Water resources play a crucial role in economic development and social production, and are also an indispensable part of people's daily lives. However, with the rapid development of industry and agriculture, large amounts of industrial wastewater are being discharged, leading to increasingly severe heavy metal pollution in water bodies. According to statistics, China generates around 40 billion tons of industrial wastewater annually, with heavy metal wastewater accounting for approximately 60%. This wastewater severely contaminates both surface and groundwater, causing a sharp decline in the total available water resources. Heavy metal wastewater typically originates from industries such as mining, metal smelting and processing, electroplating, leather production, pesticides, papermaking, painting, dyeing, nuclear technology, and petrochemicals.

Heavy metals are not biodegradable and are easily absorbed and accumulated by living organisms. Their toxicity is persistent, making them highly hazardous pollutants. If left untreated, they will pose severe threats to the ecological environment and human health. However, as valuable and essential resources, heavy metals also have high usage value. Therefore, effectively treating heavy metal pollution in water bodies, protecting human health and the environment, while recovering and recycling heavy metals to alleviate the pressure on China's resources and environment, is an issue that cannot be ignored.

Currently, there are three main methods for treating heavy metal wastewater:

a. Chemical Methods: These involve removing heavy metal ions through chemical reactions, including chemical precipitation, chemical reduction, electrochemical methods, and the use of high-molecular heavy metal adsorbents.

b. Physical Methods: These methods separate heavy metal ions without changing their chemical forms, through processes such as adsorption, concentration, and membrane separation. Techniques include adsorption, solvent extraction, evaporation and solidification, ion exchange, and membrane separation.

c. Biological Methods: These rely on microorganisms or plants to remove heavy metals through flocculation, absorption, accumulation, and enrichment. Methods include bioflocculation, phytoremediation, and biosorption.

This article discusses the application and research progress of these methods in heavy metal wastewater treatment, providing theoretical references for the management of water body heavy metal pollution.

1. Chemical Methods

1.1 Chemical Precipitation Method

The chemical precipitation method is a widely used and effective approach for industrial heavy metal wastewater treatment. It involves adding chemical reagents to the water to remove heavy metal ions through precipitation reactions, including methods such as hydroxide precipitation, sulfide precipitation, and ferrite formation.

1.1.1 Hydroxide Precipitation Method

The hydroxide precipitation method is a well-established, cost-effective, and easy-to-manage technique for treating heavy metal-containing wastewater. Mirbagherz S A and others used alkaline reagents, such as lime and sodium hydroxide, to treat wastewater containing copper and chromium. At pH values of 12 and 8.7, Cu²⁺ and Cr³⁺ ions were completely precipitated, making the wastewater meet discharge standards. Heming Chang and others used sodium hydroxide solution to gradually adjust the pH of electroplating wastewater, precipitating copper, chromium, zinc, and nickel at various pH points, significantly reducing the heavy metal content.

While hydroxide precipitation can effectively separate heavy metal ions from wastewater, it has some drawbacks: for amphoteric hydroxides, improper pH control may lead to the re-dissolution of heavy metal ions; the method is not very effective for removing heavy metals from dilute solutions; and the large volume of precipitate, high moisture content, and filtration difficulties are challenges. This method is now rarely used in heavy metal wastewater treatment.

1.1.2 Sulfide Precipitation Method

The sulfide precipitation method is faster, with low solubility of the precipitate. It allows for selective treatment of heavy metal ions and can facilitate the recovery of heavy metals through smelting. Jingwen Li used sodium sulfide precipitation to treat simulated lead-contaminated wastewater. With a reaction time of 20 minutes, a molar ratio of sodium sulfide to lead ions of 5:1, and an initial pH of 8, the lead ion removal rate was 99.72%, and the effluent met national wastewater discharge standards. However, when using sulfide for heavy metal wastewater treatment, residual precipitant in the water can form water-soluble polysulfides in excess, which, when encountering acid, produce hydrogen sulfide gas, leading to secondary pollution. 

1.1.3 Ferrite Method

The ferrite method is widely applied. It involves adding ferrous sulfate salts to heavy metal wastewater and controlling the pH and heating conditions to form stable ferrite co-precipitates with heavy metal ions. Ming Zuo and others studied the use of the ferrite method to treat wastewater containing nickel, chromium, zinc, and copper. After treatment, the effluent quality met national wastewater discharge standards of China. 

