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Influence of the physicochemical properties of clay minerals on the degree of darkening via polycondensation reactions between catechol and glycine
Applied Clay Science 46 (2009) 277–282
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Influence of the physicochemical properties of clay minerals on the degree of darkening via polycondensation reactions between catechol and glycine Akitaka Miura, Ryo Okabe, Kenji Izumo, Masami Fukushima ⁎ Division of Solid Waste, Resources and Geoenvironmental Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
a r t i c l e
i n f o
Article history: Received 17 December 2008 Received in revised form 8 August 2009 Accepted 17 August 2009 Available online 24 August 2009 Keywords: Darkening Clay minerals Lewis acid Brønsted acid Catechol Glycine
a b s t r a c t Polycondensation reactions between amino acids and phenols are an important pathway for humification, and clay minerals are able to catalyze these reactions. In the present study, we investigated the influence of the physicochemical properties of some clay minerals on polycondensation reactions between catechol (CT) and glycine (Gly). The polycondensation of CT and Gly was evaluated using the specific absorbance at 600 nm (E600) as a measure of the degree of darkening. The following materials were used in the present study: kaolin, bentonite, zeolite, Kanuma soil (allophanic soil) and aluminum silicate. In the presence of Kanuma soil, bentonite and zeolite, the degree of darkening was significantly enhanced, compared to the control (CT + Gly without clay), clearly showing that these clay minerals are able to serve as effective catalysts. The characteristics of these clay minerals were as follows: higher levels of specific surface area, and higher Fe, Ti and Ca contents. Transition metals (Fe and Ti) in the clay minerals can serve as Lewis acids, and divalent alkaline-earth metals (Ca) can contribute to the appearance of Brønsted acid sites. Therefore, contents of such metals in clay minerals are important factors in terms of enhancing darkening via polycondensation reactions between amino acids and phenols. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Losses of soil organic matter (SOM) have been reported to be related to the alternation of ecosystems in soil environments, as the result of global warming in recent years (Bellamy et al., 2005; Loya et al., 2003). This poses several problems: e.g., desertification, resulting in difficulties in retaining agricultural areas. Humic substances (HSs), the major components of SOM and related polymers, cannot serve as carbon pools in soil but can serve as nutrient storage for components such as nitrogen and minerals, for plant growth (Orlov, 1995). Thus, technologies designed to permit HSs to accumulate in soil are critical from the point of view of carbon fixation and fertilization in soil. In humification processes, residues from animals and plants undergo decomposition to lower-molecular-weight compounds, such as amino acids, saccharides and phenols. They then undergo condensation and polymerization via bio- and abiotic processes to produce dark-brown polymers, which correspond to HSs. Proposed explanations for the recombination processes between degraded amino acids and phenols include, the Ligno-protein theory, polycondensation reactions between phenols and amino acids (Tan, 2003), and the Polyphenol theory, the oxidative polymerization of quinones (Shindo and Huang, 1982). Humification processes, which involve the use of biological reactions, have been applied to the composting of organic wastes,
⁎ Corresponding author. Tel./fax: +81 11 706 6304. E-mail address: [email protected] (M. Fukushima). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.08.017
such as wood and food processing wastes (Fukushima et al., 2009b; Altieri and Espostio, 2008). The composting of organic wastes may be a useful technology for the accumulation of HSs in soil. However, spreading external microorganisms may result in the further decomposition of the original SOM, resulting in an overall decrease in soil fertilization (Brunetti et al., 2008). Hence, the use of abiotic processes is clearly the preferable approach to this problem. Clay minerals are widely distributed in soil and contribute to abiotic humification. For example, birnesite (δ-MnO2) has the ability to facilitate polycondensation reactions between phenols and amino acids (Wang and Huang, 1987, 2005). In addition, we also reported previously that an allophanic soil is able to enhance polycondensation reactions between catechol and glycine (Fukushima et al., 2009a). Polycondensation reactions between phenols and amino acids proceed via a nucleophilic reaction, which can be accelerated by the presence of acid catalysts. Thus, clay minerals can function as a solid acid-catalyst would be available for facilitating nucleophilic reactions in soils. It is well known that both Brønsted and Lewis acid sites are present on the solid surface of clay minerals. The Brønsted site corresponds to an exchangeable proton site that binds to the peripheral oxygen atom of an electronegative aluminum atom of an electronegative aluminum atom in the formation of aluminum silicate. In addition, the Lewis site can act as an electron acceptor. Wang and Huang (1989) investigated the influence of the surface treatment of kaolin, nontronite and quartz on the formation of HSs via the polymerization of hydroquinone. Their results suggest that the conditions on the surface of clay minerals are important factors. Trace
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amounts of Mn and Fe in clay minerals are known to serve as Lewis acids. Shindo and Huang (1985) noted the importance of Mn in montmorillonite or tephroite for the formation of HSs via the polymerization of quinone. In addition, Wang and Huang (2000, 2003) reported on the effects of Al, Si, Fe and Mn in clay minerals on ring cleavage and polymerization reactions of pyrogallol. The synthesis of useful clay catalysts that facilitate composting should be developed based on physicochemical factors that will enhance polycondensation reactions between amino acids and phenols. Nevertheless, the physicochemical factors of clay minerals that lead to the enhancement of polycondensation reactions between phenols and amino acids are not well understood at this time. During the formation of HSs via polycondensation reactions between phenols and amino acids, the solution becomes progressively darker (referred to as “darkening’’). The degree of darkening can be estimated from the specific absorbance at 600 nm (Kumada, 1955). The purpose of the present study was to develop a better understanding of the influence of the physicochemical properties of clay minerals on the degree of darkening for some polycondensation reactions between phenols and amino acids. To accomplish this, we analyzed the soil pH, specific surface area, cation exchange capacity (CEC), the inorganic element content and acidity of a variety of clay minerals (kaolin, bentonite, zeolite, allophanic soil and aluminum silicate) and these data were compared with the degree of darkening for the polycondensation of catechol and glycine. 2. Experimental 2.1. Materials Catechol (CT, >99% purity) and glycine (Gly, 99% purity) were purchased from Tokyo Chemical Industry Co., Ltd. and Nacalai Tesque, respectively. All other reagents were purchased from Nacalai Tesque and were used without further purification. A Kanuma soil (Allophanic soil) sample, which is commonly used for gardening, was obtained commercially. The Kanuma soil was treated according to previous reports (Fukushima and Tatsumi, 2007; Fukushima et al., 2009a). Kaolin (Kukita Chemical Industries Co., Ltd.), zeolite (Sun-Zeolite Industry Co., Ltd.), bentonite and aluminum silicate (Nacalai Tesque) were obtained commercially. Ultra-pure water, prepared using a Millipore ultra-pure water system (milli-Q) from distilled water was used in all experiments. 2.2. Incubation of CT and Gly mixture Two grams of clay mineral were placed in a 300-ml Erlenmeyer flask, and a 150 ml aliquot of phosphate buffer (pH = 7.00± 0.05) containing CT and Gly (0.05 M) was then added. The solution was incubated for 2 weeks at 30 °C. During the incubation, 1 ml aliquots of the reaction mixture were removed and transferred to a micro-tube. After centrifugation and appropriate dilution of the supernatant, total organic carbon (TOC) and UV–vis absorption spectrum were measured using a TOC-V CSH type analyzer (Shimadzu) and a V-600 type UV–vis spectrophotometer (Japan Spectroscopic Co., Ltd.), respectively. The degree of darkening (E600) can be calculated as: −1
E600 ð1g
−1
cm
Þ
Absorbance 600 nm TOCðgl−1 Þ × L
Fig. 1. FT/IR spectra of aluminum silicate, kaolin, bentonite, zeolite and Kanuma soil.
