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Adsorption of low concentration humic acid from water by palygorskite
Applied Clay Science 67–68 (2012) 164–168
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Adsorption of low concentration humic acid from water by palygorskite Mingshan Wang a, Libing Liao a,⁎, Xiuli Zhang a, Zhaohui Li b, c a b c
School of Material Sciences and Technology, China University of Geosciences (Beijing), Beijing 100083, China Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China Geosciences Department, University of Wisconsin - Parkside, Kenosha, WI 53144, USA
a r t i c l e
i n f o
Article history: Received 1 March 2011 Received in revised form 15 July 2011 Accepted 28 September 2011 Available online 24 October 2011 Keywords: Adsorption FTIR Humic acid Palygorskite Isotherm
a b s t r a c t Humic acid (HA) is a common contaminant in groundwater and surface water. Many efforts were made to use high efficiency low cost materials to remove HA from water. In this study, the effectiveness of using palygorskite to adsorb low concentrations of HA was investigated as a function of the initial HA concentration, equilibrium time, solution pH and ionic strength, temperature, and palygorskite dose. With a particle size of 0.074–0.025 mm, the maximum adsorption of HA on palygorskite was 17 mg/g at 20 °C. The adsorption equilibrium reached in 2 h at pH 6–7 and the data fitted well to the pseudo-second-order adsorption model. Higher HA adsorption was found at low pH and high ionic strength, indicating that electrostatic interaction played an important role between HA and palygorksite. The X-ray diffraction of raw mineral and spent palygorksite showed no difference in d-spacing, suggesting surface adsorption. Fourier transform infrared spectra revealed the presence of CH2 vibrations at 2850 and 2923 cm − 1, indicating direct uptake of HA by palygorskite. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Humic acid (HA) is an important component of natural organic matter (NOM) due to biological decomposition of organic matter from plants and other organisms. It represented 50–90% of organic matter in water from terrestrial sources, lakes, and rivers. The concentration of HA ranged from 20 μg/L in groundwater to 30 mg/L in surface water (Black et al., 1996). It had a great influence on the forms of metal ions in water and the stability of colloids. Common wastewater treatment methods such as chlorination might lead to significant production of by-products (Christman et al., 1983). The dominant halogenated acids detected in the acidic ether extracts were di- and trichloroacetic acids in addition to a series of non-chlorinated oxoacids and hydroxyoxoacids with a total amount of 10 mg/g when treated at pH 2 (Becher et al., 1992). Trihalomethanes were identified as one of the products after chlorination of HA containing water (Eish and Wells, 2006; Stevens et al., 1976). Therefore, removal of HA from water attracted extensive studies. The main methods to remove HA included membrane filtration (Lowe and Hossain, 2008), coagulation (Jiang et al., 2006), and oxidation (Murray and Parsons, 2004). Although these methods were used to some extent, there were some constraints. Among the processes employed in water treatment, adsorption was an important method with high removal efficiency and no harmful by-products.
⁎ Corresponding author. Tel.: + 86 10 82322104; fax: + 86 10 82322974. E-mail address: [email protected] (L. Liao). 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.09.012
A variety of natural minerals was tested as adsorbents to remove HA since 1970s. Due to the large cation exchange capacity (CEC) and high specific surface area (SSA), bentonite or pillared bentonite was commonly investigated for the potential removal of HA (Bringle et al., 2005; Doulia et al., 2009; Peng et al., 2005). Kaolinite was also studied in great detail for its removal of HA from water (Ghabbour et al., 2004; Hur and Schlautman, 2004; Taylor and Theng, 1995). Adsorption of HA by zeolite was higher when the exchangeable cation was divalent cations such as Ca 2+ in comparison monovalent cations enriched zeolites (Capasso et al., 2005; 2007). Treatment of raw bentonite with NaCl, CH3COOH, and HCOOH before HA adsorption did not affect HA adsorption while HCl treatment caused great decrease in HA adsorption (Salman et al., 2007). Strong binding and electrostatic attraction played an important role in HA adsorption after vermiculite modified by hexadecyltrimethylammonium or intercalated with poly(hydroxy iron) cations (Abate et al., 2006). Hydration and dehydration were attributed to an important mechanism for HA adsorption on kaolinite, peat, plants and soil (Ghabbour et al., 2004). Palygorskite, formed in many arid and semi-arid soils, was a phyllosilicate with a partial chain-like structure. It was a good absorbent due to its large SSA and moderate CEC. Many researches on palygorskite were focused on the removal of heavy metals such as Cd (ÁlvarezAyuso and Garcia-Sánchez, 2007), Cu and Pb (Potgieter et al., 2006), and Th (VI) (Wu et al., 2007). The removal of organic compounds such as tetracycline and cellulase by palygorskite was also reported (Chang et al., 2009; Safari Sinegani et al., 2005). The removal of high concentration HA on palygorskite was attempted, however, only the effect of pH and heat on HA removal efficiency was studied (Singer and Huang, 1989). Other factors and adsorption mechanism were not
M. Wang et al. / Applied Clay Science 67–68 (2012) 164–168
discussed. In this paper, the adsorption of low concentration humic acid by palygorskite was studied. The effect of equilibrium time, pH and ionic strength, palygorskite dose, and temperature on HA adsorption was discussed. The kinetics of HA adsorption was analyzed. The HA adsorption mechanism on palygorskite was proposed in conjunction with XRD and FTIR analyses. 2. Materials and methods
