Biosorbent for Diazo Acid Dye Removal as a Contribution to Textile Wastewater Treatment

Biosorbent for Diazo Acid Dye Removal as a Contribution to Textile Wastewater Treatment

F. Ayari and M. Trabelsi Ayadi

Laboratoire des Applications de la Chimie aux Ressources et Substances Naturelles et a` lEnvironnement (LACReSNE), Université de Carthage, Faculté de Sciences de Bizerte, Zarzouna 7021, Tunisia.

Abstract:- Adsorption of Congo red (CR) from water solution through batch adsorption experiments onto locally bentonite befor and after modification by using hexadecyltrimethylammonium (HDTMA) bromide at 3 CEC level, obtained organosmectie labeled ArPM was characterized by several methods such as X-ray diffraction (XRD), Infrared spectroscopy (IR), thermal analysis (TGA- DTG), Brunauer-Emmett-Teller (BET) and SEM, these methods were used to provide new visions into the interlayer structure of the synthesized hybrid organoclay. XRD patterns showed the changes in the d(001) spacing, which gave details of the arrangement of surfactant in the organoclays, IR spectroscopy showed the existence of HDTMA functional groups on bentonite surface. The BET surface area signicantly decreased after the modication due to the coverage of the pores of purified smectite. Acid- base potentiometric titration and zeta metric behavior of the smectite before and after organophilisation were investigated. Smectite surface which was hydrophilic in nature convert to hydrophobic after organophilisation. Hybrid material develops more positives surface charges and great basal spacing layer; therefore these materials can be very useful to remove anionic pollutants from wastewater.

After the characterization, the adsorption of a textile diazo acid dye, Congo red (CR), onto smectite and HDTMA- smectite was investigated. The maximum adsorption capacity of smectite and HDTMA-smectite for CR was remarkable at 20ºC.

Isotherm data were investigated according to Langmuir and Freundlich equations. Monolayer adsorption can be estimated for CR removal by the smectite sorbent.

Keywords-Smectite, Adsorption; Acid textile dye; Sorption isotherms; Langmuir and Freundlich models.

1. INTRODUCTION

   The problems of the environment constituted an integral part of the development strategies conceived and implemented in each country at the national level as a sector level. The industry of dyes generates rejections, pollutants, toxic heterogeneous composition, which became a source of watery pollution. It is a real degradation of environment; it is an enormous harmful effect for human health. Textile dyes present a very weak biodegrability [1] what makes the biological treatments difficult and in case of degradation, they may produce carcinogenic substances. Several processes of elimination were used; such as adsorption on activated carbon which is an effective process but expensive and produces a mud which constitutes itself an environmental threat. Therefore there has been increasing interest in research into cheaper and more readily available adsorbents. Some biological

   adsorbents have also been studied for the adsorption of reactive dyes; these include amongst others; apple pomace and wheat straw, corncob and barley husk, maize cob, wood and rice husk. These adsorbents were found to be efficient in binding with basic dyes rather than acid dyes. Research acted towards processes of treatment using natural materials. In recent years attention has been focalized in clay mineral due to its abundant availability, also according to its important surface and presence of loads on its surface and interlayers space, which gives it, the possibility of exchanging. However, most previous works are based on modication of these adsorbents via physical and chemical process, which is necessary for the adsorption of anionic dyes, as the net negative charge on the dye and clay surface brings charge repulsion and thus, resulting in lowering surface interaction and adsorption.

   A study effected on chemical classifications of dyes, shows that diazo dyes are the most toxic [2].Toxicity by exposure to the azo dyes and their metabolites is not a new fact. Since 1895, the increase in the number of cancers of bladder observed in workmen of textile industry is connected to their exposure prolonged to the azo dyes [3]. The adsorption of organic cationic dyes by clay minerals has been investigated for many years. There is very little information on the adsorption of organic anions by clay minerals in the literature. Also, there are contradictory results about the effect of pH on the adsorption of CR by various adsorbents in the literature. In this study we report the ability of locally clay mineral before and after modification to remove diazo acid dye (CR) from aqueous solution as a contribution to anionic dye removal from textile wastewaters. The clay sample collected demonstrates a good affinity with this anionic dye.

