Does variscite control phosphate availability in acidic natural waters? An experimental study of variscite dissolution rates Teresa Roncal-Herrero, Eric H. Oelkers ⇑ Ge ´ochimie et Bioge ´ ochimie Expe ´rimentale, LMTG-Universite ´ Paul Sabatier-CNRS-IRD-OMP, 14 av. Edouard Belin, 31400 Toulouse, France Received 10 March 2010; accepted in revised form 18 October 2010; available online 27 October 2010 Abstract The dissolution rates of natural, well crystallized variscite (AlPO 4 2H 2 O) were determined from the evolution of aqueous Al and P concentrations in closed and open-system mixed-flow reactors at 25 °C and pH from 1.5 to 9.0. Measured dissolu- tion rates decrease with increasing pH, from 6  10 16 mol/cm 2 /s at pH 1.5 to 5  10 17 mol/cm 2 /s at pH 5.89, and then increase with increasing pH to 4  10 16 mol/cm 2 /s at pH 9.0. Geochemical modeling calculations, performed using measured dissolution rates, indicate that it would take no more than a few weeks or months to equilibrate a mildly acidic, Al and P-free solution with variscite. Hence, variscite can buffer aqueous phosphate concentrations in mildly acidic near surface environ- ments. This conclusion is confirmed by consideration of the compositions of natural waters. Ó 2010 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Phosphorus is an essential element for life, yet, if its concentration is too high, aqueous phosphate can lead to eutrophication and environmental damage (Oelkers and Valsami-Jones, 2008). Knowledge of the dissolution and precipitation rates of the major phosphate bearing minerals should help us to better understand and potentially control the concentrations of phosphate in natural waters. Variscite (AlPO 4 2H 2 O) has been postulated to control phosphate concentrations in acidic natural waters (e.g. Lindsay, 1979). The potential role of variscite in buffering natural waters stems from its low solubility; thermodynamic calcula- tions indicate that variscite is the least soluble common phos- phate mineral at 25 °C and 3 < pH < 6 (Stumm and Morgan, 1996). An essential factor for mineral buffering is that its dis- solution/precipitation rates be sufficiently fast to assure rapid mass transfer into and out of the aqueous phase. To as- sess its potential role in buffering phosphate availability, var- iscite dissolution rates were measured at 25 °C as a function of pH in both closed and open-system reactors. The purpose of this paper is to present the results of these experiments and use them, together with field observations, to determine the role of variscite in controlling phosphate availability in acidic natural waters. Variscite is isostructural with strengite (FePO 4 2H 2 O); these two minerals form a complete solid-solution (Humin- icki and Hawthorne, 2002; Taxer and Bartl, 2004). Both minerals are orthorhombic and have a framework of alter- nating PO 4 tetrahedra and Fe or Al octahedra. Variscite can form from the interaction of phosphate-rich meteoric waters with aluminum bearing rocks (Duggan et al., 1990), and in mine soils (Alvarez Rodrı ´guez et al., 1996), bat guano deposits (Shahack-Gross et al., 2004), and caves (Moro Benito et al., 1995). Amorphous aluminum phosphate phases have been reported to form in acid soils (Kumar et al., 1991; Rodriguez et al., 1996). The rates of interaction between aqueous fluids and var- ious phosphate minerals have been studied in the past. These studies include work on the dissolution and/or 0016-7037/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.10.012 ⇑ Corresponding author. Tel.: +33 5 61 33 25 75; fax: +33 5 61 33 25 60. E-mail addresses: roncal@lmtg.obs-mip.fr (T. Roncal-Herrero), oelkers@lmtg.obs-mip.fr (E.H. Oelkers). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 75 (2011) 416–426