Palygorskite was obtained from Mingmei Minerals Co., Ltd, Anhui, Province, China. The HA was supplied by Sinopharm chemical reagent Co. Ltd. A stock solution was prepared by dissolving 1 g HA in NaOH (0.025 M). After stirring for 30 min, distilled water was added to 1 L and pH adjusted by 0.025 M HCl. After set aside over night, the solution was filtered through a filter paper with pore size between 30 and 50 μm. The dissolved HA was used in the next experiments. 2.2. Adsorption experiments Batch experiments were performed by adding palygorskite (0.5–9 g) and 100 mL HA solution to a 250 mL flask. The pH was adjusted to 3–9, the temperature to 25–65 °C, and the ionic strength was adjusted by NaCl, CaCl2, and MgCl2. Removal efficiency (%) and amount of adsorption qt (mg/g) at any time t, was calculated by the following formulas:
Removal efficiency ð% Þ ¼
qt ¼
3.2. Adsorption kinetics The adsorption of HA on palygorskite increased rapidly with time and reached equilibrium in 2 h (Fig. 1). Adsorption data were fitted to different kinetic models. The pseudo-second-order model fitted the data best with a coefficient of determination r 2 = 0.99, supporting the assumption that the adsorption of HA on palygorskite was mainly chemisorption. The pseudo-second-order model had the form of: t 1 1 ¼ þ t qt kq2 e qe
2.1. Materials
C0 −Ce 100 C0
ðC0 −Ce ÞV m
ð1Þ
ð2Þ
where C0 was the initial concentration of HA (mg/L), Ce was the HA solution concentration at any time t (mg/L), V was the solution volume (L), and m was the adsorbent mass (g). 2.3. Methods of analysis The concentration of HA was measured at 254 nm using a T6 Series UV/Vis Spectrophotometer (Puxi Co., Beijing). The pH was measured with a pH meter (pHS-3C, Shanghai Leici Instrument Factory, China). X-ray diffraction (XRD) analyses were performed on Rigaku D/Max-rA/rB X-ray Diffractometer with Ni-filtered Cu Kα radiation at 40 kV and 100 mA. Samples were scanned from 2.6° to 70° (2θ) at 4°/min with a scanning step of 0.02°/step. The Fourier transform infrared (FTIR) spectra were acquired on a Nicolet 750 infrared spectrometer using KBr pressing method at room temperature. The SSA was measured by the BET method on a Micromeritics ASAP2010 surface and pore distribution tester (Norcross, GA). The permeability coefficient was calculated with the Darcy law, by measuring the water pressure difference of certain thickness materials and flow velocity of the materials.
165
ð3Þ
where qe (mg/g) was the amount of HA adsorbed at equilibrium, k (g/ (mg∗h)) was the pseudo-second-order rate constant, and kqe2 was the initial rate (mg/(g∗h)). The k determined was 6.4 g/(mg∗h), the initial rate was 4 mg/(g∗h), and the qe was 0.8 mg/g (Fig. 1). The k value was lower than 0.5 g/(mg∗min) for HA adsorption on an acid-activated bentonite (Doulia et al., 2009), but much higher than 0.01–0.02 g/(mg∗h) for HA adsorption on zeolite in the presence of Pb 2+ and Cu2+ (Wang et al., 2008). 3.3. The effect of pH on adsorption of HA Adsorption of HA on palygorksite decreased as solution pH increased (Fig. 2). Similar observations were found for humate adsorption on a standard palygorksite PFl-1 (Singer and Huang, 1989), for HA adsorption on vermiculite (Abate and Masini, 2003), on a zeolite (Wang et al., 2008), and on montmorillonite (Salman et al., 2007). The HA consisted of heterogeneous components with a wide range of molar mass and different chemical moieties. Due to its heterogeneity, there might be a broad distribution of adsorption affinities for mineral surfaces within a particular bulk material. At lower pH, the adsorption might be controlled by carboxylate groups of HA molecules and palygorskite as well as hydrogen bonding between hydroxyl surface groups and carboxylates of HA (Gu et al., 1995; Singer and Huang, 1989). In addition, in lower pH, the charge of palygorskite was positive and it might facilitate the adsorption of negatively charged HA (Hengpraprom et al., 2006; Niu et al., 2009; Peng et al., 2005). As pH increased, dissociation of the functional groups on both HA and palygorskite increased while palygorskite became negatively charged. The electrostatic repulsion interaction between the negative charges on palygorskite and HA resulted in a decrease in HA adsorption on palygorskite. However, at a high pH, the removal efficiency of HA still showed a great adsorption capacity. 3.4. The effect of temperature on adsorption of HA As the temperature increased, the removal of HA slightly decreased, suggesting an exothermic adsorption of HA on palygorskite (Fig. 3).
3. Results and discussion 3.1. Material characterization The XRD analyses showed that the sample contained >95% of palygorskite. A permeability coefficient of 69 cm/h, an order of magnitude larger than that of the aquifer materials (4.16 cm/h) was obtained for particle size ranges of 0.5–1.4 mm. The SSA was 213 m2/g and calculated diameter of the pores using cylindrical pore model was about 9 nm.
Fig. 1. Equilibrium time of adsorption of HA on palygorskite. The line was the pseudosecond-order fit to the observed data.
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M. Wang et al. / Applied Clay Science 67–68 (2012) 164–168 Table 1 The thermodynamic parameters of HA adsorption as affected by temperature.
Preheating of palygorksite also resulted in a decrease in humate adsorption (Singer and Huang, 1989). The relationship between Kd, the distribution coefficient between the amount of HA adsorbed to the equilibrium HA concentration, and the thermodynamic parameters of adsorption was expressed as: ΔH ΔS þ RT R
Kd
296 300 306 316 321
1069 955 902 819 731
was investigated in order to determine whether the HA in water reached the groundwater standard (Fig. 4). When the concentration of HA was in the 0–20 mg/L, the permanganate index linearly correlated to the HA concentration. With a palygorskite dose of 8 g, the equilibrium HA concentration was 2.6 mg/L, equivalent to a permanganate index of 1.6 mg/L, lower than the groundwater quality standards of 3 mg/L.
