Chapter 1

Introduction to environmental chemistry

1.1 INTRODUCTION

Environmental science is a multidisciplinary branch of knowledge thatlends itself to a variety of approaches (ecological, biochemical, medical,toxicological, social, legislative, economic) in addition to the chemicaland, especially, analytical chemical standpoint adopted in this book. Inany case, environmental control calls for the joint action of social,political and economic powers, even though it relies on correct in-formation on pollutants that must be provided by analytical chemistryin accordance with the scheme of Fig. 1.1.

The analytical measurement system is a part of the overall environ-mental control system. As such, it must use appropriate methods and

EFFORTS IE F F R T Sl, A l

Fig. 1.1. The role of analytical chemistry in environmental control.

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Fig. 1.2. Environmental methods and techniques used to make analytical measurementsfor environmental control.

techniques for deriving quality information as the starting point foracquiring an exact knowledge of an environmental problem in order totake appropriate corrective or preventive measures (Fig. 1.2). The needfor environmental control has in turn fostered the development ofincreasingly sophisticated analytical methods intended to avoid "un-detected" items in analytical reports, as well as to improve qualitycontrol and assurance of measurements performed throughout theanalytical process.

Environmental chemistry can be regarded as a series of factors thataffect the distribution and interaction of elements and substancespresent in the environment, the ways they are transported and trans-ferred, and their effects on biological systems, among others. In thisrespect, the word "environment" can be used to refer to any physicalsupport that holds foreign particles, and encompasses the naturalcomposition of the support. Thus, air, water and soil, the essentialcomponents of the troposphere, hydrosphere and lithosphere,respectively, are the three natural environments also with a naturalcomposition that occasionally (or permanently) coexist with foreignelements known as "contaminants."

The role of chemical substances in controlling and regulating thebehaviour of animals and plants in natural ecosystems is a major re-search target at present. In fact, it constitutes a branch of ecology called"chemical ecology." The interactions of animals and plants with their

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TROPOSPHERE

HYDROSPHERE

LITHOSPHERE

Fig. 1.3. Physical supports that make up the biosphere.

environment can simplistically be interpreted as the release or secretionof substances that influence the behaviour, growth, etc., of otherorganisms in the group. These substances are called "allelochemicals"and include such compounds as juglone, produced by some eucalyptusspecies, and cineole, a terpene released by mint leaves that inhibits thegrowth of adjacent plants. Other substances called "pheromones" aresecreted to protect the releasing organism from predators or com-municate with other members of the same species. These concepts,however, are outside the scope of this book.

Let us thus assume our "world" or biosphere to be a system consistingof the three above-mentioned strata (lithosphere, troposphere and hy-drosphere), in close relation to one another (in fact their spans overlap tosome extent 1]). As can be seen in Fig. 1.3, which shows all possibleinteractions between the three strata, the two-way relationships est-ablished endow the system with a dynamic character that considerablycomplicates approaching and resolving the analytical problems it maypose.

One immediate inference is that each biosphere unit contaminatesthe environment and is contaminated by it. Consequently, each systemrequires control over immission and emission (the two are interrelatedvia transmission). Therefore, controlling environmental pollutionentails: (a) controlling emission by means of statutes or rules(standards, restrictions, penalties); and (b) controlling immission (e.g.via protection directives). Implementation of these measures entailsintercalating an analytical measurement or control unit in the process(Fig. 1.4).

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Fig. 1.4. Scheme of immission and emission processes, and their analytical control

1.2 DEFINITION OF CONTAMINATION AND POLLUTION

While contamination and pollution are widely used synonymously,Bowen [2] claims that contamination is the environmental release ofsubstances at measurable concentrations, whereas pollution hasperceptible effects, whether measurable or detectable, on livingorganisms. Accordingly, oxygen released during photosynthesis can beconsidered an environmental contaminant, and rose scent an environ-mental pollutant. Contamination can thus be regarded as the dispersionof a substance in the environment at a concentration that, if increased toa high enough level, can have undesirable effects or even produceenvironmental pollution.

Other authors have proposed establishing a borderline between pol-lutants and mere contaminants based on established quality standardsfor air, water and soil. Thus, based on the recommendations of the USEnvironmental Protection Agency (EPA), a contaminant is "any add-ition (or omission) that produces a deviation from the average com-position" and a pollutant is "any addition (or omission), whether ofnatural or anthropogenic origin, at a high enough concentration to exerta harmful effect on living beings (animals and plants) or things of socialinterest." Thus, CO2 in the troposphere can be considered an air con-

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taminant but not a pollutant since any harmful effects it may have at itspresent concentrations remain to be determined.

With a view to integrating pollutants in specific "systems", the abovedefinition is more suitable than a classification into "primary" and"secondary" pollutants.

The contamination concept therefore includes pollution, but escapesa definition in quantifiable terms. If contamination is taken to be therelease of substances at measurable concentrations or an addition ofsubstances that produces a deviation from the average composition,then one can term "negative contamination" anything that results in anegative deviation or a decrease in the concentration of a substance thatseparates it from its average composition. For example, chlorofluoro-carbons (CFCs) in aerosols produce a negative deviation because theydiminish the average concentrations in the biosphere through gradualdestruction of the ozone layer.

1.3 CLASSIFICATION

Except for radioactivity and noise, which are two categories of their own,all types of environmental contamination can be classified in terms ofthe chemical nature of the contaminating material. Thus, most environ-mental problems can be included in one of five broad categories, namely:

(a) Oil and oil waste related pollution.(b) Emergence of a biological oxygen demand from dispersion of

organic wastes such as sewage effluents and sludge, fecal matterpaper mill waste water, municipal rubbish, etc.

(c) Eutrophication of inland waters through soil nitrogen and phos-phorus losses.

(d) Environmental contamination by specific toxic compounds suchas inorganic acids and alkalis, organic pesticides in general, fun-gicides, herbicides, etc.

(e) Environmental dispersion of individual elements includingmetals, whether alone or in combination.

In practice, an environmental contamination problem may involvecontaminants belonging to more than one of the above categories. Forexample, metals are highly disperse in organic wastes, which in turncreate a biological oxygen demand.

This book places special emphasis on contaminants consisting oforganic compounds and metal elements in air and water (and, to a. lesserextent, soil, plants and other materials).

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Both natural and anthropogenic contaminants can be classified intotwo general groups, namely: inorganic and organic.

INORGANICCONTAMINANTS

ORGANICCONTAMINANTS

i Gases [toxic (C12, NH 3 )]

Metals Nutrients (alkaline and alkaline-earth elements)

Toxic (heavy metals)

|Anions Nutrients (halides, SO2, NO-, P anions)

Total organic matter

Hydrocarbons (PCBs, phenols)

Pesticides (chloride-containing compounds)

Surfactants

Organometals

etc.

Because our civilization relies heavily on metals, it has paid specialattention to their environmental dispersion. The pressing need to pre-serve non-renewable resources of metals - mineral ores except those ofiron and chromium may be depleted within the next 50 years - furtherstrengthens the need to prevent pollution problems posed by metaldispersion in the environment.

