A process for treating nickel wastewater containing sodium metaaluminate and heavy metal sequestering agents

Abstract

The application discloses a nickel wastewater treatment process containing sodium metaaluminate and a heavy metal chelating agent, which comprises the following steps: first filtration treatment, first nickel concentration treatment, second nickel concentration treatment, second filtration treatment, sodium metaaluminate treatment, first solid-liquid separation, high polymer chelation precipitation treatment and second solid-liquid separation. First concentrated water generated in the first nickel concentration treatment, second concentrated water generated in the second nickel concentration treatment and RO concentrated water generated by an RO membrane filtration system are transported to a mixing pool, and sodium metaaluminate treatment is performed on mixed liquid with high nickel ion concentration. Sodium metaaluminate solution is added, and efficient removal of nickel ions is realized through pH control, chemical precipitation, adsorption synergy and flocculation. RO water and primary treatment water are transported to a water production pool, and mixed liquid with low nickel ion concentration is treated through the high polymer chelation precipitation method. Heavy metal chelating agents react with residual nickel ions to form chelate precipitates, and double chemical precipitation treatment ensures efficient removal of nickel ions.

Description

Technical Field

[0001] This invention relates to the field of nickel wastewater treatment technology, specifically a nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent. Background Technology

[0002] With the rapid development of industry, nickel is increasingly used in electroplating, metallurgy, chemical industry and other fields, which leads to a continuous increase in the discharge of nickel-containing wastewater. Nickel is a heavy metal element with potential toxicity. Excessive discharge of nickel will cause serious pollution to water bodies, soil and ecological environment, and will also pose a threat to human health through accumulation in the food chain.

[0003] Currently, nickel wastewater is typically treated using the Fenton process, which involves adding Fenton's reagent (usually a mixed solution of hydrogen peroxide and ferrous ions) to the wastewater. Under suitable pH conditions, the Fenton reagent undergoes a chain reaction, generating highly oxidizing hydroxyl radicals. These hydroxyl radicals can efficiently oxidize and decompose organic matter in the wastewater, while simultaneously promoting the combination of nickel ions with hydroxide ions in the water to form nickel hydroxide precipitate. However, in current processes, the wastewater is not pre-concentrated before the Fenton process, resulting in a low concentration of nickel ions in the wastewater. This necessitates the excessive addition of Fenton's reagent to maintain reaction efficiency, leading to a decrease in reagent utilization.

[0004] In addition, some nickel ions may remain due to incomplete reaction with Fenton's reagent or insufficient precipitation stability, resulting in a nickel ion removal rate that is usually only 70%-80%, which is difficult to meet increasingly stringent environmental emission standards. Summary of the Invention

[0005] To overcome the shortcomings of existing technical solutions, this invention provides a nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent, which can effectively solve the technical problems mentioned in the background art.

[0006] The technical solution adopted by this invention to solve its technical problem is: a nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent, comprising the following steps:

[0007] Step S1: Primary filtration treatment

[0008] Wastewater containing nickel acetate is collected in a collection tank and then pumped to a security filter. The security filter removes large particulate impurities from the wastewater.

[0009] Step S2: Primary nickel concentration treatment

[0010] The wastewater treated in step S1 is transported to the primary nickel concentration system. The nickel ions in the wastewater are concentrated by the selective filtration of the membrane. The wastewater is separated into primary concentrated water and primary fresh water after concentration treatment. The primary concentrated water is discharged from the concentrated water outlet of the primary nickel concentration system and transported to the mixing tank through the concentrated water pipeline.

[0011] Step S3: Secondary nickel concentration treatment

[0012] The primary freshwater produced in step S2 is transported to the secondary nickel concentration system. The nickel ions in the primary freshwater are concentrated by the selective filtration properties of the membrane. The primary freshwater is separated into secondary concentrated water and secondary freshwater after concentration treatment. The secondary concentrated water is discharged from the concentrated water outlet of the secondary nickel concentration system and transported to the mixing tank through the concentrated water pipeline.