Fig. 2. Effect of pH on HA adsorption on palygorskite.
LnKd ¼ −
Temperature (K)
ð4Þ
where ΔH was the change in enthalpy, ΔS was the change in entropy, R was gas constant, and T was the reaction temperature in K. Increased temperature resulted in a decrease in HA adsorption, suggesting an exothermic process, similar to the HA adsorption on lanthana–alumina mixed oxide pillared bentonite (Bringle et al., 2005), but opposite to an endothermic process for HA adsorption on amine-modified polyacrylamide–bentonite composite (Anirudhan et al., 2008) and on acid-activated bentonite (Doulia et al., 2009). As Kd was much greater than 1 (Table 1) and ΔH (−10.7 kJ/mol) was negative, the calculated ΔS (0.04 kJ/(mol∗K)) was positive, indicating an increased randomness of adsorbed HA on palygorskite. A similar observation was found for HA adsorption on amine-modified polyacrylamide–bentonite composite (Anirudhan et al., 2008).
3.6. The effect of ionic strength on adsorption of HA An increase in ionic strength resulted in an increase of removal efficiency while the divalent cations had a larger effect on the adsorption of HA on palygorskite than monovalent cations (Fig. 5). Similar observations were found for HA adsorption on vermiculite (Abate and Masini, 2003) and on kaolinite and montmorillonite (Feng et al., 2005). The effect of ionic strength on HA adsorption could be attributed to several mechanisms (Capasso et al., 2005; Peng et al., 2005; Salman et al., 2007): (1) the increase of ionic strength, which resulted in minimization of the electrostatic repulsion between ionized oxygen groups and decreased molecular volume of HA, facilitated the adsorption. (2) At high ionic strength HA diffused faster due to its coiled structure. (3) An increase in ionic strength caused the compression of the thickness of the diffuse double layer which surrounded solid and liquid phases when they were in contact. Such compression helped the clay particles and HA molecules to approach each other more closely. (4) Ionization of Na +, Mg2+, and Ca2+ in aqueous solution caused the binding of water molecules and the competition between HA and water molecules for adsorption sites would be reduced. 3.7. Adsorption isotherm
3.5. The effect of palygorskite dose on adsorption of HA The removal efficiency increased with palygorskite dose (Fig. 4). Permanganate index was an important parameter to measure the content of NOM in groundwater. The third class of groundwater standard of China (GB/T 14848–93) was not exceeding 3 mg/L. The relationship between the concentration of HA and permanganate index
Fig. 3. Effect of temperature on HA adsorption on palygorskite.
Langmuir adsorption isotherm was used to describe the HA adsorption on a zeolitic tuff (Capasso et al., 2005). The linear form of Langmuir model was: Ce 1 C ¼ þ e qt KL qe qt
Fig. 4. Effect of palygorskite dose on HA adsorption.
ð5Þ
M. Wang et al. / Applied Clay Science 67–68 (2012) 164–168
167
Fig. 5. Effect of ionic strength on HA adsorption on palygorskite.
where KL was the Langmuir coefficient (L/mg). The adsorption of HA increased rapidly with the increased initial HA concentrations and followed a Langmuir adsorption (Fig. 6). The calculated HA adsorption capacity was 17 mg/g. This value was much higher than 2 mg/g of HA adsorption on clinoptilolite-rich tuffs (Capasso et al., 2007), slightly higher than 11–12 mg/g of HA adsorption on acid-activated bentonite from Greece (Doulia et al., 2009) and 8.5 mg/g on a zeolitic tuff (Capasso et al., 2005), comparable to 15–20 mg/g on kaolinite at pH 3 (Elfarissi and Pefferkorn, 2000), on a phillipsite- and chabaziterich tuff (Capasso et al., 2007), slightly smaller than 40 mg/g on a vermiculite (Abate and Masini, 2003), 78 mg/g on a zeolite (Wang et al., 2008), and 68 mg/g for humate adsorption on PFl-1 (Singer and Huang, 1989), but was much smaller than 130–200 mg/g for HA adsorption on amine-modified polyacrylamide–bentonite composite (Anirudhan et al., 2008). The good fit to the Langmuir isotherm equation suggested that site-limiting processes were responsible for HA sorption on kaolinite (Hur and Schlautman, 2004) and for HA adsorption on palygorksite in a previous study (Singer and Huang, 1989) and in this study. 3.8. The XRD pattern of adsorption HA on palygorskite After adsorbing different amounts of HA, the structure of palygorskite did not changed (data not shown), suggesting that the adsorption of HA on palygorskite was on the mineral surface. 3.9. FTIR analyses The FTIR spectra of HA-adsorbed palygorskite from 20 mg/L and 100 mg/L HA solutions showed no significant change in comparison to raw palygorskite (Fig. 7) and agreed well with the published results
Fig. 6. HA adsorption isotherm. The line was the Langmuir fit to the observed data.
Fig. 7. FTIR spectra of raw palygorskite, HA-adsorbed palygorskite and HA at 400–4000 cm− 1 (a) and 2800–3000 cm− 1 (b).
