phase cycle. For multiple quantum investigations smaller phase shifts

(see e . g . M. Hintermann, L. Braunschweiler, G. Bodenhausen, R. R. Ernst, J. Magn. Reson. 50 (1982) 316).

[I581 G. Bodenhausen, R. Freeman, D. L. Turner, J. Mugn. Reson. 27(1977)

[I591 R. Baumann, G. Wider, R. R. Ernst, K. Wbthrich, J. Mugn. Reson. 44

[I601 J . D. Mersh, J . K. M. Sanders,J. Mugn. Reson. 50(1982) 171.

are of' interest, which require, however, further hardware modifications 511.

(198 I ) 76, 402. [I571 A. Bax, G. A. Morris, J. Mugn. Reson. 42 (1981) 501.

From Environmental Analytical Chemistry to Ecotoxicology- A Plea for More Concepts and Less Monitoring and Testing

By Werner Stumm, Rene Schwarzenbach, and Laura Sigg

The development of cultural and technical civilization has been marked with increasing in- terference in hydrogeochemical cycles and the production of a growing number of chemi- cals; this is accompanied by a growing concern on the potential adverse effects of chemi- cals on biological systems. Assessment of the potential toxicological and ecological effects of pollutants is of central importance. We are of the opinion that this cannot be accom- plished by merely evaluating the harmfulness of a substance on the basis of toxicity tests with individual organisms and by monitoring analytically the environment for pollutants. We would like to encourage chemists to participate in the solution of ecotoxicological prob- lems: chemodynamical concepts permit the estimation-on the basis of physical-chemical generalizations and with the help of compound-specific data-of the fate, the distribution, the potential for bioaccumulation in the food chain, and the approximate residence time of pollutants (and thus the attainable residual concentrations) in the environment and there- fore to predict the relative risk of different pollutants.

1. Introduction

Human society runs on material and energy. In indus- trialized nations, the anthropogenic energy flow per unit area exceeds biotic energy flux (energy fixed by photosyn- thesis) by a factor of about ten; organic material produced by industry (ca. 150 kg yr-' per inhabitant, or cu. 40 g m-' yr-') is within one order of magnitude of that synthesized by nature (net primary production: ca. 300 g m-' yr-')['.21. The hydrogeochemical cycling of dissolved and suspended materials (e. g. phosphorus, heavy metals) is accelerated, and a large number of industrially synthesized chemicals are distributed by various pathways into the environment. Obviously, these anthropogenic activities, considered to be necessary for the maintenance of our civilization and cul- ture, are achieved in many instances not without perturba- tion of ecosystems and effect on human health. It has been estimatedc3] that cu. 60000 chemicals are in daily use and that this number increases by 1000 to 1500 substances ev- ery year. The evaluation of many of these substances and

[*) Prof. Dr. W. Stumm, Dr. R. Schwarzenbach, Dr. L. Sigg Institut fur Gewasserschutz und Wassertechnologie und Eidgenossische Anstalt fur Wasserversorgung, Abwasserreinigung und Gewasserschutz (EAWAG) Eidgenossische Technische Hochschule Zurich CH-8600 Diibendorf (Switzerland)

their impact on nature and human health is among the most important objectives of environmental science.

The assessment of this impact of chemical pollution and of the behavior of pollutants in the environment requires qualification and quantification in every respect of the reactions and interactions that occur. In the past, much ef- fort has been directed towards: 1) monitoring the distribu- tion and concentrations of chemical pollutants in the envi- ronment; and 2) testing the effects of individual pollutants on individual organisms. However, monitoring data can rarely be generalized unless one knows the significant in- terrelationships and interactions between the parts of an ecological system. Similarly, it is usually very difficult to assess the ecological effects of a given compound from bioassays with individual organisms under laboratory con- ditions.

In view of the great number of existing industrial chemi- cals, and considering the large amounts of environmental pollutants created every day by human activities, we need to develop and apply general concepts on the behavior and fate of pollutants and to use these concepts as an aid to procuring relevant analytical results. By evaluating the strength of natural and anthropogenic emission sources, and by identifying the relevant pathways, interactions and unit processes that govern the behavior and fate of chemi- cals in a given natural system, we can improve our ability to understand and predict the future fluxes, distribution,

380 0 Verlag Chemie GmbH. 6940 Wernheim. 1983 0570-0833/83/0505-0380 $02.50/0 Angew. Chem. Int Ed. Engl. 22 (1983) 380-389

and effects of pollutants on ecological systems and on hu- man health. This will be illustrated by some selected exam- ples primarily relevant to the aquatic environment.

