Citrate impairs the micropore diffusion of phosphate into pure and C-coated goethite Christian Mikutta * , Friederike Lang, Martin Kaupenjohann Department of Soil Science, Institute of Ecology, Berlin University of Technology, Salzufer 12, D-10587 Berlin, Germany Received 27 June 2005; accepted in revised form 13 October 2005 Abstract Anions of polycarboxylic low-molecular-weight organic acids (LMWOA) compete with phosphate for sorption sites of hydrous Fe and Al oxides. To test whether the sorption of LMWOA anions decreases the accessibility of micropores (<2 nm) of goethite (a-FeOOH) for phosphate, we studied the kinetics of citrate-induced changes in microporosity and the phosphate sorption kinetics of synthetic goe- thite in the presence and absence of citrate in batch systems for 3 weeks (500 lM of each ion, pH 5). We also used C-coated goethite obtained after sorption of dissolved organic matter in order to simulate organic coatings in the soil. We analyzed our samples with N 2 adsorption and electrophoretic mobility measurements. Citrate clogged the micropores of both adsorbents by up to 13% within 1 h of contact. The micropore volume decreased with increasing concentration and residence time of citrate. In the absence of citrate, phosphate diffused into micropores of the pure and C-coated goethite. The C coating (5.6 lmol C m 2 ) did not impair the intraparticle diffusion of phosphate. In the presence of citrate, the diffusion of phosphate into the micropores of both adsorbents was strongly impaired. We attribute this to the micropore clogging and the ligand-induced dissolution of goethite by citrate. While the diffusion lim- itation of phosphate by citrate was stronger when citrate was added before phosphate to pure goethite, the order of addition of both ions to C-coated goethite had only a minor effect on the intraparticle diffusion of phosphate. Micropore clogging and dissolution of micro- porous hydrous Fe and Al oxides may be regarded as potential strategies of plants to cope with phosphate deficiency in addition to ligand-exchange. Ó 2005 Elsevier Inc. All rights reserved. 1. Introduction Phosphate sorption to hydrous Fe oxides comprises a rapid initial adsorption to external surfaces followed by a slow reaction, which can last for days or weeks (Barrow et al., 1981; Torrent et al., 1990). The slow phosphate immobilization has been attributed to the diffusion of phosphate into microporous imperfections of the crystals, micro- and mesopores located between the crystal domains (Torrent, 1991; Strauss, 1992; Barrow et al., 1993; Fischer et al., 1996; Strauss et al., 1997; Makris et al., 2004), or the diffusion into aggregates of particles (Anderson et al., 1985; Willet et al., 1988). Torrent et al. (1990, 1992) observed that a portion of phosphate sorbed to microporous Fe oxi- des was not desorbable in 0.1 M KOH. This finding was attributed to both the slow rediffusion of phosphate out of micropores and the formation of binuclear surface com- plexes of phosphate. Also, Fuller et al. (1993) showed that the rate of the slow sorption of arsenate to ferrihydrite was limited by intraparticle diffusion. Polycarboxylic low-molecular-weight organic acids (LMWOA) successfully compete with phosphate for sorp- tion sites (Violante et al., 1991; Bhatti et al., 1998; Geelh- oed et al., 1998). This is especially relevant for the soil rhizosphere where exudation of LMWOA anions by plants and microorganisms is high. When polycarboxylic LMWOA anions are added to Al and Fe oxides or soils be- fore phosphate, a decrease in phosphate sorption is gener- ally noticed (Hue, 1991; Violante et al., 1991; Geelhoed et al., 1998; Hu et al., 2001). This may be attributed to di- rect site blocking, electrostatic repulsion or diffusion of 0016-7037/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.gca.2005.10.032 * Corresponding author. Fax: +49 30 314 73 548. E-mail address: christian.mikutta@tu-berlin.de (C. Mikutta). www.elsevier.com/locate/gca Geochimica et Cosmochimica Acta 70 (2006) 595–607