The effect of pH on the sorption of gold nanoparticles on illite

Sorption between nanoparticles(NPs)and minerals may critically affect the migration of associated elements as well as the environmental impact of NPs.Since illite is widely present in soil,sediment,and water,we have experimentally investigated the sorption behavior of citrate-coated gold nanoparticles(AuNPs)as model NPs on illite under different pH and mineral mass conditions.We demonstrated that sorption of these negatively charged AuNPs strongly depended on the suspension pH.At pH above 8,which coincided with the apparent point of zero charge(pH 7.9)of our illite sample,only marginal sorption of AuNPs was observed.At pH 3-8,significant sorption of AuNPs on illite was found,with almost complete sorption occurring at more acidic conditions(pH 3-4).TEM observations revealed that sorption took place through the attachment AuNPs on illite edges.At pH 2,AuNPs mostly formed chain-like fused structures and precipitated out of the suspension.Based upon the above pH dependence,residual organic ligand content after sorption,and complementary sorption results with positively charged AuNPs,we conclude that the sorption process is mainly driven by the electrostatic attraction between negatively charged AuNPs and positively charged illite edges,with possible competitive involvement of citrate molecules.We expect that our findings will improve our understanding of NP-mineral interaction and the environmental fate of NPs.