Kinetics of phosphate sorption to polygalacturonate-coated goethite

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 N-2 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 mu M. Pore volume and specific surface area of the goethite samples declined after PGA addition. The PGA coatings reduced the xi-potential of goethite from 42.3 to -39.6 mV at the highest C loading. With increasing PGA-C content and decreasing xi-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 intra-particle 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).