Spectroscopic Study of Effects of Goethite Surfaces on the Simulated Maillard Reaction Forming Humic-like Substances

Abstract

Progress of the simulated Maillard reaction for 0.1 mol L −1 glycine and ribose mixture solution at 80 • C for 0-7 days forming humic-like substances in the presence/absence of goethite was examined by ultraviolet-visible (UV-VIS) spectroscopy. 254 nm and 420 nm absorption intensities for the product solutions with goethite increased faster than for those without goethite, indicating enhancement of the reaction progress in the presence of goethite in several days at 80 • C. Adsorption onto goethite of reactants of the reaction were directly monitored by attenuated total reflection infrared (ATR-IR) spectroscopy with thin layers of goethite on the ATR crystal (ZnSe). IR spectral changes with time for the reactant solution (0.1 mol L −1 glycine and ribose mixture solution) on the goethite layer suggested fast adsorption within a few hours at room temperature of oxygenated components of glycine onto positively charged goethite surface. In situ heating ATR-IR kinetic measurements of humic-goethite interfaces will be useful for examining mechanisms of surface processes leading to reaction enhancement.

Gas Cell Infrared and Attenuated Total Reflection Infrared Spectroscopic Studies for Organic–Inorganic Interactions in Adsorption of Fulvic Acid on the Goethite Surface Generating Carbon Dioxide

“…S2, Supplemental Material). Thin layers of goethite were made on the ATR crystal by a similar method to our previous research, 22 which had been conducted with reference to methods by Hug 29 and Luengo et al 30 In the present study, 100 mL of a 10 g L -1 goethite suspension was dried on the ATR crystal at room temperature overnight, and the goethite layer was rinsed by pure water to remove excess goethite on the crystal surface. After drying the rinsed layer, the ATR crystal was rinsed again, and finally, thin layers of goethite remained attached on the ATR crystal (Fig.…”

“…All spectra were recorded in absorbance mode against air background to obtain pATR spectra, or pATR ¼ -log 10 I/I 0 (pATR: intensity of pATR spectrum, I: reflection intensity, I 0 : incident intensity, p means -log 10 ). 22,31 The I 0 was measured as a background air spectrum in the blank cell at room temperature. After the background measurements, pure water was injected into the ATR cell with a plastic syringe, and a pure water spectrum was measured.…”

“…On the other hand, goethite was reported to enhance the formation of humic-like substances by the Maillard-type browning in our preliminary experiments. 22 In these catalytic enhancements of interactions of organic and inorganic materials, adsorption process is generally necessary to get their associations and may also have a role to increase local concentrations of reactants and to promote concentrationdependent reactions.…”