Organic solid waste is a renewable resource as it can be transformed into a valuable product through various technologies. Composting is considered to be the most economical and effective technology for treating organic solid waste, but excessive amounts of heavy metals in organic solid waste compost are harmful to the environment. The current focus is on the addition of heavy metal passivators to organic solid waste to reduce the mobility and biotoxicity of heavy metals in situ or ex situ. The aims of this paper are to provide an overview of heavy metal passivators and their passivation mechanisms in the field of organic solid waste composting and to provide a reference for research on the control of heavy metal pollution in the treatment of organic solid waste.

In China, the treatment of pig manure for land application is an important issue. In this paper, green synthesized iron nanoparticles (G-nFe) were evaluated for their effectiveness in the aerobic composting of pig manure. G-nFe were evenly mixed into the compost, and the pH and oxidation-reduction potential (ORP) of the compost in the presence of G-nFe both decreased. FTIR showed that G-nFe promoted the formation of humus during composting. The addition of G-nFe (200 mL kg⁻¹) to the compost also promoted bacterial growth, significantly increasing the colony-forming units (CFU, up to 25.6 × 10⁴ CFU mL⁻¹) and OD600 (up to 0.634) in 5 days. Furthermore, G-nFe promoted the fermentation of the pig manure, thus reducing the phytotoxicity of the compost produced. For example, the final volatile solids (VS) degradation rate and seed germination index (GI) of the compost were the highest and reached 20.8% and 0.76%, respectively. Finally, G-nFe improved both the bacterial diversity and community richness of the compost. This indicated that the addition of G-nFe could result in the prolonging of the acidic fermentation time during composting, leading to increased compost maturity. Overall, the beneficial impact of G-nFe on compost was far greater than the potential harm to bacterial communities in the compost and perceived phytotoxicity.