The dispersion of natural elements present at trace levels in thebiosphere affects the environmental system as a whole; however, atmos-pherically dispersed elements are eventually transferred from theatmosphere to the ocean or earth. The atmosphere is thus regarded as apotential vehicle for contamination of the hydrosphere and the earth'ssurface.

Some elements of industrial use eventually reach the ocean in largeamounts. However, the water volume oceans contain is so enormousthat any ecological effects go unnoticed unless dispersion is confined in avery limited space or delayed in time. Man, like other terrestrial ani-mals, lives on soil nutrients; therefore, soil composition is crucial to him.

Let us now analyse specific sources of contamination of air and water,the two most important natural environments.

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1.4 SOURCES AND EVALUATION OF AIR CONTAMINATION

The atmosphere is a gaseous mass of fixed composition: 78.09% N2,20.95% 02, 0.93% Ar, 0.03% C0 2, 5% water vapour, negligible amountsof Ne, He, Kr and Xe, H 2 traces, radioactive emissions, nitrogen oxidesand ozone. Any of these substances at a level exceeding its normalconcentration or indeed any other substance present in the atmospherecan be considered a contaminant.

Atmospheric pollution arises from excessive concentrations of foreignsubstances in air that are hazardous (or simply disturbing or un-pleasant) to living beings. Such substances can originate from natural orartificial sources.

1.4.1 "Natural" contamination

Contamination sources of natural origin lie in one of three broadcategories, namely:

- Substances of animal origin, produced in the course of fermentationor putrefaction processes, i.e. from decomposition of organic matter byaerobic or anaerobic microorganisms. The latter oxidize proteins in-completely, giving rise to unstable or fetid substances. This categoryincludes excretion and scaling products released by insects in someseasons (spring and early summer).

- Substances of vegetable origin, which include airborne pollen clouds(mainly of the pinaceae and gramineae families) that can be transportedover long distances and deposited virtually anywhere. This is also thecase with spores and fungi. Two other potential sources are ferm-entation and putrefaction processes involving vegetable substrates. Inaddition, wood and meadow fires pollute the atmosphere directly withsmoke clouds that can span hundreds of kilometres or indirectly withcondensation nuclei.

- Substances of mineral origin. These include fine desert sand, ashclouds formed from lava sprays in volcanic eruptions, marine mists thatact as condensation nuclei by agglomeration of various salts (e.g. NaCl,N2C12, CaCl2, KBr, NaI, NI), extraterrestrial dust from meteorites thatpenetrate the atmosphere, etc.

Civilization has also created a number of environmental contam-ination problems, foremost among which are industrial pollution andurban and rural pollution.

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1.4.2 Anthropogenic contamination

As noted above, there are three main types of anthropogenic con-tamination: industrial, urban and rural.

- Industrial contamination is produced, among others, by emissionsresulting from fuel burning. The offending contaminants include CO(which is toxic), CO2, incompletely burnt hydrocarbons in the form ofaldehydes and acids, nitrogen oxides, SO2, SO3 , and small amounts of3,4-benzopyrene (a carcinogen). The last three typically arise fromburning of gas-oil and industrial materials. Thus, the iron and steelindustry releases fine particles of iron oxide, hydrogen sulphide andgaseous hydrocarbons in addition to the above-mentioned gases. Also,the cement industry contaminates the environment with alkaline andalkaline-earth sulphates, Cr or Ni in small amounts, and inert alkalinedust composed of calcareous, marly and clayey particles that is especiallydisturbing and hazardous to people. The chemical industry dumps avariety of contaminants involved in the production of: (a) oil and oil-related products (hydrocarbons, ammonia, aldehydes, carbon oxides,H2 S); (b) phosphates and phosphorus fertilizers (phosphoric acid, am-monia, nitrogen-containing compounds); (c) sulphuric (SO2, N2 0) andnitric acid (NO and NO2); and (d) detergents (dust that is usuallyalkaline and irritating). Some of the gases produced by chemical manu-facturers are suffocating, corrosive or irritating (NH 3, C12, SO 2, NO2,NO), toxic (H 2S, CO, HCN, Hg) or dazzling (benzene, CC14, trichloro-ethylene). Finally, chlorofluorocarbons (CFCs) are gradually under-mining the ozone layer.

- Urban contamination comes from various sources including smoke,exhaust, rubber sole and tyre wear, sewage gases, cleaning products,kitchen activities, tobacco, and many others.

- Rural contamination arises from gases produced by fumigation,fertilization and soil culturing practices.

1.4.3 Definition of atmospheric pollutant

Atmospheric pollutants can occur in a variety of forms, namely:- Fumes, in the form of disperse solid colloid particles resulting from

condensation, sublimation, distillation, combustion or chemical reactionvapours, and rarely exceeding 1 tm in size.

- Dust, according to the International Labour Organization's def-inition, consists of solid particles that can be dispersed or suspended in

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the air by handling, grinding, cutting, drilling, abrasion, impaction,spraying, detonation or disintegration of inorganic and organic mat-erials. Coarse dust consists of particles larger than 5 jlm, which there-fore settle very rapidly by gravity in quiescent air. Fine dust is made upof particles smaller than 5 lm (those below 0.1 Am are in Brownianmotion and diffuse in the atmosphere very much like gases).

- Emanations, viz. suspended solid particles produced by conden-sation from the gaseous state that differ from the parent substances.

- Gases and vapours. A gas is any substance that exists in the gaseousstate at 25°C and a pressure of 760 mm Hg, and a vapour is one thatoccurs as a solid or liquid under these conditions.

- Mists are formed by suspended liquid drops resulting from conden-sation of a gas into a liquid or the disintegration of a liquid into itsdisperse state by atomization, frothing or splashing.

- Aerosols, which are liquid suspensions containing both fumes andextremely fine dust that can readily be transported by air.

The way an atmospheric contaminant disperses is determined by twometeorological factors, viz. the average wind speed and atmosphericturbulence. The latter involves horizontal and vertical swirls that canmix contaminated air with clean air around it; in this way, dispersionincreases with increasing turbulence. On the other hand, contaminantconcentrations in air are inversely proportional to the wind speed.

1.5 SOURCES AND EVALUATION OF WATER CONTAMINATION

The significance of water as an integral part of the environment arisesnot only from the fact that two-thirds of the earth's surface is coveredwith it, but also from other, equally important features. Thus, water(available from seas, oceans, rivers, underground ways and rainfall) isessential to man, as it is to all living beings and the atmosphericenvironment. Water is also crucial to agriculture and economy anddictates the location of many industrial facilities. The latter use water invast amounts for a wide variety of purposes including energy prod-uction, irrigation, refrigeration and washing; some employ it as a solventor even as a raw material. Underground (non-salty) water reserves arebeing used at a higher rate than they are being replenished by rainfall.

