Environ. Sci. Technol. 2004, 38, 4120-4129
Hydration of Natural Organic Matter: Effect on Sorption of Organic Compounds by Humin and Humic Acid Fractions vs Original Peat Material MIKHAIL BORISOVER* AND ELLEN R. GRABER Institute of Soil, Water and Environmental Sciences, ARO, The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel
Natural organic matter (NOM) hydration is found to change activity-based sorption of test organic compounds by as much as 2-3 orders of magnitude, depending on the compound and the specific NOM sorbent. This is demonstrated for sorption on humin, humic acid, and the NOM source material. Hydration assistance in organic compound sorption correlates with the ability of the sorbate to interact strongly with hydrated sorbents, demonstrating the important role of noncovalent polar links in organizing the sorbent structure. Differences in hydration effect between the sorbents are caused mainly by differences in compoundsorbent interactions in the dry state. For a given compound, hydration of the sorbent tends to equalize the sorption capability of the three sorbents. No correlation was found between the strength of sorbate-sorbent interactions or the type of sorbate functional groups and the extent of sorption nonlinearity. Sorption nonlinearity compared over the same sorbed concentration range is greater on the original NOM than on either of the two extracted fractions. In elucidating sorption mechanisms on hydrated NOM, it is important to explicitly consider the participation of water molecules in organic compound interactions in the NOM phase.
Introduction Like for many macromolecular substances, hydration of natural organic matter (NOM) may result in swelling, increased flexibility, alterations in conformation, and changes in ionization status of polar functional groups. These hydration-driven changes in NOM may strongly affect sorption interactions of organic compounds with NOM by influencing intraorganic matter diffusion, sorption kinetics, and extent of sorption. Since the hydration status of NOM can vary considerably under different environmental scenarios, it is clear that understanding NOM-organic pollutant sorption interactions requires insights into the effect of NOM hydration on sorption. Among processes that can potentially contribute to the overall hydration effect on sorbate interactions in the NOM phase are competition between water and sorbate molecules, a change of sorbate speciation in the NOM phase, direct interaction between complexed water molecules and sorbate molecules in the NOM phase, a change in the * Corresponding author phone: 972-3-968-3314; fax: 972-3-9604017; e-mail:
[email protected]. 4120
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 15, 2004
total volume of the NOM phase, and a change in NOM polarity. The net effect of NOM hydration on organic compound sorption interactions is far from self-evident due to the scarcity of experimental organic compound sorption data in dry, wet, and partially hydrated NOM, and the complexity of NOM hydration phenomena (1-3). Recently, a new concept of water-disruptable noncovalent links in NOM was developed to elucidate the effect of NOM hydration on sorption of organic compounds (1, 2). The concept is the outcome of sorption studies with different organic compounds (pyridine, phenol, nitrobenzene, acetophenone, benzyl alcohol, m-nitrophenol, acetonitrile, atrazine, trichloroethylene, benzene, and tetrachloromethane; 1-4) on a model NOM under completely hydrated or dry conditions, and in active organic solvent systems under different extents of NOM solvation. In these experiments, the hydration (solvation) effect on sorption was clearly distinguished from the obvious influence of bulk solventsolute interactions by comparing sorption on a compound activity basis. For certain strongly interacting compounds (e.g., phenols, pyridine, benzyl alcohol), it was observed that sorption at a given compound activity was strongly increased upon NOM hydration (or solvation by strongly interacting organic solvents). This solvent-assisted sorption was conceived to result from formation of new sorption sites upon solvation of the NOM. This mechanism involves solvent-assisted penetration of organic sorbates into noncovalently linked NOM moieties that are not available for compound sorption in dry NOM due to strong intra-NOM interactions (e.g., H-bonding, proton-transfer phenomena, bridging via metal cations). It was derived that the driving force for solventassisted sorption is solvation of the partner of the disrupted NOM contact that does not directly interact with the sorbate (2). Importantly, this new model covers different situations such as sorbate interactions with NOM links disruptable by water molecules alone (typically considered part of NOM swelling upon hydration) and joint penetration of