KBr pellets. A KBr powder was dehydrated and dried over silica gel in a desiccator. The clay minerals and KBr were ground with a motor and pestle, and a pellet was prepared using a pellet press. Prior to recording spectra of clay samples, a pure KBr pellet was run and was used as the background spectrum. 2.3.2. pH The clay minerals were suspended in water or 1 M KCl aqueous (clay:liquid=1:2.5, w/w) and shaken for 30 min. After shaking, pH values were recorded using an AUT501 pH meter (TOA DKK). The solid to liquid ratio for the bentonite was set at 1:15 because of great swelling property. 2.3.3. Specific surface area The specific surface area of the clay minerals was calculated by a N2-BET method using a Gemini 2360 (Shimadzu) instrument. The clay minerals were dehydrated under reduced pressure (26.6 kPa) at room temperature before the measurements. 2.3.4. Cation exchange capacity (CEC) A clay mineral sample (0.2–1.0 g) was suspended in aqueous 1 M NH4Cl (40 ml) and then shaken for 30 min. After removing the aqueous phase, another aliquot of aqueous NH4Cl (1 M) was added and the sample shaken again. This procedure was repeated for a total of five times and the suspension allowed to stand overnight.
ð1Þ
where L represent the length of the light path (1 cm). 2.3. Analyses of clay minerals 2.3.1. FT–IR spectra Fourier transform infrared (FT–IR) spectra of clay minerals were recorded using an FT/IR 600 type (Japan Spectroscopic Co., Ltd.) with
Fig. 2. Kinetic curves for the degree of darkening (E600) for each of the clay minerals studied.
A. Miura et al. / Applied Clay Science 46 (2009) 277–282
Subsequently, 40 ml of aqueous NH4Cl (0.05 M) was added and the same procedure was repeated (except for standing overnight) to adjust for concentration for later measurement. After removing the aqueous phase, a 40 ml aliquot of NaNO3 (1 M) was added to release + the NH+ 4 that were adsorbed to the clay surface. The released NH4 was determined colorimetrically using the indophenol blue method. The amount of NH+ 4 released can be assumed to correspond to the CEC (Wada and Okumura, 1977; Scheiner, 1976). 2.3.5. Inorganic elements The wet-digestion of clay minerals (10 mg) was carried out using concentrated HF (0.5 ml) and HClO4 (1.0 ml) in a closed Teflon vessel at 150 °C for 5 h. For the case of kaolin, 50 mg of powdered sample, 0.75 ml of HF, 0.15 ml of HCl and 0.1 ml of HClO4 were employed. After the wet-digestion, to prevent the volatilization of SiF4 and to
279
protect the quartz torch of the inductively coupled plasma-atomic emission spectrophotometer (ICP-AES), a 4% aqueous solution of boric acid (6 ml) was added. Inorganic elements (Al, Si, Mn, Fe, Ti, and Ca) in the solution were analyzed by means of an ICP-AES (SPS7800-type, SII Nano Techonology Inc.). 2.3.6. Acidity The acidity of the clay minerals was determined using a method that was previously described by Cámera et al. (2008) with minor modification. Clay mineral samples (0.1–0.05 mg) were suspended in 50 ml of acetonitrile. Subsequently, 0.1 M n-butylamine in acetonitrile (10 μl) was added, and suspension was then stirred for 12 h. Finally, the pH value (mV) was measured using an AUT-501 pH meter (TOA DKK), connected to a glass electrode (HGS-2005, TOA DKK) and a reference electrode (HS-305DS, TOA DKK), which had a double
Fig. 3. UV–vis absorption spectra of reaction mixtures after a 2-week incubation period.
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Table 1 The bet-N2 specific surface areas, cation exchange capacities (CEC), pH values and total acidities for clay mineral samples. Sample
Zeolite Kaolin Bentonite Aluminum silicate Kanuma soil
Specific surface area (m2 g− 1)
CEC (mmol 100 g− 1)
pH Water
1 M KCl
26.82 ± 0.04 3.30 ± 0.39 34.73 ± 0.07 36.30 ± 0.05 106a
94.81 ± 0.03 2.64 ± 0.004 45.87 ± 2.64 80.99 ± 0.03 6.62 ± 0.02
6.53 ± 0.01 7.51 ± 0.04 10.05 ± 0.1 6.65 ± 0.03 6.27a
5.06 ± 0.01 6.76 ± 0.0 8.50 ± 0.13 4.35 ± 0.0 5.10 ± 0.00
Total acidity (mmol g− 1) 0.72 ± 0.16 1.05 ± 0.09 1.76 ± 0.28 1.05 ± 0.40 0.76 ± 0.03
a
Fukushima and Tatsumi, 2007.
junction (external liquid 10% CH3COOLi; internal liquid Ag/AgCl). A potentiometric titration with 0.1 M n-butylamine in acetonitrile was performed after stabilizing the potential for 20 min, in which the rate of dropping the titrant was 0.05 ml min− 1.
