Changes in cadmium and nickle adsorption over time in a periodically submerged soil

The objective of this study was to measure the rate of change of adsorptive properties of soils and sediments after submergence. Soil samples were collected from a cove which is inundated periodically each year. Samples were collected prior to submergence and analyzed for pH, Eh, organic carbon content, readily available iron and manganese oxides, free iron and manganese oxides , surface area, and the amount of cadmium and nickel adsorbed at pH 4 and the pH of the soil.

The amounts of nickel and cadmium adsorption were then investigated separately, in a 0.1 M NaNO₃ solution at the two pH levels, for eight additional sampling periods after submergence of the soils. Reducing conditions were maintained by refrigeration, exclusion of air, and by bubbling argon gas through the solutions. In a separate study, soils from four of the sampling points were tested for nickel and cadmium adsorption at pH 4 and the pH of the soil, in three different solutions (0.1 M NaNO₃, 0.1 M NaCl, and deionized water), and two different redox conditions (reduced, with Eh values of approximately 220 mV, and oxidized, with Eh values of approximately 430 mV).

Results of the adsorption:time experiment indicated a decrease in nickel and cadmium adsorption over time, with cadmium decreasing at approximately five times the rate of nickel. Data from the second study showed that adsorption decreased with increasing ionic strength and chloride concentration, and decreasing pH. Aeration of the soils lowered the pH and resulted in decreased adsorption for nickel only. The decrease in nickel adsorption was attributed to precipitation under acidic conditions of positively charged iron oxyhydroxides which repulse rather than adsorb positively charged nickel ions.

Adsorption mechanisms most affected by submergence were postulated based on these studies and previous research by other investigators. It was concluded that nickel was most affected by iron oxyhydroxide adsorption (manganese playing a secondary role), and cadmium adsorption was correlated to the amount of readily available manganese oxides and free manganese oxides. Submergence of the soils would act to increase the pH and solubilize the oxyhydroxides, resulting in a net loss of adsorption sites.