[0013] Step S4: Secondary filtration treatment

[0014] The secondary freshwater generated in step S3 is transported to the RO membrane filtration system. The high efficiency of the RO membrane is used to perform deep filtration on the secondary freshwater. The secondary freshwater is separated into RO permeate and RO concentrate through deep filtration. The RO permeate is collected to the permeate tank through the permeate pipeline, and the RO concentrate is discharged from the concentrate outlet of the RO membrane filtration system and transported to the mixing tank through the concentrate pipeline.

[0015] Step S5: Sodium aluminate treatment

[0016] Sodium aluminate solution is added to the mixing tank. The pH of the mixture is 8-12. Nickel ions react with hydroxide ions to form nickel hydroxide precipitate. Sulfuric acid is added to adjust the pH of the mixture back to 6.5-7.5. Sodium aluminate hydrolyzes to form aluminum hydroxide precipitate. Aluminum hydroxide adsorbs the previously formed nickel hydroxide. At the same time, aluminum ions and nickel ions compete with other ligands in the solution for complexation. Polyacrylamide (PAM) is added as a flocculant. After 5-10 minutes of reaction, floc formation is promoted, followed by flocculation and sedimentation after 10-30 minutes.

[0017] Step S6: Primary solid-liquid separation

[0018] Solid-liquid separation in the mixing tank is carried out by sedimentation, filtration and centrifugation. The separated liquid is the primary treated liquid after treatment with sodium aluminate. The primary treated liquid is collected into the product water tank through the product water pipeline.

[0019] Step S7: Polymer chelation precipitation treatment

[0020] The heavy metal chelating agent is added to the product water tank. The pH value of the mixed solution is 6.5-7.5. After thorough stirring and temperature control, the heavy metal chelating agent reacts chemically with the nickel ions in the RO product water and primary treatment solution to form chelate precipitates.

[0021] Step S8: Secondary solid-liquid separation

[0022] Two-stage solid-liquid separation was performed in the product water tank using sedimentation, filtration, and centrifugation methods. The separated liquid was the treated liquid after being treated by polymer chelation precipitation.

[0023] Furthermore, in step S1, the security filter has a filtration accuracy of 5-50 micrometers and a filtration velocity of 10-20 m / s. 3 / h.

[0024] Furthermore, in step S2, the membrane used in the primary nickel concentration system is a reverse osmosis membrane, and the membrane flux is set at 12-18 L / (m²). 2 The recovery rate is set at 60%-70%, the operating pressure is 1.5-2.5 MPa, and the operating temperature is 20-30℃.

[0025] Furthermore, in step S3, the membrane used in the secondary nickel concentration system is a nanofiltration membrane, and the membrane flux is set at 18-25 L / (m²). 2 The recovery rate is set at 70%-80%, the operating pressure is 1.0-1.8MPa, and the operating temperature is 20-30℃.

[0026] Furthermore, in step S4, the RO membrane filtration system uses a brackish water desalination membrane, and the membrane flux is set at 15-25 L / (m³). 2 •h), the recovery rate is set at 60%-70%, and the operating pressure is 1.5-2.5MPa.

[0027] Furthermore, in step S5, the mass ratio of aluminum ions in the sodium aluminate solution to nickel ions in the wastewater is 50-250:1, and the pH value of the mixture is increased to 8-12.

[0028] Furthermore, in step S6, when using the sedimentation method, the sedimentation time is 1-2 hours; when using the filtration method, the filter medium is a mixture of quartz sand and activated carbon, with the quartz sand particle size being 0.5-1.0 mm and the activated carbon particle size being 1-2 mm, and the filtration speed being 5-10 m / s². 3 When using centrifugation, the centrifugation speed is 2000-3000 r / min and the centrifugation time is 10-20 min.

[0029] Furthermore, in step S7, the mass concentration of the heavy metal chelating agent is 3.78-3.80 g / L, the heavy metal chelating agent is HMCA, the stirring speed is 150-250 r / min, and the stirring time is 20-40 min.