However, this method requires longer treatment times and higher temperatures (around 70°C), making it unsuitable for large-scale heavy metal wastewater treatment. It is often combined with other treatment methods. Mengjun Chen and others used a combination of the ferrite method and sulfide precipitation to treat electroplating wastewater, achieving removal rates of 94.51%, 97.78%, and 96.94% for Cu, Cr, and Ni, respectively, meeting the electroplating pollutant discharge standards.

1.2 Electrochemical Methods

The electrochemical method is a competitive water treatment technology that has developed in recent years. It applies electrolysis principles to purify wastewater through electrode reactions and the migration of heavy metal ions in the solution. With advancements in science and technology, improvements in traditional electrochemical treatment processes, and the development of new electrochemical reactors, electrochemical methods have become more effective and widely used in heavy metal wastewater treatment.

1.2.1 Electrocoagulation Method

As a relatively mature wastewater treatment process, electrocoagulation has been widely applied. Chunsheng Ding and others studied the effects of factors such as initial pH, electrolysis time, current intensity, NaCl dosage, ion coexistence, and aeration on the electrocoagulation treatment of Cr⁶⁺ and Cu²⁺ wastewater. The research indicated that, under certain pH conditions, with a current intensity of 4 A, stable removal effects could be achieved in a short time. Additionally, the coexistence of metal ions promoted the treatment of heavy metal wastewater, and appropriate aeration enhanced the removal rate. However, electrocoagulation should not be operated continuously for extended periods, as a dense film can form on the electrode surface, leading to passivation. Recently, pulsed electrocoagulation has been used to replace direct current electrocoagulation, effectively reducing concentration polarization and preventing passivation.

Yuan Qiu and others applied pulsed electrocoagulation to treat chromium-containing electroplating wastewater, achieving a chromium ion removal rate of over 99.5%, meeting discharge standards. Compared to direct current electrocoagulation, this method is more energy-efficient and has shorter treatment times. The latest research on electrocoagulation focuses on cyclic-reversing pulsed signal electrocoagulation, which combines the advantages of high-voltage pulsed electrocoagulation and enhances coagulation between metal ions and colloids, thus preventing electrode passivation.

1.2.2 Microelectrolysis

Microelectrolysis is based on chemical reactions at the electrode surface. A certain amount of active filler is added to the electrolytic cell, with heavy metal wastewater serving as the electrolyte. The active filler forms a primary battery, and electric currents flow through thousands of micro-cells on the surface of the filler. Under low-voltage direct current, electrochemical reactions and flocculation occur, effectively removing heavy metal ions from the water.

In microelectrolysis processes, common fillers include iron chips (such as cast iron or steel chips) combined with graphite or carbon particles. Jie Zhou and others applied the iron-carbon microelectrolysis method to treat chromium-containing wastewater and studied the removal of Cr⁶⁺. The results showed that the iron-carbon microelectrolysis method was effective for Cr⁶⁺ removal, with effluent Cr⁶⁺ levels below 0.1 mg/L. Compared to conventional sodium metabisulfite reduction processes, the iron-carbon microelectrolysis method saves more than 75% of the cost. Combining microelectrolysis with other methods can further enhance the treatment effect.

Shujie Huang used the microelectrolysis–alkaline neutralization precipitation method to treat low-concentration Cr⁶⁺ and Cu²⁺ electroplating wastewater. After treatment, the Cr⁶⁺ and Cu²⁺ concentrations in the wastewater met the first-level discharge standards in the GB8978-96 Intergrated Wastewater Discharge Standard. The combined electrolysis-microelectrolysis technology is one of the future directions for microelectrolysis, and research on the reaction mechanisms and process dynamics of composite microelectrolysis is a current focus in this field. 

1.2.3 Electroreduction Method

The electroreduction method, also known as cathodic reduction, operates on the principle that heavy metal ions in water migrate toward the cathode under electrostatic attraction, where they undergo reduction reactions and precipitate out. This method not only removes heavy metal ions from water but also recovers high-purity metals. However, for low-concentration heavy metal wastewater, traditional two-dimensional electrode electrolysis faces issues like low current density, low electrolysis efficiency, and high energy consumption. Electrochemical reactions are essentially electron transfer reactions occurring at the solid-liquid interface, so solving mass transfer issues at this interface has become a key challenge, and the development of high-efficiency mass transfer reactors has become a research focus.

