Progress 09/01/02 to 08/31/05
Outputs
Studies of the sorption of apolar compounds to whole soils, humic acids (HAs), modified HAs, fractionated HAs, humin, and kerogen confirmed the hypothesis that natural organic matter (NOM) behaves as a sorbent of organic molecules like a glassy polymer. Sorption isotherms are characteristically nonlinear, obey the dual mode model which has terms for dissolution and hole-filling processes, are subject to competitive effects in the presence of a second solute, and show true hysteresis. Such behavior was even observed for HAs verified to be black carbon-free. Hardening of the HA matrix by complexation with Al or adsorption on clays caused a decrease in the linearity index (Freundlich exponent). Solutes showed a 15-20 kJ/mole preference for a hole compared to a dissolution site, attributable to the free energy needed to create a cavity for the solute in the dissolution domain. Proton spin-spin (T2) relaxation NMR experiments show evidence for rigid and flexible domains in
proportions consistent with the degree of nonlinearity in the isotherm. Concentration dependent structure-activity correlation analysis revealed that sorbate molecules prefer sites richer in dipolar-polarizability influence; that is, sites richer in aromatic and polar cross-linking groups which impart rigidity to the matrix. Irreversible behaviors, including hysteresis and the conditioning effect (increased second-time sorption), were observed for all types of NOM solids, including HAs. These behaviors are characteristic of the glassy state and not characteristic of the rubbery state. They are caused by swelling (pore expansion, pore creation) by the incoming sorbate which is irreversible on the timescale of the experiment. 14-C isotope-exchange equilibria and rates were used to show that hysteresis was due to irreversible processes and not experimental artifacts. Hysteresis followed the order of expected hardness (chain-stiffness) of NOM: HA < Al-HA < kerogen. The conditioning effect
(as enhancement of organic C based partition coefficient Koc compared to an unconditioned sample) persists for at least weeks at room temperature, but relaxes on sample annealing (45 - 90 oC) in a manner similar to the relaxation of free volume and enthalpy of glassy polymers. Relaxation of the conditioning effect depended on annealing T and, at a given T, followed a double additive exponential rate law with a non-zero constant term inversely related to T. Extrapolating to environmentally relevant temperatures, it was found that Koc never completely relaxes to its original value. Sequential alkaline extraction of a forest soil yielded a series of HA with progressively lower aromatic and polar C, and progressively higher aliphatic C. The Koc of phenanthrene increased along the same series. The remaining humin had the highest aliphatic fraction and the lowest linearity index. Clays preferentially adsorb the aliphatic components of dissolved HAs, as shown spectroscopically, and the
clay-HA complexes showed enhanced Koc relative to the HA particles alone.
Impacts
The results are significant because they show that macromolecular humic substances free of black carbon can sorb nonpolar compounds nonlinearly and irreversibly due to their glassy nature. Taken as a whole, the results provide compelling evidence for the glassy nature of NOM and for irreversible pore expansion as a cause of irreversible sorption. The results underscore that nonlinear and irreversible behavior may be expected in sorption to macromolecular forms of NOM (even humic acid) that are in a glassy state, and emphasize the case that NOM is not a passive sorbent, but may be physically altered by the sorbate. The studies of sorption on Al-HA complexes, humin fractions, and clay-HA complexes indicate that close association of NOM with the mineral fraction has important effects on sorption of organic compounds. Overall, the findings of this project are expected to shift the paradigm for sorption of nonionic compounds by NOM. The findings are expected to be relevant
to many disciplines, including soil science, environmental science, and environmental engineering.
Publications
- Wang, X., T. Sato and B. Xing. Sorption and displacement of pyrene in soils and sediments. Environ. Sci. Technol. (in press, 2005).
- Pignatello, J.J.. Y. Lu, E.J. LeBoeuf, W. Huang, J. Song and B. Xing, Nonlinear and competitive sorption of apolar compounds in black carbon-free natural organic materials, J. Environ. Qual., 35: 1049-1059 (2006)
- Sander, M. and J.J. Pignatello, An Isotope Exchange Technique to Assess Mechanisms of Sorption Hysteresis Applied to Naphthalene in Kerogenous Organic Matter, Environ. Sci. Technol. 39: 7476-7484 (2005).
- Sander, M., Y. Lu, and J.J. Pignatello, Conditioning Annealing Studies of Natural Organic Matter Solids Linking Irreversible Sorption to Irreversible Structural Expansion, Environ. Sci. Technol., 40: 170-178 (2006). [Correction, 40: 6518 (2006)]
- Xing, B., and J.J. Pignatello, Sorption-Organic Chemicals, In Encyclopedia of Soils in the Environment; Hillel, D., Ed.; Elsevier Ltd.: Oxford, U.K., 2005; Vol. 3, pp 537-548 (2005).