3.2. Influences of clay minerals on the reaction kinetics In general, the color of soil can be due to organic carbons. For instance, the Andosoil that includes allophane indicates dark color due to higher content of organic carbons, more than 20% (Fukushima et al., 2006). Although the color of the Andosoil is dependent on its organic carbon, allophanic soils (i.e., Kanuma soil) that are mined from under layer of Andosoil have light yellow color. For the case of allophanic soil, the color can be due to iron oxides (Wada, 2001). In all clay minerals used in the present study, only traces of organic carbon were found in the clay minerals (0.82% for bentonite) or below the limit of detection (others). Therefore, the absorbance for dissolved organic carbon from the clay minerals was negligible, when darkening in the reaction mixtures of CT, Gly and clay minerals are monitored. This indicates that observed darkening phenomena can be attributed to the catalytic power of clay minerals. The kinetics of the degree of darkening (E600) calculated using Eq. (1) are shown in Fig. 2. The E600 values for kaolin and aluminum silicate did not vary significantly, compared to a mixture of CT and Gly in the absence of the clay minerals (control). However, for Kanuma soil, bentonite and zeolite, the E600 values clearly increased, compare to the control. These results clearly show an enhancement in darkening in the presence of Kanuma soil, bentonite and zeolite. The E600 value for zeolite initially increased and then decreased for times above 150 h. The levels of absorbance for zeolite were similar to those for bentonite during the incubation. However, for times above 150 h, the levels of TOC for zeolite were larger than those for bentonite. These results suggest that the decrease in E600 after 150 h for zeolite may be due to a higher level of TOC in the reaction mixture. It is likely that lower molecular weight components are initially adsorbed to the zeolite surface and are then desorbed, thus leading to an increase in TOC for times above 150 h. Thus, the decrease in E600 value for times above 150 h for zeolite can be attributed to an increase in the TOC term in Eq (1). Fig. 3 shows UV–vis absorption spectra for reaction mixtures, after a 2-week incubation period. For the control system at an incubation time of 0 h, a sharp peak at 275 nm corresponding to CT was observed. In spectra for kaolin and aluminum silicate, this peak was also clearly present, indicating that darkening (i.e. polycondensation between CT and Gly) had not proceeded sufficiently. However, in the presence of Kanuma soil, bentonite and zeolite, the CT peak had disappeared and the
3. Results and discussion 3.1. Characterization of clay minerals FT/IR spectra of the clay minerals are shown in Fig. 1. The following general peaks could be assigned for the clay minerals, as follows: Si–O vibration (1185–950 cm− 1 and 480–420 cm− 1), Al–O vibration, in which Si is substituted by Al in a tetrahedral configuration (800 cm− 1 region), Al–O vibration, corresponding to two octahedral configurations, in which Al dominates the octahedral configuration (700 cm− 1 and 540 cm− 1). The patterns of peaks for the Kanuma soil were similar to those for allophane-imogolite: structural O–H and/or O–H vibration of adsorbed water (3800–2800 cm− 1), H–O–H bending vibration of absorbed water (1650–1600 cm− 1), Si–Al–O stretching vibration (1200–800 cm− 1) and an absorbed region of 800–350 cm− 1. Aluminum silicate also showed a peak at 3800–2800 cm− 1, as was also observed for Kanuma soil. However, this peak was significantly smaller than the corresponding peak for the Kanuma soil. This indicates that the aluminum silicate contains a lower amount of surface OH groups that are related to proton sites. Specific peaks corresponding to an O–H stretching vibration (3700–3620 cm− 1) were observed for kaolin. The spectrum of bentonite indicated a smectite character, as evidenced by the appearance of a specific peak at 3610–3640 cm− 1. In addition, this suggests that the montmorillonite included Al because this peak in the 3640 cm− 1 region corresponds to an O–H stretching vibration. The peaks in the region 400–600 cm− 1 indicated the presence of Si–O–R3+ and/or R3+–OH due to the substitution of octahedral ions, where R3+ represents trivalent cations. For the case of the zeolite, peaks corresponding to Si–O–Si and Si– O–Al vibrations (1400–900 cm− 1 and 420–500 cm− 1) were present. Broad peaks for O–H (3400 cm− 1 region) via hydrogen binding, an O– H stretching vibration (3640 cm− 1 region) and the bending vibration of water (1650 cm− 1) were also present (The Clay Science Society of Japan, 1987; Perraki and Orfanoudaki, 2004).
Table 2 The inorganic element contents for clay mineral samples. Sample
Zeolite Kaolin Bentonite Aluminum silicate Kanuma soil a
Not detected.
Inorganic element contents (wt.%) Fe
Si
Al
Ca
Ti
Mn
1.37 ± 0.10 0.05 ± 0.0 1.19 ± 0.05 N.D.a 1.25 ± 0.35
35.05 ± 1.23 35.79 ± 0.55 34.02 ± 1.42 30.44 ± 1.12 21.24 ± 2.54
6.98 ± 0.58 10.76 ± 0.92 4.60 ± 0.71 3.19 ± 1.18 11.36 ± 3.82
0.84 ± 0.07 0.13 ± 0.01 2.22 ± 0.04 0.01 ± 0.01 0.92 ± 0.36
0.11 ± 0.02 0.06 ± 0.0 0.03 ± 0.0 N.D.a 0.10 ± 0.01
0.07 ± 0.01 N.D.a N.D.a N.D.a N.D.a
A. Miura et al. / Applied Clay Science 46 (2009) 277–282
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absorbance values gradually increased with a decrease in wavelength, as reported previously for spectra of HSs (Kumada, 1955). 3.3. pH, CEC and acidity The pH values of clay minerals indicate that a number of protons are loosely bound to the clay surface. Therefore, pH values may be regarded as an indicator of the number of Brønsted-acid sites present. The pH values for the clay minerals examined are summarized in Table 1. However, the E600 values were not pH dependent. Pushpaletha et al. (2005) reported that, in bentonite with surface modifications, the catalytic activity for benzylation reactions increased with increasing CEC and acidity. In general, benzylation is a nucleophilic reaction and such a reaction can be facilitated in the presence of H+ catalysts. The cation exchange capacity and acidity in clay samples can be considered as the amount of H+ corresponding to the Brønsted acidity. The CEC and acidity values for the clay minerals are summarized in Table 1. However, no significant correlation between CEC, acidity and E600 values were found. 3.4. Specific surface area If the catalytic power of clay minerals play a role in enhancing the darkening reaction, the nature of the surfaces would be very important. Thus, the specific surface areas of clay minerals would be predicted to be an important physicochemical parameter. The specific surface areas of each of clay minerals studied are summarized in Table 1. The specific surface area for the Kanuma soil was the largest of
Fig. 5. Correlation between the contents of Ca in clay minerals and E600 values.
all the clay minerals. The E600 values were larger clay minerals that had a larger surface area. This supports the view that darkening via polycondensation reactions between CT and Gly can be attributed to catalytic reactions on the surface of the clay. 3.5. Contents of inorganic elements Shindo and Huang (1985) reported on the contribution of Mn to the enhancement in the polymerization of quinone. Wang and Huang (2000, 2003) also reported that Fe, Mn and Al facilitate the ring-opening polymerization of pyrogallol. In addition, Pushpaletha et al. (2005) reported that the Fe in bentonite partially catalyzes benzylation reactions. Hence, contents of inorganic elements in clay minerals may be important physicochemical factors in terms of enhancing darkening. Analytical results for Al, Si, Fe, Mn, Ca and Ti in the clay minerals are summarized in Table 2. The Mn content, which can be attributed to the presence of birnesite, was in trace amounts or below the limit of detection. Hence, it appears the birnesite in the samples contributes only to a negligible extent. The Al and Si contents were not correlated to the E600 values. In the case of the polymerization of quinone, darkening of the reaction mixture was not dependent on the contents of Al or Si (Shindo and Huang, 1985). However, the Fe and Ti contents were correlated with the E600 values (Fig. 4). Such transition metals are known to function as Lewis acids. Therefore, Lewis acid sites, due to the presence of trace transition metals, can be effective in enhancing in the darkening reaction. In addition, the Ca content was also correlated with the E600 values (Fig. 5). Ca2+, which typically is present as a hydrated ion (Ca(H2O)2+ n ), is adsorbed to a cation-exchange site (Al–O ) on the surface. Therefore, Ca2+ in clay minerals may contribute to the production of H+ sites, corresponding to Brønsted acids. 4. Conclusion
Fig. 4. Correlation between the contents of Fe or Ti in clay minerals and E600 values.
