Synthesis of photocatalytic pore size-tuned ZnO molecular foams

Abstract

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.