Method for synchronously removing cadmium and thallium in industrial wastewater

Abstract

The invention belongs to the technical field of industrial wastewater treatment, and particularly relates to a method for synchronously removing cadmium and thallium in industrial wastewater. The method comprises the following steps: step 1, adding sulfide into the wastewater containing cadmium and thallium, and fully mixing and reacting until the concentration of cadmium ions in the wastewater meets the discharge requirement; 2, placing the mixed solution obtained through treatment in the step 1 under sunlight, ultraviolet light or visible light illumination, and stirring for reaction until the thallium ion concentration in the wastewater meets the discharge requirement; and 3, carrying out solid-liquid separation to obtain a mixed precipitate of the cadmium compound and the thallium compound, and treated wastewater. According to the method disclosed by the invention, Tl (I) is oxidized into Tl2O3 precipitate by utilizing the photocatalytic characteristic of CdS, so that cadmium and thallium are synchronously removed.

Description

Technical Field

[0001] The invention belongs to the technical field of industrial wastewater treatment, and in particular relates to a method for synchronously removing cadmium and thallium from industrial wastewater. Background Art

[0002] Thallium (Tl) and cadmium (Cd) are two elements that often coexist in natural minerals. With the mining of lead, zinc, copper and other mineral resources and the deepening of non-ferrous metal smelting activities, they have infiltrated into the production wastewater system in large quantities, posing a huge threat to the ecological environment.

[0003] In this type of wastewater, cadmium mainly exists in the form of divalent ions with significant toxicity, with a wide concentration range (500-3000 mg / L); thallium is more complicated, including monovalent thallium (200-300 mg / L) and trivalent thallium (300-1200 mg / L). The chemical properties of the former are similar to those of alkali metals, highly water-soluble and easily oxidized. Although the latter has limited mobility, it is almost insoluble under alkaline conditions, but its toxicity is tens of thousands of times that of monovalent thallium, making it extremely difficult to treat.

[0004] In view of the complex composition of wastewater, the current industry generally adopts oxidation-precipitation combined treatment technology. This technology converts monovalent thallium into a trivalent form that is more toxic but easier to precipitate and remove by introducing strong oxidants such as persulfate, hydrogen peroxide, potassium permanganate, and manganese dioxide, or using the strong oxidizing ability of ozone. Subsequently, by adjusting the pH of the wastewater to an alkaline environment and adding sulfide, cadmium and trivalent thallium are prompted to form insoluble sulfide precipitation, thereby achieving effective separation of the two from the wastewater.

[0005] However, although this traditional method is effective, it is also accompanied by new problems: the addition of oxidants often leads to the introduction of additional pollutants into the wastewater, such as excessive levels of manganese ions and sulfate ions, forcing companies to add treatment steps to deal with the additional substances introduced, increasing the complexity and cost of the treatment process. Even oxidants such as ozone and hydrogen peroxide that do not introduce other impurity ions have low oxidation efficiency in the absence of suitable catalysts (such as cobalt ions and ferrous ions), and the introduction of catalysts inevitably brings in new impurity ions, which require additional treatment, thereby increasing the overall treatment cost.

[0006] In view of the above problems, the development of a new method that can simultaneously remove cadmium and thallium from industrial wastewater, which requires the characteristics of simplified process, efficient treatment and economic feasibility, has become a key technical problem that needs to be urgently solved in the current field of environmental governance. Summary of the invention

[0007] In order to solve the deficiencies of the prior art, the present invention provides a method for simultaneously removing cadmium and thallium from industrial wastewater. The present invention utilizes the photocatalytic properties of CdS to oxidize Tl(I) to Tl2O3 precipitation, thereby simultaneously removing cadmium and thallium.

