## 1 Introduction  
Soil contamination by chromium (Cr) is a serious problem in China [1; 2; 3]. Cr is released into the soil by various industries, including the wood preservation, leather tanning, chromate manufacturing, and electroplating industries [4]. Cr in soil occurs primarily in its Cr(III) and Cr(VI) redox states; Cr(III) is a nutrient for plant growth, whereas Cr(VI) is a dangerous species and human carcinogen [5]. Chemical reduction removes Cr(VI) rapidly and effectively based on the use of reducing agents, such as ferrous sulfate, calcium polysulfide, or sodium bisulfate, followed by precipitation as Cr(OH)\({}_{3}\)[6]. Calcium polysulfide (CaS\({}_{5}\)) and ferrous sulfate (FeSO\({}_{4}\)) are promising reagents that have been used at many Cr-contaminated sites and for chromite ore-processing residue (COPR). The reduction of Cr(VI) with FeSO\({}_{4}\) and CaS\({}_{5}\) (denoted by its average chemical formula, CaS\({}_{5}\)) can be written as follows [6; 7]:  
\[3\text{Fe}^{2+}+\text{H}\text{Cr}\text{O}_{4}{}^{-}+7\text{H}^{+} \leftrightarrow 3\text{Fe}^{3+}+\text{Cr}^{3+}+4\text{H}_{2}\text{O} \tag{1}\] \[2\text{Cr}\text{O}_{4}{}^{2-}+3\text{CaS}_{5}+10\text{H}^{+} \leftrightarrow 2\text{Cr}(\text{OH})_{3}+155+3\text{Ca}^{2+}+2\text{H}_{2}\text{O}. \tag{2}\]  
Many studies have been performed on Cr(VI)-contaminated soils stabilized by FeSO\({}_{4}\) and CaS\({}_{5}\). However, most of them focused on the leachability and content of Cr(VI) and Cr. [PERSON] et al. [8]applied FeSO\({}_{4}\) to reduce Cr(VI) at a contaminated industrial soil site in Italy and found that FeSO\({}_{4}\) successfully lowers the amount of Cr(VI) in the soil. An alkaline digestion test showed that Cr(VI) is almost completely reduced when the Fe(II)/Cr(VI) molar ratio is 30. [PERSON] et al. [9] reported the use of FeSO\({}_{4}\) to treat Cr(VI)-contaminated soil through a column treatment. The Cr(VI) concentrations in the leachate after a toxicity characteristic leaching procedure test (TCLP) range between 0.59 and 0.7 mg/L. [PERSON] et al. [10] found that FeSO\({}_{4}\) is a useful treatment reagent in the in-situ remediation of Cr(VI)-contaminated soils in Switzerland. [PERSON] et al. [11] performed a column treatment of Cr(VI)-contaminated soil treated with CaS\({}_{5}\) and found that up to 99% Cr(VI) was reduced with an injection of CaS\({}_{5}\) at 12 times the stoichiometric requirement. [PERSON] et al. [12] reported that 62% of Cr(VI) is reduced in COPR with CaS\({}_{5}\) addition at twice the stoichiometric ratio. [PERSON] et al. [13] applied CaS\({}_{5}\) in batch studies of highly contaminated soil from a Cr plating facility in Putnam. Redox potential results showed that CaS\({}_{5}\) maintains a highly reducing environment for a prolonged period of time and that the alkaline digestion and synthetic precipitation leaching procedure concentrations are lower than Environmental Protection Agency (EPA) regulatory levels. Although these applications indicated that FeSO\({}_{4}\) and CaS\({}_{5}\) are effective reductants, insufficient information is available about the difference between the FeSO\({}_{4}\) and CaS\({}_{5}\) remediation of Cr(VI) in contaminated soil. Moreover, most works only used the TCLP or alkaline digestion test, and systematic investigations about the remediation capacities of FeSO\({}_{4}\) and CaS\({}_{5}\) on Cr(VI), based on the bioaccessibility and speciation of Cr, are lacking. The toxicity and mobility of heavy metals in soil are not only related to their total content but are also determined to a greater degree by the distribution of their speciation [14]. [PERSON] et al. [15] demonstrated that the availability and extraction effectiveness of heavy metals in soil decrease in the order of acid soluble forms > reducible forms > oxidizable forms > residual forms. This study compared the leachability and stability of Cr(VI)-contaminated soils stabilized by FeSO\({}_{4}\) and CaS\({}_{5}\).