Iron-Organic Matter Coprecipitates: Comparing Molar Enthalpies of Ion Exchange.

The interactions of organic components with mineral phases play significant functions in several biogeochemical processes. In soils ferrihydrite, often precipitate from solution containing dissolved organic matter (OM). The inclusion of small amounts of OM is found to significantly change the morphology and reactivity of natural ferrihydrite compared to its synthetic counterpart formed in the absence of OM. This study aims at a better understanding of the surface charge properties of Fe-OM coprecipitates. Specifically our objective is to quantify how humic acid (HA) inclusion via coprecipitation affects the molar enthalpies of outer-sphere exchange reactions on ferrihydrite surfaces. Since the measured heat of exchange is directly related to surface charge these results will indicate how coprecipitation alter the charge characteristics and along with other investigative techniques will reveal the underlying nature of the Fe-HA interactions. We therefore prepared a series of ferrihydrite-OM coprecipitates by titration of HA-Fe(III) solution to pH 7.5 at C/Fe ratio of 0.1, 0.3, and 0.5. We used Elliott soil HA purchased from the International Humic Substance Society (IHSS). The coprecipitates were characterized using X-ray diffraction, N₂-gas adsorption, zeta potential measurements, transmission electron microscopy, and X-ray absorption spectroscopy. The heats of exchange were obtained using a flow adsorption calorimeter. Solutions of 50 mM NaCl and NaNO₃at pH of 5.8 were used to measure the heats of Cl and NO₃ exchange for anion exchange capacity (AEC) and solutions of 50 mM NaCl and KCl for the heats of Na and K exchange for cation exchange capacity (CEC). Quantitative measurements of AEC and CEC along with calorimetrical data yielded the molar enthalpies of exchange in KJ/mol. The heats of exchange (in KJ) will also be normalized to surface area and mole of Fe and compared for different C/Fe ratio precipitates.