Optimisation of a photoelectrochemical system for the removal of pharmaceuticals in water using graphitic carbon nitride

The search for a fast and efficient process to treat water contaminated with pharmaceutical substances is ongoing and societally required due to the impact on the environment and human health. In this work, a photoelectrocatalytic (PEC) system, coupling visible-light photocatalysis and electrocatalysis with suspended and immobilised graphitic carbon nitride (g-C3N4), has been proved as a feasible alternative for the simultaneous degradation of a model persistent drug, viz. diclofenac (DCF), and the production of hydrogen peroxide (H2O2). A comprehensive study of the variables influencing PEC (visible-light emitting diode radiation and g-C3N4 load) is conducted. Moreover, the optimisation of the PEC process was accomplished through a Central Composite FaceCentred design of experiments, focusing on g-C3N4 load, immersed electrode area, and applied current intensity as key variables. From this CCFC design, the determined optimal working conditions were 0.76 g/L of g-C3N4, an electrode area of 6.38 cm2 and 58 mA of current intensity, leading to 92% DCF removal and 0.3 mM of produced H2O2 in a short period (5 min), which were experimentally validated (deviation lower than 1%). To overcome the drawbacks of operating with the g-C3N4 catalyst as powder, the synthesis of a novel material (g-C3N4@PAN) by electrospinning was carried out using polyacrylonitrile (PAN) and g-C3N4 as precursors to generate nanofibres. Characterisation techniques (Scanning Electron Microscopy, Diffuse Reflectance Fourier-Transform Infrared spectroscopy, and N2 adsorption-desorption isotherms) of the nanofibres allowed confirming a proper immobilisation assessing that the synthesis route is straightforward and enables obtaining physicochemical properties similar to those of g-C3N4 itself. The catalytic activity and reusability of g-C3N4@PAN for PEC removal of DCF were validated by operating up to 5 cycles in batch conditions without losing efficiency, and then confirmed by operating the PEC system under continuous flow mode. All results remark the great potential of the developed PEC system with a g-C3N4@PAN structure as a supported catalyst to work efficiently in different modes of operation.