A method for treating heavy metal wastewater

Abstract

The present invention relates to sewage treatment, and particularly to a method for treating heavy metal wastewater. The method comprises the following steps: adjusting the pH value of the heavy metal wastewater to 3-6; adding a substance containing phosphate radicals to enable the phosphate radicals to combine with heavy metal ions to form a negatively charged heavy metal phosphate sol; adding a substance containing ferric ions and adjusting the pH value to 5-8 to cause the ferric ions to hydrolyze and in-situ generate a positively charged ferrihydrite nanonet (abbreviated as Feh nanonet), and settling the heavy metal phosphate sol through the electrostatic adsorption of the ferrihydrite nanonet. This method can effectively remove free heavy metal ions or heavy metal complexes in heavy metal wastewater.

Description

Technical Field

[0001] The present invention relates to sewage treatment, and particularly to a method for treating heavy metal wastewater. Background Art

[0002] Heavy metal pollution has characteristics such as long-term, cumulative, latent and irreversible, which may seriously threaten the ecological environment and human health, and has become a major challenge in the field of water resources management. The components of heavy metal wastewater are complex and diverse in form, and it faces great challenges to develop economical and efficient technologies for upgrading standards and reducing emissions. Although traditional methods such as chemical precipitation and coagulation perform well in the treatment of high-concentration heavy metals, the chemical composition of the sewage is still complex and there are many types of coexisting matrices, and the treated water quality often fails to meet the discharge standards. In addition, advanced treatment technologies such as ion exchange, adsorption and membrane separation have disadvantages such as high cost and high energy consumption. Therefore, seeking a more effective and sustainable advanced treatment solution for heavy metal wastewater has become a research hotspot in the field of water treatment. Summary of the Invention

[0003] Based on this, the present invention provides a method for treating heavy metal wastewater, which solves at least one problem in the prior art.

[0004] A method for treating heavy metal wastewater provided by the present invention includes the following steps:

[0005] Adjust the pH value of the heavy metal wastewater to 3-6;

[0006] Add a substance containing phosphate ions to combine the phosphate ions with heavy metal ions to form a negatively charged heavy metal phosphate sol;

[0007] Add a substance containing ferric ions and adjust the pH value to 5-8 to hydrolyze the ferric ions to in-situ generate a positively charged ferrihydrite nanonet (abbreviated as Feh nanonet), and settle the heavy metal phosphate sol through the electrostatic adsorption of the ferrihydrite nanonet.

[0008] Due to the adoption of the above technical solutions, the embodiments of the present invention have at least the following beneficial effects:

[0009] (1) It can remove both free heavy metals and complex heavy metals through a substance containing ferric ions;

[0010] (2) It is applicable to the advanced treatment of heavy metal wastewater such as Cu, Ni, Cd, Co, Mn, Pb and Cr, and can meet the World Health Organization (WHO) drinking water standards;

[0011] (3) It can remove organic ligands in water and reduce the COD of the water body; it can also remove phosphate ions in the water body and reduce the phosphorus content in the water;

[0012] (4) The concentrations of residual Fe(III) and phosphate in the water body are relatively low, being lower than 0.02 mg / L and 0.05 mg / L respectively, meeting the sewage discharge standards;

[0013] (5) Low cost and easy to implement. Description of the Drawings

[0014] Figure 1 For the CuSO₄ solution containing 50 mg / L Cu(II) in Example 1 of the present invention 4 After being treated under different conditions, the comparative diagram of the change of the residual Cu(II) concentration in the solution with the pH value.

[0015] Figure 2 For the CuSO₄ solution containing 50 mg / L Cu(II) in Example 1 of the present invention 4 After being treated under different conditions, the comparative diagram of the change of the Zeta potential of the solution with the pH value.

[0016] Figure 3 For the CuSO₄ solution in Example 2 of the present invention treated under different stirring time and standing time conditions 4 The diagram of the change of the residual Cu(II) concentration after treatment.