Over the years, the gross pollutional load on receiving waters has increased, and its character has changed. Up to a few decades ago, most of our wastes derived predomi- nantly from human and animal excreta and other biogenic components, whereas in recent years they have become in- creasingly overloaded with the discards of modern indus- trial society (synthetic chemicals, mining products such as phosphates and metals) and by-products of energy produc- tion. This change in waste composition has occurred as a result of industrial activities having grown faster than hu- man populations. Many industrial chemicals reach receiv- ing waters indirectly (via households, agricultural drain- age, the atmosphere). Some of these substances are refrac- tory ( i . e. not readily biodegradable) and they progressively accumulate in aquatic ecosystems because self-purification and biological waste treatment are not very effective in eliminating such chemicals. We should remind ourselves that water is the most important linkage in all biogeochem- ical cycles; it acts as a solvent, catalyst and reagent in most ecosystem reactions and processes. In comparison to ter- restrial ecosystems, aquatic systems are particularly sensi- tive to chemical perturbationcz1.

2. The Ecotoxicological Task

It is the objective of ecotoxicology to assess the hazards associated with pollutants in the environment. It includes 1) the study of the effects of a given chemical on the envi- ronment, especially upon ecology and on human health,

and 2) the elucidation of the effect of the environment on the pollutant. As has been pointed out by Baughman el al.[4751 and others[6-1i1, the second part of this objective, un- til recently neglected, is of particular importance in any predictive environmental assessment.

Figure 1 illustrates synoptically the realms and perspec- tives of aquatic ecotoxicology. We consider first the release of a potential pollutant into the aquatic environment. The transfer of the pollutant into various reservoirs (air, water, soil, biota, etc.), its ultimate distribution and residual con- centration (activity or fugacity[’I) depend on the physical, chemical and biological compound-specific properties of the pollutant (vapor pressure, solubility, functional groups, Henry coefficient, lipophility, adsorbability, chemical and biological degradability) as well as on the properties of the environment (flow of water, type of interfaces, transport of particles, photosynthesis and nutrient cycles, redox inten- sity, presence of other constituents, etc.).

2.1. From Input to Exposure

Let us follow the various steps from the source to the potential ecological effects of a pollutant released into an aquatic ecosystem. The immission is measured as a flow or load (capacity factor; mass per unit time or mass per unit time and volume or area). The resulting concentration is a consequence of dilution, transport, and transformation of this chemical. At any point, the water condition is charac- terized by the entity of interacting physical, chemical, and biological factors which are all intensity factors (activity, concentration, redox intensity, velocity gradient, tempera- ture, productivity, etc.). These intensive variables, above all

A t m o s p h e r e 4 1

t Pharmacokinptics

So l ids genera t ion (photosynthesis) Ind iv idua l

organisms

S o l u t e s

Pollutant mass flux e g MT-’

Energy L i g h t

(act iv i t ies) of individual solutes PE I P H , Q

St ructure Dt versity Product ivi ty Resil I ence W A T E R

- I 4 * I

S e d i m e n t

Fig. 1. Aquatic ecosystem transfer and transformation of pollutants by different pathways and various chemi- cal, biological and physical processes. A substance introduced into the system becomes dispersed and diluted: it can become eliminated from the water by adsorption on settling particles or by escape (volatilization) into the atmosphere; it can become transformed chemically or biologically. The residual activities (concentrations) and other intensity factors ( p ~ , velocity gradient, temperature, etc.) determine causally physiological, toxicological and sociobiological effects on organisms and their communities.

Angew. Chem. lnt . Ed. Engl. 22 (1983) 380-389 38 1

the concentrations, or, more precisely, the activities of the individual constituents, determine causally the doses (con- centration x time) a n organism or an ecosystem will receive and thus, in turn, the type of community of organisms present in the ecosystem.

Thus, for an assessment of the exposure and an evalua- tion of the ecological and hygienic risk, we need to know the residual concentrations of pollutants. An estimate of the ambient concentration of a pollutant based on a pre- diction of its relevant fate and residence time (considering relevant pathways, exchange processes, biological and chemical conversions) is essential to any hazard assess- ment. The problem of estimating the dose (activity) is one of the things that distinguishes ecotoxicology from classi- cal toxicology.

The importance of the concentration (activity) of the in- dividual solutes for interpreting the cause/effect relation- ship depends-in the sense of a simple model-essentially on the thermodynamic statement that, for an equilibrium distribution, the chemical potential (related to the activity) is the same in all phases, i. e. in the solution, in the organ- ism, in the sediments, in the gas phase, etc.[”I. Under con- ditions of non-equilibrium, the difference in chemical po- tential (the activity gradient) is the driving force for the transfer of a substance from one reservoir into another, e .g . from water into an organism; but active mechanisms of transport into the inside of a cell are also possible.

