## 3 Results and Discussion  
### Sorption Isotherms and Sorption Efficiency of Arsenic  
The nanoscale drinking water treatment residuals significantly affected the sorption isotherm of As in calcareous sandy loam soil that was treated with different levels of biosolids (Figure 1). Differences in As sorption capacity were observed between different nWTR-application rates of 0.25, 0.50, and 1.00% (w/w). The initial As concentration significantly affected the As adsorption to the soil treated with both biosolids and nWTRs. When the initial As concentration increased from 5 to 800 mg/L at room temperature, the amount of As sorbed per unit mass of the treated soil also increased. The soil As concentration increase would allow more As molecules to move from soil solution to the surfaces of adsorbents in soil. Thus, the As sorption in soil is highly dependent on the initial As concentration. At a lower initial As concentration, the As mass in solution is relatively small to the amount of available sorption sites, so it was highly sorbed with available sorotion sites [33]. Importantly, if without any nWTR treatments, the soils with or without biosolids treatments had shown moderate affinity for As sorption. With the nWTR treatments, As sorption isotherms changed from S-type to H-type isotherms (Figure 1). In this study, the H-type isotherm became dominant especially at a high nWTR application rate of 1%. Therefore, the quantity of As sorbed by the soil followed a descending order of: 1% nWTR-treated soil, \(>\) 0.5% nWTR-treated soil, \(>\) 0.25% nWTR-treated soil, and \(>\) control soil (or no nWTR treatment) (Figure 1). The nWTR treatments have contributed many more sorption sites such as aluminum, iron, and calcium hydroxides in the soils, in addition to high organic matter contents and high cation exchange capacity of biosolids-amended soils (Table 1).  
The amount of sorbed As in the biosolids-amended (3%) soil was 8000, 66,100, 68,200, and 76,700 mg As/kg or 8, 66.1, 68.2, and 76.7 mg As/g soil when the soil was treated with nWTRs of 0%, 0.25%, 0.5%, and 1% (w), respectively (Figure 1). High specific surface areas of nWTRs (125 m\({}^{2}\)/g) mainly containing Al-, Ca, and/or Fe (hydr) oxides significantly enhanced the anion sorption due to nanoscale nWTR particles and the amorphous nature of metal hydroxides [24]. Arsenic sorption on metal hydroxide surfaces replaces singly coordinated OH\({}^{-}\) groups, and then undergoes a re-arrangement into a more stable binuclear bridge-type bond between cations [34]. The results from our study (desorption rate was about 1%-5% at all treatments for 800 mg As(III)/L-treated soil) generally agree with those of [PERSON] et al. [35] and [PERSON] et al. [24], showing that nWTR-adsorbed As are stable or immobilized over a long time period [26; 36].  
Figure 1: The amount of sorbed arsenic (As) in the calcareous soil amended with biosolids (3%, w/w) and with different levels of nWTRs of 0%, 0.25%, 0.5%, and 1% (w/w) at different equilibrium concentrations of As. Data shown are means and standard deviation (n = 3).The error bars were too small to be visible in the figure.  
Increasing the treatment level of nWTRs from 0.25% to 1% also likely increased the efficiency of As retention in the soil (Figure 2). For examples, at 0.25% nWTRs application rate the removal efficiency significantly (\(p<0.05\)) increased from 67.5% at As concentration 50 mg/L to 82.6% at 800 mg As/L-treated soil (Figure 2). In similar, at 0.50% nWTRs application rate the removal efficiency of As significantly (\(p<0.05\)) increased from 58.8% at As concentration 50 mg/L to 85.3% at 800 mg As/L. In addition, at 1% nWTRs application rate the removal efficiency significantly (\(p<0.05\)) increased from 95.6% at As concentration 50 mg/L to 95.6% at 800 mg As/L (Figure 2).