Aluminum-Impregnated Biochar for Adsorption of Arsenic(V) in Urban Stormwater Runoff

Biochar has recently emerged for the alleviation of various pollutants in urban runoff. However, the adsorption of arsenate (a frequently identified pollutant in urban runoff) with biochar is typically poor. In this study, an aluminum–impregnated biochar was investigated for adsorption of arsenate. Results with synthetic arsenic-polluted urban runoff show that As(V) was bound to amorphous aluminum hydroxide on biochar surface through irreversible inner-surface complexation. At pH 7.6, the adsorption kinetic data followed pseudo-second-order reaction models ($R^2\ge 0.96$), and the Langmuir and Frendulich models both fit well adsorption isotherm data ($R^2\ge 0.99$). The adsorption was little influenced by pH (4.5–8.5) and ionic strength (0–100 mM NaCl) in this study. Although phosphate and dissolved organic matter (DOM) have a potential to suppress As(V) adsorption, their inhibiting effects were insignificant under the study conditions [$0–200\mathrm{\mu g}/\mathrm{L}\mathrm{P}$, $0–40\mathrm{mg}/\mathrm{L}$ dissolved organic carbon, and $500\mathrm{\mu g}/\mathrm{L}$ As(V)]. However, it is likely that the suppression of phosphate or DOM on the As(V) adsorption becomes more significant with the increasing molar ratio of P or DOM to As in urban runoff during the realistic stormwater treatment. Besides As(V), the aluminum-impregnated biochar could effectively remove many other runoff pollutants (i.e., Pb, Zn, Cu, and ${\mathrm{P}\mathrm{O}}_4^3$) in a polluted real urban runoff. Exhausted aluminum-impregnated biochar passed the toxicity characteristic leaching procedure (TCLP) screening, suggesting that the spent adsorbent can be safely disposed of in municipal landfills. This study demonstrates that surface modification with aluminum enriches active surface sites of biochar for arsenate and expands biochar applications in stormwater treatment.