Predicting the abiotic degradability of organic pollutants in the troposphere

Chemosphere, Vol.

38, No. 6, pp...1361-1370, 1999 © 1999 Elsevier Science Ltd, .All rights reserved 0045-6535/99/$ - see f r o ~ matter

Pergamon

PII: S0045-6535(98)00538-4 P R E D I C T I N G T H E ABIOTIC DEGRADABILITY O F O R G A N I C POLLUTANTS

IN THETROPOSPHERE

Hans Giisten

Institut ftir Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe/Universit~it Karlsruhe, D-76021 Karlsruhe, Federal Republic of Germany

ABSTRACT Based on a global average of the OH and NO3 free radial concentration in the troposphere, the lifetime of organic chemicals can be calculated from the rate constant of their reaction with the free radicals. Various models for estimating the reactivity of organic compounds with the tropospheric free radicals allow a rapid estimation of their degradability. An overview on the existing models - empirical, quantitative structureactivity relationships (QSAR) with measured physico-chemical descriptors, QSAR with semiempirical quantum-chemical descriptors as well as ab initio molecular orbital calculations - is described and their limitations and range of applicability to estimate the tropospheric lifetime of an organic compound is discussed. INTRODUCTION In this century the number of chemicals has increased from less than half a million to well over 18 million compounds. Although the vast majority of the known compounds have never reached the market there is still a substantial number of chemicals in common use which might end up in the environment. The number of everyday chemicals have been estimated in the range of 20,000 - 70,000 [1]. The European Inventory of Existing Chemical Substances (EINECS) [2] contains already well over 100,000 compounds and some 30,000 chemicals are produced on larger commercial scales. It is believed that this number increases at an annual rate of about 2,000. The U.S. Environmental Protection Agency (EPA) reviews annually about 1000 - 1600 premanufacture notices for new chemicals [3]. The world-wide production of the chemical industry is on the order of 400 Million tons per year. As far as the potential risks posed by these chemicals to man and the environment is concerned, the enormous amount of existing chemicals makes clear that testing alone will not be sufficient to supply all the data needed for a proper risk assessment within an appropriate time scale. Even if we restrict risk assessment to the so-called High Production Volume Chemicals (HPVC), i.e. 1361

1362 those chemicals which are produced in quantities exceeding 1000 metric tons per year, the sheer number of existing chemicals is so large that their degradability and environmental fate can never be measured properly at the laboratory. Although the production volume is not an optimal indicator of exposure, as a reasonable first approximation to which extent chemicals may be found in our environment, the HPVC may describe their potential hazard. 1000 metric tons of an organic compound, for instance, are about the quantity to fill the volume of the troposphere within the first three kilometers to a concentration of about l ng/m 3 [4]. In the light of these facts and the huge number of environmental chemicals in the troposphere, prediction methods are needed to allow a cost-effective and quick estimation of the abiotic degradability of chemicals. Thus, an overview is given of the various prediction models for the abiotic tropospheric degradation of organic compounds.

DEGRADATION OF ORGANIC COMPOUNDS IN THE TROPOSPHERE The troposphere is the main and often the first recipient of chemical compounds from the industrialised society. Due to their larger vapour pressures the majority of chemical compounds in the atmosphere are of organic nature. At equilibrium in the environment, these organic compounds are distributed between air, water and soil depending on their partition coefficients between the different environmental compartments. In the troposphere, the volatilisation of organic compounds occurs through gas/particle partitioning, i.e. depending on the vapour pressure, a certain fraction is adsorbed at the tropospheric aerosols of natural and/or anthropogenic origin [5]. Even large organic molecules like the polychlorinated biphenyls are present at 20°C up to about 90% in the gas phase [4]. Thus, the troposphere is a highly efficient transport medium and the tropospheric lifetime of most of the organic compounds from terrestrial emissions are controlled by their reaction with the OH radical at day-time and the NO~ radical at night [6,7]. The tropospheric lifetime or residence time is defined as the time for an organic compound in the gas phase to decrease in concentration by a factor of I/e (i.e.-37%) of its initial concentration [6]. Due to the fact that the free radical concentration is in the lower or even sub-ppt range and the organic compounds are generally present in the troposphere in the ppb range, the kinetics in the degradation reaction follow a pseudo-first order behaviour, i.e. they are of linear dynamics. The tropospheric lifetime 1: is given by -c = 1/kox × [ox],

