Discussion on Treatment Technologies for Heavy Metal Wastewater and Their Future Prospects

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Discussion on Treatment Technologies for Heavy Metal Wastewater and Their Future Prospects: With the rapid economic development, the massive discharge of wastewater has led to an increase in the accumulation of heavy metals in soil and water sources, thereby exacerbating heavy metal pollution. Since heavy metals can easily bioaccumulate through the food chain, they pose a serious threat to the health of living organisms and humans. How to effectively manage heavy metal pollution has become an issue of common concern for humanity. Heavy metal pollution is also relatively severe in the wastewater from the development zone. According to tests conducted by the Changsha Environmental Monitoring Station, nickel (Ni) levels in the wastewater from this zone occasionally exceed the allowed limits. Tests on sludge revealed that the total levels of cadmium, nickel, and copper in such sludge exceed the pollution control standards set for its use in agriculture. Therefore, this paper explores the methods for treating heavy metals in this context, as well as their potential future developments. 　　Scholars at home and abroad have conducted extensive research on the control of heavy metal pollution. The wastewater treatment methods that have been developed and applied include chemical, physicochemical, and biological methods, such as chemical precipitation, electrolysis, ion exchange, membrane separation, adsorption using activated carbon and silica gel, biological flocculation, biological adsorption, and phytoremediation. Both chemical and physicochemical methods transfer residual pollutants, which can lead to secondary pollution; moreover, they are difficult to apply to large river basins affected by low concentrations of harmful heavy metals. The biological method boasts advantages such as good effectiveness, low investment and operating costs, ease of management and operation, and no generation of secondary pollution, which has drawn increasing attention. The following methods will be discussed: 1. Chemical method. The chemical method mainly includes chemical precipitation and electrolysis, and is primarily suitable for the treatment of wastewater containing high concentrations of heavy metal ions. 　　The principle of chemical precipitation involves using chemical reactions to convert heavy metals in dissolved form in wastewater into water-insoluble heavy metal compounds, and then removing these precipitates from the aqueous solution through filtration and separation. This includes methods such as neutralization precipitation, sulfide precipitation, and ferrite coprecipitation. Due to the influence of precipitants and environmental conditions, the concentration of the effluent obtained by precipitation methods often fails to meet the required standards, necessitating further treatment. The resulting precipitates must be properly treated and disposed of; otherwise, it will lead to secondary pollution. 　　Electrolysis utilizes the electrochemical properties of metals; metal ions can be separated from solutions with relatively high concentrations during electrolysis, and then utilized. Electrolysis is mainly used for the treatment of electroplating wastewater, and its drawback is that the concentration of heavy metal ions in the water cannot be reduced to very low levels. Therefore, the electrolytic method is not suitable for treating wastewater containing heavy metal ions at lower concentrations. 　　2 Physicochemical methods Ion exchange and membrane separation techniques are suitable for the treatment of wastewater containing low concentrations of heavy metal ions. 　　The ion exchange method is carried out in an ion exchanger, and it makes use of ion exchangers to function. Different types of exchange resins are installed in the exchanger as required. When a liquid containing heavy metals passes through these resins, the ions on the resins exchange places with the heavy metal ions in the water, thereby removing those heavy metal ions from the water. This method is affected by the type, yield, and cost of the exchange agent. Over the past few years, scholars at home and abroad have carried out extensive research on the development of ion exchangers. With the continuous emergence of ion exchangers, the ion exchange method is increasingly demonstrating its advantages in areas such as the advanced treatment of electroplating wastewater and the recovery of precious metal salts. 　　Membrane separation technology is a method that uses a special semipermeable membrane to separate or concentrate solvents and solutes under external pressure, without altering their chemical form in the solution; it includes electrodialysis and membrane electrolysis. Electrodialysis is a physicochemical process in which, under the action of a direct current electric field, the selective permeability of anion and cation exchange membranes to these ions is utilized to separate heavy metal ions from water. Diaphragm electrolysis is a method of electrolysis in which a membrane separates the anode and cathode of the electrolytic cell; it is essentially a combination of electrodialysis and electrolysis. The above methods all encountered problems such as electrode polarization, scaling, and corrosion during operation. 　　