Vaccari D.A., Strom P.F., Alleman J.E. Environmental Biology for Engineers and Scientists

Подождите немного. Документ загружается.

situ tec hniques. However, the key problem with these ex situ procedures is that they

require the initial excavation and relocation of the contaminated soils, and this effort

can rapidly become prohibitively expensive. Th e ﬁrst such option (Figure 16.74) is that

of slurry-phase soil bioremediation, where the extracted contaminated soil is combined

with water in an aboveground reactor and then mixed in batch fashion as a suspended

slurry to facilitate subsequent microbial degradation.

The microbes involved may not only include a variety of indigenous forms but also

specially acclimated organisms seeded into the reactor or recycled from prior treatment

batches. Nutrients and oxygen can, of course, be added to the reactor to optimize the

environmental conditions for the microorganisms that are degrading the contaminants.

After the desired level of contaminant removal is completed, the water is extracted

from the solids, and these water and solid residuals are then placed back into the ground.

This slurry-phase biological treatment scheme can provide a fairly rapid means of reme-

diating soil contaminants given its ability to maintain a homogeneous soil matrix and opti-

mal microbial contact, particularly when compared to the sort of slow-rate bioremediation

processes that might be achieved with in situ treatment of tight clay soils. However, there

is clearly a trade-off with this technology in terms of the beneﬁts of accelerated treatment

vs. the costs assoc iated with soil extraction.

The second ex situ soil bioremediation scheme (Figure 16.75) is completed as a solid-

phase proce ss in which the extracted, contaminated soils are placed in carefully engi-

neered, typically enclosed, aboveground mounds known as soil biopiles. These soil

piles are built with several meters of soil stacked vertically over an impermeable base

constructed with either well-packed clay, plastic liners, or geomembranes, as well as an

associated leachate collection system that ensures that the contaminants are not able to

leave the site. Air is then drawn through the pile using a vacuum pump tied into an under-

lying air extraction piping network. Moisture, temperature, nutrients, and oxygen levels

can then be controlled within these mounds to enhance their available rates of biodegra-

dation, and in some cases the mounds may also be mixed or turned on an occasional basis

to promote homogeneous remediation.

Figure 16.74 Ex situ engineered bioremediation using soil excavation and slurry-phase soil

treatment.

698

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

These solid-phase systems are somewhat easier to maintain and operate than is the case

with slurry-phase units, but they do require more space, and their metabolic rates are typi-

cally lower, due to their reduced effective microbial intensity. There is also a variation in

the original makeup of these piles in which the contaminated soils are mixed with various

forms of bulking material (i.e., straw, hay, corncobs, wood chips, etc.) to open the soil and

to make it more permeable to the passage of water, air, and nutrients. Here agai n, these

piles will often be developed with some form of internal aeration header, or alternatively,

mixed on a rout ine basis with mechanical turning to ensure that the pile contents are kept

aerated.

The third option for ex situ soil remediation is that of land farming. This relatively

simple process again involves the excavation of the contaminated soils, which are then

spread across land areas which are then tilled periodically much lik e farmlands to entrain

air and remix the actively remediating soil matrix. Moisture and nutrients are also mon-

itored and added as needed to promote the desired bioremediation. Given the fact that

most of these land-farming operations are maintained on unenclosed lands, there may

be instanc es in which the contaminant removal process could actually be attributable

more to volatilization than to biodegradation. If this is expected to be the case, where

the rate of contaminants would probably be unacceptably high, this process may have

to be shifted to an enclosed operation.

PROBLEMS

16.1. Call your state’s regulatory department for environmental management, protec-

tion, or health and identify a local municipal or industrial wastewater treatment

facility for which you could then obtain a 12-month span of com pliance reports

covering each of the following process streams: efﬂuent (liquid) discharge and

Figure 16.75 Ex situ engineered bioremediation using soil excavation and biopile process.

PROBLEMS 699

biosolids (residuals). Identify the stipulated parameters and values in each such

case (e.g., efﬂuent BOD, suspended solids, ammonium-nitrogen, fecal coliforms),

and then qualify the relative compliance or noncompliance maintained with each

such stream during the period of record. If, indeed, the facility has had any

noncompliance events, characterize the nature and extent of these periods. Finally,

obtain the necessary contact information (name and phone numbers) for this

facility by which you can then arrange a subsequent on-site guided tour.

