Do organic ligands affect forsterite dissolution rates? Julien Declercq, Olivier Bosc, Eric H. Oelkers ⇑ GET/Université Paul Sabatier, Observatoire Midi-Pyrénées, UMR 5563 (CNRS/UPS/IRD/CNES), 14 Avenue Edouard Belin, 31400 Toulouse, France article info Article history: Received 8 November 2012 Accepted 27 September 2013 Available online 4 October 2013 Editorial handling by M. Hodson abstract Far-from equilibrium, steady state forsterite dissolution rates were measured at pH 3 and 25 °C in aque- ous solutions containing 0.1 m/kg NaCl and up to 0.1 mol/kg of 13 distinct dissolved organic ligands in mixed-flow reactors. The organic ligands considered in this study include those common in Earth surface environments and those considered as potential catalysts for use in CO 2 sequestration efforts: acetate, oxalate, citrate, EDTA 4 , glutamate, gluconate, malonate, aspartate, tartrate, malate, alginate, salycilate and humate. The presence of up to 0.1 mol/kg of each organic ligand altered forsterite dissolution rates less than 0.2 log units, which is the estimated uncertainty of the measured rates. Results obtained in this study, therefore, suggest that the presence of aqueous organic anions negligibly affects forsterite far-from equilibrium dissolution rates in most natural environments, and indicate that forsterite carbonation may not be appreciably accelerated by organic ligand catalysis. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction A significant number of studies have been aimed at characteriz- ing forsterite dissolution rates at various solution compositions and temperatures (Luce et al., 1972; Sanemasa et al., 1972; Grand- staff, 1978, 1986; Murphy and Helgeson, 1987, 1989; Blum and Lasaga, 1988; Van Herk et al., 1989; Wogelius and Walther, 1991, 1992; Casey and Westrich, 1992; Jonckbloedt, 1998; Awad et al., 2000; Chen and Brantley, 2000; Rosso and Rimstidt, 2000; Pokrov- sky and Schott, 1999, 2000a,b; Oelkers, 2001; Giammar et al., 2005; Golubev et al., 2006; Hänchen et al., 2006; Olsen and Rims- tidt, 2008; Rimstidt et al., 2012; Saldi et al., 2013; Wang and Gaim- mar, 2013). There has been increased recent interest in these rates owing to the potential application of forsterite in carbon capture and storage efforts (e.g. Giammar et al., 2005; Oelkers and Schott, 2005; Marini, 2007; Kelemen and Matter, 2008; Oelkers et al., 2008; Dufaud et al., 2009; Prigiobbe et al., 2009; Garcia et al., 2010; King et al., 2010a,b; Daval et al., 2011; Guyot et al., 2011; Olsson et al., 2012). Forsterite is commonly thought of as the best source of the divalent metal cations required for mineral carbon- ation due to its fast dissolution rates and global abundance. The carbonation of forsterite in CO 2 -rich fluids can occur via Mg 2 SiO 4ðsÞ Forsterite þ2CO 2 þ 2H 2 O ¼ 2MgCO 3ðsÞ Magnesite þH 4 SiO 4ðaqÞ ð1Þ where the resulting aqueous silica could eventually precipitate as amorphous silica or other silicate minerals (cf. Weres et al., 1981; Teir et al., 2007; Orlando et al., 2001). Numerous studies have fo- cused on the application of reaction (1) to carbon sequestration on an industrial scale (e.g. O’Connor et al., 2000a,b; Wolf et al., 2004; Giammar et al., 2005; Maroto-Valer et al., 2005; Chen et al., 2006; Gerdemann et al., 2007; Oelkers et al., 2008; Broecker, 2012). Reaction (1) involves the coupling of two processes: forste- rite dissolution and magnesite precipitation. The overall rate of car- bonation reaction (1) is, therefore, a function of these two processes (e.g. Saldi et al., 2013). Catalysis which could accelerate the rates of forsterite dissolution thus may also accelerate its carbonation in ac- cord with reaction (1). A vast number of past studies have suggested that organic li- gands can increase the dissolution rates of silicate minerals sub- stantially (e.g. Huang and Kiang, 1972; Manley and Evans, 1986; Mast and Drever, 1987; Bennett et al., 1988; Welch and Ullman, 1993, 1996; Poulson et al., 1997; Cama and Ganor, 2006; Golubev and Pokrovsky, 2006; Golubev et al., 2006; Ganor et al., 2009; Pokrovsky et al., 2009; Schott et al., 2009). The effect of the pres- ence of organic ligands on forsterite dissolution has been consid- ered in detail by Grandstaff (1986), Wogelius and Walther (1991), Hänchen et al. (2006), Olsen and Rimstidt (2008) and Prigiobbe and Mazzotti (2011). Grandstaff (1986) reported an in- crease of forsterite dissolution rates when the reactive aqueous fluid contained citrate, EDTA, oxalate, tannic acid, succinate and phthalate at 25 °C and pH 3.5 and 4.5. Wogelius and Walther (1991) reported that the addition of either 0.05 mol/kg potassium phthalate or 10 3 mol/kg ascorbic acid increases forsterite disso- lution rates at 25 °C by 0.75 log units at pH 4, but this effect de- creased with decreasing pH. Hänchen et al. (2006) reported that 10 3 mol/kg citric acid increased forsterite dissolution rates at 90 °C by 0.25 log units at pH 3.4 and 0.5 log units at pH 4.5. Ol- sen and Rimstidt (2008) reported that 10 4 mol/kg oxalic acid in- creases forsterite dissolution rates by a factor of 6 at 25 °C and 2.5 < pH < 6.5. 0883-2927/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2013.09.020 ⇑ Corresponding author. Tel.: +33 561332575. E-mail address: oelkers@get.obs-mip.fr (E.H. Oelkers). Applied Geochemistry 39 (2013) 69–77 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem