Acid Green 25 removal from wastewater by organo-bentonite from Pacitan

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Abstract

The capability of surfactant modified-bentonite for adsorption of dyestuff from aqueous solution was investigated. The so-called “organo-bentonite” was obtained by modifying the raw bentonite obtained from Pacitan, Indonesia using cetyl trimethylammonium bromide (CTAB) and later employed as adsorbent for Acid Green 25 removal. Isotherm and kinetic experiments were carried out on three different temperatures (30, 40 and 50 °C). Langmuir and Freundlich models were chosen for isotherm equilibria data correlation, of which the former showed better suitability. On kinetic data representation, pseudo-first and second order models were used, with the last model gave better correlation.

Introduction

Dyes are used widely as coloring agents in many industries, such as textiles, cosmetics, leather, printing, foods and plastics. Due to their resistance to degradation, they might be present on wastewater at substantial quantity. Though not particularly toxic, dyes might be harmful to human beings and hazardous to aquatic organisms; not to mention their adverse aesthetic effects as they are quite visible. The presence of color also reduces aquatic diversity by blocking the passage of light through water (Khenifi et al., 2007). In some cases, aquatic organisms and plants that normally use light as an energy resource die. In human, these agents can cause several mutagenic and/or carcinogenic effects. Severe damage to reproductive system, liver, brain and dysfunction of kidneys for example, have been pointed out (Baskaralingam et al., 2006).

Acid dyes which comprise the largest class of dye in the Color Index (CI) are anionic compounds mainly used for dyeing nitrogen-containing fabrics like wool, polyamide, modified acryl and silk. These compounds are the most difficult to remove, even by activated carbon. Acid Green 25 (1, 4-di-[(2-sulfono-4-methylphenyl) amino]-9, 10-anthracenedione, disodium salt) in particular belongs to the commercial acid dye often used in textile, hair dye formulation and cosmetic product. This dye has also been known as Acid Green Anthraquinone, Alizarin Cyanine Green F, Japan Green 201 and/or D&C Green No. 5. Acid Green 25 is stable (not readily decomposed) in water at 20 °C up to 7 days. As a result, some aquatic organisms and plants' presence might be compromised. Moreover, this compound also brought about a slight or spotty discoloration in the orbital tissue of some animals at 10% aqueous solution, very mild irritation at 15% aqueous solution and dermal irritation at 50% aqueous solution in 1% CMC (carboxymethyl cellulose).

Adsorption using activated carbon or ion exchange resins is the most popular technique for dyes removal due to its efficiency. Adsorbents remove dyes from wastewater, either by adsorption (anionic dyes) or by combined adsorption and ion exchange (cationic dyes). Although most adsorbents do not remove all dyes, due to the existence of different types of dyes (non-ionic, anionic or cationic dyes), activated carbon in particular is capable of adsorbing many different dyes with high capacity. However, due to the high price and regeneration cost (as desorption of dye molecules is not easily achieved), activated carbon becomes infeasible as adsorbent (Khaled et al., 2009, Leung et al., 2009, Al-Degs et al., 2009, Amin, 2009). Therefore, another cheaper and more economic alternative adsorbent has either been sought after or developed, such as bentonite.

