Influence of biochar thermal regeneration on sulfamethoxazole and dissolved organic matter adsorption

Biochar is emerging as a cost-effective, environmentally-sustainable adsorbent for removing organic contaminants (OC) from wastewater, stormwater, and drinking water, but strategies for managing exhausted biochar are needed. Here, pine biochar generated at 850 °C was exhausted by background dissolved organic matter isolated from surface water [dissolved organic carbon ∼4.2 mg L^-1, UV-absorbance at 254 nm (UVA₂₅₄) ∼0.10 cm^-1] and sulfamethoxazole (SMX) [∼200 ng L^-1], in a column. Exhausted biochar underwent a semi-oxic-thermal-regeneration step at 600 °C (i.e., heat treatment). SMX and UVA₂₅₄ adsorption capacity and breakthrough were evaluated in rapid small-scale column tests (RSSCTs). Relative to fresh biochar, heat treated biochar that had been exhausted exhibited ∼3.5-fold and ∼3-fold greater SMX and UVA₂₅₄ adsorption capacities, respectively, and ∼3-fold increase in adsorption efficiency (i.e., mass loss-adjusted SMX adsorption capacity). When applying the heat treatment to fresh biochar, a similar improvement in adsorption capacity was observed. Adsorption capacity and BET surface area were positively correlated and continued to increase after a second exhaustion-regeneration cycle, but the adsorption efficiency remained the same due to mass loss. SMX breakthrough correlated with that of UVA₂₅₄, which provides the basis for a rapid, inexpensive method to predict OC breakthrough. Heat-treated biochar's SMX adsorption capacity was ∼20% of activated carbon's. Greater empty-bed-contact times increased the SMX adsorption capacity of heat-treated biochar. These results suggest that thermal regeneration could enhance the economic and environmental sustainability of biochar sorbents.