Published in Soil Sci. Soc. Am. J. 68:1838-1843 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Division S-2—Soil Chemistry
Disappearance of Aluminum Tridecamer from Hydroxyaluminum Solution in the Presence of Humic Acid
Noriko Yamaguchia,
Syuntaro Hiradatea,*,
Masaru Mizoguchib and
Tsuyoshi Miyazakib
a National Institute for Agro-Environmental Sciences, 3-1-3 Kan-nondai, Tsukuba, Ibaraki 305-8604, Japan
b Graduate School of Agriculture and Life Science, The Univ. of Tokyo, Yayoi, Bunkyo, Tokyo 113-8657, Japan
* Corresponding author (hiradate{at}affrc.go.jp)
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ABSTRACT
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We investigated the influences of humic acid on the removal of Al tridecamer (Al13) from a hydroxyaluminum (HyA) solution at various humic acid/Al ratios. The Al species contained in the solution were analyzed by using a liquid-state 27Al-NMR and an atomic absorption spectrometer and fractionated into three Al species: (i) Al13, (ii) Al monomer and dimer (AlSYM), and (iii) other undefined species including aggregated/precipitated Al (AlNON). By the addition of humic acid to the HyA solution, the concentration of Al13 was rapidly decreased within 0.007 d (10 min). The decrease in Al13 and the increase in AlNON were more pronounced at a higher humic acid/Al ratio. When the molar ratio of humic acid carboxylic groups to Al exceeded 0.8, Al13 was undetected from solution within 0.007 d. The formation of Al13–humic acid complexes and the aggregation/precipitation of those complexes were a predominant mechanism in removing aqueous Al13 at the early stage of the reaction. Approximately 10 mol of carboxylic groups in humic acid were required to remove 1 mol of Al13 from the HyA solution. Aqueous Al13 had greater preference in precipitating with humic acid than AlSYM. After 5 to 570 d of aging, the concentration of Al13 and AlNON also decreased and increased, respectively, both in the presence and absence of humic acid. In conclusion, aqueous Al13 would not exist in soil solution under a high humic acid condition.
Abbreviations: Al13, aluminum tridecamer • AlNON, undefined species other than Al13 or AlSYM including aggregated or precipitated Al that cannot be detected by NMR • AlSYM, Al monomer and dimer • HyA, hydroxyaluminum • NMR, nuclear magnetic resonance
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INTRODUCTION
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ACIDIC SOILS, which occupy 30 to 40% of agricultural land in the world, compromise plant growth and have a negative impact on agriculture. A major plant growth inhibitor in acidic soils is soluble Al, and the phytotoxicity of Al depends on the chemical forms of Al (Carver and Ownby, 1995; Ryan et al., 1995; Ma et al., 1997; Ginting et al., 1998). Both the amount of Al present and its speciation are important for assessing the effect of Al on plant growth.
Aluminum tridecamer (Al13), AlO4Al12
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7+12, is a partially hydrolyzed species of a soluble hydroxyaluminum (HyA) ion; it has a Keggin structure with a tetrahedrally coordinated Al atom in the center surrounded by 12 octahedrally coordinated Al (Bertsch and Parker, 1996). The phytotoxic effect of Al13 is more pronounced than that of monomeric Al (Parker et al., 1989; Shann and Bertsch, 1993). Therefore, the occurrence of aqueous Al13 in soil systems is of considerable concern. Hunter and Ross (1991) detected Al13 in the organic horizon of a forested spodosol and suggested that the presence of humified organic matter may induce the formation of Al13. Furrer et al. (2002) discovered Al13 in Al oxyhydroxide flocs that formed at streams polluted by acid drainage of high Al concentration up to 16 mmol L–1. An equilibrium calculation by Furrer et al. (1992) indicates that the formation of Al13 is quite possible in nature if the total Al concentration is >10–5 mol L–1. Nevertheless, the Al13 species is difficult to observe in soil systems. Hiradate et al. (1998) investigated the chemical species of Al in 1 M KCl extracts from seven Japanese acidic soils and showed that A13 was undetectable, even though some of the extracts satisfy the requirements for the presence of Al13 in terms of pH and concentration of Al. The reason that Al13 is rarely detected in soil systems is related to the existence of potentially interfering inorganic and organic substances for the presence of Al13. Hiradate et al. (1998) and Taniguchi et al. (1999) suggested that the coexistence of orthosilicic acid decreases the formation of Al13. Tartaric acid suppresses the formation of Al13 and promotes the formation of octahedral Al species instead (Krishnamurti et al., 1999). By means of equilibrium calculations, Gérard et al. (2001) concluded that Al13 should be negligible in natural soils and surface water due to the presence of interfering substances.
