Boron removal from hydraulic fracturing wastewater by aluminum and iron coagulation: Mechanisms and limitations

One promising water management strategy during hydraulic fracturing is treatment and reuse of flowback/produced water. In particular, the saline fiowback water contains many of the chemicals employed for fracking, which need to be removed before possible reuse as frac water. This manuscript targets turbidity along with one of the additives; borate-based cross-linkers used to adjust the rheological characteristics of the frac-fluid. Alum and ferric chloride were evaluated as coagulants for clarification and boron removal from saline flowback water obtained from a well in the Eagle Ford shale. Extremely high dosages (> 9000 mg/L or 333 mM Al and 160 mM Fe) corresponding to Al/B and Fe/B mass ratios of similar to 70 and molar ratios of similar to 28 and 13 respectively were necessary to remove similar to 80% boron. Hence, coagulation does not appear to be feasible for boron removal from high-strength waste streams. X-ray photoelectron spectroscopy revealed B-O bonding on surfaces of freshly precipitated Al(OH)(3)(am) and Fe(OH)(3)(am) suggesting boron uptake was predominantly via ligand exchange. Attenuated total reflection-Fourier transform infrared spectroscopy provided direct evidence of inner-sphere boron complexation with surface hydroxyl groups on both amorphous aluminum and iron hydroxides. Only trigonal boron was detected on aluminum flocs since possible presence of tetrahedral boron was masked by severe Al-O interferences. Both trigonal and tetrahedral conformation of boron complexes were identified on Fe(OH)3 surfaces. (C) 2017 Elsevier Ltd. All rights reserved.