   1. MATERIALS AND METHODS

      II .1 Materials

      Congo Red (CR) is a synthetic anionic secondary diazo dye, belonging to the family of reactive dyes which contain chromophoric groups.The molecular weight of CR is 696,66 g.mol-1, the maximum wavelength absorption in the field of visible is max = 497 nm. The IUPAC name of CR is [1-napthalene sulfonic acid, 3,3 -(4,4 biphenylene bis (azo))bis (4-amino-) disodium salt], its structural formula is: C32H22N6Na2O6S2.

      Congo red contains NH2 and SO3 functional groups (fig1). The CR sodium saltis responsible for dying cotton full red and is the first synthetic dye capable of directly dying cotton [4]. It has a strong affinity for

      cellulose fibers, very used in textile industry. Congo Red was used without any further purification.

      The color of CR changes from red to blue in the presence of inorganic acids. The change of color is due to the resonance between charged canonical structures [5]. The

      red color is stable in the pH range of 510 [6]. The stock solution of CR was prepared by dissolving known quantity of CR powder in distilled water, successive dilutions were made to obtain the working solution at desired concentrations.

      Clay, used in this study, is a naturally calcium smectite.

      For purified clay, raw clay was previously crushed and sieved, then suspended in polyethylene bottle containing a solution of NaCl (1 M). After 12 hours of agitation, the clay particles are separated from the stage water by centrifugation at a speed of 6000 rpm for 30 min -1. This step is repeated several times to provide near complete substitution of the interlayer ions (Ca2+, Mg2+, Li+) by sodium ion and finally obtain purified Na saturated clay. Impurities such as carbonate, quartz, calcite etc…associated to the clay fraction are also eliminated.

      The Na saturated (ArP) clay is finally washed with distilled water several times; this step is followed by a dialysis process to remove the excess of chloride ions, AgNO3 test is needed to verify the elimination of chloride ions. Obtained clay is collected in a Teflon beaker, dried at 333K and then crushed and sieved.

      Organoclays

      surfactant-modified smectite (ArPM) was prepared by mixing an amount of Na-smectite, initially dispersed in ~100 mL of deionized water, with a stoichiometric amount of surfactant (tree time the cation of exchangeable cation of ArP (3CEC)) dispersed in 200mL of deionized water, added slowly to the clay suspension, Surfactant used in this study is precisely hexadecyltrimethylammonium bromide (fig.2). The reaction mixtures were stirred for 12 h at 80ºC. After maturation, centrifugation, and several successive dialyses, the prepared organoclay materials was washed with distilled water to remove any excess surfactants, until free of broide anions (tested by AgNO3), dried at 80 ºC, ground in an agate mortar and passed through a 200 mesh sieve. The organoclay prepared was marked as H3-bent.

      5.1Ã…

      4.1Ã…

      21Ã…

      N

      H

      C

      25.3Ã…

      fig.2: Chemical structure of HDTMA

      II.2. Methods

      Composition of the clay sample was identified by X-ray diffractometer (XRD). XRD analysis was carried out using a PANalytical X'Pert High Score Plus diffractometer, CuK radiation. Composition of ArP sample was confirmed with an EDX system. The Fourier transform infrared (FTIR) spectra were acquired on a Perkin Elmer 783 dispersive spectrometer in the range of 4000400 cm-1. Cation exchange capacities (CEC) was estimated as described in previous studies [7,8]. As complimentary technicals for clay characterization, the sample morphology was recognized by Transmission electron microscopy (TEM). BET surface area (SBET) and

      pore volumes of adsorbent were measured using the physical adsorption of nitrogen by Quantachrome Autosorb-1 instrument. Zeta potential measurement using a Zetasizer 3000 HSA (Malvern Instruments) and potentiometric titration at different ionic strength were used to estimate the points of zero proton charge of used adsorbents. Uv-spectrometer was also used to determine the percent of dye removal by ArP and ArPM under several conditions such as pH, dye concentration

   2. RESULTS AND DISCUSSIONS

   1. Characterization of the adsorbents

      1. Chemical analyzes

         Purified clay was performed by means of X-ray fluorescence spectrometer (Table 1).Chemical composition is given in weight percent of oxides.

         Results stipulate the presence of silica and alumina as major constituents along with traces of potassium, magnesium and calcium oxides [9].

         Table 1: Chemical composition (%weight (w%)) of Sodium smectite ArP

         MxOy	SiO2	MgO	CaO	Fe2O3 Al2O3	Na2O	K2O
         %weight	52	4.2	0.4	4.8 15.56	1.63	0.76

         Accordingly the average structural formula of the Na exchanged puried clay is:

         Ca 0.05 Na0.145K 0.479(Si 7.87Al0. 13) (Al2. 6 Fe0. 678 Mg 0.547) O22

         The formula shows that the two types of substitutions exist (octahedral and tetrahedral substitutions) which gives a beidelliticmontmorillonitic character to this smectite.

