Environ. Sci. Technol. 2001, 35, 4260-4264
Sorption of Nonpolar Aromatic Contaminants by Chlorosilane Surface Modified Natural Minerals PETRA HUTTENLOCH,* KARL ERNST ROEHL, AND KURT CZURDA Department of Applied Geology, University of Karlsruhe, Kaiserstrasse 12, D-76128 Karlsruhe, Germany
The efficacy of the surface modification of natural diatomite and zeolite material by chlorosilanes is demonstrated. Chlorosilanes used were trimethylchlorosilane (TMSCl), tert-butyldimethylchlorosilane (TBDMSCl), dimethyloctadecylchlorosilane (DMODSCl), and diphenyldichlorosilane (DPDSCl) possessing different headgroups and chemical properties. Silanol groups of the diatomite and zeolite were modified by chemical reaction with the chlorosilanes resulting in a stable covalent attachment of the organosilanes to the mineral surface. The alteration of surface properties of the modified material was proved by measurements of water adsorption capacity, total organic carbon (TOC) content, and thermoanalytical data. The surface modified material showed great stability even when exposed to extremes in ionic strength, pH, and to pure organic solvents. Sorption of toluene, o-xylene, and naphthalene from water was greatly enhanced by the surface modification compared to the untreated materials which showed no measurable sorption of these compounds. The enhanced sorption was dependent on the organic carbon content as well as on chemical characteristics of the chlorosilanes used. Batch sorption experiments showed that the phenyl headgroups of DPDSCl have the best affinity for aromatic compounds. Removal from an aqueous solution of 10 mg/L of naphthalene, o-xylene, and toluene was 71%, 60%, and 30% for surface modified diatomite and 51%, 30%, and 16% for modified clinoptilolite, respectively. Sorption data were well described by the Freundlich isotherm equation, which indicated physical adsorption onto the lipophilic surface rather than partitioning into the surface organic phase. The chlorosilane modified materials have an apparent potential for application in environmental technologies such as permeable reactive barriers (PRB) or wastewater treatment.
Introduction Organic contaminants, especially nonionic hydrophobic organic compounds such as PAH and BTEX, the latter with relatively high water solubility, are mobile in soils and sediments with low organic matter contents, therefore bearing the risk of groundwater contamination. Development and evaluation of new materials for the sorption of organic contaminants still remains in the focus of innovative applications in water treatment and in-situ remediation technologies such as permeable reactive barriers (1, 2). * Corresponding author phone: +49 0 721 608 7610; fax: +49 0 721 606 279; e-mail:
[email protected]. 4260
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Sorptive materials to be used in permeable reactive barriers have to fulfill the sometimes conflicting conditions of being granular to ensure an adequate hydraulic permeability on one hand and being rather fine-grained to have a large surface area offering a sufficient number of sorption sites on the other hand. The sorption properties of mineral materials toward organic contaminants can often be improved significantly by surface modification. The potential of using natural zeolites and clays modified by cationic surfactants for the removal of organic contaminants from aqueous solution was studied by several investigators (3-10). The mechanism of surface modification is based on the exchange of extrastructural cations of the zeolites and clays by organic cations such as hexadecyltrimethylammonium (HDTMA) up to the external cation exchange capacity (CEC) enhancing the surface affinity for organic contaminants. The ability of HDTMA to increase the sorption capacity of zeolite materials was shown for BTEX compounds (3), for benzene, aniline, and phenol (4) and for perchloroethylene (PCE) (5). The removal of inorganic oxyanions such as sulfate, chromate, and selenate from aqueous solution by HDTMA modified zeolite was also studied (6). Li et al. (7) proposed a combination of a reductive material (zerovalent iron) with a sorptive material represented by HDTMA-modified zeolites to enhance the removal of chromate and PCE from contaminated