[0017] Figure 4 For the CuSO₄ solution in Example 3 of the present invention treated under different Cu / P molar ratios 4 The diagram of the change of the residual Cu(II) concentration after treatment.

[0018] Figure 5 For the CuSO₄ solution in Example 3 of the present invention treated under different Cu / P molar ratios 4 The diagram of the change of the Zeta potential after treatment.

[0019] Figure 6 For the CuSO₄ solution in Example 4 of the present invention treated under different Fe(III) concentrations 4 The diagram of the change of the residual Cu(II) concentration after treatment.

[0020] Figure 7 For the MnSO₄ solution containing 50 mg / L Mn(II) in Example 5 of the present invention 4 After being treated under different conditions, the comparative diagram of the change of the residual Cu(II) concentration in the solution with the pH value.

[0021] Figure 8 For the MnSO₄ solution containing 50 mg / L Mn(II) in Example 5 of the present invention 4 After being treated under different conditions, the comparative diagram of the change of the Zeta potential of the solution with the pH value.

[0022] Figure 9For the residual Mn(II), PO after treating manganese sulfate solution under different Fe(III) concentration conditions in Example 6 of the present invention 4 3- and the change diagram of Fe(III) concentration.

[0023] Figure 10 It is the removal effect diagram of free heavy metal concentration in deionized water in Example 7 of the present invention.

[0024] Figure 11 It is the removal effect diagram of free heavy metal concentration in tap water in Example 7 of the present invention.

[0025] Figure 12 It is the removal effect diagram of free heavy metal concentration in the natural water body of Ganjiang River in Example 7 of the present invention.

[0026] Figure 13 It is the change diagram of the concentrations of free copper ions and copper-containing complexes with the Fe / Cu molar ratio in Example 8 of the present invention.

[0027] Figure 14 It is the change diagram of the concentrations of free iron ions and iron-containing complexes with the Fe / Cu molar ratio in Example 8 of the present invention.

[0028] Figure 15 For Cu / PO in Example 9 of the present invention 4 3- When the molar ratio is 3 / 1, the change diagram of the concentrations of free copper and complex copper in the solution with the Fe / Cu molar ratio.

[0029] Figure 16 For Cu / PO in Example 9 of the present invention 4 3- When the molar ratios are 3 / 2 and 1 respectively, the change diagram of the concentrations of free copper and complex copper with the Fe / Cu molar ratio.

[0030] Figure 17 It is the TEM image of the flocs formed by 0.1 mM Fe(III) under the conditions of pH = 4.0, 5.0, 6.0, 7.0, 8.0, and 9.0 in Example 11 of the present invention.

[0031] Figure 18 It is the TEM image of the flocs formed by Fe(III) with concentrations of 0.1, 0.25, 0.5, 0.75, 1, 1.5, 2.5, and 5 mM under the condition that the pH is adjusted to 7.0 in Example 12 of the present invention.

[0032] Figure 19 It is the TEM image of the flocs formed by 0.1 mM Fe(III) at pH = 7.0 and 8.0 and their capture of copper phosphate sol in the water body in Example 12 of the present invention. Detailed implementation manners

[0033] The concept of the present invention and the resulting technical effects will be clearly and completely described below to fully elaborate the purpose, solution and effects of the present invention.

[0034] A heavy metal wastewater treatment method provided by the present invention includes the following steps:

[0035] S100. Adjust the pH value of the heavy metal wastewater to 3-6;

[0036] S200. Add a substance containing phosphate ions to combine the phosphate ions with heavy metal ions to form a negatively charged heavy metal phosphate sol;

[0037] S300. Add a substance containing ferric ions and adjust the pH value to 5-8 to hydrolyze the ferric ions to in-situ generate a positively charged ferrihydrite nanonet (abbreviated as Feh nanonet), and settle the heavy metal phosphate sol through the electrostatic adsorption of the ferrihydrite nanonet.

