Adsolubilization of Organic Compounds in Surfactant-Modified Alumina

1 J. Surface Sci. Technol., Vol 21, No. 1-2, pp. 97-112, 2005 2005 Indian Society for Surface Science and Technology, India Adsolubilization of Organic Compounds in Surfactant-Modified Alumina ASOK ADAK, MANAS BANDYOPADHYAY and ANJALI PAL* Civil Engineering Department, Indian Institute of Technology, Kharagpur 721302, India. Abstract Adsolubilization is the solubilization of organics into the three dimensional micelle like structures formed by the adsorption of ionic surfactants at high concentration on oppositely charged surfaces. Adsolubilization process could efficiently be used for the removal of different organic pollutants like dyes, phenolic compounds, etc. from water environment. In the present work, the adsorption characteristics of sodium dodecyl sulfate (SDS), an anionic surfactant (AS) on neutral alumina were studied in detail. Alumina was found to be an efficient adsorbent for SDS and could be used for the removal of SDS from its highly concentrated (several thousand ppm) solution. The equilibrium time found was 2 h. Though the removal efficiency was low (~ 65%) at neutral ph, but at slightly acidic condition and in the presence of NaCl, the efficiency could be increased drastically (>98%). The adsorption isotherm study showed four distinct regions. The effects of various other parameters such as adsorbent dose, the presence of different ions (Cl, NO 3, SO 4 2 and Fe 3+ ), and non-ionic surfactant on the SDS adsorption were also studied. It was observed that the adsorption capacity was increased due to the presence of these ions in general. After the adsorption of SDS on alumina, the alumina, which was then called the surfactant-modified alumina (SMA), was used for the removal of crystal violet (CV), a well-known cationic dye and phenol from aquatic environment. The kinetic studies showed that 1 h shaking time was sufficient to achieve the equilibrium for CV. The equilibrium time for phenol was 1.5 h. Studies were conducted to see the effects of adsorbent dose on the removal of CV and phenol separately using the SMA. The ph was maintained at 6.7±0.1. The SMA was found to be very efficient, and ~ 99% and ~ 90% efficiency could be achieved under optimised conditions for the removal of CV and phenol when present even at a high concentration (200 ppm for CV and 50 ppm for phenol). Effects of different parameters like SDS coverage on alumina, ph, presence of different ions, temperature, etc. were studied. To test whether the removal of CV and phenol was possible from real water using the SMA, the removal studies were conducted using CV and phenol spiked tap water samples. It was interesting to note that the removal efficiency was better for the tap water samples than for the distilled water ones. Desorption of SDS from the SMA was possible *Author for Correspondence : E-mail : anjalipal@civil.iitkgp.ernet.in. Paper presented at International Conference on Soft Matter (ICSM 2004), Kolkata, India, Nov. 18 20.

