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

- FW from Yanchang formation, Ordos Basin has very poor biodegradability.
- n-ZVI achieves simultaneous degradation of PEGs, TPHs, and FW dechlorination.
- Processes controlling degradation/removal of TPH and PEGs by n-ZVI were clarified.
- XRF spectroscopy confirms recovery of metals and non-metals by n-ZVI from FW.

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 (2 g/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 120 min reaction time. Activity of n-ZVI catalyst on degradation kinetics of total petroleum hydrocarbon (TPH) was over 6 times faster in acidic condition (Krc = 0.0029 min−1), than at natural pH of the raw FW (Krc = 0.00046 min−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.

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