Cadmium(II) Uptake from Aqueous Solution by Adsorption onto Carbon Aerogel Using a Response Surface Methodological Approach

Recently a new form of activated carbon has appeared: carbon aerogel. Use of carbon aerogel for the adsorptive removal of inorganic compounds (and especially metal ions) has not been investigated. The purpose of this study is to investigate feasibility of cadmium(II) adsorption on carbon aerogel from aqueous solution. The physicochemical properties of carbon aerogel were analyzed to obtain a better understanding of the adsorption mechanism of Cd(II) onto carbon aerogel in aqueous solution. The surface structure of carbon aerogel was analyzed by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray analysis (EDAX). Batch mode adsorption experiments were carried out to assess the adsorption behavior of Cd(II) in aqueous solution. The maximum adsorption capacity (Q0) was calculated by applying the Langmuir equation to Cd(II) in the adsorption isotherm and was found to be 15.53 mg g^1. The influence of three-process variables, namely, adsorbent concentration (0.02-0.1 g) per 50 mL of metal solution, solution pH (2.0 -10.0), and temperature (20-70 °C), on the percentage removal of Cd(II) was also examined, using a response surface methodological (RSM) approach. The Box-Behnken model was used as an experimental design, and a statistically practicable second-order polynomial equation was fitted to the model exhibiting a response-variable relationship. This was evidenced by a high R^2 value of 0.9602. Response surfaces were plotted on the basis of the fitted second-order polynomial equation. The optimum conditions for maximum adsorption of Cd(II) were found to be as follows: adsorbent concentration of 0.1 g per 50 mL of metal solution, pH range 6.0-7.0, and temperature of 70 °C. Effect of initial solution pH on Cd(II) removal was carried out to assess the adsorption behavior at different pH values. Adsorption of cadmium(II) increases with increasing initial pH from 2.0 to 10.0.