- Email: [email protected]
Simultaneous adsorption of chlorophenol and heavy metal ions on organophilic bentonite
Applied Clay Science 31 (2006) 126 – 133 www.elsevier.com/locate/clay
Simultaneous adsorption of chlorophenol and heavy metal ions on organophilic bentonite S. Andini a , R. Cioffi b , F. Montagnaro a , F. Pisciotta a , L. Santoro a,⁎ a
Dipartimento di Chimica, Università di Napoli Federico II, Complesso Universitario di Monte S.Angelo, Via Cintia, 80126 Napoli, Italy b Dipartimento per le Tecnologie, Università di Napoli Parthenope, Via Ammiraglio F. Acton 38, 80133 Napoli, Italy Received 15 December 2004; received in revised form 24 August 2005; accepted 8 September 2005 Available online 8 November 2005
Abstract Organophilic bentonite obtained by ion exchange with benzyldimethyl octadecylammonium chloride has been used for multiple adsorption of 2-chlorophenol and the metals Pb2+ and Cd2+. This is of interest for the stabilization of wastes in which simultaneous organics/heavy metals contamination occurs. In such cases cement-based processes take advantage from the addition of quaternary ammonium salts exchanged bentonite. The results have shown that any of the contaminants is adsorbed according to a multilayer cooperative mechanism. Due to its organophilic nature, exchanged bentonite is able to adsorb 2-chlorophenol to a very high extent (about 0.7 g/g) when the organic is the only solute. On the other hand, each of the two metal ions is adsorbed to a much lesser extent when alone in solution (about 22 and 2.8 mg/g for Pb2+ and Cd2+, respectively). In the case of simultaneous presence of 2chlorophenol and Pb2+ in solution, the adsorption isotherms of both solutes change to monolayer type and the adsorption capacity strongly decreases (about 0.3 g/g for 2-chlorophenol and 0.6 mg/g for Pb2+). In the case of the 2-chlorophenol/Cd2+ systems, the adsorption capacity of about 0.3 g/g for 2-chlorophenol is retained, while Cd2+ is not adsorbed at all. In all the cases in which the bentonite is in contact with one of the metal ions, it has been checked that no exchange with the ammonium ions takes place. Finally, FT-IR analysis has shown that in all the cases investigated physical adsorption takes place and no new chemical bonds are formed. © 2005 Elsevier B.V. All rights reserved. Keywords: Organophilic bentonite; Adsorption; 2-Chlorophenol; Heavy metals; Quaternary ammonium salts
1. Introduction Bentonites are argillaceous materials that can be effectively employed as adsorbents of many waste water pollutants, namely heavy metal ions and organic compounds. This outstanding capability is due to the presence of the mineral montmorillonite, the smectite clay contained in bentonite rocks. In adsorption pro⁎ Corresponding author. Tel.: +39 081 674028; fax: +39 081 674090. E-mail address: [email protected] (L. Santoro). 0169-1317/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2005.09.004
cesses, raw bentonite acts as an inorganic stationary phase able to effectively adsorb heavy metal ions (Khan et al., 1995; Tiller et al., 1984). However, the nature of bentonite can be easily turned to organophilic as the Group IA and IIA loosely bound metal ions (Li+, Na+, K+, Mg2+, Ca2+ and Ba2+) present between alumina and silica layers can be readily exchanged with quaternary ammonium ions such as [R4N]+ (Jordan, 1949; Jordan et al., 1950). The expulsion of the metal ions with coordinated water molecules strongly reduces the hydrophilicity of the clay and, if at least one of the nitrogen substituents is a long aliphatic
S. Andini et al. / Applied Clay Science 31 (2006) 126–133
chain, the interlayer space increases and the adsorption capacity increases even more. In addition, favourable interactions can arise with the R groups of the quaternary ammonium ions. Organic molecules can be more easily attracted into the expanded interlayer space and, when drawn between the layers of clay, a number of van der Waals interactions and chemical bonds can occur. The latter involve the alumina and silica oxygens which can form hydrogen bonds with the hydrogens of the adsorbed molecules, and also the transition metals of the clay which can form coordination compounds with the adsorbed organics (Gibbson and Soundararajan, 1988). Many papers are found in the literature on the use of quaternary ammonium salt (QAS) exchanged bentonites as adsorbents of many organic compounds from water and other solvents (Boyd et al., 1988; Cioffi et al., 2001a; Dékány et al., 1986a,b,c, 1996; Lagaly, 1994; Liu and Chang, 1992; Lizhong et al., 1996, 1997; Mortland et al., 1986; Natali Sora et al., 2005; Regdon et al., 1998a,b; Stockmeyer and Kruse, 1991; Wolfe et al., 1986). This rather unique characteristic of bentonites is also of great advantage for the stabilization of organic contaminated wastes. It is well known that, among the multiple options for treatment of hazardous wastes in solid, liquid or sludge form, one of the most widely used is stabilization/solidification (S/S) using a cementitious matrix to obtain a monolithic residue. This kind of treatment can allow a safer disposal of the waste and in some cases permits recycling in the field of building materials (Conner, 1990; Spence, 1993). However, while cement-based S/S is effective in the case of inorganic wastes, organic containing ones cannot be satisfactorily treated by this type of process because some interaction occurs between the organic contaminants and the cementitious matrix, thereby compromising the effectiveness of the process and the technological properties of the stabilized product. Many organic compounds are known to have a retarding effect on cement hydration reactions and adversely affect the microstructural, mechanical and leaching properties of the cementitious materials (Cullinane and Bricka, 1993; Eaton et al., 1987; Montgomery et al., 1991a; Ramachandran and Ashton, 1986; Sheffield et al., 1987). QAS exchanged bentonites can help face this problem and many studies are found in the literature on their use as pre-solidification agents in cement-based S/S processes (Alther et al., 1988; Brown et al., 1992; Caldwell et al., 1990; Calvanese et al., 2002; Cioffi et al., 2001b; Hebatpuria et al., 1999; Montgomery et al., 1988, 1991a,b; Sell et al., 1992; Sheriff et al., 1989;
127
Soundararajan et al., 1990; Spence et al., 1992). The effectiveness of modified bentonites in waste disposal is also demonstrated by some patents that have been filed using QAS-clays as adsorbents in view of this application (Beall, 1984, 1987). In some cases mixed organics/heavy metals pollution occurs, especially in sediments (Lee et al., 2000). When this happens, some concern arises on the effectiveness of QAS-bentonites in waste stabilization, as organophilicity could not be retained if heavy metal cations would replace quaternary ammonium ions by ion exchange. So far, this problem has not been addressed by the scientific community, and no papers are found in the literature on QAS-bentonite behaviour in the presence of mixed organics/heavy metals contamination, neither as adsorbents in liquid phase, nor as supporting additives in waste cement stabilization. In this paper the adsorption properties of organophilic bentonite in aqueous phase and in the presence of both organic and heavy metal contamination were studied. To this purpose, 2-chlorophenol (CP), cadmium and lead were selected as contaminants, while bentonite was made organophilic by exchange with benzyldimethyl octadecylammonium (BDMOA) chloride. Adsorption isotherms for any of the three contaminants and the effect of the two heavy metal ions (one at a time) on CP adsorption were determined. Fourier Transform Infra Red (FT-IR) spectroscopy was used as main experimental technique to assess the interactions among the adsorbent and the adsorbates. Other experimental techniques were X-ray diffraction (XRD), Ultra Violet (UV) spectroscopy, Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and High Performance Liquid Chromatography (HPLC). The instruments were a Jasco FT/IR-430 spectrometer, a Philips PW 3710 diffractometer, a Jasco UV-560 spectrometer, a Perkin Elmer ICP-OES Optima 2000 DV spectrometer and a Shimadzu Analytical HPLC System. 2. Materials and methods 2.1. Bentonite characterization The bentonite used in this work comes from a quarry located in Santa Croce di Magliano (Campobasso, Italy) and was supplied by the company “Eredi dott. Settimio Cinicola— Bentonite”. Its chemical composition is shown in Table 1. The presence of quartz and calcite impurities was noted by XRD analysis. The diffraction peak corresponding to the family of planes (001) allowed the basal spacing of the bentonite to be found at the value of 1.468 nm. The cation exchange capacity (CEC) of the bentonite was found by exchange with potassium and sodium chloride to be 0.80 meq/g.