98 Adak et al. using aqueous sodium hydroxide and desorption of CV and phenol from the exhausted SMA surface could be done by rectified spirit. Keywords : Alumina, Anionic surfactant, Sodium dodecyl sulfate, Adsorption, Surfactant-modified alumina, Crystal violet, Phenol, Removal, Adsolubilization. INTRODUCTION Surfactants are harmful to human beings, fishes and vegetation. They are responsible for causing foams in rivers and effluent treatment plants and they reduce the quality of water. Surfactants cause short-term as well as long-term changes in ecosystem. Phenols and dyes are the major constituents of wastewater of many industries. Being water soluble, they are highly mobile and are likely to reach drinking water sources downstream from discharge point. Even at low concentrations, phenols can cause severe odour and taste problems and pose risks to population. Coloured dye effluents pose a major threat to the surrounding ecosystem. Many of the dyes are extremely toxic. For example, crystal violet (CV) a dye being used for various purposes is a mutagen and a mitotic poison. Various treatment technologies like adsorption, photodegradation, coagulation-flocculation, chemical oxidation, biological process, etc. are available [1-10] for the removal of phenol and dye from the wastewater. Surfactant enhanced pollutant removal technologies recently have drawn much attention for their low energy requirement and low cost. In very dilute aqueous solution, an ionic surfactant acts much as normal electrolytes, but at higher concentrations it acts in a very different way. This behaviour is explained in terms of the formation of organized aggregates of large number of molecules called the micelle, in which the hydrophobic parts of the surfactants associate in the interior of the aggregates leaving hydrophilic parts to face the aqueous medium. The surfactant concentration at which micelles are formed is called the critical micelle concentration (CMC). The micelles have the ability to preferentially absorb organic solutes from solution [11]. This process is called solubilization. The adsorption of ionic surfactants on oppositely charged surface is generally different from ordinary adsorption process. The ionic surfactant molecules form monolayer or bilayer on solid charged surface depending upon the surfactant concentration. At low surfactant concentration the surfactant molecules are adsorbed individually, and at higher concentration the monolayer and bilayer structures are formed. These structures act as micelles, and have the potential to solubilize the organics into the three dimensional structures. The process is called adsolubilization. Hence, adsolubilization is the surface analog to solubilization, with adsorbed

Adsolubilization of Oraganic Compounds in Surfactant-Modified Alumina 99 surfactant bilayers playing the role of the micelles [12]. The monolayer structure is called the hemimicelle and the bilayer structure, the admicelle. Adsolubilization process can efficiently be used for the removal of different organic pollutants like dyes, phenolic compounds, etc. from water environment. The characteristics of adsolubilization have been studied in the past two decades for many reasons. Based on their purposes, the studies on adsolubilization can be classified into four different categories : (1) the use of adsolubilized molecules to help characterizing the adsorbed surfactant layer; (2) the use of adsolubilization in separation processes; (3) the use of adsolubilization in the formation of ultra thin films; (4) the use of adsolubilization in admicellar catalysis. There are few reports on adsolubilization where it has been exploited in separation processes. Removal of different organic pollutants like naphthalene, naphthol, 4-amino-1-naphthalenesulfonic acid, styrene, isoprene, steroids, phenanthrene, perchloroethylene (PCE), phenolic compounds, etc. could be achieved through adsolubilization [13-22] using surfactant-modified silica, zeolite, alumina, titanium oxide, montmorilonite, kaolinite, etc. Most of the research deals with the physico-chemical aspect of the removal. The present work, however, aims towards the engineering aspect. The studies have been made in two stages. In the first stage, the neutral alumina has been used for the removal of anionic surfactant (AS) from water. The alumina thus produced after the removal of AS is called the surfactant-modified alumina (SMA). This has now been used for the removal of organic pollutants such as CV and phenol. The alumina was found to be a very efficient adsorbent for AS in the first stage, and CV or phenol in the second stage. The described procedure is unique particularly in systems where the pollutants are present at high concentrations, such as in wastewater. EXPERIMENTAL Materials : Acridine orange (ACO), CV, sodium dodecyl sulfate (SDS; a representative member of AS), phenol, glacial acetic acid and toluene were from BDH (AR grade) and were used as received. All other chemicals used in this study were of high purity and used without further purification. Alumina was supplied by SRL, India and used as such without further grinding and sieving. The granulation of neutral alumina is 70 290 mesh ASTM, molecular weight is 101.96 and zero point charge (Z pc ) is 9.15. The surface area of the alumina was 900 cm 2 /g. Methods : A high precision electrical balance (Sartorious GMBH) was used for weighing. A digital ph meter (DHP-500, SICO, India) was used for ph

