CHEM 181ENVIRONMENTAL CHEMISTRY

INTRODUCTION

Definition:

Environmental Chemistry – is the study of the sources, reactions, transport, effects, and fates of chemical species in water, soil, and air environments, and the effects of technology thereon.

Environmental chemists help avoid difficulties with regulatory agencies and are instrumental in developing profitable pollution-control products and processes. Chemists must be aware of the possible effects their products and processes have upon the environment.

Chemical analysis is the vital first step in environmental chemistry research. It involves the determination of the nature and quantity of specific pollutants in the environment. Significant levels of air pollutants may consist of less than microgram per cubic meter of air. For many water pollutants, 1ppm by weight (1mg/L) maybe very high value. Others may only be a few precipitate. Chemical analysis used to study environmental systems require very low limit of detection.

Human Impact and Pollution

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Pollutants are substances present in greater than natural concentration as a result of human activity that has a net detrimental effect upon the environment or upon something of value in that environment.

Contaminants are not classified as pollutants unless they have some detrimental effect or cause deviation from normal composition of the environment.

Receptor is anything that is affected by pollutant. The source of the pollutant is important since it is the logical place to eliminate pollution

Sink is the long-time depository of pollutant if it is long-lived. For example, limestone wall is the sink for sulfuric acid which fixes as part of the wall.

CaCO3 + H2SO4 CaSO4 + H2O + CO2

Hazardous Waste is the most serious kind of pollutant that is likely to contaminate the geosphere especially the soil. It is potentially dangerous substance that has been discarded, abandoned, neglected, released or designated as a waste material or one that may interact with other substances to pose a threat.

Dealing with toxic by-products

End-of the-Pipe Solution

Reduce the release into the environment by capturing and disposing of the end –products.Example: Gaseous pollutants are captured in smoke stack and then converted to solid and used as landfills.Drawback: Pollutants are not destroyed instead, only made benign or deposited into different medium

(landfill instead of air)

Green Chemistry

1. Reformulation of synthetic routes so that toxic byproducts are not produced.Examples: a) replacement of organic solvents by water

b) replacement of heavy metal catalysts with environmentally benign substances2. Design products to make them recyclable or safely disposed of.Example: changing habits of consumers: i) segregation of waste for recycling

ii) turning from products with excessive packaging or using natural over synthetic packaging materials.

Approaches to Prevention of Pollution

1. Assimilation by Nature a. Natural systems convert them into harmless, naturally occurring substances. b. Dilution to such extent that they no longer pose threat to life. Many substances are not assimilated because:

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i) they are persistent, meaning, that they are unaltered by light, water, air, or microorganisms. Examples are: DDT, CFC, CO2, Hg .

ii) they are not dispersed uniformly but concentrated within living organisms.

2 Ban its ProductionExample: production of DDT was stopped completely. However the concentration in nature did not reach zero

level.

Toxic by-products are substances that are produced in small quantities from production, distribution, and use of commercial products.Example: Dioxin –is a by-product of bleaching wood pulp, production of pesticides, and wood, and wood preservatives.

Water Chemistry

Water covers 70% of the Earth’s surface. It is an essential part of living system and it is the medium from which life evolves and in which life exists. Energy and matter are carried through various spheres of the environment by water. Water leaches soluble constituents from mineral matter and carries them to the ocean or some distance from the source.

The Hydrologic Cycle: Sources and Uses of Water

Five parts of the hydrologic cycle where the world’s water supply is found:1. ocean - where large portion of water is found2. atmosphere - as water vapor or clouds3. ice and snow - in snowpacks, glaciers, and the polar ice caps4. surface water - in lakes, streams and reservoirs5. ground water - located in aquifers underground

There is a strong connection between hydrosphere and the lithosphere. Human activities affect both. For example, disturbance of land by conversion of grasslands and forests to agricultural land or intensification of agricultural production may reduce vegetation cover, decreasing transpiration (loss of water vapor by plants) and affecting the microclimate. The result is increased rain runoff, erosion, and accumulation of silt in bodies of water. The nutrient cycle maybe accelerated, leading to nutrient enrichment of surface waters. This in turn can affect the biological and chemical characteristics of bodies of water.