(1)

where [ox] is the concentration of the oxidising free radical(s) of the troposphere (OH and N O 3 ) and k,,x the bimolecular rate constant for the oxidising radicals in the reaction with a chemical. The rate constant of these gas-phase reactions is expressed in units of c m 3 s I. Occasionally, the literature indicates instead of lifetime the tropospheric half-life [6]. Over the last twenty years a large body of reaction rate constants for the OH and N O 3 radical reactions with organic [8-10] and inorganic compounds [11] have been measured at the laboratory. It is interesting to note that the range of measured rate constants for OH and NO3 radicals covers more than four to six orders of magnitude, respectively, i.e. they range from the gas-kinetic limit of 10-H~ to 10 -16 cm 3 s "1 [8,9,12]. The

1363 rate constants measured at the laboratory are then used to calculate the tropospheric lifetime of an organic compound using Equation 1. In Figure 1 a diagram is shown for the relation of the tropospheric lifetime ot an organic compound to the tropospheric OH radical concentration. Midday photostationary-state OH concentrations range from lxl07 species/cm3 to 1×106 species/cm3 or less, depending on ambient levels of ozone, water vapour and UV solar radiation. The NO3 radical concentration in the lower troposphere is generally one order of magnitude larger than for OH [9]. Generally, the lifetimes for tropospheric degradation of organic compounds by OH or NO3 radicals range from hours to 10 years and longer [8-10].

OH.IO s

1011

10 9 10 YeBr8

10 7 1 Month

1 D._.a.y__. . . . .

10 s

1 Hour 10 3

1

,

z

10-16

10-14

koH

z

10-12 [ crn3/rnolecule/s ]

10-1o

Figure 1: Dependency of the tropospheric lifetime on the OH radical concentration according to Equation l PREDICTING THE TROPOSPHERIC DEGRADATION OF ORGANIC COMPOUNDS On the basis of existing experimental data for the tropospheric degradability of organic compounds [8-12] predictive and computational methods are developed to estimate the tropospheric lifetime of chemical compounds. The literature provides about 500 rate constants for organic compounds in the gas-phase reaction with the OH radical and about 150 for the reaction with the NO3 radical [10]. The early predictive

1364 techniques are purely empirical, others tools base more on a scientific rational. One such tool which can be used to estimate the tropospheric degradability of new or untested chemicals is the quantitative structureactivity relationship (QSAR) methodology. The basic idea of QSAR is to develop correlations that will be valid for many classes of chemicals based on the reactivity measured for a small number of representative chemicals. Often, the known reactivity is correlated with other physical parameters of the compound in question. Generally, these parameters are readily available in the literature and are called descriptors. During the last decade, the number of QSAR studies in environmental chemistry was steadily growing [3,13,14]. A literature search over the last two decades has revealed 48 QSAR models for estimating the reaction rate constant of organic chemicals with tropospheric free radicals (OH or NO3) [15,16]. The many prediction models for the abiotic tropospheric degradation of organic compounds can be grouped to empirical models, QSAR with measured physico-chemical descriptors, -- QSAR with semiempiricat quantum-chemical descriptors,

ab iHitio molecular orbital calculations. The great ma:lority of the models were developed for a single class or a fairly small number of chemicals using spectroscopic or thermodynamic descriptors. Statistical tests (correlation coefficient, standard erro, of the estimate, etc.) showed that the range of applicability for estimating the tropospheric lifetime of new organic compounds is quite limited for most of the models published. Among the many models for the prediction o1 the abiotic degradation of organic compounds in the troposphere only three are not classspecific [15,16].

Empirical models There exists a good correlation between the reactivity of the OH radical in aqueous solution and in the gas phase [17]. Since there is a larger body of OH radical reaction in the aqueous solution with organic compounds available, these data allow a rapid estimate of the OH radical reactivity in the troposphere. Reasonable correlations were obtained that allow predictions within one order of magnitude for most of the organic compounds [ 17]. The use of functional group increments from a known data base, to compose the OH radical reactivity ol a new compound from its increments, was successfully introduced in the Atkinson's group contribution method 118,19]. This model is based on four possible reaction pathways of the OH radical with an organic compound, namely: (i) hydrogen abstraction, (ii) hydroxyl radical addition to double and triple bonds, (iii~ hydroxyl radical addition to aromatic rings, and (iv) reaction with nitrogen, sulphur, and phosphorus containing groups. It is assumed that the OH radical reactivity can be estimated by the additive group contribution approach and the overall OH reaction rate is a sum of all possible individual reaction rates. The group contribution method is based on chemical experience and statistical arguments (fitting to known data). With the exception of the electrophilic substituent constants for aromatic rings [20], the group contribution