100 Adak et al. measurements. A spectrophotometer (Thermo Spectronic UV1, UK) was used for absorbance measurement. Analytical method A rapid and reliable solvent extraction spectrophotometric method has been developed for the determination of AS [23]. Acridine orange (ACO) chemically known as 3,6-bis (dimethylamino) acridine having a colour (l max = 467 nm) has the potential for being used as an ion-pairing agent with AS. Sample solution (10 ml) containing SDS in the range of 0.1 to 6.0 ppm was transferred into a 25 ml separating funnel. ACO (5 10-3 M) and glacial acetic acid, 100 µl each, were added. Then 5 ml of toluene was added to it and shaken for 1 min. The aqueous layer was then discarded and the toluene layer was used for absorbance measurement at 467 nm. CV was estimated by the spectrophotometric method at its l max (591 nm) at neutral ph. Phenol was estimated also by the spectrophotometric method. Phenol at alkaline condition reacts with potassium ferricyanide and 4-aminoantipyrine to form a red coloured complex, which is measured by spectrophotometric method at its l max (500 nm). Experimental studies The batch experiments were carried out using synthetic samples of SDS in distilled water at 25 o C in a mechanical shaker at an agitation speed of 150 rpm. The batch studies were carried out with high initial SDS concentration (2000 ppm), which is often found in industrial wastewater. The ph of the solution was maintained at 6.7±0.1. The sorption equilibrium time was studied using adsorbent dose of 100 g/l and the shaking time was varied from 0 to 8 h. The equilibrium time was found to be 2 h and was used for further studies. Experiments were carried out to see the effects of different adsorbent doses. SDS solutions (10 ml) with varying adsorbent doses (0 to 150 g/l) were shaken for 2 h. The optimal adsorbent dose was found to be 100 g/l and this adsorbent dose was used for further studies. Studies were carried out to see the effect of ph (in the range of 2.2 to 9.3). The ph of the solutions was maintained by adding HCl or NaOH. The effects of different ions like chloride (0 to 1500 ppm, added as NaCl), nitrate (0 to 800 ppm, added as NaNO 3 ), sulfate (0 to 400 ppm, added as Na 2 SO 4 ), iron (0 to 60 ppm Fe 3+, added as [Fe 2 (SO 4 ) 3 ]) were studied. The initial SDS concentration was 2000 ppm and ph was 6.7±0.1. The effect of nonionic surfactant (Triton X-100) in the range of 0 to 3000 ppm was studied. The sorption isotherm was drawn to understand the nature of the equilibrium distribution of SDS on the surface of alumina. Presence of chloride ions and lowering the ph generally increase the adsorption capacity. So in the isotherm study sodium chloride was added in the SDS solution and ph of the solution was

Adsolubilization of Oraganic Compounds in Surfactant-Modified Alumina 101 kept low by adding HCl. The initial SDS concentration was varied from 0 to 40000 ppm. The dose of sodium chloride was 2500 ppm and the initial ph was 4.4±0.1. In all the above cases, after shaking, the samples were allowed to settle for 5 minutes and then filtered through ordinary filter paper. From the adsorption isotherm study it was found that the adsorption capacity is maximum (111.6 mg/g) when the initial concentration is 20000 ppm. Therefore a solution containing SDS of 20000-ppm concentration was taken for the preparation of SMA, which was further used for the removal of CV and phenol from aquatic environment. Alumina (200 g) was shaken for 24 h with 2-litre 20000 ppm SDS solution. To increase the removal efficiency and to have maximum loading of SDS on to alumina, the NaCl dose was kept at 2500 ppm and the ph at 4.4±0.1. After shaking, the supernatant was discarded and the alumina was washed first with tap water and finally with distilled water. Then the material was dried at 60 o C for 24 h. This material was used for the removal of CV and phenol from aquatic environment. Kinetic study was conducted to find out the equilibrium time. Effects of different parameters like adsorbent dose, surfactant coverage on alumina, ph, presence of different ions, presence of humic acid and the variation of temperature were studied. Removal of CV and phenol were carried out using tap water spiked samples. RESULTS AND DISCUSSIONS Removal of AS by alumina : Since the adsorbent dose has significant effect on the removal of SDS, the effect of this parameter was studied. The adsorbent dose was varied from 0 to 150 g/l and the initial concentration was 2000 ppm. The removal efficiency of SDS increases with the increase of adsorbent dose up to 100 g/l and after 100 g/l the removal efficiency remains almost constant. The increase in the removal efficiency with the increase of adsorbent dose can be attributed to the increased number of sites available for adsorption. The effect of ph was studied in the range 2.2 to 9.3. Lower ph is favourable for SDS adsorption and as the ph increases the adsorption capacity gradually decreases. The zero point charge (Z pc ) of alumina is 9.15. At this ph the adsorption capacity is almost zero. At low ph, the alumina particles become more positive and hence there is an increase in adsorption at lower ph [24]. The effects of different ions like chloride, nitrate, sulfate and iron were studied. The general trend is the increase of the adsorption capacity in the presence of ions. The presence of anions decreases the lateral repulsive force between the head groups of the ionic surfactant and results in more surfactant adsorption [25]. The presence of cations increases the surface charge of alumina and also results