[0030] Furthermore, in step S8, when using the precipitation method, the precipitation time is 2-3 hours; when using the filtration method, the filter medium is an ultrafiltration membrane with a pore size of 0.01-0.1 micrometers and a filtration pressure of 0.1-0.3 MPa; when using the centrifugation method, the centrifugation speed is 3000-4000 r / min and the centrifugation time is 15-25 min.

[0031] Furthermore, it also includes step S9: processing fluid detection.

[0032] The treated liquid obtained in step S8 is tested, including nickel ion concentration, pH value, and turbidity. If the nickel ion concentration, pH value, and turbidity all meet the emission standards, the treated liquid is discharged. If any of the nickel ion concentration, pH value, or turbidity does not meet the emission standards, the treated liquid is returned to step S7 for re-treatment using the polymer chelation precipitation method.

[0033] Compared with the prior art, the beneficial effects of the present invention are:

[0034] Through multi-stage treatment including primary nickel concentration, secondary nickel concentration, and RO membrane filtration system, nickel ions in wastewater are gradually concentrated. The primary concentrated water produced by primary nickel concentration treatment, the secondary concentrated water produced by secondary nickel concentration treatment, and the RO concentrated water produced by RO membrane filtration system are all transported to the mixing tank through concentrated water pipelines, which facilitates subsequent chemical precipitation treatment of the mixed liquid with high nickel ion concentration in the mixing tank.

[0035] In the sodium aluminate process, sodium aluminate solution is added to the mixing tank to provide aluminum ions and raise the pH of the mixed solution to 8-12, causing nickel ions to precipitate as nickel hydroxide. Sulfuric acid is then added to adjust the pH to 6.5-7.5, where sodium aluminate hydrolyzes to produce aluminum hydroxide. The aluminum hydroxide adsorbs nickel hydroxide, and aluminum ions compete with nickel ions for complexation, forming a composite precipitate. Polyacrylamide is added to promote floc formation, and the precipitate is separated by static sedimentation. This process removes most of the nickel ions. Subsequently, a high-molecular-weight chelation precipitation method is used, where a heavy metal chelating agent (HMCA) further reacts with the remaining nickel ions to form a chelate precipitate. This dual chemical precipitation treatment ensures efficient removal of nickel ions, enabling the nickel ion concentration in the treated solution to meet emission standards. Attached Figure Description

[0036] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] like Figure 1 As shown, this invention provides a nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent, comprising the following steps:

[0039] Step S1: Primary filtration treatment

[0040] Wastewater containing nickel acetate is collected in a collection tank and then pumped to a security filter. The security filter removes large particulate impurities by filtering the wastewater. The security filter has a filtration precision of 5-50 microns and a filtration velocity of 10-20 m / s. 3 / h;

[0041] Step S2: Primary nickel concentration treatment

[0042] The wastewater treated in step S1 is then transported to a primary nickel concentration system. The selective filtration properties of the membrane are used to concentrate nickel ions in the wastewater. The membrane used in the primary nickel concentration system is a reverse osmosis membrane, with a membrane flux set at 12-18 L / (m³). 2 •h), the recovery rate is set at 60%-70%, the operating pressure is 1.5-2.5MPa, the operating temperature is 20-30℃, and the wastewater is concentrated to separate primary concentrated water and primary fresh water. The primary concentrated water is discharged from the concentrated water outlet of the primary nickel concentration system and transported to the mixing tank through the concentrated water pipeline.

[0043] Step S3: Secondary nickel concentration treatment

[0044] The primary desalinated water produced in step S2 is then fed into a secondary nickel concentration system. The nickel ions in the primary desalinated water are concentrated using the selective filtration properties of a membrane. The membrane used in the secondary nickel concentration system is a nanofiltration membrane, with a membrane flux set at 18-25 L / (m²). 2 •h), the recovery rate is set at 70%-80%, the operating pressure is 1.0-1.8MPa, and the operating temperature is 20-30℃. The primary desalinated water is concentrated to separate secondary concentrated water and secondary desalinated water. The secondary concentrated water is discharged from the concentrated water outlet of the secondary nickel concentration system and transported to the mixing tank through the concentrated water pipeline.