1365 method has no mechanistic background. The selection of new fragments is arbitrary and no rules are available how to define a ,,new" fragment. A critical consideration of this prediction method [15,16,19] has revealed that questions concerning the range of applicability and their boundaries are still open. Internal validation, made by Atkinson [19], has shown that OH rate constants of about 90% of 485 organic compounds are predicted within a factor of two of the experimental values. However, an external validation study with 369 organic compound shows that the majority of the OH reaction rate constants are estimated within a factor of three of the experimental data [21]. Nevertheless, the group contribution method is quite successful and not class-specific. At present, there are more than 71 fragments for which the group or substituent factors are calculated and their number increases with each new class of chemical compounds. Since it is basically an empirical prediction method, for entirely new classes of chemicals experimental OH rate constants have to be measured at the laboratory in order to calculate the appropriate substituent or group factor. Unfortunately, Atkinson's method has shown larger deviations for several important classes of chemical compounds: organic compounds with 3 halogens on the same carbon atom as in DDT, chemicals with NOx groups as in nitroalkanes, phosphates, often used in herbicides, and small heterocylic rings. The group contribution method does not work for perhalogenated alkanes. The model of Atkinson yields second order rate constants for the reaction of OH radicals in units of l0 -12 cm 3 molecule-t sec-L. Since the OH radical is not the only reactive free radical in the troposphere, the tropospheric lifetime calculated by this estimation method will always give a worst case scenario. Atkinson's method was adopted by OECD (Organisation for Economic Cooperation and Development) in 1988 to be used for estimating the degradability of organic compounds in the troposphere.

QSAR with measured physieo.chemical descriptors Many QSAR models have been published that correlate spectroscopic (absorption maxima, torsional frequencies, ionisation energies, NMR shifts, etc.) [15] or thermodynamic (bond dissociation energies) [16,22] with the reaction rate constant of organic chemicals in reaction with OH and/or NO3 [¥ee radicals. The most successful and not class-specific model is based on measured ionisation energies [22-26]. Statistically significant linear correlations between the reaction rate constant for the OH and NO3 free radical reactions with organic compounds in the gas phase at 298 K and the corresponding vertical ionisation

energies (Ei) have been obtained. As shown in Figure 2 the correlation reveals two large classes of organic compounds: aromatics and aliphatics. This linear correlation for both radicals holds over at least four and six orders of magnitude in rate constants for OH and NO3, respectively. A plot of log kNo3 vs. Ei resembles that of log kon vs. Ei in Figure 2 only that it is extended to well over six orders of magnitude [25]. The large group termed ,,aliphatics" consists of alkanes, alkenes, polyenes, terpenes, alkynes, chloroalkenes, aldehydes, ethers, thiols, thioethers, plus benzaldehyde, phenol and cresols. The two regression lines reflect the major reaction mechanisms of the OH and NO3 radical with these organic compounds, namely hydrogen abstraction from ,,aliphatics" and addition to the aromatic g-electron system for ,,aromatics" [6]. Ionisation

1366 energies of organic compounds range generally from 7 to 14 eV. It is interesting to note that the small letters (a-t) in Figure 2 represent all kind of environmental reference chemicals, from p-chloroaniline (a), naphthalene (c), propylene (k) to ethane (t) [23]. Furthermore, benzaldehyde is located on the less steeper regression line of the ,,aliphatics". This illustrates, that the electrophilic OH and NO3 radicals abstract the Hatom from the aldehyde group rather than adding itself to the benzene ring, the primary step in the degradation reaction of aromatics.

15 •

;

•

14

./. 12

/

o=:,'r_ I

10

"

o/

11

10

!1} 1 I

8 Figure 2:

g

i

10

i

11 EJeV

i

i

i

12

13

14

Correlation of - log koH to El of 161 organic compounds in the gas phase [23]. o = aromatics, • = aliphatics. Letters a-t denote environmental priority pollutants [27].

Contrary to the empirical Atkinson method, this QSAR model is based on the well established frontier molecular orbital theory. Although with lesser accuracy than the Atkinson method this QSAR model seems to be applicable to any class of organic compounds. Since there is a large body of ionisation energies of organic compounds available in the literature [28,29] and they can readily be measured in the gas phase by photoelectron spectroscopy with a precision of better than 1% [29,30], the upper limit of the tropospheric lifetime of organic compounds can be estimated and used as a quick pre-screening method. Furthermore, ionisation energies can be correlated with semiempirical quantum-chemical descriptors such as the HOMO energies of a given organic compound [16]. Thus, they can be calculated using semiempirical MO algorithms.

