Fast electron transfer induced peracetic acid activation by Fe2+ for efficient degradation of refractory hydroxypropyl guar gum

Hydroxypropyl guar gum (HPG), as a refractory organic pollutant in fracturing flowback fluid, has caused serious pollution when released to water, but traditional wastewater treatment technologies exhibit low removal efficiency for HPG due to its highly stable structure. In this work, a homogenous advanced oxidation process (AOP), constructed as peracetic acid (PAA) activated by Fe2+, was developed for HPG degradation. The Fe2+/PAA system showed excellent HPG removal of 96.5 % even at a high initial concentration of 4000 mg/L, and efficient HPG removal (>95 %) was also found over a broad pH range (3-7). The good effectiveness of the Fe2+/PAA system was attributed to the production of various strong reactive species, mainly including hydroxyl radical (center dot OH) and alkoxyl radicals (CH3C(=O)OO center dot). The production of reactive species was confirmed by scavenger quenching tests and electron paramagnetic resonance (EPR) analysis. Mechanism exploration based on density functional theory (DFT) calculation reveals that the formed hybridization between O 2p and Fe 3d orbitals leads to fast electron transfer from Fe2+ to PAA, thus achieving cleavage of peroxy bond in PAA for radicals generation. Furthermore, DFT calculation on the Fukui index indicates that the reactive sites of HPG for electrophilic attack are mainly located at the oxygen atoms in the six-membered heterocycle. Finally, toxicity assessment of degradation products confirmed that the Fe2+/PAA technology achieved effective detoxification of HPG after degradation. In total, the proposed Fe2+/PAA technology not only provides a feasible solution to decontamination and detoxification of refractory pollutants in fracturing flowback fluids.