Mechanistic insights into Cd(II) and As(V) sorption on Miscanthus biochar at different pH values and pyrolysis temperatures

Abstract

Biochar has received great attention as a biosorbent, but explanations of the underlying sorption mechanisms are still unclear. Here, batch sorption of cadmium (Cd(II)) and arsenate (As(V)) to Miscanthus biochar at different pH values and pyrolysis temperatures and the sorption mechanisms were comprehensively investigated. The maximum sorption capacities for both Cd(II) and As(V) were observed under alkaline conditions. Physisorption was identified as a common sorption mechanism for both Cd(II) and As(V) irrespective of pH; however, inner sphere complexation with acidic functional groups (AFGs) and crystallized precipitation as otavite predominate at higher pH values for Cd(II), while hydrophobic attraction of arsenite and metallic As and electrostatic bridging with multivalent ions at deprotonated AFGs are presumed to be dominant sorption mechanisms for As (V). Inner-sphere complexes of Cd(II) (98.6%) and electrostatic bridging complexes of As(V) (89.5%) were the dominant sorption forms for B400, while inner-sphere complexes (45.9%) and precipitates (50.5%) of Cd(II) and physisorption and hydrophobic interactions of As (63.7%) were abundant. The results challenge the widely held notion that the sorption of anions decreases as pH increases, while that of cations increases with increasing pH. This unexpected phenomenon can be explained by reduction of As(V) and by the difference in the charge densities between As(V) and basic functional groups of the biochar. Such biochar-induced reduction would cause an unexpected risk of exposing human health and ecosystems to reduceable pollutants. These findings contribute to a better explanation for the environmental fate and behavior of inorganic pollutants in biochar applications.