It is now recognized that an understanding of hexavalent chromium pollution in the environment is tied to an understanding of how hexavalent chromium binds to geosorbent surfaces. We have applied the nonlinear optical laser spectroscopy surface second harmonic generation (SHG) to study the adsorption and desorption kinetics of submonolayer amounts of chromate interacting with the fused quartz/water interface at pH 7 and at room temperature. The chromate concentrations are varied between 10-6 and 10-5 M. The adsorption and desorption behavior of chromate at the fused quartz/water interface can be described by a Langmuir adsorption and a first-order chromate desorption model with an adsorption rate constant of 3(1) × 103 s-1 M-1 and a desorption rate constant of 0.9(7) × 10-3 s-1. At 300 K and pH 7, the resulting equilibrium constant for chromate binding is in good agreement with equilibrium constants obtained from Langmuir isotherm measurements carried out between pH 4 and 9. Thus, thermodynamic and kinetic measurements carried out in separate studies result in a chromate binding constant of 3.3 × 105 ((+17 × 105)/(- 2.1 × 105)) M-1 and a corresponding standard free energy of chromate binding to fused quartz/water interfaces of 32 (+4/-3) kJ/mol. In agreement with the general notion that chromate is highly mobile in most soil environments, a simple transport model predicts that chromate would move between 2 and 9% slower than the noninteracting groundwater phase; that is, it is poorly retained.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry