Concentration-dependent adsorption behaviors and mechanisms for ammonium and phosphate removal by optimized Mg-impregnated biochar

Using biochar as an adsorbent is a promising technique for simultaneous recovery of ammonium and phosphate from wastewater. However, deficiencies in studying the optimal preparation procedures and aqueous-condition-related adsorption mechanisms of biochar limited its large-scale application. In this study, the optimized Mg-impregnated biochar showed excellent Langmuir maximum adsorption capacities of 5.5 and 4.4 mmol/g for ammonium and phosphate, respectively, owing to the well-developed pore structures with high Mg content and the abundant oxygen-containing functional groups. Meanwhile, turning points in the curve of removal efficiency against initial concentration suggested a concentration-dependent adsorption behavior, which was also greatly affected by the solution pH and co-existing substances. Characterizing the adsorption mechanisms of biochar indicated that, when the concentrations of nutrient ions were low, a small amount of ammonium would be released, while phosphate was adsorbed by electrostatic interaction and Mg3(PO4)2 precipitation following the ratio of 1:3. In comparison, when the concentrations of nutrient ions were high, >98% of adsorbed phosphate and >80% of adsorbed ammonium were precipitated together as struvite, while the remaining adsorbed ammonium complexed with oxygen-containing functional groups. This study revealed the discrepant performances of Mg-impregnated biochar under different aqueous conditions, which could be a key influencing factor concerning its real application in nutrient recovery.