[0038] This method first combines phosphate ions with free heavy metals to generate a poorly soluble sol of negatively charged heavy metal phosphates, increasing the size of heavy metal substances; then add a substance containing ferric ions to hydrolyze the ferric ions to in-situ generate Feh nanonet; Feh nanonet captures the poorly soluble sol of heavy metal phosphates in the wastewater through electrostatic adsorption and net trapping, and the ferrihydrite nanonet slowly grows and settles, thereby achieving the removal of heavy metals in the wastewater.

[0039] In some embodiments, before step S200, the following steps are further included:

[0040] S150. Add a substance containing ferric ions to free the heavy metal ions in the heavy metal complex in the heavy metal wastewater.

[0041] It should be noted that in heavy metal wastewater, heavy metal complexes may exist. If step S200 is directly performed after step S100, it is difficult for phosphate ions to combine with this part of heavy metal complexes. Adding step S150 between step S100 and step S200 can release heavy metal ions by removing heavy metal complexes with ferric ions, thereby improving the treatment effect of heavy metal wastewater. For wastewater containing heavy metal complexes, the complexation constant of Fe 3+ with organic ligands is usually larger than that of most heavy metals. For example, the complexation constant of Fe 3+ -EDTA (lgK = 25.1) is larger than that of Ni 2+ -EDTA (lgK = 18.6) and Cu 2+-EDTA (lgK = 18.8) by several orders of magnitude; thus, under mildly acidic conditions, Fe 3+ will compete with heavy metal ions for organic ligands, promoting the dissociation of heavy metal complexes and releasing free heavy metal ions; the free heavy metal ions will then combine with phosphate ions to form negatively charged heavy metal phosphate colloids, which are subsequently captured and removed by the Feh nanonet formed by the hydrolysis of substances containing ferric ions.

[0042] In some embodiments, the heavy metal wastewater contains at least one heavy metal ion or its complex among Cu, Ni, Cd, Co, Mn, Pb, and Cr.

[0043] In some embodiments, the substance containing phosphate ions is soluble phosphate or its solution. After adding the soluble phosphate or its solution, the concentration of phosphate ions is 1 / 3 to 2 times the total charge concentration (Q, mmol / L) of heavy metals in the heavy metal wastewater. Soluble phosphates include, but are not limited to, one or more of sodium hydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, potassium hydrogen phosphate, potassium dihydrogen phosphate, and potassium phosphate.

[0044] In some embodiments, the substance containing phosphate ions is sodium dihydrogen phosphate.

[0045] In some embodiments, the substance containing ferric ions is soluble small-molecule inorganic iron salt or its solution. After adding the soluble small-molecule inorganic iron salt or its solution, the concentration range of ferric ions in the heavy metal wastewater is 0.05 to 5 mmol / L, preferably 0.1 to 1 mmol / L. Soluble small-molecule inorganic iron salts include, but are not limited to, one or more of ferric sulfate and ferric chloride.

[0046] In some embodiments, the substance containing ferric ions is ferric chloride solution.

[0047] In some embodiments, step S200 is carried out under stirring conditions. Preferably, step S200 is carried out under stirring at 500 rpm.

[0048] In some embodiments, step S300 is carried out under stirring conditions. Preferably, step S300 is carried out under stirring at 100 rpm for 30 min.

[0049] In some embodiments, after step S300, the following steps are further included:

[0050] S350, standing for more than 3 h.

[0051] In some embodiments, standing for 1 to 2 days.

[0052] Some typical embodiments are introduced below.

[0053] Example 1: Removal of free copper ions in CuSO 4 solution

[0054] Prepare multiple 1L solutions of CuSO with a Cu(Ⅱ) concentration of 50mg / L and conduct experiments according to the following conditions respectively at a stirring speed of 500rpm: 4

[0055] a) Control group: Adjust the pH to 6.0, 6.5, 7.0, 7.5, and 8.0 respectively with NaOH;

[0056] b) Fe(Ⅲ) group: Add ferric chloride solution to make the concentration of Fe in the CuSO 4 solution reach 0.1mmol / L. After mixing evenly, adjust the pH to 6.0, 6.5, 7.0, 7.5, and 8.0 respectively; 3+