2.2. The Toxic Effect

The ecological harmfulness of a substance depends on its interaction with organisms or with entire communities of organisms (Fig. 1). The intensity of this interaction de- pends on the specific structure and on the activity of the substance under consideration; but other factors such as temperature, turbulence, and the presence of other sub- stances are also important. A thorough knowledge of the basic mechanisms by which toxic agents exert their inju- rious effects (pharmacokinetics, uptake, conversion, de- gradation and separation, disturbance of regulatory mech- anisms in the biological community) is fundamental for an understanding of biological responses. Comparative toxi- cological research is necessary in order to extrapolate ef- fects observed in laboratory experiments to organisms in nature and from one organism to another or to man.

In evaluating toxicity, we need to distinguish between:

1 ) Substances which have a direct effect on humans, ani- mals and other terrestrial or aquatic organisms (here one often speaks of acute, subacute or chronic toxicity, depending on the time interval within which an effect is observed).

2) Substances which primarily affect the organization and structure of an ecosystem. Here a contaminant may im- pair the self-regulatory functions of the system or inter- fere, in a subtle way, with food chains.

Sociobiological Effects: While we have some knowledge about the impact of xenobiotic substances on individual organisms, we know less about their impact on ecosystems.

382

In considering biological communities (the biocoenoses) various intra- and inter-species interactions of a sociobio- logical nature (e . g . chemotaxis and chemoreception) have to be taken into account.

The natural distribution of organisms depends primarily on their ability to compete under given conditions and not merely on their ability to survive the physical and chemical environment; a population will be eliminated when its competitive power is reduced to such an extent that it can be replaced by another species‘13. 14). The competitive abili- ties of an organism are an interplay of its reproductive rates, which are related to food and physiological poten- tial, and the mortality rates from all sources, including pre- dation and imposed toxi~i ty”~’ . Tinbergen’lsl pointed out that there are millions of ways in which an organism can die, but that there is only a very narrow range of ways in which it can survive and leave offspring. Thus, in an eco- system, a population may be eliminated by the presence of pollutants even at apparently trivial toxicity levels if its competitive ability is marginal, or if it is the most sensitive of the competitors. Often, contaminants at very low con- centrations cause changes in the structure of the popula- tion by interfering through chemotaxis with interorganis- mic communication. For example, the survival of a fish population may be rendered impossible by a pollutant (even if it exhibits neither acute nor chronic toxicity to the particular species of fish) if it impairs the food source (zooplankton) or disturbs chemotactical stimuli or mimicks wrong signals (and thus, for example interferes with food finding).

Ecotox icology

Tra s format ion

Environmental

risk, hazard )

I --. :-.. r

I interpretation I information

C h e m specres

species

Group para meters

Collective Parameters D Specificity eb Sensitivity

Environmental Analytical Chemistry Fig. 2. Interrelationship between environmental analytical chemistry and ecotoxicology ; the establishment of cause-effect relationships hinges signifi- cantly on the ability to sensifively and selectively detect individual compo- nents in a complex environmental system.

Angew. Chem. Inl. Ed. Engl. 22 (1983) 380-389

As a consequence of the many microhabitats (niches) that are typically present in a "healthy" water, many spe- cies can survive. Because of inter-species competition, most species are present in a low population density. Poi- lution destroys microhabitats, diminishes the chance of survival for some of the species, and thus in turn reduces the competition; the more tolerant species become more numerous. This shift in the frequency distribution of the species towards a lower diversity of the ecosystem is a gen- eral consequence of the chemical impact on waters by sub- stances non-indigeneous to nature"31.

F r e e metal ions

3. The Role of Environmental Analytical Chemistry

Inorganic Organic Metal species Metal species Metal species Precipitates, complexes, complexes, bound to in the form adsorbed on particulate and chelates high molecular of highly colloids org. material , methylmetal org. mater ia l dispersed colloids detritus compounds

I Acute Toxicity I

[ C U ~ ( O H ) ~ ] ~ + [Pb(C03)21z- AgSH [CdClj' [Zn(OH)3]- IAgzs~H12- [CH3Hgr (CH, )4Pd

Example: Tetrachlorodibenzoj 1.41dioxins (TCDD)

CI&.,) CI CI &;& ;;&& CI

LD50 (Rat) : > l q / k q >01 g1kg 0 00004 g I kg

I Chemical Reactivity (Hydrolysis)

Example: Butyl chlorides CH3

CH3 -, 30 seconds

CH3CH2CH2CH2 Cl CH3CH2xCH ci CH3-C - CI CH3'

t l l p (pH7) : - lyear - 40 days

MSR MOOCR

do HzN\

c u 0' \ N H ~ P 0

Soluble org. constituents of humus

Figure 2 illustrates the interdependence between envi- ronmental analytical chemistry and ecotoxicology, i. e. the assessment of the environmental fate and the impact of chemical substances. An understanding of the interaction of chemical compounds in the natural system, that is the mode of their participation in various processes, hinges on the recognition of the compositional complexity of the en- vironment. This requires an adequate analytical method- ology, especially the ability to detect individual compo- nents (chemical species) selectively and to measure them accurately and with the greatest sensitivity.