Ensuring availability of adequate amounts of water of appropriatequality is a crucial task in protecting the environment. While quite large,the amounts of water that are currently being used are only a small

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TABLE 1.1

Maximum allowable levels (mg/L) issued by WHO for drinking water

Chloride 250a Boron 0.3 Cadmium 0.003

Sulphate 250a Iron 0.3a Cyanide 0.07

Sodium 200a Manganese 0.5 Chromium 0.05

Aluminium 0.2 a 0.1a Mercury 0.001

Ammonium 1.5a Copper 2.0 Nickel 0.02

Hydrogen sulphide 0 .05 a 1.0a Lead 0.01

Nitrate 50 Zinc 3.0a Antimony 0.005

Nitrite 3 Fluorine 1.5 Selenium 0.01

Phenols 0.01 a,1 Free residual chlorine >0.5

0.04a,2 1.0

a

0.203 Barium 0.7

0.30a ,3 Arsenic 0.01

a Comfortable limit.1 Chlorophenol.2 2,4-Dichlorophenol.3 2,3,4-Trichlorophenol

fraction of all the water available; however, there is the certain risk ofcontaminating and hence rendering much of it unusable.

The purity requirements for water vary with its projected use. Themost stringent are those for drinking water and special industrial (e.g.foodstuff and pharmaceutical) applications. On the other hand, the largeamounts of water used by the refrigeration industry, for example, neednot be so pure. Underground water is currently the purest of all as it isprotected by subsoil layers. Analytical results for water can be used toestimate the types and concentrations of its contaminants and hence itsfitness for a given use. Maximum allowed levels for each contaminant -which are the concern of not only chemists, but also physicians, hygien-ists and biologists on account of the potential hazards to the flora andfauna - and the legal rules derived from them vary widely betweencountries. Table 1.1 shows the purity demands established by the WHOfor drinking water.

Let us briefly review the main sources of water contamination -which are to some extent common to air - in two large groups, viz.natural and artificial (anthropogenic).

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1.5.1 Naturally occurring impurities

Water contains 02 and CO2, which are indispensable for fish and aquaticgreen plants, respectively. However, it also acquires gases in the form ofcondensed water vapour that precipitates as rain water or snow, inaddition to dust particles that act as condensation nuclei. On contactingthe ground, a variety of foreign substances filter through it and reachthe water beneath, where they are found as contaminants. Thesenaturally occurring substances can be of two types, namely:

- Inorganic. This group includes (a) alkaline salts such as NaCl (nearsalt deposits), and sodium and potassium salts resulting from the de-composition of natural silicates (e.g. Na and K feldspar and micas); (b)calcium and magnesium salts, which determine the so-called "waterhardness" and are expressed in degrees or as a concentration (ppm) ofCaCO3 or CaO; (c) heavy metal salts (particularly Fe and Mn bicar-bonates); (d) anions such as fluoride (which aids in the prevention ofcaries at an optimal concentration of 1 mg/L) and iodide (of interest onaccount of its association with goitre).

- Organic. Organic contaminants are usually secondary as regardswater quality since they are largely eliminated in the self-cleaningactivity of water. Organic matter in water normally originates in thedecomposition of plants that reach it for some reason. Some cellulardecay and transformation products (lignins, peptins and albumins) arecollectively designated "vegetable mould" or "humus" and include acidcomponents called "humic acids." Algae, bacteria and other small org-anisms make up a special group or organic substrates present in water.Biological analyses for these organisms and their living conditions allowwater quality to be evaluated using the so-called "saprophyte system."

1.5.2 Artificial contaminants

This type of contaminant, introduced by human activities, can be clas-sified according to its origin and chemical composition.

According to origin, there are chiefly industrial and domestic waters -the latter are dumped into municipal sewers. The two are becomingincreasingly similar in terms of impurities owing to the expanding use ofchemicals (detergents, cosmetics and insecticides) by households. Agri-cultural waters usually contain insecticides and commercially availablefertilizers that are flushed by ground water if used in excess amounts.

According to chemical composition, artificial contaminants can beclassified into two groups: inorganic and organic.

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1.5.2.1 Inorganic contaminantsInorganic artificial contaminants are mostly one of five types, namely:

(a) Acids. The primary sources of increased acidity in waters are minedrainage and acid rain. The latter is dealt with in detail in Section 1.7.1.The former is typically associated with sulphur ores (particularly pyritedeposits). Water infiltrations in pyrite mines produce sulphuric acid:

2FeS 2 + 702 + 2H 2 0 - 2FeSO 4 + 2H 2SO4 (I)

The acidity is further increased if a small amount of ferrous ion isoxidized to ferric ion, with production of additional sulphuric acid:

4FeSO 4 + 10H2 0 + 02 - 4Fe(OH) 3 + 4H 2SO4 (II)

As the sulphuric acid proceeds through minerals and rocks it dissolvescalcium and magnesium compounds, which neutralize it in part, butincrease water hardness via the soluble calcium sulphate formed:

CaCO3 + H2S0 4 -- CaSO4 + H20 + CO 2 (III)

The acid can also dissolve other metal compounds and thus con-taminate the water with more foreign species.

(b) Ammonium salts, from fertilizer factories and others that use it intheir production processes. They can also originate in the biologicaldegradation of albumins and other species that are incompletely con-verted into nitrogen. A high NH4 content in water is harmful to fish,particularly at an alkaline pH, where it exists as NH3, and is especiallypoisonous.

(c) Sodium chloride and sulphate salts from potash mines, detergentsand artificial silk (rayon) industries (particularly Na2SO 4); from oil drillsin waters containing large amounts of NaCl; or from calcium compounds(especially CaCl 2, a byproduct in the obtainment of NaOH by the Solvayprocedure).

(d) Heavy metals, of which iron is usually present in the largestamounts and comes from acid fluids used by the iron and steel industryfor polishing iron wires, sheets and related products (the fluids dissolvesome surface iron oxide and metal iron). On the other hand, this pro-cedure is used to protect the environment by precipitating colloid sus-pensions in waste waters.

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Other metals and their compounds reach waste waters as purifi-cation, mineral extraction or processing residues. Such is the case of Cr,Ni, Zn and Cd from galvanization industries. Zinc can also originatefrom pipelines and Cu from copper processing industries. Lead can reachwaste waters as a fuel burning residue in road dust. Finally, con-taminating mercury comes from sodium lye factories using the amalgammethod, artificial fibre manufacturing processes (where it acts as acatalyst), thermometer and electrical appliance factories, and someinsecticides and pharmaceuticals.

(e) Anions. Sulphate and carbonate are the two most frequent con-taminating anions encountered in waters. The WHO recommends amaximum level in water of 250 ppm for both. Chloride ions are alsofrequently present in waters, and so are nitrates. The latter, which origi-nate as inorganic species at the end of the mineralization cycle fornitrogen-containing compounds, have more critical environmentaleffects. The greatest hazard is their conversion into nitrite ions byreduction (e.g. during digestion or in contact with Zn pipes). Nitrite ionis poisonous: it produces cyanosis. Nitrates can originate in landstreated with large amounts of nitrogen fertilizers, and so can phos-phates, which, however, are usually present in very small amountsowing to the low water solubility of their commercial fertilizer formu-lations. Rather, they come from domestic detergents, which typicallycontain abundant tripolyphosphates. If phosphate-contaminated watersreach lakes, they result in the much-feared eutrophication, which isdescribed in Section 1.7.2. In any case, cyanide is the most toxic andhazardous contaminating anion by far. It usually comes from industrialwaste waters, where it occurs at high concentrations. Thus, the gal-vanizing industry uses Zn, Ni and Cr salts in the form of complexcyanides that are passed into waste water during clearance procedures.Cyanide is also a component of Ag and Au electrolytic baths used byjewellers.