[0057] c) PO 4 3- group: Add sodium dihydrogen phosphate according to the Cu / P molar ratio of 3 / 2 and adjust the pH to 6.0, 6.5, 7.0, 7.5, and 8.0 respectively;

[0058] d) PO 4 3- +Fe(Ⅲ) group: First add sodium dihydrogen phosphate according to the Cu / P molar ratio of 3 / 2, then add ferric chloride solution to make the Fe 3+ concentration reach 0.1mmol / L, and adjust the pH to 6.0, 6.5, 7.0, 7.5, and 8.0 respectively;

[0059] After adjusting the pH of each above solution, continue to stir at a speed of 100rpm for 30min, then let the solution stand for more than 1d, take the supernatant and measure the Cu(Ⅱ) concentration (C Cu(Ⅱ) ) and Zeta potential value.

[0060] As Figure 1 shown, in each group of experiments, the Cu(Ⅱ) concentration decreases with the increase of pH value. Compared with the Control group, Fe(Ⅲ) group and PO 4 3- group, the solution has a lower Cu(Ⅱ) concentration under the conditions of the PO 4 3- +Fe(Ⅲ) group.

[0061] As Figure 2 shown, under the conditions of the PO 4 3- +Fe(Ⅲ) group, the Zeta potential of the solution is closer to 0. Example 2: Removal of CuSO under different stirring time and standing time conditions​​4 Free copper ions in solution

[0062] At a stirring speed of 500 rpm, a 1 L volume of CuSO containing a Cu(Ⅱ) concentration of 50 mg / L was first stirred. 4 In the solution, 49 mg / L PO was added according to the Cu / P molar ratio of 3 / 2. 4 3- , then add ferric chloride solution to make CuSO 4 Fe in solution 3+ The concentration was 0.1mmol / L; then the solution was slowly stirred at 100rpm for 60min and then allowed to stand for 2d; samples were taken at 0, 10, 20, 30, 40, 50, 60min after the start of slow stirring and at 3h, 1d, and 2d after the start of standing, and the residual Cu(Ⅱ) concentration (C Cu(Ⅱ) ).

[0063] The change of Cu(Ⅱ) concentration with stirring time and standing time is as follows Figure 3 As shown in the figure, when the slow stirring time is 30 minutes, the concentration of Cu(Ⅱ) in the solution is lower. Moreover, the longer the standing time, the lower the concentration of Cu(Ⅱ) in the solution; when the standing time exceeds 1 day, the concentration of Cu(Ⅱ) basically reaches stability.

[0064] Example 3: Removal of CuSO under different Cu / P molar ratios 4 Free copper ions in solution

[0065] Prepare several 1L volumes of CuSO containing 50 mg / L Cu(II) 4 The solution was stirred at 500 rpm and the experiments were carried out according to the following conditions:

[0066] a)PO 4 3- Group: sodium dihydrogen phosphate was added according to the Cu / P molar ratio of 3 / 1, 3 / 2, 3 / 3, 3 / 4 and 3 / 5, and the pH was adjusted to 7.0;

[0067] b)PO 4 3- +Fe(Ⅲ) group: PO was added at a Cu / P molar ratio of 3 / 1, 3 / 2, 3 / 3, 3 / 4 and 3 / 5 respectively 4 3- Then add ferric chloride solution to make Fe 3+ The concentration was 0.1mmol / L and the pH was adjusted to 7.0;

[0068] After adjusting the pH of each of the above solutions, continue to stir at a speed of 100 rpm for 30 min, then let the solution stand for more than 1 day, take the supernatant and measure the Cu(II) concentration and Zeta potential value.

