While it is well known that chromium contamination in groundwater represents a considerable threat to the environment, little is known about the heterogeneous processes that govern chromium interaction with solid materials in soil. Using the nonlinear optical laser spectroscopy surface second harmonic generation (SHG), we have studied chromate adsorption and desorption at the fused quartz/liquid water interface in real time, at room temperature and at chromate concentrations between 1 × 10 -6 and 2 × 10 -4 M. Adsorbed chromate is spectroscopically identified via a two-photon resonance of one of its ligand-to-metal charge-transfer bands with the fundamental probe light. Adsorption isotherm measurements at 300 K result in a free chromate adsorption energy ∆G ads of 38 ( 1 kJ/mol at pH 7. Real-time kinetic measurements of chromate adsorption and desorption show reversible chromate binding to the fused quartz/water interface, consistent with the high mobility of Cr(VI) in soils and the ∆G ads determined from our adsorption isotherm measurements. The pH dependence of chromate binding to the fused quartz/water interface is discussed.

Resonantly enhanced surface second harmonic generation (SHG) measurements carried out at pH 7 and room temperature were performed to study how surface-bound carboxylic acid and methyl ester functional groups control the interaction of chromate ions with fused silica/water interfaces. These functional groups were chosen because of their high abundance in humic and fulvic acids and related biopolymers commonly found in soils. They were anchored to the silica surface using organosilane chemistry to avoid competing complexation processes in the aqueous solution as well as competitive adsorption of the organic compounds and chromate. The SHG experiments were carried out at room temperature and pH 7 while using environmentally representative chromate concentrations ranging from 1 x10(-6) to 2 x 10(-4) M. Chromate is found to bind to the acid- and ester-functionalized silica/water interfaces in a reversible fashion. In contrast to the plain silica/water interface, chromate binding studies performed on the functionalized silica/water interfaces show S-shaped adsorption isotherms that can be modeled using the Frumkin-Fowler-Guggenheim (FFG) model. This model predicts a coverage-dependent binding constant of K(ads) x exp(gtheta). Values for g are found to be 3.2(2), 2.1(2), and 1.3(2) for the carboxylic acid-, the ester-, and the nonfunctionalized silica/water interfaces, respectively, and are consistent with stabilizing lateral adsorbate-adsorbate interactions among the Cr(VI) species adsorbed to the functionalized surfaces. The FFG model allows for the parametrization of the solid-liquid partition coefficient and chromate retardation factors in silica-rich soil particles whose surfaces contain organic adlayers rich in carboxylic acid and methyl ester groups. The straightforward model presented here predicts that chromate retardation increases by up to 200% when carboxylic acid functional groups are present at the silica/water interface. Increases up to 50% are predicted for methyl ester-containing organic adlayers, and the retardation factor remains effectively near unity for the plain silica/water interface (no siloxanes present).

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.

Carbon-hydrogen (C-H) vibration modes serve as key probes in the chemical identification of hydrocarbons and in vibrational sum-frequency generation spectroscopy of hydrocarbons at the liquid/gas interface. Their assignments pose a challenge from a theoretical viewpoint. In this work, we present a detailed study of the C-H stretching region of dimethyl sulfoxide using a new ab initio molecular dynamics (AIMD) module that we have implemented in NWChem. Through a combination of AIMD simulations and static normal mode analysis, we interpret experimental infrared and Raman spectra and explore the role of anharmonic effects in this system. Comprehensive anharmonic normal mode analysis of the C-H stretching region casts doubt upon previous experimental assignments of the shoulder on the symmetric C-H stretching peak. In addition, our AIMD simulations also show significant broadening of the in-phase symmetric C-H stretching resonance, which suggests that the experimentally observed shoulder is due to thermal broadening of the symmetric stretching resonance.

Surface second harmonic generation (SHG) phase measurements are carried out on methyl ester-functionalized fused quartz/water interfaces in the presence and absence of Cr(VI). The experiments are performed at pH 7, room temperature, and a chromate concentration of 10(-4) M, which corresponds to monolayer Cr(VI) coverage. The liquid/solid interface is probed from the fused quartz side by directing the probe light field at 580 nm onto the interface together with an SHG reference signal at 290 nm that is collinear with the fundamental. The phase difference of the SHG signals generated at the interface in the presence and absence of Cr(VI) is 85 degrees, which is consistent with SHG resonance enhancement observed for the surface-bound Cr(VI) near 290 nm. The optical arrangement discussed here does not require vacuum technology or optics that compensate for the dispersion of the fundamental and the second harmonic E-fields in the two condensed-phase media. This approach is general and can be applied for analyzing thermodynamic and kinetic data derived from SHG measurements of physical and chemical processes occurring at any buried interface.