102 Adak et al. in more SDS adsorption [26]. The effect of nonionic surfactant (Triton X-100) on SDS uptake by alumina was also studied. The adsorption of SDS on alumina is enhanced at low nonionic surfactant concentration and then the adsorption capacity gradually decreases with further increase in nonionic surfactant concentration and finally there is no effect. The addition of small amount of nonionic surfactant decreases the electrostatic repulsion forces between the ionic heads of the surfactant molecules, which helps in the adsorption of more surfactant molecules on alumina surface. In addition, the hydrophobic interaction between nonionic and ionic surfactants adsorbed increases. As a result, the adsorption of one surfactant is often enhanced by the addition of small amount of another surfactant [27-28]. Fig. 1 shows the unique adsorption isotherm described earlier [24] for the adsorption of ionic surfactant on oppositely charged surfaces. These isotherms are commonly divided into four regions. The region I is a region of low adsorption densities and is sometimes referred to as the Henry s law region. In the region I the surfactants are adsorbed as monomers and do not interact with one another. The adsorption in this zone results primarily from electrostatic forces between surfactant ions and the charged solid surface. The region II is indicated by the sharp increase in the slope of the isotherm. In the region II, the adsorption is Log Adsorption Region II Region III Region IV Region I Log equilibrium surfactant concentration Fig. 1. Schematic diagram of adsorption isotherm of ionic surfactant on oppositely charged oxide surface.

Adsolubilization of Oraganic Compounds in Surfactant-Modified Alumina 103 due to the electrostatic attraction between the ions and the charged solid surface and hemimicellar association of hydrocarbon chains. For example, in the case of adsorption of sodium dodecyl sulfate on alumina, when sulfate ions equivalent in number to the surface sites have been adsorbed, the contribution due to the electrostatic attraction disappears, and the further increase in adsorption will be only due to the association between the hydrocarbon chains. Micelle like aggregates is formed on the solid surface. The transition from the region I to the region II have been given designations analogous to the critical micelle concentration (CMC) such as critical admicelles concentration (CAC) or hemimicelle concentration (HMC). The monolayered structure is called the hemimicelle and the bilayered structure is called the admicelle. The transition from the region II to the region III is marked by a decrease in the slope of the isotherm. In the region III, the surfactants are adsorbed by association between hydrocarbon chains of adsorbed surfactant and free surfactant in solution. The region IV is called the plateau adsorption region. In most systems the transition from region III to region IV occurs above the CMC of the surfactant. The adsorption isotherm of SDS on alumina surface, as obtained in our case, is shown in Fig. 2. The region I occurs at a very low equilibrium concentration, below 3.37 ppm of SDS. This region indicates the monolayer formation. Because the alumina surface is highly heterogeneous and not perfectly smooth and planar, during the monolayer formation the adsorbed SDS molecules 1000 Adsorption capacity / mg g 1 100 10 1 0.1 0.01 Region I Region II Region III Region IV 1 10 100 1000 10000 1000000 Equilibrium concentration / ppm Fig. 2. Adsorption isotherm of SDS on alumina

