Enhancing surface corrosion of zero-valent aluminum (ZVAl) and electron transfer process for the degradation of trichloroethylene with the presence of persulfate

Though covered with a compact oxide film, unmodified zero-valent aluminum (ZVAl), showed a significant synergistic effect combined with persulfate (PS) for the degradation of trichloroethylene (TCE) over a wide initial pH range (3.00-10.00) without any induction period. In ZVAl/PS system, there are oxidizing species, including PS, O-2, H2O and the target contaminant TCE, which will compete for electrons released from ZVAl corrosion. In order to really understand the reaction mechanism, in this work, the complicated surface corrosion and electron transfer processes for TCE degradation were deeply explored. Firstly, by the characterizations of SEM-EDS, TEM, size distribution, N-2 adsorption-desorption isotherms, XRD and XPS, we found that pristine ZVAl was a core-shell structure and during the reaction the core was corroded and oxidized to Al-(hydr)oxide. After reaction, the oxide film was not directly removed, but became rougher, causing the increase of particle size, specific surface area, total pore volume and average pore size of ZVAl. The addition of PS can indeed activate ZVAl surface and accelerate ZVAl corrosion rate, and acidic environment is more conducive to the corrosion, which is opposite to the case without PS. Secondly, the dominant active species and electron transfer processes for TCE degradation were identified. SO4 center dot- was generated through electron transfer from ZVAl to PS directly. O-2 may be involved in the process at acidic pH by forming O-2(center dot-) and H2O2. Hence, rather than the reductive removal by electron released from ZVAl, complete dechlorination and partial mineralization of TCE could be achieved, due to the high solubility and standard redox potential of PS which has more chance to capture electron and to the highly oxidative capacity of generated radicals.