Abstract
Red mud as a waste material is produced in large quantities by the aluminum industry. Heat activation has been used to enhance sorption capacity of red mud for its beneficial reuse as an effective sorbent. In this study, heat-activated red mud (HARM) was investigated for its Cd(II) sorption capacity under various process conditions (Cd concentration, pH and contact time) using response surface methodology (RSM). Analysis with RSM identified pH as the most important process parameter. The positive correlation between higher pH and greater Cd(II) sorption was likely due to: (i) decreased proton competition with Cd(II) for sorption sites at higher pH; (ii) enhanced sorption via ion exchange by monovalent Cd species from hydrolysis at higher pH; and (iii) improved thermodynamics of sorption at higher pH as protons are being released as products. Further analysis indicated the sorption process was thermodynamically favorable with a negative change in Gibbs free energy. Additionally, the sorption process exhibited a positive change in enthalpy, indicative of endothermic nature of sorption; this is consistent with sorption increase at higher temperature. These findings provide needed insight into the mechanisms underlying Cd(II) sorption by HARM for more effective applications of heat-activated red mud as sorbents for Cd(II) removal.
Highlights
Cadmium (Cd) has been exerting a great pressure on the environment due to its increasingly higher input flux from anthropogenic sources like battery, alloy, and metal plating industries[12,13]
To characterize the Cd(II) sorption behavior of heat-activated red mud (HARM) under various process conditions, a three-factor, three-level Box-Behnken experimental design combined with response surface methodology (RSM) was employed to develop a mathematical model
Greater Cd(II) sorption exhibited at higher pH could be attributed to weaker H+ competition with Cd(II) cations for HARM sorption sites at lower H+ concentration
Summary
Cadmium (Cd) has been exerting a great pressure on the environment due to its increasingly higher input flux from anthropogenic sources like battery, alloy, and metal plating industries[12,13]. Sorption has proven to be an effective and economical method for the removal of Cd(II) in the aqueous environment[12,16]; using mineral wastes, red mud, as sorbents to remediate toxic Cd(II) should be a promising strategy. The effectiveness of heat activation in enhancing sorption performance has been conducted on a number of other heavy metals such as nickel[18] and manganese[19]. Based on these studies, process parameters including pH, contact time, and adsorbate concentration have been shown to have a major influence on the sorption of heavy metals by heat-activated red mud. The objectives of this work are to characterize Cd(II) sorption by heat-activated red mud using a mathematical model developed by response surface methodology (RSM) and to predict sorption behavior under various process conditions
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