OTS ‐grafted zeolite membranes for diclofenac removal by air‐gap membrane distillation: from electrostatic to thermal retention mechanism

BACKGROUND The hydrophobic functionalization of ceramic membranes represents a promising strategy for pharmaceutical wastewater treatment. However, the relationship between surface chemistry and operating mode remains insufficiently understood. In this study, a zeolite microfiltration membrane with a mean pore size of 0.55 μm was grafted with n‐octadecyltrichlorosilane ( OTS ) to investigate the influence of surface hydrophobicity on filtration and air‐gap membrane distillation ( AGMD ) performance. RESULTS OTS grafting transformed the membrane surface from superhydrophilic (contact angle: 0°) to highly hydrophobic (120° ± 1.8°), as supported by thermogravimetric analysis, SEM – EDX , porosity measurements, N 2 adsorption–desorption analysis, and dry‐storage stability evaluation. During pressure‐driven filtration at 1 bar and ambient temperature, the grafted membrane exhibited low diclofenac ( DCF ) rejection (8.9%), while conductivity removal decreased from 62.6% for the ungrafted membrane to 41.1% after grafting. In contrast, AGMD operated at a feed temperature of 60 °C and a coolant temperature of 5 °C achieved 92.04% DCF rejection from the synthetic solution. For real pharmaceutical wastewater, apparent UV ‐based removal reached 86.42%, while chemical oxygen demand ( COD ) and conductivity removals were 96.45% and 88.97%, respectively. These findings are consistent with the expected separation mechanism of AGMD , although partial pore wetting or liquid leakage cannot be excluded. Water rinsing recovered 62.09% of the initial permeability, whereas sequential alkaline–acid cleaning restored it to 97.12% while largely preserving the hydrophobic surface (contact angle: 113.2° ± 2.3°). CONCLUSION Combining OTS ‐induced surface hydrophobicity with the thermal driving force of AGMD substantially improved contaminant retention compared with pressure‐driven filtration. The results highlight the potential of natural zeolite‐based ceramic membranes for pharmaceutical wastewater treatment and demonstrate the importance of appropriate cleaning strategies for permeability recovery. Further investigation of membrane wetting and liquid leakage is required to confirm long‐term separation performance and operational reliability.