Adsorption of congo red by three Australian kaolins

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Abstract

This study investigated the potential use of kaolin as alternative adsorbents for removal of congo red from wastewater. The effect of adsorbent dosage, dye concentration, pH and temperature were experimentally studied to evaluate the adsorption capacity, kinetics and equilibrium. Experimental results revealed that optimal adsorption took place at acidic pH and high dye concentration. Ceram kaolin had the highest removal efficiency among studied kaolins, followed by K15GR and Q38. The dye uptake process obeyed the pseudo-second order kinetic expression and was best described by the Langmuir isotherm. Intra-particle diffusion studies showed that the adsorption mechanism was not exclusively controlled by the diffusion step and was more likely to be governed by external mass transfer. Thermodynamic studies showed congo red adsorption on all kaolins was exothermic and spontaneous in nature. Recovered Q38 and K15GR kaolin demonstrated an enhanced adsorption capability. The results indicate that these local kaolins could be employed as low-cost alternatives for removal of anionic dyes from industrial wastewater.

Introduction

Extensive use of synthetic dyes in many industrial applications has produced large volumes of dye wastewater. Many of them are considered hazardous and require careful removal prior to discharge into receiving water bodies. Dye wastewater is commonly characterised as high in salt and organic content and low in biodegradation potential (Alinsafi et al., 2005). Direct discharge of dye effluents can cause serious problems to the environment due to contribution of high organic loading, toxicity and aesthetic pollution related to colour. Contaminated dye can absorb and reflect sunlight entering the water stream causing interference with the growth of bacteria to levels such that the biological degradation of impurities is impeded and an ecological imbalance results. The synthetic dyes usually contain azo-aromatic groups which are of extreme environmental concern due to their carcinogenic, mutagenic, inert properties. The complex aromatic structures of the dyes result in physicochemical, thermal and optical stability, and resistance to conventional wastewater treatment.

The adsorption process is an effective and attractive proposition for the treatment of dye contaminated wastewater. This method can be considered as an economical alternative since it does not require any additional pre-treatment step if low-cost adsorbents are employed. Thus, much recent research has been towards seeking a cheap and locally available material to substitute for the use of activated carbon, which is expensive and complicated to be regenerated (Crini, 2006).

The use of clays as alternative adsorbents has been receiving attention for the removal of dyes from wastewater in recent years (Alkan et al., 2007, Vimonses et al., 2008). They have advantages over commercially available adsorbents in terms of low-cost, abundant availability, high adsorption properties, non-toxicity and large potential for ion exchange, resulting from a net negative charge on the structure of the minerals. Utilisation of locally available clays could bring massive economic and environmental benefits to wastewater industries.

Anionic dyes are the most problematic because of their bright colour, water soluble reactive and acidic properties. Many studies has been carried out on adsorption of organic cationic dyes by clay minerals (Ghosh and Bhattacharyya, 2002), there is very little information on adsorption of anionic dyes (Bulut et al., 2008). In most cases, the adsorption capacity for acidic dyes is much lower than for basic dyes, due to weak interactions between the negatively charged surface on clay minerals and anionic charges on the dyes.

Kaolin has received considerable recognition as an adsorbent because of its high adsorption capacity. It is generally referred to as a clay that is mainly composed of kaolinite and a lower amount of minerals such as quartz and mica etc. It is the most abundant mineral in soils and sediments and interacts with other soil elements to contribute to the mechanical stability of the soil column (Chen et al., 2000). The adsorption properties of kaolinite are likely determined by its surface structure and the edges (Miranda-Trevino and Coles, 2003, Schoonheydt and Johnston, 2006). The edges possess a variable charge that can be correlated to the reaction between ionisable surface groups along the edges and the clay mineral surface and the ions present in aqueous solution.

Congo red (CR) or 1-naphthalenesulfonic acid, 3,3′-(4,4′-biphenylenebis (azo)) bis (4-amino-) disodium salt is a benzidine-based anionic diazo dye prepared by coupling tetrazotised benzidine with two molecules of napthionic acid (Purkait et al., 2007) (Fig. 1). This anionic dye can be metabolised to benzidine, a known human carcinogen (Mall et al., 2005). Effluent containing congo red is largely produced from textiles, printing, dyeing, paper, and plastic industries etc. (Purkait et al., 2007, Pavan et al., 2008). The treatment of contaminated congo red in wastewater is not straightforward, since the dye is generally present in sodium salt form giving it a very good water solubility. Also, the high stability of its structure makes it difficult to biodegrade and photodegrade.

In this study three Australian kaolins were tested to explore their adsorptive potential for the removal of congo red from wastewater.

Section snippets

Materials

Three commercial kaolins: Q38, K15GR and Ceram provided by Unimin Australia Ltd., Australia were investigated as to their dye adsorption capacity. Q38 is dry milled kaolin of good white colour, typically used in the manufacture of paints or products that requires a white colour. K15GR is white kaolin generally used as a pigment extender in paints and as a functional filler in rubber compounds. Lastly, Ceram is a white firing kaolin exhibiting ball clay properties. This Ceram kaolin consistently

Material characterisations

The surface morphology of the kaolins was observed by SEM analysis (Fig. 2). The SEM images illustrate the solid hexagonal flakes of the kaolinite particles. Some large kaolinite flakes were stacked together to form agglomerates as found in Q38 (Fig. 2a). K15GR represented the scattering of flaky shaped particles with consistent distribution of micropores (Fig. 2b). Ceram kaolin demonstrated a similar pattern to K15GR but rather present with fine particles to form deep and apparent pores (Fig. 2

Material recovery

Recycle of spent adsorbent is considered as an important economical aspect to minimise the material cost. High temperature calcinations can decompose the organic dyes on the surfaces or in pores of kaolin to carbon and thereafter oxidise to carbon oxides in air. However, in some cases, calcination of porous adsorbents at high temperature can bring about changes in the surface functional groups and cause pore collapse. This circumstance leads to lower specific surface area and pore volume, and

Conclusions

Three commercial Australian kaolins, namely Q38, K15Gr and Ceram, have been studied for removal of the toxic anionic dye, congo red, from aqueous solution. Ceram kaolin performed with the highest adsorption capacity in removal of the dye, followed by K15GR and Q38. The CR adsorption was described by the Langmuir model. Kinetic calculations show that adsorption followed the pseudo-second order model with a multi-step diffusion process, where external mass transfer is likely the rate-controlling

Acknowledgements

The authors are grateful to Unimin Australia Ltd. and Zeolite Australia Ltd. for kindly supplying clay minerals. This work was supported by the Australian Research Council Linkage Grant ((LP0562076) through the Water Environmental Biotechnology Laboratory (WEBL) at the University of Adelaide.

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