Removal of Nitrate and Phosphorus in Aqueous Solutions by MgO-Modified Biochars: Adsorption Isotherms

Excess nutrient loading is a global challenge affecting water quality. Biochar has emerged as an effective tool for reducing nutrient loads through adsorption, whether as a soil amendment or an edge-of-field treatment. This study evaluated biochar derived from two commonly available agricultural byproducts, walnut and pistachio shells, for their capacity to remove nitrate and phosphorus from aqueous solutions. To enhance their nutrient removal potential, the biochars were chemically modified with magnesium oxide (MgO), allowing for a comparative assessment of unmodified and modified biochars. Batch experiments revealed that unmodified walnut and pistachio biochars did not adsorb phosphorus; instead, they released phosphate into the solution, increasing the concentration by 0.019 mg/g and 0.017 mg/g, respectively. In contrast, MgO-modified biochar effectively removed phosphorus, with modified walnut biochar demonstrating a maximum phosphorus adsorption capacity of 20.40 mg/g compared to 18.80 mg/g for modified pistachio biochar. For nitrate removal, unmodified biochar performed better, with walnut and pistachio biochars showing adsorption capacities of 6.94 and 5.88 mg/g, respectively. Adsorption behavior was evaluated using the Langmuir and Freundlich isotherms. The Langmuir model generally provided the best fit, except for MgO-modified pistachio biochar, where the Freundlich model described the data more accurately. This finding provides additional insight into the adsorption process and highlights the contrasting effects of MgO modification on nitrate and phosphorus removal, offering a more accurate model for describing nutrient removal.