Phosphoric Acid-Modified Wood Biochar for Cr(VI) Retention in Aqueous Systems and Sustained Mobility Control Under...

Effective remediation of hexavalent chromium [Cr(VI)] requires materials that perform well not only in aqueous systems but also in heterogeneous soils exposed to fluctuating water flow and environmental disturbance. Here, phosphoric acid-modified wood biochar (P-BC) was evaluated against pristine biochar (BC) through physicochemical characterization, pH-dependent aqueous retention tests, soil water-retention measurements, repeated-infiltration leaching experiments, and accelerated drying-rehumidification cycles. Across the tested pH range (pH 2.0-4.0), P-BC showed slightly higher mean apparent Cr(VI) retention than BC at each tested pH. Batch experiments showed higher measured apparent Cr(VI)-derived retention for P-BC under the tested conditions: at 25 °C and an initial Cr(VI) concentration of 106.06 mg L^-1, the 360-min mean apparent retention values were 8.46 ± 0.03 mg g^-1 for BC and 10.10 ± 0.10 mg g^-1 for P-BC. At 55 °C, the model-derived Langmuir q_max values were 18.08 and 28.18 mg g^-1 for BC and P-BC, respectively. P-BC showed higher late-stage water-retention values, while a 3.5% layered application reduced cumulative Cr(VI) leaching by 82.9% relative to the unamended control. Under the accelerated drying-rehumidification protocol, low mean water-extractable Cr(VI) endpoints were observed at the higher P-BC application rates, with the 2.5% P-BC treatment yielding the lowest mean cycle-12 concentration among the tested P-BC treatments. Taken together, these findings show that phosphoric acid modification was associated with modestly higher aqueous Cr(VI)-derived retention and lower measured Cr(VI) mobility under several dynamic soil conditions. The results further indicate that amendment rate and spatial configuration are important determinants of remediation performance, supporting further evaluation of P-BC as an engineered amendment for aqueous Cr(VI) treatment and soil mobility control.