Soil is one of the resources that humans rely on for survival, and it is the foundation of the ecological environment important components. In the process of transforming nature, especially with the rapid development of industry and the expansion of global trade, the chemical industry has achieved rapid development. Subsequently, the environmental damage caused by chemical pollution has become increasingly severe. The problem of heavy metal pollution in soil has become particularly prominent. In foreign countries, bone pain caused by excessive cadmium levels and Minamata disease caused by excessive mercury levels directly threaten people's lives and health safety. The "Hunan Cadmium Rice Incident" that occurred around 2013 in Guangdong, China also sounded an alarm for people. On May 28, 2016, the State Council implemented the "Soil Pollution Prevention and Control Action Plan" (Article 10), which explicitly stipulated that pollution control and remediation should be carried out to improve the quality of regional soil environment. Especially in areas with heavy metal soil pollution, such as Taizhou City in Zhejiang Province, Huangshi City in Hubei Province, Changde City in Hunan Province, Shaoguan City in Guangdong Province, Hechi City in Guangxi Zhuang Autonomous Region, and Tongren City in Guizhou Province, priority areas should be established. Give full play to the leading role of the government, establish special funds for soil pollution prevention and control by the central and local governments at all levels, and increase support for soil pollution prevention and control work. Since the 1990s, green chemicals and related technologies have made great progress in the treatment of heavy metal pollution in soil, with prominent research related to the derivatives of aspartic acid, polyaspartic acid (PASP) and iminodisuccinic acid (IDHA).

1. Biomimetic Synthetic Chemicals - PASP

PASP is a water-soluble synthetic protein that naturally exists in the mucus of marine shellfish such as oysters. The PASP structure is free of numerous carboxyl and amino groups, with asymmetric α、β Two configurations, a versatile and environmentally friendly multifunctional biopolymer material with a wide range of applications. Widely used for plant nutrition supplementation, fertilizer efficiency enhancement, dispersed scale inhibition in the water treatment industry, soil heavy metal treatment, etc. In numerous application fields, PASP promotes crop growth. Most importantly. Given the unique chelating and dispersing metal ions function of PASP, the use of polyaspartic acid salts in the treatment of soil heavy metal pollution is increasingly valued by many researchers. At present, research on polyaspartic acid salts in this field mainly focuses on chemical and biological treatment methods.

1.1Chemical Treatment Law

The chemical treatment method for heavy metal pollution in PASP soil refers to the method of utilizing the characteristics of PASP chelating metal ions, combining them with heavy metal ions, and then using leaching or extraction methods to separate PASP heavy metal chelates from the soil, thereby removing heavy metals from the soil. When PASP is used for soil heavy metal pollution control, it is less affected by environmental pH. Cao Zhenyu's research shows that when PASP is applied to the oscillatory leaching treatment of contaminated soil, the removal rate of heavy metals by polyaspartic acid salt is higher in a lower pH environment, especially at pH 1. In a study on heavy metals in sludge from Taopu Sewage Treatment Plant in Shanghai, researchers found that PASP has good extraction performance for various heavy metals in sludge at moderate acidity. However, researchers have different opinions on the types of heavy metals that can be activated by PASP, but they can demonstrate the richness of their chelated heavy metal types from a side perspective. Zhang Hua found that by working together with hydrogen peroxide, PASP can effectively extract Zn, Ni, Cu, as well as some Cd and Cr from the sludge. Fang Yifeng et al. found through research that PASP has a good extraction effect on heavy metal ions Cd, with extraction rates exceeding 50%, and the higher the amount of PASP used, the better the extraction effect. Wen Dongdong believes that PASP can effectively remove Pb from soil, but its enhancement effect on Cu and Cr removal is not significant; The main reason for this conclusion is that PASP promotes the transformation of heavy metal Cu and Cr forms in soil, resulting in poor mobility and affecting its extraction efficiency.

