Efficient calcium removal from oilfield produced water containing scale inhibitors using a novel internal recycled rotating cathodic electrochemical reactor via enhanced mass transfer and decomplexation

In this study, a novel internal recycled rotating cathodic electrochemical softening (IRRCES) system was developed to enhance Ca²⁺ removal from oilfield produced water containing amino trimethylene phosphonic acid (ATMP) scale inhibitors. The IRRCES system was a membrane-based divided electrolytic cell. During operation, oilfield produced water first flowed into the cathode chamber with a rotating cathode for maximize utilization of HCO₃⁻ in the raw water. Cathode rotation improved OH⁻ diffusion, promoted homogenous CaCO₃ crystallization, and increased crystal size in the cathode chamber, facilitating the following solid-liquid separation. After separation in the sedimentation tank, wastewater then entered the anode chamber for enhanced decomplexation between Ca²⁺ and ATMP. Ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry (LC-Q-TOF-MS) analysis confirmed Ca-ATMP degradation via C-P bond cleavage. Extended X-ray absorption fine structure (EXAFS) analysis revealed a reduction in Ca-O bonds within the anode chamber. The decomplexed intermediates then returned to the cathode chamber for crystallization. Elemental, morphological and lattice analyses of the precipitates verified the involvement of ATMP intermediates during Ca crystallization in the cathode chamber. CP2K calculations further indicated that ATMP intermediates co-precipitated via Ca-O covalent and hydrogen bonding with CaCO₃. According to the above mechanism, the IRRCES system achieved around 90% Ca²⁺ removal efficiency, with a precipitation rate of 1545.6 gCaCO₃/m²·h for actual oilfield produced water containing ATMP with 1416 mg/L Ca²⁺, which significantly outperformed conventional electrochemical softening processes. The treated water from the IRRCES system was directly used to prepare guar gum fracturing fluid in oilfields. Overall, the IRRCES system provides a new approach for advancing the electrochemical softening of wastewater containing scale inhibitors.