(Blanco et al., 1988; Serna et al., 1977; Yariv, 1986). The most characteristic bands of palygorskite were those in 3700–3200 cm− 1 corresponding to the stretching of structural hydroxyl groups, 1635 cm− 1 due to deformation of water, and 1300–400 cm − 1 from Si, Al–O stretching. Because of the more accessible position of the coordinated water molecules, they could be easily deuterated at room temperature, leaving three absorption bands at 3625, 3595, and 3560 cm − 1 due to the structural hydroxyls (Serna et al., 1977). These bands had no apparent shift after HA adsorption, indicating that its adsorption on palygorskite did not alter the structure, similar to that of XRD observation. However, after HA adsorption from an initial HA concentration of 100 mg L two vibration bands at 2852 and 2925 cm− 1 appeared (Fig. 7). They corresponded to symmetric and asymmetric CH2 stretching vibrations (Li et al., 2008). The appearance of these two bands confirmed uptake of HA by palygorskite. Direct evidence for selective sorption of different functional groups and proteins on clay mineral surfaces was obtained using 1H NMR techniques with montmorillonite displaying a higher uptake of aromatics and proteins while more CH2 groups were observed to sorb to kaolinite surfaces (Feng et al., 2005). 3.10. Discussion The presence of HA on the vermiculite increased the adsorption of divalent metals Pb and Cd in the all pH and was attributed to surface complexation at pH 5.0 and 6.0 (Abate and Masini, 2005). The adsorption of Cd 2+ on kaolinite also increased due to formation of kaolinite humic acid complex (Taylor and Theng, 1995). On the other hand, in Pb 2+/HA system, Pb 2+ exhibited competitive adsorption with HA on the raw zeolite, resulting in reduced adsorption of both Pb 2+ and humic acid (Wang et al., 2008). For the Cu 2+/HA system, competitive adsorption and complexation between Cu 2+ and HA influenced the adsorption, resulting in decreased adsorption of HA but increased adsorption of Cu 2+ (Wang et al., 2008). The enrichment in divalent cations enhanced the ability of a zeolitic tuff to take up HA, whereas enrichment by monovalent cations reduced it, which was attributed to formation of stable bridges with the organic matter by the cations and the specific selective sorption by the tuff (Capasso et al., 2005). In the Na +, Mg 2+ and Ca 2+/HA system, they were more likely similar with kaolinite to form a palygorskite humic acid complex. But
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Na + might be the form more stable bridges with the organic matter result a slightly low adsorption of HA. The presence of divalent cations Mg2+ and Ca2+ on palygorskite should be responsible for the adsorption of HA. Ligand exchange between the carboxylated groups of the humic substances and the inorganic surface hydroxyls of vermiculite seemed to be responsible for HA adsorption on vermiculite (Abate and Masini, 2003). An attempt to separate the contributions of various modes of interactions between HA clay minerals showed that ligand exchange, van der Waals, and cation bridging accounted for approximately 32, 22%, and 41% of HA sorption on clay surfaces when Ca2+ was the background electrolyte (Feng et al., 2005). In addition, precipitation was found the dominant process of HA removal from solution by vermiculite (Abate and Masini, 2003). Regardless what was the most important mechanism for HA adsorption, palygorskite certainly showed a great promise as an adsorbent to remove low concentrations of HA from water. 4. Conclusion The maximum adsorption of HA on palygorskite was 17 mg/g at 20 °C. Adsorption reached at equilibrium in 2 h at pH 6–7 and fitted well to the pseudo-second-order adsorption model. The adsorption of HA on palygorskite increased at low pH and high ionic strength, indicated that the electrostatic interaction played an important role in interactions between HA and palygorskite. The concentration of HA in water was reduced from 20 to 2.6 mg/L, equivalent to the permanganate index 1.6 mg/L, below the groundwater quality standard of 3 mg/L. The calculated ΔH and ΔS values indicated exothermic adsorption and increase in randomness of the adsorbed HA on palygorskite. The XRD pattern and FTIR spectra of raw and HA-absorbed palygorskite indicated that the adsorption was on the external surfaces. Acknowledgments Financial supports from National Program of Control and Treatment of Water Pollution (No. 2009ZX07424-002) and the School of Water Resources and Environment of China University of Geosciences are greatly appreciated. References Abate, G., Masini, J.C., 2003. Influence of pH and ionic strength on removal processes of a sedimentary humic acid in a suspension of vermiculite. Colloids Surfaces A: Physicochemical and Engineering Aspects 226, 25–34. Abate, G., Masini, J.C., 2005. Influence of pH, ionic strength and humic acid on adsorption of Cd(II) and Pb(II) onto vermiculite. Colloids Surfaces A: Physicochemical and Engineering Aspects 262, 33–39. Abate, G., Dos Santos, L.B.O., Colombo, S.M., Masini, J.C., 2006. Removal of fulvic acid from aqueous media by adsorption onto modified vermiculite. Applied Clay Science 32, 261–270. Álvarez-Ayuso, E., Garcia-Sánchez, A., 2007. Removal of cadmium from aqueous solutions by palygorskite. Journal of Hazardous Materials 147, 594–600. Anirudhan, T., Suchithra, P., Rijith, S., 2008. Amine-modified polyacrylamide–bentonite composite for the adsorption of humic acid in aqueous solutions. Colloids Surfaces A: Physicochemical and Engineering Aspects 326, 147–156. Becher, G., Ovrum, N.M., Christman, R.F., 1992. Novel chlorination by-products of aquatic humic substances. The Science of the Total Environment 117, 509–520. Black, B.D., Harrington, G.W., Singer, P.C., 1996. Reducing cancer risks by improving organic carbon removal. Journal of American Water Works Association 88, 40–52. Blanco, C., Herrero, J., Mendioroz, S., Pajares, J., 1988. Infrared studies of surface acidity and reversible folding in palygorskite. Clays and Clay Minerals 36, 364–368.