Humic acid is a representative ligand that controls the mobility and fate of metal species in soils and aquatic systems. Phytotoxic Al species have a strong affinity with humic acid and form insoluble Al-humic acid complexes (Vance et al., 1996). Since the formation of insoluble Al-humic acid complexes reduces the concentration of Al in soil solution, humic acid has a role in preventing Al toxicity in corn (Zea mays L.) (Tan and Binger, 1986). The interactions between humic acid and Al13 have not been well elucidated so far. Hiradate and Yamaguchi (2003) revealed that the formation of Al13 was reduced by the presence of humic acid. Even though Al13 may form naturally in soil systems, humic acid has a high affinity to complex with aqueous Al13, which makes it undetectable in soil solutions. The objective of this study is to evaluate the effects of humic acid on the removal of aqueous Al13 from a HyA solution and assess the possibility of the presence of Al13 in a soil solution of humic acid-rich soils.
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MATERIALS AND METHODS
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Preparation of Humic Acid Solution
Humic acid was extracted from an allophanic Andisol, Tsukuba, Japan, according to the procedure of Yonebayashi and Hattori (1988). Inorganic impurities were removed by 0.1 mol L–1 HCl and 0.3 mol L–1 HF mixed solution. The humic acid was dissolved into a small amount of alkaline solution and dialyzed (critical molecular weight; 8000 Da), and then passed through Amberlite IR-120 resin (H+ form) and freeze-dried. Acidity, which originated from the carboxylic group of the humic acid (COOH acidity), was estimated as 4.07 mmol g–1 by the neutralization of the humic acid (H+ form) to pH 7 (Swift, 1996). The ash content determined by dry combustion at 823 K was 0.64%. The humic acid was dissolved in distilled water and brought to pH 4 by using NaOH and HClO4. The concentration of the stock humic acid solution was 2 g L–1.
Aluminum Speciation by Aluminum-27 Nuclear Magnetic Resonance and Atomic Absorption Spectrometry
Aluminum speciation was conducted by a liquid-state 27Al-NMR (JNM-
600 FT-NMR system, JEOL) at 156.25 MHz and 298 K without spinning. The sample solution (570 µL) was transferred to an NMR tube (5-mm diam.). The experimental conditions included non-decoupling, a single pulse with a flip angle for 27Al of 
/2 (13.2 µs as pulse width), an observation band of 62.5 kHz, an observation point of 32768, a pulse delay of 0.400 s, and an acquisition time of 0.524 s. The accumulated number was 500 to 2000, depending on the S/N ratio. The standard chemical shift (0 ppm) was adjusted by using 2.5 mmol L–1 AlCl3 in 0.1 mol L–1 HCl after shimming against D2O.
