A method for synchronously removing cadmium and thallium from industrial wastewater

Abstract

The present invention belongs to the technical field of industrial wastewater treatment, and particularly relates to a method for synchronously removing cadmium and thallium from industrial wastewater. The method comprises the following steps: Step 1: Add sulfide to the wastewater containing cadmium and thallium, and fully mix and react until the cadmium ion concentration in the wastewater meets the discharge requirements; Step 2: Place the mixed solution obtained in Step 1 under sunlight, ultraviolet light or visible light irradiation, stir and react until the thallium ion concentration in the wastewater meets the discharge requirements; Step 3: Perform solid-liquid separation to obtain a mixed precipitate of cadmium compounds and thallium compounds, and the treated wastewater. The present invention utilizes the photocatalytic property of CdS to oxidize Tl(I) into Tl2O3 precipitate to synchronously remove cadmium and thallium.

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 shortcomings 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 Tl 2 O3 Precipitation is carried out to remove cadmium and thallium synchronously.

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

[0009] A method for synchronously removing cadmium and thallium from industrial wastewater, comprising the following steps:

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

[0011] Step 2: Place the mixed solution obtained after being treated in Step 1 under sunlight, ultraviolet light or visible light, stir and react until the concentration of thallium ions in the wastewater meets the discharge requirements;

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

[0013] In the above technical solution:

[0014] In Step 1), the S(-II) ions generated by the hydrolysis of sulfide can convert Cd(II) and Tl(III) in the wastewater into insoluble CdS and Tl 2 S 3 precipitates, thereby reducing the concentration of cadmium ions in the wastewater below the discharge standard. However, since Tl(I) cannot be removed by sulfidation, only part of the trivalent thallium ions can be removed in this step;

[0015] In Step 2), the cadmium sulfide (CdS) precipitate generated in the first step is an important II-VI group direct bandgap semiconductor with a bandgap width of 2.42 eV at room temperature and excellent photocatalytic properties. It has a strong response ability to both visible light and ultraviolet light. After irradiating this mixed system with light, electrons (e - ) and holes (h + ) can be excited in the CdS semiconductor material. h + has strong oxidizing properties and can oxidize water molecules (H 2 O) and hydroxide ions (OH - ) into hydroxyl radicals (·OH). ·OH can also spontaneously combine into hydrogen peroxide (H 2 O 2 ). The redox potentials of a series of oxidizing radicals generated in this process are all above 2.1 V, up to 2.8 V. Under neutral and alkaline conditions, the potential of E(Tl 2 O 3 / Tl(I)) is only 0.02 V. Therefore, by using the photocatalytic effect of CdS, Tl(I) can be easily oxidized to Tl 2 O 3 precipitate;

[0016] Since sunlight contains ultraviolet light and visible light, and the treatment of industrial wastewater is built outdoors, during the day, the wastewater is fully exposed to sunlight. Therefore, in the actual treatment process, there is no need to add other oxidants. Only by using the in-situ generated CdS precipitate and irradiating with sunlight can the efficient oxidation and precipitation removal of thallium(I) be achieved.

[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 thallium(I) ions and thallium(III) 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 molar ratio 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 ultrasonic, and the mixing time is 30-180 min.

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

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

[0026] Specifically, in step 3:

[0027] The cadmium compound is CdS;

[0028] The thallium compound includes Tl 2 S 3 and Tl 2 O 3 .

[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) Without the need to additionally add an oxidant, the synchronous and efficient removal of cadmium and thallium can be achieved only through sulfidation and light irradiation.

[0033] 2) For open-air wastewater treatment enterprises, the synchronous and efficient removal of cadmium and thallium can be achieved only by adding a sulfiding agent and using natural sunlight, without being affected by the temperature.

[0034] 3) There is no need for subsequent impurity ion treatment procedures.

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

[0036] Figure 1 is a flow chart of the method for synchronously removing cadmium and thallium from industrial wastewater provided by the present invention. Detailed Embodiments

[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 intended to limit the scope of the present invention.

