The efficacy of Pb, As(V) and Sb(III ) removal by biochar is determined by solution chemistry

Biochars (BC) are cost-effective and sustainable sorbents to clean up waters polluted with metal(loid)s. Understanding the influence of water chemistry is critical in identifying processes that limit metal(loid) removal. To address this, we investigated the removal of lead [Pb], arsenate [As(V)], and antimonite [Sb(III)] using BC in the presence of various solution constituents. A design of experiments approach was used to investigate sorption for each metal(loid)-BC setup (Pb with a straw BC, As(V) with charred wood-dolomite and Sb(III) with a steam-activated wood BC) with twenty-five different background solutions varying in calcium (Ca), natural organic matter (NOM), phosphorus (P), and iron [Fe(III)] content. Background solution composition affected removal of Pb (29 to 100%) more strongly than that of As(V) (37 to 92%) and Sb(III) (20 to 70%), with the selected BC at the metal(loid) concentrations studied. Pb removal was associated with Fe(III)–NOM–Ca organo-mineral phases for solutions containing Fe(III), NOM and Ca. As(V) sorption was enhanced by Ca due to cation-bridging and reducing the competition for sorption sites by NOM and P in high NOM and/or P containing solutions. Sb(III) sorption was hindered by oxidation to Sb(V) through redox active moieties in the BC in all solutions. Sb(III) removal decreased in the presence of high Fe(III), because Fe(III)/Fe(III)–NOM phases blocked accessibility to sorption sites in the highly porous BC, and/or due to enhanced oxidation of Sb(III) to the more mobile (but less toxic) Sb(V). Ideally, the design of BC sorbents for the removal of metal(loid)s from contaminated waters should a priori consider complex solution compositions.