- Email: [email protected]
Influence of the synthesis method of Al-hydroxy intercalated clays on their fulvic acid sorption capacity
Applied Clay Science 32 (2006) 283 – 290 www.elsevier.com/locate/clay
Influence of the synthesis method of Al-hydroxy intercalated clays on their fulvic acid sorption capacity Steven Vreysen 1 , André Maes ⁎ Laboratory for Colloid Chemistry, Department of Interphase Chemistry, Catholic University of Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium Received 25 January 2005; received in revised form 23 August 2005; accepted 13 September 2005 Available online 27 March 2006
Abstract The purpose of this study was to investigate the potential use of both Al-hydroxy intercalated clays and ‘pillared clays’ (calcinated Al-hydroxy intercalated clays) for the removal of fulvic acids (FA) from water. Different preparation procedures of the Al-hydroxy intercalated clays were tested and the products were characterised (XRD and N2 sorption) and tested for their FA sorption capacity. It was observed that the drying method (oven or freeze-drying) and the calcination step were the most important parameters influencing the FA sorption capacity of the Al-hydroxy intercalated clays. The noncalcined freeze-dried Alhydroxy intercalated clays showed the largest sorption capacity for FA. Comparison with freeze-dried Wyoming bentonite and granular activated carbon showed that this freeze-dried Al-hydroxy intercalated clays are potential good sorbents for the removal of fulvic acids from water. © 2005 Elsevier B.V. All rights reserved. Keywords: Aluminium intercalated clays; Laurentian fulvic acid; Water purification; Fulvic acid sorption
1. Introduction The removal of humic substances from drinking water is important not only for aesthetic reasons but also for minimising health risks. If humic substances are present in high concentrations, they can react with chlorine used for disinfection and form trihalomethanes, which are suspected carcinogens (Wilbulswas et al., 1998). In the water treatment combined with membrane filtration processes humic substances are an important cause of membrane fouling (Abdel-Jawad et al., 1997; Hong and Elimelech, 1997; Cho et al., 1998; Huber, ⁎ Corresponding author. Tel.: +32 16 321598; fax: +32 16 321998. E-mail address: [email protected] (A. Maes). 1 Tel.: +32 16 321598; fax: +32 16 321998. 0169-1317/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2005.09.002
1998). Thus, humic substances have to be removed from drinking water and from all kinds of waters, which are to be treated by membrane filtration. Because the traditional coagulation/flocculation/sand filtration techniques are often insufficient for a complete removal of humic substances (Chakravorty and Layson, 1997), rather expensive techniques such as microfiltration or filtration over granular activated carbon are mostly used as an additional final removal. Many sorbents other than activated carbon such as natural and synthetic zeolites, clays and modified clay minerals, aluminas and resins have the potential for removing humic substances from water. This paper focuses on the use of modified clay minerals. Amin and Jayson (1996) investigated the use of Zr-pillared clays for the removal of humic substances from surface
284
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
water with a ‘humic substance’ concentration of 12 mg/ l at pH 3 and 5. At those low pH values some of the Zrpillared clays were able to take up 95% of the humic substances. Lacey et al. (1997) investigated the removal of both fulvic acids (extracted from natural waters) and Aldrich humic acids by iron and aluminium pillared clays. Alumina-based pillared clays sorbed up to five times the amount of FA that iron-based pillared clays could sorb and sorbed more FA (94 mg/g) than HA molecules (48 mg/g). However, the pH was not measured and no information concerning the background electrolyte was provided. Wilbulswas et al. (1998) studied the removal of Aldrich HA from distilled water and tap water by alumina-based, oven-dried pillared clays. The tap water was only characterised by its calcium content (12 mg/l). But, it can be questioned whether in this study pillared clays were really produced since the d001 values ranged from 10.0 Å to 14.8 Å, in contrast to expected values, which range from 16 Å up to 20 Å (Cool and Vanstant, 1998). The fact that no pillared clays were produced was also reflected in a rather low uptake of humic acid (20 mg/g). The purpose of this study is to investigate the potential use of both Al-hydroxy intercalated clays and ‘pillared clays’ (calcinated Al-hydroxy intercalated clays) for the removal of fulvic acids from water. The influence of the preparation method of the Al-hydroxy intercalated clays on their FA sorption is studied in detail.
capacity of the Wyoming bentonite (0.75 meq/g at pH 7; Muurinen and Lehikoinen, 1999). The suspensions were stirred for different time periods (‘aging times’ ranging from 1 day to 6 days). The sorbents were washed twice with 0.1 M CaCl2 and subsequently excess salts were removed by dialysis (Medicell International Ltd. tubing visking membranes, 7–30/ 32 in., cutoff = 12 000–14 000 Da) until the conductivity of the wash water was less than 50 μS/m. The sorbent was then freeze-dried or oven-dried at 378 K until constant weight (2 days). In some experiments the freeze-dried sorbent was also calcinated for 1 h at 673 K. The clays prepared with the lab-produced base hydrolysed pillaring solution are indicated as LabPil, and those prepared with the diluted commercially available pillaring solution as ComPil. The aluminium concentration used, the aging time and the drying method are also indicated. For example, a freeze-dried clay produced by a diluted commercially available solution of 0.0624 M Al and an aging time of 6 days is indicated as fd-ComPil-0.06M-6d. 2.2. Characterisation of Al-hydroxy intercalated clays XRD spectra of the WyB and the Al-hydroxy intercalated clays were recorded on a Siemens Kristalloflex 700 apparatus at 40 kV and 20 mA using CuKα radiation (λ = 0.15418 nm). The external surface areas of the WyB and the oven dried and calcined Lab-Pil-0.06M clays were determined by N2 adsorption and desorption at 77 K with an Omnisorp apparatus. 2.3. Fulvic acid adsorption experiments
2. Material and methods 2.1. Preparation of Al-hydroxy intercalated clays Commercially available unpurified MX-80 Volclay Wyoming sodium-bentonite (WyB) was used as start material for the production of Al-hydroxy intercalated clays. The clay sample contains 10% quartz, 1.4% CaCO3 and 0.34% CaSO4 impurities (Muurinen and Lehikoinen, 1999). Two pillaring agents were used: (1) a base hydrolysed Al-pillaring solution and (2) a diluted commercial Al-solution (Chlorhydrol, Reheis Chemical Co.). The base hydrolysed Al-pillaring solution with an OH/Al molar ratio of 2.25 was prepared by adding a 0.2 M NaOH solution dropwise (2 ml/min) to a 0.2 M AlCl3· 6H2O solution at room temperature while stirring vigorously. This solution turned clear after it was heated at 333 K for 3 h. At this stage the aluminium concentration was 0.0615 M and the pH was 4.4. The commercial Al-solution (Chlorhydrol) was diluted to aluminium concentrations of 0.0624 M and 3.12 M. The OH/ Al molar ratio of these solutions was 2.5 and the pH was 4.6. The pillaring solutions were added dropwise (4 ml/min) to a 1 wt.% bentonite suspension. The amount of aluminium in the solution was either 15 mmol Al/g clay or 77.8 mmol Al/g clay. Both concentrations largely exceed the cation exchange
Laurentian soil fulvic acid was provided by Ecolinc (Roxboro, Canada) and was used without further purification. FA sorption isotherms were made at room temperature on fdComPil-3.12M-1d, fd-ComPil-0.06M-1d, fd-LabPil-0.06M1d and fd-LabPil-0.006M-6d clays, a freeze dried CaWyB, od-LabPil-0.06M-1d clays and calc-LabPil-0.06M-1d and calc-ComPil-3.12M-1d clays. All batch adsorption experiments were performed in 40 ml polycarbonate centrifuge tubes to which 50 mg of sorbent was weighed and 40 ml of a FA solution was added. Four different FA solution concentrations (100, 75, 50 and 25 mg/l) were prepared in 0.005 M CaCl2 and were brought to pH 7 with 0.5 M NaOH. The tubes were shaken overnight at room temperature and were centrifuged at 137×g (1000 rpm) for 15 min. Preliminary kinetic studies showed that equilibrium was reached. The FA sorption was calculated as the difference in FA absorbance before and after sorption and centrifugation. It was verified that the applied centrifugation conditions were sufficient for a complete sorbent removal. To this end a sample without FA was run and proved to have zero absorbance. It was also independently verified that however a small amount of FA (< 5%) is centrifuged off in absence of sorbent. The sorption results were corrected for this loss.
