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Equilibrium, FTIR, scanning electron microscopy and small wide angle X-ray scattering studies of chromates adsorption on modified bentonite
Journal of Molecular Structure 740 (2005) 203–211 www.elsevier.com/locate/molstruc
Equilibrium, FTIR, scanning electron microscopy and small wide angle X-ray scattering studies of chromates adsorption on modified bentonite Marek Majdana,*, Oksana Maryuka, Stanisław Pikusa, Elzbieta Olszewskaa, Ryszard Kwiatkowskib, Henryk Skrzypekc a
Faculty of Chemistry UMCS, 20-031 Lublin, pl. M.C. Skłodowskiej 2, Poland Instytut Inzynierii Tekstylio´w i Materiało´w Polimerowych, Akademia Techniczno-Humanistyczna w Bielsku-Białej, 43-309 Bielsko-Biała, ul. Willowa 2, Poland c SEM Laboratory, Department of Zoology and Ecology, Catholic University of Lublin, 20-718 Lublin, Kras´nicka 102, Poland b
Received 2 November 2004; revised 11 January 2005; accepted 12 January 2005
Abstract The study presents a discussion about the adsorption mechanism of chromate anions on bentonite modified by hexadecyltrimethylammonium bromide (HDTMA-Br). The formation of alkylammonium chromates: HDTMAHCrO4, (HDTMA)2Cr2O7 and to the lesser extent (HDTMA)2CrO4 at the water–bentonite interface is examined based on the Scanning Electron Microscopy and surface tension measurements. The histograms of HDTMA/Cr(VI) molar ratio on the bentonite surface, found from Scanning Electron Microscopy (SEM) measurements, show that for the majority of points of bentonite surface the value of this ratio is in 1–2 range. FTIR spectra of modified bentonite samples show the change from gauche to trans conformation in the surfactant arrangement in the clay interlayer accompanying its concentration increase. In turn Small Wide Angle X-Ray Scattering (SWAXS) patterns evidently suggest incorporation of chromate anions into the interlamellar space of bentonite structure. q 2005 Elsevier B.V. All rights reserved. Keywords: HDTMA; Chromate; Bentonite; Adsorption; SEM
1. Introduction The surface modified clays have been studied by numerous researchers [1–5] during the last two decades. Introduction of cationic surfactants to the structure of clays results in such changes of their adsorptive properties that it is possible to use them in the removal of both organic and inorganic pollutants from the wastes. Organic pollutants, such as benzene, toluene, phenol or their derivatives, are effectively adsorbed on the hydrophobic parts of the structure of the modified clays. This process is well known and was sufficiently explained in literature [6]. However, adsorption of anions on clays modified by quaternary alkylammonium salts, especially chromates, is
* Corresponding author. Fax: C48 81 533 3348. E-mail address: [email protected] (M. Majdan). 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.01.044
relatively little known [7–12]. The mechanism of this process is described by the formation of a positively charged bilayer of the alkylammonium cations on the clay surface, which attracts anions from the aqueous solution. This kind of explanation is justified when loading of clay by surfactant is above its cation exchange capacity CEC. Our study shows that the existence of adsorptive property of modified clay (in this case bentonite) is possible when its CEC is not fully exhausted by alkylammonium cations. In connection with this we are suggesting a mechanism of chromates adsorption based mainly on the formation of alkylchromates. We have intentionally selected the problem of Cr(VI) adsorption for our research, because Cr(VI) metal species are negatively charged (anionic) and are therefore mobile in most aqueous environments. Traditional treatment processes are typically ineffective in removal of these anionic species. The use of surfactant-modified clays in permeable
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barriers seems to offer obvious advantages over conventional processes in removal of anionic metals from solution. One of the important characteristics distinguishing toxic metals from other pollutants is that the former are not biodegradable. Once metal ions have been emitted to the environment, their chemical form mainly decides about their potential toxicity. Chromium is present in the environment as a result of effluent discharge from steel, electroplating, tannery, oxidative dyeing, and other industrial processes. Its toxicity depends mainly on its oxidation state. This element exists mainly in two valence states—the toxic hexavalent state and the relatively safe trivalent state. The anionic form of chromium (VI) is carcinogenic to humans [13]. Therefore, it is of great interest to remove hexavalent chromium completely from the industrial effluent.
2. Experimental The clay mineral used in this study supplied by SigmaAldrich was bentonite (smectite type); with the composition: O, 54.2%; Na, 2.7%; Mg, 1.9%; Al, 11.2%; Si, 27.5%; Fe, 2.4%. The chemical composition was checked by Scanning Electron Microscopy method (see Section 2.1 for details). 2.1. Modified bentonite preparation Bentonite loaded by HDTMA-Br was prepared in the following way. To ensure that there is an excess of sodium ion concentration with relation to cation exchange capacity (CEC) the 5 g sample of bentonite was contacted with 100 ml of 1 M NaCl (Sigma Aldrich, min. 99.5%) through 24 h. The sodium form of bentonite was washed several times by water to remove the rest of sodium chloride. The presence of chlorides in washing solution was checked argentometrically. After filtration the solid residue was dried in air. In the next stage, 1 g sample of sodium– bentonite was equilibrated through 4 h. using mechanical shaker with 100 ml of 0.014 M HDTMA-Br (Sigma Aldrich, 99% purity) in 60 8C. The time of equilibration seemed to be enough to replace the majority of sodium by alkylammonium ions in bentonite sample [14]. In the next stage, the solution was filtered through the paper filter and the solid residue dried in air was used in further experiments. 2.2. Equilibrium study The adsorption isotherms of Cr(VI) were registered by a 4-h equilibration of 0.1 g samples of HDTMA-bentonite with 100 ml of K2Cr2O7 (Polskie Odczynniki Chemiczne) with concentration of Cr(VI) ranging from 0.0001 to 0.001 M (the kinetics of chromates adsorption on modified bentonite was checked earlier [15]). After 4 h. the solutions
were filtered using paper filter (Filtrak 390, Polskie Odczynniki Chemiczne) and centrifuged at 10,000 rpm for 10 min. The initial and equilibrium concentrations of Cr(VI) were determined spectrophotometrically using diphenylcarbazide [16]. The concentration of Cr(VI) in bentonite phase was found from the relationship cb Z ðcin K ceq ÞV=m; where cb, cin, ceq denote concentrations of Cr(VI) in bentonite phase, initial and equilibrium solutions. The symbols V and m relate to the volume of solution in cm3 and to the sample mass in mg. The pH values of the equilibrium solutions were controlled using combined glass electrode (Sigma Chemical Co.) connected to the pH-meter (CX-731 type, Elmetron Co.) 2.3. FTIR spectrograms The FTIR spectra of the bentonite samples were recorded in the transmission mode at room temperature on 1725X Perkin Elmer instrument using KBr pellet technique (1:20), with the resolution 2 cmK1. The KBr was dried in drier at 200 8C for 24 h, then 560 mg KBr was homogenized with bentonite sample in ball grinder. The tablets (radius 1 cm, thickness 0.1 cm) were prepared using hydraulic press. The modified bentonite samples were prepared through the equilibration of 1 g sodium–bentonite with the surfactant aqueous solutions with concentrations ranging from 0.001 to 0.01 M. 2.4. Surface tension measurements Surface tension of the aqueous solutions was detected using Sigma 70 tensiometer (KSV Instruments Ltd). The well-known Wilhelmy Plate method was applied. 2.5. Scanning Electron Microscopy of modified bentonite The chemical composition of modified bentonite in 50-ty randomly selected points of the surface was determined by standardless Scanning Electron Microscopy (SEM) method (microscope LEO SEM 1430 VP supplied with EDX detector; operating conditions of electron microprobe 20 kV, 80 mA beam current). The concentration of Al, Si, Na, Mg, Fe, Cr, C on the bentonite surface was determined using standardless version of SEM method. The electron beam penetrated the samples for about 1 mm. Vacuum 10K5 Pa was preserved during measurements. Next, the bentonite samples in their sodium and HDTMA-Cr(VI) forms were investigated. The HDTMACr(VI)-bentonite sample was prepared through the equilibration of 1 g HDTMA-bentonite with 100 ml of 0.01 M solution of K2Cr2O7 (Polskie Odczynniki Chemiczne). The solid residue after filtration of the solution was dried in air.
