Removal of ammonium and phosphate from water by Mg-modified biochar: Influence of Mg pretreatment and pyrolysis temperature

Abstract

Poplar chips before and after a magnesium (Mg) pretreatment were pyrolyzed under different temperatures to obtain pristine and Mg-modified biochar samples (i.e., 300/BC, 450/BC, 600/BC, 300Mg/BC, 450Mg/BC, and 600Mg/BC). The biochars were used to evaluate how pyrolysis temperature and Mg modification influenced their capacity to adsorb ammonium and phosphate from eutrophic waters. An increased temperature caused an increase in carbonization degree, a more developed porous structure, and a decrease in the functional groups on biochar. The Mg modification was helpful for the development of a porous structure and the preservation of functional groups. The optimal ammonium adsorption was obtained by 300Mg/BC. This was attributed to the enriched functional groups. The maximum adsorption capacity was 58.6 mg/g. The 600Mg/BC sample exhibited optimal adsorption for phosphate with a maximum adsorption capacity of 89.0 mg/g, which was ascribed to the large surface area, the positively charged surface, and the metal oxides effect. A pseudo-second-order kinetic model and Langmuir-Freundlich isothermal model well described the adsorption of ammonium and phosphate on the modified biochar.