3 Biological methods 3.1 Biological flocculation method The biological flocculation method is a pollution removal technique that utilizes microorganisms or the metabolites produced by them to achieve flocculation and precipitation. Microbial flocculants are natural polymers composed of microorganisms themselves and possess high-efficiency flocculation properties; their main components include glycoproteins, mucopolysaccharides, cellulose, and nucleic acids. Since most microorganisms have a certain linear structure, and some of their surfaces possess a high charge or strong hydrophilicity, they can bind to particles through various mechanisms, thereby achieving an excellent flocculation effect. To date, a total of 17 types of microorganisms with flocculation properties have been developed, including bacteria, molds, actinomycetes, yeasts, and algae. Among them, 12 have a flocculating effect on heavy metals. Chen Tian et al. used chitosan extracted from various microorganisms as a flocculant to recover Pb2+, Cr3+, and Cu2+ from simulated industrial wastewater. By adding 10 mg of chitosan to 200 mL of wastewater with an ion concentration of 100 mg/L, the concentrations of Cr3+ and Cu2+ in the solution were reduced to less than 0.1 mg/L, while the Pb2+ concentration was reduced to less than 1 mg/L, yielding satisfactory results. The microbial flocculation method for treating wastewater is safe and convenient, non-toxic, does not cause secondary pollution, offers good flocculation results. It also features rapid microbial growth and ease of industrial implementation. Furthermore, microorganisms can be engineered, domesticated, or used to create strains with special functions. Therefore, the microbial flocculation method has broad development prospects. 　　3.2 Biological adsorption Biological adsorption refers to the general process by which heavy metal ions are absorbed by microbial cells through a series of biochemical reactions, including complexation, chelation, ion exchange, and adsorption. The mechanisms by which these microorganisms separate metal ions from solutions include extracellular enrichment and precipitation ； Surface adsorption or complexation of cells ； Intracellular enrichment. Among these, surface adsorption or complexation occurs in both dead and living microorganisms, whereas significant accumulation inside and outside the cells usually requires that the microorganisms be active. Many studies have shown that both living and dead microorganisms possess a strong capacity to adsorb heavy metal ions. Bio-based materials used as biological adsorbents can remove heavy metals from aqueous solutions containing low concentrations of these ions, and microorganisms with practical utility for this purpose are readily available. For example, yeast during the fermentation process is an excellent biological source for biosorbents, and a large number of algae from the ocean are also inexpensive biological sources. Zhao Ling and colleagues studied the adsorption capacity of the marine red tide organism Prorocentrum micans, both in its living form and in algae killed with formaldehyde, for Cu2+, Pb2+, Ni2+, Zn2+, Ag1+, and Cd2+. Experiments showed that after 30 minutes of adsorption by Prorocentrum micans, the concentrations of these metal ions decreased significantly and reached equilibrium; both the living and dead forms of Prorocentrum micans exhibited similar adsorption capacities for these six metal ions. 　　By using a carrier to pre-treat and fix the microbial adsorbent through physical or chemical methods, the mechanical strength and chemical stability of the adsorbent are enhanced, its service life is prolonged, the depth and efficiency of wastewater treatment can be improved, and losses during the adsorption-desorption cycle are reduced. In recent years, many scholars at home and abroad have conducted research on the use of immobilized cells to treat toxic wastewater containing heavy metals. Bioadsorbents have the advantages of wide availability, low cost, strong adsorption capacity, and ease in separating and recovering heavy metals. Moreover, using dead microorganisms as the biological source facilitates their immobilization, allowing for the creation of specialized bioadsorbents that can be reused as needed. Therefore, biological adsorption has good prospects for industrial application. At present, most wastewater treatment plants in China use the activated sludge process; therefore, in areas where the removal of heavy metals is necessary, it is possible to treat wastewater containing low concentrations of heavy metals by acclimating the activated sludge (while being careful to avoid excessive heavy metals from poisoning it) and by introducing appropriate microorganisms using biological inoculation methods. 　　3.3 Plant remediation techniques The mechanisms by which plants absorb and accumulate heavy metals involve two main aspects: one is the use of the plants’ well-developed root systems to absorb and filter heavy metal-containing wastewater, thereby enabling the accumulation of these metals in the plants. Secondly, by utilizing the active principles of microorganisms and the affinity between heavy metals and microorganisms, heavy metals are converted into less toxic substances. By harvesting or removing the branches of plants that have accumulated and concentrated heavy metals, the concentration of these metals in the soil or water bodies is reduced, thereby achieving the goal of pollution control and environmental restoration. 　　There are many plants that can be utilized in plant remediation techniques, including algae, herbaceous plants, woody plants, and so on. Its main characteristics are a high tolerance to heavy metals and the ability to accumulate them; different plant species have varying capacities to absorb and accumulate different heavy metals, and their tolerance levels also differ. 　　Haoyuntao et al. isolated and screened a strain of Chlorella ellipsoidea with high resistance to heavy metals, and studied the effects of different concentrations of the heavy metals copper, zinc, nickel, and cadmium on the growth of this algae, as well as its ability to absorb and accumulate these heavy metal ions. The results showed that this algae has high tolerance to Zn2+ and Cd2+. The tolerance to the four heavy metals is in the order of zinc > cadmium > nickel > copper. This algae exhibits excellent efficiency in removing heavy metals; after being exposed to concentrations of 15 μmol/L Cu2+, 300 μmol/L Zn2+, 100 μmol/L Ni2+, and 30 μmol/L Cd2+ for 72 hours, the removal rates reached 40.93%, 98.33%, 97.62%, and 86.88%, respectively. It can be seen that this algae can be applied to the treatment of wastewater containing heavy metals. 　　