[0008] The technical solution provided by the present invention is as follows:

[0009] A method for simultaneously removing cadmium and thallium from industrial wastewater comprises the following steps:

[0010] Step 1: Add sulfide to wastewater containing cadmium and thallium, mix and react thoroughly until the cadmium ion concentration in the wastewater meets the discharge requirements;

[0011] Step 2: placing the mixed solution obtained by the treatment in step 1 under sunlight, ultraviolet light or visible light, stirring and reacting until the thallium ion concentration in the wastewater reaches the discharge requirement;

[0012] Step 3: solid-liquid separation to obtain a mixed precipitate of cadmium compounds and thallium compounds, and treated wastewater.

[0013] In the above technical solution:

[0014] In step 1), the S(-II) ions produced by sulfide hydrolysis can be used to convert Cd(II) and Tl(III) in the wastewater into insoluble CdS and Tl2S3 precipitates, thereby reducing the concentration of cadmium ions in the wastewater to below the discharge standard. However, since Tl(I) cannot be removed by sulfidation, this step can only remove part of the trivalent thallium ions.

[0015] In step 2), the cadmium sulfide (CdS) precipitate produced in the first step is an important II-VI group direct bandgap semiconductor with a bandgap width of 2.42 eV at room temperature. It has excellent photocatalytic properties and has a strong response to both visible light and ultraviolet light. After applying light to the mixed system, the CdS semiconductor material can be excited to generate electrons (e - ) and holes (h + ), h + It has strong oxidizing properties and can convert water molecules (H2O) and hydroxide ions (OH - ) is oxidized to hydroxyl radical (·OH), ·OH can also spontaneously combine to form hydrogen peroxide (H2O2). The redox potential of a series of oxidative free radicals generated in this process is above 2.1V, and the highest can reach 2.8V. Under moderately alkaline conditions, the potential of E(Tl2O3 / Tl(I)) is only 0.02V. Therefore, it is extremely easy to oxidize Tl(I) to Tl2O3 precipitation by utilizing the photocatalytic effect of CdS.

[0016] Since sunlight contains ultraviolet light and visible light, and industrial wastewater treatment is carried out outdoors, the wastewater is fully exposed to sunlight during the day. Therefore, in the actual treatment process, there is no need to add other oxidants. Only the CdS precipitate generated in situ and sunlight can be used to achieve efficient oxidation and precipitation removal of monovalent thallium.

[0017] Specifically, in step 1:

[0018] The wastewater is neutral or alkaline wastewater containing cadmium and thallium generated in industrial production;

[0019] Alternatively, the wastewater is obtained by adjusting the pH of acidic wastewater containing cadmium and thallium generated in industrial production to above 7.

[0020] Specifically, in step 1: thallium exists in the form of monovalent thallium ions and trivalent thallium ions at the same time.

[0021] Specifically, in step 1: the sulfide includes but is not limited to any one or more of sodium sulfide, potassium sulfide, sodium hydrosulfide or sodium thiosulfate.

[0022] Specifically, in step 1: the ratio of the molar amount of sulfur element in the sulfide to the total molar amount of cadmium and thallium is (1-10):1.

[0023] Specifically, in step 1: the mixing method is stirring, shaking or ultrasound, and the mixing time is 30-180 minutes.

[0024] Specifically, in step 2: the stirring reaction time is 30-240 minutes.

[0025] Specifically, in step 3: the solid-liquid separation is a combination of one or more of sand filtration, filter press, membrane filtration or centrifugal separation.

[0026] Specifically, in step 3:

[0027] The compound of cadmium is CdS;

[0028] Thallium compounds include Tl2S3 and Tl2O3.

[0029] Specifically, in the treated wastewater obtained in step 3, the concentration of cadmium ions is less than 0.5 mg / L.

[0030] Specifically, in the treated wastewater obtained in step 3, the concentration of thallium ions is less than 2 μg / L.

[0031] The beneficial effects of the present invention are:

[0032] 1) No additional oxidant is needed, and cadmium and thallium can be removed simultaneously and efficiently only through sulfidation and light irradiation.

