Mechanisms and kinetics of enhanced oxidative degradation of tetrachloroethene by iron bearing soil minerals and glutathione in hyporheic zone / Nur Dalila Mohamad

The feasibility study on the physical and chemical characterization of hyporheic zone at the Klang River Basin was conducted to evaluate the controlling factors to the oxidative degradation of PCE as natural attenuation process in the hyporheic zone. The presence of iron-bearing soil minerals (IBSMs) (magnetite (Fe3O4), hematite (Fe2O3)), electrolytes (Fe, NO3‾ and SO42‾) and natural organic matter were detected in the hyporheic zone as important environmental factors that significantly influenced the mechanisms and kinetics of oxidative degradation of PCE. The oxidative degradation of PCE was initiated by nano-IBSMs (nano-Fe3O4 and nano-Fe2O3) via Fenton-like reaction. However, PCE was incompletely degraded due to the instability of Fe3+/Fe2+ redox in the nano-IBSMs suspension. The oxidative degradation of PCE was enhanced through the coupling of redox reactions between nano-IBSMs and glutathione (GSH). The potential role of GSH as a reductant could sustain the redox of Fe3+ to Fe2+ and promote the generation of reactive oxygen species (OH• and O2‾•) for the enhanced oxidative degradation of PCE by nano-IBSMs. The oxidative degradation kinetic rate constant of PCE in the nano-Fe3O4-GSH suspension was 11.7 times faster (0.035 ± 0.001 hr−1) than that in the nano-Fe3O4 suspension (0.003 ± 0.08 hr−1). Contrarily, PCE was oxidatively degraded 600 times faster (0.014 ± 0.003 hr−1) in the nano-Fe2O3-GSH than that in the nano-Fe2O3 (0.00023 ± 0.00008 hr−1). The oxidative degradation of PCE was predominantly controlled by the concentration of OH• than the O2‾• in the nano-IBSMs-GSH suspension. The comparison of enhanced oxidative degradation of PCE between nano-Fe3O4-GSH and nano-Fe2O3-GSH revealed that the oxidative degradation kinetics of PCE by the nano-Fe3O4-GSH was 2.5 times faster (0.035 ± 0.001 hr−1) than the nano-Fe2O3-GSH (0.014 ± 0.003 hr−1), indicating nano-Fe3O4 was more reactive as iron catalyst than nano-Fe2O3. The enhanced oxidative degradation of PCE by nano-IBSMs-GSH were conducted at different environmental conditions to evaluate the kinetics of the oxidative degradation. The enhanced oxidative degradation kinetics of PCE significantly increased by the increased of concentration of nano-IBSMs and GSH in alkaline condition. Interestingly, a remarkable enhancement of the oxidative degradation of PCE by the nano-IBSMs-GSH in the presence of NO3‾ and SO42‾ due to the generation of NO3• and SO4‾• radicals. However, the presence of HA and increased concentration of PCE significantly decreased the kinetics of the oxidative degradation of PCE. PCE was completely transformed to oxalic acid as a major by-product via hydroxylation and oxidation reactions.