Environ. Sci. Technol. 1993, 2 7 , 316-326
Adsorption of Substituted Nitrobenzenes and Nitrophenols to Mineral Surf aces Stefan B. Haderleln* and Ren6 P. Schwarzenbach" Swiss Federal Institute for Water Resources and Water Pollution Control (EAWAG) and Swiss Federal Institute of Technology (ETH), CH-8600 Dubendorf, Switzerland
The adsorption of a large number of nitroaromatic compounds (NACs) to mineral surfaces, particularly to homoionic kaolinites, has been investigated. The results demonstrate that NACs may adsorb specifically and reversibly to the negatively charged siloxane surface of kaolinite. The strength of adsorption depends on the structure of the compound (Le., type of substituent) and on the type of cation adsorbed to the siloxane surface. In the presence of strongly hydrated cations (e.g., Li+, Na+, Mg2+, Ca2+,A13+),no significant specific adsorption of NACs is observed,while for more weakly hydrated cations (e.g., NH4+,K+, Rb+, Cs+),the distribution coefficient, Kd, of a given NAC, increases with decreasing free energy of hydration of the cation. We propose that electron donor-acceptor (EDA) complexes between surface oxygens of the siloxane surface and a given NAC are responsible for the observed specific adsorption. Some simple model calculations indicate that such EDA complexes may have a significant impact on the transport and the fate of NACs (and, possibly, of other organic pollutants exhibiting electron acceptor properties) in the subsurface environment.
nitrobenzenes and nitrophenols to selected mineral surfaces has been systematically investigated. Nitroaromatic compounds (NACs) are widely used as explosives, as intermediates in the synthesis of pesticides and dyes, as solvents, and as herbicides and insecticides (27-29).They have been found to be ubiquitous environmental pollutants, particularly in subsurface environments (30-33). The major goals of this study were as follows: (1) to identify the types of surface sites and interactions that are responsible for sorption of NACs to mineral surfaces, (2) to study the influence of solution pH and major ion composition on the sorption process(es), and (3) to evaluate the electronic and steric effects of substituents on the sorption behavior of NACs. Homoionic kaolinite was chosen as the major model sorbent, because this clay mineral exhibits several different types of surface sites that are representative for many minerals in the environment. In an effort to evaluate environmentally relevant mineral surfaces that exhibit more homogeneous surface sites, additional sorption experiments were carried out with pure oxide sorbents including &aluminum oxide (6-A120,), yaluminum hydroxide [y-Al(OH),, gibbsite],and amorphous silicon oxide @io2).
Introduction Sorption from aqueous solution to solid surfaces is one of the key processes determining the distribution and fate of organic pollutants in the environment. For natural sorbents exhibiting an appreciable amount of organic matter (Le., mass fraction of organic carbon, f,,, >lo-,), partitioning of a given compound into the organic phase is commonly assumed to be the major sorption mechanism, particularly when dealing with hydrophobic compounds (1-4).In such cases, predictive models have been applied with reasonable success (5-8). There are, however, situations