groundwater. Organoclays are mostly bentonites modified by cationic surfactants such as quaternary ammonium cations or similar long-chain molecules to increase their ability to sorb organic contaminants from aqueous solution (8, 9) and contaminated soils (10). Song et al. (11) used octadecyltrichlorosilane and octadecyltrimethoxysilane for characterization of montmorillonite surfaces after modification with respect for application in encapsulating systems and storage of hazardous materials. A new approach discussed in this paper is the use of chlorosilane modified diatomite and zeolite to sorb organic aromatic compounds from aqueous solution. Diatomite, also known as kieselgur, is a siliceous sedimentary rock consisting of fossilized remains of diatoms. The siliceous skeletons arise from condensing silicic acid that results in a threedimensional polymerization of silicatetrahedrons and builds a X-ray-amorphous opal structure (12). Zeolites are tectosilicates with three-dimensional alumosilicate structure containing water molecules, alkali, and alkaline earth metals in their structural framework (13). To change the character of the diatomite and zeolite surface, respectively, from polar to lipophilic the silanol groups of the mineral surface can be altered by chemical reaction. One way to achieve this is the modification of the material with chlorosilanes. Chlorosilanes are reactive chemical compounds possessing up to three nonpolar aliphatic or aromatic moieties which can be grafted covalently to the silanol groups on the mineral surface. This provides a stable chemical linkage between the organosilane and the bulk material in contrast to electrostatic interactions occurring in connection with cationic surfactants. Similar types of organosilane modified silica proved to be very effective for chromatographic purposes in reversed phase high performance liquid chromatography (HPLC) for standard chemical analytics (14, 15). Objective of the present paper is to demonstrate the efficacy of the silanization process for diatomite and zeolite.
Materials and Methods Diatomite and Zeolite. The diatomite was obtained from United Minerals. It is a calcined product (AXIS FINE) with a particle size range of 0.25-0.85 mm, consisting of 90% of 10.1021/es010131f CCC: $20.00
2001 American Chemical Society Published on Web 09/26/2001
TABLE 1. Empirical and Structural Formula and Molar Mass of the Chlorosilanes Used for Surface Modification of Diatomite and Zeolite: Experimental Conditions of the Silanization Reaction
FIGURE 1. Reaction scheme for the surface modification with chlorosilanes representative for the diatomite.
a Quantity of chlorosilane used for 10 g of diatomite or clinoptilolite in 40 mL of pyridine.
X-ray-amorphous silica. Cation exchange capacity (CEC), determined by the ammonium acetate method (16), was 5 mequiv/100 g. Representing natural zeolites, a clinoptiloliterich tuff from Northern Carpathia with a particle size range of 0.2-1.0 mm was selected for this study. The mineral content is 90% clinoptilolite, plus additional quartz, feldspar, and illite. The CEC was 145 mequiv/100 g (ammonium acetate method). Surface Modification. The chlorosilanes used were trimethylchlorosilane (TMSCl), tert-butyldimethylchlorosilane (TBDMSCl), dimethyloctadecylchlorosilane (DMODSCl), and diphenyldichlorosilane (DPDSCl) characterized by different headgroups (Fluka, purity > 98%). Their empirical formula, the headgroups, and the corresponding reaction conditions are shown in Table 1. Before surface modification the diatomite and zeolite materials were dried at 105 °C for 24 h. Ten grams of dried material was weighted into a two-necked flask and 40 mL of pyridine as well as 92 mmol of the corresponding chlorosilane were added (Table 1). Pyridine was used as an organic solvent and a buffer for the developing hydrochloric acid. The mixture was heated under nitrogen atmosphere to the temperature indicated in Table 1 (10 °C below boiling point of the most volatile component) for 24 h in a temperature controlled heating bath. The whole experimental setup was designed in a way to ensure anhydrous conditions in order to prevent hydrolysis of the chlorosilanes. Following the surface silanization, the modified material was filtered and washed with distilled water and acetone and dried at 105 °C for 24 h. Additionally, for DPDSCl the reaction time was extended to 48 h, and the whole silanization procedure was repeated to improve surface loading and sorption capacity. Moreover, instead of using the clinoptilolite as received, it was transformed into the H-form (17) before surface modification. This was achieved by dealumination with 2 M HCl for 1 h (30 mL of acid per gram zeolite) and subsequent drying at 750 °C. Sorption Tests. Sorption isotherms were prepared for o-xylene, toluene, and naphthalene for the surface modified
