Dynamic Adsorption of Cd2+ Onto Acid-Modified Attapulgite From Aqueous Solution

Abstract

AbstractIn the present study, acid-modified attapulgite was used, as an adsorbent, to remove as much Cd2+ as possible from aqueous solution. Static adsorption experiments using powdered acid-modified attapulgite, and dynamic adsorption using granular acid-modifed attapulgite, were conducted to explore the practical application of modified attapulgite in the adsorption of Cd2+. The modified attapulgite had a larger specific surface area and thinner fibrous crystals than the unmodified version. No obvious differences were noted, in terms of the crystal structure, between the natural attapulgite and the modified version. The effects of initial concentration, pH, contact time, and ionic strength on the adsorption of Cd2+ were investigated, and the results showed that the adsorption capacity of the modified attapulgite was increased with increasing pH and the initial Cd2+ concentration. The adsorption properties were analyzed by means of dynamic adsorption tests with respect to various Cd2+ concentrations and flow rates. The maximum adsorption capacity of 8.83 mg/g occurred at a flow rate of 1 mL/min and at an initial concentration of 75 mg/L. Because there was better accord between the data and a pseudo-second order model than a pseudo-first-order model, external mass transfer is suggested to be the rate-controlling process. The experimental data were also fitted for the intraparticle diffusion model, implying that the intraparticle diffusion of Cd2+ onto the modified attapulgite was also important for controlling the adsorption process. The Bohart-Adams model was more suitable than the Thomas model for describing the dynamic behavior with respect to the flow rate and the initial Cd2+ concentration. This research provided the theoretical basis for the dynamic adsorption of Cd2+ on the modified attapulgite. Compared to the powdered modified attapulgite, the dynamic adsorption by granular modified attapulgite appeared more favorable in terms of practical application.