Edwin H. Klehr School of Civil Engineering and Environmental Science The University of Oklahoma Norman, 73069
An Approach to Teaching Some of the Principles of Environmental Chemistry
Recently, environmental chemistry has begun to emerge as a distinct field of both research and teaching. There is much confusion as to what an environmental chemist does, and what should be taught in an environmental chemistry course. Moreover, there is a widespread lack of communicat,ion between chemistry departments, which tend to emphasize principles, on the one hand, and engineering groups which must use these orinci~lesand which tend to emphasize practicality and immediate problem solving. Since the majority of environmental scientists are now, and will continue to he, working with engineers, every effort should be made to uromote meaningful interdisciplinary contacts for our students. 1 suggest that environmental chemistry is properly the study of the chemistry of the movement and transformations 01 matter and energy in the physical environment. As such, it is an applied chemistry, and has close ties with other areas such as hiology, geology, and meteorology. I t must deal with both basic principles and practical applications. The purpose of this paper is to indicate how a few concepts from svstems analvsis can be used to intearate - some of the principles of environmental chemistry. The approach is llexible enough so that it can be used a t more than one level. It can serve as a generalized, semi-quantitative introduction for students with a general chemistry backzround, as a fairly rigorous course a t anadvanced level, & a format for a graduate seminar, or as a auide inresearch planning. 111 an? level. one 01 the most impt,rmnt cuncepts that needs to IIV hrourht out is that of the dynamic nature of matter and energy in the environment. There is a continuous cycling of many elements and chemical species; matter is pumped through grand and small cycles by energy obeying the laws of thermodynamics. Perturbations of gny of these cycles hy human activities (e.g. pollution) lead to some type of disruption ofthe ecosystem and loss of stability. Development of a Grand Transport~ransformatlonMatrix We shall view the environment as a system made up of interrelated compartments. Matter and energy can flow from ,me u m l w t m e n t t i ) another, and transformationscan occur Wr can formalize the approach by thc w ~ t h ~wnipar~ments. n use ~~f~trans~ortltran~formation matrices itlt .~~~ . . matrices). This is simply an attempt to identify all the pertinent input/ktput relatimships and transformations for matter or energy in a given environmental system. First of all, compartments must be selected, and the following are suggested as relevant criteria for the selection ~
~~~
.~
mosphereIAhiotic, AtmosphereIBiotic, HIA, H B , LIA,
LIB. Criticism of the above selection and formulation of definitions could profital~lyIn! the first tupic of class discussion, w ~ t h the nskiny o f v i ~ w p ~ , i nirom t s utht:r diicinlines. A brief. Ill11 well organized prksentation of the main features of each compartment a t this point could he a fine opportunity for some honest team teaching. The selection and discussion of the biotic suhcompartments should emphasize the trophic level concept, energy flows, and the recycling of nutrients (I). The trophic level approach divides organisms into categories related to energy flow and recycling (i.e. energy is "captured" by the producers and passed along to the succeeding levels). Four examples of different attempts a t categorizing trophic levels are
658 I Journal of Chemical Education
4
Saprotrophs Secondary con~umem Decomposers
U"
Table 1. TransportlTransformationMatrix AIA
A18
HIA
HIE
LIA
Out
LIB
AIA
Storage
Uptake Pption Uptake Pption Uptake Min Senling Settling Gas trans Gas trans
A18
Death Elimin
Storage 0
HIA
Evspn
Uptake Storage Uptake Senling Uptake Sorbn Chem Ppn
Suspn
Elimn
Feeding
Feeding Death
Feeding
Storage Elimn
0 0
.
-
Feeding
0
Uptake
0
Elimn
~-
Suspn Evapn
Uptake Suspn Ion Exc
Uptake
DisalnElimn
Feeding Elimn 0
?
-~
Feeding Death
~..~
out ~?
