One promising water management strategy during hydraulic fracturing is treatment and reuse of flowback/produced water. The flowback water contains many of the chemicals employed for fracking which need to be removed before possible reuse as “frac water.” This presentation targets one of the additives; borate-based cross-linkers used to adjust the rheological characteristics of the frac-fluid. Alum and FeCl3 were evaluated as coagulants for boron removal from saline flowback water obtained from the Eagle Ford shale via jar testing. Electrocoagulation was evaluated using a custom designed cell fitted with an annular electrode geometry comprising an Al anode and a perforated cylindrical 316-stainless steel cathode. For chemical coagulation, extremely high dosages corresponding to Al/B and Fe/B mass ratios of ~70 were necessary to remove ~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 and Fe(OH)3 suggesting boron uptake was predominantly via ligand exchange. ATR-FTIR 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 Al 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. For electrocoagulation, boron removal increased monotonically with Al dosage and was higher at pH 8 but remained relatively constant at ≥ 20 mA/cm2. Chloride ions induced pitting and super-Faradaic Al dissolution and their electrooxidation produced free chlorine. ATR-FTIR suggested outer-sphere and inner-sphere complexation of trigonal B(OH)3 with Al(OH)3, which was confirmed by the B-O bond shifting towards lower binding energies in XPS. Severe Al-O interferences precluded evidence for tetrahedral B(OH)4- complexation.

Boron removal from hydraulic fracturing flowback water by chemical and electrocoagulation: Mechanisms and limitations

255th National Meeting (2018)

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