[0045] Step S4: Secondary filtration treatment

[0046] The secondary freshwater produced in step S3 is then fed into an RO membrane filtration system. The high efficiency of the RO membrane allows for deep filtration of the secondary freshwater. The RO membrane filtration system uses a brackish water desalination membrane, with a membrane flux set at 15-25 L / (m³). 2 •h), the recovery rate is set at 60%-70%, the operating pressure is 1.5-2.5MPa, and the secondary freshwater is separated into RO permeate and RO concentrate through deep filtration. The RO permeate is collected to the permeate tank through the permeate pipeline, and the RO concentrate is discharged from the concentrate outlet of the RO membrane filtration system and transported to the mixing tank through the concentrate pipeline.

[0047] Step S5: Sodium aluminate treatment

[0048] Sodium aluminate solution is added to the mixing tank. The mass ratio of aluminum ions in the sodium aluminate solution to nickel ions in the wastewater is 50-250:1. The sodium aluminate solution provides aluminum ions for subsequent reactions and hydrolyzes to generate sodium hydroxide, raising the pH of the mixed solution to 8-12. Under alkaline conditions, nickel ions react with hydroxide ions to form nickel hydroxide precipitate. Adding sulfuric acid solution lowers the pH of the mixed solution to 6.5-7.5. Under neutral or weakly acidic conditions, sodium aluminate hydrolyzes to form aluminum hydroxide precipitate. Due to the large specific surface area and adsorption capacity of aluminum hydroxide, it can effectively adsorb nickel hydroxide precipitate to form a composite precipitate. At the same time, aluminum ions and nickel ions compete with other ligands in the solution to form complexes. Polyacrylamide (PAM) is added to the mixed solution as a flocculant. After 5-10 minutes of reaction, floc formation is promoted, followed by flocculation and sedimentation after 10-30 minutes.

[0049] Step S6: Primary solid-liquid separation

[0050] Solid-liquid separation in the mixing tank is achieved through sedimentation, filtration, and centrifugation. When using sedimentation, the sedimentation time is 1-2 hours. When using filtration, the filter medium is a mixture of quartz sand and activated carbon, with quartz sand particle size of 0.5-1.0 mm and activated carbon particle size of 1-2 mm. The filtration velocity is 5-10 m / s². 3 / h, when using centrifugation, the centrifugation speed is 2000-3000r / min, the centrifugation time is 10-20min, the separated liquid is the treated liquid after treatment by sodium aluminate method, and the primary treated liquid is collected into the production water tank through the production water pipeline.

[0051] Step S7: Polymer chelation precipitation treatment

[0052] After treatment with sodium aluminate in step S5, the pH of the primary treatment solution is 6.5-7.5. HMCA is used as the heavy metal chelating agent, and the mass concentration of the heavy metal chelating agent is prepared to be 3.78-3.80 g / L. The prepared heavy metal chelating agent is added to the permeate tank. After thorough stirring and temperature control, the heavy metal chelating agent reacts chemically with the nickel ions in the RO permeate and the primary treatment solution. The stirring speed is 150-250 r / min and the stirring time is 20-40 min to form a chelate precipitate.

[0053] Step S8: Secondary solid-liquid separation

[0054] Two-stage solid-liquid separation is performed in the product water tank using sedimentation, filtration, and centrifugation. When using sedimentation, the sedimentation time is 2-3 hours. When using filtration, the filter medium is an ultrafiltration membrane with a pore size of 0.01-0.1 micrometers and a filtration pressure of 0.1-0.3 MPa. When using centrifugation, the centrifugation speed is 3000-4000 r / min and the centrifugation time is 15-25 minutes. The separated liquid is the treated liquid after being treated by polymer chelation precipitation.

