Carbothermal Synthesis of Sludge Biochar Supported Nanoscale Zero-Valent Iron for the Removal of Cd(2+) and Cu(2+): Preparation, Performance, and Safety Risks

The practical application of nanoscale zero-valent iron (NZVI) is restricted by its easy oxidation and aggregation. Here, sludge biochar (SB) was used as a carrier to stabilize NZVI for Cd(2+) and Cu(2+) removal. SB supported NZVI (SB-NZVI) was synthesized using the carbothermic method. The superior preparation conditions, structural characteristics, and performance and mechanisms of the SB-NZVI composites for the removal of Cd(2+) and Cu(2+) were investigated via batch experiments and characterization analysis. The optimal removal capacities of 55.94 mg/g for Cd(2+) and 97.68 mg/g for Cu(2+) were achieved at a Fe/sludge mass ratio of 1:4 and pyrolysis temperature of 900 °C. Batch experiments showed that the SB-NZVI (1:4-900) composite had an excellent elimination capacity over a broad pH range, and that weakly acidic to neutral solutions were optimal for removal. The XPS results indicated that the Cd(2+) removal was mainly dependent on the adsorption and precipitation/coprecipitation, while reduction and adsorption were the mechanisms that play a decisive role in Cu(2+) removal. The presence of Cd(2+) had an opposite effect on the Cu(2+) removal. Moreover, the SB-NZVI composites made of municipal sludge greatly reduces the leaching toxicity and bio-availability of heavy metals in the municipal sludge, which can be identified as an environmentally-friendly material.