Characterisation and environmental application of an Australian natural zeolite for basic dye removal from aqueous solution

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Abstract

An Australian natural zeolite was collected, characterised and employed for basic dye adsorption in aqueous solution. The natural zeolite is mainly composed of clinoptiloite, quartz and mordenite and has cation-exchange capacity of 120 meq/100 g. The natural zeolite presents higher adsorption capacity for methylene blue than rhodamine B with the maximal adsorption capacity of 2.8 × 10−5 and 7.9 × 10−5 mol/g at 50 °C for rhodamine B and methylene blue, respectively. Kinetic studies indicated that the adsorption followed the pseudo second-order kinetics and could be described as two-stage diffusion process. The adsorption isotherm could be fitted by the Langmuir and Freundlich models. Thermodynamic calculations showed that the adsorption is endothermic process with ΔH° at 2.0 and 8.7 kJ/mol for rhodamine B and methylene blue. It has also found that the regenerated zeolites by high-temperature calcination and Fenton oxidation showed similar adsorption capacity but lower than the fresh sample. Only 60% capacity could be recovered by the two regeneration techniques.

Introduction

Many industries such as textile and printing use dyes and pigments and thus produce highly coloured waste effluents. Disposal of these wastes into waters causes environmental problems. Various methods for dye and colour removal, such as aerobic and anaerobic microbial degradation, coagulation and chemical oxidation, membrane separation process, electrochemical, filtration, softening and reverse osmosis have been proposed from time to time [1]. However, all of the methods suffered from one or another limitation and none of the processes described above were successful in removing colour from wastewater completely. Adsorption is probably the simplest process for dye removal. Currently, activated carbon is believed to be the most effective adsorbent. However, high cost in production and regeneration make it uneconomical [2].

Natural zeolite is an abundant resource of aluminosilicates available all over the world. Natural zeolite usually has high cation exchange capacity. There are many possible uses of the natural sedimentary zeolites such as for building stone, cement pozzolan, lightweight aggregate, oil spill cleanup, paper filler, desiccants and for gas and liquid separations. One of the most significant commercial applications of natural zeolites is the removal of heavy metal ions from wastewater [3], [4], [5], [6], [7]. In addition, natural zeolites have also been used for adsorption of ammonium [8], [9], [10] and organics [11], [12] in water. However, few investigations have been conducted in dye removal using natural zeolite [13], [14]. Some investigations have been reported using organic-zeolite composites for dye removal [13], [15], [16]. Metes et al. [14] also investigated several synthetic zeolites for cleaning printing ink wastewater and found that adsorption is independent of pore structure. ZSM-5 and NH4-beta are effective while other zeolites studied showed lower efficiency. In this paper, we report an investigation using an Australian natural zeolite for dye removal. We investigated the kinetics, equilibrium isotherm and thermodynamics as well as physical and chemical methods for regeneration of the spent natural zeolite in adsorption of basic dyes.

Section snippets

Adsorbent and dyes

A natural zeolite was obtained from Zeolite Australia Ltd. Its physicochemical properties are presented in Table 1. XRF measurement shows that the chemical compositions of the natural zeolite are SiO2, Al2O3, CaO, K2O and Fe2O3. Most of natural zeolite contains large amount of clinoptilolite, which has a three-dimensional crystal structure. It is known that natural zeolite usually has some ion exchangeable cations such as Na, K, Ca and Mg in channels. Such cations give a high cation exchange

Characterisation of natural zeolite

Table 1 presents some properties of the natural zeolite. As shown, the cation exchange capacity is 120 meq/100 g. The solid slurry pH is greater than 7, indicating that the zeolite shows negative charge in aqueous solution. XRD measurement also shows that the natural zeolite is mainly composed of clinoptiloite, quartz and mordenite. N2 adsorption isotherm and pore size distribution are presented in Fig. 2. The BET surface area and total pore volume were given in Table 1. As seen that BET surface

Conclusion

A natural zeolite has been tested for adsorption of basic dyes, methylene blue and rhodamine B. It is found that zeolite exhibits higher MB adsorption than RB due to the difference in molecular size. For methylene blue, the adsorption capacities are 6.8 × 10−5 and 7.9 × 10−5 mol/g at 30 and 50 °C, respectively. In contrast, the adsorption capacities of rhodamine B are 2.1 × 10−5 and 2.8 × 10−5 mol/g at 30 and 50 °C, respectively. Thermodynamic and kinetic calculations indicate that the adsorption is

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