      2. X-Ray diffraction

         The clay sample, before and after purification, analyzed by XRD measurements (fig.3 and fig.4) showed that clay is composed mainly of smectite, kaolinite, illite, and quartz. The smectite is deduced by the appearance of peaks at 15.3Ã… on the normal diffractogram of the raw

         sample ArB) which was displaced to 12.38 Å after purification and which was enlarged on the glycoled diffractogram to 17.69 Å. The presence of kaolinite is proved by the appearance of peaks at 7.19 and 3.57 Å on the normal and glycoled diffractograms. These peaks disappear in the sample heated at 550°C during 2 h. The appearance of peaks at 4.27and 3.35Å marked the presence of quartz, witch disappears after purification, and peaks at

         10.32 and 2.57 Å marked the presence of illite [10, 11]. Heating the sample above 550°C collapses the interlayer spacing at 10Å, this data support the clays mectite group.

         17.69A°

         17.69A°

         12.65A°

         10A°

         10A°

         3.50A°

         3.50A°

         7.11A°

         10A°

         7.11A°

         10A°

         7.11A°

         5A°

         5A°

         3.51A°

         3.51A°

         3.33A°

         3.33A°

         2.35A°

         2.35A°

         2.35A°

         2.35A°

         LN

         5A°

         5A°

         3.33A°

         3.33A°

         LG

         5A°

         5A°

         LC

         fig.3: X-raydiffractogram of samples ArB and ArP

         1. FTIR analysis

            The infrared spectra were recorded with Perkin Elmer 783 dispersion spectrometer in the range of 4000 to

            400 cm-1. Spectra were obtained by accumulating 128 scans at a resolution of 4 cm-1. FTIR spectroscopy has been employed to pinpoint the most significant modification by means of HDTMA grafting in the purified smectite (ArP). Therefore spectrum of ArP, surfactant molecule, and synthesized HDTMA modified smectite (organoclay sample ArPM) (fig. 5) were carried out and compared with each other to obtain information regarding modification of smectite with HDTMA functional groups.

            As can be seen from figure 5 and on the basis of information given by earlier works [12-13] absorption bands at 3626cm1 and 3622 in ArP and ArPM respectively are owing to hydroxyl group vibration in Mg-OH-Al, Al- OH-Al and Fe-OH-Al elements of the octahedral layer.

            Position and shape of the OH stretching band in the FT-IR spectra of smectite mineral is basically influenced by the nature of the octahedral atoms to which the hydroxyl groups are coordinated. Thus absorption band at 3626 cm-1,

            10 20 30 40

            mp

            mp

            10 A°

            10 A°

            fig 4: diffractogram of oriented s2a °les of ArP (LC: heated blade, LG glycol blade; LN: normal blade)

            found in the spectrum of ArP, is typical for smectite with large amounts of Al in the octahedral sheet.

            Broad bands at around 3430 in both ArP and ArPM were attributed to HOH stretching vibration of adsorbed water. Appearance of intense peak at around 3017 cm-1 (stretching peak of the N-H groups) in the infrared spectrum of ArPM is a suggestion of interaction between smectite structure and functional groups of surfactant.

            Peaks at around 1623 and 1648 cm1 relate to the OH deformation of water (H-O-H bending), reveals the presence of adsorbed water. These bands appear in both ArP (1623cm-1) and ArPM (1648 cm-1) [14], yet its position shifted from 1623 cm-1 to 1649cm-1 witch reects that the amount of hydrogen bonded water molecules present in the organoclay is less than those with ArP. This could be explained by that the H2O content is reduced with the replacement of the hydrated cations in the interlayer space of smectite by HDTMA+ ions. Therefore internal surface property of smectite was converted from hydrophilic to hydrophobic. In addition, comparing to HDTMA spectra, infrared spectrum of organoclay showed a pair of a strong intense vibrations at around 2921 and

            2854 cm1 attributed to symmetric and asymmetric stretching vibrations of methylene groups (CH2), which are absent in ArP, this attest transformation of purified smectite on organoclay with the surfactant cation (HDTMA+). These frequencies are extremely sensitive to the conformational ordering of the chain of cationic surfactant onto clay [15], in this case vibration bands close to micelle surfactant conguration as reported by Xi in a previous studies [16].