1.5.2.2 Organic contaminantsOrganic artificial contaminants come mainly from four differentsources:

(a) Oil and its derivatives, which can reach different types of water(sea, sewage, municipal sewer, river) in many different forms. Someoriginate in oil tanker wrecks; others, from fuel (used oil) dumping intorivers and ground waters. The solubility of these compounds in water,while limited, can play havoc with its quality. Benzol and its derivatives

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are somewhat more soluble than naphthenes and paraffins, for example.As a rule, the water solubility of these compounds decreases withincreasing molecular weight. Phenols and carboxylic acids formed byoxidation of hydrocarbons are highly soluble in water. Also, the solu-bility of some commercial products such as gasoline increases with theircontents in benzol and related substances (aromatic hydrocarbons),which can readily be detected from their odour.

The maximum allowed concentration of benzol is 5 mg/L (above thislevel, it is toxic to fish); those for gasolines range from 50 to 200 mg/L(depending on the particular type), that for naphthalin from 2.5 to 5mg/L, and that for diesel gas-oil from 50 to 100 mg/L - the product isinsoluble at the higher concentration, however.

(b) Cellulose debris. Waste water from cellulose processing factoriesusually contains the largest amounts of organic contaminants. Wood istreated with magnesium sulphite lye; while this is recovered, residuallye contains ligninsulphonic acid and soluble carbohydrates (mainlyhexoses), of which the former is the more difficult to degrade.

(c) Insecticides and pesticides. Insecticides present in waters comefrom agricultural applications (pest treatment), households, industries,etc. The most harmful effect of insecticides in waters is exerted byaccumulation in the so-called "nutrition chains" in a living medium. Infact, many insecticides are not degraded at all; they build up in animalorgans from which they enter the human nutrition chain. Long-terminsecticides, which are rather difficult to degrade (especially chlorinatedhydrocarbons such as DDT), are increasingly being replaced with othersthat are easier to decompose and rarely accumulate in the body (e.g.carbamic pesticides).

(d) Domestic detergents. Detergents are the primary source of dom-estic wastes in addition to fecal matter and meal leftovers. The inceptionof domestic detergents rapidly displaced traditional soap and posed aworrisome threat since the active substances they contain cannot bebiologically degraded rapidly enough. Rather, they accumulate in riversand form froth layers up to a few metres thick that hinder waterself-cleaning processes. Some countries have banned traditional deter-gents because they contain phosphates, which cause eutrophication; inresponse, manufacturers have developed new, biologically active deter-gents at essentially the same cost and with no detriment to cleaningpower. In any case, these new detergents also have some harmful effectson waters.

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1.5.3 Organic wastes with oxygen demand

Dissolved oxygen (DO) is indispensable for aquatic animals and plants.Its minimum acceptable level is 6 ppm. The amount of DO in watervaries with the degree of saturation, temperature and height. It istypically 9.1 ppm at 20°C at sea level, 8.2 ppm at 900 m and 7.4 ppm at1800 m [3] (Fig. 1.5).

Water heating decreases DO levels; this has a detrimental effect in theform of thermal contamination, which arises from climatic changes andthe use of water as a coolant in industrial processes, among others.Cooling water returned to a stream or river can raise its temperature byup to 12°C. Thermal contamination also increases the rate of chemicalreactions in the aquatic medium - the effect can be lethal in some cases.In any case, the primary source of DO depletion is the presence ofoxygen-demanding organic matter.

A given water mass is said to be contaminated when its DO content isbelow the level required to maintain its normal biota. The contaminantsit contains are readily degraded or decomposed by effect of bacterialactivity in the presence of oxygen. Most oxygen-demanding compoundsare of organic nature. Because they contain mainly carbon, bacterialaction produces carbon dioxide as the main oxidation product:

C 0 2 bacteria C02 (IV)

The reaction requires almost three times as much oxygen as carbon ispresent (32 g 02 per 12 g of C) to develop.

-j

m

0Lo

z

0

0 20 40

TEMPERATURE (C)

Fig. 1.5. Variation of oxygen solubility with altitude and temperature.

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TABLE 1.2

End products of the bacterial decomposition of organic matter in the presence and absenceof oxygen

Conditions C N S P

Aerobic CO2 NH 3 + HNO3 H2SO4 H3PO 3

Anaerobic CH 4 NH3 + amines H2 S PH3 and Pcompounds

While aerobic decomposition consumes DO, the effect of anaerobicorganisms on organic matter is even more detrimental (see Table 1.2).

The amount of oxygen-consuming contaminants in a given watermass is a significant parameter that can be measured via the bio-chemical oxygen demand (BOD); this is the amount of dissolved oxygenthat is used to oxidize oxygen-demanding residues and is measured byincubating a water sample at 209C for 5 days. The 02 uptake (BOD) isdetermined chemically by quantifying DO in the water before and afterincubation. Thus, a BOD of 1 ppm is typical of nearly pure water, 3 ppmof highly pure water and 5 ppm of water of dubious purity. A BOD above20 ppm suggests massive contamination and a serious hazard to publichealth.

While BOD is a measure of water quality, it takes a fairly long time todetermine and is accurate to within 20% only. Hence, it is usuallyreplaced or complemented with chemical oxygen demand (COD) andtotal organic carbon (TOC) determinations - the latter parameter isproportional to the total oxygen demand (TOD). In a COD test, theanaerobic bacteria used in BOD tests are replaced with a chemicaloxidant such as sulphuric K2Cr20 7, which ensures more powerful andthorough oxidation.

Total organic carbon is determined by catalytic burning at a hightemperature (900-1000°C). The resulting CO 2 is measured by using aninstrumental technique [4], which allows TOC to be determined within afew minutes. Figure 1.6 compares the methods used to measure thethree parameters. As can be seen, the COD profile lies above those ofBOD and TOC because the oxidation is more powerful and converts allorganic matter into inorganic substances. Also, the TOC profile liesbelow the other two as it only measures the fraction of organic andinorganic matter that is transformed into CO2. All these methods aredescribed in detail in Chapter 11.

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120-

(L 40-

0-

5 10 15 20 DAYS

Fig. 1.6. Hypothetical measurements of COD, BOD and TOC in waste effluents.

1.6 THE CHEMISTRY OF ATMOSPHERIC POLLUTION

The chemistry of atmospheric pollution involves the carbon, nitrogenand sulphur cycles, which are briefly described below.

1.6.1. The atmospheric cycle of carbon

Carbon dioxide is a natural component of air and thus regarded as anon-contaminant a priori. This gas is involved in a continuous cycle inthe atmosphere and outside it that is governed by the activity of animalsand plants. In the carbon cycle, plants use light to have CO 2 in air reactwith water in order to produce carbohydrates for storage and oxygen forrelease to the atmosphere; this process is known as photosynthesis.When plants are oxidized in natural decay, burning or animal feedingprocesses, oxygen in air is consumed and CO 2 regenerated (Fig. 1.7).