In engineering applications, three-dimensional electrode reactors are commonly used. These reactors feature fast mass transfer, low operating costs, small footprint, and high removal efficiency. They can reduce heavy metal concentrations from 100 mg/L to 0.1 mg/L within minutes. Shaofeng Zhang and others used three-dimensional electrodes to treat low-concentration acidic lead-containing industrial simulated wastewater. Under identical conditions, a foam copper cathode material in the three-dimensional electrode achieved a Pb²⁺ removal rate of 85%, significantly better than the 34% removal rate achieved with stainless steel plates in a two-dimensional electrode setup. Wu Chen and others used a small composite bipolar rectangular packed bed as a three-dimensional electrode reactor to treat zinc-containing wastewater. Under optimal conditions, the three-dimensional electrode achieved a 95.7% Zn²⁺ removal rate, meeting the Level II requirements of the Chinese "Intergrated Wastewater Discharge Standard" GB8978-88.

2. Physical Methods

2.1 Ion Exchange Method

The ion exchange method purifies wastewater by exchanging heavy metal ions with ions from ion exchange resins, thereby reducing the concentration of heavy metals in water. The driving force is the concentration difference between the ions and the affinity of the functional groups on the exchange agent for the ions. Ion exchange resins include cation exchange resins, anion exchange resins, chelating resins, and humic acid resins. In industrial wastewater treatment, ion exchange resins are mainly used for recovering heavy metals, precious metals, and rare metals. Rengaraj S and others used IRN77 and SKN1 cation exchange resins to remove and recover Cr³⁺ from nuclear power plant cooling wastewater.

Jian Wei and others treated Mn²⁺-containing wastewater with selected ion exchange resins. The method has the advantages of large exchange capacity and stable effluent quality, achieving manganese recovery. Li and others used chelating ion exchange resins Chelex 100 and IRC 748 to exchange Cu²⁺ and Zn²⁺ from solutions, with the maximum exchange capacity for Cu²⁺ being 0.88 mol/kg and 1.10 mol/kg, respectively.

Ion exchange resins can selectively recover heavy metals from water, with the effluent having much lower heavy metal ion concentrations than those treated with chemical precipitation, and generating less sludge. However, ion exchange resins have some drawbacks, such as low strength, poor high-temperature resistance, and low adsorption rates. Improving the adsorption capacity, selectivity, exchange rate, regeneration performance, and mechanical strength of ion exchange resins is a key development direction both now and in the future.

2.2 Membrane Separation Method

As a new separation technology, membrane separation can effectively purify wastewater while recovering useful substances. It also has the advantages of energy efficiency, no phase change, simple equipment, and easy operation. Therefore, it has been widely applied in wastewater treatment with great development prospects. The principle is that a semi-permeable membrane selectively allows certain solutes and solvents to pass through, separating and purifying the solution under external energy. Common membrane separation techniques used for heavy metal wastewater treatment include microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and electrodialysis.

Due to the small particle size of heavy metal ions, single membrane separation techniques are not very effective in removing them. Typically, membrane combination processes are employed. Jinbao Wan and others used a neutralization/microfiltration process to treat wastewater containing Zn²⁺ and Pb²⁺. The results showed removal rates of 90.92% and 76.55%, respectively. After adding flocculants, the removal rates increased to 99.92% and 99.77%. Yunren Qiu and others used a chelation–ultrafiltration coupled process, utilizing sodium polyacrylate as a chelating agent and aromatic polyamide ultrafiltration membranes to treat Cu²⁺ wastewater.

Research showed that at pH 6 and a P/M ratio of 22, the Cu²⁺ retention rate was over 97%. Compared to microfiltration and ultrafiltration, nanofiltration is a membrane process with higher particle retention precision and better retention of divalent and multivalent metal ions. Mehiguene and others studied the use of nanofiltration technology to separate Cu²⁺ and Cd²⁺ from wastewater. They found that when HNO₃ was added to the solution, the retention rate for Cd²⁺ was 35.2%, and for Cu²⁺ was 76.5%, achieving effective separation of copper and cadmium ions. However, concentration polarization during nanofiltration can significantly reduce water flux and desalination rate, and cause the precipitation of some insoluble salts, such as CaSO₄, on the membrane. Therefore, in practical applications, it is important to focus on developing integrated processes and optimizing the procedure.

Membrane separation technologies are efficient, energy-saving, and produce no secondary pollution, offering great potential in wastewater treatment. However, the complex composition of industrial wastewater and the stringent treatment conditions mean that membrane materials must have good separation performance and long service life. From this perspective, developing high-performance membranes with excellent anti-fouling properties is of strategic importance.

