Remediation of hexavalent chromium through adsorption by bentonite based Arquad® 2HT-75 organoclays

https://doi.org/10.1016/j.jhazmat.2010.06.110Get rights and content

Abstract

Unlike hydrophobic organic pollutants, the potential of organoclays to adsorb inorganic ionic contaminants is relatively underexplored. The present study attempts to characterise bentonite (QB) based organoclays synthesised from a commercially available, low-cost alkyl ammonium surfactant Arquad® 2HT-75 (Aq) and test their ability to adsorb hexavalent chromium (Cr (VI)) in aqueous solution. XRD, FTIR and TGA characterisation techniques prove successful modification of the bentonite structure and reveal that higher surfactant loadings gives rise to more ordered surfactant conformation in the organoclays. The zeta potential values indicate that higher surfactant loadings also create positive charges on the organoclay surfaces. Detailed isothermal and kinetic studies show that the organoclays effectively remove hexavalent chromium (Cr (VI)) from aqueous solution by both physical and chemical adsorption processes. Higher surfactant loadings provide better adsorption efficiency. The adsorption performance is reasonably efficient under the levels of pH, temperature, electrolyte concentration and natural organic matter concentration that generally prevail in contaminated soil and water. This study shows that organoclay sorbents offer good potential for remediating Cr (VI) under real environmental conditions.

Introduction

The use of chromium in industries is widespread and economically important. It is widely used in industries such as electroplating, alloy and steel manufacturing, leather tanning, metal finishing, pigment and dye synthesis and many others. Chromium commonly enters the environment in the effluents from these industries. Once released into soil and water, it is considered a major cause of environmental pollution. Chromium is listed among the 126 priority pollutants by the US EPA. It is also listed among the 25 most hazardous substances posing the greatest risk to human and ecosystem health at the priority superfund sites [1]. Chromium exists in the environment in two major oxidation states, trivalent (Cr (III)) and hexavalent chromium (Cr (VI)). Of the two, Cr (III) is an essential microelement for living organisms at low concentrations. However, its hexavalent form, which is the most dominant oxidation state in natural systems, is seriously toxic when present at concentration as low as 50 ppb in drinking water. Being a cation, Cr (III) is less mobile in the soil environment, whereas anionic Cr (VI) is highly mobile and poses a tremendous risk of ground water pollution. Cr (VI) is a known mutagen and carcinogen, shown to cross the cell membrane readily and convert to forms that can adduct with DNA causing permanent damage to the cells, leading to cancer [2]. Due to its serious environmental and health impact, industrial effluents containing Cr (VI) must be treated as a rule. Several chemical, physio-chemical and biological treatment methods have been developed and implemented for this purpose. Sorption is one of the most popular methods where an engineered sorbent material acts as a sink for the contaminant and so reduces its mobility.

Natural materials such as clays are cost-effective for immobilising toxic environmental contaminants as they are inexpensive, readily available, environmentally stable and have high adsorptive and ion exchange properties. Clay materials can also be modified using a variety of chemical/physical treatments to achieve the desired surface properties for best immobilisation of contaminants. For example, when surfaces of these clays are modified with organic molecules, the resulting products are called organoclays [3], [4], [5]. The organic modifiers used for this purpose are generally cationic quaternary amine compounds which are commonly known as surfactants.