The presence of Kanuma soil, bentonite and zeolite can enhance darkening via polycondensation reactions between CT and Gly. These clays contained relatively high levels of Fe, Ti and Ca, which contribute to the development of Lewis and Brønsted acid sites in clay minerals. The surface modification of clay minerals, as applied to organic synthesis, has been investigated in recent years (Hattori, 2001; Pushpaletha et al., 2005; Gallo et al., 2006). The goal of our study was to synthesize clay mineral catalysts that are capable of facilitating the composting of organic wastes, such as food-processing wastes. The above results reveal some of the important factors that control the catalytic activities for the synthesis of clay mineral catalysts.
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Acknowledgment This work was supported by Grants-in-Aid for Scientific Research from Japan Society for Promotion of Science (19656236). References Altieri, R., Espostio, A., 2008. Olive orchard amended with two experimental olive mill wastes mixtures: effects on soil organic carbon, plant growth and yield. Bioresour. Technol. 99, 8390–8393. Bellamy, P.H., Loveland, P.J., Bradley, R.I., Lark, R.M., Kirk, G.J.D., 2005. Carbon losses from all soils across England and Wales 1978–2003. Nature 437, 245–248. Brunetti, G., Senesi, N., Plaza, C., 2008. Organic matter humification in olive oil mill wastewater by abiotic catalysis with manganese (IV) oxide. Bioresour. Technol. 99, 8528–8531. Cámera, R., Rimada, R., Romanelli, G., Autino, J.C., Vázquez, C., 2008. Silica-supported aluminum chloride as catalyst for the tetrahydropyranylaction of thymol. Catal. Today 133–135, 822–827. Fukushima, M., Tatsumi, K., 2007. Degradation of pentachlorophenol in contaminated soil suspensions by potassium monopersulfate catalyzed oxidation by a supramolecular complex between tetra (p-sulfophenyl) porphirin iron (III). J. Hazard. Mater. 144, 222–228. Fukushima, M., Tanabe, Y., Yabuta, H., Tanaka, F., Ichikawa, H., Tatsumi, K., Watanabe, K., 2006. Water solubility enhancement effects of some polychlorinated organic pollutants by dissolved organic carbon from a soil with a higher organic carbon content. J. Environ. Sci. Health, Part A 41, 1483–1494. Fukushima, M., Miura, A., Sasaki, M., Izumo, K., 2009a. Effect of allophanic soil on humification reactions between catechol and glycine: spectroscopic investigations of reaction products. J. Mol. Struct. 917, 142–147. Fukushima, M., Yamamoto, K., Ootsuka, K., Komai, T., Aramaki, T., Ueda, S., Horiya, S., 2009b. Effect of the maturity of wood waste compost on the structural features of humic acids. Bioresour. Technol. 100, 791–797. Gallo, J.M.R., Teixeira, S., Schuchardt, U., 2006. Synthesis and characterization of niobium modified montmorillonite and its use in the acid-catalyzed synthesis of βhydroxyethers. Appl. Catal. A 311, 199–203.
Hattori, H., 2001. Solid base catalysts: generation of basic sites and application to organic synthesis. Appl. Catal. A 222, 247–259. Kumada, K., 1955. Absorption spectra of humic substances. Soil Plant Food 1, 29–30. Loya, W.M., Pregitzer, K.S., Karberg, N.J., King, J.S., Giardina, C.P., 2003. Reduction of soil carbon formation by tropospheric ozone under increased carbon dioxide levels. Nature 425, 705–707. Orlov, D.S., 1995. Humic Substances of Soil and General Theory of Humification. A.A. Balkema Publishers, Brookfield, pp. 1–7. Perraki, Th., Orfanoudaki, A., 2004. Mineralogical study of zeolites from Pentalofos area, Thrace, Greece. Appl. Clay Sci. 25, 9–16. Pushpaletha, P., Rugmini, S., Lalithambika, M., 2005. Correlation between surface properties and catalytic activity of clay catalysis. Appl. Clay Sci. 30, 141–153. Scheiner, D., 1976. Water Res. 10, 31. Shindo, H., Huang, P.M., 1982. Role of Mn(IV) oxide in abiotic formation of humic substances in the environment. Nature 298, 363–365. Shindo, H., Huang, P.M., 1985. The catalytic power of inorganic components in the abiotic synthesis of hydroquinone-derived Humic polymers. Appl. Clay Sci. 1, 71–81. Tan, K.H., 2003. Humic Matter in Soil and the Environment: Principles and Controversies. Dekker, New York, p. 114. The Clay Science Society of Japan. 1987, Nendo-Handobook (Handbook of Clay minerals), Gihodo Shuppan, Jpn, pp.25-26, 31–32, 62–65, 94–95. (in Japanese). Wada, S.-I., 2001. A procedure for separation and purification of allophane from weathered pumice. J. Clay Sci. Soc. Japan 40, 242–248 (in Japanese). Wada, K., Okumura, Y., 1977. Proceeding of the International Seminar on Soil Environment and Fertility Management in Intensive Agriculture, Tokyo, p. 811. Wang, M.C., Huang, P.M., 1987. Polycondensation of pyrogallol and glycine and the associated reactants as catalyzed by birnessite. Sci. Total Environ. 62, 435–442. Wang, M.C., Huang, P.M., 1989. Catalytic power of nontronite, kaolinite and quartz and their reaction sites in the formation of hydroquinone-derived polymers. Appl. Clay Sci. 4, 43–57. Wang, M.C., Huang, P.M., 2000. Ring cleavage and oxidative transformation of pyrogallol catalyzed by Mn, Fe, Al, Si oxides. Soil Sci. 165, 934–942. Wang, M.C., Huang, P.M., 2003. Cleavage and polycondensation of pyrogallol and glycine catalyzed by natural soil clays. Geoderma 112, 31–50. Wang, M.C., Huang, P.M., 2005. Cleavage of 14C-labeled glycine and its polycondensation with pyrogallol as catalyzed by birnessite. Geoderma 124, 415–426.