[0038] Unless otherwise specified, the test methods used in the examples are all conventional methods; the materials, reagents, etc. used, unless otherwise specified, are reagents and materials that can be obtained from commercial sources.

[0039] Example 1

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

[0041] Collect the wastewater containing cadmium and thallium, and use inductively coupled plasma optical emission spectrometry (ICP-OES) to detect the concentrations of cadmium, total thallium ions, monovalent thallium ions, and trivalent thallium ions in the wastewater. 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, after adjusting the pH value of the wastewater to alkaline with sodium hydroxide solution, the steps of the present invention are carried out, and reference can be made to Figure 1 .

[0044] Step 1: Add a certain mass of sodium sulfide (Na 2 S) solid into 1 L of the above actual wastewater in proportion, stir evenly with a magnetic stirrer, and react for 30 min;

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

[0046] Step 3: Filter and separate the generated precipitate, and use ICP-OES to detect the concentrations of cadmium and thallium in the filtrate. The results are shown in the following table.

[0047]

[0048] The treated wastewater was detected, and it was found that the residual concentration of cadmium in the treated wastewater was less than 0.48 mg / L, the residual concentration of thallium ions was less 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 synchronously 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 to 1 L of the above actual wastewater in proportion, stir evenly with a magnetic stirrer, and react for 60 min;

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

[0053] Step 3: Filter and separate the generated precipitate, and use ICP-OES to detect the concentrations of cadmium and thallium in the filtrate. The results are shown in the following table.

[0054]

[0055] The treated wastewater was detected, and it was found that the residual concentration of cadmium in the treated wastewater was less than 0.42 mg / L, the residual concentration of thallium ions was less 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 synchronously removing cadmium and thallium from industrial wastewater, the specific steps are as follows:

[0058] Step 1: Add a certain mass of sodium hydrosulfide solid to 1 L 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 min;

[0059] Step 2: Use a 500 W xenon lamp (visible light source) and a 500 W high-pressure mercury lamp (ultraviolet light source) as light sources respectively to irradiate the above mixture for 30 - 120 min;

[0060] Step 3: Filter and separate the generated precipitate, and use ICP-OES to detect the concentrations of cadmium and thallium in the filtrate. The results are shown in the following table.

[0061]

[0062]

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

[0064] Comparative Example 1

[0065] Step 1: A certain mass of sodium sulfide and sodium bisulfide solids were added to 1 L of the above actual wastewater at a molar ratio of S / (Cd + Tl) of 2.5, and stirred evenly with a magnetic stirrer for 60 min.

[0066] Step 2: The above wastewater was continuously stirred for 120 min in the dark.

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

[0068]

[0069] The results of the comparative example show that in the system without applying light, only cadmium and trivalent thallium were removed, and there was still as high as 265 mg / L of thallium in the wastewater, far from meeting the discharge standard of 2 μg / L.

[0070] It can be seen from the above examples that whether different sulfiding agents are used or ultraviolet light, visible light or sunlight is used, the cadmium and thallium in the wastewater can be reduced to the discharge standard, and the removal rate is as high as 99.99%.

[0071] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall 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; 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; Thallium exists in the form of monovalent thallium ions and trivalent thallium ions at the same time; The sulfide includes any one or more of sodium sulfide, potassium sulfide, sodium hydrosulfide or sodium thiosulfate; 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; Step 2: placing the mixed solution obtained by the treatment in step 1 under sunlight, stirring the reaction, and utilizing the photocatalytic effect of CdS to oxidize Tl(I) to Tl2O3 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; In step 3: The compound of cadmium is CdS; the compounds of thallium include Tl2S3 and Tl2O3; In the treated wastewater obtained, the concentration of cadmium ions is less than 0.5 mg / L; the concentration of thallium ions is less than 2 μg / L.

2. 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 min.

3. 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.

4. The method for synchronously 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.

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