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 N2-physisorption measurements. The surface fractal dimensions of the prepared materials were calculated from N2-adsorption-desorption isotherms. The results indicate that the fast combustion samples have larger surface fractal dimensions than calcined samples. A correlation between fractal dimensions and Co2+ ion adsorption was found. The present study demonstrates the importance of textural properties and surface fractal dimensions in the adsorption performance of adsorbents.

Calcium phosphate with hydroxyapatite structure was synthesized and its ability to adsorb Cr(VI) from aqueous solution is presented. XRD, BET, IR, TGA, SEM and EDS techniques were used to characterize the obtained material. Apure phase was obtained through a simple synthesis process. The specific surface area of the synthesized powder was found to be 64.5 m² g-1. The X-ray diffraction pattern shows that the calcium phosphate formed was nanocrystalline with an average grain size of approximately 75 nm. A fast adsorption was observed and in less than 24 h it was found that 2.41 x 10-4 meq g-1 of Cr(VI) ions were adsorbed on calcium phosphate. Desorption experiments showed that Cr(VI) adsorption decreased to 2.23 x 10-4 meq g-1 of calcium phosphate. This behavior is a consequence of partial dehydration of the synthesized material. Calcium phosphate can be effectively used for removing Cr(VI) from aqueous solutions in treatment processes of metal wastes.

Se sintetizó fosfato de calcio, con la estructura de la hidroxiapatita, y se presenta su habilidad para adsorber Cr(VI) en solución acuosa. Para caracterizar el material obtenido se usaron técnicas de DRX, BET, IR, ATG, MEB y EDS. Se obtuvo una fase pura por un proceso simple de síntesis. El área específica del polvo sintetizado fue de 64.5 m² g-1. El patrón de difracción de rayos-X muestra que el fosfato de calcio formado fue nanocristalino con un promedio de grano alrededor de 75 nm. Se observó una rápida adsorción en menos de 24 h y se encontró que 2.41 x 10-4 meq g-1 de iones Cr(VI) fueron adsorbidos sobre el fosfato de calcio. Experimentos de desadsorción mostraron que la adsorción de Cr(VI) disminuyó a 2.23 x 10-4 meq g-1 de fosfato de calcio. Este comportamiento es una consecuencia de la deshidratación parcial del material sintetizado. El fosfato de calcio puede ser usado efectivamente para la remoción de Cr(VI) de soluciones acuosas en los procesos de tratamiento de desechos de metales.

CrO4(2-) ion adsorption on Fe-treated tri-calcium phosphate was studied by batch experiments as a function of contact time, initial concentration of metal ion and temperature. Adsorption results showed that at pH 5.5 and 1.0×10-4 M chromium concentration the adsorption capacity of Fe-treated tri-calcium phosphate for CrO4(2-) ions was 7.10 × 10-3 mmol/g. Chromium adsorption data on Fe-treated tri-calcium phosphate at various initial concentration fitted the Freundlich isotherm. By temperature studies the thermodynamic parameters ΔH0, ΔG0 and ΔS0 were estimated and the obtained results showed that the adsorption reaction was endothermic and spontaneous.

Por medio de experimentos estáticos se estudió la adsorción de iones CrO4(2-) en fosfato tricálcico tratado con Fe, como una función del tiempo de contacto, concentración inicial y temperatura. Los resultados mostraron que a un pH de 5.5 y una concentración 1.0 × 10-4 M de cromo la capacidad de adsorción del fosfato tricálcico tratado con Fe para los iones cromato fue de 7.1 × 10-3 mmol/g. Los datos de adsorción del cromo en el adsorbente modificado con Fe, como una función de su concentración, se ajustaron a la isoterma de Freundlich. A través de estudios de temperatura se determinaron los parámetros termodinámicos ΔH0, ΔG0 y ΔS0 y los resultados obtenidos mostraron que el proceso de adsorción de los iones cromato es endotérmico y espontáneo.