1367

QSAR with semiempirical quantum-chemical descriptors Quantum-chemical descriptors are a new and rapidly developing class of molecular descriptors [31] which allow computer estimation of the gas-phase reactions of organic compounds with free radicals. As semiempirical quantum-chemical calculations can be performed routinely, they can provide a vast amount of molecular information about the internal electronic properties of molecules which are not available from experimental techniques. The most frequently used quantum-chemical descriptors include the energy of the highest occupied molecular orbital (HOMO), reflecting electrophilicity, and that of the lowest unoccupied molecular orbitals (LUMO), reflecting nueleophilicity, frontier orbital electron densities, electron charge distribution, dipole moments, and bond energies. According to Koopmans theorem the HOMO and LUMO energy levels can be regarded as an approximation of the organic compounds ionisation energy and electron affinity, respectively. Klamt [32,33] has developed QSARs for three of the possible reaction routes of the OH radical with organic compounds, (i) hydrogen atom abstraction, (ii) hydroxyl radical addition to double and triple bonds and (iii) hydroxyl radical addition to aromatic rings. Using the AM I semiempirical SCFMO method [34] with the MOPAC programme package [35], descriptors such as charge-limited effective HOMO energy can be readily calculated from the atomic and molecular orbital coefficients. Six new calculated descriptors have been devised which are some combinations of molecular orbital energies and atomic charges on appropriate reaction centers of the organic molecule in question. A non-linear optimisation procedure was performed to obtain regression coefficients. Compared to the estimation algorithm of Atkinson's method the overall regression coefficients for 159 compounds with the MO-method is better (r2=0.985), the standard deviation corresponds to a factor of 1.4 compared to a factor of 2.5 to 3 for the Atkinson method [32]. The so-called MOOH model of Klamt is also applicable to 93 oxygenated organics (aldehydes, ketones, alcohols, esters and carbonic acids) [33]. Newer studies have shown that semiempirical quantum--chemical methods (MNDO, PM3) can be used for QSAR models to estimate the tropospheric degradation of commercial chemicals by OH and NO3 radicals [6,16,21,36]. The predictive capabilities of the MOOH model are limited only by the semiempirical parameterisation of the AM1 method. The prediction models discussed so far yield linear or multiple linear regression equations of Y (chemical reactivity) versus X (descriptor) and the statistical uncertainty is expressed by the scatter of data. New chemometric strategies of multivariate data analysis and statistical experimental design are better tools to handle the problem of statistical significance of QSAR models than the classical linear regression analysis [14,37]. Methods such as partial least squares projection to latent variables (PLS) or principal component analysis (PCA) are used to determine the chemical domain of a QSAR model. They describe better for which chemicals the model is valid and which chemicals are outliers than the classical data analysis based on multiple regression. These chemometric technique has been used to forecast the rate of reaction between the OH radical in the gas phase with organic compounds such as halogenated aliphatic hydrocarbons [38,39] and a set of 57 unsaturated hydrocarbons including some terpenes [40]. Contrary to the multiple linear

1368 regression analysis, the PLS method can handle many co-linear descriptors which can exceed the number of chemical compounds in the data set [37].

Ab initio molecular orbital calculations With the advent of large and inexpensive computer memories, accurate ab initio molecular orbital calculations of the abstraction of hydrogen atoms by hydroxyl radicals in the gas phase from organic molecules are now performed at workstations. High-level ab initio calculations were carried out for the reaction of ethane [41], substituted aldehydes [42], halogenated ethanes and ethenes [43-45] with the hydroxyl radical in the gas phase. Contrary to semi-empirical molecular orbital calculations, the ab initio calculations can provide a realistic description of the transition states for the hydrogen abstraction reaction. Barrier heights, bond dissociation energies, and reactions enthalpies as well as the geometry and vibrational frequencies of the transition states can be calculated. In case of chloroethane the calculation predicts that the hydrogen abstraction from the weaker C-H bond on the s-carbon will result in a lower barrier height than that from the stronger C-H bond of the 13-carbon [43]. Thus, using the reaction path concept of the transition-state theory and knowledge of the potential energy surface of a particular reaction, the thermal rate constant can be obtained through a Boltzmann average of the reaction cross-section [46]. Though good quality ab initio calculations are today restricted to molecules with a maximum of about 30 atoms. The near future will allow direct and accurate calculations of the OH radical reactivity of larger molecules to be made. Furthermore, ab initio calculations will enable to predict also the structure of the reaction products. None of the QSAR methods is capable of providing this information.

Acknowledgement: Part of the own research work described here was carried out and funded within the framework of the EU-projects ,,Quantitative Structure-Activity Relationships for Predicting Fate and Effects of Chemicals in the Environment" (Contract EV5V-CT92-0211) and ,,Fate and Activity Modelling of Environmental Pollutants using Structure-Activity Relationships" (Contact ENV4-CT96-0221).

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