Speczjkityr The environmental behavior and impact of a substance (biological availability, physiological and toxi- cological effects, adsorbability, chemical, biochemical and geochemical reactivity) are strongly structure-specrfic. Fig- ure 3 illustrates that organic isomers may exhibit remarka- ble differences in toxicity, chemical reactivity and adsorp- tivity. The same is true for inorganic constituents. Figure 4

I (Ad.)sorptivity I Example: Tetrachlorophenols

c,&; @CI OH $, CI CI

pK, 5 0 4 5 40 6.35

Ki(sj(pH68) ' 10 20 180

CI CI

-

Fig. 3. Chemical and biological reactivity is strongly structure-specific. LDIo is the concentration that will produce death in half the population, f1 ,2 is the half-life; pK, is the neg. log of the acidity constant; K&) is the distribution coefficient [m' lo-' kg-'I between a solid (surface) phase (containing 2.9% organic carbon) and the solution.

shows the various forms in which metals are thought to oc- cur in natural waters. The different species have different effects. For example CuC03(aq) and Cu(aq)'+ each affect the growth of algae in quite different ways"61. Collective

Examples :

M-lipids Humic acid polymers Lakes Yellow pig- ments

saccharides M-poly-

M"+

MCO,, MS etc. on clay minerals, FeOOH or on Mn (1v) oxides 3r on biolog. -ells

Diameter range:

-10 nm- 100 nm- I m t

4 filtrable t

4 membrane filtrable

dialyzable - w i n t rue solution-

Fig. 4. A knowledge of the forms of occurrence of metals (free metal ion, inorganic or organic complex) is a prerequisite for evaluat- ing biological and chemical effects. It is operationally difficult to distinguish between dissolved and colloidally dispersed substances because membrane filters may not always retain colloids.

Angew. Chem. Ini. Ed. Engl. 22 (1983) 380-389 383

parameters such as total organic carbon, copper(i1) or total phosphorus, although often useful in establishing total loads, are not suitable for establishing cause-effect rela- tionships between chemistry and biology. For the determi- nation of individual organic chemical species, various methods capable of high selectivity (GC, G U M S , and HPLC) are available. The specification of metal ions, usually attempted by electrochemical (voltammetric) means is, on the other hand, still a difficult problem.

Sensitivity: Analytical chemistry has made remarkable progress in improving the sensitivity of detection down to concentrations of 1 0 - “ ~ . This has been a great help for studies on natural water systems, since aquatic ecosystems are more sensitive to chemical perturbation than terrestrial ones. The resorption of a substrate or the uptake of a pol- lutant by aquatic organisms is controlled by enzymatic processes, and these are affected typically by concentra- tions lower than 1 0 - 6 ~ . (The Michaelis constant for enzy- matically controlled processes is often between and 1 0 - ” ~ . ) The growth rate of algae has been shown to be affected by free C u z + ions at concentrations as low as lo-’’ to 1 0 - 1 3 ~ , while concentrations as low as ~ O - ” M are toxic to daphnia and fish[’61 (Fig. 5).

3.0 t m

n 10 0 73 I

* 01 i 6 8 10 12 14 16

-0.5’ ’ ’ ’ ” ’ ’ ’ ‘ 1 pcu -

Fig. 5. Effect of “free” Cu:; ions on the growth of algae in seawater. pCu was buffered in seawater by Cu” and various complex formers. pCu (= -log[Cu*+]) was calculated from the concentrations of Cu” and complex formers and the pH 1161.

Precision, Accuracy and Sampling: The decisive prerequi- site for any meaningful interpretation is the collection of reliable and accurate data. Because of the extremely low concentrations usually encountered, rather demanding problems have to be met in order to avoid contamination or loss of constituents in all stages, from sampling to sam- ple pretreatment and sample preservation until analytical determination. For example, as shown by Patterson et U Z . [ ‘ ~ ] , most of the results obtained before 1975 for Pb con- centrations in seawater were an order of magnitude too high.

4. Limitations of Analytical Monitoring and Toxicological Testing

Understandably, the general public requires that the bio- logical effect of pollutants be examined systematically by

384

appropriate toxicity tests and that the environment be sur- veyed by monitoring programs that search for pollutants and toxicants. Such investigations are important, but we must not overvalue our analytical possibilities and the lim- ited validity of standard toxicity tests.

We are concerned that the present preference and over- emphasis of such programs over flexible basic and explo- ratory research may hinder rather than aid our understand- ing of nature and of its changing processes”~. The syste- matic monitoring of the chemistry of a watershed or of a river or lake is an extremely involved, expensive, and time- consuming enterprise. Because of a lack of understanding of the reciprocal interactions between pollutants and the environment, much of the effort in the past had by neces- sity to be devoted to measuring, within the momentarily available “analytical window” (for a given methodology), “what is” rather than to projecting “what is likely to be” or “could be made to be”. Baseline measurements, as special cases of monitoring, are more promising because they per- mit recognition of changes in the environment. Unfortu- nately, baseline records often turn out to be less valuable than one had hoped when the measurements were ini- tiated, because one later realizes that at that time igno- rance had prevented both a meaningful definition of the problem and the selection of the best method1’].

