| In natural aqueous systems, adsorption and co-precipitation (sorption) are regarded as possible metal sequestration mechanisms that would promote in situ passive remediation. Calcite is a ubiquitous mineral on the Earth's surface, whose propensity to scavenge trace metals has been extensively investigated. Since Nickel (Ni) appears on the EPA "priority list" of pollutants, identifying factors that may control its fate in calcite-rich natural waters (e.g. groundwaters, carbonate-rich coastal and marine sediments) is critical. Unfortunately, scarce literature is dedicated to the influence of mineral properties and solution chemistry on the affinity and sorption mechanisms of Ni on the calcite surface. With that in mind, batch sorption experiments were conducted, under controlled laboratory conditions (ambient temperature of 25 +/- 2ºC, and atmospheric pCO2= 10 - 3.41atm) in calcite-saturated 0.1M and 0.7M NaCl solutions over a range of pH (7.5 to 8.9) and total Ni concentrations- [Ni] total- (1.2 x 10-4 to 3.5 x 10-6 M), to characterize the affinity of Ni for the calcite surface. Our experimental results revealed that the first 24 hours of reaction are critical to the metal-mineral interaction and the fractional Ni(II) sorption varied with solution pH and Ni(II) aqueous speciation. Notably, Ni sorption onto the calcite surface increased with pH and was well correlated to the relative abundance of the NiCO 30(aq) species (ion-pair) over the pH range studied. After 24 hours, the overall Ni fractional sorption was attenuated with an increase of the background electrolyte (NaCl) concentration. The application of adsorption isotherm models (Langmuir and Freundlich) allowed us to highlight the dependence of adsorption to the [Ni]total and solution pH. Finally, irrespective of the solution ionic strength (0.1 and 0.7 M NaCl), Ni did not desorb readily when the total Ni(II) concentration in solution exceeded 4.0 x 10-5M. |