128
S. Andini et al. / Applied Clay Science 31 (2006) 126–133
Table 1 Chemical composition of bentonite, wt.% LOI a
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2 O
MnO
Total
14.75
45.85
14.41
3.53
17.08
1.91
0.94
0.71
0.10
99.28
a
Loss on ignition at 1000 °C.
2.2. Exchanged bentonite preparation The impure bentonite was exchanged with a sodium chloride solution to obtain a Na-exchanged bentonite because the organic cations are strongly preferred in the exchange sites relative to Na+ ions (Wolfe et al., 1986). The exchange of the Na-bentonite with BDMOA-Cl was carried out by dispersing 100 g of bentonite in 4 dm3 of an aqueous solution containing QAS in amount just equal to the CEC of the clay, stirring for 8 h at 50 °C, decanting overnight, filtering, washing with distilled water and drying at 50 °C for 24 h. The solutions were analysed after the exchange to determine their Na+ contents and verify the complete exchange of the sodium ions by the quaternary ammonium cations. After the exchange, the basal spacing of the bentonite was found to increase to 2.008 nm by XRD analysis. 2.3. Adsorption experiments All the adsorption experiments were carried out at 25 °C. The adsorption isotherm of CP on the bentonite exchanged with the ammonium salt was obtained by dispersing different amounts of bentonite in 50 cm3 of CP aqueous solution (12 g/ dm3), stirring for 6 h, decanting and filtering. It was checked that liquid–solid equilibrium was reached after 6 h contact. The solutions were analysed to determine the CP equilibrium concentration by UV spectroscopy. The adsorption isotherms of Cd2+ and Pb2+ were obtained by dispersing 0.12 g of bentonite in 100 cm3 of metal solution with concentration ranging from 2 to 30 mg/dm3. After stirring for 6 h, decanting and filtering, the metal equilibrium aqueous concentration was found by means of ICP-OES. The effect of Cd2+ and Pb2+ on CP adsorption was assessed through the determination of CP isotherm in the presence of the two metal ions (one at a time) in 25 mg/dm3 concentration. The possible release of the ammonium salt due to ion exchange with the metal cations has been verified by means of UV spectroscopy and HPLC. The products obtained in the different experimental conditions have been characterized by means of FT-IR spectroscopy to assess the interactions that occur upon adsorption.
to the classification by Giles et al. (1960) based on the initial slope and the shape of the upper part of the curve, the adsorption isotherm is of class L (Langmuir), subgroup 4. It is a multilayer type isotherm, typical of those obtained by moderate interaction between the organic compound and the bentonite, followed by a cooperative effect of the adsorbed molecules in the multilayer adsorption (Gregg and Sing, 1982). Not always organophilic bentonite gives L4 type isotherms. Cases are found in the literature of L2 type (monolayer Langmuir) isotherms typical of stronger adsorbent–adsorbate interactions (Boyd et al., 1988; Liu and Chang, 1992; Mortland et al., 1986), and S2 type (S shaped) isotherms (Cioffi et al., 2001a; Dékány et al., 1996). In the latter case weak interactions occur between the organic compound and the bentonite at low solute concentration, but, once a molecule is adsorbed, adsorbent–adsorbate interactions increase and promote the adsorption in a cooperative fashion (Gregg and Sing, 1982). Finally, BDMOA-bentonite shows outstanding adsorption capacity to CP, as the concentration of the organic on the bentonite at saturation was found as high as 0.70 g of CP per gram of bentonite. This result is in agreement with those by Natali Sora et al. (2005) who found equally high adsorption capacity of similar organophilic bentonites against 2-chloroaniline. The adsorption isotherm of Pb2+ on the BDMOAbentonite is shown in Fig. 2. Even in this case the isotherm is of L4 type, typical of cooperative multilayer
3. Results and discussion 3.1. Adsorption isotherms The results of the adsorption experiments of CP on the BDMOA-bentonite are shown in Fig. 1. According
Fig. 1. Adsorption isotherm at 25 °C of 2-chlorophenol on BDMOAbentonite.
S. Andini et al. / Applied Clay Science 31 (2006) 126–133
adsorption. However, differently from the CP case, the low adsorbent loading at Pb2+ concentration up to about 0.1 mg/dm3 makes evident that solute–adsorbent interactions are quite weak at low solute concentration. Then, as this concentration increases, cooperative adsorption takes place to a much greater extent and brings the total solid loading to about 22 mg of Pb2+ per gram of bentonite. As expected, the maximum Pb2+ loading is much lower than that of CP. Evidently, the organophilic nature of the BDMOA-bentonite does not favour the adsorption of hydrated metal ions. Fig. 3 shows the adsorption isotherm of Cd2+ on BDMOA-bentonite. Once again the isotherm is of L4 type. However, the adsorption capacity reaches values as low as about 2.8 mg of Cd2+ per gram of bentonite, proving that very weak adsorbent–adsorbate interactions occur in this case. When compared to Pb2+, Cd2+ adsorption is less favoured, due to its higher charge density that increases hydrophilicity. This result is in agreement with literature data by Barbier et al. (2000) on the adsorption of these two metal ions on pure montmorillonite and commercial bentonite. In the adsorption experiments of Pb2+ and Cd2+ no release of the ammonium salt was found. This was checked by UV spectrometry that can detect the presence of the benzyl group through its absorbance at 262 nm. The influence of Pb2+ and Cd2+ on the adsorption of CP has been studied by replicating CP adsorption experiments in the presence of either 25 mg/dm3 of Pb2+ or 25 mg/dm3 of Cd2+. The results are shown in Fig. 4. The isotherms are very different from the case of the adsorption of CP alone. They turn to L2 type (Langmuir monolayer) and the formation of the monolayer takes place at the same equilibrium concentration in both cases. However, the portion of curve corresponding to multilayer cooperative adsorption completely disappears. As a consequence, the adsorption capacity considerably decrea-
Fig. 2. Adsorption isotherm at 25 °C of Pb2+ on BDMOA-bentonite.
129
Fig. 3. Adsorption isotherm at 25 °C of Cd2+ on BDMOA-bentonite.
ses from 0.70 g of CP per gram of bentonite to 0.30 and 0.32 g of CP per gram of bentonite when the solution also contains Pb2+ and Cd2+, respectively. In any CP-metal ion adsorption experiment, the metal ion equilibrium concentration was also measured to ascertain the occurrence of competition between the two adsorbates. In the case of Cd2+, no adsorption takes place at all, as no change of the initial concentration (25 mg/l) was found. This also means that no exchange of Cd2+ with quaternary ammonium ions takes place. This latter issue was also checked by HPLC analysis of the liquid phase at equilibrium. This analytical technique was necessary because the presence of the benzene ring in both CP and ammonium salt makes the relative UV signals overlap. The experiments for simultaneous Pb2+ and CP adsorption lead to different results. The analysis of the relative systems revealed that the bentonite is able to adsorb simultaneously both adsorbates, as shown by the
Fig. 4. Adsorption isotherms at 25 °C of 2-chlorophenol on BDMOAbentonite in the presence of Pb2+ and Cd2+.
130
S. Andini et al. / Applied Clay Science 31 (2006) 126–133
Pb2+ adsorption isotherm in the presence of CP reported in Fig. 5. The comparison of this figure with Fig. 2 (adsorption of Pb2+ alone) makes clear that the presence of CP makes the Pb2+ isotherm change from L4 to L2 type and strongly depresses the adsorption capacity against this metal. It dramatically decreases from about 22 mg per gram of BDMOA-bentonite at about 0.4 mg/l equilibrium concentration to about 0.6 mg per gram of BDMOA-bentonite at about 3.5 mg/l equilibrium concentration. Despite the occurrence of competitive adsorption of Pb2+, it was checked by HPLC analysis that even in this case no exchange with quaternary ammonium ions takes place. In synthesis, the experiments of simultaneous CPmetal ions adsorption demonstrated that: (a) the adsorption capacity of BDMOA-bentonite against CP is strongly reduced by the presence of both Pb2+ and Cd2+; (b) the isotherm type changes from L4 (multilayer Langmuir) to L2 (monolayer Langmuir) in both cases; (c) the effects of Pb2+ and Cd2+ are almost the same; (d) Cd2+ is not adsorbed at all in the presence of CP; (e) the adsorption capacity of BDMOA-bentonite against Pb2+ is strongly reduced by the presence of CP; and (f) no exchange takes place between the ions of each metal and the quaternary ammonium ions. The fact that the adsorption capacity of BDMOA-bentonite is strongly reduced when CP and either Cd2+ or Pb2+ are present in solution can be ascribed to documented interactions between CP and the metal ions of solute–solute type (complexation) reported by Christensen and Christensen (2000) and Krishnamurti and Naidu (2003). In view of application of organophilic bentonite as pre-solidification adsorbent in cementitious systems, the desorption of CP and/or heavy metal ions can be of concern. To this regard, the desorption of CP, due to its acid properties, may well take place in the alkaline
Fig. 5. Adsorption isotherm at 25 °C of Pb2+ on BDMOA-bentonite in the presence of 2-chlorophenol.