Nevertheless, knowledge gained from monitoring data is enhanced significantly if the data can be interpreted in a functional and rational way. To this end, a quantitative knowledge of the hydrological regime (water budget, rain- fall, evapo-transpiration, runoff, infiltration) in the area, and information on input and on transformation pathways are required. Field measurements are essential, but they should be supplemented by concurrent laboratory studies on the chemodynamic properties of selected substances of interest in order to establish comparisons between mea- sured field data and concentrations predicted from the “behavior profiles” of chemical species.

Toxicity tests are rarely designed to assess chronic toxic- ity or to evaluate impairment of self-regulatory functions of ecosystems. From a n ecological point of view, these lat- ter effects are more important than acute toxicity. Further- more, one must take into consideration that for each sub- stance (and its degradation intermediates), short and long- term tests with different organisms should be carried out in order to determine whether synergistic and antagonistic ef- fects may occur in presence of other substances or differ- ent solution variables. Each negative test result may not mean “zero toxicity”.

In addition, the sensitivity of different taxonomic groups to chemicals may vary markedly-even organisms of the same species may respond differently. Thus, results ob- tained with one kind of organism may not readily be extra- polated to other species. Obviously, new chemicals may be produced and enter the environment faster than they can be tested satisfactorily.

Although we have some knowledge of the effects of xenobiotic substances on individual organisms, our under- standing of the impairment of whole aquatic ecosystems is very limited. As we have already seen (Section 2.2), even if no acutely damaging effects on individual organisms are observed, the ecological consequences of such impairment

Angew. Chem. Int. Ed. Engl. 22 (1983) 380-389

c z 3 Field studies

~ Information or solubilty etc.) structure and dynamics of

Compound- specific parameters

of individual processes

I I I ecosystems

I natural and/or 1 anthropogenic input marker (sources, quantities compounds

1 %I-o------Ig--oJ ‘ L ~ g ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

-specification of pollutant and of possible trans- formation products

-determination of spatial and temporal variations in concentration (activity)

system parameters -determination of

-hydrolysis

- oxidationireduction - microbial transformation

- photolysis

Prediction of the -volatilization environmental behavior -adsorption/desorption of a pollutant in a given -coordination

-bioconcentration natural system

Fig. 6. Development of methods for predicting the environmental behavior of pollutants: General approach. The ecotoxicological and hygienic evaluation of pollutants requires establishing their “behavior profile” in the environment. With the help of simple models, it is in many cases possible to estimate the approximate residual concentrations.

to communities of organisms may often be more detrimen- tal over long time spans than acute toxic effects.

degradability, etc.) aid in establishing the pollution po- tential of individual substances and permit one to fore- cast the manner in which the compounds are distrib-

5. Concepts for Assessing and Predicting the Environmental Behavior of Pollutants

uted in the environment, how fast they may be degraded chemically or biologically, and thus, in turn, to predict their fate and approximate residence time. Thus, a semi-

5.1. Development of Predictive Assessment Models quantitative estimate of the ecological impact is ob- tained. In order to project the distribution of polIutants and how it is influenced by systemic variables, one needs to understand the structure and dynamics of eco- systems and the interactions between the biogeochemi- cal cycles in aquatic ecosystems.

As illustrated in Figure 6, three approaches complement each other in establishing predictive models: 1) A set of data of compound-spec& parameters (vapor

pressure, solubility, lipophility, Henry coefficient, bio-

Table I . Pertinent information to assess fate and residence time of pollutants.

Parameter

production statistics mass balance transport mixing

Transport Paths

Distribution air-water surface-water sediment-water

(ad)sorption isotherms solubility vapor pressure Henry constant lipophilicity n-octanol-water distribution coeff. mass transfer t coefficient

Molecular Transformation

microbial oxidation hydrolysis DhOtOlVSiS

biodegradability equilibrium constants rate constants light absorption

Significance

immission into environment physical fate

interface transport sedimentation bioconcentration bioaccumulation

equili- food chain brium hiolog. retention

immission into and from atmosphere dry deposition precipitation scavenging evaporation condensation

residence time half life form of occurrence kinetic

structure-reactivity correlations photodecomposition . _ I

Angew. Chem. Int . Ed. Engl. 22 (1983) 380-389 385

2) Laboratory studies in which individual processes, e. g . , with model compounds, can be investigated under con- ditions where variables are known or kept constant are necessary in order to determine reaction constants and other substance-specific parameters.