16.2. Obtain permission for, and then complete, a guided tour of the previously facility

identiﬁed in Problem 16.1 and then complete the following steps: (a) prepare a

schematic and technical description of the liquid processing stream involved; (b)

identify the sources, characteristics (e.g., inﬂuent BOD, suspended solids), and

quantities (i.e., percent municipal versus commercial versus industrial) of this

facility’s various wastewater sources; (c) identify this facility’s typical removal

efﬁciencies for BOD, suspended solids, and ammonium-nitrogen; (d) quantify the

hydraulic retention times provided by each successive liquid processing step; and

(e) qualitatively describe the apparent environmental condition of this facility’s

receiving water body and characterize the probable impact of this incoming

discharge stream.

16.3. In conjunction with the site visit and tour completed for Problem 16.2, do the

following: ( a ) prepare a schematic and technical description of the sludge

processing streams involved; (b) qualify and quantify the daily wet sludge volume,

moisture content, and volatile solids fraction, as well as the daily dry solids mass;

tion, dewatering); and (d) describe the procedure(s) used for ﬁnal disposal of this

facility’s biosolids materials.

16.4. In conjunction with the site visit and tour completed for Problem 16.2, do the

following: ( a) prepare a schematic and technical description of the disinfection

process involved; (b) characterize the involved disinf ection strategy and desired

efﬁcacy; (c) quantify the hydraulic retention time provided by this disinfection

step; and (d) quantify the requisite levels of energy and chemical consumption.

16.5. Explain how the various processes used for water and wastewater disinfection

(e.g., chlorination, bromination, ultraviolet irradiation) produce a desired pathogen

inhibition or mortality, and then explain how these ‘‘disinfected’’ organisms might

effectively be considered ‘‘dead’’ (i.e., unable to reproduce viably), although they

exhibit catabolic activity (i.e., such that they still appear to be ‘‘alive’’).

16.6. Call your state’s regulatory department for environmental management, protec-

tion, or health. Identify the location of a nearby constructed wetland used for

either wastewater treatment or stormwater management, and obtain the necessary

contact information (name and phone numbers) for this facil ity. Arrange and

complete an on-site guided tour at this facility, and then do the following: (a) pre-

pare a schematic and technical description of the ﬂow scheme involved; (b) char-

acterize the distribution and forms of plants found within this system; and

16.7. Prepare written answers to the following questions regarding subsurface-ﬂow

constructed wetlands: (a) Why are these systems commonly built with washed

700 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

‘‘river’’ gravel as the internal mediaum rather than unwashed sand, soil, or gravel;

(b) What is the porosity of a washed gravel (in percent)? [Note: you might

consider running a simple test using a small bucket ﬁlled with washed gravel set

on a bathroom scale and then weighed with and without water ﬁlling the voids).

wetland ‘‘ages’’? (d) Is the latter change in porosity associated with aging of the

wetland and plant root biomas s growth considered during initial design and

retention-time analysis?

16.8. Complete a literature review (e.g., using a Web of Science search) regarding the

options for, and uses of, molecular biology procedures and tools (e.g., DGGE,

PLFA, FISH) in terms of environmental biology research and assessment (i.e.,

covering both fundamental and applied applications), and then prepare a written

synopsis of this information.

16.9. Complete a Web-based search for an available online case study report covering a

successful soil bioremediation operation, and then prepare a written synopsis of

this information, covering the following details: a) original site background,

conditions, and scal e; (b) site ownership and regulatory factors; (c) involved

technical procedures; (d) stipulated remediation efﬁcacy criteria (if any); (e) pro-

ject time frames (start date and expected closure date); (f) postremediation

characteristics and conditions; and (g) involved costs and funding sources.

16.10. Explain why it is often necessary to supply nutrients, including nitrogen and

phosphorus, during in situ soil bioremediation operations.

16.11. Complete a Web-based search for an available online case study report covering a

successful bioﬁlter operation for air-phase contaminant treatment, and then

prepare a written synopsis of this information, covering the following details:

(a) facility background and conditions [e.g., airﬂow rates, contaminant(s) and

concentration information]; (b) involved technical procedures (i.e., process

scheme); (c) bioﬁlter media form(s) and retention times; ( d) stipulated efﬁcacy

levels; and (e) performance details observed (e.g., efﬂuent contaminant levels).