Organoclays have extensively been studied for their unique sorption behaviour towards various hydrophobic organic contaminants in the environment [6], [7], [8], [9]. However, these clay-based modified materials can also be used for the remediation of ionic contaminants such as heavy metals [10], [11], [12], [13], [14] and metalloids [15], [16], [17], [18]. For example, Lin and Juang [12] reported that sodium dodecylsulphate (SDS; an anionic surfactant) modified montmorillonite showed a much more negative zeta potential and exhibited higher affinity for Cu and Zn as compared to the naturally available montmorillonite. In another study, Oyanedel-Craver and Smith [13] found that the sorption of Cd, Pb and Zn by hexadecyltrimethylammonium (HDTMA) and benzyltrimethylammonium modified bentonite was non-linear and the adsorption of metals decreased with the increase in surfactant loadings in the organoclays from 25% to 100% of the CEC of the bentonite. Normally, naturally occurring clays are not efficient sorbents for anions because of their intrinsic negative charge. Although a very small amount of anion sorption can take place with natural clays through physical sorption, anion exchange or electrostatic binding mechanisms, it is neither significant in terms of sorption quantity nor binding strength. So, in order to immobilise anions like chromate and arsenate effectively using these materials, their surfaces are modified to possess enough positively charged sites to bind with the anions. In addition, another mechanism can come into action if the weakly held counter ions of the modifying surfactants are replaced by more strongly held adsorbate counter ions. Some specific alkyl ammonium surfactant cations may impart such favourable surface properties to natural clays to enhance their ability to hold anions. For example, Krishna et al. [16] reported improved adsorption of chromate oxyanion by HDTMA modified kaolinite, montmorillonite and pillared montmorillonite as compared to the unmodified clays. The authors [16] found that the amount of adsorption decreased with increasing pH of the solution. Atia [15] also observed similar results when they used cetylpyridinium modified bentonite to adsorb chromate and molybdate. However, the potential for remediating anionic pollutants using organoclays has yet to be fully explored. There is also a need to study the mechanism of sorption of anionic metalloids onto organoclays and the associated process parameters, so as to predict the performance of the sorbents under field conditions.

This study attempts to characterise bentonite based organoclays synthesised from a commercially available, relatively cheaper alkyl ammonium surfactant Arquad® 2HT-75 and test their adsorption abilities towards hexavalent chromium (Cr (VI)) in aqueous solution. Organic modification of the bentonite clay is established and characterised by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and zeta potential measurement. The resulting sorbents have been tested for sorption of hexavalent chromium from an aqueous solution. Sorption isotherm, kinetics and effect of process parameters such as pH, ionic strength, temperature and dissolved organic carbon (DOC) on sorption have been investigated in details. This study will help in developing novel remediation materials for removing Cr (VI) from various industrial waste waters as well as in situ immobilisation of Cr (VI) in contaminated soils and sediments.

Section snippets

Materials and synthesis

The clay used in the present study is a locally available bentonite (QB). The cation exchange capacity (CEC) value of this clay determined by the ammonia electrode method [19] is 66.7 cmol (p+) kg−1. The clay was passed through a 300 μm sieve and used without any further purification.

Two organoclays, QB-Aq1 and QB-Aq2, were prepared by modifying Queensland bentonite (QB) with a commercially available, relatively inexpensive surfactant, Arquad® 2HT-75 (Aq). Chemically this surfactant is

X-ray characterisation of the organoclays

The X-ray diffraction patterns of the unmodified bentonite (QB) and the synthesised organoclays are shown in Fig. 1. The main primary diffraction peaks attributed to (0 0 1) planes are observed at 15.02, 30.05 and 35.04 Ǻ, respectively, for QB, QB-Aq1 and QB-Aq2. Additionally, the clay products show intense XRD peak at 2θ value of 26.8°, which is attributed to the presence of quartz as impurity. In the present study, the d (0 0 1) spacing of the unmodified QB is slightly higher (15.02 Ǻ) than the

Conclusions

This study concludes that bentonite can effectively be modified by Arquad® 2HT-75, which is a relatively cheaper surfactant, for adsorbing Cr (VI) from aqueous solution. As illustrated by XRD, FTIR and TGA characterisation techniques, more ordered solid like conformation of surfactant molecules is obtained in the organoclay as the surfactant loading increases. As apparent from the measured zeta potential values, positive charge formation takes place on the bentonite surface due to surfactant

Acknowledgements

Binoy Sarkar is thankful to the University of South Australia for the award of University President Scholarship and Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC CARE) for the award of CRC CARE PhD fellowship. The authors would like to acknowledge the financial and infrastructural support from the CRC CARE and the Centre for Environmental Risk Assessment and Remediation (CERAR), University of South Australia. Suggestions from anonymous

References (40)

Cited by (139)

View all citing articles on Scopus
1

Present address: 313-855, West 16th Street, North Vancouver, BC V7P1R2, Canada.

View full text