# Remediation of Heavy Metal Contaminated Farmland Soil by Biodegradable Chelating Agent GLDA
## 1 Introduction  
In recent years, heavy metal soil pollution caused by various activities, including agricultural practices, industrial activities, and waste treatment, has become a problem of worldwide concern [1; 2; 3]. Environmental soil problems are becoming more and more serious in China [4; 5]. In the remediation of heavy metal-contaminated soil, leaching with synthetic extracts, such as aminopolycarboxylic acid chelators (aminopolycarboxylate chelants, APCs), is one effective method of removing heavy metals from the soil. It uses physical processes or both physical and chemical processes to limit pollutants in the soil [6; 7], and it is able to interact with most toxic metals. Common chelating agents include EDTA, EDTS, GLDA, NTA, and HIDS--among which, EDTA is the environmentally durable chelating agent, and the other three are biodegradable chelating agents [8]. EDTA is considered to be the most effective chelator for heavy metal removal [9; 10; 11]. This is because it can form strong complexes with divalent and trivalent metals and alkaline earth metals, such as the macronutrients calcium (Ca) and magnesium (Mg), and trace elements, such as iron (Fe), copper (Cu), zinc (Zn), nickel (Ni), lead (Pb), and cadmium (Cd) [12; 13]. It is often used as a preservative or stabilizer in cosmetics and as a food additive. However, EDTA is not easy for the environment to biodegrade, and it persists for a long time. Moreover, it is highly persistent in aquatic environments and has potential ecological risks [14]. High concentrations of EDTA may be toxic to soil, aquatic organisms, and plants [14]. It can improve bioavailability and phytotoxicity in sewage sludge or contaminated soil by changing the permeability of cell membranes [15]. Therefore, scholars and others are also constantly studying biodegradable chelating agents, and more andmore people are paying attention to the development and use of environmentally friendly chelating agents with better biodegradability and less environmental toxicity [16; 17; 18]. At present, EDDS has become the main alternative product, which degrades easily [18; 19]; however, EDDS is expensive, and the degradation effect is not as good as EDTA [20]. The cost of HIDS is high, and NTA has adverse effects on health [1; 21]. At the same time, the latest research shows that nano-sized adsorbents, such as BFR and SBR, have also been applied in the adsorption treatment of wastewater and soil, which can effectively remove toxic metals from the environment [5]. A new biodegradable, environmentally friendly chelating agent, GLDA (\(N\),\(N\)-bis(carboxymethyl)-L-glutamic acid tetrasodium salt), has attracted attention [22; 23]. It is considered to have the greatest potential to purify soil by in situ washing treatment compared to other biodegradable chelators, and its process is based on green chemistry [24]. The traditional biodegradability test (OECD 301D) shows that its consistent degradability far exceeds the 60% degradation rate that is considered to be easily biodegradable. Biodegradable chelating agents, HIDS and GLDA, can better solve the problem of ecological risks to the environment during the use of EDTA and are often used to recover potentially toxic elements or rare metals from various solid wastes [6]. GLDA is mainly made from plant materials [25]. Different from the production of EDTA, the carbon source of GLDA is mainly biological. Compared with other chelating agents, GLDA has the smallest ecological footprint (a smaller ecological footprint means a smaller impact on the environment) and is easy to biodegrade. In terms of chelating capacity, GLDA is as efficient as EDTA [6] when used to remove heavy metals from soil and sludge [7; 26; 27]. GLDA has good chelation ability for many metal ions and good biodegradability. More than 60% of GLDA can be degraded within 28 days [28]. During soil leaching and remediation, GLDA is often combined with other chelating agents to form a mixed chelating agent [29; 30].