Effect of iron nanoparticles on passivation of cadmium in the pig manure aerobic composting process

Cadmium (Cd) is a toxic metal ion in pig manure impacting on the ecosystem, and hence the immobilization of Cd by green synthesis of iron nanoparticles (G-nFe) is a potential approach. In this study, transformation of Cd (II) during the pig manure thermophilic aerobic composting process in the presence of G-nFe was investigated. The results show that the addition of G-nFe promoted the composting process and release of available phosphorus (AP). In all six experiments, obvious passivation of Cd occurred during 15 days' composting. Particularly when 500 mL kg^-1 of G-nFe was added and Cd (II) was added at 0.6%(w/w%), residual Cd increased from 0.0016% to 55.70% and exchangeable Cd decreased from 98.54% to 7.21%. Batch experiments revealed that the G-nFe promoted the transformation of Cd into a larger passivation fraction. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), SEM-Mapping and Fourier transform infrared (FTIR) analysis was used to characterize residual samples, where indicated that the passivation of Cd in compost was highly correlated with the increase of P, it can be concluded that fixing with compost resulted in the formation of Cd phosphate precipitation or co-precipitation with other phosphates.