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
The pH of the supernatant solution was measured with a 702 SM Titrino (Metrohm). The FA concentration was determined by a LKB 4053 Ultrospec K UV/VIS spectrophotometer at 280 nm using a continuous flow cuvette. A calibration curve of FA solution in 0.005 M CaCl2 was made and was subjected to the same procedure (shaking, centrifugation) as the samples used in the FA adsorption experiments. The sorption isotherms of FA onto the Al-hydroxy intercalated clays were fitted by the Langmuir equation: q¼
qmax KA C 1 þ KA
where q = the sorbed FA concentration (mg FA/g), qmax = maximum sorbed concentration, KA = a constant and C = the FA concentration in solution (mg FA/l).
3. Results and discussion 3.1. Characterisation of Al-hydroxy intercalated clays Fig. 1 shows the XRD spectra of the unpurified Wyoming bentonite and seven Al-hydroxy intercalated Wyoming bentonite clays, prepared by different procedures. Table 1 presents the basal spacings of these clay minerals and the 2θ values of the first peak. The XRD patterns show that Wyoming bentonite mainly consists of montmorillonite clay (peaks indicated with ‘M’), with some quartz impurities (peaks indicated with ‘Qz’). As expected, the 2θ values of the first peak of all 7 Alhydroxy intercalated clays were smaller than the 2θ value for WyB (7.56). Thus the d001 distances increase after intercalation of the aluminium polyhydroxo
285
Table 1 Basal spacing and corresponding 2θ values of the studied clay minerals Sorbent
d001 spacing (Å)
2θ
WyB od-LabPil-0.06M-1d fd-LabPil-0.06M-1d calc-LabPil-0.06M-1d fd-LabPil-0.06M-6d fd-ComPil-3.12M-1d fd-ComPil-3.12M-6d fd-ComPil-0.06M-1d
11.7 16.5 18.8 16.7 19.3 17.1 19.4 18.5
7.56 5.36 4.70 5.29 4.58 5.17 4.55 4.78
cations. The freeze-dried LabPil-0.06M-1d clay has a larger d001 spacing than the same LabPil clay, which was oven-dried at 378 K or calcined at 678 K. According to Kloprogge et al. (2002) this decrease in basal spacing results from the partial removal of the outer water molecules from the Al13 complex in the oven dried clays and from the conversion of these Al-polyhydroxo cations to Al-pillars in the calcined clays . The d001 spacings of the LabPil and the ComPil clays, prepared with an aging time of 6 days (LabPil-0.06M-6d and ComPil-3.12M6d), are slightly larger than those prepared with 1 day aging time (LabPil-0.06M-1d and ComPil-3.12M-1d). Fig. 2 shows the N2 adsorption/desorption isotherms made on both an oven dried and a calcined LabPil-0.06 M-1d clay. No N2 adsorption/desorption isotherms of the freeze dried Al-hydroxy intercalated clays were made because the pretreatment of the clays includes an oven-drying step at 105 °C in order to remove the adsorbed water molecules. Moreover, Pinnavaia et al. (1984) found that the BET surface of air-dried and
Fig. 1. XRD spectra of (a) WyB, (b) od-LabPil-0.06M-1d, (c) fd-LabPil-0.06M-1d, (d) fd-LabPil-0.06M-6d, (e) fd-ComPil-3.12M-1d, (f) fdComPil-3.12M-6d, (g) fd-ComPil-0.06M-1d, (h) calc-LabPil-0.06M-1d.
286
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
Fig. 2. N2 sorption and desorption isotherms of a calcined and an oven-dried LabPil-0.06M-1d.
freeze dried Al intercalated clays was similar. The adsorption isotherms in Fig. 2 are of type I in the Brunauer, Deming, Deming and Teller (BDDT) classification with a hysteresis loop type H4. Thus the pores in both LabPil clays are mainly micropores (with a diameter < 2 nm) and no macropores (diameter > 50 nm) are present. The small hysteresis indicates the presence of a small amount of mesopores (diameter 2–50 nm). The surface area of the mesopores is only 10% of the total surface area (Table 2). The specific surface area of the Al-hydroxy intercalated clays calculated with the Langmuir method, the mesopore surface area calculated by t-plot analysis of the sorption isotherms and the micropore volume are presented in Table 2. The introduction of Al-hydroxy species in the intercalated clay increased the surface area from 32.7 m2/g (WyB) to 324.2 m2/g. Upon calcining the sample a significant loss of 25% in surface area took place, mainly affecting the micropores of the starting intercalated clay, as it can be deduced from the decreasing microporous volumes and the fact that the mesopore surface area only slightly decreased. These results agree with results of N2 adsorption/desorption studies on Al intercalated and pillared saponites by Gil and Gandía (2003). They found that the pores with the smallest size present in the pillared samples are the most deteriorated ones when calcination temperature increases. In their Table 2 N2 adsorption/desorption results Sorbent
Specific surface area (m2/g)
Mesopore surface area (m2/g)
Micropore volume (ml/g)
WyB od-LabPil0.06M-1d calc-LabPil0.06M-1d
32.7 324.2
– 39.97
– 0.340
244.0
35.11
0.300
literature review they mention several mechanisms for this surface area loss as proposed by other authors. For example, the migration of the pillars to the edges of the clay tactoids upon thermal treatment of PILCs could partly block the access to the micropores.
3.2. Effect of the pillaring solution, aluminum concentration and aging time of the sorbent on the FA sorption Fig. 3 shows the sorption isotherms of freeze dried WyB in the Ca form (Ca-WyB), freeze dried ComPil clays prepared with pillaring solutions having aluminium concentrations of 3.12 M (fd-ComPil-3.12M-1d) and 0.0624 M (fd-ComPil-0.06M-1d) and freeze dried LabPil clays (fd-LabPil-0.06M-1d and fd-LabPil0.06M-6d), only prepared with low concentrated (0.0615 M Al) pillaring solutions. Table 3 shows the calculated Langmuir parameters. The Langmuir equation does satisfactorily describe the FA sorption onto the Al-hydroxy intercalated clays (R2 values: 0.928– 0.999). However, this equation cannot be used to describe the FA sorption onto the Ca-WyB (R2 = 0.805). With increasing FA sorption onto the Al-hydroxy intercalated clays the equilibrium pH changed from 5.9 to 6.5. Comparison of the Al-hydroxy intercalated clays with untreated freeze-dried Ca-WyB bentonite shows that the intercalation of Al-polyhydroxy cations inbetween the clay sheets largely enhances the sorption of FA. The sorption capacity increases from 15 mg FA/g onto Ca-WyB to 60 mg FA/g onto an Al-hydroxy intercalated clay. Both Fig. 3 and Table 3 show that there is no significant difference in the sorption behaviour of the Al-hydroxy interaclated clays for FA irrespective of the nature of the pillaring solution (Com vs. Lab), the aging time (1 day vs. 6 days) and the concentration of the pillaring solution (3.12 M vs. 0.06 M).