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Influence of the physicochemical properties of clay minerals on the degree of darkening via polycondensation reactions between catechol and glycine Akitaka Miura, Ryo Okabe, Kenji Izumo, Masami Fukushima ⁎ Division of Solid Waste, Resources and Geoenvironmental Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
a r t i c l e
i n f o
Article history: Received 17 December 2008 Received in revised form 8 August 2009 Accepted 17 August 2009 Available online 24 August 2009 Keywords: Darkening Clay minerals Lewis acid Brønsted acid Catechol Glycine
a b s t r a c t Polycondensation reactions between amino acids and phenols are an important pathway for humification, and clay minerals are able to catalyze these reactions. In the present study, we investigated the influence of the physicochemical properties of some clay minerals on polycondensation reactions between catechol (CT) and glycine (Gly). The polycondensation of CT and Gly was evaluated using the specific absorbance at 600 nm (E600) as a measure of the degree of darkening. The following materials were used in the present study: kaolin, bentonite, zeolite, Kanuma soil (allophanic soil) and aluminum silicate. In the presence of Kanuma soil, bentonite and zeolite, the degree of darkening was significantly enhanced, compared to the control (CT + Gly without clay), clearly showing that these clay minerals are able to serve as effective catalysts. The characteristics of these clay minerals were as follows: higher levels of specific surface area, and higher Fe, Ti and Ca contents. Transition metals (Fe and Ti) in the clay minerals can serve as Lewis acids, and divalent alkaline-earth metals (Ca) can contribute to the appearance of Brønsted acid sites. Therefore, contents of such metals in clay minerals are important factors in terms of enhancing darkening via polycondensation reactions between amino acids and phenols. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Losses of soil organic matter (SOM) have been reported to be related to the alternation of ecosystems in soil environments, as the result of global warming in recent years (Bellamy et al., 2005; Loya et al., 2003). This poses several problems: e.g., desertification, resulting in difficulties in retaining agricultural areas. Humic substances (HSs), the major components of SOM and related polymers, cannot serve as carbon pools in soil but can serve as nutrient storage for components such as nitrogen and minerals, for plant growth (Orlov, 1995). Thus, technologies designed to permit HSs to accumulate in soil are critical from the point of view of carbon fixation and fertilization in soil. In humification processes, residues from animals and plants undergo decomposition to lower-molecular-weight compounds, such as amino acids, saccharides and phenols. They then undergo condensation and polymerization via bio- and abiotic processes to produce dark-brown polymers, which correspond to HSs. Proposed explanations for the recombination processes between degraded amino acids and phenols include, the Ligno-protein theory, polycondensation reactions between phenols and amino acids (Tan, 2003), and the Polyphenol theory, the oxidative polymerization of quinones (Shindo and Huang, 1982). Humification processes, which involve the use of biological reactions, have been applied to the composting of organic wastes,
⁎ Corresponding author. Tel./fax: +81 11 706 6304. E-mail address: [email protected] (M. Fukushima). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.08.017
such as wood and food processing wastes (Fukushima et al., 2009b; Altieri and Espostio, 2008). The composting of organic wastes may be a useful technology for the accumulation of HSs in soil. However, spreading external microorganisms may result in the further decomposition of the original SOM, resulting in an overall decrease in soil fertilization (Brunetti et al., 2008). Hence, the use of abiotic processes is clearly the preferable approach to this problem. Clay minerals are widely distributed in soil and contribute to abiotic humification. For example, birnesite (δ-MnO2) has the ability to facilitate polycondensation reactions between phenols and amino acids (Wang and Huang, 1987, 2005). In addition, we also reported previously that an allophanic soil is able to enhance polycondensation reactions between catechol and glycine (Fukushima et al., 2009a). Polycondensation reactions between phenols and amino acids proceed via a nucleophilic reaction, which can be accelerated by the presence of acid catalysts. Thus, clay minerals can function as a solid acid-catalyst would be available for facilitating nucleophilic reactions in soils. It is well known that both Brønsted and Lewis acid sites are present on the solid surface of clay minerals. The Brønsted site corresponds to an exchangeable proton site that binds to the peripheral oxygen atom of an electronegative aluminum atom of an electronegative aluminum atom in the formation of aluminum silicate. In addition, the Lewis site can act as an electron acceptor. Wang and Huang (1989) investigated the influence of the surface treatment of kaolin, nontronite and quartz on the formation of HSs via the polymerization of hydroquinone. Their results suggest that the conditions on the surface of clay minerals are important factors. Trace
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amounts of Mn and Fe in clay minerals are known to serve as Lewis acids. Shindo and Huang (1985) noted the importance of Mn in montmorillonite or tephroite for the formation of HSs via the polymerization of quinone. In addition, Wang and Huang (2000, 2003) reported on the effects of Al, Si, Fe and Mn in clay minerals on ring cleavage and polymerization reactions of pyrogallol. The synthesis of useful clay catalysts that facilitate composting should be developed based on physicochemical factors that will enhance polycondensation reactions between amino acids and phenols. Nevertheless, the physicochemical factors of clay minerals that lead to the enhancement of polycondensation reactions between phenols and amino acids are not well understood at this time. During the formation of HSs via polycondensation reactions between phenols and amino acids, the solution becomes progressively darker (referred to as “darkening’’). The degree of darkening can be estimated from the specific absorbance at 600 nm (Kumada, 1955). The purpose of the present study was to develop a better understanding of the influence of the physicochemical properties of clay minerals on the degree of darkening for some polycondensation reactions between phenols and amino acids. To accomplish this, we analyzed the soil pH, specific surface area, cation exchange capacity (CEC), the inorganic element content and acidity of a variety of clay minerals (kaolin, bentonite, zeolite, allophanic soil and aluminum silicate) and these data were compared with the degree of darkening for the polycondensation of catechol and glycine. 2. Experimental 2.1. Materials Catechol (CT, >99% purity) and glycine (Gly, 99% purity) were purchased from Tokyo Chemical Industry Co., Ltd. and Nacalai Tesque, respectively. All other reagents were purchased from Nacalai Tesque and were used without further purification. A Kanuma soil (Allophanic soil) sample, which is commonly used for gardening, was obtained commercially. The Kanuma soil was treated according to previous reports (Fukushima and Tatsumi, 2007; Fukushima et al., 2009a). Kaolin (Kukita Chemical Industries Co., Ltd.), zeolite (Sun-Zeolite Industry Co., Ltd.), bentonite and aluminum silicate (Nacalai Tesque) were obtained commercially. Ultra-pure water, prepared using a Millipore ultra-pure water system (milli-Q) from distilled water was used in all experiments. 2.2. Incubation of CT and Gly mixture Two grams of clay mineral were placed in a 300-ml Erlenmeyer flask, and a 150 ml aliquot of phosphate buffer (pH = 7.00± 0.05) containing CT and Gly (0.05 M) was then added. The solution was incubated for 2 weeks at 30 °C. During the incubation, 1 ml aliquots of the reaction mixture were removed and transferred to a micro-tube. After centrifugation and appropriate dilution of the supernatant, total organic carbon (TOC) and UV–vis absorption spectrum were measured using a TOC-V CSH type analyzer (Shimadzu) and a V-600 type UV–vis spectrophotometer (Japan Spectroscopic Co., Ltd.), respectively. The degree of darkening (E600) can be calculated as: −1
E600 ð1g
−1
cm
Þ
Absorbance 600 nm TOCðgl−1 Þ × L
Fig. 1. FT/IR spectra of aluminum silicate, kaolin, bentonite, zeolite and Kanuma soil.
