Nitrate sorption to biochar following chemical oxidation

Biochar amendments can reduce nitrate (NO₃) leaching in agricultural soil. It has been hypothesized that functional groups on the biochar surface from oxidation can increase NO₃ sorption. This study evaluates the effect of chemical oxidation of biochar on NO₃ sorption characteristics. Eight biochars, made from wood and corn cobs, underwent sodium hypochlorite (NaClO) and hydrogen peroxide (H₂O₂) oxidation and then assessed for NO₃ sorption capacity using batch isotherm methods. The unoxidized and oxidized biochar produced at low temperatures (400 °C) had no significant NO₃ sorption. Oxidized biochars produced at higher temperatures (600 °C and 700 °C) had calculated maximum NO₃ sorption capacities (S_max) ranging from 0.50 to 3.97 mg NO₃-N g^-1. Biochar oxidations with 50 mmol NaClO g^-1 (N50) in combination with an acid wash (AW) had the largest estimated sorption capacities of 3.68, 3.97, and 1.46 mg NO₃-N g^-1 for CT_N50,AW, BW3_N50,AW, and CC3_N50,AW, respectively. Sorption capacity of wood-based biochars was higher than corn cob biochars due to increased oxidation as measured by total acid group content (TAGC). Wood biochar Smax values were correlated with ΔTAGC (R²= 0.86), with a slope of 1.2 μmol NO₃-N μmol TAGC^-1 suggesting that cationic bridging of NO₃ to oxidized sites is the primary mechanism for NO₃ sorption.