16.12. Describe the beneﬁts that might be achieved with prehumidifying an airﬂow

stream passed through a gas-phase bioﬁlter, using a water spray scrubber, in terms

of physical, chemical, and biochemical attribute factors.

16.13. Complete a Web-based search for an available online case study report covering a

successful phytoremediation operation, and then prepare a written synopsis of this

information, covering the following details: (a) original site background, condi-

tions, and scale; (b) site ownership and regulatory factors; (c) applied plant forms

and growth patterns; and (d) chronological remediation efﬁcacy.

16.14. Prepare a table qualitatively covering the various mechanisms by which biological

factors and metabolism can negatively affect the aesthetic (e.g., taste, odor, clarity)

quality of our environment, and in each such case identify possible remedial or

corrective strategies potentially able to negate or obviate these problems.

16.15. Review the global biochemical nitrogen cycle and then describe the corresponding

impacts and/or beneﬁts of the following environmental situations: (a) oxidative

nitriﬁcation com pleted within wastewater treatment facilities; (b) reductive

PROBLEMS 701

denitriﬁcation completed within wastewater treatment facilities; (c) reductive

nitrogen ﬁxation maintained by algal growth within a polluted lake; (d) concen-

trated urine release, storage, and hydrolysis relative to conﬁned animal feedlot

operations; and (e) anhydrous ammonia application associated with farm fertilizer

operations.

REFERENCES

Anderson, T. A., E. A. Guthrie, and B. T. Walton, 1993. Bioremediation, in In the rhizosphere,

Environmental Science and Technology, Vol. 27, No. 13, pp. 2630–2636.

Asimov, I., 1989. Asimov’s Chronology of Science and Discovery, Harper & Row, New York.

Atlas, R. M., and R. Bartha, 1987. Microbial Ecology, 2nd ed., Benjamin/Cummings, Menlo Park,

CA.

Bailey, J. E., and D. F. Ollis, 1977. Biochemical Engineering Fundamentals, McGraw-Hill, New

York.

Baker, M. N., 1948. The Quest for Pure Water, American Water Works Association, New York.

Bitton, G., 1999. Wastewater Microbiology, Wiley-Liss, New York.

Bitton, G., and C. P. Gerba, 1984. Groundwater Pollution Microbiology, Wiley, New York.

Bryant, E. A., G. P. Fulton, and G. C. Budd, 1992. Disinfection Alternatives for Safe Drinking Water,

Van Nostrand Reinhold, New York.

CH2M-HILL, 1984. A Comparison of Trickling Filter Media, CH2M-Hill, Inc., Denver, CO.

Characklis, W. G., and K. C. Marshall, 1990. Bioﬁlms, Wiley-Interscience, New York.

Code of Federal Regulations, 1993. Standards for the Disposal of Sewage Sludge, 40CFR, Title 40,

Parts 257 and 503, U.S. government printing ofﬁce, Washington, DC.

Cole, S., 1998. The emergence of treatment wetlands, Environmental Science and Technology/News,

May 1, pp. 218A–223A.

Crites, R., and G. Tchobanoglous, 1998. Small and Decentralized Wastewater Management Systems,

WCB McGraw-Hill, New York.

Darleym, W. M., 1982. Algal Biology: A Physiological Approach, Blackwell Scientiﬁc, Oxford.

Datar, M. T., M. N. Rao, and S. Reddy, Vermicomposting: a technological option for solid waste,

1997. Management Journal, of Solid Waste Technology and Management, Vol. 24, No. 2.

Eikelboom, D. H., and H. J. J. van Buijsen, 1981. Microscopic Sludge Investigation Manual, TNO

Research Institute for Environmental Hygiene, Delft, The Netherlands.

Fox, J. C., P. R. Fitzgerald, and C. Lue-Hing, 1981. Sewage Organisms: A Color Atlas, Metropolitan

Sanitary District of Greater Chicago, Chicago.

Gaudy, A. F. Jr., and E. T. Gaudy, 1988. Elements of Bioenvironmental Engineering, Engineering

Press, San Jose, CA.

Grady, C. P. L., G. T. Daigger, and H. C. Lim, 1999. Biological Wastewater Treament, 2nd ed. Marcel

Dekker, New York.