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Adsorption of low concentration humic acid from water by palygorskite Mingshan Wang a, Libing Liao a,⁎, Xiuli Zhang a, Zhaohui Li b, c a b c
School of Material Sciences and Technology, China University of Geosciences (Beijing), Beijing 100083, China Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China Geosciences Department, University of Wisconsin - Parkside, Kenosha, WI 53144, USA
a r t i c l e
i n f o
Article history: Received 1 March 2011 Received in revised form 15 July 2011 Accepted 28 September 2011 Available online 24 October 2011 Keywords: Adsorption FTIR Humic acid Palygorskite Isotherm
a b s t r a c t Humic acid (HA) is a common contaminant in groundwater and surface water. Many efforts were made to use high efficiency low cost materials to remove HA from water. In this study, the effectiveness of using palygorskite to adsorb low concentrations of HA was investigated as a function of the initial HA concentration, equilibrium time, solution pH and ionic strength, temperature, and palygorskite dose. With a particle size of 0.074–0.025 mm, the maximum adsorption of HA on palygorskite was 17 mg/g at 20 °C. The adsorption equilibrium reached in 2 h at pH 6–7 and the data fitted well to the pseudo-second-order adsorption model. Higher HA adsorption was found at low pH and high ionic strength, indicating that electrostatic interaction played an important role between HA and palygorksite. The X-ray diffraction of raw mineral and spent palygorksite showed no difference in d-spacing, suggesting surface adsorption. Fourier transform infrared spectra revealed the presence of CH2 vibrations at 2850 and 2923 cm − 1, indicating direct uptake of HA by palygorskite. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Humic acid (HA) is an important component of natural organic matter (NOM) due to biological decomposition of organic matter from plants and other organisms. It represented 50–90% of organic matter in water from terrestrial sources, lakes, and rivers. The concentration of HA ranged from 20 μg/L in groundwater to 30 mg/L in surface water (Black et al., 1996). It had a great influence on the forms of metal ions in water and the stability of colloids. Common wastewater treatment methods such as chlorination might lead to significant production of by-products (Christman et al., 1983). The dominant halogenated acids detected in the acidic ether extracts were di- and trichloroacetic acids in addition to a series of non-chlorinated oxoacids and hydroxyoxoacids with a total amount of 10 mg/g when treated at pH 2 (Becher et al., 1992). Trihalomethanes were identified as one of the products after chlorination of HA containing water (Eish and Wells, 2006; Stevens et al., 1976). Therefore, removal of HA from water attracted extensive studies. The main methods to remove HA included membrane filtration (Lowe and Hossain, 2008), coagulation (Jiang et al., 2006), and oxidation (Murray and Parsons, 2004). Although these methods were used to some extent, there were some constraints. Among the processes employed in water treatment, adsorption was an important method with high removal efficiency and no harmful by-products.
⁎ Corresponding author. Tel.: + 86 10 82322104; fax: + 86 10 82322974. E-mail address: [email protected] (L. Liao). 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.09.012
A variety of natural minerals was tested as adsorbents to remove HA since 1970s. Due to the large cation exchange capacity (CEC) and high specific surface area (SSA), bentonite or pillared bentonite was commonly investigated for the potential removal of HA (Bringle et al., 2005; Doulia et al., 2009; Peng et al., 2005). Kaolinite was also studied in great detail for its removal of HA from water (Ghabbour et al., 2004; Hur and Schlautman, 2004; Taylor and Theng, 1995). Adsorption of HA by zeolite was higher when the exchangeable cation was divalent cations such as Ca 2+ in comparison monovalent cations enriched zeolites (Capasso et al., 2005; 2007). Treatment of raw bentonite with NaCl, CH3COOH, and HCOOH before HA adsorption did not affect HA adsorption while HCl treatment caused great decrease in HA adsorption (Salman et al., 2007). Strong binding and electrostatic attraction played an important role in HA adsorption after vermiculite modified by hexadecyltrimethylammonium or intercalated with poly(hydroxy iron) cations (Abate et al., 2006). Hydration and dehydration were attributed to an important mechanism for HA adsorption on kaolinite, peat, plants and soil (Ghabbour et al., 2004). Palygorskite, formed in many arid and semi-arid soils, was a phyllosilicate with a partial chain-like structure. It was a good absorbent due to its large SSA and moderate CEC. Many researches on palygorskite were focused on the removal of heavy metals such as Cd (ÁlvarezAyuso and Garcia-Sánchez, 2007), Cu and Pb (Potgieter et al., 2006), and Th (VI) (Wu et al., 2007). The removal of organic compounds such as tetracycline and cellulase by palygorskite was also reported (Chang et al., 2009; Safari Sinegani et al., 2005). The removal of high concentration HA on palygorskite was attempted, however, only the effect of pH and heat on HA removal efficiency was studied (Singer and Huang, 1989). Other factors and adsorption mechanism were not
M. Wang et al. / Applied Clay Science 67–68 (2012) 164–168
discussed. In this paper, the adsorption of low concentration humic acid by palygorskite was studied. The effect of equilibrium time, pH and ionic strength, palygorskite dose, and temperature on HA adsorption was discussed. The kinetics of HA adsorption was analyzed. The HA adsorption mechanism on palygorskite was proposed in conjunction with XRD and FTIR analyses. 2. Materials and methods