Fig. 6. FT-IR spectra of raw bentonite (a), BDMOA-Cl (b) and BDMOA-bentonite (c).
environment of hydrating cement. The extent of CP desorption from BDMOA-bentonite loaded with 0.7 g of the organic per gram of bentonite in a 0.1 M NaOH solution was found about 28% (Maffucci, 1999). It is believed that this result may not be of concern, as the pH of hydrating cement is one–two units lower than that of the NaOH solution used in the desorption experiment referred to above. Furthermore, pH is even lower in a S/S system were cement is mixed with waste. Equally, no concern should arise from heavy metal ions desorption, as it is well known that cementitious systems are particularly well suited for the stabilization of these pollutants. 3.2. Infrared and X-ray characterization FT-IR spectroscopy was used for characterizing the materials and the type of interactions that occur in the different experimental conditions tested. The FT-IR spectra of raw bentonite, BDMOA-Cl and BDMOA-bentonite are reported in Fig. 6. The spectrum of raw bentonite (trace a in Fig. 6) shows typical infrared bands of montmorillonite due to ỌH stretching at 3620 and 3423 cm− 1 and ṢiỌSi bending at 1043 cm− 1 (Farmer, 1974). Raw bentonite shows also a wide and intense band at 1430 cm− 1 and two weaker bands at 914 and 874 cm− 1. These bands are due to ỌH bending of, respectively, Ca2+, Mg2+ and Fe3+ hydration water (Farmer, 1974). Trace b in Fig. 6 is the FT-IR spectrum of BDMOA-Cl and shows two sharp bands characteristic of the aliphatic C̣H stretching at 2920 and 2852 cm− 1 and some very weak bands in the region 3056–3008 cm− 1 due to the aromatic C̣H stretching (Bellamy, 1958). The spectrum of BDMOAbentonite results in trace c of Fig. 6 which, with the exception of the bands at 1430, 914 and 874 cm− 1 that
S. Andini et al. / Applied Clay Science 31 (2006) 126–133
completely disappear, is the almost perfect superimposition of the two previous traces, meaning that the ion exchange of Na-bentonite with BDMOA-Cl takes place as expected. Also, the FT-IR results show that no new bonds are formed between BDMOA and bentonite after exchange. Fig. 7 shows the results of FT-IR characterization of BDMOA-bentonite after adsorption of CP, Pb2+ and Cd2+ in the conditions of maximum loading. Trace a in Fig. 7 is the spectrum of BDMOA-bentonite and is the same as trace c in Fig. 6. Trace b (Fig. 7) is the result of CP infrared analysis and is mainly characterized by many strong bands in the region 1660–650 cm− 1. Trace c is the spectrum of CP-loaded BDMOA-bentonite at maximum sorption (0.7 g/g). It is seen that all the infrared bands characteristic of CP spectrum disappear. According to Montgomery et al. (1991b), this result may be due to the fact that van der Waals interactions can promote so compact and tight packing within the exchanged clay interlayer space that adsorbed CP vibrational motion is strongly hindered. In CP adsorption only physical interactions occur, as no new infrared band is observed in trace c with respect to traces a and b. Traces d and e are relative to Pb2+ and Cd2+ loaded BDMOA-bentonite and both differ from trace a only for the presence of an infrared band at 1384 cm− 1. This band is due to the presence of NO3− as the metals
131
adsorption experiments were carried out with the relative nitrates. Once again, no extra bands are present after adsorption, meaning that only physical adsorption takes place for both metals. BDMOA-bentonite after adsorption of CP, Pb2+ and 2+ Cd was characterized by means of XRD analysis, too. In the case of CP adsorption, the basal spacing was found to decrease from 2.008 nm to 1.890 nm in the condition of maximum loading. This may be due to the fact that CP adsorption increases the organophilic character of BDMOA-bentonite causing the expulsion of interlayer residual water and denser packing. This is in agreement with the results of FT-IR characterization which showed CP to be tightly packed within the clay interlayer spacings. In the cases of Pb2+ and Cd2+ adsorption no change of basal spacing was found, meaning that the adsorption of the two metals does not interfere with the molecules arrangement within the interlayer space. 4. Conclusions The adsorption of 2-chlorophenol on benzyldimethyl octadecylammonium exchanged bentonite follows a L4 mechanism (multilayer cooperative). Due to the bentonite organophilicity, loading as high as 0.7 g/g can be achieved. On the other hand, the adsorption capacity against lead and cadmium is from one to two orders of magnitude lower. Even in cases of the two metals the adsorption isotherm is of L4 type. The adsorption of 2-chlorophenol changes from L4 multilayer cooperative to L2 monolayer when the organic is in the presence of Pb2+ or Cd2+. Also, the adsorption capacity strongly reduces. The same happens for Pb2+ in the presence of 2-chlorophenol. However, in the same conditions Cd2+ is not adsorbed at all. Interactions of adsorbent–adsorbate and adsorbate–adsorbate (solute–solute) type are responsible for this behaviour. When in contact with any of the heavy metal ions, the bentonite does not release ammonium ions by ion exchange and its organophilic nature is retained. FT-IR analysis has shown that no new chemical bonds are formed after adsorption, and also that 2chlorophenol, once adsorbed, is tightly packed within the clay interlayer space. Acknowledgements
Fig. 7. FT-IR spectra of BDMOA-bentonite (a), 2-chlorophenol (b), 2chlorophenol loaded bentonite (c), Pb2+ loaded bentonite (d) and Cd2+ loaded bentonite (e).
