Sorption and speciation of palladium under high ionic strength conditions

Both crystalline and sedimentary rocks are being considered as potential host rocks for a deep geological repository in Canada. Deep-seated sedimentary rocks in the Michigan Basin,Ontario, Canada contain highly saline ground and pore waters.The relatively high ionic strength (I) of these waters may influence speciation and rock matrix sorption properties. To this end, laboratory sorption experiments were conducted to examine sorption of palladium (Pd (II)) on sodium bentonite, illite and Ordovician age shale as a function of pH and solution ionic strength. Solutions with pH values in the range of 5 to 9 and I ranging from 0.1 to 4 M were considered. Experiments were performed under aerobic conditions at 25^oC, and the Eh value of the solution ranged from 470 to 500mV. The data from sorption experiments were used to validate surface complexation models developed in PHREEQC with the JAEA thermodynamic database (TDB). The sorption of Pd on bentonite, shale and illite all showed strong dependence on I. Palladium sorption on all solids decreased with increasing I, however the rate of decrease was greatly reduced beyond an I of 2M. The surface complexation models for montmorillonite (the major constituent clay mineral of sodium bentonite), and illite (the major constituent clay mineral of shale) also predicted a strong dependence on I. In the experiments, the sorption of Pd on bentonite, shale and illite showed a dependence on pH. Between I of 0.1M and 1M, sorption was observed to decrease with increasing pH. At I of 4M, sorption appears to peak around a pH of 7.The speciation of Pd was also evaluated using PHREEQC with the JAEA TDB. At low I (0.1 M), when pH is less than 6, negatively charged PdCl₂^- is predicted to be the dominant species and when pH is greater than 6 neutral Pd(OH)₂ is dominant. As I increases, the pH value of the transition point from PdCl₂^- dominance to Pd(OH)₂ dominance increases. (author)