in which adsorption to mineral surfaces may be equally important or may even dominate the overall sorption process. This is the case for environments in which very little organic material is present, as is encountered, for example, in many aquifers or in clay mineral liners of landfills (6,9-12).Furthermore, for compounds that interact specifically with surface sites [e.g., by surface complexation or ion-exchange reactions (13,14)],adsorption to mineral surfaces may, in general, not be neglected. Finally, adsorption to mineral surfaces may be an important factor in determining the overall rate of heterogeneous hydrolysis or redox reactions of organic pollutants (15-18). Unfortunately, mechanistic models for describing adsorption of solutes to specific mineral surfaces are available for only a few classes of organic compounds that interact specifically with surface sites. These compounds include'carboxylic acids (19,20), alkylammonium cations (21), and N-heterocyclic aromatic compounds (22-24). Many of the studies in which sorption of hydrophobic or hydrophilic organic compounds to mineral phases has been investigated have been confined to rather phenomenological descriptions of the sorptive process (10,25,26)and therefore do not allow general conclusions to be drawn. In this paper, we report the results of laboratory batch experiments in which the sorption of a series of substituted
Experimental Section Chemicals. The model compounds (names and abbreviations are given in Table I) used in this study and the companies from which they were purchased follow. Fluka AG (Buchs, Switzerland): 2NP, 4-C1-2NP, 3-N02-2NP, 4-N02-2NP, 6-N02-2NP, 3-NP, 4-COOH-3NP, 4-NP, 3Me-4NP, DNOC, 2-Me-NB, 3-Me-NB, 4-Me-NB, 4-Et-NB, 2-Cl-NB, 3-Cl-NB, 4-Cl-NB, 4-Br-NB, 2-N02-NB, 3NO,-NB, 4-Me-Ph, 2-Cl-Ph, 3-C1-Ph, 4-Cl-Ph, 1,3-DCB, 1,3,5-TCB,1,2,3,4-TeCB, 1,2,3,5-TeCB. Aldrich Chemical Co. (Steinheim, Germany): 2-Me-4NP, 4-sBu-2NP, 4OMe-2NP, 5-F-2NP, 4-CF3-2NP,2,6-DINO-4-TERB, NB, 2-Phen-NB, 3-Phen-NB, 4-N02-NB,4-OCH3-NB,4-CNNB, 1,3-DNN, 1,5-DNN, l,&DNN, NCH. Ega Chemie (Steinheim, Germany): 3-Me-2NP, 4-Me-2NP, 4-CHO2Np, 2,5-DNP. Merck-Schuchardt (Darmstadt, Germany): 5-Me-2NP, 4-CHO-NB, 4-CH3CHO-NB,catechol. Riedel de Haen AG (Seelze, Germany): DINOSEB, 2,4-DINO6-TERB. Cosmital SA (Marly, Switzerland): 6-Me-2NP. 4-n-Bu-NB and 4-n-Oct-NB were provided by Dr. Lee Wolfe, EPA, Athens. Inorganic chemicals were purchased from Merck-Schuchardt. All chemicals had the highest purity available (297%) and were used as received. Sorbents and Treatment of Surfaces. Sorption experiments were conducted with the pure (hydr)oxides 6Al,03, ?-Al(OH), (gibbsite), and amorphous SiOz and the clay mineral kaolinite [A12Si205(0H)4].Important properties of these sorbents are summarized in Table 11. Kaolinite was available as China Clay Supreme from the English Clays Lovering Pochin & Co. Ltd (St. Austell/ Cornwall, UK) and was a well-crystallized weathering product from granitic bedrocks (34).An ideal kaolinite crystal consists of a nonexpandable, layered structure that contains octahedrally coordinated A13+ and tetrahedrally coordinated Si4+in a 1:l stoichiometric ratio. Kaolinite
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Environ. Sci. Technol., Vol. 27, No. 2, 1993
0013-936X/93/0927-0316$04.00/0