Palygorskite was obtained from Mingmei Minerals Co., Ltd, Anhui, Province, China. The HA was supplied by Sinopharm chemical reagent Co. Ltd. A stock solution was prepared by dissolving 1 g HA in NaOH (0.025 M). After stirring for 30 min, distilled water was added to 1 L and pH adjusted by 0.025 M HCl. After set aside over night, the solution was filtered through a filter paper with pore size between 30 and 50 μm. The dissolved HA was used in the next experiments. 2.2. Adsorption experiments Batch experiments were performed by adding palygorskite (0.5–9 g) and 100 mL HA solution to a 250 mL flask. The pH was adjusted to 3–9, the temperature to 25–65 °C, and the ionic strength was adjusted by NaCl, CaCl2, and MgCl2. Removal efficiency (%) and amount of adsorption qt (mg/g) at any time t, was calculated by the following formulas:
Removal efficiency ð% Þ ¼
qt ¼
3.2. Adsorption kinetics The adsorption of HA on palygorskite increased rapidly with time and reached equilibrium in 2 h (Fig. 1). Adsorption data were fitted to different kinetic models. The pseudo-second-order model fitted the data best with a coefficient of determination r 2 = 0.99, supporting the assumption that the adsorption of HA on palygorskite was mainly chemisorption. The pseudo-second-order model had the form of: t 1 1 ¼ þ t qt kq2 e qe
2.1. Materials
C0 −Ce 100 C0
ðC0 −Ce ÞV m
ð1Þ
ð2Þ
where C0 was the initial concentration of HA (mg/L), Ce was the HA solution concentration at any time t (mg/L), V was the solution volume (L), and m was the adsorbent mass (g). 2.3. Methods of analysis The concentration of HA was measured at 254 nm using a T6 Series UV/Vis Spectrophotometer (Puxi Co., Beijing). The pH was measured with a pH meter (pHS-3C, Shanghai Leici Instrument Factory, China). X-ray diffraction (XRD) analyses were performed on Rigaku D/Max-rA/rB X-ray Diffractometer with Ni-filtered Cu Kα radiation at 40 kV and 100 mA. Samples were scanned from 2.6° to 70° (2θ) at 4°/min with a scanning step of 0.02°/step. The Fourier transform infrared (FTIR) spectra were acquired on a Nicolet 750 infrared spectrometer using KBr pressing method at room temperature. The SSA was measured by the BET method on a Micromeritics ASAP2010 surface and pore distribution tester (Norcross, GA). The permeability coefficient was calculated with the Darcy law, by measuring the water pressure difference of certain thickness materials and flow velocity of the materials.
165
ð3Þ
where qe (mg/g) was the amount of HA adsorbed at equilibrium, k (g/ (mg∗h)) was the pseudo-second-order rate constant, and kqe2 was the initial rate (mg/(g∗h)). The k determined was 6.4 g/(mg∗h), the initial rate was 4 mg/(g∗h), and the qe was 0.8 mg/g (Fig. 1). The k value was lower than 0.5 g/(mg∗min) for HA adsorption on an acid-activated bentonite (Doulia et al., 2009), but much higher than 0.01–0.02 g/(mg∗h) for HA adsorption on zeolite in the presence of Pb 2+ and Cu2+ (Wang et al., 2008). 3.3. The effect of pH on adsorption of HA Adsorption of HA on palygorksite decreased as solution pH increased (Fig. 2). Similar observations were found for humate adsorption on a standard palygorksite PFl-1 (Singer and Huang, 1989), for HA adsorption on vermiculite (Abate and Masini, 2003), on a zeolite (Wang et al., 2008), and on montmorillonite (Salman et al., 2007). The HA consisted of heterogeneous components with a wide range of molar mass and different chemical moieties. Due to its heterogeneity, there might be a broad distribution of adsorption affinities for mineral surfaces within a particular bulk material. At lower pH, the adsorption might be controlled by carboxylate groups of HA molecules and palygorskite as well as hydrogen bonding between hydroxyl surface groups and carboxylates of HA (Gu et al., 1995; Singer and Huang, 1989). In addition, in lower pH, the charge of palygorskite was positive and it might facilitate the adsorption of negatively charged HA (Hengpraprom et al., 2006; Niu et al., 2009; Peng et al., 2005). As pH increased, dissociation of the functional groups on both HA and palygorskite increased while palygorskite became negatively charged. The electrostatic repulsion interaction between the negative charges on palygorskite and HA resulted in a decrease in HA adsorption on palygorskite. However, at a high pH, the removal efficiency of HA still showed a great adsorption capacity. 3.4. The effect of temperature on adsorption of HA As the temperature increased, the removal of HA slightly decreased, suggesting an exothermic adsorption of HA on palygorskite (Fig. 3).
3. Results and discussion 3.1. Material characterization The XRD analyses showed that the sample contained >95% of palygorskite. A permeability coefficient of 69 cm/h, an order of magnitude larger than that of the aquifer materials (4.16 cm/h) was obtained for particle size ranges of 0.5–1.4 mm. The SSA was 213 m2/g and calculated diameter of the pores using cylindrical pore model was about 9 nm.
Fig. 1. Equilibrium time of adsorption of HA on palygorskite. The line was the pseudosecond-order fit to the observed data.
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M. Wang et al. / Applied Clay Science 67–68 (2012) 164–168 Table 1 The thermodynamic parameters of HA adsorption as affected by temperature.
Preheating of palygorksite also resulted in a decrease in humate adsorption (Singer and Huang, 1989). The relationship between Kd, the distribution coefficient between the amount of HA adsorbed to the equilibrium HA concentration, and the thermodynamic parameters of adsorption was expressed as: ΔH ΔS þ RT R
Kd
296 300 306 316 321
1069 955 902 819 731
was investigated in order to determine whether the HA in water reached the groundwater standard (Fig. 4). When the concentration of HA was in the 0–20 mg/L, the permanganate index linearly correlated to the HA concentration. With a palygorskite dose of 8 g, the equilibrium HA concentration was 2.6 mg/L, equivalent to a permanganate index of 1.6 mg/L, lower than the groundwater quality standards of 3 mg/L.