[0069] As Figure 4 and Figure 5 shown, the Cu(II) concentration gradually decreases with the decrease of the Cu / P ratio, and the Zeta potential value of the solution also gradually decreases; moreover, the Cu(II) concentration decreases more under the conditions of PO 4 3- +Fe(III) group than that of the PO 4 3- group, indicating that the presence of Fe(III) can reduce the Cu(II) concentration to a lower level. Figure 4 It also shows that under the conditions of the PO 4 3- +Fe(III) group, when the Cu / P ratio is less than 3 / 2, the Cu(II) concentration is lower than 1 mg / L.

[0070] Example 4: Removal of free copper ions in CuSO 4 solution

[0071] At a stirring speed of 500 rpm, add PO 4 to multiple 1-L solutions containing 50 mg / L of Cu(II) in the form of CuSO 4 3- at a Cu / P molar ratio of 3 / 2, and then add ferric chloride solution to make the Fe 3+ concentrations be 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 mmol / L respectively, and adjust the pH to 7.0; continue to stir at a speed of 100 rpm for 30 min, then let the solution stand for more than 1 day, take the supernatant and measure the Cu(II) concentration.

[0072] As Figure 6 shown, when the Fe(III) concentration exceeds 0.1 mmol / L, the Cu(II) concentration can be reduced to 0.1 mg / L; further increasing the dosage of Fe(III), the Cu(II) concentration slightly increases instead.

[0073] Example 5: Removal of free manganese ions in a high-concentration manganese sulfate solution

[0074] This example is basically the same as Example 1, except that the copper sulfate solution with a Cu(II) concentration of 50 mg / L is replaced with a manganese sulfate solution with a Mn(II) concentration of 50 mg / L; add sodium dihydrogen phosphate according to the Mn / P molar ratio of 3 / 2; measure the Mn(II) concentration (C Mn(Ⅱ) ).) and Zeta potential value.

[0075] As Figure 7 and Figure 8 shown, compared with the Control group, Fe(Ⅲ) group and PO 4 3- group, under the condition of PO 4 3- +Fe(Ⅲ) group, the solution has a lower Mn(Ⅱ) concentration; under the condition of PO 4 3- group, the Zeta potential in the solution is lower.

[0076] Example 6: Removal of free manganese ions from low-concentration manganese sulfate solution

[0077] Under rapid stirring at 500 rpm, 20.15 mg / L PO 4 3- was added to a 1 L manganese sulfate solution with a Mn(Ⅱ) concentration of 10 mg / L according to a Mn / P molar ratio of 3 / 4, and then ferric chloride solution was added to make the Fe 3+ concentrations 0.1, 0.5, 1, 1.5 and 2.5 mmol / L respectively, and the pH was adjusted to 8.0; then the stirring was continued at a speed of 100 rpm for 30 min, and the solution was allowed to stand for more than 1 d, and the supernatant was taken and the residual Mn(Ⅱ), Fe(Ⅲ) and PO 4 3- concentrations were measured respectively.

[0078] As Figure 9 shown, after the manganese sulfate solution was treated with PO 4 3- +Fe(Ⅲ), the Mn(Ⅱ) concentration could be reduced from 10 mg / L to below 0.3 mg / L, and the residual Fe(Ⅲ) concentration in the solution was lower than 0.2 mg / L, and the PO 4 3- concentration was lower than 0.30 mg / L.

[0079] Example 7: Removal of multiple heavy metal ions from water

[0080] Mixed heavy metal (Pb(Ⅱ), Cr(Ⅲ), Cd(Ⅱ), Cu(Ⅱ), Mn(Ⅱ) and Hg(Ⅱ)) solutions were prepared respectively with deionized water, tap water and Jiangxi River water, and the concentration of each heavy metal (M) was 10 mg / L; sodium dihydrogen phosphate was added according to an M 总 / P molar ratio of 6 / 7, and rapid stirring was carried out for 3 - 5 min; then 0.5 mM Fe(Ⅲ) was added, and rapid stirring was carried out for 3 - 5 min; subsequently, it was adjusted to pH 8, and slow stirring was carried out for 15 - 30 min; after standing for 30 min, the supernatant was taken and the concentrations of each heavy metal were tested.