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
287
Fig. 3. Sorption isotherms of FA onto a fd-ComPil-3.12M-1d, fd-ComPil-0.06M-1d, fd-LabPil-0.06M-1d, fd-LabPil-0.006M-6d and a fd-CaWyB in 0.005 M CaCl2. The pH ranged from 5.9 to 6.5. Lines are drawn to guide the eye.
The foregoing results are in accordance with observations of Pinnavaia et al. (1984), who found that, although 27Al NMR spectroscopy indicates the polyhydroxo cation nuclearity to be larger in commercially available Al-solutions than in base hydrolysed Al-solutions, both reagents gave Al-hydroxy intercalated products with similar aluminium contents, pore sizes, thermal stabilities and catalytic properties. On the basis of these observations Pinnavaia et al. (1984) concluded that these similarities suggest that the same type of hydroxo cations, probably Al13 Keggin ions, is formed on the intracrystal surfaces. Unpublished results Vreysen and Maes (submitted for publication-a) also showed by zeta potential measurements and potentiometric titrations that the nature of the intercalated Al species in a fd-LabPil-0.06M-1d or a fd-ComPil3.12M-1d clay is similar. The aluminium concentration of the pillaring solution does not affect the pillaring process since the aluTable 3 Langmuir parameters Sorbent
qmax (mg/g)
KA
R2
fd-CaWyB fd-LabPil-0.06M-1d fd-ComPil-3.12M-1d fd-ComPil-0.06M-1d fd-LabPil-0.06M-6d od-LabPil-0.06M-1d calc-LabPil-0.06M-1d calc-ComPil-3.12M-1d
44.4 59.9 58.8 64.2 61.7 16.4 50.8 53.6
0.0125 0.759 0.732 0.720 0.761 2.517 0.393 0.168
0.805 0.955 0.928 0.992 0.999 0.941 0.985 0.988
minium concentration significantly exceeds the cation exchange capacity of the clay (17 mmol Al/g clay and 77.8 mmol Al/g clay). An aging time of 1 day appears to be enough for a sufficient intercalation of the aluminium polyoxo cations in-between the clay sheets. The slightly increasing basal spacing with increasing aging time (Table 1) indicates that a small ‘structural effect’ takes place, but this effect does not significantly affect the FA sorption capacity of the clays. This ‘structural effect’ is probably caused by a change in nature of the polyhydroxo cations in both the base hydrolysed and the diluted commercial pillaring solution with time. For a diluted commercial solution this change in the nature of the polyhydroxo cations is more pronounced than for a base hydrolysed solution (Pinnavaia et al., 1984; Schoonheydt et al., 1993). This could explain the larger difference in basal spacing between the ComPil clays of 1 and 6 days aging time and the LabPil clays prepared with an aging time of 1 and 6 days (Table 1). These differences in basal spacing between the ComPil clays of 1 and 6 days aging time and the LabPil clays prepared with an aging time of 1 and 6 days, however has no systematic effect on the FA sorption as already shown in Fig. 3. 3.3. Effect of the drying procedure on the FA sorption Fig. 4 shows sorption isotherms of FA onto freezedried and oven-dried LabPil-0.06M-1d clays. The Langmuir parameters of the isotherms are given in Table 3. The maximum FA sorption capacity of oven-
288
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
Fig. 4. Sorption isotherms of FA onto an oven-dried (pH 5.0–5.6) and a freeze-dried LabPil-0.06M-1d clay (pH 5.9–6.5) in 0.005 M CaCl2. Lines are drawn to guide the eye.
dried Al-hydroxy intercalated clays (18 mg FA/g) is significantly lower than the one of freeze-dried Al-hydroxy intercalated clays (60 mg FA/g). It has also to be mentioned that the equilibrium pH of the suspension of oven-dried Al-hydroxy intercalated clays (5.0–5.6) is slightly lower than the one of freeze-dried Al-hydroxy intercalated clays (5.9–6.5). This small pH difference cannot explain the large difference in sorption capacity between oven dried and freeze dried Al-hydroxy intercalated clays as shown in a separate study on the sorption mechanism of FA on these Al-hydroxy intercalated clays (Vreysen and Maes, submitted for publication-b). This study shows that FA are sorbed onto Al-hydroxy intercalated clays by ligand exchange and calcium bridging. The observed pH increase with increasing FA sorption is caused by the released OH− ions from ligand exchange reactions between the FA molecules and the intercalated Al species. The larger sorption capacity of freeze dried samples for FA is in agreement with structural differences between the oven-dried and the freeze-dried clays. Pinnavaia et al. (1984) found that this difference in pore openings could not be observed by the adsorption of small molecules such as N2, since the BET surface of airdried and freeze dried Al intercalated clays was similar. However, from adsorption experiments with organic molecules of different kinetic diameters they concluded that oven-drying yields products with pore openings greater than 0.62 nm but less than 0.92 nm. In contrast, freeze-dried products exhibit adsorption capacities for molecules with a kinetic diameter >1 nm. Thus, freeze drying preserves the structure of the Al intercalated clay in suspension, giving rise to edge-to-edge or face-to-
edge aggregation, whereas oven-drying optimises the face-to-face aggregation and also results in a different pore structure. The diameter of Suwannee River fulvic acid has been determined to be 1.76 nm using small angle X-ray scattering by Aiken et al. (1995) and 1.64 nm by Thurman et al. (1982). Domingos et al. (2004) report an average molecular weight of 1000 Da for Laurentian Fulvic acid. This corresponds with a diameter of 1 nm. Therefore the larger sorption capacity of freeze-dried samples for FA can be attributed to their much broader pore range. 3.4. Effect of the calcination on the FA sorption Fig. 5 presents sorption isotherms of FA onto both calcined and non-calcined fd-LabPil-0.06M-1d and fdComPil-3.12M-1d clays. The calculated Langmuir parameters are shown in Table 3. The calcination decreases the maximum sorption capacity of Al-hydroxy intercalated clays from 60 mg FA/g (uncalcined) to 50 mg FA/g (calcined). Compared to the non-calcined samples the equilibrium pH of the calcined samples (6.3–7.1) is slightly increased. This small pH increase cannot fully explain the decreased FA sorption. The decrease in sorption capacity can probably be explained by the decrease in specific surface area upon calcination (Table 2) and/or the transformation of Al-poly-hydroxo cations to Aloxide pillars. Upon calcination dehydration and dehydroxylation reactions of the charged pillar precursors occur to give neutral Al-oxide pillars (Cool and Vanstant, 1998). The decreased FA sorption capacity is probably best explained by a different FA sorption onto Al polyhydroxo cations and neutral Al-oxide
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
289
Fig. 5. Sorption isotherms of FA onto both a calcined (pH 6.3–7.1) and a non-calcined LabPil-0.06M-1d and ComPil-3.12M-1d freeze-dried clays (pH 5.9–6.5), in 0.005 M CaCl2.
pillars. In agreement with previous observations, little or no difference is observed between the ComPil and the LabPil clays. The comparison of the FA sorption in the pH range 5.9–7.1 on different Al-hydroxy intercalated clays leads us to conclude that the non-calcined freeze-dried Alhydroxy intercalated clays showed the largest sorption capacity for FA. Fig. 6 compares the sorption isotherms of such a non-calcined freeze dried Al-hydroxy intercalated clay (fd-LabPil-0.06M-1d) with a freeze dried WyB in the calcium form (fd-CaWyB) and a granular
activated carbon (Organosorb 10, DESOTEC). The FA sorption capacity of the freeze dried Al-hydroxy intercalated clay (60 mg/g) is twice as large as the FA sorption capacity of the granular activated carbon (30 mg/g) and three times as large as the FA sorption capacity of the freeze dried CaWyB (20 mg/g). This indicates that freeze dried Al-hydroxy intercalated clays are potential good sorbents for the removal of fulvic acids from water. To remove 99% of FA from typical surface water a sorbent concentration of about 0.5 g/l would be required.