Storage
Desorbn
Gas Trans LIB
0
lo" -E X ~
.-
- -
HIE LIA
In addition, the number of compartments should be as small as possible, hut not so small that essential information is lost. The atmosphere, hydrosphere, and lithosphere are chosen as the three major compartments of the total environmental system. Biotic (living) and abiotic (nonliving) could he used as subdivisions, giving a total of six suhcompartments: At-
3
Producers Producers Phagotrophs Primary consumers
Common to all four schemes are the processes of production (conversion of simple inorganic substances and sunlight into living matter); utilization (feeding and metabolism); decomposition (breakdown of complex organic substances and extraction of energy); and recycling (mineralization of nutrients). In sim~listic terms. matter and enerev flow throueh " a group of living organisms. We might even go so far as to say that a biotic comoartment is a soohisticated chemical reactor with several feedback loops. . At this point, classroom assignments could include a brief discussion of each trophic level of different biological systems such as oceans, lakes, forests, meadows, tundra, and agrisystems (I). At a more advanced level, example chemical reactions could be worked out for each of the processes (2).
I ) A nmpartment must he definable, usingterms meaningful not only 10 chemists, but to other disciplinesas well. 2 ) Each axmmrtment must he separable, both conceptually and
operationally. :I A compartment must be subject to verifiable sampling and meaauremenl
2
1
Autatrophs Producers Heterotrophs Macroconsumers Decomposers Microconsumers
-~
0
Elimn ?
Storage -
0 ~
0 -
~
-
0
Once a selection of subcompartments has been made, a grand transportltransformation matrix can he formed to deal with the movement of matter in the entire system. See Table 1. To form this matrix. we arrange all suhcom~artmentsin rows and columns. An ';out" compartment is included to account for matter (or energv) -. flows into and out of our system. Each element of the array (intersection of row and column) signifies a wossihle flow of matter from the subcompartment ofthe row t'o that of the column. he diagonal hoxes istorages) are used to indicate movements or transformations w i t h i n a compartment. A rule of the game is that transfers or flows shall he single step processes, a i d shall not include a transformation (change in speciation or chemical form). It should he no surprise that it is not always clear just what is a "single step process." Table 1 represents a tentative statement of those interactions deemed significant and needine further studv. As an example of the interpr&ation of th&hle, consider the suhcomoartment LIA (abiotic lithoswhere). According- to the table, matter (and hence, energy) moves from this compartment to others as follows From LIA
to
"in
AIA AIB HIA
suspension, evaporation, gas transfer uptake suspension, ion exchange, desorption, dissolution uptake storage uptake
H/B LIA LIB
The total list of processes (fluxes) includes a t this point: evaporationlsuspension, settlinglsuspension, precipitation1 dissolution, sorptionldesorption, ion exchange, uptake, feeding, elimination, and death. The chemical terms should have an obvious meaning to chemistry students, hut they may have to be defined for the nonchemistry participants. The biological terms may call for some simple definitions U ~ t o k e the : movement of non-living organic or inorganic matter
saprophytes. Feeding and Grazing: the ingestion of live organisms by other organisms; includes all1 predatorlprey relationships, Elimination: the passage of abiotic matter out of living organisms; includes urination, defecation, perspiration, exhaling. Death: difficult to define, but fairly obvious in denotation. When an organism dies, the body becomes part of an abiotic suhcampartment.
Each box of the matrix raises a number of questions: What do we know about the implied transfer? What disciplines are properly concerned with the transfer? Where in the literature shall we look for aowrooriate .. . information? Is the flow significant, and in what sense? (e.g. absolute amounts of matter transferred. transfer rates. control of other Drocesses, characteristics of sending or receiving compartments). ~ o e the s implied process have a name? Can it he formulated in quantitative terms? Can the process he measured? How? What factors control the onset, duration, extent, rate, and termination of the process? These questions can lead to meaningful class discussion and team homework assignments. Ideally, reams shot~ldho iorn~edwhich inclurls a many t)a~kyruundi as positble, thereby providing an upportunity for interdisci. d i n i m interactions. The level of the d:scussion will det~end on the level of the class. It can he done in general, qualitative terms in a sophomore course, or more rigorously and for specific circumstances in advanced courses. To help students see some of the complexities of the flows between environmental compartments, it may he helpful to compare an environmental system with an electrical system, as is done in Table 2.