Aluminum species were separated into three fractions (Al13, AlSYM, and AlNON) according to Hiradate et al. (1998). Octahedral Al in an Al monomer and dimer, which gives a peak at approximately 0 ppm, was referred to as AlSYM. A sharp peak at approximately 63 ppm was attributed to tetrahedral aluminum (AlO4) in the center of Al13. Signals from the other 12 octahedral and shell-structured Al in the Al13 are too broad to be detected by 27Al-NMR because the electric field gradient of these types of Al is relatively high and the octahedral may be distorted. Quantitative determination of AlSYM and Al13 was conducted by comparing the area of their peaks with that of the reference (2.5 mmol L–1 AlCl3 in 0.1 mol L–1 HCl). The Al concentration incorporated in Al13 was calculated by multiplying the concentration of tetrahedral Al detected at 63 ppm by 13. The detection limit for AlSYM and Al13 (as AlO4) was about 20 µmol L–1. Other undefined species, including aggregated or precipitated Al that cannot be detected by NMR, are referred to as AlNON. The AlNON was calculated by subtracting the sum of Al13 and AlSYM from the total Al added to the system. The total dissolved Al concentration was determined by a polarized Zeeman atomic absorption spectrometer (Hitachi, Z-5010, Hitachi, Tokyo) after filtration through a 0.22-µm membrane filter (hydrophilic polyethersulfone membrane, type GP; Millipore, Billerica, MA) and dilution by 0.1 mol L–1 HClO4. The amounts of precipitated Al were calculated by subtracting the amounts of total dissolved Al from those of Al initially added to the system. This results in four fractions: Al13, AlSYM, AlNON (solution), and AlNON (particulate).
Preparation of the Hydroxyaluminum–Humic Acid Mixture
A HyA solution was prepared by partial neutralization of a mixture of 10 or 20 mmol L–1 Al(ClO4)3 (3 mL) and 1.2 mol L–1 NaClO4 (500 µL) with 120 mmol L–1 NaOH to an OH/Al ratio of 2.1, and the mixture was equilibrated for 3 d at ambient temperature. All solvent used was CO2–free distilled water prepared by boiling, and the neutralization reactions were conducted under a CO2–free condition. Aluminum speciation by NMR showed that 87% of the total Al was converted into Al13 and that 13% was AlSYM. The stock humic acid solution and distilled water were added to the HyA solution at five different COOH/Al ratios of 0.07 to 0.8. The final volume of the mixed solution and total Al concentration were 6 mL and 5 or 10 mmol L–1, respectively. Before the addition of humic acid, the pH values of 5 and 10 mmol L–1 HyA solution were 4.35 and 4.28, respectively. The mixtures of HyA and humic acid solution were aged with occasional shaking, and pH measurement and Al speciation analysis were conducted after 0.007 (10 min), 0.04 (1 h), 5, 30, 210, and 570 d of aging. For the samples of selected COOH/Al ratios and aging periods, the amounts of precipitated Al were determined as described above. Aluminum speciation analysis by NMR was also conducted for the filtrate that passed through a 0.22-µm membrane filter after 5 d of aging at COOH/Al ratios of 0.15 and 0.38.
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RESULTS AND DISCUSSION
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In the present study, disappearance of Al13 was monitored at total Al concentration of 5 and 10 mmol L–1. These Al concentrations are higher than the average Al concentration in natural systems; nonetheless, experimental conditions in this study are standardized in terms of humic acid COOH/Al ratios to address the effect of humic acid on the preservation of Al13. The COOH/Al ratios used in this study are in the range of those of natural soil systems.
Immediately after the addition of humic acid to the HyA solution, all dissolved dark brown humic acid was turned into precipitates. By using a solid-state 27Al-NMR, Hiradate and Yamaguchi (2003) demonstrated that Al13 was incorporated into the precipitate formed by the addition of Al13 solution to H+–type humic acid solution. Therefore, we assume that the dark brown precipitate was mainly composed of an Al13–humic acid complex. If Al13 and AlSYM are loosely complexed with precipitated HA, they could be detected by liquid-state NMR, as in the study by Toma et al. (1999), who reported that Al13 adsorbed on a cation exchange resin was detected as a broad peak by liquid-state 27Al-NMR. To elucidate whether AlSYM and Al13 detected in HyA-HA suspension were attributed to the precipitate of an Al-HA complex or aqueous Al, NMR spectra of the suspension were compared with those of the filtrate. The chemical shift values and concentrations of Al13 and AlSYM in the HyA-humic acid mixture were not affected by filtration through a 0.22-µm membrane filter (Table 1). Therefore, AlSYM and Al13 detected in the suspension were attributed only to aqueous AlSYM and Al13 in our experiment.
