Adsorption and surface complexation modeling of palladium, rhodium and platinum in surficial semi-arid soils and sediment

Sorption isotherms, time-dependent adsorption and surface complexation modeling studies were used to investigate the post-depositional mobility of three of the platinum group-elements (Pd, Rh, and Pt) in semi-arid soil and sediment samples with varying surface properties. The acidity constants (Log Ka1 and Log Ka2), optimized from batch titration data, ranged from 4.69 to 5.34 for Log Ka1 and from −6.51 to −7.61 for Log Ka2, suggesting the occurrence of both protonation and deprotonation reactions on the solid surfaces. Partition coefficients and removal rates of the metals had a general trend of Pd > Pt > Rh. The sediment sample, with the highest clay content and exchangeable cation concentrations, also had the highest affinity for the metals. The times required for sediment to adsorb 63% of the metals were 2.63 h, 4.08 h and 10.64 h for Pd, Pt and Rh, respectively. The FITEQL program successfully optimized the conditional binding constants of the metals on the solids from batch adsorption data. The constants decreased in the order of Pd > Rh > Pt, which was consistent with the observed high affinity of the solids for Pd. The modeling results also showed that aqueous Pd was the least sensitive to pH followed by Rh and Pt. However, metal adsorption below the points of zero net proton charges (ca. pH 6.7) is attributable to the involvement of permanent negatively charged binding sites in the adsorption process. Notably, partition coefficients, removal rates and conditional binding constants all showed a high affinity of Pd for the solids. A similarity between the model outputs and the batch adsorption data indicates the suitability of the model for describing the mobility and retention of the three metals in semi-arid soils and sediments.