Speciation and removal of chromium from aqueous solution by white, yellow and red UAE sand

Removal of Cr(III) and Cr(VI) from aqueous solutions by different types of sand as a low cost abundant adsorbent was investigated. White, yellow and red sands from the United Arab Emirates (UAE) were employed at various adsorbent/metal ion ratios. The effect of contact time, pH, temperature, metal concentration and sand dosage was studied. The optimal pH for adsorption was ca. 5.0 for Cr(III) and 2.0 for Cr(VI). The optimal adsorption time for both ions was ca. 3 h. Even at the optimal pH, adsorption of Cr(VI) on all sand forms was very low (removal <= 10%) and could not be fitted to any of the common isotherms. While at pH 5.0 Cr(VI) was not adsorbed at all, Cr(III) was totally removed. Adsorption of Cr(III) by the three sand forms obeyed Lagregren first order kinetics. The rate constants at 25.0 degrees C were 3.95 x 10(-4), 2.28 x 10(-5) and 7.75 x 10(-6) s(-1) for white, yellow and red sand, respectively. For Cr(III), the Langmuir isotherm gave the best fit for adsorption. At 25.0 degrees C, the maximum mass of Cr(III) removed per gram of sand (Q(max)) was 62.5, 9.80 and 2.38 (mg/g) for white, yellow and red sand, respectively. Delta H degrees was 14.5, 51.2 and 45.8 kJ mol(-1) and Delta S degrees was 24.0, 136 and 111 J K(-1) mol(-1) for adsorption on white, yellow and red sand, respectively. The results are discussed in terms of the chromium species present in the solution and the effect of sand particle distribution and metal composition on adsorption efficiency. (c) 2009 Elsevier B.V. All rights reserved.