Concomitant degradation of complex organics and metals recovery from fracking wastewater: Roles of nano zerovalent iron initiated oxidation and adsorption

Abstract

This work examines the characteristics of fracking wastewater from an emerging shale play in Central China, and the catalytic activity of nano-sized zerovalent iron (n-ZVI) particles to degrade major organic components of fracking wastewater (FW), and to simultaneously recover/remove metals and potential toxic elements. Addition of optimized concentration of n-ZVI (2g/L) to the raw FW led to COD reductions of 30% at pH 4, and 54% at pH 3 (with addition of H2O2) respectively within 120min reaction time. Activity of n-ZVI catalyst on degradation kinetics of total petroleum hydrocarbon (TPH) was over 6 times faster in acidic condition (Krc=0.0029min−1), than at natural pH of the raw FW (Krc=0.00046min−1). Meanwhile, oxidant-assisted degradation of the FW TPH reached higher degradation amounts (C/C0=0.191) at half the time required for treatment without oxidant addition (C/C0=0.218), and thus, implies a reduce treatment cost at shorter reaction time. Moreover, n-ZVI initiated oxidation led to rapid degradation of the FW polyethylene glycols (93.7% PEGs removal), as verified by liquid chromatography/tandem triple quadrupole mass spectrometry (LC–MS/MS), and possible degradation pathway of PEGs by n-ZVI was deduced. Similarly, n-ZVI recover metals and remove potential halocarbon-forming elements like chlorine as confirmed by X-ray fluorescence (XRF) analysis. Furthermore, the n-ZVI catalyst essentially increased biodegradability index of the FW at lower pH, and in the presence of oxidants. Therefore, pre-treatment of FW with n-ZVI represents a potential and cost-effective treatment option for the reuse of fracking wastewaters.