Concentration dependent effect of humic acid on the transformations of silver nanoparticles

Influence of humic acid (HA) concentration on the colloidal stability and dissolution of silver nanoparticles (AgNPs) was evaluated in a series of long-term experiments. The stability of freshly synthesized and uncoated AgNPs was investigated as a function of HA concentration (0–250 mg/L). In the absence of HA, AgNPs underwent oxidative dissolution and aggregation disturbing the colloidal stability of AgNPs. In the HA concentration range of 1–20 mg/L, HA stabilized AgNPs due to the inhibition of the release of ionic Ag ensuring the persistence of AgNPs in the aquatic environment for a longer period. At higher concentrations of HA (20–250 mg/L) in the soil environment, AgNPs were wrapped by the HA molecules, increasing the hydrodynamic diameter of the AgNPs. Due to the suppression of dissolution of AgNPs, primary particles remained stable in the environment for a prolonged period with no change in the primary particle size and morphology. Furthermore, it was observed that ionic Ag in the solution can be reduced to form fresh, secondary AgNPs at the lower levels of HA in the aquatic environment, in the presence of sunlight. This study is important in understanding the role of HA on the stability of AgNPs in the natural environment.