[0055] Step S9: Processing fluid detection

[0056] The treated liquid obtained in step S8 is tested, including nickel ion concentration, pH value, and turbidity. If the nickel ion concentration, pH value, and turbidity all meet the emission standards, the treated liquid is discharged. If any of the nickel ion concentration, pH value, or turbidity does not meet the emission standards, the treated liquid is returned to step S7 for re-treatment using the polymer chelation precipitation method.

[0057] The following is a nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent according to the present invention, showing the changes in nickel concentration (mg / L), conductivity (mS / cm), and pH value of the wastewater in each step;

[0058]

[0059] Compared to traditional technologies:

[0060] Through multi-stage treatment including primary nickel concentration, secondary nickel concentration, and RO membrane filtration system, nickel ions in wastewater are gradually concentrated. The primary concentrated water produced by primary nickel concentration treatment, the secondary concentrated water produced by secondary nickel concentration treatment, and the RO concentrated water produced by RO membrane filtration system are all transported to the mixing tank through concentrated water pipelines, which facilitates subsequent chemical precipitation treatment of the mixed liquid with high nickel ion concentration in the mixing tank.

[0061] In the sodium aluminate process, sodium aluminate solution is added to the mixing tank to provide aluminum ions and adjust the pH to 8-12, causing nickel ions to precipitate as nickel hydroxide. Sulfuric acid is then added to adjust the pH to 6.5-7.5, where sodium aluminate hydrolyzes to produce aluminum hydroxide. The aluminum hydroxide adsorbs nickel hydroxide, and the aluminum and nickel ions compete to complex, forming a complex precipitate. Polyacrylamide is added to promote floc formation, and the precipitate is separated by static sedimentation. This process removes most of the nickel ions. Subsequently, a high-molecular-weight chelation precipitation method is used, where a heavy metal chelating agent (HMCA) further reacts with the remaining nickel ions to form a chelate precipitate. This dual chemical precipitation treatment ensures efficient removal of nickel ions, enabling the nickel ion concentration in the treated liquid to meet emission standards.

[0062] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent, characterized in that, Includes the following steps: Step S1: Primary filtration treatment Wastewater containing nickel acetate is collected in a collection tank and then pumped to a security filter. The security filter removes large particulate impurities from the wastewater. Step S2: Primary nickel concentration treatment The wastewater treated in step S1 is transported to the primary nickel concentration system. The nickel ions in the wastewater are concentrated by the selective filtration of the membrane. The wastewater is separated into primary concentrated water and primary fresh water after concentration treatment. The primary concentrated water is discharged from the concentrated water outlet of the primary nickel concentration system and transported to the mixing tank through the concentrated water pipeline. Step S3: Secondary nickel concentration treatment The primary freshwater produced in step S2 is transported to the secondary nickel concentration system. The nickel ions in the primary freshwater are concentrated by the selective filtration properties of the membrane. The primary freshwater is separated into secondary concentrated water and secondary freshwater after concentration treatment. The secondary concentrated water is discharged from the concentrated water outlet of the secondary nickel concentration system and transported to the mixing tank through the concentrated water pipeline. Step S4: Secondary filtration treatment The secondary freshwater generated in step S3 is transported to the RO membrane filtration system. The high efficiency of the RO membrane is used to perform deep filtration on the secondary freshwater. The secondary freshwater is separated into RO permeate and RO concentrate through deep filtration. The RO permeate is collected to the permeate tank through the permeate pipeline, and the RO concentrate is discharged from the concentrate outlet of the RO membrane filtration system and transported to the mixing tank through the concentrate pipeline. Step S5: Sodium aluminate treatment Sodium aluminate solution is added to the mixing tank. The pH of the mixture is 8-12. Nickel ions react with hydroxide ions to form nickel hydroxide precipitate. Sulfuric acid is added to adjust the pH of the mixture back to 6.5-7.