The authors wish to thank Dr. Filippo Cinicola for supplying the bentonite, Prof. Flavia Nastri for the help and assistance in HPLC analysis and Dr. Francesco
132
S. Andini et al. / Applied Clay Science 31 (2006) 126–133
Modestia for the help and assistance in ICP-OES analysis. References Alther, G.R., Evans, J.C., Pancoski, S.E., 1988. Organically modified clays for stabilization of organic hazardous wastes. Proc. 9th Nat. Conf. “Superfund ‘88”, Washington, DC, 28–30 November, pp. 440–445. Barbier, F., Duc, G., Petit-Ramel, M., 2000. Adsorption of lead and cadmium ions from aqueous solution to the montmorillonite/water interface. Colloids and Surfaces. A, Physicochemical and Engineering Aspects 166, 153–159. Beall, G.W., 1984. Organic Waste Disposal, U.S. Patent No. 4473477. Beall, G.W., 1987. Method of Immobilizing Organic Contaminants to Form a Non-flowable Matrix, U.S. Patent No. 4650590. Bellamy, L.J., 1958. The Infra-red Spectra of Complex Molecules. Methuen & Co. Ltd, London. Boyd, S.A., Shaobai, S., Lee, J.F., Mortland, M.M., 1988. Pentachlorophenol sorption by organo-clays. Clays and Clay Minerals 36 (2), 125–130. Brown, R.E., Jindal, B.S., Bulzan, J.D., 1992. A critical review of the effectiveness of stabilization and solidification of hazardous organic wastes. ASTM Special Technical Publication, vol. 1123. ASTM, Philadelphia, U.S.A., pp. 43–60. Caldwell, R.J., Coté, P.L., Chao, C.C., 1990. Investigation of solidification for the immobilization of trace organic contaminants. Hazardous Waste and Hazardous Materials 7 (3), 273–282. Calvanese, G., Cioffi, R., Santoro, L., 2002. Cement stabilization of tannery sludge using quaternary ammonium salt exchanged bentonite as pre-solidification adsorbent. Environmental Technology 23, 1051–1062. Cioffi, R., Costanzo, S., Maffucci, L., Santoro, L., 2001a. Adsorption of the organic fraction of a tannery sludge by means of organophilic bentonite. Environmental Technology 22, 83–89. Cioffi, R., Maffucci, L., Santoro, L., Glasser, F.P., 2001b. Stabilization of chloro-organics using organophilic bentonite in a cement-blast furnace slag matrix. Waste Management 21, 651–660. Conner, J.R., 1990. Chemical Fixation and Solidification of Hazardous Wastes. Van Nostrand Reinhold, New York. Christensen, J.B., Christensen, T.H., 2000. The effect of pH on the complexation of Cd, Ni and Zn by dissolved organic carbon from leachate-polluted groundwater. Water Research 34 (15), 3743–3754. Cullinane, M.J., Bricka, R.M., 1993. Principles and Practices for Petroleum Contaminated Soils. Lewis Publishers, Boca Raton, Florida. Dékány, I., Szántó, F., Weiss, A., Lagaly, G., 1986a. Interactions of hydrophobic layer silicates with alcohol-benzene mixtures: I. Adsorption isotherms. Berichte der Bunsen-Gesellshaft für Physikalische Chemie 90, 422–427. Dékány, I., Szántó, F., Nagy, L.G., 1986b. Sorption and immersional wetting on clay minerals having modified surface. Journal of Colloid and Interface Science 109 (2), 376–384. Dékány, I., Szántó, F., Weiss, A., Lagaly, G., 1986c. Interactions of hydrophobic layer silicates with alcohol–benzene mixtures: II. Structure and composition of the adsorption layer. Berichte der Bunsen-Gesellshaft für Physikalische Chemie 90, 427–431. Dékány, I., Farkas, A., Király, Z., Klumpp, E., Narres, H.D., 1996. Interlamellar adsorption of 1-pentanol from aqueous solution on
hydrophobic clay mineral. Colloids and Surfaces. A, Physicochemical and Engineering Aspects 119, 7–13. Eaton, H.C., Walsh, M.B., Tittlebaum, M.E., Cartledge, F.K., Chalasani, D., 1987. Organic interference of solidified/stabilized hazardous wastes. Environmental Monitoring and Assessment 9 (2), 133–142. Farmer, V.C., 1974. The Infrared Spectra of Minerals. Mineralogical Society, London. Gibbson, J.J., Soundararajan, R., 1988. The nature of chemical bonding between modified clay minerals and organic waste materials. American Laboratory Magazine 20 (7), 38–46. Giles, C.H., MacEwan, T.H., Nakhwa, S.N., Smith, D., 1960. Studies in adsorption: Part XI. A system of classification of solution adsorption isotherms and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids. Journal of Chemical Society 3973–3993. Gregg, S.J., Sing, K.S.W., 1982. Adsorption, Surface Area and Porosity. Academic Press, New York. Hebatpuria, V.M., Arafat, H.A., Bishop, P.L., Pinto, N.G., 1999. Leaching behaviour of selected aromatics in cement-based solidification/stabilization under different leaching tests. Environmental Engineering Science 16 (6), 451–463. Jordan, J.W., 1949. Organophilic bentonites: I. Swelling in organic liquids. Journal of Physical and Colloid Chemistry 53, 294–306. Jordan, J.W., Hook, B.J., Finlayson, C.M., 1950. Organophilic bentonites: II. Organic liquid gels. Journal of Physical and Colloid Chemistry 54, 1196–1208. Khan, S.A., Riaz-Ur-Rehman, Khan, M.A., 1995. Adsorption of chromium(III), chromium(IV) and silver(I) on bentonite. Waste Management 15, 271–282. Krishnamurti, G.S.R., Naidu, R., 2003. Solid-solution equilibria of cadmium in soils. Geoderma 113 (1), 17–30. Lagaly, G., 1994. Bentonites: adsorbents of toxic substances. Progress in Colloid and Polymer Science 95, 61–72. Lee, C.L., Song, H.J., Fang, M.D., 2000. Concentrations of chlorobenzenes, hexachlorobutadiene and heavy metals in surficial sediments of Kaohsiung coast, Taiwan. Chemosphere 41 (6), 889–899. Liu, M.L., Chang, H.F., 1992. Study on treatment of organic wastewater with modified bentonite adsorbent. Fundamentals of Adsorption, Proc. IVth Int. Conf. On Fundamentals of Adsorption, Kyoto (Japan), 17–22 May 1992, pp. 381–388. Lizhong, Z., Jianying, Z., Yimin, L., Xueyou, S., Wenbin, Q., 1996. Organobentonites as adsorbents for some organic pollutants and its application in wastewater treatment. Journal of Environmental Sciences 8 (3), 378–383. Lizhong, Z., Yimin, L., Jianying, Z., 1997. Sorption of organobentonites to some organic pollutants in water. Environmental Science and Technology 31, 1407–1410. Maffucci, L., (1999). PhD thesis, University of Naples Federico II, Naples, Italy. Montgomery, D.M., Sollars, C.J., Sheriff, T.S., Perry, R., 1988. Organophilic clays for the successful stabilisation/solidification of problematic industrial wastes. Environmental Technology Letters 9, 1403–1412. Montgomery, D.M., Sollars, C.J., Perry, R., 1991a. Optimization of cement-based stabilization/solidification of organic-containing industrial wastes using organophilic clays. Waste Management and Research 9, 21–34. Montgomery, D.M., Sollars, C.J., Perry, R., Tarling, S.E., Barnes, P., Henderson, E., 1991b. Treatment of organic-contaminated indus-
S. Andini et al. / Applied Clay Science 31 (2006) 126–133 trial wastes using cement-based stabilization/solidification. Waste Management and Research 9, 113–125. Mortland, M.M., Shaobai, S., Boyd, S.A., 1986. Clay-organic complexes as adsorbents for phenol and chlorophenols. Clays and Clay Minerals 34 (5), 581–585. Natali Sora, I., Pelosato, R., Zampori, R., Botta, D., Dotelli, G., Vitelli, M., 2005. Matrix optimisation for hazardous organic waste sorption. Applied Clay Science 28, 43–54. Ramachandran, V.S., Ashton, H.E., 1986. Trends in building material research. Materials and Structures 19, 337–342. Regdon, I., Király, Z., Dékány, I., Lagaly, G., 1998a. Microcalorimetric studies of S/L interfacial layers: thermodynamic parameters of the adsorption of butanol–water on hydrofobized clay minerals. Progress in Colloid and Polymer Science 109, 214–220. Regdon, I., Dékány, I., Lagaly, G., 1998b. A new way for calculating the adsorption capacity from surface excess isotherms. Colloid and Polymer Science 276, 511–517. Sell, N.J., Revall, M.A., Bentley, W., McIntosh, T.H., 1992. Solidification and stabilization of phenol and chlorinated phenol contaminated soil. ASTM Special Technical Publication, vol. 1123. ASTM, Philadelphia, U.S.A., pp. 73–85. Sheffield, A., Makena, S., Tittlebaum, M.E., Eaton, H.C., Cartledge, F.K., 1987. Effects of three organic on selected physical properties of type I portland cement. Hazardous Waste and Hazardous Materials 4 (3), 273–286.
133
Sheriff, T.S., Sollars, C.J., Montgomery, D.M., Perry, R., 1989. The use of activated charcoal and tetra-alkylammonium-substituted clays in cement-based stabilization/solidification of phenols and chlorinated phenols. ASTM Special Technical Publication, vol. 1033. ASTM, Philadelphia, U.S.A., pp. 273–286. Soundararajan, R., Barth, E.F., Gibbson, J.J., 1990. Using an organophilic clay to chemically stabilize waste containing organic compounds. Hazardous Materials Control 3, 42–45. Spence, R.D., 1993. Chemistry and Microstructure of Solidified Waste Forms. Lewis Publishers, Boca Raton, Florida. Spence, R.D., Gilliam, T.M., Morgan, I.L., Osborne, S.C., 1992. Stabilization/solidification of wastes containing volatile organic compounds in commercial cementitious waste forms. ASTM Special Technical Publication, vol. 1123. ASTM, Philadelphia, U. S.A, pp. 61–72. Stockmeyer, M., Kruse, K., 1991. Adsorption of Zn and Ni ions and phenol and diethylketones by bentonites of different organophilicities. Clay Minerals 26, 431–434. Tiller, K.G., Gerth, J., Brümmer, G., 1984. The relative affinities of Cd, Ni and Zn for different soil clay fractions and goethite. Geoderma 34, 17–35. Wolfe, T.A., Demirel, T., Baumann, E.R., 1986. Adsorption of organic pollutants on montmorillonite treated with amines. Journal of the Water Pollution Control Federation 58, 68–76.
