Citrate impairs the micropore diffusion of phosphate into pure and C-coated goethite

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 (α-FeOOH) for phosphate, we studied the kinetics of citrate-induced changes in microporosity and the phosphate sorption kinetics of synthetic goethite in the presence and absence of citrate in batch systems for 3 weeks (500 μM 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 N₂ 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 μmol 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 limitation 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 microporous hydrous Fe and Al oxides may be regarded as potential strategies of plants to cope with phosphate deficiency in addition to ligand-exchange.