            Existence of peak at around 1473 cm-1 in ArPM spectrum (which is absent in ArP), specifying the presence of C-N vibrations of tertiary amines. The spectrum was

            CH2 symetric streaching

            C-H bending

            CH2 asymetric streaching

            similar to those of solid HDTMA because the chains were densely packed; this observation clearly indicates that the surface modication of ArP is achieved by surfactant.

            Adsorption band at around 720cm-1 correspond to the methylene rocking mode, independent of chain conformation, this mode is split due to interchain interaction between contiguous CH2 groups of adjoining chains [15].

            The bands at around 1110 and 1030 cm1, represent the Si

            Al-OH-Al

            Si-OH-Si

            H2O

            bending

            NH streaching

            H to Si-O-Al

            H to Si-O-Si

            O coordination vibrations and the stretching of SiO in the SiOSi groups of the tetrahedral sheet, respectively. Also, SiOAl (octahedral) and SiOSi bending vibrations were observed at 520 and 470 cm1, respectively, for both ArP and organo-smectite, as in other silica and silicate systems.

            fig.5: FT-IR Sectra of HDTMA molecule, ArP and ArPM

         2. XRD of organosmectite

            The XRD patterns of ArP and ArPM were recorded in the range of 2 to 10° (2) at a scanning speed of 1°min-1(fig.6) using diffractometer with Cu K radiation.

            ——–(b)——— ——–(c)———

            T O T

            ——–(a)———

            fig 6 : XRD of ArP and ArPM

            The basal diffraction peak, d001,of ArP emerges at 6.8(2) with the distance of 12.38Ã… shifts to the higher values (34 Ã…) point out that the penetration of the surfactant cations into the interlayer space of bentonite occurs, sodium ions on the interlayer of the smectite are randomly or selectively replaced with HDTMA cations gives organophilic clays.

            The values of d001 increase gradually (12.38Ã…

            16.82 Ã… 34Ã…). The expansion in the basal spacing (d=d001-9.6) varied from d=0.278nm for ArP to reach maximum d =2.44 nm for ArPM.

            Expansions of bentonite layers are related to the arrangements of HDTMA molecules (fig6) [17-18].The arrangements of the surfactant molecules in the interlayer region of the smectite can be distinguished by comparing the interlayer expansions (d) and molecular dimension of the surfactant.

            According to the data of van der Waals radius, covalent bond radius and bond angle, the steric configuration, size and shape of HDTMA organic cations could be calculated (fig.2) [19]. When the HDTMA+ is lying flat, the height of alkyl-chain is 4.1Ã… and the 'nailhead' is 5.1Ã….

            accordingly peak at 16.82Ã… (d002) hold an interlayer space of ~0.722nm, in the ArPM hybrid sample, suggest lateral bilayer of organic ion arrangement, since the height of the

            lateral bilayer is 0.81 nm [20]; Lateral bilayer (LB) is that the protruding methyl inserts into cavity between organic cations or into the hexagonal hole of basal oxygen plane [21], consequently, the alkyl chains may come close together and arrange in lateral bilayer model (fig6 (b)). In view of that, the height of LB model depends on the height of the double layers of alkyl chains rather than that of cation end of HDTMA+.

            Reflection at 34Ã… in ArPM relate to the (001) peaks suggesting an evolution of paraffin-type monolayer arrangement with a maximum interlayer expansion of 24.4 Ã… (fig6.c).

            Angles, , between the alkyl chain and basal surface was also estimated, taking into account the length of HTDMA+ (25.3Ã…) and the height of TOT layer (9.6Ã…),

            74.6° for the paraffin monolayer arrangement (fig6.c).

            Result suggests that interlayer cations in smectite should be replaced by HDTMA+ entirely, since there is no basal spacing at ~ 12.38Ã… corresponding to ArP found in the XRD patterns of hybrid sample. Consequently, we recommend that there is no Na-smectite layer in the hybrid prepared in the range of 3CEC. Adsorption mechanism of the surfactant with the clay sheets can be estimated by the following graphic (fig.7).

            fig7. Adsorption mechanism of surfactant by clay layer

         3. Scanning electron microscope ( SEM)

            SEM photomicrographs obtained for Na-smectitet and organo sample are illustrated in fig. 8. Surface of ArP looks very smooth with fluffy appearance and curved plates. After cationic surfactant insertion reaction significant changes were observed. Surface morphology loses its foliated structure and became rougher with many

            small and aggregated particles and the plates become relatively flat, this is probably occurred due to the reduction in certain crystalline domains of the clay particles with the surfactant packing density in the interlayer, the curved plates in ArP will change to flat layers.

