Elevated or unnatural levels of arsenic (As) and phosphorus (P) concentrations in soils and waterbodies from anthropogenic sources can present significant hazards for both natural ecosystems and human food production. Effective, environmentally friendly, and inexpensive materials, such as biochar, are needed to reduce mobility and bioavailability of As and P. While biochar features several physicochemical properties that make it an ideal contaminant sorbent, certain modifications such as mineral-impregnation can improve sorption efficiencies for targeted compounds. Here, we conducted sorption experiments to investigate and quantify the potential utility of magnesium (Mg) for improving biochar sorption efficiency of P and As. We synthesized a Mg-modified walnut shells-derived biochar and characterized its ability to remove As and P from aqueous solutions, thereby mitigating losses of valuable P when needed while, at the same time, immobilizing hazardous As in ecosystems. SEM-EDX, FTIR and elemental analysis showed morphological and functional changes of biochar and the formation of new Mg-based composites (MgO, MgOHCl) responsible for improved sorption potential capacity by 10 times for As and 20 times for P. Sorption efficiency was attributed to improved AEC, higher SSA, chemical forms of sorbates and new sorption site formations. Synthetized Mg-composite/walnut shell-derived biochar also removed >90% of P from real samples of wastewater, indicating its potential suitability for contaminated waterbody remediation.

The recovery of sludge produced in the wastewater treatment process in WWTPs is often limited by the high content of toxic forms of contaminants of both an inorganic and organic nature. One of the options for the effective treatment of the world’s ever-increasing quantities of sewage sludge is the pyrolysis process. Thermochemical conversion of sewage sludge is emerging as a promising method for treating these heterogeneous and highly complex wastes with increasing research work. Pyrolysis-treated sewage sludge (PM) prepared at 603–615 °C was characterized by pH, EC, and CHN-S analysis; total and bioavailable concentrations of P and heavy metals (Cd, Cu, Fe and Zn); fractionation of bound forms of P and heavy metals in the material and determination of the presence of polycyclic aromatic hydrocarbons (PAHs). The studied material was subjected to ecotoxicological tests (Daphnia pulex L.) and cultivation tests (Lactuca sativa L.). Elemental analysis revealed the concentrations of heavy metals in PM: Fe (137,600 mg/kg), Zn (2602 mg/kg), Cu (582 mg/kg), Cr (107 mg/kg), Pb (87 mg/kg), Ni (67 mg/kg), As (<1 mg/kg), Hg (<2 mg/kg) and Cd (<1 mg/kg). The highest values of extractability of the investigated heavy metals from PM were found in the cases of Zn (HCl) and Fe (Mehlich 3), both values not exceeding 500 mg/kg. BCR sequential extraction showed the major concentrations of Cu and Fe were predominantly bound in the residual fraction (F4) and Zn in the reducible fraction (F2) of PM. The results of heavy metal bioavailability suggest that the addition of PM does not negatively affect the growth of lettuce biomass and the metal contents of plant tissues. Based on the results obtained, the pyrolysis material prepared from municipal sewage sludge seems to be a promising and innovative soil additive and a potential alternative to conventional inorganic fertilizers.