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2.6. Small wide angle X-ray scattering measurements (SWAXS) The small angle X-ray scattering (SAXS) measurements were carried out with the compact Kratky camera equipped with the SWAXS optical system of HECUSMBRAUN (Austria). The Cu target X-ray tube, operated at: UZ40 kV, IZ30 mA (PW1830 Philips generator), ˚ . The primary was used as a radiation source lZ1.542 A beam was monochromatised by the Ni filter and the scattered radiation was detected by the linear position sensitive counter OED-51. The resolution of the ˚ (i.e. the Bragg spacing measurements was 750 A corresponding to the first measurement point). The transmission method was employed for the determination of the transmittance factor (T) of the samples.
3. Results and discussion 3.1. Equilibrium study We have checked the influence of the pH level on the chromates adsorption on modified bentonite and found that up to pHZ6 the concentration of CrO2K ions in the 4 adsorbent phase is constant (Fig. 1). Beginning from this pH value a sudden drop of the chromates concentrations in the adsorbent phase appears, which results probably from the concurrent formation of the alkylammonium hydroxide (HDTMA)OH, according to reaction: HDTMAHCrO4 C OHK 4 HDTMAOH C HCrOK 4
(1)
Therefore, from practical viewpoint, it is necessary to perform the adsorption of chromates from solutions with the pH level ranging from 2 to 6. However, we cannot be absolutely sure, that the competition between OH– and HCrOK 4 is the main reason for the change of Cr(VI)
Fig. 1. The influence of the pH on the Cr(VI) concentration in (c(b)) the equilibrium bentonite phase.
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2K and CrO2K adsorption. The affinities of HCrOK 4 , Cr2O7 4 ions to the modified bentonite are probably different, with the preference of the first two species in acid solutions; so for higher pH levels and consequent CrO2K ions concen4 tration increase, the Cr(VI) adsorption decrease is observed. The adsorption isotherms of chromates on bentonite in the presence of different anions (with the same 0.005 meq/dm3 concentrations) in the aqueous phase are given on Fig. 2. The parameters of the isotherms: amax, K, n (where amax, K and n are, respectively: maximum concentration of Cr(VI) in the adsorbent phase, adsorption constant K and parameter of the surface heterogeneity) were calculated from the ‘best fit’ of the experimental data found from well-known Langmuir–Fredundlich equation
cb Zamax ðKceq Þn =ð1 C ðKceq Þn Þ; where cb, ceq denote the equilibrium concentrations of Cr(VI) in the bentonite and aqueous phase respectively and are presented in Table 1. The following decreasing trend of adsorption maximum amax of chromates is observed: H2OZClKOH2POK 4 O K 2K 3K ˝ nsted strong bases: SO2K 4 ONO 3 OCO 3 OPO4 . Bro PO3K and CO2K are at the end of this order. They have a 4 3 strong influence on the chromates adsorption as a result of the pH effect and a consequent alkylammonium hydroxide formation. The comparison of the remaining monovalent anions is also interesting. According to Leontidis, nitrate ion [17] classified as water-structure-breaker (chaotropic) ion, is much more strongly bound by cationic surfactant headgroups than ClK ion, known as water-structure-maker (cosmotropic) ion. The sequence: Cl KOH 2PO K 4 O K SO2K 4 ONO3 probably reflects the increasing strength of anions interaction with HDTMA cation headgroup. If we look at the values of surface heterogeneity parameters n, we can notice that in all cases they are higher than 1. It is well known from the classical physical chemistry, that in the case of the Langmuir–Freundlich equation, when nO1, the lateral interactions between adsorbate molecules appear. Therefore, we can presume that there is a common interaction of the molecules of alkylchromates formed on the bentonite surface and built into the bentonite internal structure. The adsorption isotherms of the chromates at different temperatures are given on Fig. 3. There is an evident decrease of the concentration of Cr(VI) in the bentonite phase with the temperature increase. Exothermicity of the adsorption results from the strong electrostatic interaction on the negatively charged Cr(VI) ions with the positively charged alkylammonium ions HDTMAC. We have estimated the thermodynamical parameters for the chromates adsorption through the analysis of temperature dependence on the adsorption constant K. This procedure, based on the analysis of ln K vs. 1/T plot, was described by Krishna [14]. The values of DH8, DG8 and DS8 (enthalpy, free energy, entropy) for chromates adsorption on modified bentonite
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Fig. 2. The adsorption isotherms of Cr(VI) on the bentonite in the presence of different anions in the aqueous phase (concentration of anions 0.005 meq/1dm3; K 2K K 2K 3K 1, H2O; 2, H2POK 4 ; 3, Cl ; 4, SO4 ; 5, NO3 ; 6, CO3 ; 7, PO4 ).