- Zhu, D. and J.J. Pignatello, A Concentration-Dependent Multi-Term Linear Free Energy Relationship for Sorption of Organic Compounds to Soils Based on the Hexadecane Dilute-Solution Reference State, Environ. Sci. Technol., 39: 8817-8828 (2005).
- Kang, S. and B. Xing. 2005. Phenanthrene sorption to sequentially extracted soil humic acids and humin. Environ. Sci. Technol. 39: 134-140.
- Wang, K. and B. Xing. 2005. Structural and sorption characteristics of adsorbed humic acid on clay minerals. J. Environ. Qual. 34: 342-349.
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Progress 01/01/04 to 12/31/04
Outputs
It is postulated that some forms of natural organic matter (NOM) exist in a glassy state and exhibit sorbent behavior consistent with this state. We examined sorption of two apolar compounds in three samples of macromolecular NOM to test whether history-dependent (irreversible) behaviors, including hysteresis and the conditioning effect, agree with a pore deformation/creation hypothesis applicable to glassy organic solids such as synthetic polymers. The compounds are 1,2,4-trichlorobenzene (TCB) and naphthalene (NAPH). The NOM samples are a soil humic acid (H-HA), an Al3+-exchanged form of the same humic acid (Al-HA), and a low-rank coal (Beulah-Zap lignite, BZL). The HAs are believed free of environmental black carbon. The degree of nonlinearity in the isotherm and the ratio of hole-filling to solid phase dissolution increased in the order of hardness (stiffness) of the solid: H-HA < Al-HA < BZL. Independent of solid, solutes show a 15-20 kJ/mole preference for a
hole site compared to a dissolution site. This preference is attributable to the free energy needed to create a cavity in the dissolution domain to accommodate the solute. All solids showed hysteresis and the conditioning effect. The conditioning effect refers to enhanced re-sorption after pretreatment with a conditioning agent (in this case, chlorobenzene). Conditioning the sample results in increased sorption and greater contribution of hole-filling relative to dissolution. Progressively extracted humic fractions (humic acids and humins) from several soils and two compost samples were characterized. Compost samples were free of charcoal, thus their humic fractions contained no charcoal. Even for a given soil or compost sample, humic fractions showed tremendous heterogeneity in terms of elemental composition, polarity, and structure as determined by spectroscopic analyses. Humin, which accounted for a major fraction of organic carbon of all samples, appeared to have much larger
organic carbon-normalized sorption coefficients than soils and their humic acids. Preliminary experiments showed that humic-clay complex (synthetic humin) also appeared to have higher sorption of organic compounds than humic acid alone. Detailed spectroscopic analysis and sorption work are underway to determine why humin fractions apparently have high sorption of organic contaminants.
Impacts
The results are significant because they show that macromolecular humic substances free of black carbon can sorb nonpolar compounds nonlinearly and irreversibly due to their glassy nature. The hysteresis and conditioning effect experimental results are consistent with the glassy character of the solids. The effects of original hole population, matrix stiffness, and solute concentration on the hysteresis index and on the magnitude of the conditioning effect support pore-deformation as the underlying cause of sorption irreversibility. Pore deformation involves concurrent processes of hole expansion and creation of new holes by the incoming sorbate (or conditioning agent), which may be irreversible on the timescale of the experiment. The results underscore that nonlinear and irreversible behavior may be expected in sorption to macromolecular forms of NOM (even humic acid) that are in a glassy state, and emphasize the case that NOM is not a passive sorbent, but may be
physically altered by the sorbate. The studies of sorption on the humic fractions and synthetic humin indicate that humin fractions may play a dominant role in the sorption of organic contaminants in soils and sediments. Overall, the results of this project are expected to shift the paradigm for sorption of nonionic compounds to NOM.
Publications
- Lu, Y. and J.J. Pignatello, History-dependent Sorption in Humic Acids and a Lignite in the Context of a Polymer Model for Natural Organic Matter. 2004. Environ. Sci. Technol. 38: 5853-5862.
- Kang, S.H., D. Amarasiriwardena, P.L. Veneman, and B. Xing. 2003. Characterization of ten sequentially extracted humic acids and a humin from a soil in western Massachusetts. Soil Sci. 168: 880-887.
- Lu, Y. and J.J. Pignatello, Sorption of Apolar Aromatic Compounds to Soil Humic Acid Particles Affected by Polyvalent Metal Ion Crosslinking. 2004. J. Environ. Qual., 33: 1314-1321.
