Kaolin/Activated Carbon Hybrid Ceramic Membranes: Enhanced Permeability, Metformin Apparent Removal, and Fouling Behavior

Metformin, a widely prescribed antidiabetic drug, is a representative small (≈0.5–0.6 nm), hydrophilic (log P ≈ −1.43), and positively charged (pKₐ ≈ 12.4) emerging contaminant. Because its molecular size is well below the pore size of ultrafiltration membranes (>2 nm), it cannot be efficiently removed by size exclusion alone, and its high solubility and poor biodegradability further complicate conventional treatment. In this context, hybrid ceramic membranes incorporating adsorptive functionalities offer a promising alternative. However, the fouling behavior of such hybrid systems remains quantitatively uncharacterized. This study provides a quantitative assessment of flux decline and fouling reversibility in kaolin-based membranes containing 0, 5, and 10 wt.% activated carbon, evaluated for metformin removal under dead-end filtration at 4 bars, using a synthetic feed concentration of 20 mg L−1 at neutral pH (≈6.8) over 60 min. Increasing AC content raises open porosity from 29.1% (M0%) to 34.2% (M5%) and 39.1% (M10%), and water permeability from 3.3 to 10.9 and 28.4 L m−2 h−1 bar−1, respectively, without significantly compromising mechanical integrity (>20 MPa). The porosity enhancement is accompanied by a systematic improvement in apparent metformin removal, consistent with a contribution from carbon- and phosphorus-containing phases detected after thermal treatment (which provides abundant surface functional groups for electrostatic and π–π interactions) and is consistent with the observed improvement in permeability and apparent metformin removal. Consequently, metformin apparent removal after 60 min increases from 56.1% (M0%) to 76.2% (M5%) and 91.1% (M10%), while total fouling decreases from 38.5% to 20.5% and 11.0%, and irreversible fouling from 23.1% to 13.6% and 6.0%, respectively.