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DIVISION S-2 - SOIL CHEMISTRY

Temperature Effects on Nickel Sorption Kinetics at the Mineral–Water Interface

^a National Risk Management Research Lab., USEPA, 5995 Center Hill Avenue, Cincinnati, OH 45268
^b Dept. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303

Corresponding author (Scheckel.Kirk{at}epa.gov)

In recent years, innovative studies have shown that sorption of metals onto natural materials results in the formation of new mineral-like precipitate phases that increase in stability with aging time. While these findings have demonstrated the usefulness of current state-of-the-art molecular-scale methods for confirming macroscopic data and elucidating mechanisms, basic kinetic and thermodynamic parameters for the formation of the metal precipitates have not been examined. This study examined Ni-sorption kinetics on pyrophyllite, talc, gibbsite, amorphous silica, and a mixture of gibbsite and amorphous silica over a temperature range of 9 to 35°C. Using the Arrhenius and Eyring equations, we calculated the energy of activation (E_a) and enthalpy (H), entropy (S), and free energy of activation (G), related to the formation of the Ni precipitates. Based on values of E_a (93.05 to 123.71 kJ mol^-1) and S (-27.51 to -38.70 J mol^-1), Ni sorption on these sorbents was surface-controlled and an associative mechanism. The H values (90.60 to 121.26 kJ mol^-1) suggest, as indicated by E_a values, that an energy barrier was present for the system to overcome in order for the reaction to occur. Additionally, the large, positive G values suggest there is an energy barrier for product formation. Although metal precipitation reactions often occur in the natural environment, this study shows that the rate of these reactions depends strongly on temperature.

Abbreviations: ICP, inductively coupled plasma spectrometry • LDH, layered double hydroxide • XRD, x-ray diffraction

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