104 Adak et al. may arrange themselves irregularly with varied orientations [29]. The transition from region I to II indicates hemimicelle or admicelle formation. Here, conversion of monolayer to bilayer occurs. The transition from region II to III occurs at an equilibrium concentration of 30.2 ppm (1.05 10-4 M) and the transition from zone III to IV occurs at 8839 ppm (3 10-2 M) of SDS. Generally the transition from region III to IV occurs above the CMC (10-3 to 10-2 M). It is important to note that in many systems of alumina, after monolayer saturation of SDS, the isotherm shoots up to form multilayers without attainment of further saturation [29]. From the isotherm study the maximum adsorption capacity was found to be 111.6 mg/g and it occurs when initial concentration is 20000 ppm. The effect of initial SDS concentration on the adsorption was studied at varying ph and at varying NaCl dose. In the first case, the ph was 6.7±0.1 in the absence of NaCl. In the second case, the ph was 6.7±0.1 and NaCl dose was 2500 ppm. In the third case, the ph was 4.4±0.1 and NaCl dose was 2500 ppm. Here NaCl was added and the ph was kept low to get more adsorption capacity. The adsorbent dose was 100 g/l. Initial concentration of SDS was varied from 0 to 40000 ppm. The removal efficiency increases with the increase of initial concentration and after certain initial concentration it starts decreasing (Fig. 3). For the first case, the maximum removal was 65% at an initial concentration of 4000 ppm. For the second case, the maximum removal was 84% at an initial concentration of 3000 ppm. For the third case, the maximum removal was 98.4% at an initial concentration of 2000 ppm. In the three cases the maximum removal efficiency occurs at three different initial concentrations. This is due to the fact 100 3 Removal efficiency / % 80 60 40 20 0 2 1 0 10000 20000 30000 40000 Initial concentration of SDS / mg l 1 Fig. 3. Removal of SDS at different initial SDS concentration. 1, ph = 6.7; 2, ph = 6.7, NaCl dose 2500 mg l 1 ; 3, ph = 4.4, NaCl dose 2500 mg l 1.

Adsolubilization of Oraganic Compounds in Surfactant-Modified Alumina 105 that lowering of ph and increasing the NaCl dose increases the removal efficiency. The removal efficiency at the initial concentration of 2000 ppm is 61% for the first case, 81% for the second case and 98.4% for the third case. Removal of CV by SMA While studying the removal of CV using SMA, in all the experiments the ph of the solution was kept at 6.7±0.1, temperature at 25±2 o C and shaking speed at 150 rpm. The initial CV concentration was taken as 200 ppm, which is often found in dye bearing wastewater [1]. Experiments were carried out to find the dose of SMA to remove CV from water environment and the adsorbent dose was varied from 0 to 20 g/l. The shaking time was 1 h. The optimum adsorbent dose was found to be 6 g/l and the removal efficiency was found to be > 99% (Fig. 4). But the untreated alumina can hardly remove CV from aquatic environment (~6%). The kinetic study was performed with optimum adsorbent dose of 6 g/l and the shaking time was varied from 0 to 105 min. The equilibrium time was found to be 1 h (Fig. 5) and it was used for further studies. It was found that Removal of CV / % 100 80 60 40 20 2 1 0 0 2 4 6 8 Dose of SMA / g l 1 Fig. 4. Effect of adsorbent dose on CV removal by SMA. 1, Distilled water; 2, Tap water. the removal of CV by SMA followed the second order reaction kinetics. Experiments were conducted to see the effect of surfactant coverage on alumina on the removal of CV. In this study, the adsorbent dose at 6 g/l and the surfactant coverage was varied from 0 to 111.6 mg/g. It was noticed that with the increase of surfactant coverage, the CV removal increased, but the rate of increase gradually decreased (Fig. 6). With the increase of surfactant coverage, more CV molecules could be accommodated in the bilayer structure formed on

