Abstract
Clay minerals can be modified organically by a cationic surfactant resulting in materials known as organoclays. The organoclays have been used as adsorbents of most of the organic contaminants in the aqueous solution and oxyanions of the heavy metal. In this study, a Colombian bentonite was modified with hexadecyltrimethylammonium bromide to obtain an organobentonite, and its capacity to adsorb Cr(VI) oxyanions in the aqueous solution was evaluated. The effect of pH, stirring speed, adsorbent amount, contact time, and ionic strength were investigated at 25°C. Stirring speeds above 200 rpm, contact times greater than 120 min, and the addition of NaCl (0.1 to 2.0 mM) did not have a significant effect on Cr(VI) removal. The influence of the adsorbent amount and pH on Cr(VI) adsorption was studied by the response surface methodology (RSM) approach based on a complete factorial design 32. Results proved that the Cr(VI) adsorption follows a quadratic model with high values of coefficient of determination (R2 = 95.1% and adjusted R2 = 93.9%). The optimal conditions for removal of Cr(VI) from an aqueous solution of 50 mg/L were pH of 3.4 and 0.44 g amount of the adsorbent. The adsorption isotherm data were fitted to the Langmuir and Freundlich adsorption isotherm models, and the model parameters were evaluated. The maximum adsorption capacity of Cr(VI) onto organobentonite calculated from the Langmuir model equation was 10.04 ± 0.34 mg/g at 25°C. The results suggest that organobentonite is an effective adsorbent for Cr(VI) removal, with the advantage of being a low-cost material.
Highlights
Adsorption process has attracted attention of many researchers because of its low cost, ease of operation, design flexibility, and high efficiency for removal of inorganic and organic pollutants from wastewater [1, 4, 7]. Clay minerals and their modified derivatives are a family of materials which can be used for the adsorption of most of the chemical contaminants from the aqueous solution [7]
MacıasQuiroga et al [16] and Otavo-Loaiza et al [19] have reported similar basal spacing values for this bentonite and the bentonite modified with HDTMA-Br. e increase in the interlayer spacing indicates that the HDTMA+ cations intercalated in the interlamellar space were arranged in the form of a pseudo-trimolecular layer [20]
The ability of organobentonite prepared from bentonite and a cationic surfactant (HDTMA-Br) to remove oxyanions of Cr(VI) in the aqueous solution was investigated
Summary
E organobentonite was prepared by cationic exchange between the bentonite homoionized with sodium (CEC of the Na-Bent of 63.02 meq/100 g) and a cationic organic surfactant, hexadecyltrimethylammonium bromide (HDTMA-Br). E proportion of the surfactant used in the synthesis of organobentonite was 1.5 times the proportion of cation exchange capacity of sodium bentonite, a value recommended in literature [18], with a procedure similar to that used in a previous study [19]. E organobentonite (denoted as HDTMA-Bent) was separated by centrifugation and washed repeatedly until a negative bromide test was obtained with 0.1 M of AgNO3. E adsorbent was separated by centrifugation at 5000 rpm for 5 min, and the supernatant solution was filtered in a 0.22 μm membrane and carefully transferred to glass flasks to determine the concentrations of chromium present in the aqueous medium. Where Ci and Ce are the initial and equilibrium concentrations of the Cr(VI) in the liquid phase (mg/L), V is the aqueous phase volume (L), and W is the mass (g) of the adsorbent
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