[0081] As Figures 10 - 12As shown, PO 4 3- +Fe(Ⅲ) can remove mixed heavy metal ions in different water bodies. Except for Hg(Ⅱ), the residual concentrations of other heavy metals are lower than the concentrations specified in the World Health Organization (WHO) drinking water standard.

[0082] Example 8: Promote the dissociation of copper ions in copper-containing complexes by ferric ions

[0083] Add ferric chloride to Cu-EDTA solutions with a Cu(Ⅱ) concentration of 10 mg / L and a Cu / EDTA molar ratio of 3 / 2 at Fe / Cu molar ratios of 1 / 1, 3 / 2, 2 / 1, 3 / 1, and 4 / 1, respectively, and stir rapidly for 3 - 5 min; then adjust the pH to 8 and stir slowly for 15 - 30 min; after standing for 30 min, take the supernatant and test the concentrations of various heavy metals.

[0084] As Figure 13 and Figure 14 shown, as the Fe / Cu molar ratio increases, the concentrations of free Cu(Ⅱ) and complexed Fe(Ⅲ) in the solution gradually increase ( Figure 13 ), while the concentrations of complexed Cu(Ⅱ) and free Fe(Ⅲ) gradually decrease ( Figure 14 ), indicating that Fe(Ⅲ) can break the Cu-EDTA complex and convert complexed Cu(Ⅱ) into free Cu(Ⅱ).

[0085] Example 9: Remove free copper ions and copper-containing complexes in water

[0086] Add ferric chloride to Cu-EDTA solutions with a Cu(Ⅱ) concentration of 10 mg / L and a Cu / EDTA molar ratio of 3 / 2 at Fe / Cu molar ratios of 1 / 1, 3 / 2, 2 / 1, 3 / 1, and 4 / 1, respectively, and stir rapidly for 3 - 5 min; then add PO 4 3- at Cu / PO 4 3- molar ratios of 3 / 1, 3 / 2, and 1, respectively; then adjust the pH to 8 and stir slowly for 15 - 30 min; after standing for 30 min, take the supernatant and test the concentrations of various heavy metals.

[0087] As Figure 15 and Figure 16 shown, after adding phosphate ions, both complexed copper and free copper in the solution decrease as the Cu / PO 4 3- molar ratio increases. At a Cu / PO 4 3-When the molar ratio is 3 / 1 and the Fe / Cu molar ratio is 4 / 1, the total copper concentration (complexed + free) can be reduced to less than 1 mg / L( Figure 15 ). This result indicates that the combination of PO 4 3- + Fe(III) can remove the complexed heavy metals in water.

[0088] Example 10: Removal of heavy metal ions from electroplating wastewater

[0089] For the electroplating wastewater from an industrial park in Wanan City, Ji'an City, Jiangxi Province, first add 2 mg / L of PO 4 3- , then add 0.5 mmol / L of Fe(III), adjust the pH to 7.0, and settle for 30 min. The pollutant indexes of the water sample before and after treatment are shown in Table 1.

[0090] Table 1 Pollutant indexes of electroplating wastewater before and after treatment

[0091]

[0092] The results show that the method of the present invention can reduce the concentrations of Ni and Cr in electroplating wastewater from about 1 mg / L to 0.0375 and 0.0227 mg / L, respectively. At the same time, it has a good removal effect on COD, total phosphorus, and total nitrogen in the water sample.

[0093] Example 11: Formation of Feh nanonet under different pH conditions

[0094] Under a stirring speed of 500 rpm, 1 mol / L of NaOH was added dropwise to 6 portions of ferric sulfate solution with a volume of 1 L and a Fe(III) concentration of 0.1 mmol / L, and the pH values were adjusted to 4.0, 5.0, 6.0, 7.0, 8.0, and 9.0, respectively; then, the stirring speed was reduced to 100 rpm, and stirring was continued for about 30 min and then allowed to stand; after sedimentation was completed, the flocs in the water were taken for transmission electron microscope (TEM) morphology analysis.