106 Adak et al. Removal of CV / % 100 80 60 40 20 2 1 0 0 20 40 60 80 100 120 Time / min. Fig. 5. Removal of CV by SMA with respect to time. 1, Distilled water; 2, Tap water. 100 Removal of CV / % 80 60 40 20 0 0 20 40 60 80 100 120 SDS coverage on alumina / mg g 1 Fig. 6. Effect of surfactant coverage on CV removal by SMA the alumina surface and hence removal efficiency gradually increased. The removal was maximum at a surfactant coverage corresponding to saturation plateau [22]. The ph of an aqueous medium is an important factor that may influence the uptake of the adsorbate. Studies were conducted in the ph range, 2.6 to 10.8. The ph of the solutions was maintained by adding HCl or NaOH. At higher ph, CV became colourless, and the colour could be regained by adding acids. In this study, initial CV concentration was fixed at 200 ppm and the adsorbent dose at

Adsolubilization of Oraganic Compounds in Surfactant-Modified Alumina 107 3 g/l. The removal of CV increased with the increase of ph and again it gradually decreased. But at ph > 9.15 SDS molecules were desorbed from the alumina surface causing less removal. The effects of different ions like chloride, nitrate, hydrogen phosphate, sulfate, iron, and manganese were studied. The general trend was the increase of the removal efficiency in the presence of anions and decrease of removal efficiency in the presence of cations. The anions act as salting out agent, which increase the removal efficiency. The cations are preferentially adsorbed by the hydrophilic head groups of the SDS molecules in bilayer formation on alumina due to their smaller hydrated radii than those of CV molecules [15, 30]. Effect of humic acid in the range of 0 to 10 ppm was studied. It was found that the removal of CV was little increased by the presence of humic acid. Removal of CV by SMA was studied at three different temperatures : 15 o C, 25 o C and 35 o C. In this study, initial CV concentration was fixed at 200 ppm and the adsorbent dose was 4 g/l. The temperature in the range, 15-35 o C had no effect on the CV removal. Experiments were also carried out to see the removal of CV spiked in tap water. ph of tap water was 7.0±0.1, turbidity was 19 NTU, TDS was 330 mg/ l and hardness was 120 mg/l as CaCO 3. The effect of adsorbent dose was studied using 0 to 12 g/l of adsorbent. The shaking time was 1 h and the temperature was 25±2 o C. It was very interesting to observe that the removal of CV was more in tap water in comparison to that obtained in distilled water (Fig. 4). This is because of the fact that in tap water large quantity of TDS is present, which can act as a salting out agent. Kinetic studies were carried out with adsorbent dose of 6 g/l. The shaking time was varied from 0 to 105 minutes. The equilibrium time was found to be 45 min for tap water (Fig. 5). Again the presence of salts in tap water made the reaction faster than in the case of distilled water. The removal of CV by SMA followed the second order kinetics. Removal of phenol by SMA Removal of phenol by SMA was studied under ph at 6.7±0.1, temperature at 25±2 o C and shaking speed at 150 rpm. The initial concentration of phenol was taken as 50 ppm, which is often found in wastewater [2]. Experiments were carried out to find the dose of SMA to remove phenol from water environment. The adsorbent dose was varied from 0 to 24 g/l. The shaking time was 1.5 h. The optimum adsorbent dose was found to be 12 g/l and was thus used for further studies. The removal efficiency was found to be > 90% (Fig. 7). The equilibrium time was found to be 1.5 hours (Fig. 8) and was used for further studies. Kinetic study showed that the removal of phenol by SMA followed the second order reaction kinetics.

