Speciation, controlling steps and pathways of silver release from the sludge generated from coagulation of wastewater spiked with silver nanoparticles.

Sludge generated in wastewater treatment facilities is an integral part for the introduction of silver nanoparticles (AgNPs) to the terrestrial environment, which would cause some adverse ecosystem responses. The understanding of silver release process from the sludge is important to evaluate their risks. In this study, the amount and speciation of the released silver were investigated by taking the sludge generated by wastewater coagulation with AgNPs added (denoted as sludge) an example, and kinetic analysis and density functional theory (DFT) calculations were first used to explore the controlling steps and pathways about the silver release. The results showed that sludge could release the dissolved silver and the colloidal silver. Beside Cl, Ca in the leaching solution could enhance the silver release of sludge, especially for the colloidal silver. The released colloidal silver restricted in size from 40 nm to 100 nm with irregular shape. Although the oxidative dissolution of Ag was the origin of the silver release pathways from the sludge, the silver diffusion was the controlling step due to the spontaneous binding between silver and the hydrolysates of polyaluminium chloride in sludge. However, Ca in the leaching solution could occupy the binding site of silver on sludge, which would increase the diffusion rate of silver over the oxidative rate of Ag. With this condition, the controlling step of silver release from sludge turned to the oxidative dissolution of Ag. Our findings are important to assess the fate of AgNPs in wastewater treatment as well as sludge applications.