Electrode Corrosion, pH, and Dissolved Oxygen Dynamics, and Hardness/Silicon Removal during Aluminum Electrocoagulation of Hypersaline Produced Water

Abstract

Hypersaline produced water with >100,000 mg/L total dissolved solid concentration arising from unconventional oil and gas operations in the Permian Basin, Texas, was electrocoagulated with an aluminum anode and cathode. Anodic aluminum dissolution, formation of a (hydr)oxide passivation layer, and morphology and physicochemical properties of electrodes pre- and post-electrocoagulation were thoroughly characterized by microscopy, spectroscopy, and electrochemical techniques over a 10-fold variation in current density (2–20 mA/cm²) and a four-fold change in charge loading (CL) (∼270–1080 C/L). In addition to the anticipated oxidative anodic electrodissolution, both electrodes underwent chemical dissolution, leading to super-Faradaic aluminum dosing and lowering the bulk pH, contrary to the oft-cited advantage of electrocoagulation over conventional alum coagulation. The remarkably high concentration of chloride ions (∼68,000 mg/L) significantly influenced anodic dissolution behavior primarily by damaging the passive aluminum oxide layer leading to pitting corrosion. Importantly, organic compounds in the produced water negligibly impacted anodic aluminum (electro)dissolution. Not only the total CL but also the current affected pitting. Passing more current (and higher current densities) increased the chemical dissolution of aluminum, enhancing super-Faradaic behavior, and simultaneously increased the surface area and depth of pits (at constant CL) but had negligible effects on the floc size and morphology. The dependence of pitting and Faradaic efficiency on current constitutes a novel finding and is specific to hypersaline solutions as ohmic overpotentials were insufficient to trigger side reactions. Post-electrocoagulation, electrodes repassivated by consuming dissolved oxygen, resulting in a thicker and more conductive (hydr)oxide layer, characterized as an n-type semiconductor via Mott–Schottky analysis. Electrocoagulation effectively removed silicon (∼90%) by forming aluminosilicate flocs. Calcium and magnesium were removed by cathodic electrodeposition albeit to substantially smaller extents (∼20%) and strontium removal was negligible.