Hammer, D. A., 1989. Constructed Wetlands for Wastewater Treatment: Municipal, Industrial and

Agricultural, Lewis Publishers, Chelsea, MI.

Harker, A. R., and Y. Kim, 1990. Trichloroethylene degradation by two independent aromatic-

degrading pathways in Alcaligenes eutrophus JMP134, Applied and Environmental Microbiology,

Vol. 56, No. 4, pp. 1179–1181.

Horan, N., 1990. Biological Wastewater Treatment Systems: Theory and Operation, Wiley, New York.

702

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

Jackson, L., 1997. Why choose phytoremediation: looking through the eyes of the customer, IBC’s

2nd Annual Conference on Phytoremediation, Seattle, WA, June 18–19.

Jenkins, D., M. G., Richard, and G. T. Daigger, 1993. Manual on the Causes and Control of Activated

Sludge Bulking and Foaming, Lewis Publishers, Boca Raton, FL.

Kostecki, P., and E. Calabrese, 1991. Hydrocarbon Contaminated Soils and Groundwater, Vol. 1,

Lewis Publishers, Chelsea, MI.

Metcalf and Eddy, 2003. Wastewater Engineering: Treatment and Reuse, 4th ed., revised by

G. Tchobanoglous, F. L. Burton, and H. D. Stensel, McGraw-Hill, New York.

National Research Council, 1993. In-Situ Bioremediation: When Does It Work? National Academy

Press, Washington, DC.

Palmisano, A. C. and M. A. Barlaz, 1996. Microbiology of Solid Waste, CRC Press, Chelsea, MI.

Rittman, B. E., and P. L. McCarty, 2001. Environmental Biotechnology: Principles and Applications,

McGraw-Hill, New York.

Schnoor, J. L., L. A. Light, S. C. McCutcheon, N. L. Wolfe, and L. H. Carreira, 1995. Phytoremedia-

tion of organic and nutrient contaminants, Environmental Science and Technology, Vol. 29, No. 7,

pp. 318A–323A.

Speece, R. E., 1996. Anaerobic Biotechnology, Archae Press, Nashville, TN.

Staps, J. J. M., 1990. International Evaluation of In-Situ Biorestoration of Contaminated Soil and

Groundwater, Report 738708006, National Institute of Public Health and Environmental Protec-

tion, Bilthoven, The Netherlands.

Strom, P. F., and M. S. Finstein, 1995. New Jersey’s Manual on Composting Leaves and Management

of other Yard Trimmings, New Jersey Department of Environmental Protection, Trenton, NJ,

81pp. [Available free online at http://www.state.nj.us/dep/dshw/rrtp/compost/front.htm]

U.S. EPA, 1998. Interim Enhanced Surface Water Treatment Rule, Ofﬁce of Water, U.S. Environ-

mental Protection Agency, Washington, DC.

U.S. EPA, 1999. Alternative Disinfectants and Oxidants Guidance Manual, Ofﬁce of Water, U.S.

Environmental Protection Agency, Washington, DC.

U.S. EPA, 2000. Constructed Wetlands Treatment of Municipal Wastewaters, National Risk Manage-

ment Research Laboratory, Ofﬁce of Research and Development, U.S. Environmental Protection

Agency, Cincinnati, OH.

Verschueren, K., 1983. Handbook of Environmental Data on Organic Chemicals, Van Nostrand

Reinhold, New York.

Water Environment Federation, 1991. Design of Municipal Wastewater Treatment Plants, Manual of

Practice 8, WEF, Alexandria, VA.

Water Environment Federation, 1996. Wastewater Disinfection, prepared by the Task Force on

Wastewater Disinfection, Technical Practice Committee, Municipal Subcommittee, WEF,

Alexandria, VA.

White, G. C., 1992. The Handbook of Chlorination and Alternative Disinfectants, Van Nostrand

Reinhold, New York.

Zehnder, A. J., 1988. Biology of Anaerobic Microorganisms, Wiley-Interscience, New York.

REFERENCES 703

THE SCIENCE OF POISONS

Toxicology is the study of physical or chemical agents that produce adverse effects

on biological systems. This deﬁnition is a bit overly broad for our purpose, since, for

example, it could include nontoxic oxygen-demanding substances that can kill ﬁsh by

depleting oxygen in a stream. In this chapter and the several that follow we only examine

agents that affect individual organisms directly. Indirect effects, such as eutrophication or

deoxygenation, are described in Chapter 15.

