[Prior art]

Of the waste liquids generated from nuclear power plants, those called high conductivity waste liquids are reduced in volume by evaporative concentration and then solidified. This effluent contains a high concentration of salt, with radioactive material and detergent concentrated therein. Decontamination waste liquid generated when decontaminating equipment such as steam generators and tanks contains heavy metal ions, chelating agents such as EDTA, organic acids such as formic acid, oxalic acid, and citric acid, or those. It contains salt. This heavy metal often contains radionuclides. In addition, when decontaminating soil decontaminated with heavy metals, decontamination waste liquid containing heavy metal ions and organic matter is generated. The radioactive waste liquid is finally solidified by means of cement solidification or the like of the residue obtained by evaporation and concentration. However, when a chelating agent or an organic acid is present, the radionuclide is likely to be eluted when the solidified body comes into contact with water, and it is desired to prevent it for underground disposal of the solidified body. As a method for decomposing the organic matter contained in the waste liquid, the applicant has developed a method of oxidative decomposition by acting hydrogen peroxide in the presence of a catalyst, and has already disclosed it (Japanese Patent Laid-Open No. 61-1042).
99). This method is a safe and useful treatment method if it is allowed to consume expensive hydrogen peroxide, but it increases the amount of waste due to the addition of catalyst and oxidant to the waste liquor, and therefore the subsequent evaporation. The concentration process load is heavy. When heavy metals are contained, a treatment for insolubilization is required after the oxidative decomposition step.

[Problems to be Solved by the Invention]

An object of the present invention is, as one solution to the above-mentioned problem relating to waste liquid treatment, utilizing electrolysis to simultaneously perform insolubilization of heavy metals and oxidative decomposition of organic substances, without increasing the amount of waste, and expensive oxidation. An object of the present invention is to provide a method for treating a waste liquid containing heavy metals and organic substances without consuming an agent.

[Means for Solving the Problems]

The method for treating a waste liquid containing a heavy metal and an organic substance according to the present invention is a method in which a waste liquid containing a heavy metal ion and a water-soluble organic substance is electrolyzed with a direct current at a pH of 4 to 9 so that a heavy metal ion is metalized Alternatively, it consists of depositing as a hydroxide and oxidatively decomposing an organic substance at the anode to convert it mainly into carbon dioxide and water.

[Work]

This will be described in detail with reference to the drawings. As shown in the flow chart of FIG. 1, the waste liquid received in the tank (1) is supplied to the electrolytic cell (4) for electrolysis. At this time, adjust the pH to 4-9
The range is. The pH needs to be 4 or more to insolubilize metal ions and 9 or less to suppress the generation of oxygen gas. If necessary, add acid or alkali from each tank (2,3).
Sulfate or ammonium salt is present in the waste liquid (9) and usually exhibits an electric conductivity suitable for electrolysis, but when the salt concentration is low and the electric conductivity is too small, an electrolyte is further added. The selection of the electrode material is particularly important for the anode (5), and if oxygen gas is generated at the anode, it does not contribute to the oxidation reaction, and elution of the electrode must be avoided as much as possible, so the material is selected from this viewpoint. Noble metals such as platinum, iron oxide (Fe ₃ O ₄ ) and lead dioxide (PbO ₂ ) can be used, but PbO ₂ is useful for oxidizing organic substances. PbO ₂ is preferably used by supporting it on a material such as titanium that can be used as an insoluble electrode by electrodeposition or other means. The material of the cathode (6) is arbitrary and, for example, stainless steel is easy to use. Regarding electrolysis, not only pressure but also normal pressure is preferable from the viewpoint of safety, and the temperature is from room temperature to the boiling point of the liquid.
Higher temperatures favor the oxidation of organics. A current density of 1 to 10 A / dm ² is suitable. If the current density is too low, the progress of electrolysis is slow and it is not practical, and if it is too large, the current efficiency is lowered. The voltage will vary depending on the conditions such as the electric conductivity of the liquid, the current density, and the distance between the electrodes, but it will usually be about 4 to 6V. Electrolysis causes the following metal electrodeposition or hydroxide generation reaction at the cathode. Taking a divalent metal ion as an example, M ⁺⁺ + 2e ⁻ → M M ⁺⁺ + 2OH ⁻ → M (OH) ₂ On the other hand, at the anode, an oxidation reaction of an organic substance occurs as follows. _{^{20H - → H 2 O + O}} · + 2e C m H n + O · → CO 2 + H 2 (O · represents an oxygen atom.). When the electrolyte is weakly alkaline, metal ions may be oxidized on the anode side to form an oxide (for example, Mn ⁺⁺ is converted to MnO ₂ ). To avoid this, a cation impermeable diaphragm (not shown) may be provided between both electrodes. It is needless to say that the electrolysis operation can be carried out by either a batch system or a continuous system, but it is practical to carry out the electrolysis process for an optimum time in view of both the degree of decomposition or electrodeposition removal and the energy efficiency. Usually, about several to 10 hours is appropriate. After electrolysis, if necessary, an acid or alkali neutralizing agent is added to neutralize the electrolytic solution. When the generated metal or oxide is separated from the electrode and suspended in the electrolytic solution, it may be filtered by the filter (7) so as not to interfere with the subsequent process. This filtration may be performed while circulating the liquid by the pump (8) during electrolysis.

