An investigation of water uptake on clays minerals using ATR-FTIR spectroscopy coupled with quartz crystal microbalance measurements

It is becoming increasingly clear that the heterogeneous chemistry of mineral dust aerosol is a function of relative humidity (f(RH)) as water on the surface of the mineral dust particles can enhance or inhibit its reactivity depending on the reaction. Since clay minerals make up a significant component of the mineral dust aerosol, it is important to understand water uptake on this large fraction of dust present in the Earth's atmosphere. In this study, bulk and surface properties of several types and sources of clay minerals are characterized using a variety of techniques, including surface area measurements, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX) analysis, X-Ray diffraction (XRD), Mossbauer spectroscopy, and X-ray photoelectron spectroscopy (XPS). For these well-characterized clays, ATR-FTIR spectroscopy is combined with quartz crystal microbalance (QCM) mass measurements to investigate water uptake as a f(RH). Similar measurements were also done for alpha-Al2O3 and synthetic NaY zeolite for comparison. Water uptake on the clay minerals, although variable and dependent on both the type and source of the clay, is greater than that found for alpha-Al2O3, a metal oxide, and in many cases similar or greater than NaY zeolite, a crystalline porous material. The presence of cations with large hydration energies significantly increases the water uptake capacity for the clay minerals. The atmospheric implications of these results are discussed.