3 ) Field studies are necessary in order to determine system parameters and to measure the spatial and temporal dis- tribution of pollutants and to identify the forms of oc- currence (species) of these chemicals.

principles of physics, chemistry, and biology make possible the explicit incorporation of the effects of changes in land and water environments. In principle, causal models are more readily transferable in space and time. A mixed- model approach will be necessary for predicting the per- formance of complex systems.

Estimation of the residence time of a pollutant in a body of water (or in another reservoir of the environment) with the help of such models is of utmost importance because the residual concentration of the pollutant depends on the residence time. The latter is influenced by hydrological factors (e.g., type of dispersion and loss through the out- let) and by non-hydrological processes (e. 9. adsorption on settling particles or biomass and chemical and biological transformations).

The process of modelling is essential. Empirical models require fitting to on-site data and continuous updating with further collection of data. They have low capacity for transferability in time and space and for extrapolation beyond the range of data. Causal models based on the

Spatial and Temporal Distribution of Po I I uta nt

Systemic

Polar substances

Var la bles

Non-polar substances

L

- fk org C content

a) Sorpt ion o n sett ing particles

concentration

I b) Transfer a t t h e gas - water interface

concentration

c 61\ m

- log molecular

. volume

q, - 0 7 -

Henrr coefficient

4-

._ 11 ,+ Q rn 0 -

log Iip0phiIit)r octanol - water

4 c) Bioaccumulation

0 - m

trophic level distance

coefficient d ) lnvolment in

nutrient cycles

PI-- log conc phosphale

L--__) relative concentration

e) Biodegradation

L distance time

f ) Hydrolysis, photolysis. !K chemical oxidat ion -

a.

r 0 " Y

hydrolysis

--F-

Fig. 7 . Illustration of the dependence of spatial and temporal distribution of pollutants on compound-specific and systemic variables: a) The ex- tent of adsorption of polar substances as a function of pH is different for a cationic, anionic or molecular (weak acid) adsorbate; for non-polar li- pophilic substances, the extent of adsorption increases with increasing organic C content of the adsorbing particle; b) the concentration gradient at the water/atmosphere interface indicates in which direction the transfer of volatile substances occurs. The specific transfer coefficient is de- pendent on the Henry coefficient and on the molar volume of the substance as well as on the system-specific parameters such as water turbu- lence; c) the accumulation of a substance in the food chain is related to its lipophility or to the reciprocal of its solubility; d) nutrients, including certain heavy metals are, for example in oceans, withdrawn from the water during photosynthesis in the top water layers and returned as a conse- quence of respiration in the same proportions (corresponding to the composition of the biomass) in the deeper water layers; e) biodegradation is autocatalytic and proportional to the microbial biomass available; f) chemical and photochemical transformation processes are influenced by many solution variables, especially pH, light extinction, and concentration of singlet oxygen.

386 Angew. Chem. Int . Ed. Engl. 22 (1983) 380-389

5.2. Examples of Relevant Processes

Basic physical, chemical and biological concepts, and unit processes may be modelled in order to predict the be- havior of environmental pollutants. Table 1 and Figure 7 identify some of the relevant processes that need to be con- sidered. Some of these processes will be briefly outlined:

Adsorption: Particles acting as scrubbing agents by scav- enging are important in regulating the concentrations of pollutants. Thus, in water systems, all substances that are strongly adsorbed have small residence times relative to that of HzO. Many xenobiotic substances, hydrophobic substances such as polychlorinated hydrocarbons, as well as polar substances such as lignin sulfonates from the pa- per industry, or heavy metals, are strongly sorbed to sus- pended particles. The settling of biogenic particles to; gether with clays is responsible for the dynamic mecha- nism of elimination of heavy metals in lakes and oceans through transport into the sediments. Surface chemical theory aids in quantifying the extent of the reactions oc- curring at interfaces[“’. Often, a linear adsorption iso- therm

can be used to describe the distribution between solid sur- face and solution. For polar substances and ions, equilib- rium constants have been experimentally determined or approximated from stability constants in electrolyte solu- tions; the extent of adsorption can thus be predicted as a function of pH and solution variables. The sorption of hy- drophobic pollutants on settling particles or sediments and biota can usually be related to the lipophility of a sub- stance, i .e. the tendency to become soluble in lipids and fats, and the organic carbon content, fo,, of the adsorbing particles (Fig. 8). As a measure for lipophility, one usually uses the n-octanol/water partition coefficient, KO,:

As a measure of the distribution coefficient, K,, for many groups of substances, equations of the type

may be used[”l, where a and b are constants and K , is ex- pressed in units of volume per mass dry weight and f,, as weight fraction (foe = organic carbon content of solid mate- rial).