are: K35.52 kJ/mol, K3.12 kJ/mol and K0.11 kJ/(mol K), respectively. Therefore, it is clear, that enthalpy is favourable for chromates adsorption, contrary to entropy. Negative entropy change probably results from the fact, that chromate anions lose their translative and oscillative freedom during attraction by positive alkylammonium cations. At this point we would like to emphasize that our thermodynamical parameters of Cr(VI) adsorption are completely different from those found by Krishna [14]. In his experiments, Krishna found positive entropy change following the adsorption of Cr(VI) on montmorillonite, the same type of clay as bentonite. Thermodynamical parameters were calculated by the author based on the change of distribution ratio Kd of chromium species at different temperatures, so contrary to us this author used only one initial concentration of chromates. In our experiment, we have registered the adsorption isotherms of Cr(VI) changes with the temperature. We believe that adsorption constants K reflect more strongly the essence of thermodynamics than the distribution ratios Kd, since they refer to the adsorption behaviour of chromium at different concentrations. There is also a second reason, why we are skeptical in relation to the results of Krishna. We suspect, that the results described in Ref. [14] refer to the nonequilibrium conditions of adsorption, since the author used the period of 60 min for the equilibration of the aqueous and solid phases. We have found that the period of 4 h. is necessary to equilibrate the modified bentonite and Cr(VI) containing aqueous phase. The solid lines on the plots refer to the ‘best fit’ found from the Langmuir–Freundlich equation. The heterogeneity parameters n are: 1.3, 1.3, 1.4, 1.2 for 293, 313, 323 and 333 K, respectively, so one can conclude about the strong interaction between alkyl chains of the surfactant cation, which is independent of the temperature.
3.2. Scanning electron microscopy of the bentonite surface The interelement correlations concentrations are given on Fig. 4. Sodium and alkylammonium cations concentrations are related to Al and given as molar ratios. The correlation Na/Al vs HDTMA/Al, which is statistically significant, shows a negative trend. Therefore, one can conclude that the exchange reaction between Na C and R 4 N C cations occurs on the HDTMA-Cr(VI)-bentonite surface. In turn the statistically significant correlation Fe vs Cr is positive and it results from the formation of basic chromate KFe3(CrO4)2(OH)6 in places of the surface abundant in Fe. The formation of this compound, which is insoluble in water, is described in the literature [18]. It is evident from the histogram of the molar ratio: HDTMA/Cr(VI) (Fig. 5), that for the majority of points on the bentonite surface, the value of this ratio is in 1–2 range. One can suggest a scheme of the chromates adsorption according to the following reactions
Table 1 Parameters of Langmuir–Freundlich equation describing adsorption of Cr(VI) on the HDTMA modified bentonite in the presence of different anions in the aqueous phase amax (mol/g)
K (molK1)
n
Medium
pH
0.00035 0.00035 0.00034 0.00032 0.00029 0.00013 0.00009
43,175 25,140 33,320 17,578 7854 5255 3733
1.3 1.5 1.7 2.1 2.4 1.5 1.3
H2 O ClK H2POK 4 SO2K 4 NOK 3 CO2K 3 PO3K 4
6.5 6.20 5.70 6.20 6.15 10.00 10.20
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Fig. 3. The adsorption isotherms of Cr(VI) on the HDTMA modified bentonite for different temperatures. K ðHDTMAÞ2 BtBr C HCrOK 4 4 ðHDTMAÞ2 BtHCrO4 C Br
(2) 2ðHDTMAÞ2 BtBr C Cr2 O2K 7 4 ½ðHDTMAÞ2 Bt2 Cr2 O7 C 2BrK
(3)
HDTMAC C HCrOK 4 4 HDTMAHCrO4
(4)
2HDTMAC C CrO2K 4 4 ðHDTMAÞ2 2CrO4
(5)
2HDTMAC C Cr2 O2K 7 4 ðHDTMAÞ2 Cr2 O7
(6)
where the reactions (2) and (3) are typically ion-exchange in character and refer to the exchange of bromide ions from
HDTMA-Br adsorbed on the bentonite surface through Van der Waals interactions with alkylammonium cations in species HDTMABt/HDTMABr (where HDTMABt refers to HDTMA-bentonite). These reactions are widely described in the literature [19] and refer rather to the conditions when the concentration of surfactant on the clay surface exceeds the CEC. Therefore, the participation of the ion-exchange reactions in the overall process of the chromates adsorption, described in our experiment, is rather minor. Alkylammonium chromates formed according to reactions: (4)–(6) interact probably with the hydrophobic bentonite matrix through the Van der Waals forces, forming species: HDTMAHCrO 4/HDTMABt, (HDTMA) 2Cr 2 O 7/HDTMABt, (HDTMA)2 CrO 4/HDTMABt. This scheme of reactions is justified if we inspect the distribution
Fig. 4. Interelement correlations concentrations on the HDTMA-modified bentonite surface, loaded with Cr(VI) (r and p relate to correlation coefficient and statistical significance factor, respectively).
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3.3. Interfacial phenomena analysis
Fig. 5. Histogram of molar ratio of HDTMA/Cr(VI) on the HDTMA modified bentonite surface.