0 1993 American Chemical Society
Table I. Octanol/Water Partition Constants, UV/Visible Absorption Maxima, Extinction Coefficients, Acidity Constants, and K d Values of Neutral and Nondissociated Solutes Investigated in This Study compound
abbrev
(M-l cm.-l)
PK.3 (I = 0.05 M)
Kd (L kg-') (Cs+-kaolinite)'
2-nitrophenol 3-methyl-2-nitrophenol 4-methyl-2-nitrophenol 5-methyl-2-nitrophenol 6-methyl-2-nitrophenol 4-sec-butyl-2-nitrophenol 4-methoxy-2-nitrophenol 4-chloro-2-nitrophenol 5-fluoro-2-nitrophenol 4-(trifluoromethyl)-2-nitrophenol 2,4-dinitrophenol 2,5-dinitrophenol 2,6-dinitrophenol 3,4-dinitrophenol 3-nitrophenol 4-carboxy-3-nitrophenol 4-nitrophenol 2-methyl-4-nitrophenol 3-methyl-4-nitrophenol 6-methyl-2,4-dinitrophenol 6-sec-butyl-2,4-dinitrophenol 6-tert-butyl-2,4-dinitrophenol 4-tert-butyl-2,6-dinitrophenol
2NP 3-Me-2NP 4-Me-2NP 5-Me-2NP 6-Me-2NP 4-sBu-2NP 4-OMe-2NP 4-Cl-2NP 5-F-2NP 4-CF3-2NP 4-NO2-2NP 5-NO2-2NP 6-NO2-2NP 3-NO2-4NP 3NP 4-COOH-3NP 4NP 2-Me-4NP 3-Me-4NP DNOC DINOSEB 2,4-DINO-6-TERB 2,6-DINO-4-TERB
log KO, ,A, (nm) Nitrophenols 1.89" 278" 2.29' 269" 2.37" 282" 2.31" 294" 2.42" 290 3.87" 282" 2.02" 281" 2-46" 272" 1.91" 280" 2.34" 266" 1.67" 261" 1.80a 273" 1.226 250 1.34e 226 2.006 226 1.26' 230 2.04" 226" 2.43e 228 2.48" 231" 2.12" 270" 3.59e 271 3.54e 271 3.20e 254
nitrobenzene 2-nitrotoluene 3-nitrotoluene 4-nitrotoluene 4-ethylnitrobenzene 4-n-butylnitrobenzene 2-nitrobiphenyl 3-nitrobiphenyl 2-chloronitrobenzene 3-chloronitrobenzene 4-chloronitrobenzene 4-bromonitrobenzene 1,2-dinitrobenzene l,3-dinitrobenzene 1,4-dinitrobenzene 4-nitroanisole 4-cyanonitrobenzene 4-nitrobenzaldehyde 4-acetylnitrobenzene
NB 2-Me-NB 3-Me-NB 4-Me-NB 4-Et-NB 4-n-But-NB 2-Phen-NB 3-Phen-NB 2-C1-NB 3-C1-NB 4-Cl-NB 4-Br-NB 2-NOZ-NB 3-NOz-NB 4-NO2-NB 4-OCHyNB 4-CN-NB 4-CHO-NB 4-CHsCO-NB
Nitrobenzenes 1.86b 267 2.306 265 2.42b 273 2.40b 285 2.86e 285 3.89" 285 3.6ge 246 3.79e 246 2.24b 259 2.43b 263 2.40b 274 2.60" 283 1.58b 256 1.4g6 242 1.48b 264 2.03b 316 1.196 260 1.45e 266 1.4gb 268
7600 5800 7200 9700 9300 9400 21900 21100 3400 7300 12600 9000 >10000 >10000 >10000 8700 11000 16100 9600
3.5 3.4 18 54 62 75 3.6 58 6 24 44 52 1.7 18009 -4000k 340 5209 730 -21009*k
1,3-dinitronaphthalene 1,5-dinitronaphthalene 1,8-dinitronaphthalene
1,3-DNN 1,5-DNN 1,8-DNN
Dinitronaphthalenes 2.74e 340 2.74e 331 2.74e 328
3030 4000 3700
240009J 150009J 48
nitrocyclohexane 1,2-dihydroxybenzene 4-methylphenol 2-chlorophenol 3-chlorophenol 4-chlorophenol 1,3-dichlorobenzene 1,3,5-trichlorobenzene 1,2,3,4-tetrachlorobenzene 1,2,3,5-tetrachlorobenzene
NCH catechol 4-Me-Ph 2-C1-Ph 3-Cl-Ph 4-Cl-Ph 1,3-DCB 1,3,5-TCB 1,2,3,4-TeCB 1,2,3,5-TeCB
Other Compounds 1.62" 265 0.956 276 1.92b 278 2.17b 216 (sh)h 2.4g6 218 (sh)h 2.426 225 3.48c 324 (sh)h 4.02c 220 (sh)h 4.55' 232 (sh)h 4.65c 232 (sh)h
ern
~
6250" 2200" 6400" 7900" 6100 5050" 5650" 5850" 6000" 5100" 11650" 10700" 8800 8200 8800 17800 9700" 9900 7600" 13800" 10500 >10000 >10000
1900 8000 3500 10100 7000 9600 4800 4900 9000 9000
7.23" 7.00" 7.63" 7.34" 7.65 7.59" 7.40" 6.44" 6.30" 5.66" 3.94" 5.18" 3.70d 5.32 8.35d 6.6711.30 7.16d 7.58 7.33" 4.31" 4.62" 4.621 4.23f
35 8.9 130 450 120 4.7 1100 120 2209 33 L9OOopJ
160009J 170009J 8.3 23 13009 34 67 409 180009~~ 54 18 8.6
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