Near-Stoichiometric Adsorption of Phosphate by Silica Gel Supported Nanosized Hematite

Decreasing the size of oxide particles to nanoscale enables one to ensure maximal reaction rates and depths in the solid due to the enhancement of the specific surface area and the increase of the diffusion rate caused by shortening diffusion paths. For that reason, one may suggest that in the case of adsorption onto nanoparticles, the complete conversion of sorbents to stoichiometric compounds would become possible. Adsorption of phosphate ions onto nanosized hematite supported on silica gel surface has been studied. Modification of silica gel surface by hematite in the quantity of 0.62 and 1.25 mass% has been carried out by means of a citric acid aided method. The morphology of the samples obtained has been characterized using DTA, XRD, SEM, and low temperature desorption of nitrogen. It has been found in batch experiments that unlike natural and synthetic hematites, for which the limiting adsorption values do not exceed 14？mg/g, silica gel supported nanosized hematite adsorbs 400–2000？mg of the phosphate ions per gram, thus forming near-stoichiometric iron phosphates on the surface of the support. The degree of conversion of hematite to iron phosphates is greater in acidic media and at a lower surface coverage, when hematite crystallites are small and better accessible by adsorbate. 1. Introduction Numerous metal oxides are capable of absorbing inorganic and organic contaminants and due to this ability they are widely used as scavengers of toxic impurities. Advances in nanoscale science and engineering have raised hopes that many of the current problems involving water quality could be resolved or greatly ameliorated using nanosorbents, nanocatalysts, nanostructured catalytic membranes, and nanoparticle-enhanced filtration techniques resulting from the development of nanotechnology [1]. Decreasing the size of oxide particles to nanoscale enables one to ensure maximal reaction rates and depths in the solid due to the enhancement of the specific surface area and the increase of the diffusion rate caused by shortening diffusion paths [1, 2]. For that reason, one may suggest that in the case of adsorption onto nanoparticles, the complete conversion of sorbents to stoichiometric compounds would become possible. However, in spite of these striking benefits, the direct application of nanosized oxides in dynamic sorption is out of question in view of enormous hydraulic resistances of nanopowders. Therefore, studies in the field of nanoadsorptives can be divided into two major areas. The first area includes the works describing potentialities of nanoparticles