[0033] 2) For open-air wastewater treatment enterprises, the simultaneous and efficient removal of cadmium and thallium can be achieved simply by adding sulfiding agents and utilizing natural sunlight, without being affected by temperature.

[0034] 3) No subsequent impurity ion treatment process is required.

[0035] 4) The removal rates of cadmium and thallium are higher than 99.9%, the residual concentration of cadmium is lower than 0.5 mg / L, and the residual concentration of thallium is lower than 2 μg / L. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The present invention provides a flow chart of a method for simultaneously removing cadmium and thallium from industrial wastewater. DETAILED DESCRIPTION

[0037] The principles and features of the present invention are described below. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0038] Unless otherwise specified, the test methods used in the examples are conventional methods; the materials and reagents used are commercially available reagents and materials unless otherwise specified.

[0039] Example 1

[0040] A method for simultaneously removing cadmium and thallium from industrial wastewater, the specific steps are as follows:

[0041] Wastewater containing cadmium and thallium was collected, and the concentrations of cadmium and total thallium ions as well as monovalent thallium and trivalent thallium ions in the wastewater were detected using inductively coupled plasma optical emission spectroscopy (ICP-OES). The results are as follows:

[0042] project pH Cd(mg / L) Tl(I)(mg / L) Tl(III)(mg / L) parameter 5.7 2570 267 756

[0043] Since the original wastewater is acidic, the pH value of the wastewater is adjusted to alkaline with sodium hydroxide solution, and then the steps of the present invention are carried out. Figure 1 .

[0044] Step 1: Add a certain mass of sodium sulfide (Na2S) solid into 1L of the above actual wastewater in proportion, stir evenly with a magnetic stirrer, and react for 30 minutes;

[0045] Step 2: Move the above reaction device to outdoor sunlight and irradiate for 60 minutes;

[0046] Step 3: The precipitate was separated by filtration, and the concentrations of cadmium and thallium in the filtrate were detected by ICP-OES. The results are shown in the table below.

[0047]

[0048] The treated wastewater was tested and the results showed that the residual cadmium concentration in the treated wastewater was lower than 0.48 mg / L, the residual thallium ion concentration was lower than 1.62 μg / L, and the removal rates of cadmium ions and thallium ions were both higher than 99.99%.

[0049] Example 2

[0050] A method for simultaneously removing cadmium and thallium from industrial wastewater, the specific steps are as follows:

[0051] Step 1: Add a certain mass of sodium hydrosulfide (NaHS) solid into 1L of the above actual wastewater in proportion, stir evenly with a magnetic stirrer, and react for 60 minutes;

[0052] Step 2: Move the above reaction device to outdoor sunlight and irradiate for 60 minutes;

[0053] Step 3: The precipitate was separated by filtration, and the concentrations of cadmium and thallium in the filtrate were detected by ICP-OES. The results are shown in the table below.

[0054]

[0055] The treated wastewater was tested and the results showed that the residual cadmium concentration in the treated wastewater was lower than 0.42 mg / L, the residual thallium ion concentration was lower than 1.67 μg / L, and the removal rates of cadmium ions and thallium ions were both higher than 99.99%.

[0056] Example 3

[0057] A method for simultaneously removing cadmium and thallium from industrial wastewater, the specific steps are as follows:

[0058] Step 1: Add a certain mass of sodium hydrosulfide solid into 1L of the above actual wastewater at a molar ratio of S / (Cd+Tl) of 2.5, stir evenly with a magnetic stirrer, and react for 60 minutes;

[0059] Step 2: using a 500W xenon lamp (visible light source) and a 500W high-pressure mercury lamp (ultraviolet light source) as light sources, respectively, to irradiate the mixed solution for 30-120 minutes;

[0060] Step 3: The precipitate was separated by filtration, and the concentrations of cadmium and thallium in the filtrate were detected by ICP-OES. The results are shown in the table below.