diatomite and clinoptilolite and the untreated raw materials. Dried material (0.3 g) was placed into a 20 mL glass vial and treated with 20 mL of aqueous solution of the organic contaminants (concentration ranges for o-xylene 1-30 mg/ L, toluene 1-100 mg/L, and naphthalene 1-20 mg/L). The vial was crimp-sealed using a Teflon-lined septum. The samples were shaken at room temperature. Kinetic batch sorption experiments showed that 24 h equilibration time was sufficient. After equilibrium was reached, the samples were centrifuged at 2500 rpm. Duplicate samples and blanks were prepared for each solution concentration. Sorption of the organic contaminants was calculated from the difference between initial and final solute concentrations. The results were corrected for the blanks. Stability Experiments. Sorption of the organic compounds to the organophilic material was also determined in batch experiments with matrix solutions at pH 3 and pH 10 (adjusted by HCl and NaOH, respectively) and with matrix solutions of different ionic strengths (0.01 M and 1 M CaCl2). Further samples were treated with pure organic solvent (oxylene), regenerated at 105 °C for 24 h, and again subjected to sorption tests. In addition, following the batch experiments the material was regenerated several times, and again the sorption behavior was determined. For all stability experiments the reaction time was 4 d. Analytics. Before analyzing the equilibrium solute concentrations, 15 mL of the batch solution was extracted with 5 mL of isooctane (for o-xylene and toluene) and cyclohexane (for naphthalene), respectively. Then 2 mL of the organic solution was filled in a 2 mL sample vial. The organic compounds were analyzed using a Hewlett-Packard 5890 gas chromatograph equipped with a flame ionization detector, a 10 m long HP-5 capillary column (5% phenyl methylsilicone), and an autosampler. The temperature of the injector and the detector was 250 °C, the heating program (oven) for o-xylene and toluene was 40-110 °C (heating rate 5 °C/min to 70 °C and 20 °C/min to 110 °C, retention time 7.3 and 3.8 min) and for naphthalene 120 °C to 190 °C (heating rate 10 °C/min, retention time 2.8 min). Total organic carbon (TOC) content of solid samples (crushed and dried at 60 °C) was determined using a carbon analyzer from Elementar Analysensysteme GmbH. Water adsorption capacity was determined with the device of Enslin/ Neff. Simultaneous thermoanalysis (STA) was performed in oxygen atmosphere with a STA 409 from Netzsch. Samples (0.06 g) were air-dried and crushed. The temperature program started at 20 °C with a heating rate of 5 °C/min and ended at 800 °C.
Results and Discussion The basic silanization reaction for diatomite is shown in Figure 1. Silicon compounds have a high affinity to oxygen. Especially chlorosilanes react readily with free hydroxyl groups to form a thermodynamically stable silicon-oxygen bond. The separation of chlorine from the silane and hydrogen from the silanol group, forming HCl, enables a covalent attachment of a SiR3-group (chlorosilane) to a silanol group (mineral surface), forming a Si-O-Si-C moiety (Figure 1). By this process many different nonpolar headgroups (like VOL. 35, NO. 21, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Properties of Natural, Untreated Diatomite Compared to Surface Modified Diatomite diatomite
TOC water adsorption naphthalene content capacity sorptiona chlorosilane [weight %] [weight %] [%]
untreated surface TMSCl modified TBDMSCl DPDSCl DMODSCl
0.1 1.3 2.1 1.6 5.8
120 9 0 0 0
NDb 20 17 56 33
a Naphthalene removed from a 10 mg/L aqueous naphthalene solution (0.3 g diatomite treated 24 h with 20 mL of solution). b ND, not detected.
