Inhibition of calcite growth by alginate L.Z. Lakshtanov a,b,⇑ , N. Bovet a , S.L.S. Stipp a a NanoGeoScience, Department of Chemistry, Copenhagen University, Universitetsparken 5, DK-2100 Copenhagen K, Denmark b Institute of Experimental Mineralogy, 142432 Chernogolovka, Russia Received 28 November 2010; accepted in revised form 13 April 2011; available online 20 April 2011 Abstract The kinetics of calcite precipitation in the presence of alginate was investigated using the constant composition technique. In the concentration range investigated (0.0002–0.005 g L 1 ), alginate inhibits calcite precipitation. The extent of inhibition increased with increased alginate concentration and decreased solution supersaturation. Alginate adsorption, derived from normalized calcite precipitation rates, is described satisfactorily by the Langmuir adsorption model. At lowest supersatura- tion, alginate adsorption onto calcite probably reaches its maximal uptake of 7.5E-4 g m 2 , corresponding to surface coverage of one molecule for each 200–300 nm 2 , depending on the molecular mass of alginate. This means that one alginate molecule can be bound over 100–150 Ca surface sites. Initially, on the surface of the inhibited calcite, XPS identified alginate but after further time in solution, when the system had recovered, XPS demonstrated that it disappeared from the surface, presumably buried under the newly formed calcite. The alginate affinity constant decreases with increasing supersaturation, evidence for incomplete adsorption. A simple model based on competition between growth and desorption effectively describes the observed change in the adsorption constant. Ó 2011 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Calcite precipitation affects many environmental and geological systems. It is known to regulate the fate and mobility of anthropogenic pollutants, especially heavy met- als, by adsorption as well as incorporation during coprecip- itation (Davis et al., 1987; Tesoriero and Pankow, 1996; Rimstidt et al., 1998; Curti, 1999; Lakshtanov and Stipp, 2004). On the other hand, impurities or additives can have a significant effect on the rates of nucleation and growth, as well as on crystal size and morphology (House, 1986; Amjad, 1987; Didymus et al., 1993; Butler et al., 2006). In most cases, this effect manifests itself in the inhibition of crystal growth. Inhibitors of calcite precipitation are of interest in a number of fields, ranging from engineering sciences to med- icine and geochemistry and a lack of fundamental under- standing about how trace components interact with natural solids such as rocks, soil and sediments limits our ability to predict reactions in natural systems. In particular, the inhibitory effect of sea water species such as magnesium and orthophosphate (Baker et al., 1980; Wilbur and Bernhardt, 1984; Morse et al., 2007) could explain the ex- tremely slow recrystallization rates of chalk. Moreover, there is evidence that biopolymers produced by bacteria and algae, particularly the polysaccharides, significantly in- hibit crystallization of calcium carbonate (Gunthorpe and Sikes, 1986; Welch et al., 1999; Kanakis et al., 2001; Manoli and Dalas, 2002; Butler et al., 2006). Alginate is produced by brown algae and it is known to affect calcite growth. In the work reported here, we investi- gated the effect of the sodium salt of alginic acid, a linear copolymer of mannuronic (M) and guluronic (G) acids (Fig. 1). These appear as blocks of M and G in any sequence and of any length. Poly-G blocks are mainly responsible for chelating cations in the solution. Poly-M blocks are able to 0016-7037/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.04.014 ⇑ Corresponding author at: NanoGeoScience, Department of Chemistry, Copenhagen University, Universitetsparken 5, DK- 2100 Copenhagen K, Denmark. Tel.: +45 35 32 02 19; fax: +7 49652 49687. E-mail addresses: ll@nano.ku.dk, leonid@iem.ac.ru (L.Z. Lakshtanov). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 75 (2011) 3945–3955