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Clays modified by cationic surfactants have been widely used for the removal of organic and inorganic anionic contaminants. However, their suitability for the removal of antimonite from aqueous solutions has not been systematically studied. In this study, hexadecylpyridinium chloride modified montmorillonite (HDPy+-M) and hexadecylpyridinium bromide modified zeolite (HDPy+-Z) were used to measure Sb(III) uptake from solutions containing 0.5–2.5 mM antimonite. Adsorption isotherms of antimonite were studied using the Langmuir and Freundlich equations. Adsorption kinetics were investigated using pseudo-first-order, pseudo-second-order, Elovich and intra-particle diffusion models. The results of X-ray diffraction showed a large interlayer expansion for HDPy+-M, whereas the X-ray patterns of HDPy+-Z remained unchanged. Uptake of Sb(III) by both HDPy+-M and HDPy+-Z could be fitted well to the Langmuir isotherm, while the kinetics of adsorption could be described well using the pseudo-second-order model. Maximum adsorption capacities for Sb(III) uptake by HDPy+-M and HDPy+-Z were calculated to be 108.7 and 61.34 mg g−1, respectively. The results of the kinetic studies revealed that Sb(III) adsorption to HDPy+-Z was found to be quite fast and the reaction reached equilibrium in 8 h, whereas for HDPy+-M equilibration was attained within 24 h. The adsorption of antimonite onto both HDPy+-M and HDPy+-Z was found to be selective in the presence of Cl−1 and SO4−2 competitive anions. Considering the high affinity for Sb(III) uptake from solutions containing high concentrations of antimonite, both HDPy+-M and HDPy+-Z could be used as promising adsorbents for environmental applications.

An organoclay was prepared using montmorillonite and hexadecylpyridinium chloride (HDPyCl) and tested for the removal of nitrate and perchlorate anions from aqueous solutions. The cationic surfactant modified organoclay was prepared at room temperature using HDPyCl corresponding to 4 times the cation exchange capacity (CEC) of Na-montmorillonite. Powder X-ray diffraction (XRD) analysis of the above organoclay showed a large basal spacing of 40.27 A with the intercalation of HDPy cations in the interlayers probably as a result of a paraffin-type bilayer arrangement. The adsorption data of nitrate and perchlorate were fitted to the Langmuir and Freundlich adsorption isotherms and pseudo-first order and pseudo-second order kinetic models to better understand their adsorption mechanisms. The nitrate and perchlorate uptakes by this organoclay could be described well using the Langmuir isotherm while their uptake kinetics fitted well to the pseudo-second order model. The maximum adsorption capacities of nitrate and perchlorate by the organoclay, HDPy-montmorillonite were calculated at 0.67 and 1.11 mmol g− 1, respectively. Nitrate and perchlorate uptake kinetics were found to be fast as equilibrium was reached within 4 h. Furthermore, the uptakes of nitrate and perchlorate by HDPy-montmorillonite were found to be highly selective in the presence of Cl−, SO42 − and CO32 −, the most abundant naturally occurring anions. Therefore, the HDPy-montmorillonite could be used as a highly efficient adsorbent for the separation of nitrate and perchlorate from drinking or waste water and ground water.

The main objective of the present study was to evaluate the effectiveness of synthetic Zn–Al–SO 4 layered double hydroxide (LDH) for the removal of arsenite and arsenate from a simulated soil solution. The Zn–Al–SO 4 was synthesized using a hydrothermal method and the adsorption of arsenic was studied using Langmuir and Freundlich models. Moreover, the kinetics of adsorption was tested using pseudo-first, pseudo-second, intraparticle diffusion and Elovich models. The results of the experiments revealed that the adsorption isotherms of arsenite and arsenate on the Zn–Al–SO 4 LDHs can be described well with Langmuir isotherm and maximum adsorption capacities were calculated to be 34.24 and 47.39 mg g − 1 for arsenite and arsenate, respectively. The adsorption kinetics of arsenate followed pseudo-second order while arsenite uptake showed better correlation with intra-particle diffusion model. Furthermore, the effects of two major coexisting and competing divalent anions such as SO 4 2 − and CO 3 2 − on the uptakes of arsenite and arsenate were studied and the results showed that CO 3 2 − had greater adverse effect on the uptakes of arsenite and arsenate by Zn–Al–SO 4 than SO 4 2 − . Based on X-ray diffraction results, the main underplaying adsorption mechanism of arsenite and arsenate might be the exchange on the external surfaces and edges of Zn–Al–SO 4 LDHs, but possibly some topotactic exchange may occur through the substitution of arsenic anions with sulfate in the interlayer region.