Simultaneous adsorption of chlorophenol and heavy metal ions on organophilic bentonite S. Andini a , R. Cioffi b , F. Montagnaro a , F. Pisciotta a , L. Santoro a,⁎ a
Dipartimento di Chimica, Università di Napoli Federico II, Complesso Universitario di Monte S.Angelo, Via Cintia, 80126 Napoli, Italy b Dipartimento per le Tecnologie, Università di Napoli Parthenope, Via Ammiraglio F. Acton 38, 80133 Napoli, Italy Received 15 December 2004; received in revised form 24 August 2005; accepted 8 September 2005 Available online 8 November 2005
Abstract Organophilic bentonite obtained by ion exchange with benzyldimethyl octadecylammonium chloride has been used for multiple adsorption of 2-chlorophenol and the metals Pb2+ and Cd2+. This is of interest for the stabilization of wastes in which simultaneous organics/heavy metals contamination occurs. In such cases cement-based processes take advantage from the addition of quaternary ammonium salts exchanged bentonite. The results have shown that any of the contaminants is adsorbed according to a multilayer cooperative mechanism. Due to its organophilic nature, exchanged bentonite is able to adsorb 2-chlorophenol to a very high extent (about 0.7 g/g) when the organic is the only solute. On the other hand, each of the two metal ions is adsorbed to a much lesser extent when alone in solution (about 22 and 2.8 mg/g for Pb2+ and Cd2+, respectively). In the case of simultaneous presence of 2chlorophenol and Pb2+ in solution, the adsorption isotherms of both solutes change to monolayer type and the adsorption capacity strongly decreases (about 0.3 g/g for 2-chlorophenol and 0.6 mg/g for Pb2+). In the case of the 2-chlorophenol/Cd2+ systems, the adsorption capacity of about 0.3 g/g for 2-chlorophenol is retained, while Cd2+ is not adsorbed at all. In all the cases in which the bentonite is in contact with one of the metal ions, it has been checked that no exchange with the ammonium ions takes place. Finally, FT-IR analysis has shown that in all the cases investigated physical adsorption takes place and no new chemical bonds are formed. © 2005 Elsevier B.V. All rights reserved. Keywords: Organophilic bentonite; Adsorption; 2-Chlorophenol; Heavy metals; Quaternary ammonium salts
1. Introduction Bentonites are argillaceous materials that can be effectively employed as adsorbents of many waste water pollutants, namely heavy metal ions and organic compounds. This outstanding capability is due to the presence of the mineral montmorillonite, the smectite clay contained in bentonite rocks. In adsorption pro⁎ Corresponding author. Tel.: +39 081 674028; fax: +39 081 674090. E-mail address: [email protected] (L. Santoro). 0169-1317/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2005.09.004
cesses, raw bentonite acts as an inorganic stationary phase able to effectively adsorb heavy metal ions (Khan et al., 1995; Tiller et al., 1984). However, the nature of bentonite can be easily turned to organophilic as the Group IA and IIA loosely bound metal ions (Li+, Na+, K+, Mg2+, Ca2+ and Ba2+) present between alumina and silica layers can be readily exchanged with quaternary ammonium ions such as [R4N]+ (Jordan, 1949; Jordan et al., 1950). The expulsion of the metal ions with coordinated water molecules strongly reduces the hydrophilicity of the clay and, if at least one of the nitrogen substituents is a long aliphatic
S. Andini et al. / Applied Clay Science 31 (2006) 126–133
chain, the interlayer space increases and the adsorption capacity increases even more. In addition, favourable interactions can arise with the R groups of the quaternary ammonium ions. Organic molecules can be more easily attracted into the expanded interlayer space and, when drawn between the layers of clay, a number of van der Waals interactions and chemical bonds can occur. The latter involve the alumina and silica oxygens which can form hydrogen bonds with the hydrogens of the adsorbed molecules, and also the transition metals of the clay which can form coordination compounds with the adsorbed organics (Gibbson and Soundararajan, 1988). Many papers are found in the literature on the use of quaternary ammonium salt (QAS) exchanged bentonites as adsorbents of many organic compounds from water and other solvents (Boyd et al., 1988; Cioffi et al., 2001a; Dékány et al., 1986a,b,c, 1996; Lagaly, 1994; Liu and Chang, 1992; Lizhong et al., 1996, 1997; Mortland et al., 1986; Natali Sora et al., 2005; Regdon et al., 1998a,b; Stockmeyer and Kruse, 1991; Wolfe et al., 1986). This rather unique characteristic of bentonites is also of great advantage for the stabilization of organic contaminated wastes. It is well known that, among the multiple options for treatment of hazardous wastes in solid, liquid or sludge form, one of the most widely used is stabilization/solidification (S/S) using a cementitious matrix to obtain a monolithic residue. This kind of treatment can allow a safer disposal of the waste and in some cases permits recycling in the field of building materials (Conner, 1990; Spence, 1993). However, while cement-based S/S is effective in the case of inorganic wastes, organic containing ones cannot be satisfactorily treated by this type of process because some interaction occurs between the organic contaminants and the cementitious matrix, thereby compromising the effectiveness of the process and the technological properties of the stabilized product. Many organic compounds are known to have a retarding effect on cement hydration reactions and adversely affect the microstructural, mechanical and leaching properties of the cementitious materials (Cullinane and Bricka, 1993; Eaton et al., 1987; Montgomery et al., 1991a; Ramachandran and Ashton, 1986; Sheffield et al., 1987). QAS exchanged bentonites can help face this problem and many studies are found in the literature on their use as pre-solidification agents in cement-based S/S processes (Alther et al., 1988; Brown et al., 1992; Caldwell et al., 1990; Calvanese et al., 2002; Cioffi et al., 2001b; Hebatpuria et al., 1999; Montgomery et al., 1988, 1991a,b; Sell et al., 1992; Sheriff et al., 1989;
127
Soundararajan et al., 1990; Spence et al., 1992). The effectiveness of modified bentonites in waste disposal is also demonstrated by some patents that have been filed using QAS-clays as adsorbents in view of this application (Beall, 1984, 1987). In some cases mixed organics/heavy metals pollution occurs, especially in sediments (Lee et al., 2000). When this happens, some concern arises on the effectiveness of QAS-bentonites in waste stabilization, as organophilicity could not be retained if heavy metal cations would replace quaternary ammonium ions by ion exchange. So far, this problem has not been addressed by the scientific community, and no papers are found in the literature on QAS-bentonite behaviour in the presence of mixed organics/heavy metals contamination, neither as adsorbents in liquid phase, nor as supporting additives in waste cement stabilization. In this paper the adsorption properties of organophilic bentonite in aqueous phase and in the presence of both organic and heavy metal contamination were studied. To this purpose, 2-chlorophenol (CP), cadmium and lead were selected as contaminants, while bentonite was made organophilic by exchange with benzyldimethyl octadecylammonium (BDMOA) chloride. Adsorption isotherms for any of the three contaminants and the effect of the two heavy metal ions (one at a time) on CP adsorption were determined. Fourier Transform Infra Red (FT-IR) spectroscopy was used as main experimental technique to assess the interactions among the adsorbent and the adsorbates. Other experimental techniques were X-ray diffraction (XRD), Ultra Violet (UV) spectroscopy, Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and High Performance Liquid Chromatography (HPLC). The instruments were a Jasco FT/IR-430 spectrometer, a Philips PW 3710 diffractometer, a Jasco UV-560 spectrometer, a Perkin Elmer ICP-OES Optima 2000 DV spectrometer and a Shimadzu Analytical HPLC System. 2. Materials and methods 2.1. Bentonite characterization The bentonite used in this work comes from a quarry located in Santa Croce di Magliano (Campobasso, Italy) and was supplied by the company “Eredi dott. Settimio Cinicola— Bentonite”. Its chemical composition is shown in Table 1. The presence of quartz and calcite impurities was noted by XRD analysis. The diffraction peak corresponding to the family of planes (001) allowed the basal spacing of the bentonite to be found at the value of 1.468 nm. The cation exchange capacity (CEC) of the bentonite was found by exchange with potassium and sodium chloride to be 0.80 meq/g.
128
S. Andini et al. / Applied Clay Science 31 (2006) 126–133
Table 1 Chemical composition of bentonite, wt.% LOI a
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2 O
MnO
Total
14.75
45.85
14.41
3.53
17.08
1.91
0.94
0.71
0.10
99.28
a
Loss on ignition at 1000 °C.