KBr pellets. A KBr powder was dehydrated and dried over silica gel in a desiccator. The clay minerals and KBr were ground with a motor and pestle, and a pellet was prepared using a pellet press. Prior to recording spectra of clay samples, a pure KBr pellet was run and was used as the background spectrum. 2.3.2. pH The clay minerals were suspended in water or 1 M KCl aqueous (clay:liquid=1:2.5, w/w) and shaken for 30 min. After shaking, pH values were recorded using an AUT501 pH meter (TOA DKK). The solid to liquid ratio for the bentonite was set at 1:15 because of great swelling property. 2.3.3. Specific surface area The specific surface area of the clay minerals was calculated by a N2-BET method using a Gemini 2360 (Shimadzu) instrument. The clay minerals were dehydrated under reduced pressure (26.6 kPa) at room temperature before the measurements. 2.3.4. Cation exchange capacity (CEC) A clay mineral sample (0.2–1.0 g) was suspended in aqueous 1 M NH4Cl (40 ml) and then shaken for 30 min. After removing the aqueous phase, another aliquot of aqueous NH4Cl (1 M) was added and the sample shaken again. This procedure was repeated for a total of five times and the suspension allowed to stand overnight.
ð1Þ
where L represent the length of the light path (1 cm). 2.3. Analyses of clay minerals 2.3.1. FT–IR spectra Fourier transform infrared (FT–IR) spectra of clay minerals were recorded using an FT/IR 600 type (Japan Spectroscopic Co., Ltd.) with
Fig. 2. Kinetic curves for the degree of darkening (E600) for each of the clay minerals studied.
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Subsequently, 40 ml of aqueous NH4Cl (0.05 M) was added and the same procedure was repeated (except for standing overnight) to adjust for concentration for later measurement. After removing the aqueous phase, a 40 ml aliquot of NaNO3 (1 M) was added to release + the NH+ 4 that were adsorbed to the clay surface. The released NH4 was determined colorimetrically using the indophenol blue method. The amount of NH+ 4 released can be assumed to correspond to the CEC (Wada and Okumura, 1977; Scheiner, 1976). 2.3.5. Inorganic elements The wet-digestion of clay minerals (10 mg) was carried out using concentrated HF (0.5 ml) and HClO4 (1.0 ml) in a closed Teflon vessel at 150 °C for 5 h. For the case of kaolin, 50 mg of powdered sample, 0.75 ml of HF, 0.15 ml of HCl and 0.1 ml of HClO4 were employed. After the wet-digestion, to prevent the volatilization of SiF4 and to
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protect the quartz torch of the inductively coupled plasma-atomic emission spectrophotometer (ICP-AES), a 4% aqueous solution of boric acid (6 ml) was added. Inorganic elements (Al, Si, Mn, Fe, Ti, and Ca) in the solution were analyzed by means of an ICP-AES (SPS7800-type, SII Nano Techonology Inc.). 2.3.6. Acidity The acidity of the clay minerals was determined using a method that was previously described by Cámera et al. (2008) with minor modification. Clay mineral samples (0.1–0.05 mg) were suspended in 50 ml of acetonitrile. Subsequently, 0.1 M n-butylamine in acetonitrile (10 μl) was added, and suspension was then stirred for 12 h. Finally, the pH value (mV) was measured using an AUT-501 pH meter (TOA DKK), connected to a glass electrode (HGS-2005, TOA DKK) and a reference electrode (HS-305DS, TOA DKK), which had a double
Fig. 3. UV–vis absorption spectra of reaction mixtures after a 2-week incubation period.
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Table 1 The bet-N2 specific surface areas, cation exchange capacities (CEC), pH values and total acidities for clay mineral samples. Sample
Zeolite Kaolin Bentonite Aluminum silicate Kanuma soil
Specific surface area (m2 g− 1)
CEC (mmol 100 g− 1)
pH Water
1 M KCl
26.82 ± 0.04 3.30 ± 0.39 34.73 ± 0.07 36.30 ± 0.05 106a
94.81 ± 0.03 2.64 ± 0.004 45.87 ± 2.64 80.99 ± 0.03 6.62 ± 0.02
6.53 ± 0.01 7.51 ± 0.04 10.05 ± 0.1 6.65 ± 0.03 6.27a
5.06 ± 0.01 6.76 ± 0.0 8.50 ± 0.13 4.35 ± 0.0 5.10 ± 0.00
Total acidity (mmol g− 1) 0.72 ± 0.16 1.05 ± 0.09 1.76 ± 0.28 1.05 ± 0.40 0.76 ± 0.03
a
Fukushima and Tatsumi, 2007.
junction (external liquid 10% CH3COOLi; internal liquid Ag/AgCl). A potentiometric titration with 0.1 M n-butylamine in acetonitrile was performed after stabilizing the potential for 20 min, in which the rate of dropping the titrant was 0.05 ml min− 1.
