In this study, arsenic (As) adsorption in 2 calcareous soils with different properties (A and B) under amendment with sewage sludge (SS) and its biochar was investigated. The spectroscopic analysis showed that SS and its biochar have a rough surface and several organic functional groups. The kinetic study demonstrated a rapid initial uptake rate of As in the first 4 h, followed by a slower adsorption rate that reached equilibrium after 16–64 h in different treatments. When SS, native biochar, and modified biochar were added to soil A, which had a lower organic matter and cation exchange capacity (0.64% and 12 Cmolc kg−1, respectively), the percent of As adsorption after 88 h was 13.16, 13.74, and 15.12, respectively while the corresponding values for soil B, which had a higher organic matter and cation exchange capacity (3.52% and 16.9 Cmolc kg−1, respectively), were 14.76, 17.36, and 24.00, respectively. The cumulative As adsorbed (qe, ex) was in the range of 56.7–120.0 mg kg−1 with an average of 76.9 mg kg−1, while the predicted amounts by pseudo-second-order equation (qe, cal) were in the range of 59.0–120.6 mg kg−1 with an average of 78.0 mg kg−1. This indicates that the pseudo-second-order equation describes the kinetic adsorption of As in control and amended soils well. The distribution coefficient (Kd) values of As adsorption varied from 1.65 l kg−1 in the control soil A to 6.14 l kg−1 in soil B amended with modified biochar. According to maximum adsorption (Qmax) predicted by the Langmuir equation, soil B amended with modified biochar had the highest capacity for As adsorption (177.03 mg kg−1). Modification of the biochar increased the As adsorption in soils A and B from 87.45 to 98.30 mg kg−1 and from 119.3 to 177.03 mg kg−1, respectively. High capacity of soils in As adsorption could be due to the presence of iron oxides in the SS. The main innovative aspect of this study is application of SS biochar as a soil amendment in As-affected lands. The findings of this research can be useful for farmers and environmental organizations.s.