of different Cr(VI) forms in the aqueous solution for the different pH values. Ramsey and coworkers [20] found, that 2K in the pH range from 2 to 6, the anions HCrOK 4 and Cr2O7 are the dominant forms of Cr(VI) in solution, whereas for neutral and alkaline solutions CrO2K predominates. 4 The metal concentrations in bentonite and HDTMACr(VI)-bentonite, defined as Me/Al molar ratio are given in Table 2. We can notice the change of Me/Al values merely for Na, if we compare bentonite and HDTMA-Cr(VI)bentonite forms. Therefore, one can conclude that sodium ion is exchanged from the bentonite structure through reaction: NaBt C HDTMAC4 HDTMABt C NaC
The surface tension changes of the equilibrium aqueous phases with various surfactant concentrations are given on Fig. 6. The surfactant concentrations were evaluated from plots of surface tension of HDTMAbromide solutions versus log (concentrations HDTMAbromide). This is a routine procedure applied in registration of interfacial adsorption of surfactants [21]. There is an evident increase of surface tension (expressed in mN/m) with the equilibrium chromate concentration. The increase of surface tension results from the decrease of alkylammonium cations HDTMAC concentration in the aqueous and interfacial phase due to the formation of alkylammonium chromate, which in turn is interfacially inactive. The straight-line correlation between log cs and log Cr(VI), where cs refers to equilibrium surfactant concentration, is negative. One can conclude that the alkylammonium chromate existing as undissociable ionic pair is formed in the interfacial region: bentonite–water, which is then transferred to the bentonite phase. The whole process can be
(7)
whereas the remaining metals: Mg, Fe, Al remain unexchanged. If we assume, that 100% of CEC refers to 0.41 value of Na/Al in bentonite (this statement is obvious after inspection of Table 2), then the 0.11 value of Na/Al in HDTMA-Cr(VI)-bentonite may be considered as 27% CEC ‘occupied’ by NaC ions. Thus it has been found for the first time, that the adsorption of chromates on modified bentonite is possible in conditions, when only part of CEC (below 100%) is in the alkylammonium form. It is interesting to note, that Si/Al, Fe/Al, Mg/Al molar ratios are constant, so during the modification of bentonite the aluminosilicate matrix preserves its skeletal chemical composition. Table 2 The metal concentrations on bentonite and HDTMA-Cr(VI)-bentonite surface Metal concentrations
Bentonite
HDTMA-Cr(VI)-bentonite
Na/Al Mg/Al Fe/Al Si/Al
0.41 0.19 0.10 2.39
0.11 0.20 0.09 2.39
Fig. 6. The surface tension changes of the aqueous phase with the Cr(VI) increase (a) surface tension expressed in mN/m and (b) surfactant concentration cs in the aqueous phase.
M. Majdan et al. / Journal of Molecular Structure 740 (2005) 203–211
schematically described as K HDTMAC in C HCrO4 in 4 ðHDTMAHCrO4 Þin
(8)
ðHDTMAHCrO4 Þin 4 ðHDTMAHCrO4 Þb ;
(9)
where indexes: in, b refer to the interfacial and bentonite phase. 3.4. FTIR spectral data The FTIR spectra of the modified bentonite for the different equilibrium concentrations of HDTMA in the solid phase are given on Fig. 7. The HDTMA concentrations in the solid phase were related to CEC through the relationship: HDTMACECZ[HDTMA]/CEC, where [HDTMA] refers to molar concentration of HDTMA (its summary cationic and HDTMA-Br forms) in the solid phase, whereas CECZ0.0012 m/g. Two distinct bands: 2931, 2857 cmK1
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are observed for the concentrations of surfactant ranging from 0.0001 to 0.001 mol/g. These bands result from asymmetric and symmetric stretching vibrations of CH2 absorption bands of amine chains [22]. Their intensity increases with the surfactant concentration and its packing density in the solid phase. The down wavelength shift of these bands (Fig. 8) is probably the consequence of the change in the arrangement of surfactant in the clay interlayer from disordered (gauche) to ordered (trans) form [22]. One can conclude, that for relatively low concentration of surfactant (below 0.5 CEC) the arrangement of the surfactant in the interlayer is a lateral-monolayer form with gauche conformation. The amine ions are parallel to the interlayer and individually separated [22]. The interaction between amine molecules is very weak. For the higher concentration of surfactant (above 0.5 CEC) the lateral-monolayer arrangement varies at first to lateralbilayer (0.7 CEC) and then to paraffin-type bilayer
Fig. 7. FTIR spectra of HDTMA modified bentonite (a) FTIR spectra for HDTMACEC concentrations: 0.017(bottom), 0.034, 0.043, 0.0513, 0.06, 0.068, 0.077, 0.085, 0.171, 0.256, 0.341, 0.427, 0.513, 0.598. 0.684, 0.796 and (b) the change of the wavenumber position for 2857 and 2930 bands with HDTMACEC concentration increase.
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Fig. 8. SWAXS spectrum of bentonite, HDTMA modified bentonite, HDTMA-Cr(VI) modified bentonite.
(1.0 CEC) with trans conformation. The increase in the amine concentration results in the increased Van der Waals interchain interaction and consequently in the increased number of ordered conformers. 3.5. Analysis of SWAXS patterns The SWAXS pattern of unmodified bentonite has the ˚ , which disappears after modification by peak at 11.5 A ˚ , results from the HDTMA-Br (Fig. 8). The new peak at 20 A insertion of HDTMA-Br into interlamellar space with the bilayer arrangement of alkylammonium cations [23]. In turn 2K 2K results in the adsorption of Cr2O2K 7 , HCrO4 , CrO4 further broadening of the interlamellar space and the ˚ peak. Dichromate is the most spatially appearance of 31.5 A distributed of these three ions. The mean length of Cr–O ˚ [24], therefore for the whole ion, bond in this ion is 1.7 A ˚ with the array of four Cr–O bonds, the length is about 7 A taking into account the placement of negative charge on oxygen atoms and change of the zig-zag structure of the ion to nearly straight-line structure during interaction of negatively charged terminal oxygens with positive surfactant headgroups on the opposite sides of bimolecular complex (Fig. 9). As a result the d(001) basal spacing of ˚ . In reality we bentonite should increase by about 14 A ˚ observe an 11.5 A increase in basal spacing value when comparing HDTMA-Cr(VI)-bentonite and HDTMA-bentonite. This value is a little bit lower than expected, which results from the fact that, apart from dichromates (HDTMA) 2Cr 2O 7 also the molecules of chromates: (HDTMA)(HCrO4 ) and (HDTMA) 2CrO 4, with less extended geometry, are present in the interlayer space.
Fig. 9. Arrangements of surfactant alkylammonium cations and Cr(VI) anions in the interlamellar space of the bentonite structure (top, HDTMAbentonite, bottom- HDTMA-Cr(VI)-bentonite).
4. Conclusions 1. Adsorption of Cr(VI) on the HDTMA-modified bentonite proceeds mainly through the formation of alkylammonium chromates HDTMAHCrO4 and dichromates (HDTMA)2Cr2O7. The formation of basic chromate KFe3(CrO4)2(OH)6 is also suggested, however, its influence on the overall process of Cr(VI) adsorption is rather minor, since Fe/Al molar ratio remains stable. 2. Adsorption of chromates and dichromates on the bentonite is typically interfacial in character. These compounds formed in bentonite–water interface phase are transferred to the bentonite phase. The aluminosilicate matrix of the bentonite preserves its chemical composition during the adsorption of surfactant and subsequent adsorption of Cr(VI) anions. 3. The influence of the anions on the Cr(VI) adsorption on bentonite, except for CO2K and PO3K 3 4 , is the result of their water structure breaking or structure making properties. However, from the practical viewpoint the case of carbonates or phosphates salts is very important,
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because their solutions can be used for the regeneration of bentonite loaded with Cr(VI).
