Adsorption and desorption kinetics of rhodium (III) and platinum (IV) in turbid suspensions: potential tracers for sediment transport in estuarine flumes

Dissolved Rh (III) and Pt (IV) were added to suspensions (140 g L− 1) of cohesive and non-cohesive estuarine sediments and their time-dependent uptake monitored. Adsorption of Rh and Pt onto cohesive sediments was rapid and near complete removal (> 95%) occurred within 24 h and 48 h, respectively. The chemical response times (Tresp: the time required to reach 63% of the equilibrium) were in the range 3.5 to 6 h. Uptake of Rh and Pt onto non-cohesive sediment was slower, with Tresp values of 25 and 33 h, respectively, and extents of adsorption were significantly less than for cohesive sediments. Almost complete recovery of adsorbed Rh and Pt, on both sediment types, was obtained using concentrated HNO3, whereas 1 mol L− 1 HCl yielded only partial recovery. Time-dependent desorption experiments, involving both sediment types doped with Rh and Pt (and in some cases Ni and Zn), were conducted in tap water and in tap water amended with 5 g L− 1 NaCl at a particle concentration of 1 g L− 1. The cohesive sediments released approximately 1.5% of Rh and about 1% of Pt at equilibrium, with Tresp values in the range 0.4 h to 0.6 h, whereas, the non-cohesive sediments released 1.5% Rh and 2.6% Pt at equilibrium, with Tresp ≤ 1 h. The extents of desorption of Ni and Zn were significantly higher (maximum 14%) and were dependent on the NaCl concentration. Rhodium and Pt are taken up irreversibly onto cohesive sediments, and to some extent on non-cohesive sediments, and are quantitatively recovered by concentrated HNO3. This suggests that Rh and Pt can be used as tracers in studies of particle movement in laboratory flumes, thereby improving models predicting the behaviour of contaminated sediments in turbid estuaries.