Effect of Phosphate, Sulfate, Arsenate, and Pyrite on Surface Transformations and Chemical Retention of Gold Nanoparticles (Au–NPs) in Partially Saturated Soil Columns

The understanding of engineered nanoparticle (ENP) fate and transport in soil–water environments is important for the evaluation of potential risks of ENPs to the ecosystem and human health. The effects of pyrite grains and three types of oxyanionssulfate, phosphate, and arsenateon the retention of citrate-coated gold nanoparticles (citrate–Au–NPs) were studied in partially saturated soil column experiments. The mobility of Au–NP was found to be in the order: Au–NP–sulfide (originating from pyrite) > Au–NP–sulfate > citrate–Au–NP > Au–NP–arsenate > Au–NP–phosphate. Chemical retention mechanisms, including hydrogen bonding and calcium bridging, are proposed and discussed. The retention of Au–NPs in soil columns increases with the increased ability of transformed Au–NP surfaces to create strong hydrogen bonding through adsorbed oxyanions with soil surfaces. Oxyanions were also found to reduce aggregation and aggregate size of Au–NPs upon interaction with Ca²⁺ solution. While the effects of cationic substances on ENP transport and stability have been studied frequently, the results here demonstrate that anionic substances have a substantial effect on Au–NP transport and stability. Furthermore, this study highlights the importance of examining ENPs under environmentally relevant condition, and the significant effect of ENP transformations on their mobility in soils.