Adsorption and fractionation of Pt, Pd and Rh onto inorganic microparticles and the effects of macromolecular organic compounds in seawater

Adsorption and fractionation of Pt, Pd and Rh (defined here as platinum group elements, PGEs) onto the representative inorganic microparticles, including Fe2O3, MnO2, CaCO3, SiO2, Al2O3 and kaolinite in seawater were investigated. The effects of macromolecular organic compounds (MOCs) as the representatives of organic matter, including humic acids (HA), bovine serum albumin (BSA) and carrageenan, on the adsorption were also studied considering that organic matter is ubiquitous in seawater and indispensable to marine biogeochemical cycles. In the absence of MOCs, the representative mineral particles Fe2O3 and MnO2 had the strongest interaction with PGEs. The adsorption of PGEs onto the representative biogenic particles SiO2 and CaCO3 and lithogenic particles Al2O3 and kaolinite was similar or weaker than onto the mineral particles. MOCs inhibited the interaction between PGEs and the particles except for Pt and Pd onto the biogenic particles in artificial seawater. This impediment may be closely related to the interaction between particles, MOCs and elements. The partition coefficient (log Kd) of Pt was similar (∼4.0) in the presence of MOCs, indicating that the complexation between Pt and MOCs was less important than hydrolysis or adsorption onto the acid oxide particle surface. Rh tended to fractionate onto the mineral and lithogenic particles in the presence of HA and carrageenan, while Pd was more likely to fractionate onto the biogenic particles. However, BSA enhanced the fractionation tendency of Pd onto the mineral particles. The results indicate that the adsorption behavior of Pd onto inorganic particles was significantly affected by the composition or the type of MOCs. Hence, the interaction between PGEs and inorganic particles may be greatly affected by the macromolecular organic matter in the ocean.