Effect of humic acid and transition metal ions on the debromination of decabromodiphenyl by nano zero-valent iron: kinetics and mechanisms

E-waste sites are one of the main sources of the pollutant decabromodiphenyl ether (BDE209); contaminated farmland and water bodies urgently need to be remediated. As a potential in situ remediation technology, nano zero-valent iron (nZVI) technology effectively removes PBDEs. However, the humic acid (HA) and heavy metals in the contaminated sites affect the remediation effects. In this study, we explored the influence of HA and transition metals on the removal of PBDEs by nZVI. The specific surface area and average size of the nZVI particles we prepared were 35 m2/g and 50–80 nm, respectively. The results showed that HA inhibited the removal of PBDEs; as the concentration of HA increased, its inhibitory effect intensified and the k obs decreased. However, the three metal ions (Cu2+, Co2+, and Ni2+) enhanced the removal of PBDEs. The enhancement effect was followed the order Ni2+ > Cu2+ > Co2+. As the concentration of metal ions increased, the promotion effect improved. The synergistic effect of HA and the metal ions was manifested in the combination of the inhibitory effect and the enhancement effect. The values of the first-order kinetic constants (k obs) under the combined effect were between the values of the rate constants under the individual components. The inhibitory mechanism was the chemisorption of HA, i.e., the benzene carboxylic and phenolic hydroxyl groups in HA occupied the surfactant reactive sites of nZVI, thus inhibiting the removal of BDE209. The promotion mechanism of Cu2+, Co2+, and Ni2+ can be explained by their reduction to zero valence on the nZVI surface; furthermore, Ni2+ strongly affects the debromination and dehydrogenation of BDE209, leading to a stronger promotability than Cu2+or Co2+.