Fig. 6. Sorption isotherms of FA onto a fd-LabPil-0.06M-1d, a fd-CaWyB and a Granular Activated Carbon (Organosorb 10, DESOTEC) (pH 5.9–7.1).
290
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
Organo-clays are also known to have a large sorption capacity for humic substances (Önkal-Engin et al., 2000; Jaruwong and Wibulswas, 2003). However, in these papers mostly the sorption of humic acid is studied, which sorbs to the organo-clays by hydrophobic sorption. Humic acid is known to be more hydrophobic than fulvic acid. The organo-clays thus have a much larger affinity for humic acid than for fulvic acid (Zhao and Vance, 1998). It is therefore difficult to compare our experimental results with these literature data in a quantitative manner. 4. Conclusion The drying method and the calcination of the Alhydroxy intercalated clays were the most important parameters influencing the FA sorption capacity of the Al-hydroxy intercalated clays. Oven-dried Al-hydroxy intercalated clays showed a smaller sorption capacity for FA compared to freeze-dried Al-hydroxy intercalated clays because of the smaller pore openings in the former. Calcination lead to a change in the nature of the intercalated species and a decreased surface area and basal spacing of the clay, which resulted in a decreased sorption capacity of the Al-hydroxy intercalated clays for FA. The nature of the pillaring solution and the aging time of the sorbent had no influence on the FA sorption capacity of the Al-hydroxy intercalated clays. Comparison with freeze dried CaWyB and granular activated carbon showed that freeze dried Al-hydroxy intercalated clays are potential good sorbents for the removal of fulvic acids from water. Acknowledgments The authors acknowledge the KULeuven Geconcerteerde Onderzoeksacties (GOA 2000/007 and 2005/013) for their financial support and Reheis Chemical Co. for providing a sample of the Chlorhydrol solution. References Abdel-Jawad, M., Ebrahim, S., Al-Atram, F., Al-Shammari, S., 1997. Pretreatment of the municipal wastewater feed for reverse osmosis plants. Desalination 109, 211–223. Aiken, G.R., Brown, P.A., Noyes, T.I., Pinckney, D.J., 1995. Molecular size and weight of fulvic and humic acids from the Suwannee river. In: Averett, R.C., Leenheer, J.A., McKnight, D. M., Thorn, K.A. (Eds.), Humic Substances in the Suwannee River, Georgia: Interactions, Properties, and Proposed Structures. US Geological Survey, Denver, pp. 89–97. Amin, S., Jayson, S.S., 1996. Humic substance uptake by hydrotalcites and PILCs. Water Research 30 (2), 299–306.
Chakravorty, B., Layson, A., 1997. Ideal feed pretreatment for reverse osmosis by continuous microfiltration. Desalination 110, 143–150. Cho, J., Amy, G., Pellegrino, J., Yoon, Y., 1998. Characterisation of clean and natural organic matter (NOM) fouled NF and UF membranes, and foulants characterisation. Desalination 188, 101–108. Cool, P., Vanstant, E.F., 1998. Al-hydroxy intercalated clays: preparation, characterization and applications. Molecular Sieves 1, 165–288. Domingos, R.F., Benedetti, M.F., Croué, J.P., Pinheiro, J.P., 2004. Electrochemical methodology to study labile trace metal/natural organic matter complexation at low concentration levels in natural waters. Analytica Chimica Acta 521 (1), 77–86. Gil, A., Gandía, L.M., 2003. Microstructure and quantitative estimation of the micropore-size distribution of an aluminapillared clay from nitrogen adsorption at 77 K and carbon dioxide adsorption at 273 K. Chemical Engineering Science 58, 3059–3075. Hong, S., Elimelech, M., 1997. Chemical and Physical aspects of natural organic matter (NOM) fouling of nanofiltration membranes. Journal of Membrane Science 132, 159–181. Huber, S.A., 1998. Evidence for membrane fouling by specific TOC constituents. Desalination 119, 229–234. Jaruwong, P., Wibulswas, R., 2003. Influence of organo-clay's carbon number on the adsorption of humic acid. Asian Journal of Energy and Environment 4 (1–2), 41–59. Kloprogge, J.T., Evans, R., Hickey, L., Frost, R.L., 2002. Characterisation and Al-pillaring of smectites from Miles, Queensland (Australia). Applied Clay Science 20, 157–163. Lacey, A.L., Hayes, M.H.B., Vaidyanathan, L.V., 1997. Preparation of iron Al-hydroxy intercalated clays and their applications for sorption of humic substances. In: Hayes, M.H.B., Wilson, W.S. (Eds.), Humic Substances, Peats, and Sludges. Health and Environmental Aspects. The Royal Society of Chemistry, Cambridge, pp. 219–225. Muurinen, A., Lehikoinen, J., 1999. Porewater chemistry in compacted bentonite. Engineering Geology 54, 207–214. Önkal-Engin, G., Wibulswas, R., White, D.A., 2000. Humic acid uptake from aqueous media using hydrotalcites and modified montmorillonite. Environmental Technology 21, 167–175. Pinnavaia, T.J., Tzou, M.-S., Landau, S.D., Raythatha, R.H., 1984. On the pillaring and delamination of smectite clay catalysts by polyoxo cations of aluminium. Journal of Molecular Catalysis 27, 195–212. Schoonheydt, R.A., Van den Eynde, J., Tubbax, H., Leeman, H., Struyckens, M., Lenotte, I., Stone, W.E.E., 1993. The Al pillaring of clays: Part I. Pillaring with dilute and concentrated Al solutions. Clays and Clay Minerals 41 (5), 598–607. Thurman, E.M., Wershaw, R.L., Malcolm, R.L., Pinckney, D.J., 1982. Molecular size of aquatic humic substances. Organic Geochemistry 4, 27–35. Vreysen, S., Maes, A., submitted for publication-a. Influence of pH on the aluminium speciation in freeze dried Al-hydroxy intercalated clays. Applied Clay Science. Vreysen, S., Maes, A., submitted for publication-b. Sorption mechanism of fulvic acid onto freeze dried Al-hydroxy intercalated clays. Applied Clay Science. Wilbulswas, R., White, D.A., Rautiu, R., 1998. Removal of humic substances from water by alumina-based Al-hydroxy intercalated clays. Environmental Technology 19, 627–632. Zhao, H., Vance, G.F., 1998. Sorption of trichloroethylene by organoclays in the presence of humic substances. Water Research 32 (12), 3710–3716.
