Efficient removal of ammonium in aqueous solution by ultrasonic magnesium-modified biochar

Ammonium is one of the most important nutrients in agricultural wastewater, and its excessive existence will pose a threat to ecosystem and even human life. The cavitation effect generated by ultrasonic waves with huge energy can break the carbon bonds of biochar and thus increase the abundance of functional groups. In order to adsorb ammonium effectively in water, we used ultrasound to improve biochar or magnesium-loaded biochar performance in removing wastewater pollutant. Batch experiments with material characterization and adsorption models were conducted to assess adsorption characteristics. The nano-biochar (NBC) was obtained after the ultrasonic modification, with porosity, cation exchange capacity (CEC), zero point of charge (pHpzc), and functional groups (–OH, C–O, C–O–H, C–H) effectively being improved, indicating OH– and H+ were separated from water and bound to biochar to form functional groups during ultrasonic cavitation. The nano-Mg-biochar (NMBC) also increased the functional group of metal oxides. The ammonium adsorption of NBC and NMBC was dominated by intraparticle diffusion. The observed ammonium adsorption capacity of NBC and NMBC was best described by the Langmuir isotherm, implying the adsorption derived from the energetically homogeneous surface. Moreover, the maximum adsorption capacities for NH4+ on NBC, and NMBC were 13.589, and 23.777 mg·g−1, respectively. Ultrasound improved the maximum ammonium adsorption capacity of raw biochar and Mg-biochar through electron attraction and ion exchange, respectively. Notably, ultrasonic modification causes a more uniform distribution of chemical crystals in the biochar and broken carbon bond to combine with more functional groups, which would be a significant way to reorganize functional groups directionally and optimize the adsorption performance of materials.