Degradation of BTEX in groundwater by nano-CaO2 particles activated with L-cysteine chelated Fe(III): enhancing or inhibiting hydroxyl radical generation

Abstract

The simultaneous oxidation performance of benzene, toluene, ethylbenzene, and xylene (BTEX) by nanoscale calcium peroxide particles (nCaO2) activated with ferric ions (Fe(III)) and the mechanism of the enhancement of BTEX degradation by L-cysteine (L-cys) were investigated. The batch experimental results showed that the nCaO2/Fe(III)/L-cys process was effective in the destruction of BTEX in both ultrapure water and actual groundwater. A proper amount of L-cys could enhance BTEX degradation due to the promotion of Fe(II)/Fe(III) redox cycles by the participation of L-cys, but an excessive presence of L-cys would cause inhibition. Adding 1.0 mM L-cys to the nCaO2/Fe(III) system, the concentration of Fe(II) increased to 1.15 mM instantly. Simultaneously, the yield of HO• produced by the 1.0 mM L-cys-containing system was 0.066 mM at 180 min reaction, higher than that without L-cys (0.049 mM). When excess L-cys (5.0 mM) was added to the system, the amount of Fe(II) increased to 3.73 mM because excessive L-cys caused a large amount of Fe(III) in the system to be reduced. However, the yield of HO• decreased to 0.043 mM since excessive Fe(II) could conversely scavenge HO• to produce Fe(III) again. EPR tests and quenching results indicated that HO• was the dominant reactive species in the nCaO2/Fe(III)/L-cys system. For the removal of BTEX, the optimal molar ratio of nCaO2/ Fe(III)/L-cys was 10.5/20/1 based on calculation by response surface methodology (RSM). Finally, the BTEX destruction pathway was proposed according to the detected intermediates by liquid chromatography–mass spectrometry (LC–MS).