            ArP ArPM

            20.0µm

            20.0µm

            fig.8: SEM images of Na-Smectite and organosmectite

         4. X-ray photoelectron spectroscopy (XPS) characterization

            XPS was a sensitive technique applied to probe the surface chemical composition and chemical states of the prepared organomaterial (ArPM) as a result of ion exchange reactions. Figure 9 shows the XPS survey scans of Na-Smectite (ArP), HDTMAB and representative organoclay ArPM.

            The survey scan divulge the presence of sodium, silicon, aluminium, iron, and magnesium in the Na- Smectite, the XPS result is in excellent agreement with our chemical analysis result of Na-Smectite. It has also indentified the presence of carbon, nitrogen, and bromine in HDTMAB. In addition, there is a minor amount of oxygen in HDTMAB and carbon in ArP, resulting from adsorbed CO2 [22].

            Trace amount of magnesium and iron are seen in the scans of ArPM, whereas the sodium peak is disappeared. Both magnesium and iron are located in the ArP structure rather than in the interlayer. The absence of sodium ions in the ArPM confirms the sodium ions are exchanged when cationic surfactants are introduced in the interlayer this result corroborates with those of XRD.

            In addition the Al/Si ratio dropped from 0.32 (ArP), this value is in good agreement with the chemical analysis of ArP (0.3), to 0.26 (ArPM). As the intercalation of surfactants increases in the interlayer, the distance between Al-O(OH) octahedral sheets and two Si-O tetrahedral sheets in the structure of ArP is expanded, and hence, the detecting ratio of Al and Si is lower.

            fig.9: XPS survey scans of HDTMAB, ArP and the representative ArPM

         5. Potentiometric titration

            Surface charge density H of the clay sample was determined by potentiometric acidbase titration using NaCl as background electrolyte at constant ionic strengths for different concentrations (0.5, 0.1 and 0.01M) to guess the effect of ionic strength on the surface charge density H and to determine the pH were H=0. This pH value is referred to the point of zero net proton charge, noted pHPZC.

            Titration curves were similar in shape for the range pH used (fig.10) and concord very well with previous works [23]. In the acidic pH range, the degree of protonation increased with increasing ionic strength and the opposite was observed in the alkaline pH range. The dependence of the surface charge density on the ionic strength was due to the fact that ionic strength had signicant and contrasted effects on the dissolved species

            Al, Si, Fe and Mg [24]. ArPM has a surface charge density higher than ArP. This high surface charge density was produced by protonation and deprotonation of inner-sphere ArPM hydroxyl surface complex formed at variable-charge hydroxyl groups at the edges [23].

            PZC of the organobentonite moved to the basic pH according to the HDTMA+ intercalation (fig. 9) in comparaison with that of ArP. PZC increase from 6.6 (ArP) to ~9 (ArPM).

            Modifications of Na-smectite by Surfactant have an important role in surface charge behavior of the adsorbent. The loading of surfactants causes a transformation of clay surface property from hydrophilic to hydrophobic. Hydrophobicity suppressed the formation of bound HO layers on the surface.

            fig. 10: Potentiometric titration curves versus pH at different ionic strengths of (a) ArP anf (b) ArPM, at 279 K

         6. Zeta potentiometric

            I=0.5

            I=0.1

            I=0.01

            The zeta potential of the ArP and ArPM were measured at various pH (Fig. 11). The purified smectite has a point of zero charge (p.z.c.) of 6.5, after modication by HDTMA surfactant at 3 CEC level the p.z.c. moved to 9.2, these values were in excellent agreement with those of potentiometric titration.

            fig.11. Effect of pH on the zeta potential of ArP and ArPM.

         7. Specific surface area SBET and cation exchange capacity (CEC)

         Specic surface area (figures not shown), pore volume, and CEC of the ArP and hybrid organoclay are presented in table 2.

         Surface area drops dramatically from 110 cm3g-1 (ArP) to 0.64cm3g-1 (ArPM). It is also found that the pore volume decrease significantly. This is due to the fact that interparticle pores of smectite are covered and the interlamellar spaces are blocked, lead to inhibition of the passage of N2 molecules. Micropores surface area (Sp) and micropores volumes (Sp) wre annulled after the organophilic modification.