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Equilibrium, FTIR, scanning electron microscopy and small wide angle X-ray scattering studies of chromates adsorption on modified bentonite Marek Majdana,*, Oksana Maryuka, Stanisław Pikusa, Elzbieta Olszewskaa, Ryszard Kwiatkowskib, Henryk Skrzypekc a
Faculty of Chemistry UMCS, 20-031 Lublin, pl. M.C. Skłodowskiej 2, Poland Instytut Inzynierii Tekstylio´w i Materiało´w Polimerowych, Akademia Techniczno-Humanistyczna w Bielsku-Białej, 43-309 Bielsko-Biała, ul. Willowa 2, Poland c SEM Laboratory, Department of Zoology and Ecology, Catholic University of Lublin, 20-718 Lublin, Kras´nicka 102, Poland b
Received 2 November 2004; revised 11 January 2005; accepted 12 January 2005
Abstract The study presents a discussion about the adsorption mechanism of chromate anions on bentonite modified by hexadecyltrimethylammonium bromide (HDTMA-Br). The formation of alkylammonium chromates: HDTMAHCrO4, (HDTMA)2Cr2O7 and to the lesser extent (HDTMA)2CrO4 at the water–bentonite interface is examined based on the Scanning Electron Microscopy and surface tension measurements. The histograms of HDTMA/Cr(VI) molar ratio on the bentonite surface, found from Scanning Electron Microscopy (SEM) measurements, show that for the majority of points of bentonite surface the value of this ratio is in 1–2 range. FTIR spectra of modified bentonite samples show the change from gauche to trans conformation in the surfactant arrangement in the clay interlayer accompanying its concentration increase. In turn Small Wide Angle X-Ray Scattering (SWAXS) patterns evidently suggest incorporation of chromate anions into the interlamellar space of bentonite structure. q 2005 Elsevier B.V. All rights reserved. Keywords: HDTMA; Chromate; Bentonite; Adsorption; SEM
1. Introduction The surface modified clays have been studied by numerous researchers [1–5] during the last two decades. Introduction of cationic surfactants to the structure of clays results in such changes of their adsorptive properties that it is possible to use them in the removal of both organic and inorganic pollutants from the wastes. Organic pollutants, such as benzene, toluene, phenol or their derivatives, are effectively adsorbed on the hydrophobic parts of the structure of the modified clays. This process is well known and was sufficiently explained in literature [6]. However, adsorption of anions on clays modified by quaternary alkylammonium salts, especially chromates, is
* Corresponding author. Fax: C48 81 533 3348. E-mail address: [email protected] (M. Majdan). 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.01.044
relatively little known [7–12]. The mechanism of this process is described by the formation of a positively charged bilayer of the alkylammonium cations on the clay surface, which attracts anions from the aqueous solution. This kind of explanation is justified when loading of clay by surfactant is above its cation exchange capacity CEC. Our study shows that the existence of adsorptive property of modified clay (in this case bentonite) is possible when its CEC is not fully exhausted by alkylammonium cations. In connection with this we are suggesting a mechanism of chromates adsorption based mainly on the formation of alkylchromates. We have intentionally selected the problem of Cr(VI) adsorption for our research, because Cr(VI) metal species are negatively charged (anionic) and are therefore mobile in most aqueous environments. Traditional treatment processes are typically ineffective in removal of these anionic species. The use of surfactant-modified clays in permeable
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barriers seems to offer obvious advantages over conventional processes in removal of anionic metals from solution. One of the important characteristics distinguishing toxic metals from other pollutants is that the former are not biodegradable. Once metal ions have been emitted to the environment, their chemical form mainly decides about their potential toxicity. Chromium is present in the environment as a result of effluent discharge from steel, electroplating, tannery, oxidative dyeing, and other industrial processes. Its toxicity depends mainly on its oxidation state. This element exists mainly in two valence states—the toxic hexavalent state and the relatively safe trivalent state. The anionic form of chromium (VI) is carcinogenic to humans [13]. Therefore, it is of great interest to remove hexavalent chromium completely from the industrial effluent.
2. Experimental The clay mineral used in this study supplied by SigmaAldrich was bentonite (smectite type); with the composition: O, 54.2%; Na, 2.7%; Mg, 1.9%; Al, 11.2%; Si, 27.5%; Fe, 2.4%. The chemical composition was checked by Scanning Electron Microscopy method (see Section 2.1 for details). 2.1. Modified bentonite preparation Bentonite loaded by HDTMA-Br was prepared in the following way. To ensure that there is an excess of sodium ion concentration with relation to cation exchange capacity (CEC) the 5 g sample of bentonite was contacted with 100 ml of 1 M NaCl (Sigma Aldrich, min. 99.5%) through 24 h. The sodium form of bentonite was washed several times by water to remove the rest of sodium chloride. The presence of chlorides in washing solution was checked argentometrically. After filtration the solid residue was dried in air. In the next stage, 1 g sample of sodium– bentonite was equilibrated through 4 h. using mechanical shaker with 100 ml of 0.014 M HDTMA-Br (Sigma Aldrich, 99% purity) in 60 8C. The time of equilibration seemed to be enough to replace the majority of sodium by alkylammonium ions in bentonite sample [14]. In the next stage, the solution was filtered through the paper filter and the solid residue dried in air was used in further experiments. 2.2. Equilibrium study The adsorption isotherms of Cr(VI) were registered by a 4-h equilibration of 0.1 g samples of HDTMA-bentonite with 100 ml of K2Cr2O7 (Polskie Odczynniki Chemiczne) with concentration of Cr(VI) ranging from 0.0001 to 0.001 M (the kinetics of chromates adsorption on modified bentonite was checked earlier [15]). After 4 h. the solutions
were filtered using paper filter (Filtrak 390, Polskie Odczynniki Chemiczne) and centrifuged at 10,000 rpm for 10 min. The initial and equilibrium concentrations of Cr(VI) were determined spectrophotometrically using diphenylcarbazide [16]. The concentration of Cr(VI) in bentonite phase was found from the relationship cb Z ðcin K ceq ÞV=m; where cb, cin, ceq denote concentrations of Cr(VI) in bentonite phase, initial and equilibrium solutions. The symbols V and m relate to the volume of solution in cm3 and to the sample mass in mg. The pH values of the equilibrium solutions were controlled using combined glass electrode (Sigma Chemical Co.) connected to the pH-meter (CX-731 type, Elmetron Co.) 2.3. FTIR spectrograms The FTIR spectra of the bentonite samples were recorded in the transmission mode at room temperature on 1725X Perkin Elmer instrument using KBr pellet technique (1:20), with the resolution 2 cmK1. The KBr was dried in drier at 200 8C for 24 h, then 560 mg KBr was homogenized with bentonite sample in ball grinder. The tablets (radius 1 cm, thickness 0.1 cm) were prepared using hydraulic press. The modified bentonite samples were prepared through the equilibration of 1 g sodium–bentonite with the surfactant aqueous solutions with concentrations ranging from 0.001 to 0.01 M. 2.4. Surface tension measurements Surface tension of the aqueous solutions was detected using Sigma 70 tensiometer (KSV Instruments Ltd). The well-known Wilhelmy Plate method was applied. 2.5. Scanning Electron Microscopy of modified bentonite The chemical composition of modified bentonite in 50-ty randomly selected points of the surface was determined by standardless Scanning Electron Microscopy (SEM) method (microscope LEO SEM 1430 VP supplied with EDX detector; operating conditions of electron microprobe 20 kV, 80 mA beam current). The concentration of Al, Si, Na, Mg, Fe, Cr, C on the bentonite surface was determined using standardless version of SEM method. The electron beam penetrated the samples for about 1 mm. Vacuum 10K5 Pa was preserved during measurements. Next, the bentonite samples in their sodium and HDTMA-Cr(VI) forms were investigated. The HDTMACr(VI)-bentonite sample was prepared through the equilibration of 1 g HDTMA-bentonite with 100 ml of 0.01 M solution of K2Cr2O7 (Polskie Odczynniki Chemiczne). The solid residue after filtration of the solution was dried in air.