R1 ) methyl, phenyl, and R2 ) methyl, tert-butyl, dimethyloctadecyl, or Cl) can be introduced to the surface of an inorganic bulk material. The more free silanol groups are present on the surface of the material the more organosilane molecules can be attached. Generally, aliphatic or aromatic moieties can be present in the organosilane which increase the sorption capacity for organic compounds. The possible types of interactions between the headgroups and organic contaminants can be of hydrophobic nature (van der Waals forces) or can be enhanced further by attraction of aromatic moieties (π-π electron stacking interactions). Preliminary experiments were performed in order to find the chlorosilane type leading to the best sorption behavior for hydrophobic aromatic compounds. The diatomite surface was modified with chlorosilanes possessing different headgroups (Table 1). Physical and chemical properties of the surface modified diatomite changed dramatically dependent on the chlorosilanes employed. Water adsorption capacity, TOC content, and the extent of naphthalene sorption from a 10 mg/L aqueous naphthalene solution are shown in Table 2 for surface modified diatomite compared to natural diatomite. TOC increased from 0.1% of the untreated diatomite up to 5.8% in case of the dimethyloctadecylchlorosilane (DMODSCl) treated sample, whereas the water adsorption capacity was reduced from 120% for the natural diatomite to zero due to the resulting lipophilic nature of the modified diatomite surface (Table 2). The sorption experiments with naphthalene showed that the sorption behavior for hydrophobic aromatic compounds is dependent not only on the increased TOC content of the treated samples but also on the chemical characteristics of the corresponding chlorosilanes. Small methyl or tert-butyl groups of trimethylchlorosilane and tert-butydimethylchlorosilane, respectively, have a low affinity for aromatic organic compounds although TOC was increased up to 1.3% and 2.1%, respectively (Table 2). The dimethyloctadecylsilane group showed a slightly increased affinity compared to the latter silane groups although here the TOC was highest with 5.8 %. DMODSCl incorporates an 18-carbon chain in its octadecyldimethyl head, comparable in length and polarity to the cationic surfactant HDTMA (16-carbon chain group), which was often used for surface modification of zeolites and clay minerals by cation exchange (3-7, 9, 10). The increased alkyl chain length and thus the higher hydrophobicity achieved by the dimethyloctadecyl group results in an enhanced sorption capacity. However, this can even be exceeded by applying diphenyldichlorosilane (DPDSCl) with two small aromatic headgroups to the diatomite surface. Despite the relatively low TOC content of 1.6% the sorption capacity was nearly doubled which is most likely a consequence of the presence of the two phenyl groups. Recent studies also ascertained (18-20) that sorption of hydrophobic organic compounds on natural sediments was also dependent 4262
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TABLE 3. Optimization (Control: Naphthalene Sorption) of the Surface Modification Conditions for Diatomite and Clinoptilolitec material surface modified diatomite
reaction TOC content naphthalene time [h] [weight %] sorptiona [%] 24 48 2 × 48b
untreated clinoptilolite surface modified clinoptilolite 24 2 × 48b surface modified clinoptilolite 2 × 48b (H-form)
1.6 2.1 2.8 0.1 1.1 1.6 1.7
56 62 70 0 35 42 54
a Naphthalene removed from a 10 mg/L aqueous naphthalene solution (0.3 g of diatomite treated 24 h with 20 mL of solution). b Two successive treatments for 48 h each. c Chlorosilane used: DPDSCl.