Influence of the synthesis method of Al-hydroxy intercalated clays on their fulvic acid sorption capacity Steven Vreysen 1 , André Maes ⁎ Laboratory for Colloid Chemistry, Department of Interphase Chemistry, Catholic University of Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium Received 25 January 2005; received in revised form 23 August 2005; accepted 13 September 2005 Available online 27 March 2006
Abstract The purpose of this study was to investigate the potential use of both Al-hydroxy intercalated clays and ‘pillared clays’ (calcinated Al-hydroxy intercalated clays) for the removal of fulvic acids (FA) from water. Different preparation procedures of the Al-hydroxy intercalated clays were tested and the products were characterised (XRD and N2 sorption) and tested for their FA sorption capacity. It was observed that the drying method (oven or freeze-drying) and the calcination step were the most important parameters influencing the FA sorption capacity of the Al-hydroxy intercalated clays. The noncalcined freeze-dried Alhydroxy intercalated clays showed the largest sorption capacity for FA. Comparison with freeze-dried Wyoming bentonite and granular activated carbon showed that this freeze-dried Al-hydroxy intercalated clays are potential good sorbents for the removal of fulvic acids from water. © 2005 Elsevier B.V. All rights reserved. Keywords: Aluminium intercalated clays; Laurentian fulvic acid; Water purification; Fulvic acid sorption
1. Introduction The removal of humic substances from drinking water is important not only for aesthetic reasons but also for minimising health risks. If humic substances are present in high concentrations, they can react with chlorine used for disinfection and form trihalomethanes, which are suspected carcinogens (Wilbulswas et al., 1998). In the water treatment combined with membrane filtration processes humic substances are an important cause of membrane fouling (Abdel-Jawad et al., 1997; Hong and Elimelech, 1997; Cho et al., 1998; Huber, ⁎ Corresponding author. Tel.: +32 16 321598; fax: +32 16 321998. E-mail address: [email protected] (A. Maes). 1 Tel.: +32 16 321598; fax: +32 16 321998. 0169-1317/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2005.09.002
1998). Thus, humic substances have to be removed from drinking water and from all kinds of waters, which are to be treated by membrane filtration. Because the traditional coagulation/flocculation/sand filtration techniques are often insufficient for a complete removal of humic substances (Chakravorty and Layson, 1997), rather expensive techniques such as microfiltration or filtration over granular activated carbon are mostly used as an additional final removal. Many sorbents other than activated carbon such as natural and synthetic zeolites, clays and modified clay minerals, aluminas and resins have the potential for removing humic substances from water. This paper focuses on the use of modified clay minerals. Amin and Jayson (1996) investigated the use of Zr-pillared clays for the removal of humic substances from surface
284
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
water with a ‘humic substance’ concentration of 12 mg/ l at pH 3 and 5. At those low pH values some of the Zrpillared clays were able to take up 95% of the humic substances. Lacey et al. (1997) investigated the removal of both fulvic acids (extracted from natural waters) and Aldrich humic acids by iron and aluminium pillared clays. Alumina-based pillared clays sorbed up to five times the amount of FA that iron-based pillared clays could sorb and sorbed more FA (94 mg/g) than HA molecules (48 mg/g). However, the pH was not measured and no information concerning the background electrolyte was provided. Wilbulswas et al. (1998) studied the removal of Aldrich HA from distilled water and tap water by alumina-based, oven-dried pillared clays. The tap water was only characterised by its calcium content (12 mg/l). But, it can be questioned whether in this study pillared clays were really produced since the d001 values ranged from 10.0 Å to 14.8 Å, in contrast to expected values, which range from 16 Å up to 20 Å (Cool and Vanstant, 1998). The fact that no pillared clays were produced was also reflected in a rather low uptake of humic acid (20 mg/g). The purpose of this study is to investigate the potential use of both Al-hydroxy intercalated clays and ‘pillared clays’ (calcinated Al-hydroxy intercalated clays) for the removal of fulvic acids from water. The influence of the preparation method of the Al-hydroxy intercalated clays on their FA sorption is studied in detail.
capacity of the Wyoming bentonite (0.75 meq/g at pH 7; Muurinen and Lehikoinen, 1999). The suspensions were stirred for different time periods (‘aging times’ ranging from 1 day to 6 days). The sorbents were washed twice with 0.1 M CaCl2 and subsequently excess salts were removed by dialysis (Medicell International Ltd. tubing visking membranes, 7–30/ 32 in., cutoff = 12 000–14 000 Da) until the conductivity of the wash water was less than 50 μS/m. The sorbent was then freeze-dried or oven-dried at 378 K until constant weight (2 days). In some experiments the freeze-dried sorbent was also calcinated for 1 h at 673 K. The clays prepared with the lab-produced base hydrolysed pillaring solution are indicated as LabPil, and those prepared with the diluted commercially available pillaring solution as ComPil. The aluminium concentration used, the aging time and the drying method are also indicated. For example, a freeze-dried clay produced by a diluted commercially available solution of 0.0624 M Al and an aging time of 6 days is indicated as fd-ComPil-0.06M-6d. 2.2. Characterisation of Al-hydroxy intercalated clays XRD spectra of the WyB and the Al-hydroxy intercalated clays were recorded on a Siemens Kristalloflex 700 apparatus at 40 kV and 20 mA using CuKα radiation (λ = 0.15418 nm). The external surface areas of the WyB and the oven dried and calcined Lab-Pil-0.06M clays were determined by N2 adsorption and desorption at 77 K with an Omnisorp apparatus. 2.3. Fulvic acid adsorption experiments
2. Material and methods 2.1. Preparation of Al-hydroxy intercalated clays Commercially available unpurified MX-80 Volclay Wyoming sodium-bentonite (WyB) was used as start material for the production of Al-hydroxy intercalated clays. The clay sample contains 10% quartz, 1.4% CaCO3 and 0.34% CaSO4 impurities (Muurinen and Lehikoinen, 1999). Two pillaring agents were used: (1) a base hydrolysed Al-pillaring solution and (2) a diluted commercial Al-solution (Chlorhydrol, Reheis Chemical Co.). The base hydrolysed Al-pillaring solution with an OH/Al molar ratio of 2.25 was prepared by adding a 0.2 M NaOH solution dropwise (2 ml/min) to a 0.2 M AlCl3· 6H2O solution at room temperature while stirring vigorously. This solution turned clear after it was heated at 333 K for 3 h. At this stage the aluminium concentration was 0.0615 M and the pH was 4.4. The commercial Al-solution (Chlorhydrol) was diluted to aluminium concentrations of 0.0624 M and 3.12 M. The OH/ Al molar ratio of these solutions was 2.5 and the pH was 4.6. The pillaring solutions were added dropwise (4 ml/min) to a 1 wt.% bentonite suspension. The amount of aluminium in the solution was either 15 mmol Al/g clay or 77.8 mmol Al/g clay. Both concentrations largely exceed the cation exchange
Laurentian soil fulvic acid was provided by Ecolinc (Roxboro, Canada) and was used without further purification. FA sorption isotherms were made at room temperature on fdComPil-3.12M-1d, fd-ComPil-0.06M-1d, fd-LabPil-0.06M1d and fd-LabPil-0.006M-6d clays, a freeze dried CaWyB, od-LabPil-0.06M-1d clays and calc-LabPil-0.06M-1d and calc-ComPil-3.12M-1d clays. All batch adsorption experiments were performed in 40 ml polycarbonate centrifuge tubes to which 50 mg of sorbent was weighed and 40 ml of a FA solution was added. Four different FA solution concentrations (100, 75, 50 and 25 mg/l) were prepared in 0.005 M CaCl2 and were brought to pH 7 with 0.5 M NaOH. The tubes were shaken overnight at room temperature and were centrifuged at 137×g (1000 rpm) for 15 min. Preliminary kinetic studies showed that equilibrium was reached. The FA sorption was calculated as the difference in FA absorbance before and after sorption and centrifugation. It was verified that the applied centrifugation conditions were sufficient for a complete sorbent removal. To this end a sample without FA was run and proved to have zero absorbance. It was also independently verified that however a small amount of FA (< 5%) is centrifuged off in absence of sorbent. The sorption results were corrected for this loss.
