Acid Polysaccharide Coatings on Microporous Goethites

Abstract

Organic coatings on Fe oxides can decrease the accessibility of intraparticle pores for oxyanions like phosphate. We hypothesized that the slow sorption of phosphate to goethite coated with polygalacturonate (PGA) is controlled by the accessibility of external goethite surfaces to phosphate rather than by diffusion of phosphate into micropores (Ø < 2 nm). We studied the phosphate sorption kinetics of pure and PGA‐coated goethites that differed in their microporosity (N2 at 77 K, 46 vs. 31 mm3 g−1). Because drying may affect the structure or surface coverage of PGA, we also tested the effect of freeze‐drying on the slow phosphate sorption. The samples were examined by gas adsorption (N2, CO2) and electrophoretic mobility measurements. Phosphate sorption and PGA‐C desorption were studied in batch experiments for 3 wk at pH 5. In PGA‐coated samples, the slow phosphate sorption was independent of micropore volume. Phosphate displaced on average 57% of PGA‐C within 3 wk. Similar to phosphate sorption, the PGA‐C desorption comprised a rapid initial desorption, which was followed by a slow C desorption. Sorption competition between phosphate and presorbed PGA depended on the <10‐nm porosity and the C loading of the adsorbent. The efficacy of phosphate to desorb PGA generally increased after freeze‐drying. We conclude for PGA‐coated goethites that (i) freeze‐drying biased the slow phosphate sorption by changing the structure/surface coverage of PGA, and (ii) within the time frame studied, micropores did not limit the rate of the slow phosphate sorption. Rather, the slow, gradual desorption of PGA and/or the diffusion of phosphate through PGA coatings controlled the slow phosphate sorption to PGA‐coated goethite.