F OR MATO

tosynthe.;

H,OHYDROCARBONS

+ +CO

o 0,

Fig. 1.7. Scheme of the carbon cycle.

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ACOD

ICC

Therefore, atmospheric CO 2 levels remain fairly constant unless alteredby some human activity.

In fact, man alters the carbon cycle in deforesting - which decreasesthe available vegetable mass - burning fossil fuels and converting lime-stone into cement. Deforestation decreases the ability of nature toremove atmospheric CO 2, whereas the other two activities increase it.Most of the CO2 released to the atmosphere comes from industrialburning processes. The amount is on a steady increase that is giving riseto the "hothouse effect", one of the most severe environmental problemsfaced at present, which is described in the following section.

Anthropogenic carbon monoxide is usually the result of one of thefollowing chemical processes:

(a) Incomplete burning of compounds containing the gas. The form-ation of carbon oxides (COx) is a straightforward process only if thereactants are pure carbon and oxygen. The burning process is highlycomplex; however, it can be simplistically represented as follows:

k,

2C + 02 -~ 2CO (V)k2

2CO+ O - 2CO2 (VI)sunlight

Reaction (V) is 10 times faster than (VI), so CO is an intermediate inthe overall reaction that tends to accumulate if the fuel and air arepoorly mixed and local oxygen-deficient zones are present as a result.

(b) A chemical reaction between CO2 and a carbon-containing mat-erial at a high temperature:

high T

CO 2 C --- 2CO (VII)

This reaction takes place very rapidly at high temperatures (e.g. in ablast furnace, where CO is used as a reductant to produce iron from itsores (iron oxides). However, part of the gas may leak into the atmos-phere and contaminate it.

(c) Dissociation of CO 2 into CO and 02 at a high temperature. Underfavourable conditions in the presence of enough oxygen for completeburning, this reaction can act as a source of carbon monoxide. In fact, ata high temperature, CO 2 and CO are in equilibrium:

high T

CO 2 T- CO+O (VIII)

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Therefore, if a mixture of the two is abruptly cooled down, any COpresent will remain since the time needed to restore equilibrium at a lowtemperature is very much longer.

The fact that atmospheric CO levels remain fairly constant suggeststhat part of the gas is removed by some mechanism. Atmosphericreactions are known to be too slow to eliminate significant amounts ofCO. Thus, reaction (VI) only removes 0.1% of available CO per hour ofsunlight - which is an indispensable reaction ingredient. Oceans arenow known to be CO sources rather than sinks. In addition, higherplants have never been shown to be able to abstract CO from theirsurrounding atmosphere. Finally, many soils can withdraw atmosphericCO via a biological mechanism; in fact, 14 fungal types are known to beinvolved in the removal of CO by facilitating or effecting its oxidation toCO2 .

The effects of carbon monoxide on plants are insignificant; exposinghigher plants to CO vapours at concentrations up to 100 ppm for 3 weekswas found to have no detrimental effect on them - in addition, the usualatmospheric CO concentrations are much lower than those tested. Onthe other hand, CO has a much more harmful effect on man; above 100ppm, the gas can be lethal. The effects at levels well below 100 ppm (theusual atmospheric concentration) have started to be unveiled.

Carbon monoxide is a health hazard on account of its ability to reactwith hemoglobin (Hb) in blood to form carboxyhemoglobin (COHb),which reduces the ability of blood to transfer oxygen (O2Hb) from thelungs to somatic cells and CO2 (CO2Hb) from the latter to the former.Because the affinity of CO for Hb is over 200 times higher than that of02, COHb will be preferentially formed over O2Hb. A COHb level of10-80% in blood results in headaches, fatigue, sleeplessness, breathingfailure or even death. The COHb content in blood is directly pro-portional to the concentration of CO in inhaled air.

1.6.2 The atmospheric cycle of nitrogen

Most NO and NO2 of anthropogenic origin results from oxidation ofatmospheric nitrogen in burning processes at a high temperature.Atmospheric oxygen and nitrogen react to yield NO, which furtherreacts with oxygen to form NO2 according to the following scheme:

N2 + 2 - 2NO (IX)

19

AIR

+0

SOLAR /\UV hv + NO2 -- NO + 03 ---- 2 AR

LIGHT

Fig. 1.8. Photolytic cycle of nitrogen.

2NO + 02 -> 2NO2 (X)

Both NO and NO2 are toxic and take part in atmospheric photochemicalreactions. The two are designated collectively NOx . The amounts of N20in the atmosphere are usually negligible; in addition, this is a non-toxicgas that is involved in no photochemical reaction, nor produced by manin significant amounts.

Once in the atmosphere, NO and NO2 enter a sequence of naturalphotochemical reactions that diminish the concentration of the formerand augment that of the latter. The interactions between sunlight andNO2 are known as the "photolytic cycle of NO2 ." The cycle involves thefollowing steps (Fig. 1.8): (1) NO2 absorbs energy in the form of solar UVradiation; (2) the energy absorbed cleaves NO 2 molecules into NO mol-ecules and oxygen atoms (O), which are highly reactive; (3) atomicoxygen reacts with atmospheric oxygen (02) to give ozone (03), which isa secondary contaminant; and (4) ozone reacts with NO to yield NO2 and02, thereby closing the cycle. The reactions involved are thus as follows:

hv

NO 2 - NO + O (XI)

0 + O2(air) 3 (XII)

03 + NO - NO 2 + 02 (XIII)

The net effect of this cycle is the rapid recycling of NO 2, so, in theabsence of competing atmospheric reactants, the cycle need not have anet effect. The environmental concentrations of NO and NO2 musttherefore remain constant since NO and 03 are probably formed and

20

destroyed in identical amounts. The competitive reactions in questioninvolve hydrocarbons released by the same sources as NOx which un-balance the cycle; NO is converted into NO2 more rapidly than the latterdissociates into NO and O as a result of hydrocarbons reacting with Oand 03 to give free radicals that are the precursors for photochemicaloxidants. This also gives rise to other significant oxidants that make upphotochemical smog.

Atmospheric NOx species are primarily eliminated by conversion intonitric acid. This is the origin of acid rain, which is described in detail inSection 1.7.1, and the chief mechanism for NOX removal from theatmosphere. The mechanism by which nitric acid (acid rain) is formedfrom NOx remains somewhat obscure. It is believed to involve a directinteraction between NO2 and H20 according to

2NO2 + H20O HNO 3 + HNO2 (XIV)

3NO2 + H2 0 -2HNO 3 + NO (XV)

These reactions are not fast enough to account for the amount of HNO3produced. One alternative, rapid mechanism, involves a reaction bet-ween NO2 and 03 to yield the intermediate N205, which could sub-sequently be dissolved in H 20 to give HNO 3 according to

2NO2 + 03 N2 0 5 + 02 (XVI)

N205 + H20 - 2HNO 3 (XVII)

Available data suggest that soil can withdraw NO2 from the air,thereby effecting natural elimination. Soil might also be a sink for NO,as it is for CO via a fungus-mediated mechanism.