Furthermore, we are mainly concerned here with environmental pollutants. The ﬁeld of

toxicology also deals with other toxins, such as pharmaceuticals, food additives, and those

that occur naturally. Of particular interest are xenobiotics. Various forms of radiation, if

capable of deposit ing enough energy to break chemical bonds, can also produce toxic

effects. Radiation with sufﬁcient energy is called ionizing radiation or just high-energy

radiation. Examples include ultraviolet, x-ray, and gamma radiation from the electromag-

netic spectrum, and high-energy particles such as alpha or beta radiation (helium nuclei

and electrons, respectively) from radioactive decay.

This chapter focuses on the general principles of toxicological effects at and below the

organism level (e.g., biochemical, cellular, organ systems). In subsequent chapters we

detail higher-level effects such as ecosystem-wide changes, or organism effects that are

speciﬁc to particular groups of organisms, such as aquatic or mammalian.

Toxicology is an interesting combination of the qualitative and the quantitative. A

major activity in toxicology is examining exposed organisms to determine the ‘‘how’’

and ‘‘what’’ of a toxin’s effect: ultimately, it is hoped, to the molecular level of under-

standing. Another large area of activity is the measurement of toxic responses, in either

laboratory experiments or in ﬁeld measurements. These responses are usually studied in

probabilistic terms using the tools of statistics. Toxicity tests involve exposing organisms

Environmental Biology for Engineers and Scientists, by David A. Vaccari, Peter F. Strom, and James E. Alleman

704

to varying amo unts of toxicants and measuring the probability of a particular response at

each dosage.

Detection of cause-and-effect relationships between toxicants and possible effects in

human populations is the job of the intersecting ﬁeld of epidemiology. Epidemiology

is the study of disease occurrence. (Epidemiology is concerned not only with chemical

or physical agents of disease, but also other causes, such as infectious agents.) Epidemio-

logy uses statistical tools such as analysis of variance (ANOVA) and regression to detect

such relationships empirically. Empirical evidence, however, is not proof of cause and

effect. Proof of a direct effect requires conﬁrmation by independent evidence, such as

is provided by the other two toxicological activities. If all three point in the same direc-

tion, this makes a strong case. Quite often they are performed in the following order:

Epidemiology Detection of a correlation between exposure to a toxin

and some adverse effect, from ﬁeld obser vations

Bioassay Laboratory veriﬁcation of the cause-and-effect relation-

ship found by epidemiology

Biochemical/physiological Determination of the mechanism of toxic effect

The sequence above is, however, a reactive situation. We are increasingly using a

proactive stance, in which the bioassay comes ﬁrst. In this way we will know if a chemic al

is toxic, and how, and prevent exposure befor e it can occur in the ﬁeld.

17.1 MECHANISMS OF TOXICITY

We can disti nguish between the mechanism of toxicity and the toxic effect. The toxic

effect is the adverse reaction that is observed, whereas the mechanism is the underlying

process that leads to the effect. Put another way, the mechanism is the cause, usually at

the molecular level, and the effect is the result. In some cases, as in biochemical effects,

the distinction can be arbitrary.

Ultimately, toxins act by reacting with speciﬁc compounds in the organism. The result

may be an alteration to a metabolite, enzyme, or cell structure. The site where the toxin

acts is called the target tissue or target organ. The speciﬁc compounds in the target

tissue that the toxins react with are called receptors. Organs vary in their sensitivity

because they may differ in numbers or types of receptors, toxin transport, or organ func-

tion. For example, neurons depend on mitochondrial activity for their ATP and are there-

fore more sensitive to substances, such as carbon monoxide, that affect oxygen transport.

The liver and kidney receive more blood than do other organs, making them a frequent

target. Rapidly dividing cells such as bone marrow and intestinal mucosa are more

sensitive to genotoxins.

Some toxins are relatively nonselective; they exert their effect on any tissue they con-

tact. This tends to be the case for more highly reactive toxins, such as strong oxidizers,

irritants, or free-radical producers. Even nonselective toxins, however, can exert their

damage selectively either at the tissue ﬁrst contacted or at other susceptible tissues.