1.2Biological Governance Law

The biological treatment method for heavy metal pollution in PASP soil refers to using PASP as an auxiliary means for biological treatment of heavy metal pollution in soil. By utilizing the regulatory effect of PASP on biological enzymes in the crop or the improvement effect of PASP on soil, PASP can combine with metal ions such as Fe, Zn, Mn in the soil to form exogenous biological enzymes for crops, thereby promoting the improvement of crop yield and quality, and enhancing the absorption of heavy metals by crops, Thus, it is a method for controlling heavy metals in soil. As a widely used synergistic agent in China, PASP has an unquestionable effect on crop growth, which provides inspiration for researchers in heavy metal treatment research.

Xu Li's research shows that PASP can promote the growth of vetiver grass, increase the chlorophyll content of vetiver grass, strengthen plant photosynthesis, especially under low concentration Cu conditions. PASP can promote vetiver grass growth and to some extent alleviate the damage of Cu to vetiver grass tissue. Zhang Xin et al. found that within a certain concentration range, the activation ability of PASP for Pb and Cd increases with the increase of PASP concentration; At the same time, it was found in pot experiments that PASP has a significant strengthening effect on the remediation of heavy metal contaminated soil by corn. Xu Weiwei et al. believe that the sharing of PASP and FeCl3 has a good effect on Cd pollution, and unlike other chemical agents, the use of PASP treatment can significantly improve crop biomass growth. Dou Qiaohui found that under Cu and Cd stress, the application of polyaspartic acid salt in tomatoes can not only balance plant nutrition, improve enzyme activity in organisms, promote crop growth, but also improve tomato quality, reduce the absorbable content of Cu and Cd, which is beneficial for the management of soil heavy metal pollution. 

2. Green chelating agent - IDHA

Chelating agents are one of the most widely used chemicals, covering almost all industries such as pharmaceuticals, chemicals, textiles, daily chemicals, papermaking, food, leather, rubber, agriculture, oil fields, mining, soil treatment, etc. Traditional chelating agents mainly include ethylenediaminetetraacetic acid and its salts (EDTA), hypoaminotriacetic acid and its salts (NTA), diethylenetriaminepentaacetic acid and its salts (DTPA), citric acid, tartaric acid, etc; Among them, EDTA has become the most widely used chelating agent due to its excellent chelating ability and excellent cost-effectiveness. However, the EDTA production process is severely polluted and difficult to degrade in the natural environment, which can cause serious environmental pollution and may cause the leaching of heavy metal substances into the groundwater system after application, thereby posing a certain risk to human health. In addition, wastewater containing EDTA will carry harmful metals from underwater sludge into the water body after discharge, causing new hazards to human and ecological health; Therefore, the European Union has issued relevant regulations requiring the concentration of EDTA in rivers to be between 10 and 100 μ G/L, with a concentration of 1-10 in the lake μ G/L is the most stringent requirement among all artificial compounds. With the strengthening of environmental awareness, people are gradually beginning to take action on this. EU Directive 1999/476/ECL187/52 explicitly prohibits the use of EDTA in multiple industries such as food, medicine, and textiles. At the same time, it restricts its use in the washing industry and gradually strengthens research on green chemicals. In just a few years, many new types of chemicals with chelating properties have emerged worldwide, with IDHA being a representative of them. IDHA has relatively stable chemical properties and can maintain good stability in strong acid and alkali media. Compared with EDTA, it has two prominent characteristics: (1) it has a tetracarboxylic acid ligand structure, moderate chelating ability, and is easy to achieve chelation and de chelation of metal ions. The chelation constant for general metal ions is slightly lower than EDTA, but some ions such as Cu2+have higher chelation constants than EDTA; (2) Non toxic, harmless, clean production process, easily biodegradable, and can be completely decomposed into biodegradable amino acids and succinic acid. At present, this chemical has gradually been applied in various fields such as agriculture, printing and dyeing, papermaking, daily chemicals, water treatment, and heavy metal pollution. The reports on the remediation of heavy metal pollution in soil by IDHA mainly focus on biological and chemical treatment methods.