Both NO and NO2 are detrimental to plants, man and materials. Infact, exposure to NO2 causes necrosis of plant tissues. NO decreasesphotosynthesis (CO2 absorption) by 60-70%. These effects are observedat high concentrations since the effects of low NO, levels (below 1 ppm)on plants remain to be ascertained.

On the other hand, NO_ are known to affect human health and belethal at high enough concentrations (both NO and NO2 are toxic toman). No single case of human poisoning by NO has even been reported,however. This gas is neither irritating nor hazardous at its normalatmospheric concentrations. The real danger arises from its oxidation to

21

NO2 . Atmospheric NO 2 is not detrimental to health at "normal" levels;however, an excessive concentration can result in olfactory perception,nasal irritation, disturbances or acute pain of the lungs, pulmonaryedema (fluid accumulation) and ultimately death.

NOx have a bleaching action on dyes and fabric. Also, high nitratelevels can result in tensile failure through corrosion of the Cu-Nitwisted-pair wires typically used in the relays of telephone networks.

1.6.3 The atmospheric cycle of sulphur

Sulphur is one other major element in the chemistry of atmosphericpollution. Sulphur dioxide that reaches the atmosphere by burning ofsulphur compounds (industrial origin) or from volcanic eruptions(natural origin), and H2S from rotting plants or industrial processes, areoxidized to S03, which subsequently reacts with water vapour to formsulphuric acid according to the following simplified reaction sequencefor the formation of SO, and H2SO4:

S + 02 - SO 2 (XVIII)

2SO2 + 02 - 2SO3 (XIX)

SO 3 + H20 F- H2 SO4 (XX)

Sulphuric acid in turn can react with atmospheric NH3 to give(NH4 )2SO4. Sulphates are collected or flushed by rain water (as is H2SO4itself), thereby contributing to acid rain. Figure 1.9 schematizes theatmospheric cycle of sulphur. Hydrogen sulphide is rapidly oxidized toSO2 via ozone naturally present in the atmosphere as a result of adisruption in the catalytic cycle of nitrogen.

Reaction (XIX) proceeds to a limited extent at room temperature.Also, at the typically high combustion temperatures (1200gC), the in-stability of SO 3 causes the process to revert. Accordingly, this reactionshould take several days to produce SO3 from atmospheric SO2 . This isnot the case since the reaction has been shown to complete within a fewhours as a result of (a) catalytic oxidation and (b) photochemical oxi-dation'. The former is believed to take part largely in solution (withinwater drops) or after the gas is absorbed on the surface of suspendedsolid particles; however, this is still to be ascertained.

22

SOx FORMATION

I 1 IO0 H,O NH, CATALYTIC PHOTOCHEMICAL

H,S SO, .-. SO, _-- HSO, (NH,),SO, OXIDATION OXIDATION(Mn, V, Fe, Cr, Cu)

S

Z65

z1

'///'/i/ ///

Fig. 1.9. Scheme of the sulphur cycle.

The catalytic oxidation of SO2 in water drops is believed to take placevia molecular 02 as the oxidant, Fe and Mn salts acting as catalysts:

Fe,Mn

2 SO2 + 2 H20 + 02 - 2 H2 SO4 (XXI)

The process is started by the SO3 initially formed rather than by SO2

- the latter has a high affinity for water and is thus rapidly dissolved.This results in the formation of a mist of sulphuric acid droplets thatincreases in size as it collides with more water molecules. The mist actsas a dissolution medium for SO2 and 02, and is largely responsible for thehazy appearance of sulphur-contaminated air. The mist can also dissolvelarge amounts of SO2 , the water solubility of which exceeds those ofother air contaminants (100 mL of water at 202C can dissolve 11.3 g ofSO2 but only 0.0006 g of NO and 0.003 g of CO, for example).

Iron, manganese and other metals (Cr, V, Cu) are frequently presentin coal fly ash. The oxygen-mediated conversion of SO 2 into H2SO4 inwater containing V5 + can be explained with the aid of the followingmechanism [5]:

2V 5+ + SO2 + 02- - 2V4 + + SO 3 (XXII)

SO 3 + H20 - H2SO4 (XXIII)

23

I

2V 4 + 0 2 2 + 0 2-2 2 T±2 V+

the overall reaction being

SO2 H20 + 0 2 -- H2SO4 (XXV)

Chromium oxide, ferric oxide and oxides of other metals are alsoknown to be catalysts for the conversion of SO 2 into sulphate ion. Inaddition, Mn(II) and Cu(II) play a prominent role in the environmentalproduction of H2SO4 .

At a high acid concentration (above 1 M), the oxidation process stops;this is believed to be partly due to the low solubility of SO2 in acids.However, in the presence of metal oxides or NH3 (which is naturallypresent in the atmosphere and highly soluble in water), the oxidationprocess is resumed because both types of substance neutralize theexisting acidity:

MgO + H2SO4 - MgSO4 + H2 0 (XXVI)

2NH3 + H2 SO4 - (NH 4 )2SO4 (XXVII)

The photochemical oxidation of SO2 may be a faster process. It resultsfrom interactions of the oxide with oxidants in photochemical smog. Theformation of natural mists in urban environments decreases SO2 levelsand increases atmospheric sulphate contents. The main photochemicaloxidants are 03 and peroxyacid nitrates (R-CO-O-O-NO2, where R is ahydrocarbon residue that is atmospherically produced from a hydro-carbon with the aid of light). These oxidants oxidize substances that areresistant to atmospheric oxygen.

SOx species (whether SO2 or SO3) have detrimental effects of man,animals and plants. High concentrations of SO 2 produce necrotic areasin plants that ultimately dry up and take on a whitish or ivory colour. Atlow concentrations, leaves turn yellow by effect of SO2 hindering chloro-phyll synthesis. Sulphuric acid mists also damage leaves; in contact withmist or dew-sprinkled leaves, the acid leaves yellow stains.

Occasionally, the joint action of two contaminants is more detri-mental than the summation of their individual effects by virtue ofsynergy. Such is the case with SO 2 and 03, as well as with mixtures ofSO 2 and NO2. Synergistic effects account for the discrepancies observedbetween laboratory results (based on the individual action of contam-

24

(XXIV)

inants) and the actual behaviour of plants exposed to contaminantmixtures in a natural environment.

Most of the health hazards posed by SOx affect the respiratory system.Concentrations between 8 and 20 ppm gradually irritate the throat andeyes, and provoke immediate coughing. There is a clear-cut correlationbetween the incidence of respiratory infections among children andenvironmental SO2 levels. Sulphates are even more hazardous; in fact,even at very low concentrations (8-10 1tg/m 3 ), they have adverse effectson asthmatic patients, old people and individuals with chronic resp-iratory complaints.

As regards effects on materials, paint drying and hardening timesincrease with exposure to SO2. The corrosion of metals such as iron, steeland zinc is accelerated by the presence of the oxide. Also, sulphuric acidattacks building materials. Thus, carbonate-containing materials arereadily converted into soluble sulphates:

CaCO, (limestone) + H2SO4 - CaSO4 + CO2 + H2 0 (XXVIII)

Leather absorbs SO2; as a result, it becomes more fragile and even-tually cracks. Also, SO2 absorbed on paper is oxidized to H2 SO4 , whichturns it whitish, fragile and brittle.

