Adsorption kinetics of platinum group elements onto macromolecular organic matter in seawater

Abstract

Adsorption kinetics of the interaction between Pt, Pd and Rh (defined here as platinum group elements, PGEs) ions and macromolecular organic compounds (MOCs, >10 kDa), including humic acid, carrageenan and bovine serum albumin, and different cutoff fractions of natural organic matter (>1 kDa and >3 kDa) obtained from seawater using centrifugal ultrafiltration devices were investigated. For a given element, all the adsorption kinetics did not reach equilibrium except the interaction between Pt and >1 kDa cutoff, and between Pd and humic acid. For all the tested MOCs, the adsorption kinetics could be divided into two stages, a rapid adsorption process in the first 8 h and the desorption stage after the first 8 h until the equilibrium. The change trend of partition coefficient (log 10 K d ) values with experiment time was consistent with that of the kinetic curves. However, in the interaction between PGE ions and natural dissolved organic matter (NDOM), an obvious difference in the change trends of log 10 K d and kinetic curves was observed. It indicated that the partition behavior of PGE ions interacting with NDOM in seawater was a combined effect of different organic constituents. The adsorption and log 10 K d of PGEs in the >1 kDa NDOM fraction were higher and more stable than those in the >3 kDa NDOM fraction. The results also indicated that the 1-3 kDa NDOM may dominate the interaction between PGEs ions and NDOM. Moreover, no kinetic model could perfectly simulate the adsorption process. It indicated that the colloidal struction and morphology of MOCs or NDOM in seawater might be inhomogeneous. Hence, the interaction between PGE ions and organic matter in seawater was a complicated process and needs further research.

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

“…3 and 4). In BSA-particle binary system, the inhibition effect of BSA on the adsorption of Pd onto the particles was smaller than that of HA, which was consistent with the results of the previous research that the interaction between BSA and Pd was weaker than that between HA and Pd (Liu et al, 2019).…”

“…The chemical forms of Pt, Pd and Rh in the stock solution were Cl-forms and the oxidation states were þ2, þ2 and þ 3, respectively. The labwares used in this study were cleaned strictly according to Li et al (2015) and Liu et al (2019). They were washed with Decon® 90 (2% in DIW), then infused in 10% HNO 3 and 10% HCl for 48 h, followed by rinsing with DIW for more than five times, and finally dried in a Class 100 clean bench.…”

“…Hence, humic acid, mainly with aromatic ring structures (Wood, 1996;Thimsen and Keil, 1998;Vermeer and Koopal, 1998;Kleber et al, 2007), blocked the available surface of Fe/Mn oxide particles (Takahashi et al, 1999;Tomb acz et al, 2004;Qin et al, 2015), and weakened the adsorption of Rh, Pd and Pt onto the particles. In addition, irrespective of particles, there were complex interactions between PGEs and HA in seawater (Liu et al, 2019), which implies that HA competes with particles for adsorbing the ions of PGEs in seawater.…”

“…The results of zeta potential measurement showed that the values of the CaCO 3 and Al 2 O 3 single kind of particle systems changed greatly from 10.8 to À9.01 mV and À4.64 to À30.1 mV respectively when carrageenan was added, indicating an increase in affinity to cations caused by carrageenan. A strong interaction between Pd and Rh ions and carrageenan was found in Liu et al (2019). Therefore, the formation of PGEs-carrageenan complexes in carrageenan-biogenic and -lithogenic particle binary systems may influence the adsorption of PGEs onto particles greatly.…”

“…Third, the competitive adsorption occurred between PGEs-MOCs and PGEsparticles. MOCs and the colloids they formed could adsorb PGEs in seawater (Zakharova, 1987;Liu et al, 2019), which led to a reduction in the distribution of PGEs onto the inorganic particles through the competitive adsorption. In addition, the presence of MOCs in the solution could change the surface properties of the particles (Fein et al, 1999;Tomb acz et al, 2004;Feng et al, 2005;Janot et al, 2012;Sanderman et al, 2014).…”