3.2. Influences of clay minerals on the reaction kinetics In general, the color of soil can be due to organic carbons. For instance, the Andosoil that includes allophane indicates dark color due to higher content of organic carbons, more than 20% (Fukushima et al., 2006). Although the color of the Andosoil is dependent on its organic carbon, allophanic soils (i.e., Kanuma soil) that are mined from under layer of Andosoil have light yellow color. For the case of allophanic soil, the color can be due to iron oxides (Wada, 2001). In all clay minerals used in the present study, only traces of organic carbon were found in the clay minerals (0.82% for bentonite) or below the limit of detection (others). Therefore, the absorbance for dissolved organic carbon from the clay minerals was negligible, when darkening in the reaction mixtures of CT, Gly and clay minerals are monitored. This indicates that observed darkening phenomena can be attributed to the catalytic power of clay minerals. The kinetics of the degree of darkening (E600) calculated using Eq. (1) are shown in Fig. 2. The E600 values for kaolin and aluminum silicate did not vary significantly, compared to a mixture of CT and Gly in the absence of the clay minerals (control). However, for Kanuma soil, bentonite and zeolite, the E600 values clearly increased, compare to the control. These results clearly show an enhancement in darkening in the presence of Kanuma soil, bentonite and zeolite. The E600 value for zeolite initially increased and then decreased for times above 150 h. The levels of absorbance for zeolite were similar to those for bentonite during the incubation. However, for times above 150 h, the levels of TOC for zeolite were larger than those for bentonite. These results suggest that the decrease in E600 after 150 h for zeolite may be due to a higher level of TOC in the reaction mixture. It is likely that lower molecular weight components are initially adsorbed to the zeolite surface and are then desorbed, thus leading to an increase in TOC for times above 150 h. Thus, the decrease in E600 value for times above 150 h for zeolite can be attributed to an increase in the TOC term in Eq (1). Fig. 3 shows UV–vis absorption spectra for reaction mixtures, after a 2-week incubation period. For the control system at an incubation time of 0 h, a sharp peak at 275 nm corresponding to CT was observed. In spectra for kaolin and aluminum silicate, this peak was also clearly present, indicating that darkening (i.e. polycondensation between CT and Gly) had not proceeded sufficiently. However, in the presence of Kanuma soil, bentonite and zeolite, the CT peak had disappeared and the
3. Results and discussion 3.1. Characterization of clay minerals FT/IR spectra of the clay minerals are shown in Fig. 1. The following general peaks could be assigned for the clay minerals, as follows: Si–O vibration (1185–950 cm− 1 and 480–420 cm− 1), Al–O vibration, in which Si is substituted by Al in a tetrahedral configuration (800 cm− 1 region), Al–O vibration, corresponding to two octahedral configurations, in which Al dominates the octahedral configuration (700 cm− 1 and 540 cm− 1). The patterns of peaks for the Kanuma soil were similar to those for allophane-imogolite: structural O–H and/or O–H vibration of adsorbed water (3800–2800 cm− 1), H–O–H bending vibration of absorbed water (1650–1600 cm− 1), Si–Al–O stretching vibration (1200–800 cm− 1) and an absorbed region of 800–350 cm− 1. Aluminum silicate also showed a peak at 3800–2800 cm− 1, as was also observed for Kanuma soil. However, this peak was significantly smaller than the corresponding peak for the Kanuma soil. This indicates that the aluminum silicate contains a lower amount of surface OH groups that are related to proton sites. Specific peaks corresponding to an O–H stretching vibration (3700–3620 cm− 1) were observed for kaolin. The spectrum of bentonite indicated a smectite character, as evidenced by the appearance of a specific peak at 3610–3640 cm− 1. In addition, this suggests that the montmorillonite included Al because this peak in the 3640 cm− 1 region corresponds to an O–H stretching vibration. The peaks in the region 400–600 cm− 1 indicated the presence of Si–O–R3+ and/or R3+–OH due to the substitution of octahedral ions, where R3+ represents trivalent cations. For the case of the zeolite, peaks corresponding to Si–O–Si and Si– O–Al vibrations (1400–900 cm− 1 and 420–500 cm− 1) were present. Broad peaks for O–H (3400 cm− 1 region) via hydrogen binding, an O– H stretching vibration (3640 cm− 1 region) and the bending vibration of water (1650 cm− 1) were also present (The Clay Science Society of Japan, 1987; Perraki and Orfanoudaki, 2004).
Table 2 The inorganic element contents for clay mineral samples. Sample
Zeolite Kaolin Bentonite Aluminum silicate Kanuma soil a
Not detected.
Inorganic element contents (wt.%) Fe
Si
Al
Ca
Ti
Mn
1.37 ± 0.10 0.05 ± 0.0 1.19 ± 0.05 N.D.a 1.25 ± 0.35
35.05 ± 1.23 35.79 ± 0.55 34.02 ± 1.42 30.44 ± 1.12 21.24 ± 2.54
6.98 ± 0.58 10.76 ± 0.92 4.60 ± 0.71 3.19 ± 1.18 11.36 ± 3.82
0.84 ± 0.07 0.13 ± 0.01 2.22 ± 0.04 0.01 ± 0.01 0.92 ± 0.36
0.11 ± 0.02 0.06 ± 0.0 0.03 ± 0.0 N.D.a 0.10 ± 0.01
0.07 ± 0.01 N.D.a N.D.a N.D.a N.D.a
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absorbance values gradually increased with a decrease in wavelength, as reported previously for spectra of HSs (Kumada, 1955). 3.3. pH, CEC and acidity The pH values of clay minerals indicate that a number of protons are loosely bound to the clay surface. Therefore, pH values may be regarded as an indicator of the number of Brønsted-acid sites present. The pH values for the clay minerals examined are summarized in Table 1. However, the E600 values were not pH dependent. Pushpaletha et al. (2005) reported that, in bentonite with surface modifications, the catalytic activity for benzylation reactions increased with increasing CEC and acidity. In general, benzylation is a nucleophilic reaction and such a reaction can be facilitated in the presence of H+ catalysts. The cation exchange capacity and acidity in clay samples can be considered as the amount of H+ corresponding to the Brønsted acidity. The CEC and acidity values for the clay minerals are summarized in Table 1. However, no significant correlation between CEC, acidity and E600 values were found. 3.4. Specific surface area If the catalytic power of clay minerals play a role in enhancing the darkening reaction, the nature of the surfaces would be very important. Thus, the specific surface areas of clay minerals would be predicted to be an important physicochemical parameter. The specific surface areas of each of clay minerals studied are summarized in Table 1. The specific surface area for the Kanuma soil was the largest of
Fig. 5. Correlation between the contents of Ca in clay minerals and E600 values.
all the clay minerals. The E600 values were larger clay minerals that had a larger surface area. This supports the view that darkening via polycondensation reactions between CT and Gly can be attributed to catalytic reactions on the surface of the clay. 3.5. Contents of inorganic elements Shindo and Huang (1985) reported on the contribution of Mn to the enhancement in the polymerization of quinone. Wang and Huang (2000, 2003) also reported that Fe, Mn and Al facilitate the ring-opening polymerization of pyrogallol. In addition, Pushpaletha et al. (2005) reported that the Fe in bentonite partially catalyzes benzylation reactions. Hence, contents of inorganic elements in clay minerals may be important physicochemical factors in terms of enhancing darkening. Analytical results for Al, Si, Fe, Mn, Ca and Ti in the clay minerals are summarized in Table 2. The Mn content, which can be attributed to the presence of birnesite, was in trace amounts or below the limit of detection. Hence, it appears the birnesite in the samples contributes only to a negligible extent. The Al and Si contents were not correlated to the E600 values. In the case of the polymerization of quinone, darkening of the reaction mixture was not dependent on the contents of Al or Si (Shindo and Huang, 1985). However, the Fe and Ti contents were correlated with the E600 values (Fig. 4). Such transition metals are known to function as Lewis acids. Therefore, Lewis acid sites, due to the presence of trace transition metals, can be effective in enhancing in the darkening reaction. In addition, the Ca content was also correlated with the E600 values (Fig. 5). Ca2+, which typically is present as a hydrated ion (Ca(H2O)2+ n ), is adsorbed to a cation-exchange site (Al–O ) on the surface. Therefore, Ca2+ in clay minerals may contribute to the production of H+ sites, corresponding to Brønsted acids. 4. Conclusion
Fig. 4. Correlation between the contents of Fe or Ti in clay minerals and E600 values.
