Kinetics of Phosphate Sorption to Polygalacturonate‐coated Goethite

Abstract

Biogenetic polysugars may affect the sorption characteristics of soil mineral particles in the rhizosphere. We hypothesized that polygalacturonate [PGA, (C6H7O6)n−] coatings on goethite reduce the diffusion of phosphate into the pores of the adsorbent. Goethite was preloaded with PGA (0–10 mg C g−1). The samples were characterized by N2 and CO2 adsorption, electrophoretic mobility measurements, and scanning electron microscopy/energy dispersive X‐ray analysis (SEM‐EDX). The phosphate sorption kinetics was studied with batch experiments over 2 wk at pH 5 and an initial phosphate concentration of 250 μM Pore volume and specific surface area of the goethite samples declined after PGA addition. The PGA coatings reduced the ζ‐potential of goethite from 42.3 to −39.6 mV at the highest C loading. With increasing PGA‐C content and decreasing ζ‐potential the amount of phosphate sorbed after 2 wk decreased linearly (P < 0.001). Sorption of phosphate to pure and PGA‐coated goethite showed an initial fast sorption followed by a slow sorption reaction. At the smallest C loading (5.5 mg C g−1) the portion of phosphate retained by the slow reaction was smaller than for the treatment without any PGA, while at higher C loadings the fraction of slowly immobilized phosphate increased. Our results suggest that at low C‐loadings PGA impaired the intraparticle diffusion of phosphate. In contrast, the slow step‐by‐step desorption of PGA (<52% within 2 wk) or the diffusion of phosphate through PGA coatings or both are rate limiting for the slow phosphate reaction at C loadings > 5.5 mg C g−1