1.7 PROBLEMS ARISING FROM WATER AND AIR POLLUTION

This section deals briefly with some of the more important environ-mental challenges faced at present in chemical terms.

1.7.1 Acid rain

The origin of acid rain is the presence in air of SO2 and NO. released infuel burning processes (especially in industrial areas); as water vapourcondenses, it sweeps the acids formed between these compounds andwater, viz. H2SO4 and HNO3 (Fig. 1.10), according to the above-describedmechanisms. Alternative, more detailed pathways for the conversionshave been provided [6].

Cloud particulates act as condensation nuclei onto which water con-denses or ice is formed. This is one inherent way in which air self-cleans:the so-called "rain-out" effect. One alternative mechanism involvessweeping of atmospheric particulates by falling rain drops or snow flakes

25

Fig. 1.10. Acid rain production.

that deposit them on the ground; this is called the "washout" effect andis an effective means for removing atmospheric particles larger than1 jim in diameter. Smaller particles are swept out of the rain way by thewind. On the other hand, the rain-out effect is the origin of water acidity.

The "acid rain" concept has been expanded to "acid precipitation",which includes not only "wet precipitation" (viz. pollutants deposited byrain water or snow), but also "dry precipitation" (gaseous or solidpollutants such as aerosols and dust) in the absence of water.

The normal pH of water, 5.6, is the result of the presence of CO2 in theair; the gas is dissolved to form carbonic acid. However, pH values as lowas 4 have been measured in acid rain. The acidity of rain water is thus ameasure of air quality. It should be borne in mind that a change in pH byone unit entails a ten-fold change in the hydrogen ion concentration.Thus, if the water pH decreases from 5 to 4 it is as the result of the H+

supply being increased by a factor of 10. Acid precipitation has con-siderably increased since the beginning of this century owing to thegrowing amounts of contaminants such as NOx and SO, that are beingreleased to the atmosphere. Acid rain pollution levels in Europe peakedin the late 1970s [7]. International agreements signed afterwards haveled to diminished release of sulphur oxides and hence to decreased acidprecipitation in recent decades.

Acid rain is detrimental to ecological systems. It can diminish soilfertility by removing of essential nutrients. There are reported cases offish deaths caused by highly acid rain water.

One other adverse effect is weathering of stone monuments (seeScheme 1.I). Studies have revealed the effects to vary in strength andspeed depending on the building material concerned [8]; thus,

26

EFFECTS OF ACID PRECIPITATION I

DECREASED SOIL FERTILITY

HINDERS AQUATIC LIFE

DAMAGES STONE MONUMENTS

MECHANICAL DAMAGE

CRYSTALLIZATION OF.T. I RTINIF .iF'

Scheme 1.1. Adverse effects of acid precipitation.

calcareous substances in marble and related materials (limestone,sandstone and granite), which are among the most frequently usedfor building monuments [51, have been found to deteriorate via twoprincipal mechanisms: (a) chemical dissolution of calcite; and (b) themechanical action of soluble salts formed during dissolution, whichsubsequently crystallize in stone pores. The extent to which eachmechanism contributes to weathering of a statue or monument isdifficult to evaluate accurately.

The thickness of embedded SO2 and NO2 layers in marble is ameasure of the rate at which the material is attacked by these pollu-tants. Calculated constant rates allow the thickness of embedded layersto be calculated over a wide range of SO 2 and NO2 concentrations [9].Experiments have shown that, even in the presence of excess atmos-pheric NO2 over SO 2, the amount of CaSO4 produced exceeds that ofCa(NO3)2, thereby indicating that SO 2 has a stronger weathering effecton marble relative to NO2 [9]. This may be the result of metals in fly ashacting as catalysts in the oxidation of sulphurous acid to sulphate ion [5]according to reaction (XXV) above.

Finally, acid rain has elicited abundant literature including proposalsfor control strategies [10], papers and reports [11], and editorials inspecialist journals [12].

1.7.2 Eutrophication

Eutrophication is caused by substances containing phosphorus or nitro-gen (e.g. detergents including phosphates in their formulations) on

27

l

f

TABLE 1.3

Composition of commonplace domestic detergents

Surfactants (e.g. alkylbenzenesulphonic acids) 10-15 %

Polyphosphates pentasodium triphosphate) 30-40 %

Bleaching agents (sodium perborate) 20-30 %

Optical brighteners 0.1-0.3 %

Anti-greying agents (methylcarboxycellulose) 0.5-2.0 %

Anti-corrosion agents (potassium silicate) 3-5 %

Anti-foaming agents (behenate) 2-3 %

Stabilizers (ethylenediaminetetraacetate, magnesium silicate) 0.2-2.0 %

a "Biologically active" detergents contain 1-2% of enzymes.

dumping into waste waters [13]. The phenomenon reflects in excessive,uncontrolled growth of algae and aquatic plants. While eutrophicationenriches waters with nutrients, the phenomenon becomes an environ-mental threat if it takes place abruptly to an exceedingly large extent.

In fact, algae and aquatic plants containing excessive amounts ofnutrients are not only unpleasant to the eye and unfit for recreationaluses, but also a source of disagreeable odours and flavours as theycorrupt and eventually die, producing oxygen-demanding organicmatter in the process. Not only phosphorus and nitrogen, but also othernutrients such as K, S and C can result in eutrophication if theirconcentration is allowed to rise in an uncontrolled manner.

One of the main sources of eutrophication is detergents. As theygradually supersede soap, they are posing a worrying threat becausethey cannot be biologically degraded at a high enough rate; in addition totheir eutrophic action, residual detergent forms thick froth layers thatimpede water self-cleaning. The eutrophic effect of detergents is due tothe presence of polyphosphates (Na5 P 301 0 ), which are hydrolysed tonormal (and non-toxic) orthophosphate:

P3 0lo + 2H2 0 4-2HPO- (XXIX)

Similarly, perborate releases 02 and becomes normal borate. Theother components of detergents (mostly organic compounds) increasethe organic matter content in water until they are thoroughly degraded.Table 1.3 shows the typical composition of conventional detergents.

The action ofPO4- ion is in principle advantageous as it prevents theformation of metal complexes - thereby reducing water hardness -

28

and enhances detergency, in combination with its own cleaning power.However, phosphates dumped into waste waters eventually result ineutrophication (particularly when they reach large water masses such asthose in lakes).

There have been several attempts at replacing phosphates with othertypes of compounds in detergent formulations. Nitrilotriacetic acid(NTA) is an inexpensive, readily degradable alternative that also formssoluble, stable complexes with metals (particularly Ca and Mg). It wasintroduced in the USA in the early 1970s but was banned at the end ofthat decade because, according to one report, it caused fetal anomalieson injection as a Cd complex to mice. While the concentration levels usedin detergents were later shown to be completely harmless, the ban wasnever lifted in the USA, even though NTA use was allowed in othercountries including Canada, Finland and Sweden.

