Influences of surface-coated fulvic and humic acids on the adsorption of metal cations to SiO2 nanoparticles

Humic substances such as fulvic acid (FA) and humic acid (HA) exist universally in the environment and play an important role in the interfacial reactions of contaminants. In this study, influences of surface-coated FA and HA on the adsorption of metal cations of Cu(II), Pb(II) and Zn(II) on SiO2 nanoparticles (nano-SiO2, 20 nm) were investigated. The results showed that nano-SiO2 had high adsorption capacities for the metal cations following the sequence of Pb(II) > Cu(II) > Zn(II), irrespective of coating with FA or HA. Nano-SiO2 had a strong affinity for FA and HA, and coating with FA and HA significantly enhanced the adsorption of the metal cations on nano-SiO2. Cation adsorption on the original and coated nano-SiO2 followed the same pronounced increasing trend with increasing pH. X-ray absorption spectroscopy (XAS) analysis by taking Cu(II) as an example revealed that Cu(II) was adsorbed on nano-SiO2 by forming inner-sphere complexes; whereas its adsorption on FA/HA coated nano-SiO2 was through complexing with the hydroxyl moieties and organic functional groups of FA/HA as bridges. This work demonstrates that FA and HA can significantly change the adsorption capacities and mechanisms for metal cations and thus likely influence their behaviors in the environment.