This may be the mode of action for chemical disinfectants such as chlorine.

Biochemical changes may be reversible or irreversible. Physicochemical interactions

tend to be reversible. Examples include carbon monoxide poisoning of hemoglobin and

narcosis induced by hydrophobic solvents. Covalent bonding and degradation reactions

MECHANISMS OF TOXICITY 705

between toxins and cellular substances are more likely to be irreversible. Examples are the

bonding of mercury, lead, and cadmium to sulfhydryl (SH) groups in proteins, and the

damage to lung cell components by ozone.

Covalent Bonding Examples of toxicity due to covalent bonding include protein dena-

turing by heavy metals and the interference of cyanide and sulﬁde with the cytochrome

system. Mer cury, lead, and cadmium bond to sulfhydryl (SH) groups in proteins. Since

these groups are important in protein folding, this causes denaturing of proteins, leading

to profound effects. HCN and H

S bond to iron in cytochromes. This halts the electron

transport system, which is responsible for most of the energy yield of respiration. Others

examples, which are described further below, are the reaction of pesticides with the

enzyme acetylcholinesterase and reactions involving cellular DNA.

Physicochemical Bonding Bonds between toxins and cellular substances involving

van der Waals, hydrogen, and polar bonds also create toxic effects by modifying normal

biochemical function. Examples are the complexation of metals and the effect of solvents

on plasma membranes.

Multiple physicochemical bonds with a single molecule or complex is called chela-

tion. Chelation can alter the distribution of substances, especially heavy metals, by

effectively changing their physicochemical behavior (solubility, lipophi licity, etc.). This

can affect excretion or can sequester substances within the organism. Enzyme activity

can be susceptible to this chelation since many enzyme cofactors are inorganic metals.

For example, the solvent carbon disulﬁde, CS

, reacts with amino acids to form

thiazolidone and thiocarbamate compounds, which chelate zinc. Rats exposed to CS

rapidly lost Zn in the urine due to the increased solubility of the complex. Thiozolidone

is also thought to make copper less available as an enzyme cofactor. The complexing

agent ethylenediaminetetraacetic acid (EDTA) is used in cases of lead poisoning to hasten

excretion of that toxic metal.

A phenomenon called metal shift is the effect in which a toxicant causes metals to

decrease in one set of organs and increase in another. For example, feeding vanadium

to rats at levels below 150 ppm resulted in iron moving into the liver and spleen, whereas

above 250 ppm it caused iron concentrations in those organs to fall as low as one-third of

the normal content. Metal shift might be caused by chelation or competitive absorption.

Enzyme Disruption The normal action of enzymes can be disrupted in a number of

ways. A toxin can bind, reversibly or irreversibly, to the active site or the binding site

of a cofactor, or it may bind elsewhere, causing conformational changes affecting the

active site. The toxin can also have its effect by reacting directly with a cofactor, changing

its reactivity or availability. For example, ﬂuoride inhibits enolase, one of the enzymes of

glycolysis, presumably by complexing with its Mg

2þ

cofactor. Finally, a toxin may mimic

a metabolite or cofactor, competing with the normal metabolite for the active site. This is

the case with ﬂuorocitrate, as described in Section 18.5.

An interesting comparison of effects occurs with two classes of insecticides, both of

which affect the same enzyme, one irreversibly and the other reversibly. Both of these

classes form covalent bonds with the neurotransmitter-degrading enzyme acetylcholines-

terase, inhibiting it (Figure 17.1). Symptoms produced are muscular twitching, weakness,

and ultimately paralysis. One class, the organophosphates, forms bonds that are practi-

cally irreversible. Carbamate insecticides, on the other hand, bond to the same enzyme

706 THE SCIENCE OF POISONS

OCH

acetylcholine

extremely

fast

HO C H

choline

CCH

extremely

fast

HO C CH

acetate

+ R

HO P

fast

extremely

slow

organophosphate

ROCNHCH

OCNHCH

ROH

HO O C NH CH

slow

slo w

carbamate

(a)

(b)

(c)

Figure 17.1

(a) Normal hydrolysis of acetocholine by the enzyme acetocholinesterase;

(b) reaction of organophosphate pesticides with

enzyme; (c

) reaction of carbamate pesticides with enzyme.

707