Sorption of cadmium from aqueous solution by surfactant-modified carbon adsorbents

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Abstract

Sorption capacities for cadmium, Cd(II) on indigenously prepared, steam activated, untreated, surfactant-modified carbon powder, from husk and pods of Moringa oleifera were investigated. The optimized conditions for all the experimental runs were pH 8.0 ± 0.2, temperature 30 ± 0.5 °C, contact time 120 min, agitation speed 160 rpm, initial metal concentration 30 mg L−1 and adsorbent dosage 1.0 g L−1, respectively. Maximum Cd(II) removal, 98.0% was observed when cetyltrimethyl ammonium bromide (CTAB), cationic surfactant-treated carbon was used as an adsorbent. The Cd(II) removal percentages for sodium dodecyl sulphate (SDS), anionic surfactant, Triton X-100 (non-ionic surfactant) treated and untreated powder activated carbons were found to be 95.60, 81.50 and 73.36%, respectively. SEM images and BET surface area, porosity and pore volume measurements have revealed that surfactant-treated carbons have superior porosity and enhanced surface area than untreated carbons. The sorption data were correlated better with the Langmuir adsorption isotherm than Freundlich isotherm with R2 values ranging from 0.91 to 0.98.

Introduction

The contamination of the environment and specifically water by toxic heavy metals, has persistently posed a global challenge due to increasing environmental pollution [1]. Unlike organic pollutants, the majority of which are susceptible to bio-degradation, heavy and noble metals are not bio-degradable [2], [3]. They are toxic to aquatic flora and fauna even if they are present in relatively low concentrations. Some of these are capable of being assimilated, stored and concentrated by organisms. Cadmium, Cd(II), for example can be accumulated in human body, causing erothrocyte destruction, nausea, salivation, diarrhea and muscular cramps, renal degradation, chronic pulmonary problems and skeletal deformity [4]. Today, cadmium emissions have increased dramatically, as cadmium-containing products are rarely re-cycled, but often dumped together with household waste. Cigarette smoking is also a major source of cadmium exposure. Many states of health are linked to elemental imbalance leading to the generation of multifarious diseases [5], [6]. Therefore, measures should be taken to reduce the pollution caused by heavy metal contamination in general and cadmium exposure in particular, to minimize the risk of adverse effects on health and the environment.

Numerous efforts including the development of chemical processes such as electrofloatation, chemical precipitation, ion exchange, biosorption, co-precipitation/adsorption, flocculation and membrane filtration have been developed to reduce the hazardous effects of toxic metals [7]. Adsorption is considered as the most practical and economical way to remove heavy metals. The ability of activated carbon to scavenge the pollutant's molecules is mainly attributed to higher specific surface area. Activated carbon from cheaper and readily available resources like, coke, peat, wood, saw dust, coconut shell [8], rice husk [9], papaya wood [10], etc. have been successfully employed. Various kinds of agricultural, domestic and industrial wastes such as sugarcane bagasse, Moringa oleifera (M. oleifera) husk and pods [11], cotton stalk, wastes rubber [12] have been utilized in the preparation of activated carbon. In Pakistan, Moringa is represented by only two species: M. concanensis and M. oleifera. The dried seeds can be crushed to produce high-quality vegetable oil and the resulting press-cake mixed with water and strained to form a coagulant for water treatment [13], [14], [15]. The residue-containing seed husk is currently discarded as waste. It has been reported that the simple steam pyrolysis procedure [11], [16] can produce high-quality microporous activated carbons from both the waste husks and pods of M. oleifera.

Surface-active substance or surfactants are amphipathic substances with lyophobic and lyophilic groups making them capable of adsorbing at the interfaces between liquids, solids and gases. They form self-associated clusters, which normally lead to organized molecular assemblies, monolayers, micelles, vesicles, liposomes and membranes. Depending upon the nature of hydrophilic group, they can be anionic (negative charge), cationic (positive charge), non-ionic (no apparent charge), and Zwitterionic (both charges are present). The critical micelle concentration (CMC) is the concentration of an amphiphilic component in solution at which the formation of aggregates (micelles, round rods, lamellar structures, etc.) in the solution is initiated [17], [18]. For these characteristics, surfactant-modified adsorbents are not only superior in terms of removal efficacy than the conventional adsorbents, but also encourage selective adsorption [11]. As it is nowadays, other than for environmental remediation, surfactants are being utilized in almost every industry including paints, textiles, cosmetics, pharmaceuticals, agrochemicals, fibers, plastics, petroleum, food and mineral processing. Therefore, emerging technologies represent a vibrant and challenging area in understanding the physico-chemical properties of conventional and non-conventional (gemini, non migratory, etc) surfactants and phase behavior of surfactant based processes involved in the preparation of emulsions, dispersions, suspensions and their subsequent characteristics i.e. stabilization, formation, breaking of interfaces, wetting, spreading, solubilization and adhesion. Moreover, surfactants are also very useful in chromatographic separations of the tedious compounds like pesticides. Their use in the chemical modification of adsorbents can lead to the selective separation and recovery of precious and noble metals as well.

Adsorption behavior can be explained in terms of adsorption isotherms. The linearized form of the Langmuir isotherm equation assumes that every adsorption site is equivalent and the ability of a particle to bind is independent of whether or not adjacent sites are occupied. On the other hand, Freundlich isotherm model assumes that the adsorption takes place on a heterogeneous surface [19]. The objectives of this study were to investigate the effect of surfactant modification on the structural morphology and surface area of indigenously produced activated carbon. This will be accomplished by evaluating the Cd(II) removal efficiency by analyzing the sorption data with the help of Langmuir and Freundlich models.

Section snippets

Preparation and structural characterization of activated carbon

Mature pods of M. oleifera-containing seeds to be used for the preparation of the carbon adsorbents were collected from plants, within the campus of University of Agriculture, Faisalabad, Pakistan. Seeds were separated from the husk and pods. Activated carbon was produced based on the method as described by Warhurst et al. [11], [16]. The precursor (30 g husk material and 10 g pods) was heated in a furnace to eliminate volatiles (carbonization) and concurrently activated by steam flowing from the

Structural characterization

SEM images of steam activated, SDS, Triton X-100 and CTAB-modified activated carbon are presented in Fig. 1. It is evident from the obtained SEM images that surfactant modification is significantly responsible to alter the physico-chemical properties and porosity of the materials.

BET surface area and porous structure characteristics of the adsorbents are summarized in Table 1. The results not only show that the modified carbon adsorbents have greater BET surface area as compared to the

Conclusion

This study presents the sorption of Cd(II) from aqueous solution using indigenously prepared and surfactant-modified carbon adsorbents from husk and pods of M. oleifera. Adsorption was found to be strongly dependent on pH, adsorbate, adsorbent dosage and contact time. A single step steam pyrolysis performed in this study proven to be an easy and economical method for the conversion of agricultural waste materials like pods and husk of M. oleifera into good quality micro-porous activated carbon.

Acknowledgement

The author (M. Nadeem) is grateful to University of Agriculture, Faisalabad for granting the leave to pursue higher studies.

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