2.2. Exchanged bentonite preparation The impure bentonite was exchanged with a sodium chloride solution to obtain a Na-exchanged bentonite because the organic cations are strongly preferred in the exchange sites relative to Na+ ions (Wolfe et al., 1986). The exchange of the Na-bentonite with BDMOA-Cl was carried out by dispersing 100 g of bentonite in 4 dm3 of an aqueous solution containing QAS in amount just equal to the CEC of the clay, stirring for 8 h at 50 °C, decanting overnight, filtering, washing with distilled water and drying at 50 °C for 24 h. The solutions were analysed after the exchange to determine their Na+ contents and verify the complete exchange of the sodium ions by the quaternary ammonium cations. After the exchange, the basal spacing of the bentonite was found to increase to 2.008 nm by XRD analysis. 2.3. Adsorption experiments All the adsorption experiments were carried out at 25 °C. The adsorption isotherm of CP on the bentonite exchanged with the ammonium salt was obtained by dispersing different amounts of bentonite in 50 cm3 of CP aqueous solution (12 g/ dm3), stirring for 6 h, decanting and filtering. It was checked that liquid–solid equilibrium was reached after 6 h contact. The solutions were analysed to determine the CP equilibrium concentration by UV spectroscopy. The adsorption isotherms of Cd2+ and Pb2+ were obtained by dispersing 0.12 g of bentonite in 100 cm3 of metal solution with concentration ranging from 2 to 30 mg/dm3. After stirring for 6 h, decanting and filtering, the metal equilibrium aqueous concentration was found by means of ICP-OES. The effect of Cd2+ and Pb2+ on CP adsorption was assessed through the determination of CP isotherm in the presence of the two metal ions (one at a time) in 25 mg/dm3 concentration. The possible release of the ammonium salt due to ion exchange with the metal cations has been verified by means of UV spectroscopy and HPLC. The products obtained in the different experimental conditions have been characterized by means of FT-IR spectroscopy to assess the interactions that occur upon adsorption.
to the classification by Giles et al. (1960) based on the initial slope and the shape of the upper part of the curve, the adsorption isotherm is of class L (Langmuir), subgroup 4. It is a multilayer type isotherm, typical of those obtained by moderate interaction between the organic compound and the bentonite, followed by a cooperative effect of the adsorbed molecules in the multilayer adsorption (Gregg and Sing, 1982). Not always organophilic bentonite gives L4 type isotherms. Cases are found in the literature of L2 type (monolayer Langmuir) isotherms typical of stronger adsorbent–adsorbate interactions (Boyd et al., 1988; Liu and Chang, 1992; Mortland et al., 1986), and S2 type (S shaped) isotherms (Cioffi et al., 2001a; Dékány et al., 1996). In the latter case weak interactions occur between the organic compound and the bentonite at low solute concentration, but, once a molecule is adsorbed, adsorbent–adsorbate interactions increase and promote the adsorption in a cooperative fashion (Gregg and Sing, 1982). Finally, BDMOA-bentonite shows outstanding adsorption capacity to CP, as the concentration of the organic on the bentonite at saturation was found as high as 0.70 g of CP per gram of bentonite. This result is in agreement with those by Natali Sora et al. (2005) who found equally high adsorption capacity of similar organophilic bentonites against 2-chloroaniline. The adsorption isotherm of Pb2+ on the BDMOAbentonite is shown in Fig. 2. Even in this case the isotherm is of L4 type, typical of cooperative multilayer
3. Results and discussion 3.1. Adsorption isotherms The results of the adsorption experiments of CP on the BDMOA-bentonite are shown in Fig. 1. According
Fig. 1. Adsorption isotherm at 25 °C of 2-chlorophenol on BDMOAbentonite.
S. Andini et al. / Applied Clay Science 31 (2006) 126–133
adsorption. However, differently from the CP case, the low adsorbent loading at Pb2+ concentration up to about 0.1 mg/dm3 makes evident that solute–adsorbent interactions are quite weak at low solute concentration. Then, as this concentration increases, cooperative adsorption takes place to a much greater extent and brings the total solid loading to about 22 mg of Pb2+ per gram of bentonite. As expected, the maximum Pb2+ loading is much lower than that of CP. Evidently, the organophilic nature of the BDMOA-bentonite does not favour the adsorption of hydrated metal ions. Fig. 3 shows the adsorption isotherm of Cd2+ on BDMOA-bentonite. Once again the isotherm is of L4 type. However, the adsorption capacity reaches values as low as about 2.8 mg of Cd2+ per gram of bentonite, proving that very weak adsorbent–adsorbate interactions occur in this case. When compared to Pb2+, Cd2+ adsorption is less favoured, due to its higher charge density that increases hydrophilicity. This result is in agreement with literature data by Barbier et al. (2000) on the adsorption of these two metal ions on pure montmorillonite and commercial bentonite. In the adsorption experiments of Pb2+ and Cd2+ no release of the ammonium salt was found. This was checked by UV spectrometry that can detect the presence of the benzyl group through its absorbance at 262 nm. The influence of Pb2+ and Cd2+ on the adsorption of CP has been studied by replicating CP adsorption experiments in the presence of either 25 mg/dm3 of Pb2+ or 25 mg/dm3 of Cd2+. The results are shown in Fig. 4. The isotherms are very different from the case of the adsorption of CP alone. They turn to L2 type (Langmuir monolayer) and the formation of the monolayer takes place at the same equilibrium concentration in both cases. However, the portion of curve corresponding to multilayer cooperative adsorption completely disappears. As a consequence, the adsorption capacity considerably decrea-
Fig. 2. Adsorption isotherm at 25 °C of Pb2+ on BDMOA-bentonite.
129
Fig. 3. Adsorption isotherm at 25 °C of Cd2+ on BDMOA-bentonite.
ses from 0.70 g of CP per gram of bentonite to 0.30 and 0.32 g of CP per gram of bentonite when the solution also contains Pb2+ and Cd2+, respectively. In any CP-metal ion adsorption experiment, the metal ion equilibrium concentration was also measured to ascertain the occurrence of competition between the two adsorbates. In the case of Cd2+, no adsorption takes place at all, as no change of the initial concentration (25 mg/l) was found. This also means that no exchange of Cd2+ with quaternary ammonium ions takes place. This latter issue was also checked by HPLC analysis of the liquid phase at equilibrium. This analytical technique was necessary because the presence of the benzene ring in both CP and ammonium salt makes the relative UV signals overlap. The experiments for simultaneous Pb2+ and CP adsorption lead to different results. The analysis of the relative systems revealed that the bentonite is able to adsorb simultaneously both adsorbates, as shown by the
Fig. 4. Adsorption isotherms at 25 °C of 2-chlorophenol on BDMOAbentonite in the presence of Pb2+ and Cd2+.
130
S. Andini et al. / Applied Clay Science 31 (2006) 126–133
Pb2+ adsorption isotherm in the presence of CP reported in Fig. 5. The comparison of this figure with Fig. 2 (adsorption of Pb2+ alone) makes clear that the presence of CP makes the Pb2+ isotherm change from L4 to L2 type and strongly depresses the adsorption capacity against this metal. It dramatically decreases from about 22 mg per gram of BDMOA-bentonite at about 0.4 mg/l equilibrium concentration to about 0.6 mg per gram of BDMOA-bentonite at about 3.5 mg/l equilibrium concentration. Despite the occurrence of competitive adsorption of Pb2+, it was checked by HPLC analysis that even in this case no exchange with quaternary ammonium ions takes place. In synthesis, the experiments of simultaneous CPmetal ions adsorption demonstrated that: (a) the adsorption capacity of BDMOA-bentonite against CP is strongly reduced by the presence of both Pb2+ and Cd2+; (b) the isotherm type changes from L4 (multilayer Langmuir) to L2 (monolayer Langmuir) in both cases; (c) the effects of Pb2+ and Cd2+ are almost the same; (d) Cd2+ is not adsorbed at all in the presence of CP; (e) the adsorption capacity of BDMOA-bentonite against Pb2+ is strongly reduced by the presence of CP; and (f) no exchange takes place between the ions of each metal and the quaternary ammonium ions. The fact that the adsorption capacity of BDMOA-bentonite is strongly reduced when CP and either Cd2+ or Pb2+ are present in solution can be ascribed to documented interactions between CP and the metal ions of solute–solute type (complexation) reported by Christensen and Christensen (2000) and Krishnamurti and Naidu (2003). In view of application of organophilic bentonite as pre-solidification adsorbent in cementitious systems, the desorption of CP and/or heavy metal ions can be of concern. To this regard, the desorption of CP, due to its acid properties, may well take place in the alkaline
Fig. 5. Adsorption isotherm at 25 °C of Pb2+ on BDMOA-bentonite in the presence of 2-chlorophenol.