[0095] As Figure 17 shown, when the pH of the Fe(III) solution is adjusted to 5.0, Feh nanonet can be formed in the solution; and as the pH increases, the pores of the Feh nanonet gradually shrink and aggregate. That is, Fe(III) can form Feh nanonet in the range of pH 5 - 8.

[0096] Example 12: Formation of Feh nanonet under different Fe(III) concentration conditions

[0097] At a stirring speed of 500 rpm, 1 M NaOH was added dropwise to 6 portions of iron(III) sulfate solutions with a volume of 1 L and iron(III) concentrations of 0.1, 0.25, 0.5, 0.75, 1.0, 1.5, 2.5, and 5 mmol / L, respectively, to adjust the solution pH to 7.0. Then, the stirring speed was reduced to 100 rpm, and after continuing to stir for about 30 min, the solution was allowed to stand. After sedimentation was completed, the flocs in the water were taken for transmission electron microscopy (TEM) morphology analysis.

[0098] As Figure 18 shown, with the increase in the iron(III) concentration, the size of the Fe(OH) 3 skeleton that composes the nanonet gradually increases and aggregates.

[0099] As Figure 19 shown, the Feh nanonet can capture heavy metal phosphate sols from water. Further, combined with Figures 17 - 19 the TEM results, it can be seen that in the pH range of 5 - 8 and under relatively low iron(III) concentrations of 0.01 - 2 mM, the formed Feh nanonet can capture negatively charged sols in water, such as heavy metal phosphates and organic colloids.

[0100] As described above, these are only the preferred embodiments of the present invention. The present invention is not limited to the above - described embodiments. As long as the same or equivalent means are used to achieve the technical effects of the present invention, they should fall within the protection scope of the present invention. Within the protection scope of the present invention, various different modifications and changes can be made to its technical solutions and / or implementation manners.

Claims

1. A method for treating heavy metal wastewater, characterized in that: The following steps are involved: S100, adjusting the pH value of heavy metal wastewater to 3-6; S200, adding a phosphate-containing substance to combine the phosphate with the heavy metal ion to form a negatively charged heavy metal phosphate sol; S300, adding a substance containing trivalent iron ions, and adjusting the pH value to 5-8, so as to hydrolyze the trivalent iron ions and generate positively charged ferric hydroxide nano-mesh flocs in situ, and precipitating the heavy metal phosphate sol through the electrostatic adsorption of the ferric hydroxide nano-mesh flocs.

2. The method according to claim 1, characterized in that Before step S200, the method further includes the following steps: S150. Add substances containing trivalent iron ions to liberate heavy metal ions of heavy metal complexes in heavy metal wastewater.

3. The method according to claim 1 or 2, characterized in that: Heavy metal wastewater contains at least one heavy metal ion or its complex among Cu, Ni, Cd, Co, Mn, Pb and Cr.

4. The method according to claim 1 or 2, characterized in that: The phosphate-containing substance is a soluble phosphate or a solution thereof. After the soluble phosphate or a solution thereof is added, the concentration of the phosphate is made to be 1 / 3 to 2 times of the total charge concentration of the heavy metals in the heavy metal wastewater.

5. The method according to claim 4, characterized in that The substance containing phosphate is sodium dihydrogen phosphate.

6. The method according to claim 1 or 2, characterized in that: The substance containing trivalent iron ions is a soluble small molecule inorganic iron salt or its solution. After adding the soluble small molecule inorganic iron salt or its solution, the concentration of trivalent iron ions in the heavy metal wastewater is adjusted to a range of 0.05 to 5 mmol / L.

7. The method according to claim 6, characterized in that The substance containing trivalent iron ions is ferric chloride solution.

8. The method according to claim 1 or 2, characterized in that: Step S200 is performed under stirring conditions.

9. The method according to claim 1 or 2, characterized in that: Step S300 is performed under stirring conditions.

10. The method according to claim 1 or 2, characterized in that: After step S300, the method further includes the following steps: S350, let stand for more than 3 hours.

Images