## 3 Results and Discussion
### Effect of nWTRs on As Fractionation in Biosolids-Amended Soil  
Figure 3 shows fractionation changes of As in biosolids-amended soil treated with different levels of nWTRs. The As fractionation result in the control soil was significantly different from those in the soils treated with biosolids and nWTRs. In the control soil, exchangeable As was 4.1%, carbonate-bound As was 22.8%, Fe-Mn oxides-bound As was 8.9%, organic matter-bound As was 8.1%, and residual As was 56.1%. In the biosolids-amended soil, exchangeable As was 8.1%, carbonate-bound As was 22.1%, Fe-Mn oxides-bound As was 12.5%, organic matter-bound As was 30.9%, and residual As was 26.5%. However, when the biosolids-amended soil was treated with nWTRs, As fractionation has been significantly affected (Figure 3). The nWTR treatment of 0.25% significantly increased the residual fraction of As (RS) up to 73.7%, while other As fractions decreased in comparison with the soils with or without biosolids treatments. Further, increasing the nWTR application rate from 0.5% to 1% drastically increased the stable and immobilized residual fraction (RS-As) of As from 82.5% to 87.7%, respectively (Figure 3), due its accumulation in silicates [28].  
The relatively high percentage of carbonate-bound As in the soils was mainly due to high calcium carbonate content (326 g/kg) in the studied soil (Table 1). The overall mobility and potential  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Wavenumber Range, cm\({}^{-1}\)**} & \multicolumn{3}{c}{**The FTIR Spectral of nWTRs/Biosolids-Treated Soil**} \\ \cline{2-5}  & **Before** & **After** & **Difference** & **Functional Groups** \\  & **As-Saturation** & **As-Saturation** & **Difference** & **Functional Groups** \\ \hline
3411-3200 & 3411 & 3412 & 1 & C-H methyl and methylene group \\
3000-2921 & 2921 & 2921 & 0 & Bounded hydroxyl group (OH) \\
1737-1625 & 1622 & 1621 & \(-\)1 & C-H stretching \\
1508-1330 & 1508 & 1508 & 0 & Symmetric binding of CH\({}_{3}\) \\
1508-1330 & 1462 & 1462 & 0 & Carboxyl group \\
1508-1330 & 1320 & 1320 & 0 & \\
1330-1375 & 1374 & Absent & - & C-O stretching of COOH \\
1290-1000 & 1246 & 1245 & \(-\)1 & O-H alcohol and aliphatic ether \\
1290-1000 & 1053 & 1055 & 2 & C-O stretching of COOH \\
800-600 & 780 & Absent & - & stretching vibration of NH\({}_{2}\) \\
800-600 & 608 & 609 & 1 & stretching vibration of NH\({}_{2}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: The FTIR spectral characteristics of 1% nWTRs/biosolids-treated soil before and after As sorption (with initial As concentration of 800 mg/L).  
Figure 3: Percentage of arsenic (As) fractions in the calcareous soil amended with biosolids (3%, w/w) and treated with different levels of nWTRs of 0%, 0.25%, 0.50%, and 1% (w/w). OM: organic matter; RS: residual fraction.  
bioavailability of As in the studied soil amended with 3% biosolids and treated with different levels of nWTRs followed a descending order of: control soil without nWTR treatment, \(>\) 0.25% nWTR treated soil, \(>\) 0.5% nWTR-treated soil, and \(>\) 1% nWTR-treated soil (Figure 3). These results were generally in accordance with previous research findings [37; 38; 39], showing the effect of nWTR treatments on the bioavailability of some heavy metals. It was concluded that nWTRs can significantly reduce the bioavailability of As, Cr, and Hg in contaminated calcareous soil.