Behavior of ZnO Nanoparticles in Aqueous Environments: Influence of pH and Adsorption of Humic Acid

The surface charge and average size of ZnO was studied with a function of pH and time. The interactions between ZnO and Suwannee River humic acid (SRHA) were further investigated under a range of environmentally relevant conditions with the pH(PZC) as a point of reference. The anionic charge carried by aquatic humic substances plays a major part in the aggregation and disaggregation of metal oxide nanoparticles. At low concentrations of SRHA (<0.05 mg/L) the positively charged ZnO aggregates were rapidly adsorbed below the pH(PZC). With similar SRHA concentrations, at pH=pH(PZC), SRHA was able to control the suspension behavior of the ZnO and promoted partial disaggregation in small volumes. This was more distinguishable as SRHA form a surface coating on the nanoparticles and enhances stability via electrostatic stabilization mechanism which induces the disaggregation behavior in the ZnO nanoparticles and hence, decreases the aggregate size.