Ozone-functionalized acacia wood sawdust biochar for total nitrogen adsorption from pig wastewater

Abstract The growing environmental problem of nitrogen-rich pig wastewater, with total nitrogen (TN) concentrations ranging from 500 to 1,500 mg l −1 and ammonia (NH 4 + -N) concentrations between 50 and 70 mg l −1 , requires innovative treatment solutions. This study examines the effectiveness of ozone-modified biochar (SPO) derived from acacia wood sawdust in removing nitrogen, with an oxygen uptake rate increased from 17.11 wt% to 22.28 wt% at 5 l min −1 . Under conditions (pH 7, 0.2g dosage/50 ml, 270 min contact time), SPO achieves a maximum adsorption rate of 32.38 mg g −1 and a removal rate of 46.39 mg l −1 at an initial TN of 279 mg l −1 . Characterisation by SEM, EDS and FTIR revealed a porous, oxygen-rich surface with functional groups (e.g., -OH, -COOH) that facilitate both chemisorption and multilayer adsorption. The kinetic studies favoured the pseudo-second-order model (R 2 = 0.9310) and the Elovich model (R 2 = 0.9484), while heterogeneous adsorption was indicated by the Freundlich isotherm (R 2 = 0.9612). Machine learning models, including the extreme gradient boosting (XGB) model with an R 2 of 0.998, predicted the removal efficiency and identified adsorption dose (average SHAP = 7.736) and nitrogen concentration (average SHAP = 3.607) as the main drivers. The study shows the potential of SPO as a sustainable adsorbent, confirmed by experimental data and predictive modelling, to mitigate the harmful effects of pig wastewater. SPO demonstrated moderate reusability over five adsorption–desorption cycles, retaining 53.27% of its initial capacity (17.35 mg g −1 from 32.57 mg g −1 ) at an estimated production cost of 0.15–0.25 USD/kg, outperforming commercial zeolite (0.5–1.0 USD/kg) and activated carbon (1–2 USD/kg) in cost-effectiveness for scalable swine wastewater treatment.