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The present study involves an investigation on the comparison of a Mexican clinoptilolite-heulandite zeolitic mineral and the modified zeolitic material with the surfactant hexadecyltrimethylammonium bromide (HDTMA) for the removal of cadmium from aqueous solutions. The effects of pH and contact time on the adsorption process were examined. The optimum pH for adsorption was found to be 7. Cadmium retention reached equilibrium in 32 h and the rate of cadmium adsorption by the zeolites was rapid in the first 10 h. Elovich's model best described the reaction rate. Batch adsorption experiments conducted at room temperature showed that the adsorption pattern followed the Langmuir -Freundlich isotherm model. The cadmium retention capacity decreased very slightly when the zeolite surface was modified with the surfactant HDTMA, and this material has advantages for its use in the removal of some other contaminants, such as anions and nonpolar organic compounds, like phenols. The results showed that natural zeolite and the surfactant modified zeolite could be considered as potential adsorbents for cadmium removal from aqueous solutions.

Nanometric magnetite (nm-Fe 3 O 4) particles were prepared by the reverse coprecipitation synthesis method, obtaining particle sizes that ranged from 4 to 8.5 nm. In This article is part of the Topical Collection on

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Boehmite was used for the removal of fluoride ions from aqueous solutions in a batch system. The pH, contact time, and fluoride concentration in the removal of fluoride ions by boehmite were evaluated. The removal of fluoride ions by boehmite was the highest between the pH values of 4.5 and 7.5. The kinetic fluoride sorption from aqueous solutions by boehmite was best described by the pseudo-secondorder model, and equilibrium was reached in about 24 h. The Freundlich model described the isotherm sorption process; the results indicate that the sorption mechanism is chemisorption on a heterogeneous material.

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The interaction of the herbicide paraquat dichloride (PQ,substrate) with p-tert butylcalix[6,8]arenes (L, receptor) was investigated in both the solution and solid states. The isolated paraquat calixarene complexes were characterised by UV-visible, 1H NMR, ESI-MS, Luminescence and IR spectroscopies and elemental analysis. The stoichiometry of complexes 1 and 2 was 1:1 (1 herbicide:1 calixarene) and both revealed a biexponential luminescence decay with lifetimes depending on the size and the conformational particularity of the calixarenes. Molecular modelling suggested that both calixarenes interact with the herbicide through cation-π interaction. PQ is included in the p-tert-butylcalix[8]arene cavity, a situation favoured by its pinched conformation in polar solvent while it is partially included in the p-tert-butylcalix[6]arene cavity because of its in-out cone conformation. The theoretical results, in particular using MOPAC procedures, were in agreement with the experimental findings.