Photodegradation of malachite green using UV–vis light from two microwave-powered electrodeless discharge lamps (MPEDL-2): Further investigation on products, dominant routes and mechanism

In this research, we choose photolysis degradation (PD) course and H2O2-enhanced PD (HPD) course using UV–vis light from two microwave-powered electrodeless discharge lamps (MPEDL-2) as simplified models. The results verified that (1) ∼70% and 99.7% of MG were removed in PD and HPD courses within 5 min irradiation, and PD course was proved to be dependent on OH and O2-, (2) major routes were related to transformation of MG into malachite green carbinol base (MGCB), which could be further decomposed into 4-dimethylaminobenzophenone (DLBP), and (3) after 0.5 min irradiation, PD course verified that ∼30% and 1% of MG was converted into MGCB and DLBP; for comparison, HPD course demonstrated that ∼12.9% and 28.5% of MG was transformed into MGCB and DLBP. Accordingly, we draw conclusions that degradation mechanism of MG could be revised as seven steps with MGCB as a key product and pH variation of aqueous dyes solution played minor role. Additionally, these conclusions were also enlightened for degradation of MG by advanced oxidation process.

► We model PD and HPD courses for comparatively disclosing the photodegradation mechanism of MG.
► In PD course, less ROS mainly transformed MG into MGCB, which was further decomposed into DLBP.
► In HPD course, more ROS enhanced the transformation of MG into MGCB and decomposition of MGCB into DLBP.
► After 0.5 min irradiation, different routes to the discoloration of MG was evaluated and compared during PD and HPD courses.
► The degradation mechanism was also enlightened for AOP-based degradation of MG.