Fig. 2. Effect of pH on HA adsorption on palygorskite.
LnKd ¼ −
Temperature (K)
ð4Þ
where ΔH was the change in enthalpy, ΔS was the change in entropy, R was gas constant, and T was the reaction temperature in K. Increased temperature resulted in a decrease in HA adsorption, suggesting an exothermic process, similar to the HA adsorption on lanthana–alumina mixed oxide pillared bentonite (Bringle et al., 2005), but opposite to an endothermic process for HA adsorption on amine-modified polyacrylamide–bentonite composite (Anirudhan et al., 2008) and on acid-activated bentonite (Doulia et al., 2009). As Kd was much greater than 1 (Table 1) and ΔH (−10.7 kJ/mol) was negative, the calculated ΔS (0.04 kJ/(mol∗K)) was positive, indicating an increased randomness of adsorbed HA on palygorskite. A similar observation was found for HA adsorption on amine-modified polyacrylamide–bentonite composite (Anirudhan et al., 2008).
3.6. The effect of ionic strength on adsorption of HA An increase in ionic strength resulted in an increase of removal efficiency while the divalent cations had a larger effect on the adsorption of HA on palygorskite than monovalent cations (Fig. 5). Similar observations were found for HA adsorption on vermiculite (Abate and Masini, 2003) and on kaolinite and montmorillonite (Feng et al., 2005). The effect of ionic strength on HA adsorption could be attributed to several mechanisms (Capasso et al., 2005; Peng et al., 2005; Salman et al., 2007): (1) the increase of ionic strength, which resulted in minimization of the electrostatic repulsion between ionized oxygen groups and decreased molecular volume of HA, facilitated the adsorption. (2) At high ionic strength HA diffused faster due to its coiled structure. (3) An increase in ionic strength caused the compression of the thickness of the diffuse double layer which surrounded solid and liquid phases when they were in contact. Such compression helped the clay particles and HA molecules to approach each other more closely. (4) Ionization of Na +, Mg2+, and Ca2+ in aqueous solution caused the binding of water molecules and the competition between HA and water molecules for adsorption sites would be reduced. 3.7. Adsorption isotherm
3.5. The effect of palygorskite dose on adsorption of HA The removal efficiency increased with palygorskite dose (Fig. 4). Permanganate index was an important parameter to measure the content of NOM in groundwater. The third class of groundwater standard of China (GB/T 14848–93) was not exceeding 3 mg/L. The relationship between the concentration of HA and permanganate index
Fig. 3. Effect of temperature on HA adsorption on palygorskite.
Langmuir adsorption isotherm was used to describe the HA adsorption on a zeolitic tuff (Capasso et al., 2005). The linear form of Langmuir model was: Ce 1 C ¼ þ e qt KL qe qt
Fig. 4. Effect of palygorskite dose on HA adsorption.
ð5Þ
M. Wang et al. / Applied Clay Science 67–68 (2012) 164–168
167
Fig. 5. Effect of ionic strength on HA adsorption on palygorskite.
where KL was the Langmuir coefficient (L/mg). The adsorption of HA increased rapidly with the increased initial HA concentrations and followed a Langmuir adsorption (Fig. 6). The calculated HA adsorption capacity was 17 mg/g. This value was much higher than 2 mg/g of HA adsorption on clinoptilolite-rich tuffs (Capasso et al., 2007), slightly higher than 11–12 mg/g of HA adsorption on acid-activated bentonite from Greece (Doulia et al., 2009) and 8.5 mg/g on a zeolitic tuff (Capasso et al., 2005), comparable to 15–20 mg/g on kaolinite at pH 3 (Elfarissi and Pefferkorn, 2000), on a phillipsite- and chabaziterich tuff (Capasso et al., 2007), slightly smaller than 40 mg/g on a vermiculite (Abate and Masini, 2003), 78 mg/g on a zeolite (Wang et al., 2008), and 68 mg/g for humate adsorption on PFl-1 (Singer and Huang, 1989), but was much smaller than 130–200 mg/g for HA adsorption on amine-modified polyacrylamide–bentonite composite (Anirudhan et al., 2008). The good fit to the Langmuir isotherm equation suggested that site-limiting processes were responsible for HA sorption on kaolinite (Hur and Schlautman, 2004) and for HA adsorption on palygorksite in a previous study (Singer and Huang, 1989) and in this study. 3.8. The XRD pattern of adsorption HA on palygorskite After adsorbing different amounts of HA, the structure of palygorskite did not changed (data not shown), suggesting that the adsorption of HA on palygorskite was on the mineral surface. 3.9. FTIR analyses The FTIR spectra of HA-adsorbed palygorskite from 20 mg/L and 100 mg/L HA solutions showed no significant change in comparison to raw palygorskite (Fig. 7) and agreed well with the published results
Fig. 6. HA adsorption isotherm. The line was the Langmuir fit to the observed data.
Fig. 7. FTIR spectra of raw palygorskite, HA-adsorbed palygorskite and HA at 400–4000 cm− 1 (a) and 2800–3000 cm− 1 (b).
