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Department of Soil and Water, Connecticut Agricultural Experiment Station, 123 Huntington Street, New Haven, CT 06511
* Corresponding author (Joseph.Pignatello{at}po.state.ct.us).
Received for publication May 19, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: Al-HA, aluminum-ion-exchanged humic acid • DCB, 1,3-dichlorobenzene • DMM, dual-mode model • HA, humic acid • H-HA, hydrogen-ion-exchanged humic acid • OC, organic carbon • TCB, 1,2,4-trichlorobenzene
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Sorption in rubbery solids occurs by solid-phase dissolution and generates isotherms that are linear, noncompetitive, and reversible. Sorption in glassy solids occurs by a dual mechanism that includes dissolution and hole-filling processes. The holes are cavities of nanometer to subnanometer scale that are envisioned to be distributed within the dissolution domain of the solid. Sorption in glassy solids is typically nonlinear, competitive, and hysteretic. According to the dual-mode concept, the hole-filling process accounts for nonideality of sorption. Nonlinearity, competition in bisolute systems, and hysteresis have been observed by many investigators in various soil and isolated SOM samples; further explanation and key references are given in, for example, Miller and Pedit (1992), Xing et al. (1996), Xing and Pignatello (1997), Huang et al. (1997), Huang and Weber (1997), Kan et al. (1998), Weber et al. (1998), Xia and Ball (1999), and Xia and Pignatello (2001).
The glass transition temperature (Tg) of a solid depends on matrix rigidity (i.e., the flexibility of its macromolecules). Properties that decrease flexibility include high molecular weight, presence of unsaturated bonds (especially aromatic groups), and cross-linking (Rudin, 1999). Cross-linking ties strands together, increasing rigidity and density. Humic substances in soil environments contain matrix polyvalent metal ions such as Al3+, Fe3+, Ca2+, Mg2+, and others. Polyvalent metal ions can function as cross-linking agents through their ability to coordinate to multiple functional groups (mainly carboxyl and phenolic groups) located on different strands of the humic backbone. Polyvalent metal ion cross-linking is hypothesized to increase the glassy character of humic substances and therefore enhance nonlinearity, competition, and hysteresis of hydrophobic compound sorption. We note that kerogens and coals are highly cross-linked through covalent bonds, and typically give isotherms of pronounced nonlinearity and hysteresis.
To test the cross-linking hypothesis we set out to determine sorption behavior of certain compounds on a particulate humic acid (HA) in its metal-ion free protonated form (H-HA) compared with the same material in its fully Al3+–exchanged form (Al-HA). We used Al3+ as an example of the metal ions available in the environment that can act as cross-linking agents. Humic acids extracted from soil and reconstituted in particulate form are solids that appear to lie low on the hardness ("glassiness") scale among the different forms of natural organic matter; their glass transition temperatures are relatively low (43–72°C; LeBoeuf and Weber, 1997, 2000), though still above room temperature, and sorption in them is reported to be slightly nonlinear (Xing and Pignatello, 1997; Huang and Weber, 1997; Gunasekara et al., 2003).