108 Adak et al. 100 Removal of Phenol / % 80 60 40 20 0 2 1 0 4 8 12 16 20 24 28 Adsorbent dose / g l 1 Fig. 7. Effect of adsorbent dose on phenol removal by SMA. 1, Distilled water; 2, Waste water. Removal of Phenol / % 100 80 60 40 20 2 1 0 0 20 40 60 80 100 120 140 160 180 200 Time / min. Fig. 8. Removal of phenol by SMA with respect to time. 1, Distilled water; 2, Waste water. Experiments were conducted to see the effect of surfactant coverage on alumina on the removal of phenol in the range 0 to 111.6 mg/g. Similar to the case of CV, here it was also noticed (Fig. 9) that with the increase of surfactant coverage the phenol removal increased, but the rate of increase gradually decreased. With the increase of surfactant coverage, more phenol molecules can be accommodated in the bilayer structure formed on the alumina surface and hence removal efficiency gradually increases [22].

Adsolubilization of Oraganic Compounds in Surfactant-Modified Alumina 109 100 Removal of Phenol / % 80 60 40 20 0 0 20 40 60 80 100 120 SDS coverage on alumina / mg g 1 Fig. 9. Effect of surfactant coverage on phenol removal by SMA The effect of ph was studied in the range, 2.3 to 11.4. The ph of the solutions was maintained by adding HCl or NaOH. The removal of phenol decreases with the increase of ph. At lower ph phenol is adsorbed as molecular form. As ph increases phenol is converted into negatively charged phenolate ions and the negatively charged head groups of SDS molecules adsorbed on the alumina surface repulse these ions. The effects of different ions like chloride, nitrate, sulfate, hydrogen phosphate, iron, and magnesium were studied. As in the case of CV removal, the general trend was the increase of the removal efficiency in the presence of anions, and slight decrease of removal efficiency in the presence of cations. The anions act as salting out agent, which increase the removal efficiency. Humic acid in the range of 0 to 10 ppm and temperature in the range 15-35 o C had no effect in the removal of phenol by SMA. Experiments were also carried out to see the removal of phenol spiked in tap water (same composition as mentioned in earlier section). The effect of adsorbent dose was studied using 0 to 24 g/l of adsorbent. The shaking time was 1.5 h and the temperature was 25±2 o C. Very similar to the case of CV, the removal of phenol was more in tap water (~96%) in comparison to that obtained in distilled water (Fig. 7). This is because of salting-out effect of dissolved solids present in tap water. Kinetic studies were carried out with adsorbent dose of 12 g/l. The equilibrium time was found to be 1.5 h (Fig. 8). The removal of CV by SMA followed the second order kinetics.

