## 1 Introduction  
Cadmium (Cd) and lead (Pb) are two typical potential toxic metals (PTMs) [1] that often compound with each other and cause soil pollution. PTMs firstly accumulate in the soil, and then lead to deterioration of the soil ecosystem and a decline in crop yields and quality [2]. Finally, it enters human body through the food chain and thus endangers humans' physical health [3; 4]. As a noxious carcinogenic pollutant, Cd is classified as a category I human carcinogen [5; 6]. Numerous studies on the health effects of Cd showed that its adverse effects on the bones and kidneys [7; 8; 9; 10]. Pb is a potent human neurotoxin which when accumulated in the blood results in neurocognitive damage including intelligence quotient (IQ) deficits, behavioral problems and, at very high levels, death [11]. Therefore, it is particularly important to prevent PTMs from getting into plants so as to ensure food safety. The toxicity of PTMs not only has a closed relation with their total amount, but also involves their speciation in soil [12]. The biological effectiveness of PTMs greatly depends on its speciation [13]. Once in the soil, PTMs are adsorbed by initial fast reactions (minutes, hours), followed by slow adsorption reactions (days, years) and are, therefore, redistributed into different chemical forms with varying bioavailability, mobility, and toxicity [14; 15]. This distribution is believed to be controlled by reactions of PTMs in soils such as (i) mineral precipitation and dissolution, (ii) ion exchange, adsorption, and desorption, (iii) aqueous complexation, (iv) biological immobilization and mobilization, and (v) plant uptake [16]. According to [PERSON] et al. [17] and [PERSON] et al. [18], soil in situ stabilization technology is used to reduce the activity of PTMs by adding some kinds of material to the soil contaminated by PTMs, which can transform these metals into a form that is difficult to exploit through a series of chemical reactions. By doing so, the biological effectiveness of PTMs can be reduced. [PERSON] et al. [19] reported that 10-40 t/hm\({}^{2}\) biochar dosage can significantly reduce the percentage of Cd inside wheat seed by 24.8%-44.2%, improve soil fertility [20], and reduce the proportion of CaCl\({}_{2}\) leaching Cd in soil by 10.1%-40.2%. Using the _Miscanthus_ straw to make biochar to remediate Cd, Pb, and Zn in soil, the percentage of the bioavailability of Cd and Pb decrease by 71% and 92% respectively [21]. Moreover, biochar can also induce root growth of plants [22; 23; 24; 25]. The stabilizers of ferric salt such as FeSO\({}_{4}\), Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\), and FeCl\({}_{3}\) are commonly used in soil stabilization [26; 27]. The iron compound has been widely recognized as an antidote that can effectively reduce the mobility of As [28]. However, studies on the stabilization effect of Cd and Pb in soil by iron-based materials are relatively few and the results are divided. Studies have found that ferrihydrite can enhance the adsorption capacity of Cd in soil with the help of Fe(III)-reducing bacteria, and it is also possible to produce a mixed mineral phase of CdCO\({}_{3}\)-FeCO\({}_{3}\)-CaCO\({}_{3}\)[29; 30]. [PERSON] et al. [29; 31] use phosphate, FeSO\({}_{4}\) and Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) to stabilize Pb and As in soil, as they find that Pb and As cannot be stabilized by solely used one of the stabilizers. The percentage of the exchangeable content of Pb and As can only be reduced by using two stabilizers together. However, other studies have found that ferric salt can increase the dissolution of Cd and Pb in soil. [PERSON] et al. [32] found that ferrous sulphate fertilization significantly increased Cd concentrations in rice grains. [PERSON] and [PERSON] have shown that mixing ferrous sulfate with lime can increase the uptake of Cu, Zn and Pb by plant [33]. At present, many organic materials such as animal excrement, plant straw, etc.