Encouraging preliminary results in support of the metal ion cross-linking hypothesis have appeared. Yuan and Xing (2001) report that nonlinearity and hysteresis of naphthalene, phenanthrene, and 1-naphthol are enhanced when humic acid particles are suspended in Al3+ solution compared with Ca2+ solution, although they did not verify exchange of Al3+, and did not test metal-free H-HA. The Al-HA used in our study was prepared by flocculating dissolved H-HA with Al3+ according to a published method that affords a material shown by Al nuclear magnetic resonance spectroscopy to contain Al ions coordinated to organic groups rather than incorporated in Al oxide polymers (Vilge-Ritter et al., 1999; Masion et al., 2000). While it is not possible to quantify the degree of cross-linking, it is reasonable to postulate that it is significant due to the chelation effect, which favors complexation of a coordinating group with a nearby complexed cation over a free cation in solution. Sorption of naphthalene and 1,2,4-trichlorobenzene (TCB) are quantitatively evaluated with respect to linearity, competition, and reversibility. In addition, we apply the dual-mode model to sorption isotherms and demonstrate that cross-linking increases the affinity of solutes for the hole domain compared with the dissolution domain. We find that nonlinearity, competition, and irreversibility are all enhanced in the cross-linked solid.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Preparation and Characterization of Humic Acids
The H-HA was prepared from a peat soil (Terric Haplosaprist) collected in Amherst, MA, in May 2001. The soil was air-dried and passed through a 2-mm sieve. It contained 18.9% organic carbon (OC) on a dry weight basis. Following International Humic Substances Society (IHSS)-recommended methods (Swift, 1996), HA was extracted by shaking the soil in N2–purged 0.1 M NaOH (1:10 w/v) at 21°C for 24 h. After settling overnight the suspension was centrifuged at 700 x g for 30 min and the supernatant was decanted. The recovered supernatant was then acidified to pH 1.0 with concentrated HCl to precipitate H-HA. The precipitate was separated by centrifugation at 700 x g for 20 min, and washed repeatedly with 0.1 M HCl followed by distilled, deionized water. The solid H-HA was redissolved in a minimum amount of 0.1 M NaOH, diluted with distilled, deionized water, and then pumped through a cation exchange resin (Na+–saturated Dowex 50WX8 resin; Fluka Chemical Co., Buchs, Switzerland) to remove metal ions. The dissolved HA in the effluent was reprecipitated by HCl as described above. The obtained solid H-HA was de-ashed twice (24 h each time) with 0.1 M HCl–0.3 M HF solution, followed by thorough rinsing with distilled, deionized water. Finally, the H-HA was freeze-dried, ground, and stored in a refrigerator.
The Al-HA was prepared by the method of Vilge-Ritter et al. (1999) and Masion et al. (2000). The H-HA was dissolved in distilled, deionized water by addition of a minimum amount of 10 M NaOH, diluted with distilled, deionized water to approximately 1 g/L of organic carbon, and adjusted to pH 6 with 0.1 M HCl. The HA was then flocculated at pH 6 with 0.01 M Al2(SO4)3. The Al dose (20 mmol/g OC) was comparable with doses used in the cited studies (8.4–42 mmol Al/g OC). Briefly, the flocculant was added to the HA solution under rapid mixing condition (140 rpm controlled by a magnetic stirrer), and without delay the pH was re-adjusted to 6 with NaOH. After a 3-min period of rapid mixing followed by a 30-min period of slow mixing at 40 rpm, the Al-HA flocs were allowed to age and settle overnight. Residual dissolved organic carbon was less than 1% of initial. The flocs were rinsed repeatedly with distilled, deionized water to remove excess Al salt, centrifuged at 700 x g for 15 min, and then freeze-dried and ground.
The obtained H-HA and Al-HA were characterized for Al, Fe, Ca, Mg, and Na by inductively coupled plasma–optical emission spectroscopy (ICP–OES) after HNO3 digestion, and for other elements and ash by Galbraith Laboratories (Knoxville, TN) using standard microanalytical techniques. The results of elemental analyses of both solids are shown in Table 1. Total metal content of H-HA was less than 1% (w/w). The Al content was 3.1 g/kg for H-HA and 102 g/kg for Al-HA. Organic C content was 51.7% for H-HA and 26.2% for Al-HA. The relatively high ash content of H-HA is believed to be mainly attributed to the residual Al, Fe, and Si minerals that are strongly incorporated in the HA matrix and are difficult to remove even by HF and HCl treatments. Comparison of the Al, Fe, and Si contents before and after de-ashing indicated that 64, 34, and 49% of Al, Fe, and Si, respectively, were removed by the de-ashing process.
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-Al2O3 (1-µm particle size) were reported by Mader et al. (1997). If all the Al in the Al-HA product ended up as polymer, and assuming the polymer was not highly porous, the resulting sorption would correspond to only 1.1 x 10–3% (naphthalene) and 7 x 10–4% (TCB) of observed sorption. Since we observe significantly different sorption behavior on Al-HA, we can attribute these differences to alteration of humic structure by Al. Complexation of organo groups by Al is supported by the highly efficient ability of Al to precipitate the humic acid. The total acidity of H-HA was found to be 13.7 mmol/g OC by alkalimetric titration (Lu and Allen, 2002). The total Al in Al-HA is 14.4 mmol/g OC. Thus, the exchange ratio of H+ for Al3+ is approximately 1:1. If all Al in the solid were involved in cross-linkages, this exchange ratio suggests the average Al chain length is 2. However, there is likely to be a distribution of chain lengths, but they should be limited to a relatively low number because the 1:1 exchange ratio has to be maintained. Cross-linking is not limited to monomeric Al species. For example, it is quite reasonable to expect a cross-link of the type: humic–O–Al–O–Al–O–humic.
