Iron-modified barley straw biochar for nitrate and phosphate removal from water

Excessive nitrate and phosphate in water pose serious environmental and health risks, requiring effective and sustainable removal methods. This study investigates the efficiency of iron-modified biochar derived from barley straw (Fe-BSBC) for removing these pollutants from water. The influence of contact time, pH, adsorbent dosage, and competing anions on adsorption performance was tested in batch experiments. At pH 6 and 23 ± 1 °C, with an initial adsorbate concentration of 15 mg/L and adsorbent dosages of 5 g/L for phosphate and 15 g/L for nitrate, equilibrium was achieved within 8 h for phosphate and 24 h for nitrate. Fe-BSBC demonstrated adsorption capacities of 13.7 mg/g for phosphate and 2.0 mg/g for nitrate, outperforming most of the previously reported biochar adsorbents. Isotherm modelling indicated that the Sips model best described the adsorption process, suggesting multilayer and heterogeneous adsorption. Predicted maximum adsorption capacities were 22.0 mg/g for phosphate and 4.07 mg/g for nitrate. Kinetic data aligned with the pseudo-second-order model, indicating chemisorption as the primary mechanism. Electrostatic attraction was identified as the main mechanism for nitrate adsorption, evidenced by a decrease in zeta potential after nitrate uptake and supported by FTIR, EDS, and XRD characterisation. Conversely, phosphate removal was mainly driven by ligand exchange, leading to the formation of Fe-O-P complexes, alongside electrostatic interactions. Overall, Fe-BSBC presents a cost-effective and scalable water treatment solution that supports the Sustainable Development Goals.