## 1 Introduction  
Soil contaminated by heavy metals, due to both natural sources and human activities such as mining industry, waste water irrigation, and sewage sludge applications, can influence animals' ground water and human health via the biological chain [1; 2; 3]. Lead (Pb) exposure has been of major public concern due to its well-established adverse effects on human health (e.g., affecting nervous, skeletal, and enzymatic systems) [4; 5]. In Pb-contaminated sites such as mining and smelting areas, childhood Pb poisoning is also worthy of concern; research indicates that the average soil intake rate for children ranges from 1.2 to 23 mg per day via hand-to-mouth behaviors and inhalation of resuspended soil particles [6; 7]. Therefore, remediation of lead-contaminated soil is an urgent need. Various remediation technologies have been developed over the past few decades, such as physical remediation, chemical remediation, bioremediation, and combined repair, which could be applied to the remediation of lead-contaminated soil [8; 9; 10; 11; 12]. Among them, soil leaching is a quite effective remediation technology that can be widely used in the field of the remediation for heavy-metal-contaminated soils because of its advantages such as stable remediation efficiency, permanent remediation, ease of operation, and short repair cycle [13; 14]. In particular, the key factor of remediation efficiency is the choice of eluting agent, and the repair mechanism and repair capacity vary according to the eluting agent. There are four main kinds of eluting agent, namely inorganic eluting agents, low-molecular-weight organic acids, artificial chelating agents, and biosuffactants [15; 16; 17]. Recently, many studies have researched the leaching remediation of contaminated soil using different eluting agents. As a typical artificial chelating agent, EDTA can form stable complexes with most heavy metals in a higher pH range. The equilibrium constant of the Pb-EDTA complex is 19.0, which could further promote the dissolution of Pb from the soil and inhibit re-adsorption into the soil [18]. A 0.05 mol\(\cdot\)L\({}^{-1}\) concentration of EDTA was used to extract lead from the waste residue of a lead smelter, and the leaching efficiency reached 72% when the leaching time was 1 h [19]. Although EDTA has a high remediation efficiency for lead-contaminated soils, its application prospect is limited due to its poor biodegradability. Due to the high cost and low remediation efficiency, biosurfactants are rarely used solely for remediation of lead-contaminated soil, but is normally used combined with other eluting agents [20; 21]. Inorganic eluting agents are the most frequent eluents used in the leaching remediation of heavy-metal-contaminated soil. Among them, inorganic acid has a strong acid-dissolving effect; moreover, some inorganic acids have complexing ability, so they could effectively and quickly remove heavy metals from soil at a low cost. [PERSON] used HCl to repair soil with a lead-pollution intensity of 105.38 mg\(\cdot\)kg\({}^{-1}\) and found that the remediation effect of low-concentration HCl was poor. When the concentration of HCl was 0.8 mol\(\cdot\)L\({}^{-1}\), a remediation efficiency of Pb of 90.46% was achieved [22]. HCl always had a good remediation effect on soil contaminated with heavy metal. For soil with a lead pollution intensity of 64,195 mg\(\cdot\)kg\({}^{-1}\), the remediation efficiency could reach 83% under 6 mol\(\cdot\)L\({}^{-1}\) HCl, but when the concentration of HCl was 1 mol\(\cdot\)L\({}^{-1}\), the remediation efficiency was only 35% [23]. Although increasing the concentration of HCl will have a better remediation effect on heavy-lead-contaminated soil, it also might destroy the physical, chemical, or biological properties of the soil due to its acidity and corrosive property. Numerous studies have shown that low-molecular-weight organic acids could remove heavy metal ions in contaminated soil via chelation reactions.