Synthesis of photocatalytic pore size-tuned ZnO molecular foams

The safe and efficient removal of organic micropollutants, such as pharmaceuticals, pesticides or caffeine from wastewater remains a major technological and environmental challenge. Here, the synthesis of self-supporting ZnO foam monoliths by direct incorporation of air into the forming gel is presented for the first time. These foams, labelled as MolFoams, showed a highly porous and interconnected structure, allowing for high solution flow rates and fast degradation kinetics of carbamazepine, a widely used pharmaceutical compound, used here as a model micropollutant. Altering the concentration of CTAB used in the formulation of the gels allowed controlling the size of the macropores of the MolFoam in the 0.69-0.84 mm range. Smaller macropores within the MolFoam structure were highly beneficial for the degradation of carbamazepine with pseudo first-order degradation kinetics of 5.43 × 10⁻³ min⁻¹ for the MolFoams with the smallest macropore size. The best foams were tested in a recirculating reactor, with an optimal flow rate of 250 mL min⁻¹, resulting in a quantum yield of 0.69 and an electrical energy of 21.3 kW h m⁻³ per order, in addition to high mechanical and chemical stability. These results surpass the performance of photocatalytic slurries and immobilised systems, showing that self-supporting, photocatalytic foams can be an effective solution for the removal of organic micropollutants in wastewater.