Desorption characteristics of phosphate and ammonium from sludge-based biochar

It is effective to adsorb phosphate and ammonium from water by sludge-based biochar, while the desorption performance has not been studied systematically. Biochar in this study was prepared through the co-pyrolysis of sludge and walnut shells to remove NH4+ and PO43− from water. The desorption characteristics of NH4+ and PO43− from the post-adsorption sludge-based biochar were investigated. The effects of the adsorption condition (concentration of adsorption solution) and desorption conditions (pH value of desorption solution and desorption temperature and time) on desorption performance were examined. Several techniques were performed to characterise the properties of the post-adsorption sludge-based biochar. The adsorption amount of the pure sewage sludge biochar (SBC) for PO43− and the biochar derived from the co-pyrolysis of sewage sludge and walnut shell with the mixing ratio of 3:1 (MBC3-1) for NH4+ were 14.19/ 23.75 mg/g and 9.28/ 16.23 mg/g, respectively, when the concentrations of the adsorbates were 100 and 500 mg/L. The desorption experiments showed that the acidic condition (pH = 2) was beneficial for PO43− and NH4+ desorption. The highest desorption ratio reached 7.58% for PO43− and 2.18% for NH4+. The desorption of PO43− was endothermic, whereas that of NH4+ was exothermic. The desorption amounts of PO43− and NH4+ decreased and increased, respectively, with the increase in desorption time. This study of the desorption characteristics of PO43− and NH4+ in sludge-based biochar provides a theoretical basis for the subsequent utilisation of sludge-based biochar.