Temperature and Aging Effects on the Surface Speciation of Cd(II) at the Goethite-Water Interface

摘要

Cadmium (Cd) adsorption at the goethite-water interface was studied as a function of aging period (16 h vs. 180 d (d: days) ) and equilibrium temperature (20°C vs. 40° C) using extended X-ray absorption fine structure (EXAFS) spectroscopy. Structural parameters gleaned from non-linear least-squares fitting of EXAFS data revealed that Cd ions form single edge-sharing CECd-Fe) surface complexes on goethite after 16 h regardless of equilibrium temperature. After 180 d, both 20 and 40° C EXAFS samples show the formation of double-edge sharing (2Ecd^Fe) surface complexes indicated by circa one Fe atom at an average radial distance (Rcd-Fe) of 3.30 A in addition to circa one Fe atom at 3.13 A. Contributions from double-corner sharing (2CCd-Fe) surface complexes at 3.80±0.05A remained minor, which we ascribed to differing reaction conditions of the samples in this vs. previous studies. With respect to recently published observations regarding the desorption behavior of Cd from goethite as a function of aging period and temperature, we conclude that the formation of'2ECd-Fe surface complexes over time provides increased stability to Cdions sorbedon the goethite surface, which is in good agreement with recent density functional theory calculations The effect of temperature on Cd desorption from goethite is likely due to the increased electrostatic attraction of Cd ions to the goethite surface at 40°C as a result of a decreasing point of zero charge with temperature.