(Blanco et al., 1988; Serna et al., 1977; Yariv, 1986). The most characteristic bands of palygorskite were those in 3700–3200 cm− 1 corresponding to the stretching of structural hydroxyl groups, 1635 cm− 1 due to deformation of water, and 1300–400 cm − 1 from Si, Al–O stretching. Because of the more accessible position of the coordinated water molecules, they could be easily deuterated at room temperature, leaving three absorption bands at 3625, 3595, and 3560 cm − 1 due to the structural hydroxyls (Serna et al., 1977). These bands had no apparent shift after HA adsorption, indicating that its adsorption on palygorskite did not alter the structure, similar to that of XRD observation. However, after HA adsorption from an initial HA concentration of 100 mg L two vibration bands at 2852 and 2925 cm− 1 appeared (Fig. 7). They corresponded to symmetric and asymmetric CH2 stretching vibrations (Li et al., 2008). The appearance of these two bands confirmed uptake of HA by palygorskite. Direct evidence for selective sorption of different functional groups and proteins on clay mineral surfaces was obtained using 1H NMR techniques with montmorillonite displaying a higher uptake of aromatics and proteins while more CH2 groups were observed to sorb to kaolinite surfaces (Feng et al., 2005). 3.10. Discussion The presence of HA on the vermiculite increased the adsorption of divalent metals Pb and Cd in the all pH and was attributed to surface complexation at pH 5.0 and 6.0 (Abate and Masini, 2005). The adsorption of Cd 2+ on kaolinite also increased due to formation of kaolinite humic acid complex (Taylor and Theng, 1995). On the other hand, in Pb 2+/HA system, Pb 2+ exhibited competitive adsorption with HA on the raw zeolite, resulting in reduced adsorption of both Pb 2+ and humic acid (Wang et al., 2008). For the Cu 2+/HA system, competitive adsorption and complexation between Cu 2+ and HA influenced the adsorption, resulting in decreased adsorption of HA but increased adsorption of Cu 2+ (Wang et al., 2008). The enrichment in divalent cations enhanced the ability of a zeolitic tuff to take up HA, whereas enrichment by monovalent cations reduced it, which was attributed to formation of stable bridges with the organic matter by the cations and the specific selective sorption by the tuff (Capasso et al., 2005). In the Na +, Mg 2+ and Ca 2+/HA system, they were more likely similar with kaolinite to form a palygorskite humic acid complex. But
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Na + might be the form more stable bridges with the organic matter result a slightly low adsorption of HA. The presence of divalent cations Mg2+ and Ca2+ on palygorskite should be responsible for the adsorption of HA. Ligand exchange between the carboxylated groups of the humic substances and the inorganic surface hydroxyls of vermiculite seemed to be responsible for HA adsorption on vermiculite (Abate and Masini, 2003). An attempt to separate the contributions of various modes of interactions between HA clay minerals showed that ligand exchange, van der Waals, and cation bridging accounted for approximately 32, 22%, and 41% of HA sorption on clay surfaces when Ca2+ was the background electrolyte (Feng et al., 2005). In addition, precipitation was found the dominant process of HA removal from solution by vermiculite (Abate and Masini, 2003). Regardless what was the most important mechanism for HA adsorption, palygorskite certainly showed a great promise as an adsorbent to remove low concentrations of HA from water. 4. Conclusion The maximum adsorption of HA on palygorskite was 17 mg/g at 20 °C. Adsorption reached at equilibrium in 2 h at pH 6–7 and fitted well to the pseudo-second-order adsorption model. The adsorption of HA on palygorskite increased at low pH and high ionic strength, indicated that the electrostatic interaction played an important role in interactions between HA and palygorskite. The concentration of HA in water was reduced from 20 to 2.6 mg/L, equivalent to the permanganate index 1.6 mg/L, below the groundwater quality standard of 3 mg/L. The calculated ΔH and ΔS values indicated exothermic adsorption and increase in randomness of the adsorbed HA on palygorskite. The XRD pattern and FTIR spectra of raw and HA-absorbed palygorskite indicated that the adsorption was on the external surfaces. Acknowledgments Financial supports from National Program of Control and Treatment of Water Pollution (No. 2009ZX07424-002) and the School of Water Resources and Environment of China University of Geosciences are greatly appreciated. References Abate, G., Masini, J.C., 2003. Influence of pH and ionic strength on removal processes of a sedimentary humic acid in a suspension of vermiculite. Colloids Surfaces A: Physicochemical and Engineering Aspects 226, 25–34. Abate, G., Masini, J.C., 2005. Influence of pH, ionic strength and humic acid on adsorption of Cd(II) and Pb(II) onto vermiculite. Colloids Surfaces A: Physicochemical and Engineering Aspects 262, 33–39. Abate, G., Dos Santos, L.B.O., Colombo, S.M., Masini, J.C., 2006. Removal of fulvic acid from aqueous media by adsorption onto modified vermiculite. Applied Clay Science 32, 261–270. Álvarez-Ayuso, E., Garcia-Sánchez, A., 2007. Removal of cadmium from aqueous solutions by palygorskite. Journal of Hazardous Materials 147, 594–600. Anirudhan, T., Suchithra, P., Rijith, S., 2008. Amine-modified polyacrylamide–bentonite composite for the adsorption of humic acid in aqueous solutions. Colloids Surfaces A: Physicochemical and Engineering Aspects 326, 147–156. Becher, G., Ovrum, N.M., Christman, R.F., 1992. Novel chlorination by-products of aquatic humic substances. The Science of the Total Environment 117, 509–520. Black, B.D., Harrington, G.W., Singer, P.C., 1996. Reducing cancer risks by improving organic carbon removal. Journal of American Water Works Association 88, 40–52. Blanco, C., Herrero, J., Mendioroz, S., Pajares, J., 1988. Infrared studies of surface acidity and reversible folding in palygorskite. Clays and Clay Minerals 36, 364–368.
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