Na2S-modified biochar for Hg(II) removal from wastewater: A techno-economic assessment

Biochar is an effective agent for removing mercury (Hg(II)) from industrial wastewater. However, its cost is usually high which restricts large-scale applications. It is urgent to perform techno-economic assessments for quantifying the cost-effectiveness ratio of biochar material, which would produce cheaper, more effective, and feasible biochar to push it from lab to market. In this work, sodium sulfide (Na2S)-modified biochar (SBC) was produced with a single pyrolyzing step by mixing sawdust with Na2S. Hg removal efficiency, purifying mechanism, and potential costs were compared between SBC and unmodified wood biochar (UBC). Results indicate that SBC prepared at 300 °C (SBC300) had the highest Hg removal efficiency, 96.4 %, which was well described by the pseudo-second-order model. SBC300 had a maximum adsorption capacity of 54.34 mg/g. Incorporation with sulfur-containing functional groups and precipitation on biochar surface accounted for most Hg removal. Techno-economic assessments proved that SBC300 had a high cost-effectiveness ratio, costing USD 1.74/1 g Hg for removing from wastewater, and is an alternative for industrial production and large-scale applications.