on the content as well as the compositionsas defined by aromaticity and polaritysof the organic matter in soils and sediments. Due to the results obtained with the different chlorosilanes, DPDSCl was chosen for all further studies. Reaction conditions were varied in order to improve the sorption capacity for hydrophobic aromatic compounds. The reaction time was increased from 24 h to 48 h, and the whole procedure was repeated with single modified material (double treatment). Additionally, the clinoptilolite was dealuminized to the H-form before surface modification. Resulting TOC content and sorption behavior for naphthalene are presented in Table 3. For surface modified diatomite and clinoptilolite, TOC content as well as sorption capacity for naphthalene increased for the longer reaction time of 48 h and the double treatment. Naphthalene sorption was enhanced by 20% for double treatment with DPDSCl relative to surface alteration for 24 h for diatomite and 16% in case of clinoptilolite, respectively. Additionally, the dealumination of clinoptilolite, resulting in a higher amount of silanol groups, improved sorption capacity by 29% relative to clinoptilolite with no acid pretreatment. The TOC contentscaused by DPDSClsis 60% higher for diatomite compared to clinoptilolite due to a higher amount of surface silanol groups. Simultaneous thermoanalysis (STA) results for untreated and surface modified diatomite are given in Figure 2. The surface modified clinoptilolite showed similar behavior during the STA analysis (not shown). The STA measurements clearly represent the changes of physical and chemical properties of the modified compared to the natural, untreated materials. The figure shows the weight loss curves (TG), the corresponding differential curves (DTG), and further the differential thermoanalytical curve (DTA) which indicates endothermic and exothermic reactions during heating the sample to 800 °C. Initial loss of water from the untreated diatomite (2.5%) is indicated by an endothermic reaction peak in the temperature range of 50-250 °C (Figure 2a). The dehydration process in this temperature range is also characteristic for clay minerals and zeolites (21). In comparison, the organophilic diatomite (Figure 2b) shows no measurable weight loss up to 200 °C. In the range of 200-600 °C the organophilic diatomite shows a weight loss of approximately 7% due to the volatilization of diphenyl groups. A significant reaction can also be observed for the break of the covalent bond between the silane and the mineral surface. The break of the bonding occurs in two steps indicated by two minima in the DTG curve at 335 °C and 544 °C. This behavior can be explained by two different bonding arrangements of the DPDSCl which possesses two chlorine groups to join with surface silanol groups (Figure 3).
FIGURE 2. Thermoanalytical data for (a) untreated diatomite and (b) diatomite surface modified with DPDSCl. FIGURE 4. Sorption isotherms of toluene, o-xylene, and naphthalene for (a) diatomite and (b) clinoptilolite surface modified by DPDSCl.
TABLE 4. Freundlich Parameters for the Adsorption of Toluene, o-Xylene, and Naphthalene on Diatomite and Clinoptilolite Surface Modified with DPDSCl (Double Treatment, 48 h Each Treatment), Compared to the Water Solubility of the Organic Compounds (r2 > 0.99 for All Isotherms)
FIGURE 3. Possible binding modes of DPDSCl to the surface silanol groups: (a) double joints to two adjacent surface silanol groups; (b1) simple covalent bond to the diatomite surface, or (b2) to an organosilanol group after two successive treatments. In the case of double bonding to two adjacent surface silanol groups (Figure 3a) more energy was needed to break this covalent bond than for single bonding (Figure 3b). Double joined chlorosilanes on mineral surfaces were also described by Ogawa et al. (22). The second peak of the DTG was accompanied by an exothermic peak of the DTA at 550 °C (Figure 2). No clear DTA peak could be assigned to the first DTG minimum. The surface modified diatomite and clinoptilolite, respectively, showed no significant change of their sorption behavior toward aromatic compounds when exposed to extremes in pH and ionic strength. Also after treatment with pure organic solvent and subsequent regeneration no change in sorption behavior could be observed. The material in general could be regenerated at 60 °C for 24 h with no measurable loss in sorption ability. It can be concluded from these studies that the surface modification of the two materials exhibits great stability even under extreme conditions which enables the application in permeable reactive barriers and wastewater treatment plants.