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
The pH of the supernatant solution was measured with a 702 SM Titrino (Metrohm). The FA concentration was determined by a LKB 4053 Ultrospec K UV/VIS spectrophotometer at 280 nm using a continuous flow cuvette. A calibration curve of FA solution in 0.005 M CaCl2 was made and was subjected to the same procedure (shaking, centrifugation) as the samples used in the FA adsorption experiments. The sorption isotherms of FA onto the Al-hydroxy intercalated clays were fitted by the Langmuir equation: q¼
qmax KA C 1 þ KA
where q = the sorbed FA concentration (mg FA/g), qmax = maximum sorbed concentration, KA = a constant and C = the FA concentration in solution (mg FA/l).
3. Results and discussion 3.1. Characterisation of Al-hydroxy intercalated clays Fig. 1 shows the XRD spectra of the unpurified Wyoming bentonite and seven Al-hydroxy intercalated Wyoming bentonite clays, prepared by different procedures. Table 1 presents the basal spacings of these clay minerals and the 2θ values of the first peak. The XRD patterns show that Wyoming bentonite mainly consists of montmorillonite clay (peaks indicated with ‘M’), with some quartz impurities (peaks indicated with ‘Qz’). As expected, the 2θ values of the first peak of all 7 Alhydroxy intercalated clays were smaller than the 2θ value for WyB (7.56). Thus the d001 distances increase after intercalation of the aluminium polyhydroxo
285
Table 1 Basal spacing and corresponding 2θ values of the studied clay minerals Sorbent
d001 spacing (Å)
2θ
WyB od-LabPil-0.06M-1d fd-LabPil-0.06M-1d calc-LabPil-0.06M-1d fd-LabPil-0.06M-6d fd-ComPil-3.12M-1d fd-ComPil-3.12M-6d fd-ComPil-0.06M-1d
11.7 16.5 18.8 16.7 19.3 17.1 19.4 18.5
7.56 5.36 4.70 5.29 4.58 5.17 4.55 4.78
cations. The freeze-dried LabPil-0.06M-1d clay has a larger d001 spacing than the same LabPil clay, which was oven-dried at 378 K or calcined at 678 K. According to Kloprogge et al. (2002) this decrease in basal spacing results from the partial removal of the outer water molecules from the Al13 complex in the oven dried clays and from the conversion of these Al-polyhydroxo cations to Al-pillars in the calcined clays . The d001 spacings of the LabPil and the ComPil clays, prepared with an aging time of 6 days (LabPil-0.06M-6d and ComPil-3.12M6d), are slightly larger than those prepared with 1 day aging time (LabPil-0.06M-1d and ComPil-3.12M-1d). Fig. 2 shows the N2 adsorption/desorption isotherms made on both an oven dried and a calcined LabPil-0.06 M-1d clay. No N2 adsorption/desorption isotherms of the freeze dried Al-hydroxy intercalated clays were made because the pretreatment of the clays includes an oven-drying step at 105 °C in order to remove the adsorbed water molecules. Moreover, Pinnavaia et al. (1984) found that the BET surface of air-dried and
Fig. 1. XRD spectra of (a) WyB, (b) od-LabPil-0.06M-1d, (c) fd-LabPil-0.06M-1d, (d) fd-LabPil-0.06M-6d, (e) fd-ComPil-3.12M-1d, (f) fdComPil-3.12M-6d, (g) fd-ComPil-0.06M-1d, (h) calc-LabPil-0.06M-1d.
286
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
Fig. 2. N2 sorption and desorption isotherms of a calcined and an oven-dried LabPil-0.06M-1d.
freeze dried Al intercalated clays was similar. The adsorption isotherms in Fig. 2 are of type I in the Brunauer, Deming, Deming and Teller (BDDT) classification with a hysteresis loop type H4. Thus the pores in both LabPil clays are mainly micropores (with a diameter < 2 nm) and no macropores (diameter > 50 nm) are present. The small hysteresis indicates the presence of a small amount of mesopores (diameter 2–50 nm). The surface area of the mesopores is only 10% of the total surface area (Table 2). The specific surface area of the Al-hydroxy intercalated clays calculated with the Langmuir method, the mesopore surface area calculated by t-plot analysis of the sorption isotherms and the micropore volume are presented in Table 2. The introduction of Al-hydroxy species in the intercalated clay increased the surface area from 32.7 m2/g (WyB) to 324.2 m2/g. Upon calcining the sample a significant loss of 25% in surface area took place, mainly affecting the micropores of the starting intercalated clay, as it can be deduced from the decreasing microporous volumes and the fact that the mesopore surface area only slightly decreased. These results agree with results of N2 adsorption/desorption studies on Al intercalated and pillared saponites by Gil and Gandía (2003). They found that the pores with the smallest size present in the pillared samples are the most deteriorated ones when calcination temperature increases. In their Table 2 N2 adsorption/desorption results Sorbent
Specific surface area (m2/g)
Mesopore surface area (m2/g)
Micropore volume (ml/g)
WyB od-LabPil0.06M-1d calc-LabPil0.06M-1d
32.7 324.2
– 39.97
– 0.340
244.0
35.11
0.300
literature review they mention several mechanisms for this surface area loss as proposed by other authors. For example, the migration of the pillars to the edges of the clay tactoids upon thermal treatment of PILCs could partly block the access to the micropores.
3.2. Effect of the pillaring solution, aluminum concentration and aging time of the sorbent on the FA sorption Fig. 3 shows the sorption isotherms of freeze dried WyB in the Ca form (Ca-WyB), freeze dried ComPil clays prepared with pillaring solutions having aluminium concentrations of 3.12 M (fd-ComPil-3.12M-1d) and 0.0624 M (fd-ComPil-0.06M-1d) and freeze dried LabPil clays (fd-LabPil-0.06M-1d and fd-LabPil0.06M-6d), only prepared with low concentrated (0.0615 M Al) pillaring solutions. Table 3 shows the calculated Langmuir parameters. The Langmuir equation does satisfactorily describe the FA sorption onto the Al-hydroxy intercalated clays (R2 values: 0.928– 0.999). However, this equation cannot be used to describe the FA sorption onto the Ca-WyB (R2 = 0.805). With increasing FA sorption onto the Al-hydroxy intercalated clays the equilibrium pH changed from 5.9 to 6.5. Comparison of the Al-hydroxy intercalated clays with untreated freeze-dried Ca-WyB bentonite shows that the intercalation of Al-polyhydroxy cations inbetween the clay sheets largely enhances the sorption of FA. The sorption capacity increases from 15 mg FA/g onto Ca-WyB to 60 mg FA/g onto an Al-hydroxy intercalated clay. Both Fig. 3 and Table 3 show that there is no significant difference in the sorption behaviour of the Al-hydroxy interaclated clays for FA irrespective of the nature of the pillaring solution (Com vs. Lab), the aging time (1 day vs. 6 days) and the concentration of the pillaring solution (3.12 M vs. 0.06 M).