Sorption and Desorption Experiments
Sorption Isotherms
Detailed sorption isotherms of TCB and naphthalene on H-HA and Al-HA were conducted in 24-mL glass screw-cap vials with teflon-backed silicone septa. Each isotherm consisted of 25 to 28 points, covering a range of three or four powers of 10 in concentration, with the high end approaching the water solubility, Sw. Sorption of TCB to H-HA and Al-HA was performed in 0.005 M NaCl solution at an unadjusted pH of 6. The H-HA remained largely in the H+ form during the sorption experiments because the pH remained in the range 5.8 to 6 and only slight color developed in solution. The Al-HA solution remained colorless and at constant pH of 6. Sorption of naphthalene to H-HA and Al-HA was performed at pH 3 adjusted with HCl. The lower pH was used as a precaution here to eliminate possible effect of dissolved organic matter (color development) that was observed in the TCB experiment at pH 6. We added NaN3 (200 mg/L) to inhibit biodegradation. The water-to-solid ratios were adjusted to achieve 40 to 75% of added solute sorbed. Fifty milligrams H-HA or 80 mg Al-HA was added for the TCB experiment and 50 mg H-HA or 100 mg Al-HA was added for the naphthalene experiment, followed by addition of enough liquid phase (approximately 23 mL) to almost eliminate the headspace. An additional series of samples without sorbent was employed as control over the same test concentration range to correct for systematic errors.
After a 48-h sorbent wetting period, the sorbate was added in methanol carrier to each vial. Methanol concentration was identical in all vials of a given experiment and kept below 0.001 mole fraction to eliminate cosolvent effects. After addition of sorbate, the vials were rotated at 6 rpm in an incubator at 20 ± 1°C for 7 d (TCB) or 8 d (naphthalene). Uptake rate experiments indicated that these times are more than adequate to reach apparent equilibrium (Fig. 1). For TCB, apparent equilibrium was reached in less than 2 d in both solids. Similar results were obtained for naphthalene, tested with H-HA solid only.
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Desorption
After completion of the sorption period, desorption at selected concentrations was conducted in duplicate vials using a single-step, centrifuge-withdraw-refill method. For TCB, 68 or 89% of the supernatant was replaced with fresh liquid phase for the duplicate vials; for naphthalene, 85% of the supernatant was replaced for each of the vials. The equilibration times were the same as those used in the sorption step.
Competitive Sorption of Tri- and Dichlorobenzene
Competitive sorption experiments were performed for H-HA and Al-HA at a constant initial concentration of the target compound, TCB (approximately 0.4 mg/L, which is approximately 10–2Sw), with varying initial concentrations (0–26 mg/L) of the competing sorbate, DCB. All vials in a series contained the same amount of sorbent, 50 mg H-HA or 100 mg Al-HA. Other conditions were as described above. After equilibration, TCB and DCB in the liquid phase were determined by GC–ECD.
Sorption Models
Sorption isotherms were fit to both the Freundlich model and the simple dual-mode model (DMM) by weighted nonlinear regression (weighting factor, w = 1/qmeasured2).
The Freundlich model is given by:
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The simple DMM includes solid-phase dissolution described by a linear term and hole-filling described by a Langmuir term, and is expressed as:
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The ratio (S0b/KD) in Eq. [2] represents the effective affinity of sorbate for the hole-filling domain relative to the dissolution domain in the Henry's law region of the isotherm (Lu and Pignatello, 2002); thus, since the hole-filling domain is wholly responsible for nonlinear behavior, (S0b/KD) is an alternative index of nonlinearity. The higher the ratio (S0b/KD) the greater the predominance of hole-filling, and the greater the nonlinear effect.