Exchange with the Atmosphere: Transfer of volatile or- ganic substances from aquatic systems to the atmosphere or vice versa can readily be examined in the laboratory. The equilibrium distribution of a compound is given by the Henry coefficient H :

[A,], [Aaq] and [A&] are, respectively, the concentration of A in the gas phase, in aqueous solution and in a saturated solution (=solubility); pA and p i are, respectively, the partial pressure of A and the vapor pressure of pure A (or

K;(s) AS A FUNCTION OF ORGANiC CARBON CONTENT f,, (s)

/ /

/

,

o 1,2, 4 , 5 -Tetrachlorobenzene + 1, 2 , 4 - Trichlorobenzene A 1,4- Dichlorobenzene

Chlorobenzene

Fig. 8. The distribution of non-polar organic substances between aquatic sol- ids and water (as given by the distribution coefficient K , ) is dependent upon the lipophility of the compound and the organic C content of the adsorbing material (fa,= weight fraction). The solid phases considered here are coastal sea and lake sediments (I - 9, river sediments (6, 8), solids from aquifers (7, 9- 12) and activated sludge (AS). The octanol/water distribution coefficients are, respectively 500, 2400, 11 200, and 52000 for chlorobenzene, 1.4-dichlo- robenzene, 1,2,4-trichlorobenzene, and 1,2,4,5-tetrachlorobenzene.

the vapor pressure in equilibrium with a solution saturated with A).

The rate of transfer F of compound A across the water gas phase boundary can be expressed by

where KL is the overall gas transfer coefficient (KL de- pends on the hydrodynamic conditions ; for most volatile substances KL is of a similar order of magnitude as that of oxygen). Substances having a Henry coefficient larger than ca. 0.1 are transferred relatively rapidly from waters of high turbulence to the atmosphere (see example in Fig. 9a). Less volatile compounds ( e .g . DDT; H - 5 x lo-“) have, with regard to gas exchange with the atmosphere, a longer residence time in water; nevertheless, such relatively not very volatile substances “slowly” escape into the atmo- sphere where they are transported over large distances.

387 Angew. Chem. Int. Ed. Engl. 22 (1983) 380-389

Figure 9b illustrates both; the transfer of a substance from the atmosphere into a lake water, and the “volatilization” of another substance from the lake.

“1 300

201f5 --+180%

loo: 0 30 0 30 6 31 2 Distance lkm]

t o to +IS to +30 Time [ min 1

rlocI 5 10 15 20

I Atmasphered

’il a *....--

f 15 a g 20

25

30 0 I0 20 30 40 50 60 70

C l n g / l I Fig. 9. Gas exchange between surface water and atmosphere is an important process for volatile organic substances. a) Concentration profile of tetrachlo- roethylene and 1,4-dichlorobenzene along a stretch (1.2 km; average depth 0.42 m) of the River Glatt (Switzerland). Some cascades are responsible for efficient gas exchange. The decrease in concentrations is mainly caused by the escape of these substances into the atmosphere (tl,2 (tetrachloroethy- lene)=6S min; (1,4-dichlorobenzene)=80 min).-b) Exchange of non- polar organic substances between Lake Zurich and the atmosphere. While l,3-xylene (a component of gasoline) invades the lake water from the atmo- sphere, tetrachloroethylene (used in dry cleaning and metal degreasing), a water pollutant, escapes from the water into the atmosphere.

Microbial Degradation: Among the molecular transfor- mations, biodegradability is one of the most important fac- tors determining the residence time of organic pollutants in the receiving waters. Today, detailed knowledge of the microbiology, the mechanisms and the kinetics of the bio-

H19Cg-@OCH2 CH, (OCH, CH,), OCH2CH2 OH

Poly(ethyiene oxide) with a 4-nonylphenyl end group (non-iontc detergents)

microbial degradation

e g 1 H19Cg-@ 0 CHp CH2 OH

2-(&NonyIphenoxy) ethanol persistent , ecologically objectionabie degradation

intermediates

Fig. 10. Example of a group of substances which as a consequence of biolog- ical waste treatment is converted into a highly objectionable, relatively re- fractory degradation intermediate [20].

388

transformation of organic pollutants under natural condi- tions is still lacking. Many substances are readily altered to the extent that bacteria use their original structural entity, but the metabolites so formed are non- or slowly degrada- ble. For example, as is illustrated in Figure 10, certain non- ionic detergents yield ecologically objectionable degrada- tion intermediates that persist in the water much longer than the parent compound[201.

Chemical Transformations: In order to predict the reac- tivity of organic pollutants, it is often possible to utilize structure reactivity correlations because many organic chemicals share common reaction patterns and reactivities among like-structured chemicals; within a class of com- pounds, the reactivity of a functional group depends on the type and number of substituents. For example, the ki- netic hydrolysis constant of organic phosphate esters of the type (CH30),P(0)-OR can be correlated with the acidity constant of the leaving group ROH[241. Further- more, predictions on the dependence of kinetic constants of chemical reactions upon solution variables can often be made (e . g., p H dependence of hydrolysis).

