Electrode Corrosion,\npH, and Dissolved Oxygen Dynamics,\nand Hardness/Silicon Removal during Aluminum Electrocoagulation of\nHypersaline Produced Water

Hypersaline produced water with >100,000 mg/L total\ndissolved solid\nconcentration arising from unconventional oil and gas operations in\nthe Permian Basin, Texas, was electrocoagulated with an aluminum anode\nand cathode. Anodic aluminum dissolution, formation of a (hydr)oxide\npassivation layer, and morphology and physicochemical properties of\nelectrodes pre- and post-electrocoagulation were thoroughly characterized\nby microscopy, spectroscopy, and electrochemical techniques over a\n10-fold\nvariation in current density (2–20 mA/cm<sup>2</sup>) and a\nfour-fold change in charge loading (CL) (∼270–1080 C/L).\nIn addition to the anticipated oxidative anodic electrodissolution,\nboth electrodes underwent chemical dissolution, leading to super-Faradaic\naluminum dosing and lowering the bulk pH, contrary to the oft-cited\nadvantage of electrocoagulation over conventional alum coagulation.\nThe remarkably high concentration of chloride ions (∼68,000\nmg/L) significantly influenced anodic dissolution behavior primarily\nby damaging the passive aluminum oxide layer leading to pitting corrosion.\nImportantly, organic compounds in the produced water negligibly impacted\nanodic aluminum (electro)dissolution. Not only the total CL but also\nthe current affected pitting. Passing more current (and higher current\ndensities) increased the chemical dissolution of aluminum, enhancing\nsuper-Faradaic behavior, and simultaneously increased the surface\narea and depth of pits (at constant CL) but had negligible effects\non the floc size and morphology. The dependence of pitting and Faradaic\nefficiency on current constitutes a novel finding and is specific\nto hypersaline solutions as ohmic overpotentials were insufficient\nto trigger side reactions. Post-electrocoagulation, electrodes repassivated\nby consuming dissolved oxygen, resulting in a thicker and more conductive\n(hydr)oxide layer, characterized as an n-type semiconductor via Mott–Schottky\nanalysis. Electrocoagulation effectively removed silicon (∼90%)\nby forming aluminosilicate flocs. Calcium and magnesium were removed\nby cathodic electrodeposition albeit to substantially smaller extents\n(∼20%) and strontium removal was negligible.