The chemical nature and pore structure of bentonites generally determine their adsorption ability. Yet, due to the hydrophilicity induced by the exchangeable metal cations, bentonites normally are not effective in adsorbing organic compounds such as dyes. Therefore, to improve the adsorption capacity of these adsorbents, surface modification has been conducted and investigated (Juang et al., 2002, Lee et al., 2002, Yilmaz and Yapar, 2004, Özcan et al., 2004, Yoo et al., 2004, Erdemoglu et al., 2004, Zeng et al., 2006, Sanchez-Martin et al., 2006, Richards and Bouazza, 2007, Khenifi et al., 2007, Akimbaeva and Ergozhin, 2007, Ma and Zhu, 2007, Zhu et al., 2008, Koyuncu, 2008). Two types of surface modification exist: (1) Impregnation of organic molecules on bentonite surface, classified as a physical process and (2) organo-functionalization or grafting of organic molecules on bentonite surfaces, classified as a chemical process. Impregnation or organic modification process is accomplished through the replacement of inorganic exchangeable cations, like Na+, K+, Al3+ and Ca2+, by organic cations, typically with quaternary ammonium cations. The surface nature of these bentonites can be changed from hydrophilic to hydrophobic or organophilic by organo-functional molecules with surface hydroxyl groups, Lewis and Bronsted acidic sites, etc., by grafting organic groups on the bentonite mineral surface (Erdemoglu et al., 2004). The adsorption ability of bentonite can also be improved by treatment with strong inorganic acid, usually carried out at high temperature. When bentonites are acid-activated by hot mineral acid solutions, hydrogen ions attack the aluminosilicate layers via the interlayer region. This process alters the structure, chemical composition, and physical properties of the bentonite while increasing its adsorption capacity resulting in the so-called “organo-bentonite”. Organo-bentonite used for wastewater treatment applications in today's industry is strongly recommended due to their local availability, technical feasibility, engineering applications and cost effectiveness. Numerous studies have investigated the use of organo-bentonite as potential sorbent for organic contaminants in a wide variety of environmental applications (Juang et al., 2002, Lee et al., 2002, Erdemoglu et al., 2004, Yoo et al., 2004, Yilmaz and Yapar, 2004, Özcan et al., 2004, Sanchez-Martin et al., 2006, Zeng et al., 2006, Khenifi et al., 2007, Richards and Bouazza, 2007, Ma and Zhu, 2007, Akimbaeva and Ergozhin, 2007, Koyuncu, 2008, Zhu et al., 2008). These studies revealed organo-bentonite's performance and prospects as adsorbents.

Bentonite deposit in East Java, Indonesia, is mainly of Ca–Mg type bentonite. This type of bentonite is generally suitable as a raw material for adsorbent and bleaching earth. In East Java, bentonite reserves can be found in several areas such as Pacitan, Ponorogo, Blitar, Trenggalek, etc., with total reserve more than 500 million tons. Currently, the major use of this material is for the purification of crude palm oil and as drilling mud. As far as we know, systematic studies on the modification of bentonite originated from Indonesia for acid dyes removal purpose has not yet existed. It is our hope that via this study, a better understanding on this aspect can be obtained. Therefore, this paper is focused on the preparation and characterization of “organo-bentonite” made from this bentonite. Different process variables were used to obtain bentonite with specific chemical and surface characteristic nature followed by bentonite modification using cationic surfactants. The standard procedures for characterizing porous materials were then employed to characterize the bentonite and organo-bentonite. The organo-bentonite was subsequently employed for acid dye adsorption equilibria and kinetic study.

Section snippets

Materials

Acid Green 25 was obtained from Semarang, Indonesia and used without any pretreatment. The surfactant was cetyl trimethylammonium bromide (CTAB) which was purchased from Sigma-Aldrich Singapore (purity 99%). Bentonite was acquired from mining site on Pacitan, East Java, Indonesia. Initially, as-collected bentonite was dried in forced-circulation oven at 110 °C to remove excess moisture content. The drying process was carried out for 24 h. Subsequently, dried bentonite was crushed using JANKE &

Physical characterizations of adsorbent

The specific surface area and pore size distribution were determined by nitrogen adsorption at its boiling point using Quadrasorb SI. Nitrogen sorption isotherms for both bentonite and organo-bentonite were shown in Fig. 1. Both bentonite and organo-bentonite have mesoporous structures. Mesoporous structure was shown by a hysteresis between adsorption and desorption curve after relative pressure (P/P0) of 0.4. The difference between bentonite and organo-bentonite lies on its volumetric

Conclusion

The so-called “organo-bentonite” made from bentonite obtained within Pacitan, Indonesia was probed in terms of its adsorption potential to remove Acid Green 25 from aqueous solution. The Langmuir model fitted well the experimental data. The pseudo-second order model represented the experimental kinetic data in a better manner than the pseudo-first order model, indicating chemisorption as the controlling step of adsorption.

Acknowledgements

The authors acknowledge the funding of this work through the National Taiwan University of Science and Technology–Widya Mandala Surabaya Catholic University joint research project with contract no RP09-01 and the Directorate General of Higher Education Republic of Indonesia through competitive grant for international publication with contract number 676/SP2H/PP/DP2M/VII/2009. This work is also partially funded by the Directorate Higher Education, Indonesia through PKMP Student Grant 2009.

References (27)

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