Herbaceous plants that have the ability to adsorb heavy metal ions include Eichhoria crassipes Somis and Typha orientalis Presl, among others. Cattail is an internationally recognized and widely used plant for pollution control, possessing special structures and functions such as fleshy leaves and well-developed palisade tissue. Cattail plants, growing for long periods in wastewater containing high concentrations of heavy metals, develop special structures to withstand such harsh conditions; they can also regulate certain physiological processes to adapt to the toxic effects of pollution. Zhao Wenrui et al. studied the stability of the broad-leaf cattail artificial wetland system in treating the mineral processing wastewater from the Fankou lead-zinc mine in Shaoguan, Guangdong. Results of 10 years of monitoring show that this system can effectively purify lead-zinc mine wastewater. The untreated wastewater contained high concentrations of harmful metals such as lead, zinc, and cadmium; after passing through the constructed wetland, the quality of the water at the outlet improved significantly, with purification rates for lead, zinc, and cadmium reaching 99.0%, 97.0%, and 94.9% respectively. By analyzing the annual and monthly variation trends of its pH and the mass fractions of Pb, Zn, Cd, Hg, and As, it was found that the variations in these parameters in the wastewater after treatment through the wetland were small on both an annual and monthly basis, and all values remained below the **emission standards for industrial wastewater. This indicates that the wetland exhibits high stability in wastewater purification. 　　The use of woody plants to treat polluted water bodies offers advantages such as effective purification, the ability to handle large volumes of water, minimal impact from climate conditions, and low risk of causing secondary pollution, which has led to increasing attention for them. The experimental results conducted by Hu Huanbin and others show that both reed and Chinese fir possess a strong capacity to accumulate the heavy metals lead and cadmium, with the woody plant Chinese fir demonstrating a better purification effect than the herbaceous plant reed. Zhou Qing et al. studied the responses of 5 evergreen tree species to cadmium pollution stress. The experimental results showed that under high concentrations of cadmium, various physiological and biochemical parameters of the leaves of these 5 tree species, such as chlorophyll content, cytoplasmic membrane permeability, catalase activity, and cadmium accumulation levels, underwent significant changes. Among them, boxwood, begonia, and fir exhibited greater resistance to cadmium pollution compared to camphor and holly. Heavy metal wastewater treatment technologies based primarily on woody plants can prevent toxic and harmful substances from entering the food chains of humans and livestock, thus avoiding secondary pollution. These techniques allow for targeted cultivation; while treating pollutants, they also help to improve the environment and generate economic benefits, making them an ideal method for environmental restoration. 　　4 Trends and Prospects in the Treatment of Heavy Metal Wastewater I. Biological methods will become the dominant approach Although chemical, physicochemical, and biological methods can all be used to treat and recover heavy metals from wastewater, biological methods are preferred due to their low cost, high efficiency, ease of management, absence of secondary pollution, and contribution to the improvement of the ecological environment. Furthermore, through the application of technologies such as genetic engineering and molecular biology, organisms can be endowed with enhanced adsorption, flocculation, and remediation capabilities. Therefore, the biological method has broader development prospects. 　　II. Integration of several technologies for treating heavy metal wastewater – Heavy metal wastewater is a type of resource, and many heavy metals are quite expensive. If the heavy metals in wastewater are recovered as a resource, it not only addresses the problem of heavy metal pollution but also brings certain economic benefits. Electrochemical methods can meet these requirements for treating heavy metal wastewater, but since the concentration of heavy metals in such wastewater is generally low, using conventional electrochemical methods results in low current efficiency and high energy consumption. Therefore, to meet the increasingly stringent environmental regulations and achieve wastewater reuse as well as heavy metal recovery, several technologies can be integrated to treat heavy metal wastewater, taking advantage of the strengths of each technology. Tung Chung-Ching et al. achieved significant results by integrating micelle-enhanced ultrafiltration for the removal of copper ions from aqueous solutions. Zhang Yongfeng employed an integrated technology of complexation-ultrafiltration-electrolysis to treat heavy metal wastewater; the concentrate obtained through ultrafiltration could be used for the recovery of heavy metals via electrolysis, thereby achieving both the reuse of wastewater and the recovery of heavy metals, and offering a new approach for effectively dealing with heavy metal wastewater. 　　We should make full use of the synergistic purification effects of microorganisms and plants in nature, supplemented by physical or chemical methods, to find effective ways to purify heavy metals. Strict control and supervision are exercised over the sources of heavy metal pollution, and physical and chemical methods are used to treat wastewater containing high concentrations of heavy metals at those sources, preventing such wastewater from entering the urban drainage system. This reduces governance costs and eases the processing burden on secondary wastewater treatment plants, achieving good economic and environmental benefits. In existing environmental remediation projects, improvements and modifications to the heavy metal treatment processes can be considered in order to address the issues related to those metals. In unfinished environmental remediation projects where heavy metal treatment is necessary, the removal of such metals should be taken into account from the outset of the project, in order to achieve more effective pollution control and environmental restoration.