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
287
Fig. 3. Sorption isotherms of FA onto a fd-ComPil-3.12M-1d, fd-ComPil-0.06M-1d, fd-LabPil-0.06M-1d, fd-LabPil-0.006M-6d and a fd-CaWyB in 0.005 M CaCl2. The pH ranged from 5.9 to 6.5. Lines are drawn to guide the eye.
The foregoing results are in accordance with observations of Pinnavaia et al. (1984), who found that, although 27Al NMR spectroscopy indicates the polyhydroxo cation nuclearity to be larger in commercially available Al-solutions than in base hydrolysed Al-solutions, both reagents gave Al-hydroxy intercalated products with similar aluminium contents, pore sizes, thermal stabilities and catalytic properties. On the basis of these observations Pinnavaia et al. (1984) concluded that these similarities suggest that the same type of hydroxo cations, probably Al13 Keggin ions, is formed on the intracrystal surfaces. Unpublished results Vreysen and Maes (submitted for publication-a) also showed by zeta potential measurements and potentiometric titrations that the nature of the intercalated Al species in a fd-LabPil-0.06M-1d or a fd-ComPil3.12M-1d clay is similar. The aluminium concentration of the pillaring solution does not affect the pillaring process since the aluTable 3 Langmuir parameters Sorbent
qmax (mg/g)
KA
R2
fd-CaWyB fd-LabPil-0.06M-1d fd-ComPil-3.12M-1d fd-ComPil-0.06M-1d fd-LabPil-0.06M-6d od-LabPil-0.06M-1d calc-LabPil-0.06M-1d calc-ComPil-3.12M-1d
44.4 59.9 58.8 64.2 61.7 16.4 50.8 53.6
0.0125 0.759 0.732 0.720 0.761 2.517 0.393 0.168
0.805 0.955 0.928 0.992 0.999 0.941 0.985 0.988
minium concentration significantly exceeds the cation exchange capacity of the clay (17 mmol Al/g clay and 77.8 mmol Al/g clay). An aging time of 1 day appears to be enough for a sufficient intercalation of the aluminium polyoxo cations in-between the clay sheets. The slightly increasing basal spacing with increasing aging time (Table 1) indicates that a small ‘structural effect’ takes place, but this effect does not significantly affect the FA sorption capacity of the clays. This ‘structural effect’ is probably caused by a change in nature of the polyhydroxo cations in both the base hydrolysed and the diluted commercial pillaring solution with time. For a diluted commercial solution this change in the nature of the polyhydroxo cations is more pronounced than for a base hydrolysed solution (Pinnavaia et al., 1984; Schoonheydt et al., 1993). This could explain the larger difference in basal spacing between the ComPil clays of 1 and 6 days aging time and the LabPil clays prepared with an aging time of 1 and 6 days (Table 1). These differences in basal spacing between the ComPil clays of 1 and 6 days aging time and the LabPil clays prepared with an aging time of 1 and 6 days, however has no systematic effect on the FA sorption as already shown in Fig. 3. 3.3. Effect of the drying procedure on the FA sorption Fig. 4 shows sorption isotherms of FA onto freezedried and oven-dried LabPil-0.06M-1d clays. The Langmuir parameters of the isotherms are given in Table 3. The maximum FA sorption capacity of oven-
288
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
Fig. 4. Sorption isotherms of FA onto an oven-dried (pH 5.0–5.6) and a freeze-dried LabPil-0.06M-1d clay (pH 5.9–6.5) in 0.005 M CaCl2. Lines are drawn to guide the eye.
dried Al-hydroxy intercalated clays (18 mg FA/g) is significantly lower than the one of freeze-dried Al-hydroxy intercalated clays (60 mg FA/g). It has also to be mentioned that the equilibrium pH of the suspension of oven-dried Al-hydroxy intercalated clays (5.0–5.6) is slightly lower than the one of freeze-dried Al-hydroxy intercalated clays (5.9–6.5). This small pH difference cannot explain the large difference in sorption capacity between oven dried and freeze dried Al-hydroxy intercalated clays as shown in a separate study on the sorption mechanism of FA on these Al-hydroxy intercalated clays (Vreysen and Maes, submitted for publication-b). This study shows that FA are sorbed onto Al-hydroxy intercalated clays by ligand exchange and calcium bridging. The observed pH increase with increasing FA sorption is caused by the released OH− ions from ligand exchange reactions between the FA molecules and the intercalated Al species. The larger sorption capacity of freeze dried samples for FA is in agreement with structural differences between the oven-dried and the freeze-dried clays. Pinnavaia et al. (1984) found that this difference in pore openings could not be observed by the adsorption of small molecules such as N2, since the BET surface of airdried and freeze dried Al intercalated clays was similar. However, from adsorption experiments with organic molecules of different kinetic diameters they concluded that oven-drying yields products with pore openings greater than 0.62 nm but less than 0.92 nm. In contrast, freeze-dried products exhibit adsorption capacities for molecules with a kinetic diameter >1 nm. Thus, freeze drying preserves the structure of the Al intercalated clay in suspension, giving rise to edge-to-edge or face-to-
edge aggregation, whereas oven-drying optimises the face-to-face aggregation and also results in a different pore structure. The diameter of Suwannee River fulvic acid has been determined to be 1.76 nm using small angle X-ray scattering by Aiken et al. (1995) and 1.64 nm by Thurman et al. (1982). Domingos et al. (2004) report an average molecular weight of 1000 Da for Laurentian Fulvic acid. This corresponds with a diameter of 1 nm. Therefore the larger sorption capacity of freeze-dried samples for FA can be attributed to their much broader pore range. 3.4. Effect of the calcination on the FA sorption Fig. 5 presents sorption isotherms of FA onto both calcined and non-calcined fd-LabPil-0.06M-1d and fdComPil-3.12M-1d clays. The calculated Langmuir parameters are shown in Table 3. The calcination decreases the maximum sorption capacity of Al-hydroxy intercalated clays from 60 mg FA/g (uncalcined) to 50 mg FA/g (calcined). Compared to the non-calcined samples the equilibrium pH of the calcined samples (6.3–7.1) is slightly increased. This small pH increase cannot fully explain the decreased FA sorption. The decrease in sorption capacity can probably be explained by the decrease in specific surface area upon calcination (Table 2) and/or the transformation of Al-poly-hydroxo cations to Aloxide pillars. Upon calcination dehydration and dehydroxylation reactions of the charged pillar precursors occur to give neutral Al-oxide pillars (Cool and Vanstant, 1998). The decreased FA sorption capacity is probably best explained by a different FA sorption onto Al polyhydroxo cations and neutral Al-oxide
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
289
Fig. 5. Sorption isotherms of FA onto both a calcined (pH 6.3–7.1) and a non-calcined LabPil-0.06M-1d and ComPil-3.12M-1d freeze-dried clays (pH 5.9–6.5), in 0.005 M CaCl2.
pillars. In agreement with previous observations, little or no difference is observed between the ComPil and the LabPil clays. The comparison of the FA sorption in the pH range 5.9–7.1 on different Al-hydroxy intercalated clays leads us to conclude that the non-calcined freeze-dried Alhydroxy intercalated clays showed the largest sorption capacity for FA. Fig. 6 compares the sorption isotherms of such a non-calcined freeze dried Al-hydroxy intercalated clay (fd-LabPil-0.06M-1d) with a freeze dried WyB in the calcium form (fd-CaWyB) and a granular
activated carbon (Organosorb 10, DESOTEC). The FA sorption capacity of the freeze dried Al-hydroxy intercalated clay (60 mg/g) is twice as large as the FA sorption capacity of the granular activated carbon (30 mg/g) and three times as large as the FA sorption capacity of the freeze dried CaWyB (20 mg/g). This indicates that freeze dried Al-hydroxy intercalated clays are potential good sorbents for the removal of fulvic acids from water. To remove 99% of FA from typical surface water a sorbent concentration of about 0.5 g/l would be required.
