Mg-impregnated rice husk biochar in fixed-bed columns: high-capacity continuous-flow removal of nitrate and phosphate from wastewater

Wastewater contaminated with nitrate and phosphate requires effective removal to prevent eutrophication in water bodies, a process that can lead to significant environmental and economic issues. Magnesium-modified rice husk biochar was prepared, characterized, and used for the removal of nitrate and phosphate from both synthetic and real wastewater through fixed-bed adsorption. The influence of operational variables such as flow rate (5.5–20 mL/min), bed height (4–8 cm), and initial influent concentration (20–100 mg/L) on breakthrough curves was investigated. The results revealed that as flow rate and initial concentration increased, the breakthrough curves became steeper, while their slopes decreased as bed height increased. Adsorption capacities of 9.4 mg/g and 78 mg/g were found for nitrate and phosphate, respectively, for synthetic water. Additionally, the competitive removal of nitrate and phosphate from real wastewater was studied, finding adsorption capacities of 1.4 mg/g for nitrate and 2.4 mg/g for phosphate, with a flow rate of 5.5 mL/min, bed heights ranging from 4 to 8 cm, natural pH (6.2), ambient temperature (20 °C), and respective nitrate and phosphate concentrations of 14.9 mg/L and 3.3 mg/L, being the main finding of this research the application of the studied biochar to real wastewater based on the prior study of the operational variables of the adsorption process in fixed-bed columns. Overall, Thomas, Bohart-Adams, Yoon-Nelson, and Clark models provided good fits for the breakthrough curves, enabling their application for the design and implementation of alternative treatments in wastewater treatment plants.