         2+

         2+

         The cation exchange capacity (CEC) of ArP and the hybrid organoclay, estimated by using Cu(EDA)2 and ammonium acetate methods, show the same magnitude. CEC decreases from 98.6 (ArP) to 6meq/100g (ArPM). These results confirm those of acid-base potentiometric titration. The PZC of ArPM was basic (table 2), adsorption of the surfactant modify the surface character. The increase of pHPZC after surfactant treatment indicates that the organoclay becomes more positive and the surface properties of the adsorbent change from hydrophilic to hydrophobic by the substitution of Na+ interlayer cation by HDTMA+ cation. Oragnoclay generate higher adsorption capacity for anionic compound. Similar remarks were reported in many previous studies [20-21-25].

         Table2. Textural parameters and CEC studied of ArP and ArPM and pHPZC

         samples	SBET	Sµp	Vp		Vµp	CEC (meq/100g)	CEC (meq/100g)	pHPZC
         	(m2g-1)			(cm3g-1)		Ammonium acetate method	2+ method Cu(EDA)2	Potentiometric	Zeta metric
         ArP	110	8.7	0.138		0.016	98.6	97.56	6.6	6.5
         ArPM	0.63	–	0.001		–	6	8	9	9.2

         From XRD, CEC and SBET results it can be estimative that HDTMA+ occupies interlayer space, via cation exchange with Na+ interlayer and surface adsorption sites. This can well explain the dramatic surface area and pore volume decrease of organoclay compared to those of starting clay.

   1. ADSORPTION STUDIES

   1. Sorption isotherms

      Adsorption isotherms were determined at 25°C by shaking 100mg of the adsorbent (ArP) with 50ml of CR solution with variant concentrations for 60min at natural pH (pH=6.2). After shaking at 5000tr/min, the solution was separated from the solid by centrifugation. The residual concentrations in the supernatants were determined by UV- visible spectrophotometer at 498 nm, and the solid phases were analyzed by FTIR spectroscopy at the range 400 to 4000nm-1.

      ArPM

      qe (mgg-1)

      qe (mgg-1)

      ArP

      Ce (µmolL-1)

      fig.12: Adsorption isotherm of CR by ArP and ArPM

      Adsorption isotherms of congo red by ArP (fig.12), expressing the adsorbed amounts (qe) as a function of equilibrium concentration (Ce), is L-type [26]. In this kind of isotherm, the initial portion provides information about the availability of the active sites to the adsorbate and the plateau suggests a monolayer formation. The initial curvature indicates that a large amount of dye is adsorbed at a lower concentration as more active sites of ArP are available. As the concentration increases, it becomes difcult for a dye molecule to nd vacant sites, and so monolayer formation occurs.

      Adsorption of CR takes place possibly by means of surface exchange reactions until the surface functional sites are completely full, thereafter dye molecules diffuse into the smectite layers for further interactions and/or reactions such as ion-exchange, complexation interactions [27-28]. This reaction mechanism may be partly due to complexation between the negatively charged groups (D SO3) in CR and positively charged Al2O3 groups on the smectite surface or to ion exchange between positively charged groups (D+NH3 or +NH=N) in CR (fig.13) and Na+ ions, initially present in the exchange position of the smectite.

      Fig.13: Protonated Congo red ammonium rich variety at natural pH

      According to previous work the hydrolysis of Si OH or AlOH bonds along the clay lattices produces the surface charge. Depending on the silica structure and the pH of the solution, the net surface charge can be either positive or negative. Adsorption of cations was favoured at pH > pHPZC, while the adsorption of anions was favoured at pH < pHPZC.

      D-SO3-

      D-SO3-

      In the aqueous solution, the acid dye is rst dissolved and the sulfonate groups of the acid dye (D SO3Na) are dissociated and converted to anionic dye ions at natural solution pH of 6.2. The pHPZC of ArP was 6.5 at this condition Arp develop more positive charge witch explain the amout of dye removal. Adsorption mecanism can be estimated and reprisented by figure.14.

