Deciphering CaO-induced peroxydisulfate activation for destruction of halogenated organic pollutants in a low energy vibrational mill

Abstract

• CaO-PDS features high reactivity in low energy vibrational mill. • The CaO addition much enhances 2,4-DCP dechlorination and mineralization. • PDS is activated via electron transfer, base activation, and energy transfer. • CaO both activates PDS and transforms SO 4 •− surf to •OH surf. • CaO-PDS enables destruction of persistent halogenated organic pollutants. Mechanochemical (MC) treatment utilizing calcium oxide (CaO) and peroxydisulfate (PDS) for halogenated organic pollutants (HOPs) has been developed for pure substance degradation and contaminated soil remediation. However, the role of CaO and the mechanism of PDS activation in solid phase reaction remain unclear. This study was dedicated to decipher CaO-induced PDS activation for destruction of typical HOPs such as 2,4-dichlorophenol (2,4-DCP), in a low energy single-cylinder horizontal vibrational mill. With mass ratio of CaO:PDS:2,4-DCP = 6.3:6.7:1, CaO-PDS as co-milling agents destructs 2,4-DCP with much higher degradation and dechlorination rate than CaO (about 13.9- and 121.2-fold) or PDS (about 2.5- and 9.4-fold) only. According to detection of intermediates and measurement of total organic carbon, the CaO addition shifts the degradation pathway into dechlorination and ring-open routes, and increase mineralization efficiency from 64% (PDS only) to 96%. Quenching test suggests •OH surf is the dominant oxidative species with CaO-PDS as co-milling agents, which enables complete oxidative destruction. PDS in such MC system is activated mainly via electron transfer (∼50.6%), followed by base activation (∼40.5%) and energy transfer (＜ 8.9%). CaO plays a dual role in activating PDS and transforming SO 4 •− surf to •OH surf . Finally, CaO-PDS shows high reactivity in destructing persistent HOPs like polybrominated phenols and polychlorinated benzenes, which expands its potential in practical application. This study may guide the selection of effective oxidative co-milling agents for mineralization of solid HOPs by mechanochemistry, facilitating safe disposal of obsolete HOPs.