Kinetic Studies of Chromium (VI) Binding to Carboxylic Acid- and Methyl Ester-Functionalized Silica/Water Interfaces Important in Geochemistry

Abstract

Real-time kinetic measurements of hexavalent chromium binding to fused silica surfaces functionalized with carboxylic acid and methyl ester terminal groups are performed in situ using resonantly enhanced surface second harmonic generation (SHG) at pH 7 and 300 K. These functional groups were chosen because of their high abundance in humic acids and related biopolymers. Kinetic measurements are conducted in the submonolayer regime using chromate solution concentrations ranging from 1 x 10(-6) to 2 x 10(-5) M. The adsorption rates were analyzed using the standard Langmuir model and the Frumkin-Fowler-Guggenheim model. The desorption kinetics are consistent with a first-order process. These results indicate that hexavalent chromium mobility in carboxylic acid- and ester-rich soil environments increases with decreasing chromate concentrations. Based on the measured half-lives of the adsorbed Cr(VI) species, remobilization of bound hexavalent chromium due to natural or anthropogenic events that lower the chromate concentration in the aqueous phase can occur within minutes.

Beyond the Gouy–Chapman Model with Heterodyne-Detected Second Harmonic Generation

“…The application of second harmonic generation (SHG) and sum frequency generation (SFG) to charged aqueous interfaces has been an area of substantial interest for several decades. − The field has been greatly influenced by the foundational work of Eisenthal and co-workers, who interpreted the SHG signal generated from the fused silica/water interface, E SHG , as consisting of a second-order component, χ (2) , and an interfacial potential-dependent third-order component, χ (3) , using the following model Here, Φ(0) is the interfacial potential present at the zero plane of the interface, referenced to zero potential in the bulk solution. The χ (2) term in eq originates from molecules that are net oriented at the interface.…”

pH-Dependent Inversion of Hofmeister Trends in the Water Structure of the Electrical Double Layer

“…The chemistry of water at the mineral/water interface is complex, largely due to deviations in the behavior of water molecules at the interface from their counterparts in the bulk liquid. − In particular, the solution pH and electrolyte composition are among the most important parameters that affect the behavior of water at the silica/water interface. Many studies have examined the effect of pH and electrolyte composition on dissolution rates of silica and quartz, − with possible mechanisms of dissolution related to cation hydration and changes in water orientation at the silica/aqueous interface. , Consequently, elucidating the structural properties of water at the silica/aqueous interface is crucial in understanding the interfacial phenomena that play significant roles in chemical separations as well as geochemical, petrochemical, environmental, and biological processes. − …”

Aqueous proton transfer across single-layer graphene

“…A detailed description of our SHG setup has been described previously 60 61 62 63 . Briefly, we use a regeneratively amplified Ti:sapphire system (Hurricane, Spectra Physics) that operates at a kHz repetition rate to produce 120 fs pulses to pump an optical parametric amplifier (OPA-CF, Spectra Physics) tuned to produce 600 nm light.…”