.~-~-~~~-" ~~~~~
~~~
~
~
Table 2. Comparison of an Electrical System with an Environmental-System
p p p p p
"em
~
~
Electrical System Environmental System - - .~-~ -~~ ~~
Homogeniely of compments
Components are homogeneous. (resistors, capacitars, etc.) Flows wimin compartments are of no concern.
Transformation
The item tracked (an electron) does not change form.
1s01ation
Individual components can be studied reliably in isolation (separated from the system). Components have behavior independent of location Insystem (e.g.A resistor is always a resistor with behavior described as R = VIO. A priori knowledge of connections (e.g, wiring diagrams).
Dependenceon locatim
Flow patterns
~
Compartments are not homogeneous (eg. the biotic hycrosphere). Movements within compartmenis must be considered. Chemical and physical processes occur within compartments, changing the farm of the item tracked. (e.g. nitrate to ammina groups). Natural components usually cannot be studied in isolation. Behavior of natural corn ponsnts Is highly dependent on location.
Connections not always known; complex webs.
Table 3. Example List of Processes Which Can Occur within Subcompatiments-. Biologicsllchemicai- .~ Physicailchemical Chemical ~
Storage Translational movement Diffusion Mixing. dilution
Acidlbase Redox Complex formation Ion exchange Sarptiwldesorption
Metabolism
41 this point, i t should he stressed that energ\. is the "push" for these tlows. .4 discusciot~oi energy will depend I,IIthe 1c.n.l of thecuurse. At an inrroductory level, it ihould incIude:ditferent kinds ol'energy, the tirit and second laws ut thermodynamtcs, and thermochmnistry (31.At ,I more ndmnred I w 4 it ihould include a discussim uf iree energy. tntrtrpy, crlterts fur snontnneitv. eauilihrium states. and steady states 141. The preskntation ihoild be slanted to'fit the nee& of the course and the backeround of the students. Energv flow diagrams such as that ievised by H. Odum ( 3 , 5 )sh&ld be incLded; they can be used a t whatever level of sophistication is appropriate for the class. Examining Processes within Compartments
The next step in the use of the grand tlt matrix is to "open the boxes" and look a t some of the processes within suhcompartments. I have found it effective to stress the idea of chemical, physicallchemical, and hiologicallchemical processes as major categories, and talk of classes of reactions. Table 3 is an example list of processes which can occur within suhcomnartments. After a discussion of each process or reaction by the lecturer, assienments could include a literature search for hard details " of each as it occurs in nature. Many of the chemical reactions listed are usually discussed from an equilibrium point uf view such as that found in the excellent hook "Aquatic Chemistry," by Stumm and Morgan ( 4 ) . In particular, the techniques of the master variable diagrams for different classes of reactions, and predominant area diagrams are admirably suited to a course such as this one. The development of such diagrams for specific acidlbase, redox, or complex formation reactions in the environment make fine class assignments. Acidlbase behavior of the inorganic carbon system; the soluhility of Volume 54, Number 1 I. November 1977 1 859
calcium carhonate; the redox behavior of iron; the solubility of irtm in the presence of carhonate, hydroxide, and sulfide are example reactions I have used in my classes with good success. Here again, the level of the assignment depends on the level of the class. The grand tlt matrix should have indicated a generalized approach to the study of the environment, pointed out the significant processes and reactions which occur, and emphasized the need for interdisciplinary teamwork and the relevance of several disciplines. Studying the Behavior of Particular Chemical Elements in a System
The generalizations developed above can be applied to more specific situations, using much the same matrix technique. A slightly modified matrix can he used to study the behavior of an element such as nitrogen in a lake. The steps in the formation of the N-matrix would he 1)
ldentifv the svstem:
2) List a
