Distinctive aging and inhibiting effects of microplastics between fresh and sulfidated nano-zero valent iron for various metal adsorption

Nano zero-valent iron (nZVI) is one of the most widely applied nanomaterials in environmental engineering for heavy metal and organic pollutants removal. However, its in-situ application is often compromised to the increasingly complex environmental systems, such as dissolved oxygen, coexisting (oxy)anions and emerging pollutants such as microplastics (MPs). To figure out measures to overcome the inhibition by MPs, sulfidation of nZVI was chosen as a solution to these drawbacks as sulfidation could enhance surface conductivity and electron transfer from the core of nZVI, and improve the electron selectivity. Batch experiments were carried out, indicating polyvinyl chloride (PVC) inhibited the metal sequestration efficiency of nZVI and resulted in desorption of metals (especially in Zn (II) system). After sulfidation, however, such inhibition on Zn (II) from PVC was only observed before equilibrium, while inhibition completely disappeared on the other metals. With the employment of S-nZVI, Cu (II), Pb (II), Zn (II) could reach a complete (100%) removal within 24 h even under the interference of PVC. Therefore, sulfidation assisted to overcome the inhibition of PVC on nZVI, which was attributed to enhanced reducing, stronger adsorption, higher affinity of metals with FeS layer, and the anti-ageing characteristics presented by S-nZVI. These findings suggest sulfidation is a promising method to overcome the inhibition of PVC MPs. Mechanisms identified between S-nZVI and metal ions in the presence of MPs may trigger studies to examine further insights to promote heavy metal removal using S-nZVI in the face of similar intricate surrounding environments.