## 3 Test Methods  
The soil cation exchange capacity (CEC) and MnO\({}_{2}\) content were determined using standard methods [24]. Soil acid digestion was performed to determine the Cr and Mn content in soil according to EPA Method 3050B [25]. Nitrogen adsorption-desorption measurements were determined by a surface area analyzer (Nova 1000e, Quantachrome Instruments, USA). The chemical composition of the samples was measured by an X-ray fluorescence (XRF) scan. The size distribution of the waste particles was measured using a Malvern MS3000 laser diffraction particle size analyzer. pH values for all soil were measured as per ASTMD4972-01 [26]. The toxicity characteristic leaching procedure (TCLP) of Cr was conducted as per USEPA Method 1311 [27].The bioaccessibility test was performed according to the U.S. EPA (2007) protocol [28] and the British Geological Survey [29]. The Cr(VI) content of contaminated soil was measured using the USEPA Method 3060A alkaline digestion method [30].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Items** & **Raw Soil** & **Chromium-Contaminated Soil** \\ \hline \multirow{6}{*}{Physicochemical properties} & Water content/\% & 20.78 & — \\  & pH & 8.53 & 7.76 \\  & Specific gravity & 2.72 & 2.79 \\  & Liquid limit/\% & 41.63 & 40.18 \\  & Plastic limit/\% & 21.84 & 21.33 \\  & Mn (mg/kg) & 798.36 & 797.48 \\  & C.E.C (near/100 g) & 9.12 & 9.87 \\ \hline \multirow{3}{*}{Mechanical properties} & Optimum moisture content/\% & 19.53 & 18.95 \\  & Maximum dry density/(g/cm\({}^{3}\)) & 1.72 & 1.73 \\  & Brunauer-Entnet-Teller specific & & \\  & surface area (m\({}^{2}\)/g) & 30.74 & 29.62 \\ \hline \multirow{3}{*}{Grain-size distribution (\%)} & Clay content (\(c\)0.005 mm) & 4.62 & 3.23 \\  & Silt content (0.005–0.075 mm) & 74.29 & 71.76 \\  & Sand content (0.075–2 mm) & 21.09 & 25.01 \\ \hline \multirow{6}{*}{Chemical composition (\%)} & Al\({}_{2}\)O\({}_{3}\) & 22.12 & 21.67 \\  & Si\({}_{2}\)O\({}_{2}\) & 64.2 & 64.37 \\ \cline{1-1}  & K\({}_{2}\)O\({}_{3}\) & 2.78 & 2.85 \\ \cline{1-1}  & CaO & 1.43 & 1.42 \\ \cline{1-1}  & TiO\({}_{2}\) & 0.84 & 0.86 \\ \cline{1-1}  & MnO & 0.12 & 0.13 \\ \cline{1-1}  & Fe\({}_{2}\)O\({}_{3}\) & 8.51 & 8.59 \\ \cline{1-1}  & Cr\({}_{2}\)O\({}_{3}\) & — & 0.11 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physicochemical and mechanical properties of raw soil and chromium-contaminated soil.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Test No.** & **Reductant** & **Dosage (\%)** \\ \hline
1 & FeSO\({}_{4}\) & 0 \\
2 & FeSO\({}_{4}\) & 1 \\
3 & FeSO\({}_{4}\) & 3 \\
4 & FeSO\({}_{4}\) & 5 \\
5 & CaS\({}_{5}\) & 0 \\
6 & CaS\({}_{5}\) & 1 \\
7 & CaS\({}_{5}\) & 3 \\
8 & CaS\({}_{5}\) & 5 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Experimental design for the stability study.  
The Cr(VI) concentration in the filtrate was measured using U.S. EPA Method 7196A colorimetric analyses [31]. The modified European Community Bureau of Reference (BCR) sequential extraction procedure was conducted as per the method recommended by [PERSON] et al. [32]. The sequential extraction procedure consisted of four steps, which corresponded to the exchangeable, reducible, oxidizable, and residue fractions.