Temperature effects on chlorinated-benzene sorption to hydrophobic surfaces

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Sorption and desorption of chlorinated benzenes were investigated in a series of column experiments using porous silica that had phenyl groups bonded to the surface; the mass-fraction organic carbon was 0.016. Both sorption and desorption curves were asymmetrical, but they were mirror images of each other for most experiments, indicating good sorption reversibility. The resulting breakthrough curves were fit to an advection-dispersion mathematical model, with sorption as a first-order, reversible reaction. Significantly greater tailing in the chlorinated-benzene breakthrough curves versus the salt-tracer ones was evidence of slow sorption and desorption. ΔH° values for di-, tri- and tetra-chlorobenzene were 13–21 kJ mol −1, indicative of strong van der Waals binding. Despite these small values, slow desorption was attributed to slow binding and release rather than diffusion through the bonded organic phase. Desorption rates decreased in going from di- to tri- to tetra- to penta-chlorobenzene. This decrease was significantly more than the decrease in molecular-diffusion coefficients in the same series, suggesting a chemical rather than a physical rate control. There was less difference in sorption rates through the series, suggesting an inverse relation between partition coefficient and desorption rate. ΔG° values were −17 to −23 kJ mol −1, giving TΔS° values of about 4 kJ mol −1. Thus enthalpic contributions to sorption appear to be of greater importance than entropic contributions.