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2.6. Small wide angle X-ray scattering measurements (SWAXS) The small angle X-ray scattering (SAXS) measurements were carried out with the compact Kratky camera equipped with the SWAXS optical system of HECUSMBRAUN (Austria). The Cu target X-ray tube, operated at: UZ40 kV, IZ30 mA (PW1830 Philips generator), ˚ . The primary was used as a radiation source lZ1.542 A beam was monochromatised by the Ni filter and the scattered radiation was detected by the linear position sensitive counter OED-51. The resolution of the ˚ (i.e. the Bragg spacing measurements was 750 A corresponding to the first measurement point). The transmission method was employed for the determination of the transmittance factor (T) of the samples.
3. Results and discussion 3.1. Equilibrium study We have checked the influence of the pH level on the chromates adsorption on modified bentonite and found that up to pHZ6 the concentration of CrO2K ions in the 4 adsorbent phase is constant (Fig. 1). Beginning from this pH value a sudden drop of the chromates concentrations in the adsorbent phase appears, which results probably from the concurrent formation of the alkylammonium hydroxide (HDTMA)OH, according to reaction: HDTMAHCrO4 C OHK 4 HDTMAOH C HCrOK 4
(1)
Therefore, from practical viewpoint, it is necessary to perform the adsorption of chromates from solutions with the pH level ranging from 2 to 6. However, we cannot be absolutely sure, that the competition between OH– and HCrOK 4 is the main reason for the change of Cr(VI)
Fig. 1. The influence of the pH on the Cr(VI) concentration in (c(b)) the equilibrium bentonite phase.
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2K and CrO2K adsorption. The affinities of HCrOK 4 , Cr2O7 4 ions to the modified bentonite are probably different, with the preference of the first two species in acid solutions; so for higher pH levels and consequent CrO2K ions concen4 tration increase, the Cr(VI) adsorption decrease is observed. The adsorption isotherms of chromates on bentonite in the presence of different anions (with the same 0.005 meq/dm3 concentrations) in the aqueous phase are given on Fig. 2. The parameters of the isotherms: amax, K, n (where amax, K and n are, respectively: maximum concentration of Cr(VI) in the adsorbent phase, adsorption constant K and parameter of the surface heterogeneity) were calculated from the ‘best fit’ of the experimental data found from well-known Langmuir–Fredundlich equation
cb Zamax ðKceq Þn =ð1 C ðKceq Þn Þ; where cb, ceq denote the equilibrium concentrations of Cr(VI) in the bentonite and aqueous phase respectively and are presented in Table 1. The following decreasing trend of adsorption maximum amax of chromates is observed: H2OZClKOH2POK 4 O K 2K 3K ˝ nsted strong bases: SO2K 4 ONO 3 OCO 3 OPO4 . Bro PO3K and CO2K are at the end of this order. They have a 4 3 strong influence on the chromates adsorption as a result of the pH effect and a consequent alkylammonium hydroxide formation. The comparison of the remaining monovalent anions is also interesting. According to Leontidis, nitrate ion [17] classified as water-structure-breaker (chaotropic) ion, is much more strongly bound by cationic surfactant headgroups than ClK ion, known as water-structure-maker (cosmotropic) ion. The sequence: Cl KOH 2PO K 4 O K SO2K 4 ONO3 probably reflects the increasing strength of anions interaction with HDTMA cation headgroup. If we look at the values of surface heterogeneity parameters n, we can notice that in all cases they are higher than 1. It is well known from the classical physical chemistry, that in the case of the Langmuir–Freundlich equation, when nO1, the lateral interactions between adsorbate molecules appear. Therefore, we can presume that there is a common interaction of the molecules of alkylchromates formed on the bentonite surface and built into the bentonite internal structure. The adsorption isotherms of the chromates at different temperatures are given on Fig. 3. There is an evident decrease of the concentration of Cr(VI) in the bentonite phase with the temperature increase. Exothermicity of the adsorption results from the strong electrostatic interaction on the negatively charged Cr(VI) ions with the positively charged alkylammonium ions HDTMAC. We have estimated the thermodynamical parameters for the chromates adsorption through the analysis of temperature dependence on the adsorption constant K. This procedure, based on the analysis of ln K vs. 1/T plot, was described by Krishna [14]. The values of DH8, DG8 and DS8 (enthalpy, free energy, entropy) for chromates adsorption on modified bentonite
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Fig. 2. The adsorption isotherms of Cr(VI) on the bentonite in the presence of different anions in the aqueous phase (concentration of anions 0.005 meq/1dm3; K 2K K 2K 3K 1, H2O; 2, H2POK 4 ; 3, Cl ; 4, SO4 ; 5, NO3 ; 6, CO3 ; 7, PO4 ).