solute and solvent molecules into NOM contacts that are less available for only sorbate or solvent molecules. Depending on the ability of sorbate molecules to interact with NOM and compete with water molecules for sorption sites, there will be a tradeoff between solvent-assisted penetration of organic compound molecules into polar contacts and competition between sorbate and solvent molecules for new sites at those disrupted contacts (2, 3). The importance of polar NOM noncovalent links in organizing the NOM phase and in dictating the hydration effect on sorption of organic compounds was supported by recent observations that the greater a compound’s ability to undergo specific interactions with NOM, the greater the hydrationassisted sorption effect (3). This is because penetration of organic compounds into polar contacts solvated by water must involve competition with water molecules. This conceptual model was also strengthened by the introduction of a new sorption isotherm model that describes sorption interactions of organic compounds on NOM (5, 6). Previous experiments were performed on a single model NOM (International Humic Substances Society, IHSS, Pahokee peat). Thus, the major objective of the current research was to examine different extents of the hydration effect on sorption of organic compounds for different NOMs. To achieve this goal, the hydration effect was studied on humin and humic acid extracted from the IHSS peat. There are many studies in the literature where aqueous sorption of different organic compounds was examined on humin 10.1021/es035357s CCC: $27.50
2004 American Chemical Society Published on Web 06/19/2004
TABLE 1. Properties of Organic Compounds
compd
molar vol,a cm3/mol
molar refraction,b cm3/mol
aqueous solubility, mg/L
log Kowc
-(log Hh)d
-(log Hw)d
log Pe
-(∆ log HOC)b
acetophenone nitrobenzene benzyl alcohol m-nitrophenol
116.3 102.2 103.4 93.7
36.5 32.9 32.6 35.4
5740c 1949c 40100c 12150h
1.60 1.85 1.10 2.00
4.50 4.56 4.22 5.69
3.36 3.02 4.86 7.06
-0.40f -0.61f -1.02g -3.20i
1.5 2.3 3.4 5.5
a Compound densities from ref 14, 15-20 °C. b Reference 13. c Reference 15. d Converted from Ostwald coefficients from ref 16. e Logarithm of the saturated vapor pressure over the liquid, 25 °C (mmHg). f Reference 17. g Extrapolated from 29.5 °C from ref 18. h Reference 3. i Calculated on the basis of the reported aqueous solubility, aqueous Henry constant, fusion enthalpy (19.2 kJ/mol; 19), and melting point (97 °C; 14).
and humic acid (e.g., refs 7-11 and others). However, to the best of our knowledge, there are no studies comparing sorption isotherms of different organic compounds on dry and completely hydrated humin and humic acid. Considering the evident differences in physical-chemical properties between isolated NOM fractions and the source material, comparison of sorption of organic compounds on these NOM fractions under hydrated and nonhydrated conditions will be illustrative for testing the variability of hydration effects on different NOMs. This study will demonstrate how consideration of two counteracting sorption mechanisms, namely, solvent-assisted sorbate penetration into NOM polar contacts on one hand, and sorbate/solvent competition for new sites at those disrupted contacts on the other, is helpful in understanding sorption phenomena in NOM.
Experimental Section General Procedures. Sorption data for four compounds on humin and two compounds on humic acid measured from water (hydrated systems) and n-hexadecane (nonhydrated, dry systems) are reported. The hydration effect is established for each compound by comparing sorption isotherms measured under hydrated and nonhydrated conditions on a compound activity basis. Procedures for sorption experiments, analytical determinations, and solubility measurements are the same as provided in previous papers (2, 3). Materials. Humic acid (HA) and humin (HUM) were prepared from IHSS Pahokee peat (83% organic matter; 49% organic carbon (OC), 3.3% N, 4.3% H, 0.5-1.2% S; 3) according to the procedure described earlier (0.5 M NaOH, eight consecutive extractions; 12). Humic acid and humin fractions were freeze-dried and γ-irradiated prior to sorption experiments. The humin fraction represented 10% of the initial peat amount, in excellent agreement with humin content reported for the same peat (0.1 M NaOH, 50 consecutive extractions; 7). Humic acid contained 42% C, 2.7% N, 3.6% H, and 0% S, and humin contained 47.6% C, 2.5% N, 4.7% H, and 0% S (dry weight basis, elemental analysis, Carlo Erba, EA-1108). Carbon contents of humic acid and humin are less by 4-5% as compared with carbon contents of humic acid and humin reported in ref 7. This difference may be related to the effect of the more concentrated NaOH solution used herein, and may be associated with NOM oxidation involving a reduction in organic carbon. Moisture contents of freezedried humic acid and humin samples were 2-3% w/w as determined by oven-drying at 105 °C. m-Nitrophenol (m-NO2C6H4OH; 99%, BDH Chemicals Ltd., Poole, England), nitrobenzene (C6H5NO2; 99%, Aldrich), acetophenone (C6H5C(O)CH3; purum, 98% GC, Fluka), and benzyl alcohol (C6H5CH2OH; ACS reagent, Sigma) were selected as probe sorbing compounds. Compounds were selected according to their similar electronic polarizabilities (as expressed by similar molar refractions), similar molar volumes, and increasing ability to undergo specific interactions with NOM (3). Compound ability to undergo specific interactions with NOM is correlated to the difference between