Bioaccumulation: Of particular importance in assessing the ecological or toxicological relevance of a pollutant is the question whether the substance is incorporated into the biota. Such incorporation may lead to a retention of the pollutant and its biomagnification in the food chain. In- corporation into biomass is related to the lipophility as measured by the n-octanol/water distribution coeffi- cientl2l1 (Fig. 1 I). The tendency of certain pollutants to be- come accumulated in the food chain is much stronger in aquatic ecosystems than in terrestrial ones because the products of photosynthesis are consumed to a much larger extent by herbivores in waters than on land, where the plants are mostly degraded by microorganisms.

2 ,4 ,2 ,4 ‘ -PCB 0

hexachloro

n-rlrhlnm- r -’-’ ‘1-1 -

I I 1 I 1 lo2 103 lo4 105 lo6 107 Octanol/Water partition coefficient

Fig. 11. The lipophility of a substance, as measured by the octanol/water dis- tribution coefficient, is an essential parameter for predicting biomagnifica- tion (biostorage and accumulation in the food chain) (calculated from data by Chiou ef al. t211). 2,4,2’,4’-PCB = 2,4,2‘,4‘-tetrachlorobiphenyl.

6. The Consequences

Man’s life is accompanied by his concern for safety. The evaluation of ecological and health effects caused by expo- sure to pollutants is extremely difficult because of our lim- ited knowledge of the impact of chemicals on aquatic eco- systems. As we have tried to show, standard toxicity tests

Angew. Chem. In!. Ed. Engl 22 (1983) 380-389

with single organisms cannot be used to predict the subtle and often chronic impacts of substances on humans and ecology.

As long as our insight into the effect of chemicals on na- ture is so limited, we must remain very cautious in adopt- ing tolerance levels; in establishing “minimum adverse ef- fect levels”, we must demand safety factors that are ade- quate to protect against the unanticipated effects of yet un- suspected biologically active compounds in the environ- ment. On the other hand, there exists for every substance- even if not accessible experimentally-a certain level of exposure below which damage is highly improbable. The concern for ecosystem protection and safeness cannot ex- ceed a reasonable incremental cost/benefit ratio. The final decision for this balance between ecological and toxicolog- ical measures and economics and the public propensity for risk acceptance is after all political ; but the environmental scientist should procure better scientific criteria for this de- cision-making process. Exploratory multidisciplinary re- search on comparative toxicology, on the cycles of energy and nutrients in ecological communities, and on the im- pact of chemicals and other stresses upon structure and species distribution frequency can eventually yield criteria that permit the establishment of no adverse effect toler- ance levels for substances non-indigeneous to natural sys- tems.

The recommendation made here, i.e. to predict, on the basis of substance-specific parameters, the fate, the distri- bution, the tendency for bioaccumulation and the approxi- mate residence time of potential pollutants, and to estimate tKeir most likely attainable residual concentrations permits a comparison of various substances concerning their rela- tive risks and impairment potential. The confidence in such assessments can be enhanced if field measurements give relative agreement between concentrations predicted and those observed.

Biodegradability and lipophility (lipophilic storage in organisms) are among the most important compound-spe- cific criteria. The best and perhaps most economically fea- sible way to safeguard aquatic ecosystems and humans against unanticipated effects of organic pollutants is to re- quire: 1 ) that all substances that are mass-produced and may become dispersed into the environment because they are being used in households, small trades, and agriculture (e. g . detergents, cleaning fluids, agrochemicals) have a sat- isfactory biodegradability; and 2) that in industrial pro- duction, the recovery or safe disposal of intermediates and residuals becomes an integral part of the production proc- ess.

In establishing priorities on policies in environmental protection, the question arises: What should be of greater concern, human health or “health” of the ecosystem? In

answering this question, one needs to consider that the maintenance of resilient ecosystems with a sufficient diver- sity of organisms, i. e. systems that can resist structural and functional displacement, is a prerequisite for the well-be- ing and health of humans.

It is encouraging to find that the ideas and concepts pre- sented here-it may not have been sufficiently evident that much of our ideas have been influenced by the suggestions of Baughmun‘51, Blumed’], Mackuy[’], HapkeI6’, and Tinseley“*I-are, at least partially, gaining some accept- ance by the Organization for Economic Cooperation and Development (OECD)‘3.221, the American Society for Test- ing Materials (ASTM)[’], and the US Environmental Pro- tection Agencyfz3].

The expansion of chemistry will continue to shape our life styles. In the use of chemicals and in further develop- ments, we have to accept the boundary conditions imposed by ecology and human health.

Received: November 12, 1982 [A 453 IE] German version: Angew. Chem. 95 (1983) 345

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Angew. Chem. Int. Ed. Engl. 22 (1983) 380-389 389