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Table 1. Chemical shift values and concentrations of aluminum tridecamer (Al13) and Al monomer and dimer (AlSYM) before (suspension) and after (filtrate) filtrate through a 0.22-µm membrane filter (10 mmol L–1 hydroxyaluminum [HyA] solution, aging period: 5 d).
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Figure 1
shows the changes in the concentrations of Al13, AlNON, and AlSYM and in pH with aging for the mixture of 10 mmol L–1 HyA solution and humic acid. The concentration of Al13 decreased rapidly in 0.007 d (10 min) after the addition of humic acid to the system and remained constant for 5 d (Fig. 1a); the concentration of Al13 was then gradually decreased after 5 d. The disappearance of Al13 by the addition of humic acid was divided into three stages, as shown in Fig. 1. The same trends were observed for the mixture of 5 mmol L–1 HyA solution and humic acid (data not shown).
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Fig. 1. Changes in the concentrations of (a) aluminum tridecamer (Al13), (b) undefined species other than Al13 or AlSYM including aggregated or precipitated Al that cannot be detected by NMR (AlNON), and (c) Al monomer and dimer (AlSYM) and (d) in pH with aging time (10 mmol L–1 hydroxyaluminum [HyA] solution). Humic acid carboxy group versus Al ratios (COOH/Al) are 0 (open square), 0.07 (open triangle), 0.15 (open inverted triangle), and 0.38 (open circle).
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Stage I
Stage I corresponded to the reaction stage from 0 to 0.007 d, including a rapid decrease in Al13 and an increase in AlNON (Fig. 1a,b). The amounts of precipitated Al formed by the addition of humic acid were increased with increasing the COOH/Al ratio and accounted for 57 to 93% of AlNON after 0.007 d of aging (Fig. 2a)
. Therefore, the predominant mechanism of Al13 disappearance from the HyA solution in 0.007 d was considered to be the aggregation/precipitation of Al13–humic acid complexes. The decreasing pH at Stage I (Fig. 1d) suggested that H+ was released from a protonated carboxyl group of humic acid by the exchange reaction with Al13. The concentration of AlSYM was slightly increased at Stage I (Fig. 1c). It is likely that the released H+ would cause degradation of Al13, increasing in concentration of AlSYM 
(Furrer et al., 1999; Yamaguchi et al., 2003).
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Fig. 2. Concentrations of undefined species other than Al13 or Alsym including aggregated or precipitated Al that cannot be detected by NMR (AlNON) (open triangle) and precipitated Al (open circle) as a function of the humic acid carboxyl group versus Al ratios (COOH/Al) after 0.007 and 570 d of aging [10 mmol L–1 hydroxyaluminum (HyA) solution].
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Figure 3
shows the concentrations of Al13, AlSYM, and AlNON after 0.007 d as a function of the COOH/Al ratio. The concentration of Al13 decreased with increasing COOH/Al ratio (Fig. 3a), whereas that of the AlNON increased (Fig. 3b). At a COOH/Al ratio of 0.78, no Al13 was detected in solution. In contrast to the strong COOH/Al ratio dependence of the concentrations of Al13 and AlNON, the concentration of AlSYM was relatively unchanged with the COOH/Al ratios used (Fig. 3c), suggesting that little AlSYM was complexed with humic acid. This indicates that humic acid has a greater preference for Al13 than AlSYM when both are present. A higher positive charge of Al13 (+7) than AlSYM (approximately +3 for the monomer, approximately +5 for the dimer) would be a major cause of the preferential complexation of Al13 with humic acid. The charge distribution, ionic potential, size, and shape of Al13 could affect the preferential complexation of humic acid with Al13 over AlSYM.