2.3 Adsorption Method

The adsorption method involves using porous materials as adsorbents to remove heavy metal ions from wastewater. Activated carbon is the most widely used and earliest employed adsorbent, with a large surface area and high treatment efficiency, but it is expensive and difficult to regenerate, which limits its development in wastewater treatment. Therefore, the search for low-cost adsorbents with good adsorption properties has become a hot research topic in recent years.

Currently, inexpensive materials such as mineral materials, industrial waste, and agricultural and forestry waste are commonly used as adsorbents. Zeolite is one of the earliest porous minerals used in heavy metal wastewater treatment. Its skeletal structure gives it a large surface area and strong adsorption capability. Jon R Kiser and others treated Cr⁶⁺-contaminated wastewater using Fe²⁺-modified zeolite. After modification, the zeolite's capacity to adsorb Cr⁶⁺ reached 0.3 mmol/g, significantly improving its adsorption ability. In recent years, some industrial and agricultural waste materials, due to their abundant sources and low costs, have also been widely used to treat heavy metal wastewater. Marisa and others used hydrothermal pretreatment on fly ash to study the adsorption capacity of modified fly ash. The results showed that the removal rates of Cu²⁺ and Mn²⁺ were 99% and 85%, respectively. Rosangela A and others used untreated yellow passion fruit shells as adsorbents to treat Cr³⁺ and Pb²⁺ in aqueous solutions, with maximum adsorption capacities of 85.1 mg/g and 151.6 mg/g, respectively. Dahiya S and others used treated crab shells and areca nut shells to adsorb Pb²⁺ and Cu²⁺ from aqueous solutions. At equilibrium, the maximum adsorption capacities of areca nut shells for Pb²⁺ and Cu²⁺ were 18.33 mg/g ± 0.44 mg/g and 17.64 mg/g ± 0.31 mg/g, respectively.

Currently, the adsorption method is mainly non-selective, meaning it does not specifically target heavy metal pollutants for removal and cannot selectively remove specific heavy metal ions from particular types of wastewater. In many real wastewater cases, there are one or two predominant heavy metal pollutants. From an environmental protection and resource recovery perspective, the use of selective adsorption to treat heavy metal wastewater is of significant importance.

3. Biological Methods

Biological methods are techniques that utilize the chemical structure and composition of biological materials to adsorb heavy metal ions from water, including plant remediation, biological flocculation, and biological adsorption. As an important purification method, biological processes offer several advantages, such as simple equipment, no secondary pollution, a wide and inexpensive range of materials, and high economic efficiency, making them a promising approach for treating heavy metal wastewater with broad application prospects.

3.1 Phytoremediation

Phytoremediation refers to the use of plants to absorb, precipitate, and accumulate heavy metals to treat contaminated water. The plants typically used in phytoremediation are large aquatic higher plants, such as algae and water hyacinths. Rai et al. and Dwivedi et al. found that water hyacinth is an excellent plant for heavy metal accumulation, capable of accumulating Cu, Mo, Cr, Cd, and As at concentrations of 62, 5, 13, 11, and 0.05 μg/g, respectively. Soltan et al. studied the adsorption of Pb²⁺, Zn²⁺, and Cu²⁺ from wastewater by water hyacinth and demonstrated that the carboxyl and hydroxyl groups on the amino acids in the plant cells chelate with heavy metal ions.

Phytoremediation not only avoids secondary pollution but also contributes to the improvement of the ecological environment. In addition to treating pollution, it can generate some economic benefits. However, factors such as wastewater concentration and pH need to be further studied to understand their effects on phytoremediation.

3.2 Microbial Flocculation

Microbial flocculation is a method that utilizes microorganisms or their metabolic products to flocculate and precipitate heavy metals. The adsorption of heavy metals by microorganisms depends on two factors: the properties of the microbial adsorbents themselves and the affinity of the metal ions for the microorganisms.

Currently, 17 species of microorganisms with flocculation ability have been developed, including bacteria, fungi, actinomycetes, and yeasts. As a new water treatment technology, microbial flocculants have been widely used in treating heavy metal wastewater. Chatterjee et al. used Bacillus spores to treat wastewater containing Cr³⁺, Co²⁺, and Cu²⁺, with removal rates of 80.8%, 79.71%, and 57.14%, respectively. Huang et al. used the fruiting bodies of Auricularia auricula as adsorbents to treat simulated wastewater, achieving maximum adsorption capacities of 221, 73.7, and 63.3 mg/g for Pb²⁺, Cu²⁺, and Cd²⁺, respectively, under optimal experimental conditions.