2.1Biological Governance Law

Liu Xiaona believes that IDHA (salt) treatment of corn plants significantly increases the Cd concentration in the aboveground parts compared to the blank control and EDTA treatment, and also significantly improves the Cu concentration in the aboveground and root parts compared to the blank control and EDTA treatment, which helps to accelerate the management of heavy metals in soil. Tian Haoqi has demonstrated through experiments that IDHA (salt) can activate fixed As and Cd in soil, promoting plant absorption of heavy metals.

2.2Chemical Treatment Law

The chemical treatment method has the characteristic of quickly removing heavy metals from contaminated soil, which is widely used and can completely solve problems. However, how to effectively separate chelating agents from heavy metals and recycle them is a challenge.

Through continuous research, researchers have found that new IDHA has the potential to solve the aforementioned problems: (1) IDHA has high chelation efficiency. According to research, the extraction efficiency of IDHA (salt) for Cd in power plant sludge under certain conditions is 68%. At the same time, under the condition of adding 1.2% phosphoric acid, the extraction efficiency of IDHA for Cu and Ni in sludge is significantly improved, with extraction rates exceeding 90%. Duan Gaoqi found through research that IDHA has a good removal effect on heavy metals in power plant sludge, especially when the total molar ratio of IDHA to heavy metals is 8:1 and a small amount of H3PO4 is added, the removal effect is the best. (2) IDHA is easy to elute and achieve separation. Hu Xiaojun regards IDHA as the main component of environmentally friendly leaching solution. Under neutral soil acidity conditions, IDHA has good elution ability for heavy metals in the soil, with single leaching removal rates above 90%. It can efficiently elute heavy metals in the soil, and found that IDHA can be completely degraded by microorganisms in the environment without causing pollution. It is an ideal environmentally friendly heavy metal soil remediation leaching substance. (3) IDHA can change the existing forms of heavy metals and has the potential to fundamentally solve the pollution of heavy metals. Wang Guiyin et al. found through research that IDHA can effectively remove heavy metals from contaminated soil and reduce the environmental risk of residual heavy metals. It can reduce the residual amounts of water-soluble, exchangeable, and carbonate bound Cd, Pb, and Zn in soil. Chen Chunle et al. also obtained similar results.

3. Prospection
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Compared with existing soil heavy metal ion chelating agents, PASP and IDHA have their unique characteristics: (1) these two substances have moderate chelating ability and are easier to separate from heavy metal ions in later treatment; (2) These two substances are easy to degrade, and the degraded product is a mixture of aspartic acid and maleic acid, which can be used by crops or microorganisms without residues and will not cause organic pollution to the soil; (3) These two substances have a biological promoting effect and can be used as auxiliary means for controlling soil heavy metal pollution; (4) Among these two substances, the chemical method function of IDHA may be superior to the biological method function, while PASP is the opposite. Through relevant research, the combination of different remediation methods can effectively improve the efficiency of heavy metal pollution control, such as the mixed remediation method of microbial remediation agent and chemical remediation agent, the biochar material bioremediation method (Yatuocao), the highly active zeolite microbial remediation method, and the microbial (Aspergillus flavus) plant (Ryegrass) remediation method.

Therefore, the author believes that the combination of the above products can organically combine biological and chemical methods, which not only reflects the fast and efficient nature of chemical methods, but also reflects the safety and green nature of biological methods, and can form a new form of governance for biological chemical methods. The author believes that the application of PASP and IDHA in the treatment of heavy metal pollution in soil can be attempted through a biochemical method, which means that after the two are used together, the biological efficiency of PASP and the chemical extraction efficiency of IDHA can be utilized to jointly promote the treatment of heavy metal pollution. Although some of the reports mentioned in this study are still in the research stage, with the implementation of relevant regulations in national policies such as the "Soil Ten Principles" and the enhancement of support, the prospects of using PASP and IDHA to treat heavy metal contaminated soil will become better and better.