Example 1 In an acrylic resin-based electrolytic cell, a positive electrode made by electrodepositing PbO ₂ on Ti and a negative electrode made of a stainless steel plate were placed in opposition. The electrode area is 0.3 dm ² for both electrodes. A solution containing 250 mg / EDTA and 13% by weight Na ₂ SO _{4 was} added with Co.
(NO ₃ ) ₂ was added so that the Co ion concentration was 100 mg /, and the simulated waste liquid and H ₂ SO ₄ were added to adjust the pH to about 4. Put 200 ml of this solution in the above electrolyzer, tank voltage 4.0V, current
Electrolysis was performed under the condition of 1.5A. The temperature of the liquid is 30 ° C. The TOC component concentration and Co ion concentration of the solution were measured every hour to obtain the graph of FIG. After 6 hours, the Co ion insolubilization rate reached 80% and the EDTA decomposition rate reached 82%.

Example 2 By adjusting the amount of H ₂ SO ₄ or NaOH added to the simulated waste liquid of Example 1, three types of liquid having pHs of 3.5, 5.0 and 9.0 were prepared. Electrolysis was performed under the same conditions as above, and the insolubilization rate of Co ions was measured. The results are shown in FIG. 3 together with the data of Example 1. From the graph in the figure, it can be seen that pH is more advantageous for insolubilizing heavy metals.

Example 3 200 ml of a waste liquid containing 10 g / EDTA and 30 g / Na ₂ SO ₄ was electrolyzed in the above electrolytic cell under the conditions of a cell voltage of 4.5 to 5.0 V and a current of 1.5 A. The decomposition temperature of EDTA was compared by keeping the liquid temperature at 30 °, 50 ° and 70 ° C., respectively. Meanwhile, the pH of the liquid is 5 to
It was in the range of 6. The decomposition rate reached at each time is shown in FIG. The graph in the figure shows that the higher the liquid temperature, the higher the decomposition.

【The invention's effect】

According to the method of the present invention, it is possible to simultaneously perform electrodeposition removal of metal and oxidative decomposition of organic matter from a waste liquid containing heavy metals and organic matter, and the treatment is completed by simple steps. The radionuclide in the heavy metal becomes an electrodeposited metal and becomes insoluble, so that safe treatment is realized. Decomposition of organic substances also progresses to a high degree, harmless water and carbon dioxide, and since it is not necessary to add an oxidizer or a catalyst, the amount of waste is not increased and the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a flow chart of an apparatus for explaining the processing method of the present invention. FIGS. 2 to 4 are data for carrying out the present invention. FIG. 2 shows changes in residual Co ion concentration and TOC concentration with respect to electrolysis time, and FIG. 3 shows changes in Co ion concentration with electrolysis time at different pH values. And FIG. 4 is a graph showing the electrolysis rate of the chelating agent with respect to the electrolysis time at different liquid temperatures. 1 ... Waste liquid tank, 2 ... Acid tank 3 ... Alkaline tank, 4 ... Electrolyzer 5 ... Anode, 6 ... Cathode 7 ... Filter, 8 ... Pump 9 ... Waste liquid

Claims (4)

[Claims] 1. A waste liquid containing heavy metal ions and a water-soluble organic substance is electrolyzed with a direct current at a pH of 4 to 9 to precipitate heavy metal ions as a metal or hydroxide at the cathode, In the method of treating waste liquid containing heavy metals and organic matter, which comprises oxidatively decomposing organic matter and converting it mainly into carbon dioxide and water. 2. The treatment according to claim 1, wherein the waste liquid is a decontamination waste liquid for equipment contaminated with radioactive substances, contains a radionuclide as a heavy metal ion, and the water-soluble organic matter is an organic acid, its salt or a chelating agent. Method. 3. The treatment method according to claim 1, wherein lead dioxide is used as an electrolysis anode, and the temperature is room temperature to the boiling temperature of waste water and the current density is 1 to 10 A / dm ² . 4. The processing method according to claim 1, wherein electrolysis is performed by placing a diaphragm between both electrodes.