Microbial flocculants have several unique advantages over traditional flocculants, including high efficiency, non-toxicity, easy biodegradability, a wide range of flocculation targets, and no secondary pollution after use. However, challenges remain, such as difficulties in preserving live flocculants, high production costs, and issues related to large-scale industrial production.

Future research should focus on exploring the flocculation mechanism and dynamics to guide the development of new super-flocculants. By utilizing genetic engineering and fermentation engineering, high-efficiency flocculant-producing strains can be selected to improve flocculation activity, reduce flocculant dosage, and lower production costs.

3.3 Biological Adsorption

Biological adsorption is a relatively novel method for treating heavy metal pollution in water, gaining increasing attention due to its high efficiency and low cost. This method uses the chemical structure and composition of certain biological materials to adsorb heavy metal ions from water, followed by solid-liquid separation to remove the heavy metals. It is suitable for treating large volumes of low-concentration heavy metal wastewater. The adsorption mechanisms include complexation, chelation, ion exchange, and electrostatic attraction.

Research has been conducted on various biological materials for heavy metal adsorption, including bacteria, fungi, yeasts, algae, and agricultural and forestry waste. These materials exhibit varying degrees of adsorption capacity for different heavy metals.

Fan Ruimei et al. found that Bacillus clausii can effectively adsorb Zn²⁺ in aqueous solutions, with an adsorption capacity of 57.5 mg/g at pH 4.5 and equilibrium reached in about 30 minutes. Melgar et al. demonstrated that the large mushroom (Lentinus edodes) could effectively adsorb Zn²⁺, Cu²⁺, Hg²⁺, Cd²⁺, and Pb²⁺ from aqueous solutions, achieving maximum removal rates of 84%, 96%, 85%, 84%, and 89%, respectively, after 15 minutes.

Studies have shown that algae can adsorb one or more metal ions. Romera et al. studied the adsorption properties of six different algae species for Cd²⁺, Ni²⁺, Zn²⁺, Cu²⁺, and Pb²⁺ in aqueous solutions. The results showed that at an algae concentration of 0.5 g/L, the adsorption performance for heavy metal ions was optimal, with the adsorption sequence being Pb > Cd ≥ Cu > Zn > Ni. Besides bacteria, fungi, and algae, low-cost agricultural and forestry waste has also attracted interest due to its high porosity and large surface area, which allow it to physically adsorb metal ions. Furthermore, the active substances in agricultural and forestry waste contribute to heavy metal adsorption.

Guohui Wang used chestnut shells to treat Cr⁶⁺-contaminated wastewater. At pH 2 and a temperature of 30°C, with 0.4 g of chestnut shells, the removal rate of Cr⁶⁺ reached over 99%. Chestnut shells showed a significant removal effect on Cr⁶⁺ over a wide range of initial concentrations. Xiaoli Jiang et al. used modified corn straw as an adsorbent to treat Cu²⁺-contaminated simulated wastewater. The results showed that the maximum removal rate of Cu²⁺ by corn straw was over 90%. Ghimire et al. prepared a phosphate-modified orange juice residue loaded with Fe³⁺ for adsorption, studying its adsorption properties for As³⁺ and As⁵⁺, achieving an adsorption capacity of 1.21 mmol/g.

Currently, biological adsorption for heavy metal wastewater treatment is still at the laboratory research stage, and the adsorption mechanism is not yet fully understood. Future research should focus on understanding the adsorption mechanisms of plant materials, the optimal adsorption conditions for industrial production, desorption and heavy metal recovery technologies, and the development of cost-effective and efficient treatment processes and equipment. This will help ensure the large-scale application of plant-based adsorbents in industrial wastewater treatment.

4. Conclusion

Chemical precipitation is a widely used and technically mature water treatment method, but it is suitable for treating high-concentration heavy metal wastewater and tends to produce large amounts of sludge. Membrane separation, as an efficient water treatment technology, has received widespread attention, but it is costly and operationally complex. Ion exchange offers high selectivity and can remove various heavy metals, but the cost of resins is relatively high, and regeneration costs are also substantial. Biological methods are economically efficient, easy to manage, and free from secondary pollution, offering broader development prospects.

In summary, there are many methods for treating heavy metal wastewater, each with its own advantages and disadvantages. Therefore, it is essential to choose an appropriate method based on the specific situation or combine several methods to achieve better treatment results. Additionally, heavy metals are valuable resources with high utility, and researchers should focus more on the study of technologies for recycling and resource recovery of heavy metals.