In any case, whether NTA is completely safe remains uncertain.There is evidence that substituting it for sodium tripolyphosphate leadsto NTA concentrations of 10-20 ppm in domestic waters when in factbiological degradation of the compound is incomplete (90%) above 8ppm. This has an adverse effect on water purification processes intendedto remove heavy metals, which are readily complexed by NTA. Finally,NTA degradation products include carbon- and nitrogen-containingsubstances that are plant nutrients and may cause additional problems.

1.7.3 The ozone hole

This environmental problem is also caused by organic pollutants: chloro-fluorocarbons (CFCs), which produce "negative" contamination as theydeplete the ozone layer in the stratosphere and allow more solar UVradiation to pass through.

After several years monitoring the ozone layer, the British AntarcticSurvey measured an anomalous drop in 03 levels in 1982. In 1984,member scientists of this institution confirmed the trend for such levels todecrease [14]. Soon, CFCs were confirmed to be responsible for graduallydestroying the protective ozone layer over the Antarctic [15,16]. Currentozone levels in the layer are roughly one-half of those measured in themid-1970s. Recent reports continue to reflect ozone losses over the Arctic,though at a much lower rate than in the 1980s [17]. The ozone layer liesin the stratosphere, 15-20 km above the earth's surface. Because atmo-spheric chemistry involves light-sensitive reactions, experiments mustbe carried out during the day and night [18].

29

Chlorofluorocarbons were developed on a commercial scale in the1930s as safe alternatives to SO3 and NH3 used as coolants (NH3 isinflammable and both are toxic). Since then, use of CFCs has beenextended to other applications including cleaning of electronic andmechanical components, plastic swelling in foam rubber manufacturing.The initial success of CFCs relies on a number of assets; thus, thesecompounds are uninflammable, non-corrosive, non-explosive and veryscarcely toxic, and have an adequate vapour pressure and flexibility thatis compatible with many building materials. Their high stability andchemical inertness soon led to massive use that was eventually re-stricted on scientific grounds only. Among other effects, their highchemical stability hinders the formation of photochemical oxidants inurban areas [19].

One of the major applications of CFCs as coolants is storage, distri-bution and transport of foodstuffs, blood, drugs and donated organs. Italso helps increase home living comfort and work productivity. Theirlow toxicity, non-inflammability and efficient dispersion also makesthem suitable aerosol propellants [19].

Overall, CFC production and consumption have been restrictedworldwide [19]. In fact, their use in aerosols in 1988 was less thanone-third that in 1974. As a rule, CFC consumption rose until 1974,when the effects on the ozone layer were first observed.

Some of the above statements are seemingly paradoxical: if CFCs areso highly stable, how can they possibly destroy the ozone layer? In whatway? In fact, there is no proven decomposition mechanism for CFCs inthe lowest atmospheric stratum (the troposphere). According toRowland and Molina, of the University of California, CFCs may remainin the atmosphere until they are transferred to the stratosphere, wherethey may be photochemically decomposed with release of chlorine atoms[19]. Subsequently, via a series of photochemical reactions, the chlorineatoms would interact with ozone to give 02 and the intermediate CO1*,which has been detected elsewhere and is crucial to the catalytic loss(destruction) of O.. The mechanism can be schematized as follows [18]:

CFC h Cl + R (XXX)

Cl* +03 - ClO + 02 (XXXI)

ClO + 0 -Cl + C* + 02 (XXXII)

C1 + 02 - P (XXXIII)

30

catalytic| I , reactions

(280-310 nm) hv '

// / z/

I/ ,'·'

I .I , ,

,.,','/ EARTH \'.

Fig. 1.11. Schematic depiction of ozone layer formation.

Because 03 absorbs UV radiation at 280-310 nm, a net decrease in theamount of 03 may increase the amount of solar UV light that falls on theearth's surface and hence have an adverse effect on animals and plants.The process is schematically illustrated in Fig. 1.11. Alternative reactionmechanisms and the influence of atmospheric NO, in the process havealso been discussed.

Stratospheric ozone levels are measured by means of various tech-niques, namely [18]: UV absorption, chemiluminescence (ethylene + 03-- hv, Xmax = 450 nm), electrochemical methods (amperometry), etc. TheDobson spectrophotometer has been used for stratospheric measure-ments since 1950. The amount of 03 is measured in Dobson units (DU, 1DU = 10- 3 atm/cm = 2.7 x 1016 molecules/cm 2 ). The spectrophotometeruses two monochromators to calibrate incident radiation at two dif-ferent wavelengths. Measurements are made from the ratio of the 03absorptivity at the two selected wavelengths (e.g. 305.5 and 325.4 nm) inorder to eliminate Rayleigh dispersion.

After the Montreal protocol was signed in September 1987 and rat-ified in January 1989, the signatory countries agreed to reduce CFCconsumption by 50% (of the 1986 figure) over a 10-year period [19]. Theimportance of the problem has called for the cooperation of NATO inozone layer protection programmes [20].

Meanwhile, development of CFC substitutes [19,21] such as hydro-fluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs), whichhave much shorter decay half-lives (2-20 years versus 100 years forCFCs) and hence are bound to introduce little chlorine into the strato-sphere, has been promoted. In any case, one must consider whether thedecomposition products remaining in the troposphere are all harmless.

31

Because HFCs contain no chlorine, there is theoretically no reason whythey should interact with 0 3 .

1.7.4 The hothouse effect

The hothouse effect arises from the fact that release of increased con-centrations of CO 2 to the atmosphere in human activities can result inclimatic changes through alterations of the earth's surface temperature.

Nearly all the light that reaches the earth's surface is in the visiblespectral region and is returned to the atmosphere as light of a longer(infrared) wavelength in the form of heat. Because CO 2 absorbs infraredradiation, the more carbon dioxide exists in the atmosphere, the moreheat will be retained and the hotter the atmosphere will be. The hot-house effect thus originates from an interaction between increasingamounts of atmospheric CO2 and light leaving the earth.

In fact, carbon dioxide acts as a one-way filter; consequently, one canpredict that the temperatures of the atmosphere and the earth will riseas the amounts of atmospheric CO2 grow. Available data reflect a cease-less trend to increasing 02 levels. In a recent report [71, an increase inthe CO2 concentration by a factor of 2 or even up to 4 - depending onspecific feedback mechanisms - each century was predicted. Also, theaverage temperature could rise by 3.5-5.3°C each century as growingamounts of CO2 accumulate in the atmosphere, oceans and terrestrialbiota.

However, the actual decrease in average annual temperatures seemsto contradict the predicted effects of the increased atmospheric concen-trations of CO2 measured each year. This may be the result of increasedcloudiness and particulate concentrations in the atmosphere cooling theearth, among others. Particulates disperse radiation and re-direct it tospace; the effect may decrease the amount of light that reaches the earthowing to the increasing amounts of particulates that are contaminatingthe atmosphere.

There remain many unanswered questions as regards the net effect ofatmospheric pollutants on climate and weather. The effects involvedmay be quite marked or cancel one another. Also, the observed temp-erature oscillations may be the reflection of a natural, as yet unknowncycle. There is insufficient evidence to associate increased atmosphericCO2 levels with climatic changes, however. In any case, one should bearin mind that human activities can bring about atmospheric alterationsliable to produce significant thermal changes.

32

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