Fig. 6. Sorption isotherms of FA onto a fd-LabPil-0.06M-1d, a fd-CaWyB and a Granular Activated Carbon (Organosorb 10, DESOTEC) (pH 5.9–7.1).
290
S. Vreysen, A. Maes / Applied Clay Science 32 (2006) 283–290
Organo-clays are also known to have a large sorption capacity for humic substances (Önkal-Engin et al., 2000; Jaruwong and Wibulswas, 2003). However, in these papers mostly the sorption of humic acid is studied, which sorbs to the organo-clays by hydrophobic sorption. Humic acid is known to be more hydrophobic than fulvic acid. The organo-clays thus have a much larger affinity for humic acid than for fulvic acid (Zhao and Vance, 1998). It is therefore difficult to compare our experimental results with these literature data in a quantitative manner. 4. Conclusion The drying method and the calcination of the Alhydroxy intercalated clays were the most important parameters influencing the FA sorption capacity of the Al-hydroxy intercalated clays. Oven-dried Al-hydroxy intercalated clays showed a smaller sorption capacity for FA compared to freeze-dried Al-hydroxy intercalated clays because of the smaller pore openings in the former. Calcination lead to a change in the nature of the intercalated species and a decreased surface area and basal spacing of the clay, which resulted in a decreased sorption capacity of the Al-hydroxy intercalated clays for FA. The nature of the pillaring solution and the aging time of the sorbent had no influence on the FA sorption capacity of the Al-hydroxy intercalated clays. Comparison with freeze dried CaWyB and granular activated carbon showed that freeze dried Al-hydroxy intercalated clays are potential good sorbents for the removal of fulvic acids from water. Acknowledgments The authors acknowledge the KULeuven Geconcerteerde Onderzoeksacties (GOA 2000/007 and 2005/013) for their financial support and Reheis Chemical Co. for providing a sample of the Chlorhydrol solution. References Abdel-Jawad, M., Ebrahim, S., Al-Atram, F., Al-Shammari, S., 1997. Pretreatment of the municipal wastewater feed for reverse osmosis plants. Desalination 109, 211–223. Aiken, G.R., Brown, P.A., Noyes, T.I., Pinckney, D.J., 1995. Molecular size and weight of fulvic and humic acids from the Suwannee river. In: Averett, R.C., Leenheer, J.A., McKnight, D. M., Thorn, K.A. (Eds.), Humic Substances in the Suwannee River, Georgia: Interactions, Properties, and Proposed Structures. US Geological Survey, Denver, pp. 89–97. Amin, S., Jayson, S.S., 1996. Humic substance uptake by hydrotalcites and PILCs. Water Research 30 (2), 299–306.
Chakravorty, B., Layson, A., 1997. Ideal feed pretreatment for reverse osmosis by continuous microfiltration. Desalination 110, 143–150. Cho, J., Amy, G., Pellegrino, J., Yoon, Y., 1998. Characterisation of clean and natural organic matter (NOM) fouled NF and UF membranes, and foulants characterisation. Desalination 188, 101–108. Cool, P., Vanstant, E.F., 1998. Al-hydroxy intercalated clays: preparation, characterization and applications. Molecular Sieves 1, 165–288. Domingos, R.F., Benedetti, M.F., Croué, J.P., Pinheiro, J.P., 2004. Electrochemical methodology to study labile trace metal/natural organic matter complexation at low concentration levels in natural waters. Analytica Chimica Acta 521 (1), 77–86. Gil, A., Gandía, L.M., 2003. Microstructure and quantitative estimation of the micropore-size distribution of an aluminapillared clay from nitrogen adsorption at 77 K and carbon dioxide adsorption at 273 K. Chemical Engineering Science 58, 3059–3075. Hong, S., Elimelech, M., 1997. Chemical and Physical aspects of natural organic matter (NOM) fouling of nanofiltration membranes. Journal of Membrane Science 132, 159–181. Huber, S.A., 1998. Evidence for membrane fouling by specific TOC constituents. Desalination 119, 229–234. Jaruwong, P., Wibulswas, R., 2003. Influence of organo-clay's carbon number on the adsorption of humic acid. Asian Journal of Energy and Environment 4 (1–2), 41–59. Kloprogge, J.T., Evans, R., Hickey, L., Frost, R.L., 2002. Characterisation and Al-pillaring of smectites from Miles, Queensland (Australia). Applied Clay Science 20, 157–163. Lacey, A.L., Hayes, M.H.B., Vaidyanathan, L.V., 1997. Preparation of iron Al-hydroxy intercalated clays and their applications for sorption of humic substances. In: Hayes, M.H.B., Wilson, W.S. (Eds.), Humic Substances, Peats, and Sludges. Health and Environmental Aspects. The Royal Society of Chemistry, Cambridge, pp. 219–225. Muurinen, A., Lehikoinen, J., 1999. Porewater chemistry in compacted bentonite. Engineering Geology 54, 207–214. Önkal-Engin, G., Wibulswas, R., White, D.A., 2000. Humic acid uptake from aqueous media using hydrotalcites and modified montmorillonite. Environmental Technology 21, 167–175. Pinnavaia, T.J., Tzou, M.-S., Landau, S.D., Raythatha, R.H., 1984. On the pillaring and delamination of smectite clay catalysts by polyoxo cations of aluminium. Journal of Molecular Catalysis 27, 195–212. Schoonheydt, R.A., Van den Eynde, J., Tubbax, H., Leeman, H., Struyckens, M., Lenotte, I., Stone, W.E.E., 1993. The Al pillaring of clays: Part I. Pillaring with dilute and concentrated Al solutions. Clays and Clay Minerals 41 (5), 598–607. Thurman, E.M., Wershaw, R.L., Malcolm, R.L., Pinckney, D.J., 1982. Molecular size of aquatic humic substances. Organic Geochemistry 4, 27–35. Vreysen, S., Maes, A., submitted for publication-a. Influence of pH on the aluminium speciation in freeze dried Al-hydroxy intercalated clays. Applied Clay Science. Vreysen, S., Maes, A., submitted for publication-b. Sorption mechanism of fulvic acid onto freeze dried Al-hydroxy intercalated clays. Applied Clay Science. Wilbulswas, R., White, D.A., Rautiu, R., 1998. Removal of humic substances from water by alumina-based Al-hydroxy intercalated clays. Environmental Technology 19, 627–632. Zhao, H., Vance, G.F., 1998. Sorption of trichloroethylene by organoclays in the presence of humic substances. Water Research 32 (12), 3710–3716.