surface modified diatomite
surface modified clinoptilolite
compound
water solubility [mg/L]
KF [µmol1-N g-1LN]
NF [-]
KF [µmol1-N g-1LN]
NF [-]
toluene o-xylene naphthalene
515 175 30
0.05 0.22 0.41
0.88 0.78 0.79
0.03 0.11 0.21
0.85 0.66 0.81
To describe the sorption behavior of the surface modified materials adsorption isotherms were prepared for o-xylene, toluene, and naphthalene. Freundlich’s equation for nonlinear sorption was applied to the results in the linearized form
log CS ) log KF + NF log CW where CS is the amount of the solute sorbed per unit mass of the sorbent, CW is the equilibrium solute concentration, and KF and NF are empirical parameters specific to the sorption material used for each material. The Freundlich parameters are derived from the intercept and the slope, respectively, of the line which is obtained by plotting log CS versus log CW. For NF ) 1 a linear isotherm results, with KF becoming the distribution coefficient Kd. Freundlich sorption isotherms obtained for o-xylene, toluene, and naphthalene with surface modified diatomite and clinoptilolite by DPDSCl (double treatment) are shown in Figure 4. For all isotherms the linear correlation coefficients were better than 0.99. The resulting Freundlich parameters and the solubility of the target organic contaminants (23) are compiled in Table 4. Untreated diatomite and clinoptilolite did not show any affinity for these organic solvents. VOL. 35, NO. 21, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The sorption behavior of surface modified diatomite and clinoptilolite with DPDSCl followed the trend naphthalene > o-xylene > toluene. The KF values indicate that the sorption capacity is correlated with the water solubility of the organic compounds (Table 4). This is due to the hydrophobic sorption of aromatic compounds which increased with decreasing solubility of the solvent (24, 25). Chiou et al. (20, 26) stated that linear isotherms for hydrophobic organic compounds were evidence for partitioning (dissolving) into an organic phase. The nonlinearity of the isotherm (N * 1) of the modified material suggests that the diphenyl groups, which are attached covalently to the mineral surface, act as a hydrophobic surface rather than a hydrophobic phase. Therefore the hydrophobic organic compounds were most likely adsorbed physically on the diphenyl groups of the modified surface and were not partitioned into the organic phase. A better coverage of the diatomite surface with DPDSCl (as proved by an increased TOC content) resulted in a better sorption capacity compared to the surface modified clinoptilolite. Nonlinear sorption of toluene, benzene, and naphthalene due to surface adsorption had previously been observed on surfactant modified clay (27), for sorption of hydrophobic organic compounds on humic substances bound to mineral surfaces (19), for BTEX compounds on powdered activated carbon and clay minerals (28), and R-naphthol on natural soils and sediments (24). In the present study the sorption from an aqueous solution of 10 mg/L of naphthalene, o-xylene, and toluene was for surface modified diatomite 71%, 60%, and 30% and for modified clinoptilolite 51%, 30%, and 16%, respectively. The results of this study show that the phenyl groups of clinoptilolite and diatomite surface modified with DPDSCl have the best affinity to the aromatic compounds due to the similar aromatic structure. The chlorosilane was attached to the mineral surface by chemical reaction of the silanol groups of the mineral surface, resulting in a stable covalent bond and excellent chemical and physical properties. Sorption behavior was greatly enhanced by chemical treatment compared to natural minerals, which had no measurable sorption capacity for the studied organic compounds. Although more research is needed, chlorosilane modified clinoptilolite and diatomite, respectively, are potential sorbents for aromatic organic compounds with application in wastewater treatment or permeable reactive barriers.
Acknowledgments This research was supported by a grant of the Graduate College “Ecological Water Resources Management” of DFG (Deutsche Forschungsgesellschaft). Special thanks to Oliver Huttenloch, Max Planck Institute and University of Dortmund, Germany, who advised on chemical problems.
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Received for review May 8, 2001. Revised manuscript received August 6, 2001. Accepted August 14, 2001. ES010131F
