MgO-Biochar for Selective Orthophosphate Removal from Swine Wastewater: Optimization and Environmental Assessment

Excess dissolved reactive orthophosphate (DRP) from swine wastewater drives eutrophication, yet its selective removal remains challenging. This study develops a magnesium oxide (MgO)-modified pine biochar for efficient DRP capture and recovery. MgO-PB, CaO-PB, and MgO/CaO-PB were produced by co-pyrolyzing <1 mm pinewood with 2% Mg, Ca, or combined Mg/Ca hydroxides at 450 °C. All Mg-containing biochars exhibited high DRP removal capacities (∼20–24 mg/g) across three simulated orthophosphate systems (10–100 mg/L, pH 5–9), achieving up to ∼99% removal. MgO-PB outperformed MgO/CaO-PB, with a Langmuir maximum capacity of 36.34 mg/g for the mixed H2PO4–/HPO42 system. Removal was then optimized directly in pretreated real swine wastewater by central composite design and validated in quintuplicate. Under the optimum (0.3 g, pH 6.82, 6.5 h), the predicted removal (91.44%) was confirmed experimentally (93.04 ± 0.77%, n = 5), reducing DRP from 28.36 mg/L (after pretreatment of the raw wastewater from 79.63 mg/L) to 1.97 ± 0.22 mg/L, an overall reduction of 97.5%. Mg leaching was limited (∼9%), and the PAH content (1.40 × 10–3 mg/kg BaP-equivalent) was below the US EPA soil screening level, supporting the phosphate-loaded adsorbent as a potential material for soil amendment, pending long-term stability and agronomic confirmation.