Abstraction of nitrates and phosphates from water by sawdust- and rice husk-derived biochars: Their potential as N- and P-loaded fertilizer for plant productivity in nutrient deficient soil

Biochars were produced by pyrolyzing the rice husk and sawdust at two temperatures of 300 °C (low-temperature) and 700 °C (high-temperature) and tested for the removal performance for nitrates and phosphates in water via batch sorption experiments. The resulting N- and P-loaded biochars were then applied to soil to evaluate their impact on maize growth. Adsorption kinetics experiments indicated a relatively high rate of adsorption of nitrates (up to 2.469 mg g⁻¹ h⁻¹) onto biochars than phosphates (up to 0.456 mg g⁻¹ h⁻¹). The high-temperature biochar showed greater adsorption capacity (up to 95.42 mg g⁻¹) for nitrates, while low-temperature biochar exhibited more adsorption (up to 47.18 mg g⁻¹) for phosphates. Chemisorption was the dominant mechanism for the adsorption of nitrate onto high-temperature biochars, as indicated by the best fitting of the sorption data to the pseudo-second-order model (R² > 0.96). Phosphate adsorption onto low-temperature biochars was mainly governed by pore diffusion and chemisorption, as predicted from the best fits to the intra-particle diffusion (R² ≥ 0.96) and pseudo-second-order (R² ≥ 0.84) models, respectively. The results of plant growth experiments indicated positive effects on maize productivity in soils amended with N- and P-loaded biochars, which was confirmed by the increase in the fresh and dry biomass weights (up to 803 and 733 %, respectively) and greater uptake of essential nutrients (Zn up to 554 %, Mn up to 954 %, Fe up to 325 %, Ca up to 537 % and Mg up to 531 %) in maize plants as compared to the control.