Effects of Humic Acid and Natural Sunlight Irradiation on the Behaviour of Zinc Oxide Nanoparticles in the Aqueous Environment

The unique properties of ZnO nanoparticles have attracted scientists’ interest to produce on a large-scale. Household items, cosmetics, consumer products, and electric sensors are some products that utilize these ZnO nanomaterials. Eventually, ZnO nanoparticles will be released into the environment in various ways. Once released, ZnO nanoparticles would dissociate into Zn2+ ions, which are toxic to aquatic organisms. The presence of humic acid and exposure to sunlight could affect the dissolution of ZnO nanoparticles. Two sizes of commercial ZnO nanoparticles (< 50 nm and < 100 nm) were chosen to study the influence of humic acid and sunlight on the dissolution. In the presence of humic acid, the dissolution of both sizes is higher, with 67 % and 39 % Zn2+ dissolved for < 50 nm and < 100 nm, respectively. The concentration of Zn2+ ions seems to be consistent or stable when exposed to sunlight. However, the humic acid enhanced the release of Zn2+ ions. Langmuir isotherm model best fitted for the humic acid's sorption onto the ZnO nanoparticles with the process been favorable.