Fig. 6. FT-IR spectra of raw bentonite (a), BDMOA-Cl (b) and BDMOA-bentonite (c).
environment of hydrating cement. The extent of CP desorption from BDMOA-bentonite loaded with 0.7 g of the organic per gram of bentonite in a 0.1 M NaOH solution was found about 28% (Maffucci, 1999). It is believed that this result may not be of concern, as the pH of hydrating cement is one–two units lower than that of the NaOH solution used in the desorption experiment referred to above. Furthermore, pH is even lower in a S/S system were cement is mixed with waste. Equally, no concern should arise from heavy metal ions desorption, as it is well known that cementitious systems are particularly well suited for the stabilization of these pollutants. 3.2. Infrared and X-ray characterization FT-IR spectroscopy was used for characterizing the materials and the type of interactions that occur in the different experimental conditions tested. The FT-IR spectra of raw bentonite, BDMOA-Cl and BDMOA-bentonite are reported in Fig. 6. The spectrum of raw bentonite (trace a in Fig. 6) shows typical infrared bands of montmorillonite due to ỌH stretching at 3620 and 3423 cm− 1 and ṢiỌSi bending at 1043 cm− 1 (Farmer, 1974). Raw bentonite shows also a wide and intense band at 1430 cm− 1 and two weaker bands at 914 and 874 cm− 1. These bands are due to ỌH bending of, respectively, Ca2+, Mg2+ and Fe3+ hydration water (Farmer, 1974). Trace b in Fig. 6 is the FT-IR spectrum of BDMOA-Cl and shows two sharp bands characteristic of the aliphatic C̣H stretching at 2920 and 2852 cm− 1 and some very weak bands in the region 3056–3008 cm− 1 due to the aromatic C̣H stretching (Bellamy, 1958). The spectrum of BDMOAbentonite results in trace c of Fig. 6 which, with the exception of the bands at 1430, 914 and 874 cm− 1 that
S. Andini et al. / Applied Clay Science 31 (2006) 126–133
completely disappear, is the almost perfect superimposition of the two previous traces, meaning that the ion exchange of Na-bentonite with BDMOA-Cl takes place as expected. Also, the FT-IR results show that no new bonds are formed between BDMOA and bentonite after exchange. Fig. 7 shows the results of FT-IR characterization of BDMOA-bentonite after adsorption of CP, Pb2+ and Cd2+ in the conditions of maximum loading. Trace a in Fig. 7 is the spectrum of BDMOA-bentonite and is the same as trace c in Fig. 6. Trace b (Fig. 7) is the result of CP infrared analysis and is mainly characterized by many strong bands in the region 1660–650 cm− 1. Trace c is the spectrum of CP-loaded BDMOA-bentonite at maximum sorption (0.7 g/g). It is seen that all the infrared bands characteristic of CP spectrum disappear. According to Montgomery et al. (1991b), this result may be due to the fact that van der Waals interactions can promote so compact and tight packing within the exchanged clay interlayer space that adsorbed CP vibrational motion is strongly hindered. In CP adsorption only physical interactions occur, as no new infrared band is observed in trace c with respect to traces a and b. Traces d and e are relative to Pb2+ and Cd2+ loaded BDMOA-bentonite and both differ from trace a only for the presence of an infrared band at 1384 cm− 1. This band is due to the presence of NO3− as the metals
131
adsorption experiments were carried out with the relative nitrates. Once again, no extra bands are present after adsorption, meaning that only physical adsorption takes place for both metals. BDMOA-bentonite after adsorption of CP, Pb2+ and 2+ Cd was characterized by means of XRD analysis, too. In the case of CP adsorption, the basal spacing was found to decrease from 2.008 nm to 1.890 nm in the condition of maximum loading. This may be due to the fact that CP adsorption increases the organophilic character of BDMOA-bentonite causing the expulsion of interlayer residual water and denser packing. This is in agreement with the results of FT-IR characterization which showed CP to be tightly packed within the clay interlayer spacings. In the cases of Pb2+ and Cd2+ adsorption no change of basal spacing was found, meaning that the adsorption of the two metals does not interfere with the molecules arrangement within the interlayer space. 4. Conclusions The adsorption of 2-chlorophenol on benzyldimethyl octadecylammonium exchanged bentonite follows a L4 mechanism (multilayer cooperative). Due to the bentonite organophilicity, loading as high as 0.7 g/g can be achieved. On the other hand, the adsorption capacity against lead and cadmium is from one to two orders of magnitude lower. Even in cases of the two metals the adsorption isotherm is of L4 type. The adsorption of 2-chlorophenol changes from L4 multilayer cooperative to L2 monolayer when the organic is in the presence of Pb2+ or Cd2+. Also, the adsorption capacity strongly reduces. The same happens for Pb2+ in the presence of 2-chlorophenol. However, in the same conditions Cd2+ is not adsorbed at all. Interactions of adsorbent–adsorbate and adsorbate–adsorbate (solute–solute) type are responsible for this behaviour. When in contact with any of the heavy metal ions, the bentonite does not release ammonium ions by ion exchange and its organophilic nature is retained. FT-IR analysis has shown that no new chemical bonds are formed after adsorption, and also that 2chlorophenol, once adsorbed, is tightly packed within the clay interlayer space. Acknowledgements
Fig. 7. FT-IR spectra of BDMOA-bentonite (a), 2-chlorophenol (b), 2chlorophenol loaded bentonite (c), Pb2+ loaded bentonite (d) and Cd2+ loaded bentonite (e).
