Influence of Textural Properties and Surface Fractal Dimensions on the Cobalt Adsorption Behavior of Rice Hull Ash Prepared via Solid Combustion

Abstract

Rice hull ash was prepared via a fast chemical combustion method with ammonium nitrate, urea and milled rice hull. Urea was used as the chemical fuel. The mixture was heated to various temperatures (100 to 1000 °C), and the resulting products were compared with products obtained via calcination, in which samples were heated to the same temperatures for 2 hours. Changes in the structure, morphology, texture and surface fractal dimensions with respect to temperature were examined and found to be highly relevant for adsorption applications. The prepared powders were characterized physicochemically by X-ray diffraction and N 2 -physisorption measurements. The surface fractal dimensions of the prepared materials were calculated from N 2 -adsorptiondesorption isotherms. The results indicate that the fast combustion samples have larger surface fractal dimensions than calcined samples. A correlation between fractal dimensions and Co 2+ ion adsorption was found. The present study demonstrates the importance of textural properties and surface fractal dimensions in the adsorption performance of adsorbents.Keywords: rice hull ash (RHA), cobalt adsorption capacity, combustion process, calcination treatment, surface fractal dimensions IntroductionRice hull (RH) is a low-cost agricultural waste obtained from rice hull mills. RH is used by various industries as a boiler fuel to produce steam. Burning rice hull for long periods generate energy results in the waste product rice hull ash (RHA), which is rich in silica and has a considerable cation exchange capacity due to the polar groups present on its surface. Srivastava et al. 1 and An et al. 2 reported that RHA is an effective adsorbent of metal ions from water. It is well known that the efficiency and cost-effectiveness during a waste water adsorption process is determined by choosing suitable potential adsorbent particles with improved structural and textural characteristics. 3 In the search for materials with appropriate physico-chemical characteristics to be used as efficient and low cost adsorbents many efforts, including the development of chemical processes, have been developed to reduce the hazardous effects of toxic pollutants. 4,5 On the other hand, combustion synthesis is a rapid chemical process based on the explosive decomposition of urea or other organic fuels.6 This decomposition provides heat for calcinations and produces high temperatures. 7 Combustion synthesis has been used to prepare a variety of advanced ceramics, composites, alloys, and nanomaterials.8 Chemical combustion provides an attractive alternative to other complex synthesis methods, requiring minimal use of specialized equipment and allowing for rapid preparation while producing a product with optimal structural and functional properties. It has been shown that the combustion process favors the retention of metallic chemical species on adsorbent solids due to their improved textural characteristics.9,10 Materials prepared by chemical combustion have attracted increasing interest because of their ...