Interactive effect of pH and cation valence in background electrolyte solutions on simazine sorption to Miscanthus biochar produced at two different pyrolysis temperatures

Biochar has considerable sorption efficiency for organic pollutants; however, the effect of physicochemical characteristics and their alteration by environmental conditions on the sorption mechanism is still unknown. We investigated the pH-dependent sorption of simazine on Miscanthus biochar produced at two pyrolysis temperatures (400 and 700 degrees C; hereafter B-400 and B-700) under two different electrolytes and interpreted the sorption mechanism. The surface charge density (SCD) decreased more in B-400 than in B-700 at higher pH due to more deprotonation of acidic functional groups (AFGs), but greater decreases were observed in B-700 than in B-400 from pH 2 to pH 3 as a result of alkali salts deposition. The decrease in K-F with increasing pH showed that simazine sorption decreased as van der Waals forces because the surface of biochar carried a greater negative SCD, which repulses simazine molecules due to the enhanced deprotonation of AFGs. At a given pH, K-F was lower in CaCl2 than in NaCl due to the formation of larger metal-biochar complexes, resulting in enhanced blocking of pores available for simazine sorption. We believe that knowledge of the pH-dependence of SCD and accessibility of biochar pores could help better interpret the behavior of simazine-like pollutants in soil and aquatic environments.