Removal of HDPE Microplastics via Al–Al Electrocoagulation in Synthetic Water and Real Municipal Wastewater: Process Optimization, Energy Trade-Offs, and Sludge Characterization

Microplastic (MP) contamination in wastewater is an emerging environmental concern. This study evaluated electrocoagulation (EC) using Al–Al electrodes to remove high-density polyethylene microplastics (HDPE-MP) from synthetic water and real municipal wastewater. A Box–Behnken design coupled with response surface methodology was used to assess the effects of initial pH, current density (J), initial HDPE-MP concentration, and NaCl concentration. In real municipal wastewater, the highest average HDPE-MP removal (88.5%) was achieved at pH 7, together with COD and turbidity reductions of 58.9% and 72.3%, respectively. NaCl concentration had a limited direct effect on HDPE-MP removal. Increasing the nominal current-density level did not proportionally improve removal efficiency, while average energy consumption increased from 9.11 kWh/m3 at the nominal level of 5 mA/cm2 to 33.06 kWh/m3 at the nominal level of 20 mA/cm2. XRD analysis of the recovered sludge showed poor crystalline Al-based phases and diffraction features consistent with semicrystalline HDPE. Overall, Al–Al electrocoagulation showed promising performance for HDPE-MP removal from complex wastewater matrices, although operating conditions should be optimized to balance removal efficiency and energy consumption.