110 Adak et al. Regeneration study After the removal of AS, alumina can be regenerated using aqueous NaOH. Exhausted alumina (500 g/l) was shaken for 1 h at 150 rpm with 0.25 M NaOH solution. The Z pc of alumina is 9.15, which indicates that the removal of AS by alumina is possible at ph >9.15. It was noticed that ~ 90% of AS was desorbed from the surface of alumina. After regeneration, the same alumina could be reused for AS removal but in this case the adsorption capacity was found to be 98 mg/ g, whereas for fresh alumina it was 111.6 mg/g. After second use the same alumina could be regenerated using the same procedure and could be again used for AS removal. This time the adsorption capacity was found to be 81 mg/g. During the NaOH treatment process, however, alumina was dissolved in trace amount (< 0.1 ppm of Al 3+ was found to be present in the supernatant). The regeneration of exhausted SMA (after CV/phenol removal) was done using either rectified spirit or acetone. It was observed that neither rectified spirit nor acetone could desorb SDS from the exhausted SMA but they could desorb CV or phenol. The reason behind this is that the ph of the solution was lower than Z pc (9.15). In case of CV, 2 g of exhausted SMA was shaken with 8 ml of organic solvent (i.e., 250 g/l) for 60 minutes at a temperature of 25 o C. The regeneration efficiency was found to be 86% and 65% for rectified spirit and acetone respectively. Hence, rectified spirit is a better choice for regenerating SMA. The regenerated CV could be collected and reused. The organic solvent could be distilled off using a Soxhlet apparatus and the same solvent could be recycled for desorption studies, i.e., for regenerating SMA from CV-adsorbed SMA. Regenerated SMA was again used for CV removal and was found to have the efficiency in the range of 58-70% (whereas the removal efficiency was ~99% using fresh SMA). In case of phenol, 5 g of exhausted SMA was shaken with 20 ml of rectified spirit or acetone (i.e., 250 g/l) for 60 minutes at a temperature of 25 o C. The regeneration efficiency was found to be 79% and 48% for rectified spirit and acetone respectively. Regenerated SMA was again used for phenol removal and it was found to have the efficiency in the range of 50-75% (whereas the removal efficiency was ~90% using fresh SMA). After the removal of CV or phenol from SMA, if SMA was treated with 0.25 M NaOH desorption of both SDS and CV/ phenol occurred along with slight dissolution of alumina. CONCLUSION Alumina was found to be very efficient adsorbent for the removal of AS from its highly concentrated wastewater. Under optimised condition ~98% removal efficiency for AS could be achieved. The equilibrium time was found to be 2 h.

Adsolubilization of Oraganic Compounds in Surfactant-Modified Alumina 111 The adsorption isotherm study showed distinct four regions. AS forms bilayer structures on the surface of alumina, which in turn could solubilize organic molecules through adsolubilization. This process has been used for the effective removal of CV and phenol from wastewater upto ~99% and ~90% respectively while present in high concentration (200 ppm for CV and 50 ppm for phenol). The kinetic studies showed that 1 h shaking time was sufficient to achieve the equilibrium for CV and it was 1.5 h for phenol. Effects of different parameters like SDS coverage on alumina, ph, presence of different ions, temperature, etc. were studied on the removal of CV and phenol. With the increase of SDS coverage, removal efficiency increases. Presence of anions in the solution increases the removal efficiency and presence of cations decreases the removal efficiency. There is no effect of temperature on the removal process. It was interesting to note that the removal efficiency was better for tap water compared to that using distilled water. The removal efficiency of CV was more than phenol due to less solubility of CV than phenol. Desorption of SDS from exhausted alumina was possible using aqueous sodium hydroxide and desorption of CV and phenol from the exhausted SMA surface could be done by rectified spirit. REFERENCE 1. V. K. Garg, R. Gupta, A. B. Yadav and R. Kumar, Bioresource Technol., 89, 121 (2003). 2. S. Rengaraj, S. H. Moon, R. Sivabalan, B. Arabindoo and V. Murugesan, Waste Management, 22, 543 (2002). 3. S. F. Kang, C. H. Liao and S. T. Po, Chemosphere, 41, 1287 (2000). 4. C. Wu, X. Liu, D. Wei, I. Fan and L. Wang, Wat. Res., 35(16), 3927 (2001). 5. S. Papic, N. Koprivanac, A. L. Bozic and A. Metes, Dyes and Pigments, 62, 291 (2004). 6. T. A. Ozbelge, O. H. Ozbeze and S. Z. Baskaya, Chem. Eng. Processing, 41, 719 (2002). 7. I. A. Salem, Chemosphere, 44, 1109 (2001). 8. C. B. Maugans and A. Akgerman, Wat. Res., 31(12), 3116 (1997). 9. S. Ledakowicz, M. Solecka and R. Zylla, J. Biotechnol., 89, 175 (2001). 10. R. L. Droste, K. L. Kennedy, J. Lu, and M. Lentz, Wat. Sci. Tech., 38(8-9), 359 (1998). 11. J. H. Fendler and E. J. Fendler, Catalysis in Micellar and Macromolecular Systems, Academic Press, New York (1975).

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