Removal of nitrogen and phosphorus from water by sludge-based biochar modified by montmorillonite coupled with nano zero-valent iron

Biochar prepared by thermal decomposition of sewage sludge is a new adsorbent for sludge resource utilization and aquatic environmental treatment. In this study, sewage sludge was used as raw material for the preparation of pyrolysis sludge-based biochar. In addition, montmorillonite (Mt) and nano zero-valent iron (nZVI) were used to modify sludge-based biochar in different combinations to improve its adsorption capacity of nitrogen and phosphorus in water, and the best modification scheme was selected. The physicochemical properties of the biochar were analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results showed that Mt and nZVI could improve the specific surface area and enrich the surface functional groups. The results on kinetics and equilibrium isotherms showed that sludge-based biochar modified by Mt coupled with nZVI (nZVI@MBC) had the best adsorption effect on NH4+ and PO43-. And its theoretical maximum saturation adsorption capacity for NH4+ and PO43- was 34.84 mg·g-1 and 294.12 mg·g-1, respectively. Our results were in the forefront of existing literature on modified sludge-based biochar. The adsorption mechanisms of nZVI@MBC for NH4+ and PO43- mainly include ligand exchange, electrostatic attraction and ionic bond. In addition, nZVI@MBC can also remove PO43- by chemical precipitation and coprecipitation of iron corrosion products.