5. Sodium aluminate hydrolyzes to form aluminum hydroxide precipitate. Aluminum hydroxide adsorbs the previously formed nickel hydroxide. At the same time, aluminum ions and nickel ions compete with other ligands in the solution for complexation. Polyacrylamide (PAM) is added as a flocculant. After 5-10 minutes of reaction, floc formation is promoted, followed by flocculation and sedimentation after 10-30 minutes. Step S6: Primary solid-liquid separation Solid-liquid separation in the mixing tank is carried out by sedimentation, filtration and centrifugation. The separated liquid is the primary treated liquid after treatment with sodium aluminate. The primary treated liquid is collected into the product water tank through the product water pipeline. Step S7: Polymer chelation precipitation treatment The heavy metal chelating agent is added to the product water tank. The pH value of the mixed solution is 6.5-7.

5. After thorough stirring and temperature control, the heavy metal chelating agent reacts chemically with the nickel ions in the RO product water and primary treatment solution to form chelate precipitates. Step S8: Secondary solid-liquid separation Two-stage solid-liquid separation was performed in the product water tank using sedimentation, filtration, and centrifugation methods. The separated liquid was the treated liquid after being treated by polymer chelation precipitation.

2. The nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent according to claim 1, characterized in that, In step S1, the security filter has a filtration accuracy of 5-50 micrometers and a filtration velocity of 10-20 m / s. 3 / h.

3. The nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent according to claim 1, characterized in that, In step S2, the membrane used in the primary nickel concentration system is a reverse osmosis membrane, and the membrane flux is set at 12-18 L / (m²). 2 The recovery rate is set at 60%-70%, the operating pressure is 1.5-2.5MPa, and the operating temperature is 20-30℃.

4. The nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent according to claim 1, characterized in that, In step S3, the membrane used in the secondary nickel concentration system is a nanofiltration membrane, and the membrane flux is set at 18-25 L / (m²). 2 The recovery rate is set at 70%-80%, the operating pressure is 1.0-1.8MPa, and the operating temperature is 20-30℃.

5. The nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent according to claim 1, characterized in that, In step S4, the RO membrane filtration system uses a brackish water desalination membrane, and the membrane flux is set at 15-25 L / (m³). 2 •h), the recovery rate is set at 60%-70%, and the operating pressure is 1.5-2.5MPa.

6. The nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent according to claim 1, characterized in that, In step S5, the mass ratio of aluminum ions in the sodium aluminate solution to nickel ions in the wastewater is 50-250:1, and the pH value of the mixture is increased to 8-12.

7. The nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent according to claim 1, characterized in that, In step S6, when using the sedimentation method, the sedimentation time is 1-2 hours; when using the filtration method, the filter medium is a mixture of quartz sand and activated carbon, with quartz sand particle size of 0.5-1.0 mm and activated carbon particle size of 1-2 mm, and the filtration speed is 5-10 m / s. 3 When using centrifugation, the centrifugation speed is 2000-3000 r / min and the centrifugation time is 10-20 min.

8. The nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent according to claim 1, characterized in that, In step S7, the mass concentration of the heavy metal chelating agent is 3.78-3.80 g / L, the heavy metal chelating agent is HMCA, the stirring speed is 150-250 r / min, and the stirring time is 20-40 min.

9. The nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent according to claim 1, characterized in that, In step S8, when using the precipitation method, the precipitation time is 2-3 hours; when using the filtration method, the filter medium is an ultrafiltration membrane with a pore size of 0.01-0.1 micrometers and a filtration pressure of 0.1-0.3 MPa; when using the centrifugation method, the centrifugation speed is 3000-4000 r / min and the centrifugation time is 15-25 min.

10. The nickel wastewater treatment process containing sodium aluminate and a heavy metal chelating agent according to claim 1, characterized in that, It also includes step S9: Processing fluid detection The treated liquid obtained in step S8 is tested, including nickel ion concentration, pH value, and turbidity. If the nickel ion concentration, pH value, and turbidity all meet the emission standards, the treated liquid is discharged. If any of the nickel ion concentration, pH value, or turbidity does not meet the emission standards, the treated liquid is returned to step S7 for re-treatment using the polymer chelation precipitation method.

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