      Clay layer

      Clay layer

      Na+

      SiOH2+

      SiOH2+

      fig.14 : CR adsorption mechanism

      Uptake of CR by ArP has been confirmed by FTIR study (fig. 15). Results show a news absorption bands in ArP FTIR spectrum after dye adsorption, in

      c

      comparaison with FTIR spectrums of ArP and CR, these bands linked to specific functionals groups of CR structure ( table 3).

      b

      a

      Fig 15: FTIR spectrums of: (a) CR powder only, (b) ArP befor CR adsorption and (c) ArP after CR adsorption

      Table 3: Most important adsorption bandes of CR functional groups

      Functional groups	(C=C)Ar	(N-H)	(C-H)Ar
      Frequency (cm-1)	1667	1500	837-914 -726

   2. Effect of pH on dye removal

      From our previous study and other works in the literature dye adsorption is strongly dependent on pH solution. This can be attributed to the chemical form of dye in solution and functional groups present on the adsorbent surface at a specic pH. CR is a diazo anionic dye and is a pH sensitive, exposure to HCl causes color change from red to blue, due to * transition of azo group shift to higher wavelength because of protonation [29]. At lower pH CR become cationic and shows two tautomeric form [30], i.e. ammonium rich variety and azonium variety (fig.16).

      In this work, effect of initial pH of dye solution on the amount of adsorbed dye was studied at 25°C over a range of pH from 2 to 12. Experiments were conducted on suspensions of 0.1g of ArP in 50ml of dye solution with initial concentration of 69.66mgL-1. The suspensions were stirred for 1h and then centrifuged. Dye concentrations in the supernatant were measured by UV-visible spectrophotometer at appropriate max [30]. The pH was adjusted by adding a few drops of NaOH or HCl before each experiment.

      The amount of dye adsorbed per unit mass of adsorbent at equilibrium as shown in figure 17 increased as pH was increased from 2 to 5, reaching a maximum at pH range 4- 5 with adsorption efficiency and 92% of dye removal. We have noted an almost complete discoloration of the solution for pH range 5 to 6. The maximum adsorption can be stimated at pH = 5.7.

      At lower pH the negatively charged silica sites of bentonite are neutralized by H+ ions and alumina sites are positives, the number of negatively surface charged decrease, However significant CR adsorption was reached. In another hand, at lower pH CR (dipolar molecule H3N+-R- SO3-) present tow protonated tautomeric species (fig 16b): an ammonium form, where the protons attached to the amino nitrogen and an azonium form, they were attached to the -azo nitrogen. Many functional N-sites are present on CR and some of nitrogen form hydrogen bonds by accepting protons from the solution and some are protonated to form cationic species.

      .

      Fig16: Protonated Congo Red at (a) pH>5.5, (b) pH<5.5[29-3

      Therefore the maximum adsorption of cationic dye at

      lower pH may be due to the mechanism of cation exchange, replacing Na+, initially present in the interlayer space of bentonite. Thereafter, adsorption decreased with increasing pH. Above pH 6, the percentage of dye removal drops. The overhead setup canbe attributed to the changes in the polarity of the electric double layer on both the silica and alumina contents of the clay from positive to negative. The high negatively charged adsorbent surface sites (at high pH) did not favor the adsorption of deprotonated CR due to electrostatic repulsion. Also, an abundance of OH- ions in basic solution creates a competitive environment with anionic ions of CR for the adsorption sites causing a significant reduction in the removal of CR dye from the solution.

      It can be suggested that NH2, -N=N-, -HN-N and SO3- groups of CR were involved in the adsorption

      Qads (mgg-1)

      Qads (mgg-1)

      pH

      fig 17: Effect of pH on CR removal by ArP

   3. Modelization of experimentals data

   Equilibrium data of Congo red adsorption on purified bentonite (ArP) were analyzed using the linearized form of Freundlich and Langmuir models.

   (i)- The Langmuir isotherm [31] assumes monolayer adsorption on a uniform surface with a finite number of

   adsorption sites. Once a site is occupied, no further sorption can take place at that site, the surface will eventually reach a saturation point where the maximum adsorption of the surface will be achieved. The linear form of the Langmuir isotherm model (Eq1) is given in table 5.

   Accordingly experimental adsorption data were analyzed using the linearized form of Langmiur isotherm, obtained by plotting versus Ce (fig18). The calculated

   Langmuir isotherm constants and the corresponding coefficient of correlation, R2 are shown in table 4.

   (ii)- The Freundlich model: is applicable to monolayer (chemisorption) and multilayer adsorption (physisorption) and is based on the assumption that the adsorbate adsorbs onto the heterogeneous surface of an adsorbent [32]. The linear form of Freundlich equation (Eq2) is given in table 4.

   The equilibrium data were further analyzed using the linearized form of Freundlich isotherm, by plotting lnqeversus lnCe (fig19). The calculated Freundlich isotherm constants (KF ,n) and the corresponding coefficient of correlation, R2 are shown in table 5.