Quantification of Hysteresis—Index of Irreversibility
The index of irreversibility (I) was computed according to the method of Braida et al. (2003) by calculating the tendency to desorb in the real case compared with the hypothetical fully reversible and no-desorption cases. The tendency to desorb is given by the local rate of change (slope) of the isotherm at the point of interest. Thus:
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Noticeable in Fig. 2 and 3 and Table 2 is that, for both compounds, sorption per unit mass of sorbent is weaker to Al-HA than H-HA. This is primarily due to dilution of the organic matter by Al (the OC content of Al-HA is about half that of H-HA; Table 1). When sorbed concentrations are normalized to OC, the TCB isotherm for Al-HA is almost coincident with that for H-HA (figure not shown). Hence, the OC-normalized Freundlich affinity coefficients (KF,OC) of TCB in the two solids are almost the same (Table 2). In the case of naphthalene, however, the OC-normalized isotherm for H-HA is shifted appreciably upward (approximately 25–60%) of that for Al-HA throughout the experimental concentration range, and the KF,OC for H-HA is about 50% larger than the KF,OC for Al-HA (Table 2). It is possible that the difference in behavior of the two compounds reflects specific interactions of naphthalene with organic matter that are unavailable (or less important) for TCB. This possibility will be addressed in a future report in connection with data on soils.
Sorption–Desorption Hysteresis
Desorption branches emanating from selected sorption points are shown in Fig. 3 and the index of irreversibility (I) as a function of solute concentration is plotted in Fig. 4. In H-HA both TCB and naphthalene isotherms show no hysteresis except at high concentration (>1 mg/L). In Al-HA, however, both compounds show appreciable hysteresis. This is true hysteresis since artifacts such as degradation and insufficient time allowed for molecular diffusion to reach steady state have been eliminated.
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Figure 4 shows a minimum in the I vs. log C curve in most cases. The cause of this minimum is speculative but may be rationalized in the following way. First, irreversible deformation of existing holes decreases with concentration due to progressive softening of the matrix as the glass transition concentration Sg (the point at which the solid is converted to the rubbery state at the experimental temperature) is approached (Xia and Pignatello, 2001). Second, new holes are created in the matrix and their creation increases with concentration (possibly exponentially) up to the Sg (Xia and Pignatello, 2001; Lu and Pignatello, 2002). Third, the hole domain, which is responsible for hysteresis, becomes increasingly less important relative to the dissolution domain, vanishing at the Sg. The combination of these trends is likely to be dependent on solute and sorbent, but can result in the observed minimum in the I vs. log C curve.
Competitive Sorption between Tri- and Dichlorobenzene
1,2,4-Trichlorobenzene sorption in the presence of a competing co-solute, DCB, is shown in Fig. 5. The ordinate is Kd–normalized to its value in the absence of DCB. The abscissa is the ratio of moles DCB sorbed per mole TCB sorbed in the absence of DCB 
. Plotting the data in this way allows us to compare the degree of competition between the two sorbents. We find that the competitive effect occurs in both HA materials, and that the effect is greater in Al-HA than in H-HA. The fact that competition is observed in H-HA is further evidence that this organic matter is glassy, since competitive sorption is not characteristic of the rubbery state. Enhancement of competition by Al-cross-linking agrees with the prediction of the DMM applied to the single-solute isotherm of TCB that cross-linking imparts greater affinity for solute of the hole-filling domain relative to the dissolution domain. The observation is consistent with other nonideal sorption behaviors discussed previously, and further reinforces the link between nonideal sorption and glassy character of SOM.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Second, the results show that nonlinearity, competition, and hysteresis in HA are all enhanced by cross-linking with Al3+. This is consistent with the hypothesis that polyvalent metal ion cross-linking creates a more rigid-chain structure. Application of a glassy polymer sorption model (dual-mode model) reveals that cross-linking increases the affinity of solutes for the hole domain relative to the dissolution domain. Hence, this study supports a link between nonideal sorption and the glassy character of soil organic matter. Our results in general agree with those of Yuan and Xing (2001) with respect to the effect of Al3+ on sorption nonlinearity and hysteresis, although different Al3+–exchange methods were used in the two studies. However, the positive correlation between nonlinearity and hysteresis shown in their study (comparison among naphthalene, phenanthrene, and -naphthol) was not seen when comparing TCB and naphthalene here.
Lastly, this study shows the importance of metal ions on humic structure with respect to the sorption of organic compounds. Polyvalent metal ions are abundant in soils and coordinate to soil organic matter. Other polyvalent metal ions may have a similar effect as observed here for Al.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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