[0061]

[0062]

[0063] The treated wastewater was tested and the results showed that the residual cadmium concentration in the treated wastewater was lower than 0.47 mg / L, the residual thallium ion concentration was lower than 1.60 μg / L, and the removal rates of cadmium ions and thallium ions were both higher than 99.99%.

[0064] Comparative Example 1

[0065] Step 1: Add a certain mass of sodium sulfide and sodium hydrosulfide solids into 1L of the above actual wastewater at a molar ratio of S / (Cd+Tl) of 2.5, stir evenly with a magnetic stirrer, and react for 60 minutes;

[0066] Step 2: Keep the wastewater away from light and continue stirring for 120 minutes;

[0067] Step 3: The precipitate was separated by filtration, and the concentrations of cadmium and thallium in the filtrate were detected by ICP-OES. The results are shown in the table below.

[0068]

[0069] The comparative results show that in the system without light application, only cadmium and trivalent thallium are removed, and the thallium content in the wastewater is still as high as 265 mg / L, far below the emission standard of 2 μg / L.

[0070] It can be seen from the above embodiments that no matter whether different sulfiding agents or ultraviolet light, visible light or sunlight are used, cadmium and thallium in wastewater can be reduced to the discharge standard, with a removal rate of up to 99.99%.

[0071] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for simultaneously removing cadmium and thallium from industrial wastewater, characterized in that: The following steps are involved: Step 1: adding sulfide to wastewater containing cadmium and thallium, mixing them thoroughly to react until the cadmium ion concentration in the wastewater meets the discharge requirements; Step 2: placing the mixed solution obtained by the treatment in step 1 under sunlight, ultraviolet light or visible light, stirring and reacting until the thallium ion concentration in the wastewater reaches the discharge requirement; Step 3: solid-liquid separation to obtain a mixed precipitate of cadmium compounds and thallium compounds, and treated wastewater.

2. The method for synchronously removing cadmium and thallium from industrial wastewater according to claim 1, characterized in that: In step 1: The wastewater is neutral or alkaline wastewater containing cadmium and thallium generated in industrial production; Alternatively, the wastewater is obtained by adjusting the pH of acidic wastewater containing cadmium and thallium generated in industrial production to above 7.

3. The method for synchronously removing cadmium and thallium from industrial wastewater according to claim 1, characterized in that: In step 1: thallium exists in the form of monovalent thallium ions and trivalent thallium ions at the same time.

4. The method for synchronously removing cadmium and thallium from industrial wastewater according to claim 1, characterized in that: In step 1: the sulfide includes but is not limited to any one or more of sodium sulfide, potassium sulfide, sodium hydrosulfide or sodium thiosulfate.

5. The method for synchronously removing cadmium and thallium from industrial wastewater according to claim 1, characterized in that: In step 1: the ratio of the molar amount of sulfur element in the sulfide to the total molar amount of cadmium and thallium is (1-10):

1.

6. The method for synchronously removing cadmium and thallium from industrial wastewater according to claim 1, characterized in that: In step 1: the mixing method is stirring, shaking or ultrasound, and the mixing time is 30-180 minutes.

7. The method for synchronously removing cadmium and thallium from industrial wastewater according to claim 1, characterized in that: In step 2: the stirring reaction time is 30-240 min.

8. The method for simultaneously removing cadmium and thallium from industrial wastewater according to claim 1, characterized in that: In step 3: the solid-liquid separation is a combination of one or more of sand filtration, filter press, membrane filtration or centrifugal separation.

9. The method for simultaneously removing cadmium and thallium from industrial wastewater according to claim 1, characterized in that: In step 3: The compound of cadmium is CdS; Thallium compounds include Tl2S3 and Tl2O3.

10. The method for simultaneously removing cadmium and thallium from industrial wastewater according to any one of claims 1 to 9, characterized in that: In the treated wastewater obtained in step 3: The concentration of cadmium ions is less than 0.5 mg / L; The concentration of thallium ions is less than 2μg / L.

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