The presence of Kanuma soil, bentonite and zeolite can enhance darkening via polycondensation reactions between CT and Gly. These clays contained relatively high levels of Fe, Ti and Ca, which contribute to the development of Lewis and Brønsted acid sites in clay minerals. The surface modification of clay minerals, as applied to organic synthesis, has been investigated in recent years (Hattori, 2001; Pushpaletha et al., 2005; Gallo et al., 2006). The goal of our study was to synthesize clay mineral catalysts that are capable of facilitating the composting of organic wastes, such as food-processing wastes. The above results reveal some of the important factors that control the catalytic activities for the synthesis of clay mineral catalysts.
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Acknowledgment This work was supported by Grants-in-Aid for Scientific Research from Japan Society for Promotion of Science (19656236). References Altieri, R., Espostio, A., 2008. Olive orchard amended with two experimental olive mill wastes mixtures: effects on soil organic carbon, plant growth and yield. Bioresour. Technol. 99, 8390–8393. Bellamy, P.H., Loveland, P.J., Bradley, R.I., Lark, R.M., Kirk, G.J.D., 2005. Carbon losses from all soils across England and Wales 1978–2003. Nature 437, 245–248. Brunetti, G., Senesi, N., Plaza, C., 2008. Organic matter humification in olive oil mill wastewater by abiotic catalysis with manganese (IV) oxide. Bioresour. Technol. 99, 8528–8531. Cámera, R., Rimada, R., Romanelli, G., Autino, J.C., Vázquez, C., 2008. Silica-supported aluminum chloride as catalyst for the tetrahydropyranylaction of thymol. Catal. Today 133–135, 822–827. Fukushima, M., Tatsumi, K., 2007. Degradation of pentachlorophenol in contaminated soil suspensions by potassium monopersulfate catalyzed oxidation by a supramolecular complex between tetra (p-sulfophenyl) porphirin iron (III). J. Hazard. Mater. 144, 222–228. Fukushima, M., Tanabe, Y., Yabuta, H., Tanaka, F., Ichikawa, H., Tatsumi, K., Watanabe, K., 2006. Water solubility enhancement effects of some polychlorinated organic pollutants by dissolved organic carbon from a soil with a higher organic carbon content. J. Environ. Sci. Health, Part A 41, 1483–1494. Fukushima, M., Miura, A., Sasaki, M., Izumo, K., 2009a. Effect of allophanic soil on humification reactions between catechol and glycine: spectroscopic investigations of reaction products. J. Mol. Struct. 917, 142–147. Fukushima, M., Yamamoto, K., Ootsuka, K., Komai, T., Aramaki, T., Ueda, S., Horiya, S., 2009b. Effect of the maturity of wood waste compost on the structural features of humic acids. Bioresour. Technol. 100, 791–797. Gallo, J.M.R., Teixeira, S., Schuchardt, U., 2006. Synthesis and characterization of niobium modified montmorillonite and its use in the acid-catalyzed synthesis of βhydroxyethers. Appl. Catal. A 311, 199–203.
Hattori, H., 2001. Solid base catalysts: generation of basic sites and application to organic synthesis. Appl. Catal. A 222, 247–259. Kumada, K., 1955. Absorption spectra of humic substances. Soil Plant Food 1, 29–30. Loya, W.M., Pregitzer, K.S., Karberg, N.J., King, J.S., Giardina, C.P., 2003. Reduction of soil carbon formation by tropospheric ozone under increased carbon dioxide levels. Nature 425, 705–707. Orlov, D.S., 1995. Humic Substances of Soil and General Theory of Humification. A.A. Balkema Publishers, Brookfield, pp. 1–7. Perraki, Th., Orfanoudaki, A., 2004. Mineralogical study of zeolites from Pentalofos area, Thrace, Greece. Appl. Clay Sci. 25, 9–16. Pushpaletha, P., Rugmini, S., Lalithambika, M., 2005. Correlation between surface properties and catalytic activity of clay catalysis. Appl. Clay Sci. 30, 141–153. Scheiner, D., 1976. Water Res. 10, 31. Shindo, H., Huang, P.M., 1982. Role of Mn(IV) oxide in abiotic formation of humic substances in the environment. Nature 298, 363–365. Shindo, H., Huang, P.M., 1985. The catalytic power of inorganic components in the abiotic synthesis of hydroquinone-derived Humic polymers. Appl. Clay Sci. 1, 71–81. Tan, K.H., 2003. Humic Matter in Soil and the Environment: Principles and Controversies. Dekker, New York, p. 114. The Clay Science Society of Japan. 1987, Nendo-Handobook (Handbook of Clay minerals), Gihodo Shuppan, Jpn, pp.25-26, 31–32, 62–65, 94–95. (in Japanese). Wada, S.-I., 2001. A procedure for separation and purification of allophane from weathered pumice. J. Clay Sci. Soc. Japan 40, 242–248 (in Japanese). Wada, K., Okumura, Y., 1977. Proceeding of the International Seminar on Soil Environment and Fertility Management in Intensive Agriculture, Tokyo, p. 811. Wang, M.C., Huang, P.M., 1987. Polycondensation of pyrogallol and glycine and the associated reactants as catalyzed by birnessite. Sci. Total Environ. 62, 435–442. Wang, M.C., Huang, P.M., 1989. Catalytic power of nontronite, kaolinite and quartz and their reaction sites in the formation of hydroquinone-derived polymers. Appl. Clay Sci. 4, 43–57. Wang, M.C., Huang, P.M., 2000. Ring cleavage and oxidative transformation of pyrogallol catalyzed by Mn, Fe, Al, Si oxides. Soil Sci. 165, 934–942. Wang, M.C., Huang, P.M., 2003. Cleavage and polycondensation of pyrogallol and glycine catalyzed by natural soil clays. Geoderma 112, 31–50. Wang, M.C., Huang, P.M., 2005. Cleavage of 14C-labeled glycine and its polycondensation with pyrogallol as catalyzed by birnessite. Geoderma 124, 415–426.