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Fig. 3. Concentrations of (a, d) aluminum tridecamer (Al13), (b, e) undefined species other than Al13 or AlSYM, including aggregated or precipitated Al that cannot be detected by NMR; (AlNON), and (c, f) Al monomer and dimer (AlSYM) as a function of humic acid carboxyl group versus Al ratios (COOH/Al) after 0.007 (a, b, c) and 570 d (d, e, f) of aging. Total Al concentrations are 5 (closed triangle) and 10 mmol L–1 (closed circle).
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There was a linear relationship between the moles of Al13 that disappeared within 0.007 d and the amount of humic acid added, except when 21 µmol of COOH was added to the 5 mmol L–1 HyA solution (Fig. 4)
. This can be explained by the excess humic acid in relation to the amount of Al13 present in the system. Omitting this point, the regression equation is given as follows:
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Fig. 4. The relationship between the amounts of humic acid added and moles of aluminum tridecamer (Al13) lost from solution within 0.007 d of aging. Total Al concentrations are 5 (open circle) and 10 mmol L–1 (closed circle).
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This indicates that almost 10 COOH groups are required to remove one Al13 from a HyA solution. A positive charge of Al13 (+7) might be neutralized by a negative charge of the dissociated COOH group conjugated with humic acids.
Stage II
At stage II (0.007–5 d), the concentrations of Al13, AlSYM, and AlNON were almost constant (Fig. 1). Almost all the carboxyl groups that were able to complex with Al would complete the precipitation with Al13 at Stage I. Stage II would be a quasi-equilibrium state in terms of aqueous Al13, AlNON, and AlSYM. In the absence of humic acid, the concentrations of Al13, AlNON, AlSYM, and pH were almost unchanged through this stage. Although the concentrations of Al13, AlSYM, and AlNON were constant, the pH of the system was increased back up to approximately 4.2 in the presence of humic acid (Fig. 1d). Degradation of Al13 would lead to the release of OH–; however, the aqueous concentration of Al13 was unchanged at Stage II. Using solid-state 27Al NMR, Hiradate and Yamaguchi (2003) proved that Al13 in the humic acid-Al13 complex gradually decomposed with time. Therefore, it is likely that the Al13 that precipitated at Stage I was degraded in the solid phase, releasing OH– to the solution.
Stage III
At Stage III (5–570 d), the concentrations of Al13 decreased, and the concentrations of AlNON were increased, both in the absence and presence of humic acid (Fig. 1a,b). In the absence of humic acid, the concentration of AlSYM increased (Fig. 1c), and the pH of the system decreased with aging (Fig. 1d). The decreasing pH resulted from H+ release by the hydrolysis reaction of Al to form colloidal Al hydroxide, and the released H+ caused the degradation of Al13 to form AlSYM. In the absence of humic acid, the increase in the AlSYM accounted for 15% of Al13 reduction after 570 d of aging. Therefore, the other 85% of the disappeared Al13 would be transformed into AlNON, probably in the form of colloidal Al hydroxides. This is supported by the fact that, in the absence of humic acid, the majority of the AlNON after 570 d of aging was particulate (Fig. 2b).
When the COOH/Al ratio was higher than 0.6, no Al13 was detectable in the 5 mmol L–1 HyA solution after 570 d of aging (Fig. 3d). This indicates that Al13 would likely disappear from solution under humic acid-rich condition.