The authors wish to thank Dr. Filippo Cinicola for supplying the bentonite, Prof. Flavia Nastri for the help and assistance in HPLC analysis and Dr. Francesco
132
S. Andini et al. / Applied Clay Science 31 (2006) 126–133
Modestia for the help and assistance in ICP-OES analysis. References Alther, G.R., Evans, J.C., Pancoski, S.E., 1988. Organically modified clays for stabilization of organic hazardous wastes. Proc. 9th Nat. Conf. “Superfund ‘88”, Washington, DC, 28–30 November, pp. 440–445. Barbier, F., Duc, G., Petit-Ramel, M., 2000. Adsorption of lead and cadmium ions from aqueous solution to the montmorillonite/water interface. Colloids and Surfaces. A, Physicochemical and Engineering Aspects 166, 153–159. Beall, G.W., 1984. Organic Waste Disposal, U.S. Patent No. 4473477. Beall, G.W., 1987. Method of Immobilizing Organic Contaminants to Form a Non-flowable Matrix, U.S. Patent No. 4650590. Bellamy, L.J., 1958. The Infra-red Spectra of Complex Molecules. Methuen & Co. Ltd, London. Boyd, S.A., Shaobai, S., Lee, J.F., Mortland, M.M., 1988. Pentachlorophenol sorption by organo-clays. Clays and Clay Minerals 36 (2), 125–130. Brown, R.E., Jindal, B.S., Bulzan, J.D., 1992. A critical review of the effectiveness of stabilization and solidification of hazardous organic wastes. ASTM Special Technical Publication, vol. 1123. ASTM, Philadelphia, U.S.A., pp. 43–60. Caldwell, R.J., Coté, P.L., Chao, C.C., 1990. Investigation of solidification for the immobilization of trace organic contaminants. Hazardous Waste and Hazardous Materials 7 (3), 273–282. Calvanese, G., Cioffi, R., Santoro, L., 2002. Cement stabilization of tannery sludge using quaternary ammonium salt exchanged bentonite as pre-solidification adsorbent. Environmental Technology 23, 1051–1062. Cioffi, R., Costanzo, S., Maffucci, L., Santoro, L., 2001a. Adsorption of the organic fraction of a tannery sludge by means of organophilic bentonite. Environmental Technology 22, 83–89. Cioffi, R., Maffucci, L., Santoro, L., Glasser, F.P., 2001b. Stabilization of chloro-organics using organophilic bentonite in a cement-blast furnace slag matrix. Waste Management 21, 651–660. Conner, J.R., 1990. Chemical Fixation and Solidification of Hazardous Wastes. Van Nostrand Reinhold, New York. Christensen, J.B., Christensen, T.H., 2000. The effect of pH on the complexation of Cd, Ni and Zn by dissolved organic carbon from leachate-polluted groundwater. Water Research 34 (15), 3743–3754. Cullinane, M.J., Bricka, R.M., 1993. Principles and Practices for Petroleum Contaminated Soils. Lewis Publishers, Boca Raton, Florida. Dékány, I., Szántó, F., Weiss, A., Lagaly, G., 1986a. Interactions of hydrophobic layer silicates with alcohol-benzene mixtures: I. Adsorption isotherms. Berichte der Bunsen-Gesellshaft für Physikalische Chemie 90, 422–427. Dékány, I., Szántó, F., Nagy, L.G., 1986b. Sorption and immersional wetting on clay minerals having modified surface. Journal of Colloid and Interface Science 109 (2), 376–384. Dékány, I., Szántó, F., Weiss, A., Lagaly, G., 1986c. Interactions of hydrophobic layer silicates with alcohol–benzene mixtures: II. Structure and composition of the adsorption layer. Berichte der Bunsen-Gesellshaft für Physikalische Chemie 90, 427–431. Dékány, I., Farkas, A., Király, Z., Klumpp, E., Narres, H.D., 1996. Interlamellar adsorption of 1-pentanol from aqueous solution on
hydrophobic clay mineral. Colloids and Surfaces. A, Physicochemical and Engineering Aspects 119, 7–13. Eaton, H.C., Walsh, M.B., Tittlebaum, M.E., Cartledge, F.K., Chalasani, D., 1987. Organic interference of solidified/stabilized hazardous wastes. Environmental Monitoring and Assessment 9 (2), 133–142. Farmer, V.C., 1974. The Infrared Spectra of Minerals. Mineralogical Society, London. Gibbson, J.J., Soundararajan, R., 1988. The nature of chemical bonding between modified clay minerals and organic waste materials. American Laboratory Magazine 20 (7), 38–46. Giles, C.H., MacEwan, T.H., Nakhwa, S.N., Smith, D., 1960. Studies in adsorption: Part XI. A system of classification of solution adsorption isotherms and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids. Journal of Chemical Society 3973–3993. Gregg, S.J., Sing, K.S.W., 1982. Adsorption, Surface Area and Porosity. Academic Press, New York. Hebatpuria, V.M., Arafat, H.A., Bishop, P.L., Pinto, N.G., 1999. Leaching behaviour of selected aromatics in cement-based solidification/stabilization under different leaching tests. Environmental Engineering Science 16 (6), 451–463. Jordan, J.W., 1949. Organophilic bentonites: I. Swelling in organic liquids. Journal of Physical and Colloid Chemistry 53, 294–306. Jordan, J.W., Hook, B.J., Finlayson, C.M., 1950. Organophilic bentonites: II. Organic liquid gels. Journal of Physical and Colloid Chemistry 54, 1196–1208. Khan, S.A., Riaz-Ur-Rehman, Khan, M.A., 1995. Adsorption of chromium(III), chromium(IV) and silver(I) on bentonite. Waste Management 15, 271–282. Krishnamurti, G.S.R., Naidu, R., 2003. Solid-solution equilibria of cadmium in soils. Geoderma 113 (1), 17–30. Lagaly, G., 1994. Bentonites: adsorbents of toxic substances. Progress in Colloid and Polymer Science 95, 61–72. Lee, C.L., Song, H.J., Fang, M.D., 2000. Concentrations of chlorobenzenes, hexachlorobutadiene and heavy metals in surficial sediments of Kaohsiung coast, Taiwan. Chemosphere 41 (6), 889–899. Liu, M.L., Chang, H.F., 1992. Study on treatment of organic wastewater with modified bentonite adsorbent. Fundamentals of Adsorption, Proc. IVth Int. Conf. On Fundamentals of Adsorption, Kyoto (Japan), 17–22 May 1992, pp. 381–388. Lizhong, Z., Jianying, Z., Yimin, L., Xueyou, S., Wenbin, Q., 1996. Organobentonites as adsorbents for some organic pollutants and its application in wastewater treatment. Journal of Environmental Sciences 8 (3), 378–383. Lizhong, Z., Yimin, L., Jianying, Z., 1997. Sorption of organobentonites to some organic pollutants in water. Environmental Science and Technology 31, 1407–1410. Maffucci, L., (1999). PhD thesis, University of Naples Federico II, Naples, Italy. Montgomery, D.M., Sollars, C.J., Sheriff, T.S., Perry, R., 1988. Organophilic clays for the successful stabilisation/solidification of problematic industrial wastes. Environmental Technology Letters 9, 1403–1412. Montgomery, D.M., Sollars, C.J., Perry, R., 1991a. Optimization of cement-based stabilization/solidification of organic-containing industrial wastes using organophilic clays. Waste Management and Research 9, 21–34. Montgomery, D.M., Sollars, C.J., Perry, R., Tarling, S.E., Barnes, P., Henderson, E., 1991b. Treatment of organic-contaminated indus-
S. Andini et al. / Applied Clay Science 31 (2006) 126–133 trial wastes using cement-based stabilization/solidification. Waste Management and Research 9, 113–125. Mortland, M.M., Shaobai, S., Boyd, S.A., 1986. Clay-organic complexes as adsorbents for phenol and chlorophenols. Clays and Clay Minerals 34 (5), 581–585. Natali Sora, I., Pelosato, R., Zampori, R., Botta, D., Dotelli, G., Vitelli, M., 2005. Matrix optimisation for hazardous organic waste sorption. Applied Clay Science 28, 43–54. Ramachandran, V.S., Ashton, H.E., 1986. Trends in building material research. Materials and Structures 19, 337–342. Regdon, I., Király, Z., Dékány, I., Lagaly, G., 1998a. Microcalorimetric studies of S/L interfacial layers: thermodynamic parameters of the adsorption of butanol–water on hydrofobized clay minerals. Progress in Colloid and Polymer Science 109, 214–220. Regdon, I., Dékány, I., Lagaly, G., 1998b. A new way for calculating the adsorption capacity from surface excess isotherms. Colloid and Polymer Science 276, 511–517. Sell, N.J., Revall, M.A., Bentley, W., McIntosh, T.H., 1992. Solidification and stabilization of phenol and chlorinated phenol contaminated soil. ASTM Special Technical Publication, vol. 1123. ASTM, Philadelphia, U.S.A., pp. 73–85. Sheffield, A., Makena, S., Tittlebaum, M.E., Eaton, H.C., Cartledge, F.K., 1987. Effects of three organic on selected physical properties of type I portland cement. Hazardous Waste and Hazardous Materials 4 (3), 273–286.
133
Sheriff, T.S., Sollars, C.J., Montgomery, D.M., Perry, R., 1989. The use of activated charcoal and tetra-alkylammonium-substituted clays in cement-based stabilization/solidification of phenols and chlorinated phenols. ASTM Special Technical Publication, vol. 1033. ASTM, Philadelphia, U.S.A., pp. 273–286. Soundararajan, R., Barth, E.F., Gibbson, J.J., 1990. Using an organophilic clay to chemically stabilize waste containing organic compounds. Hazardous Materials Control 3, 42–45. Spence, R.D., 1993. Chemistry and Microstructure of Solidified Waste Forms. Lewis Publishers, Boca Raton, Florida. Spence, R.D., Gilliam, T.M., Morgan, I.L., Osborne, S.C., 1992. Stabilization/solidification of wastes containing volatile organic compounds in commercial cementitious waste forms. ASTM Special Technical Publication, vol. 1123. ASTM, Philadelphia, U. S.A, pp. 61–72. Stockmeyer, M., Kruse, K., 1991. Adsorption of Zn and Ni ions and phenol and diethylketones by bentonites of different organophilicities. Clay Minerals 26, 431–434. Tiller, K.G., Gerth, J., Brümmer, G., 1984. The relative affinities of Cd, Ni and Zn for different soil clay fractions and goethite. Geoderma 34, 17–35. Wolfe, T.A., Demirel, T., Baumann, E.R., 1986. Adsorption of organic pollutants on montmorillonite treated with amines. Journal of the Water Pollution Control Federation 58, 68–76.