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Progress 01/01/03 to 12/31/03
Outputs
Irreversible sorption was linked to the glassy character of NOM in a study of hysteresis and the conditioning effect of 1,2,4-trichlorobenzene (TCB) and naphthalene (Naph) in three NOM samples: a soil H+ humic acid (H-HA); Al3+-exchanged form of the same humic acid (Al-HA); and Beulah-Zap lignite (BZL), a low-rank coal. These samples increase in that order in their glass transition temperature (measure of hardness) and are free of environmental black carbon. Nonlinearity and hysteresis increased with hardness (H-HA < Al-HA < BZL). Sorption was compared in the original solid and solid pre-treated with a conditioning agent, chlorobenzene, which was subsequently removed to below detection. All solids exhibited the conditioning effect. Sorption of TCB and Naph to conditioned solids was in each case enhanced, and the isotherm was more nonlinear than sorption to the original solid. The enhancement observed for the conditioned solids largely disappeared after thermal
relaxation. The rate of relaxation of a chlorobenzene-conditioned soil (Pahokee peat) using 1,2,4-trichlorobenzene as the test solute increased with temperature. Relaxation to approximately the original state could be achieved in hours at 90 0C. Experiments with structurally modified soil and compost humic acids (HA) and phenanthrene were conducted. All sorption isotherms were nonlinear, even for the compost HA free of black carbon. In addition, competitive sorption between phenanthrene and pyrene was observed for all HAs. Data from proton spin-spin (T2) relaxation NMR experiments provide further evidence for the relationship between the glassy character of organic matter and isotherm nonlinearity as we proposed (dual-mode sorption model) in our earlier work; rigid (glassy) domains of soil organic matter impart nonlinearity. We are also studying various fractions of organic matter from soil and compost samples. We exhaustively extract HAs and characterize the sequentially extracted
HAs and residual humins. We observed that aliphatic carbons of HAs increased with progressive extractions and reached maximum at humin. In addition, polarity of HAs as measured by O/C ratio (elemental analysis) and carboxylic carbons (NMR) decreased (i.e., more hydrophobic) with increasing number of extractions. These data are important because they may explain the high organic carbon-normalized sorption coefficients for humin factions of soil (sediment) organic matter. Sorption experiments with these HAs and humins are currently underway.
Impacts
The results are significant because they show that macromolecular humic substances free of black carbon can sorb nonpolar compounds nonideally, presumably due to their glassy nature. The hysteresis and conditioning effect experimental results are consistent with the glassy character of the solids and support the irreversible hole expansion/hole creation mechanism as the cause of history-dependent behavior.
Publications
- Gunasekara, A.S. and B. Xing. 2003. Sorption and Desorption of naphthalene by soil organic matter: Importance of aromatic and aliphatic components. J. Environ. Qual. 32: 240-246.
- Gunasekara, A.S. M.J. Simpson, and B. Xing. 2003. Identification and characterization of sorption domains in soil organic matter using structurally modified humic acids. Environ. Sci. Technol. 37: 852-858.
- Kang, S.H., D. Amarasiriwardena, P.L. Veneman, and B. Xing. 2003. Characterization of ten sequentially extracted humic acids and a humin from a soil in western Massachusetts. Soil Sci. (in press).
- Lu, Y. and Pignatello, J.J. (2004). Sorption of apolar aromatic compounds to soil humic acid particles affected by aluminum (III) ion crosslinking, J. Environ. Qual., in press.
- Tomson, M.B., H.E. Allen, C.W. English, W.J. Lyman, and J.J. Pignatello. (2003) Contaminant-Soil Interactions. Chapter 7 in: Contaminated Soils: From Soil-Chemical Interactions to Ecosystem Management, R.P. Lanno, Edit.; Society of Environmental Toxicology and Chemistry Press; Pensacola, FL.
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Progress 09/01/02 to 12/31/02
Outputs
We have conducted sorption experiments on a mineral soil and its organic fractions (humin and humic acid). Naphthalene sorption by all these sorbents was nonlinear. Aromaticity was lowest in soil humin as determined by 13C solid-state NMR, but this organic matter fraction had the most nonlinear isotherm and highest sorption (on an organic carbon basis). These results differ from the early literature reports which suggested that the aromatic component of natural organic matter is responsible for both high sorption and isotherm nonlinearity. Also, the humin had a highest degree of hysteresis. Hysteresis (isotherm nonsingularity) is a confounding issue that undermines the commonplace assumption of sorption reversibility in soils. We showed that a soil humic acid, and the Al-exchanged humic acid (both in particulate form) exhibit the 'conditioning effect', which refers to enhancement of sorption following exposure of the sorbent to the same or similar compound. The
conditioning effect has been used to support a pore deformation mechanism for hysteresis in glassy synthetic polymers. Chlorobenzene was the conditioning agent and naphthalene was the test solute. The naphthalene isotherm in the conditioned soil was shifted upward and was less linear than that in the non-conditioned control. We propose that hysteresis in humic substances, like in glassy polymers, is due to pore deformation by the solute.
Impacts
The results contribute to the growing body of evidence that soil organic matter has glassy character that affects its sorbent properties. (The glassy components may predominate in aliphatic as opposed to aromatic regions of the humic solid.) This general finding will alter the paradigm for sorption processes, which, in turn, will have an impact on our ability to predict the fate and biological availability of chemicals introduced to the soil. Pore deformation as a cause of hysteresis implies that desorption rates may depend in part on changes within the three-dimensional matrix of organic matter.
Publications
- No publications reported this period
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