   Table.5: Langmuir and Freundlich parameters

   Langmiur model				Freundlich model
   Linearized eqations
   = 1 + (Eq1)				lnqe = 1lnCe + lnKF (Eq2)
   qm: maximum adsorption capacity (mgg-1)		KL Langmuir constant (Lmg-1)	Ce :equilibrium concentration (mgL-1)		n: adsorption intencity	KF:Freundlich constant (Lmg-1)
   R2	RL	KL	qm	R2	1/n	KF
   0.979	0.026	0.537	372.13	0.892	0.406	387.22

   As can be noted from modelization results, experimentals data concord very well with Langmuir model (R2= 0.979), in an other hand the value of maximum adsorption capacity, qm, determined from Langmuir plot close estimately more than the half of the experimental value. Hence it can be deduced that CR adsorption phenomena in ArP sample, wtitout any further modefication, is a monolayer adsorption mecanisme with a good rate of removal.

   Essential characteristics of Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor RL that is given by the following equation (Eq5) :

   RL = 1/ (1+ KLC0) where C0 (mgL-1) is the initial concentration of adsorbate, and KL is Langmuir constant. The value of RLindicates the shape of the isotherm which is [33]: (i) unfavorable (RL> 1), (ii) linear (RL= 1), (iii) favorable (0 < RL< 1), (iv) or irreversible (RL= 0). The RL values are observed to be in the range 01(table 5), indicating that the adsorption was a favorable process.

   IV.5 Adsorption of CR by ArPM

   The same conditions of CR adsorption experiment by ArPM as the adsorption of this dye by ArP was used to test the performance of this

   Ce(mgL-1)

   fig14: Lngmuir plots of CR by ArP

   V. CONCLUSION

   lnq

   Adsorption results reveals that the locally collected bentonite, without any further modification, can be used as adsorbent to remove anionic organic compound from wastewater, the ratio of CR removal by ArP achieve 58% with 0.1g, since it develop a remarkable positive charge density compared to other low cost adsorbent, further more organosmectite appear very important for this approach, employed as adsorbent for CR removal from aqueous water it give total discoloration for the dye solution (98.7% of dye removal), the amount of dye removal close approximately to initial dye concentration used.

   lnC

   fig15: Freundlichplots of CR by ArP

   low cost nanoadsorbent for anionic dye removal (fig.8). Adsorption of CR at natural pH=6.2 onto HDTMA- smectite (ArPM) is greater than the amount retained by ArP (fig.9), it represent two time the amount of CR removal by ArP. This can be explained by the hydrophobic character, the greater interlayer expansion and the high positive charge surface that developed this adsorbent. pHpzc (9.2) and d001 (24.4Ã…) of ArPM facilities CR diffusion into interlayer space. The higher adsorption capacity of the dye onto ArPM was estimated due to the electrostatic attraction between the negatively charged -SO3- of anion dye and interaction between the cationic surfactant head groups (-N+(CH3)3). The maximum adsorption reach

   ~100% and total discoloration was noted.

   When ArPM was saturated with HDTMAB, the specic surface area was only 0.63m2g-1. However, the adsorption capacity of CR is as high. Congo red molecules were adsorbed into the interlamellar, this information was confirmed by XRD since the d001 of ArPM after CR adsorption increased from 34Ã… to 45 Ã… (figure not presented).

   +

   45Ã…

   +

   fig.9 : Estimation mechanism of CR sorption by ArPM

   As reported by Fu [34] the two primary amines (NH2) attached to the two naphthalene rings located at the two ends of the CR molecule can be protonated (NH3) at the initial pH of 6 and could have stronger basicity, which could result in the attraction between the protonated amine (NH3) and the negatively charged surface (SiO2 sites) of the adsorbent. They also proposed that electrostatic attraction was the primary mechanism, while other mechanisms could also be involved in the biosorption of CR.

   Adsorption data of ArP concord very well with Langmuir model (R2= 0.979), in an other hand the value of maximum adsorption capacity, qm, determined from Langmuir plot close to more than a half of the experimental value. Hence it can be deduced that CR adsorption phenomena in ArP sample, wtitout any further modefication, is a monolayer adsorption mecanisme with a good rate of removal. Chimisorption processe was figured out for CR removal. From our point of view ArP can be used without modification yet we use more amount of adsorbent ArP) to reach 100% of CR remonal.

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