When the COOH/Al ratio was lower than 0.6, Al13 still existed in solution even after 570 d of aging, although all Al13 completely disappeared from solution in the absence of humic acid after 210 d of aging (Fig. 1a). This implied that the disappearance of Al13 could be retarded by the presence of humic acid. There is not enough evidence, however, to conclude that humic acid directly retarded the disappearance of Al13. Rather, in the absence of humic acid, the colloidal Al hydroxide formed would provide crystal nuclei for the growth of Al hydroxide precipitate, accelerating the disappearance of Al13, whereas the humic acid retarded Al hydroxide precipitation. In the presence of humic acid, the amounts of precipitated Al did not significantly increase at Stage III (Fig. 5)
. Singer and Huang (1990) reported that the extent of precipitation of Al hydroxide polymorphs substantially decreased with increasing humic acid concentration, which is in accordance with our results. Due to the lack of Al hydroxide precipitate, humic acid retarded the precipitation of aqueous Al13 (Fig. 5). Another mechanism for the preservation of aqueous Al13 in the presence of humic acid is its buffering action, which maintains a constant pH. The suspension pH was maintained at about 4.3 in the presence of humic acid, whereas it dropped to 3.75 in the absence of humic acid after 570 d of aging (Fig. 1d). In the absence of humic acid, H+ would be released into the solution by the hydrolysis reaction of Al because the buffer capacity is quite small. In the presence of humic acid, the dissolved Al13 would be preserved because the suspension pH was maintained favorable for the existence of A113 by the buffer action of humic acid.
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Fig. 5. The concentration of precipitated Al as a function of time [10 mmol L–1 hydroxyaluminum (HyA) solution]. The humic acid carboxyl group versus Al ratios (COOH/Al) are 0 (open square), 0.07 (open triangle), 0.22 (open inverted triangle), and 0.39 (open circle).
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Possibility of the Presence of Aluminum Tridecamer in Soil Solutions
The presence of Al13 in soil and aquatic environments has been questioned because of the inhibitory effects of low molecular-weight organic and inorganic molecules on the formation and persistence of Al13 in soil systems (Thomas et al., 1991; Masion et al., 1994a, 1994b, 1994c; Molis et al., 1996; Hiradate et al., 1998; Taniguchi et al., 1999; Krishnamurti et al., 1999; Amirbahman et al., 2000; Yamaguchi et al., 2003). The present study has added the insight that Al13 cannot be preserved in soil solution when sufficient humic acid to complex with Al13 is present. However, when the amount of humic acid was relatively low (COOH/Al < 0.6, [Al total] = 5 to 10 mmol L–1), humic acid retarded the slow reaction that transforms Al13 into colloidal Al hydroxide by maintaining a pH that is favorable for the existence of Al13 and inhibiting the formation of crystal nuclei. Our results showed that the concentration of Al13 continued to decrease with aging both in the presence and absence of humic acid.
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CONCLUSIONS
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Humic acid accelerated the disappearance of dissolved Al13 from a HyA solution. At the early stage of reaction within 0.007 d, the predominant mechanism for the removal of Al13 by humic acid was an aggregation–precipitation reaction to form precipitated Al13–humic acid complexes. When the COOH/Al ratio was higher than 0.8, all the Al13 disappeared from a HyA solution within 0.007 d. Approximately 10 humic acid carboxyl groups are required to precipitate one dissolved Al13 from HyA solution. Humic acid was more effective in removing Al13 than AlSYM from the solution when both were present. After a quasi-equilibrium state of 5 d, aqueous Al13 was transformed into other polymeric Al species, both dissolved and particulate. Even though the soil solution satisfied the requirements for the presence of Al13 in terms of the pH and concentration of Al, Al13 disappeared from solution by forming insoluble humic acid–Al13 complexes. Therefore, Al13 would not be found in a soil solution when a sufficient amount of humic acid is present in soils.
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ACKNOWLEDGMENTS
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This work was partly supported by a Grant-in-Aid for Scientific Research, No. 15208008, from the Japanese Ministry of Education, Science, Culture, Sports, Science and Technology.
Received for publication April 5, 2002.
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ABSTRACT
INTRODUCTION