the logarithm of the gas phase-organic carbon distribution coefficient (log HOC) for a given compound and the log HOC for a nonpolar compound (aromatic hydrocarbon or halogensubstituted hydrocarbon) having the same molar refraction (∆ log HOC; 13). Properties of organic sorbates are provided in Table 1. Sorption was examined from water (Millipore) and n-hexadecane (n-C16H34; 99+%, Aldrich). Use of n-Hexadecane as an Inert Solvent for Sorption Experiments. n-Hexadecane is an inert solvent that is incapable of specific interactions. Compound distribution coefficients (Kd, ratio between sorbed concentration and solution concentration) measured in n-hexadecane systems significantly exceed any possible n-hexadecane sorption by NOM, such that the potential masking of probe compound sorption by n-hexadecane sorption is considered insignificant (2). The dry inert hydrocarbon system is preferred to a gasphase system because of the long sorption uptake kinetics involved (2, 3). We have previously shown there is little chance for nonpolar n-hexadecane to affect sorption interactions of polar compounds with a dry NOM sorbent (2, 3): (i) Vapor sorption isotherms obtained for nonpolar compounds (including a saturated hydrocarbon) on dry peat are linear up to the maximally measured activities of 0.6 (4, 20), demonstrating that interactions of nonpolar compounds with dry NOM are far from saturating sorption sites (if any). This is suggestive that hydrocarbons will not be able to compete with more polar organic compounds for sorption sites on peat NOM. (ii) Little effect of n-hexadecane on sorption of polar compounds is expected due to the significant sizeexclusion effect on solvent solubility in peat which was found for liquids with molar volumes greater than 93 cm3/mol (21; n-hexadecane molar volume 292.8 cm3/mol). (iii) Replacing n-hexadecane by n-hexane (molar volume 130.6 cm3/mol) had no effect on sorption interactions of m-nitrophenol with NOM (3). (iv) n-Hexadecane had no effect on sorption of pyridine on Pahokee peat when sorption was compared on an activity basis from an acetonitrile-saturated solution in n-hexadecane (99% n-hexadecane; unit acetonitrile activity) and the pure acetonitrile phase (2). Sorption Experiments. Sorption was examined in 12 systems (solute/sorbent/solvent): (I) m-nitrophenol/HA/ water, (II) m-nitrophenol/HA/n-hexadecane, (III) m-nitrophenol/HUM/water, (IV) m-nitrophenol/HUM/n-hexadecane, (V) nitrobenzene/HA/water, (VI) nitrobenzene/HA/ n-hexadecane, (VII) nitrobenzene/HUM/water, (VIII) nitrobenzene/HUM/n-hexadecane, (IX) acetophenone/HUM/ water, (X) acetophenone/HUM/n-hexadecane, (XI) benzyl alcohol/HUM/water, and (XII) benzyl alcohol/HUM/nhexadecane. Details of the sorption kinetics and equilibrium experiments are summarized in Table 2. Sorbate concentrations in the sorbent phase were determined by the difference between the initial and equilibrium solution concentrations and reported on a dry sorbent weight basis. Compound concentration reduction in control vials not containing VOL. 38, NO. 15, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4121
TABLE 2. Experimental Conditions for the Sorption Experiments
series
solute/sorbent/ solvent
I II III IV V VI VII VIII IX X XI XII
m-nitrophenol/HA/water m-nitrophenol/HA/n-hexadecane m-nitrophenol/HUM/ water m-nitrophenol/HUM/n-hexadecane nitrobenzene/HA/water nitrobenzene/HA/n-hexadecane nitrobenzene/HUM/water nitrobenzene/HUM/n-hexadecane acetophenone/HUM/water acetophenone/HUM/n-hexadecane benzyl alcohol/HUM/water benzyl alcohol/HUM/n-hexadecane
b
initial solute time: isotherm/ initial solute solid:liquid concn range, kinetics/ concn for sorption sorbed fraction of av loss, a ratio, g/mL mg/L equilibrium, h kinetics, mg/L the total amt, % % 1:27 1:200 1:33 1:125 1:30 1:7 1:150 1:170 1:22 1:4 1:4 1:4
8-6350 10-230 10-6470 11-251 27-983 206-20000 27-983 205-20000 12-1611 265-5990 500-6000 241-2990
456/456/24 1128/1128/600 624/624/96 1152/1152/800 384/384/8 936/936/430 270/270/16 510/510/50 168/ndb/nd 336/nd/nd 168/nd/nd 336/nd/nd
535 99 530 94 216 1000 220 1197 nd nd nd nd
40-65 60-85 40-80 30-60 40-60 30-50 20-50 40-65 30-40 30-50 50-60 20-35
