ZnO nanoparticles for photodegradation of humic acid in water

Humic acid (HA) is the most important precursor of toxic disinfection byproducts upon chlorination. Removing HA from water body is therefore critical in drinking water acquisition. In this research, ZnO nanoparticles are employed for photocatalysis under UV light at neutral pH to remove HA from a water environment. Almost 100% degradation of HA was achieved using 0.3 g/L of ZnO in 180 min with UV-A and UV-C light. Under identical experimental conditions, total organic carbon (TOC) removals reach 67% and 21% with UV-A and UV-C light, respectively. A higher degree of mineralization of HA is achieved with UV-A light although the degradation of HA is slightly better with UV-C light. This indicates that ZnO/UV-A has relatively low selectivity to degrade different compounds, including various intermediates from HA degradation. The use of UV-A light is therefore recommended for ZnO as it possesses higher mineralization ability. Negligible TOC is observed on the ZnO surface after photocatalytic reactions. In contrast, the adsorption of HA in dark conditions reaches 42% in 180 min. This strongly indicates that the adsorption of HA plays an important role in the photocatalytic degradation of HA, but it is not the main process for HA removal.