Retention and transport of mercury and nickel in soils

Abstract

Nickel (Ni) is one of many trace metals widely distributed in the environment. High concentrations of Ni in soils and aquifers have been observed worldwide, causing several potential human health impacts. Better understanding of Ni transport in soils and aquifers is necessary to assess and remediate insitu environmental contamination. The movement of Ni in soils and aquifers is highly dependent on adsorption-desorption reactions in the solid phase. In this study, kinetic batch, sequential extractions, and miscible displacement experiments were conducted to investigate the effect of several of environmental factors including soil type, reaction time and competing ions, on the fate of Ni in soils. In addition, forward and inverse modeling efforts were made to mathematically predict the reactivity of Ni transport in soils. Based on batch study results, adsorption of Ni was highly nonlinear and strongly kinetic. The comparison of Ni sorption on soil followed the sequences: Windsor < Olivier < Webster, which was related to soil propertities (CEC, clay content, pH and organic matter). Desorption of Ni from all soils were hysteretic in nature which is an indication of lack of equilibrium retention and/or irreversible or slowly reversible processes. A sequential extraction procedure provided evidence that a significant amount of Ni was irreversibly adsorbed on all soils. Moreover, a multi-reaction model (MRM) with equilibrium, kinetic and irreversible sorption successfully described the adsorption kinetics of Ni in Windsor, Olivier and Webster soils and was capable of predicting the desorption of Ni from these soils. Column transport experiments indicated strong Ni retardation followed by slow release or extensive tailing of the breakthrough curves (BTCs). We evaluated several MRM formulations for prediction capability of Ni retention and transport in soils and concluded that nonlinear reversible, along with a consecutive or concurrent irreversible reactions were the dominant mechanisms. The use of batch rate coefficients as model parameters for the predictions of Ni BTCs underestimated the extent of retention and overestimated the extent of Ni mobility for all soils. When utilized in an inverse mode, the MRM model provided good predictions of Ni BTCs and the distribution of Ni with soil depth in soil columns. In natural soil and water environments the competition between Ni and Cadmium (Cd) has the potential of increasing Ni mobility and bioavailability. Our results from batch experiments demonstrated that rates and amounts of Ni adsorption by these soils were significantly reduced by increasing Cd additions. The presence of Cd