are: K35.52 kJ/mol, K3.12 kJ/mol and K0.11 kJ/(mol K), respectively. Therefore, it is clear, that enthalpy is favourable for chromates adsorption, contrary to entropy. Negative entropy change probably results from the fact, that chromate anions lose their translative and oscillative freedom during attraction by positive alkylammonium cations. At this point we would like to emphasize that our thermodynamical parameters of Cr(VI) adsorption are completely different from those found by Krishna [14]. In his experiments, Krishna found positive entropy change following the adsorption of Cr(VI) on montmorillonite, the same type of clay as bentonite. Thermodynamical parameters were calculated by the author based on the change of distribution ratio Kd of chromium species at different temperatures, so contrary to us this author used only one initial concentration of chromates. In our experiment, we have registered the adsorption isotherms of Cr(VI) changes with the temperature. We believe that adsorption constants K reflect more strongly the essence of thermodynamics than the distribution ratios Kd, since they refer to the adsorption behaviour of chromium at different concentrations. There is also a second reason, why we are skeptical in relation to the results of Krishna. We suspect, that the results described in Ref. [14] refer to the nonequilibrium conditions of adsorption, since the author used the period of 60 min for the equilibration of the aqueous and solid phases. We have found that the period of 4 h. is necessary to equilibrate the modified bentonite and Cr(VI) containing aqueous phase. The solid lines on the plots refer to the ‘best fit’ found from the Langmuir–Freundlich equation. The heterogeneity parameters n are: 1.3, 1.3, 1.4, 1.2 for 293, 313, 323 and 333 K, respectively, so one can conclude about the strong interaction between alkyl chains of the surfactant cation, which is independent of the temperature.
3.2. Scanning electron microscopy of the bentonite surface The interelement correlations concentrations are given on Fig. 4. Sodium and alkylammonium cations concentrations are related to Al and given as molar ratios. The correlation Na/Al vs HDTMA/Al, which is statistically significant, shows a negative trend. Therefore, one can conclude that the exchange reaction between Na C and R 4 N C cations occurs on the HDTMA-Cr(VI)-bentonite surface. In turn the statistically significant correlation Fe vs Cr is positive and it results from the formation of basic chromate KFe3(CrO4)2(OH)6 in places of the surface abundant in Fe. The formation of this compound, which is insoluble in water, is described in the literature [18]. It is evident from the histogram of the molar ratio: HDTMA/Cr(VI) (Fig. 5), that for the majority of points on the bentonite surface, the value of this ratio is in 1–2 range. One can suggest a scheme of the chromates adsorption according to the following reactions
Table 1 Parameters of Langmuir–Freundlich equation describing adsorption of Cr(VI) on the HDTMA modified bentonite in the presence of different anions in the aqueous phase amax (mol/g)
K (molK1)
n
Medium
pH
0.00035 0.00035 0.00034 0.00032 0.00029 0.00013 0.00009
43,175 25,140 33,320 17,578 7854 5255 3733
1.3 1.5 1.7 2.1 2.4 1.5 1.3
H2 O ClK H2POK 4 SO2K 4 NOK 3 CO2K 3 PO3K 4
6.5 6.20 5.70 6.20 6.15 10.00 10.20
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Fig. 3. The adsorption isotherms of Cr(VI) on the HDTMA modified bentonite for different temperatures. K ðHDTMAÞ2 BtBr C HCrOK 4 4 ðHDTMAÞ2 BtHCrO4 C Br
(2) 2ðHDTMAÞ2 BtBr C Cr2 O2K 7 4 ½ðHDTMAÞ2 Bt2 Cr2 O7 C 2BrK
(3)
HDTMAC C HCrOK 4 4 HDTMAHCrO4
(4)
2HDTMAC C CrO2K 4 4 ðHDTMAÞ2 2CrO4
(5)
2HDTMAC C Cr2 O2K 7 4 ðHDTMAÞ2 Cr2 O7
(6)
where the reactions (2) and (3) are typically ion-exchange in character and refer to the exchange of bromide ions from
HDTMA-Br adsorbed on the bentonite surface through Van der Waals interactions with alkylammonium cations in species HDTMABt/HDTMABr (where HDTMABt refers to HDTMA-bentonite). These reactions are widely described in the literature [19] and refer rather to the conditions when the concentration of surfactant on the clay surface exceeds the CEC. Therefore, the participation of the ion-exchange reactions in the overall process of the chromates adsorption, described in our experiment, is rather minor. Alkylammonium chromates formed according to reactions: (4)–(6) interact probably with the hydrophobic bentonite matrix through the Van der Waals forces, forming species: HDTMAHCrO 4/HDTMABt, (HDTMA) 2Cr 2 O 7/HDTMABt, (HDTMA)2 CrO 4/HDTMABt. This scheme of reactions is justified if we inspect the distribution
Fig. 4. Interelement correlations concentrations on the HDTMA-modified bentonite surface, loaded with Cr(VI) (r and p relate to correlation coefficient and statistical significance factor, respectively).
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3.3. Interfacial phenomena analysis
Fig. 5. Histogram of molar ratio of HDTMA/Cr(VI) on the HDTMA modified bentonite surface.
