Synergistic removal of ammonium and phosphate using ultrasonic Mg-based biochar composite: Mechanism, behavior and green recycling

Efficient phosphate and ammonium removal is crucial to control eutrophication. With the gradual development of green chemical engineering, the reutilization of by-products following nutrient removal is becoming a new hotspot. Magnesium-based biochar composites are envisaged to play a significant role as media in green chemical engineering. In this study, ultrasonic cavitation was used to enhance the adsorption sites of magnesium-based biochar composites, improving their adsorption capacity for ammonium and phosphate, and their ability for green recycling. The resulting ultrasonic magnesium-based biochar composite (UMBC) was characterized for its ability to adsorb ammonium and phosphate and reusability through batch experiments. 1200 W and 60 min (UMBC-5) was demonstrated as the optimized ultrasonic power and time, respectively, for the modification of magnesium-based biochar (MBC) through carbon bond breakage. With increasing ultrasonic power and time (from UMBC-1 to UMBC-5), the removal rate (RR) of phosphate saw an improvement from 93.0% to 96.6%, while the RR of ammonium substantially increased from 23.9% to 50.6%. Both pseudo-first-order and pseudosecond-order kinetic models fitted the ammonium and phosphate adsorption process on UMBC-5 well. Ultrasound greatly affected electrostatic attraction between ammonium and UMBC-5 via improving the surface hydrophilic functional groups. The main forms of NP-loaded UMBC-5 are phosphate (58.4%) and struvite (15.51%, high-quality slow-release fertilizer). It not only efficiently recovered nutrients but also significantly increased plant fresh biomass and soil moisture through the combination of abundant hydrophilic functional groups, achieving the green recycling of adsorbate by-products.