of different Cr(VI) forms in the aqueous solution for the different pH values. Ramsey and coworkers [20] found, that 2K in the pH range from 2 to 6, the anions HCrOK 4 and Cr2O7 are the dominant forms of Cr(VI) in solution, whereas for neutral and alkaline solutions CrO2K predominates. 4 The metal concentrations in bentonite and HDTMACr(VI)-bentonite, defined as Me/Al molar ratio are given in Table 2. We can notice the change of Me/Al values merely for Na, if we compare bentonite and HDTMA-Cr(VI)bentonite forms. Therefore, one can conclude that sodium ion is exchanged from the bentonite structure through reaction: NaBt C HDTMAC4 HDTMABt C NaC
The surface tension changes of the equilibrium aqueous phases with various surfactant concentrations are given on Fig. 6. The surfactant concentrations were evaluated from plots of surface tension of HDTMAbromide solutions versus log (concentrations HDTMAbromide). This is a routine procedure applied in registration of interfacial adsorption of surfactants [21]. There is an evident increase of surface tension (expressed in mN/m) with the equilibrium chromate concentration. The increase of surface tension results from the decrease of alkylammonium cations HDTMAC concentration in the aqueous and interfacial phase due to the formation of alkylammonium chromate, which in turn is interfacially inactive. The straight-line correlation between log cs and log Cr(VI), where cs refers to equilibrium surfactant concentration, is negative. One can conclude that the alkylammonium chromate existing as undissociable ionic pair is formed in the interfacial region: bentonite–water, which is then transferred to the bentonite phase. The whole process can be
(7)
whereas the remaining metals: Mg, Fe, Al remain unexchanged. If we assume, that 100% of CEC refers to 0.41 value of Na/Al in bentonite (this statement is obvious after inspection of Table 2), then the 0.11 value of Na/Al in HDTMA-Cr(VI)-bentonite may be considered as 27% CEC ‘occupied’ by NaC ions. Thus it has been found for the first time, that the adsorption of chromates on modified bentonite is possible in conditions, when only part of CEC (below 100%) is in the alkylammonium form. It is interesting to note, that Si/Al, Fe/Al, Mg/Al molar ratios are constant, so during the modification of bentonite the aluminosilicate matrix preserves its skeletal chemical composition. Table 2 The metal concentrations on bentonite and HDTMA-Cr(VI)-bentonite surface Metal concentrations
Bentonite
HDTMA-Cr(VI)-bentonite
Na/Al Mg/Al Fe/Al Si/Al
0.41 0.19 0.10 2.39
0.11 0.20 0.09 2.39
Fig. 6. The surface tension changes of the aqueous phase with the Cr(VI) increase (a) surface tension expressed in mN/m and (b) surfactant concentration cs in the aqueous phase.
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schematically described as K HDTMAC in C HCrO4 in 4 ðHDTMAHCrO4 Þin
(8)
ðHDTMAHCrO4 Þin 4 ðHDTMAHCrO4 Þb ;
(9)
where indexes: in, b refer to the interfacial and bentonite phase. 3.4. FTIR spectral data The FTIR spectra of the modified bentonite for the different equilibrium concentrations of HDTMA in the solid phase are given on Fig. 7. The HDTMA concentrations in the solid phase were related to CEC through the relationship: HDTMACECZ[HDTMA]/CEC, where [HDTMA] refers to molar concentration of HDTMA (its summary cationic and HDTMA-Br forms) in the solid phase, whereas CECZ0.0012 m/g. Two distinct bands: 2931, 2857 cmK1
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are observed for the concentrations of surfactant ranging from 0.0001 to 0.001 mol/g. These bands result from asymmetric and symmetric stretching vibrations of CH2 absorption bands of amine chains [22]. Their intensity increases with the surfactant concentration and its packing density in the solid phase. The down wavelength shift of these bands (Fig. 8) is probably the consequence of the change in the arrangement of surfactant in the clay interlayer from disordered (gauche) to ordered (trans) form [22]. One can conclude, that for relatively low concentration of surfactant (below 0.5 CEC) the arrangement of the surfactant in the interlayer is a lateral-monolayer form with gauche conformation. The amine ions are parallel to the interlayer and individually separated [22]. The interaction between amine molecules is very weak. For the higher concentration of surfactant (above 0.5 CEC) the lateral-monolayer arrangement varies at first to lateralbilayer (0.7 CEC) and then to paraffin-type bilayer
Fig. 7. FTIR spectra of HDTMA modified bentonite (a) FTIR spectra for HDTMACEC concentrations: 0.017(bottom), 0.034, 0.043, 0.0513, 0.06, 0.068, 0.077, 0.085, 0.171, 0.256, 0.341, 0.427, 0.513, 0.598. 0.684, 0.796 and (b) the change of the wavenumber position for 2857 and 2930 bands with HDTMACEC concentration increase.
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Fig. 8. SWAXS spectrum of bentonite, HDTMA modified bentonite, HDTMA-Cr(VI) modified bentonite.
(1.0 CEC) with trans conformation. The increase in the amine concentration results in the increased Van der Waals interchain interaction and consequently in the increased number of ordered conformers. 3.5. Analysis of SWAXS patterns The SWAXS pattern of unmodified bentonite has the ˚ , which disappears after modification by peak at 11.5 A ˚ , results from the HDTMA-Br (Fig. 8). The new peak at 20 A insertion of HDTMA-Br into interlamellar space with the bilayer arrangement of alkylammonium cations [23]. In turn 2K 2K results in the adsorption of Cr2O2K 7 , HCrO4 , CrO4 further broadening of the interlamellar space and the ˚ peak. Dichromate is the most spatially appearance of 31.5 A distributed of these three ions. The mean length of Cr–O ˚ [24], therefore for the whole ion, bond in this ion is 1.7 A ˚ with the array of four Cr–O bonds, the length is about 7 A taking into account the placement of negative charge on oxygen atoms and change of the zig-zag structure of the ion to nearly straight-line structure during interaction of negatively charged terminal oxygens with positive surfactant headgroups on the opposite sides of bimolecular complex (Fig. 9). As a result the d(001) basal spacing of ˚ . In reality we bentonite should increase by about 14 A ˚ observe an 11.5 A increase in basal spacing value when comparing HDTMA-Cr(VI)-bentonite and HDTMA-bentonite. This value is a little bit lower than expected, which results from the fact that, apart from dichromates (HDTMA) 2Cr 2O 7 also the molecules of chromates: (HDTMA)(HCrO4 ) and (HDTMA) 2CrO 4, with less extended geometry, are present in the interlayer space.
Fig. 9. Arrangements of surfactant alkylammonium cations and Cr(VI) anions in the interlamellar space of the bentonite structure (top, HDTMAbentonite, bottom- HDTMA-Cr(VI)-bentonite).
4. Conclusions 1. Adsorption of Cr(VI) on the HDTMA-modified bentonite proceeds mainly through the formation of alkylammonium chromates HDTMAHCrO4 and dichromates (HDTMA)2Cr2O7. The formation of basic chromate KFe3(CrO4)2(OH)6 is also suggested, however, its influence on the overall process of Cr(VI) adsorption is rather minor, since Fe/Al molar ratio remains stable. 2. Adsorption of chromates and dichromates on the bentonite is typically interfacial in character. These compounds formed in bentonite–water interface phase are transferred to the bentonite phase. The aluminosilicate matrix of the bentonite preserves its chemical composition during the adsorption of surfactant and subsequent adsorption of Cr(VI) anions. 3. The influence of the anions on the Cr(VI) adsorption on bentonite, except for CO2K and PO3K 3 4 , is the result of their water structure breaking or structure making properties. However, from the practical viewpoint the case of carbonates or phosphates salts is very important,
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because their solutions can be used for the regeneration of bentonite loaded with Cr(VI).
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