The Properties of Water: a Unique SubstanceProperty Effect and SignificanceExcellent solvent Transport of nutrients and waste products making

biological processes possible in aqueous medium

Highest dielectric constant High solubility of ionic substances and their ionization

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of any common liquid in solution

Higher surface tension than Controlling factor in physiology ; governs drop and any other liquid surface phenomena

Transparent to visible and Colorless, allowing light required for photosynthesis to longer-wavelength UV reach considerable depths in bodies of water

Maximum density as a liquid Ice floats; vertical circulation restricted in stratified 4 degrees Celsius bodies of water

Higher heat of evaporation than Determines transfer of heat and water molecules any other material between the atmosphere and bodies of water

Higher latent heat of fusion than Temperature stabilized at the freezing point of water any other liq. except ammonia

Higher heat capacity than any Stabilization of temperature of organisms and other liq. except ammonia geographical regions

The Characteristics of Bodies of Water

The physical conditions of a body of water strongly influence the chemical and biological processes that occur in water.

Surface water - occurs primarily in streams, lakes, and reservoirs.Wetlands - are flooded areas in which the water is shallow enough to enable growth of bottom-rooted plantsEstuaries - consists of arms of the ocean into which streams flow. The mixing of fresh and salt water gives the

unique chemical and biological properties. They are the breeding ground of much marine life, which makes the preservation very important.

Water’s unique temperature-density relationship results in the formation of distinct layers within nonflowing bodies of water.

Thermal Stratification - the phenomenon in which during the summer a surface layer (epilimnion) is heated by solar radiation and because of its lower density, floats upon the bottom layer (hypolimnion).

When an appreciable temperature difference exists between the two layers they do not mix but behave independently and have very different chemical and biological properties. The epilimnion , which is exposed to light may have a heavy growth of algae. Because of photosynthetic activity of algae, the epilimnion contains a high level of dissolved oxygen and generally is aerobic. In the hypolimnion, bacterial action on biodegradable organic material may cause the water to become anaerobic. As a consequence, chemical species in a relatively reduced form tend to predominate.

The shear-plane, or layer between epilimnion and hypolimnion is called the thermocline. During the autumn when the epilimnion cools, a point is reached at which the temperature of the epilimnion and hypolimnionare equal. This disappearance of thermal stratification causes the entire body of water to behave as a hydrological unit, and the resultant mixing is called overturn. An overturn may also generally occur in spring. The physical

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and chemical properties of the body of water become much more uniform and a number of changes may result. Biological activity may increase from the mixing of nutrients. Changes in water composition may cause disruption in water treatment processes.

Aquatic Life

Classification of living organisms in an aquatic ecosystem:1. Autotrophic organisms - utilize solar energy (producers) or chemical energy to fix elements from simple,

nonliving inorganic material (CO2, NO3-,H2PO4

-/HPO42-) into complex life molecules that compose living

organisms.2. Heterotrophic organisms - utilize the organic substances produced by autotrophic organisms as energy

sources and as the raw materials for the synthesis of their own biomass.

Decomposers (or reducers) - subclass of heterotrophic organisms and consist chiefly of bacteria and fungi. They breakdown material of biological origin to the simple compounds originally fixed by the autotrophic organisms.

Productivity - The ability of a body of water to produce living material. It results from a combination of physical and chemical factors. Water of low productivity is desirable for water supply or for swimming. Relatively high is required for the support of fish.

Eutrophication - a set of condition of excessive productivity which can result in choking by weeds and can cause odor problems. The growth of algae may become quite high with the result that the concurrent decomposition of dead algae reduces oxygen levels in the water to very low values.

Factors affecting aquatic life:1. Temperature - very low water temperatures result in very low biological processes while very high are

fatal to most organisms.2. Transparency - is particularly important in determining the growth of algae.3. Turbulence - is an important factor in mixing processes and transport of nutrients and waste products in

water. Some small organisms (plankton) depend upon water currents for their own mobility.

Dissolved oxygen (DO) - the key substance in determining the extent of and kinds of life in a body of water. Oxygen deficiency is fatal to many aquatic animals such as fish. The presence of oxygen is equally fatal to many kinds of anaerobic bacteria.

Biochemical oxygen demand (BOD) - an important water-quality parameter. It refers to the amount of oxygen utilized when the organic matter in a given volume of water is degraded biologically.

Carbon Dioxide is produced by respiratory processes in water and sediments and can also enter water from the atmosphere. CO2 is required for synthetic production of biomass by algae and in some cases is a limiting factor. High levels of CO2 produced by the degradation of organic matter in water can cause excessive algal growth and productivity. The level of nutrient in water determines its productivity. Aquatic plant life requires an adequate supply of carbon (CO2), nitrogen (nitrate), phosphorous (orthophosphate), and trace elements such as iron. In many cases, phosphate is a limiting case and is controlled in attempts to limit excess productivity.

Salinity of water also determines the kinds of life forms present. Irrigation waters may pick up harmful levels of salt. Marine life tolerates salt water while freshwater organisms are intolerant to salt.

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Matter and Cycles of Matter

Major division of material cycles:1. Endogenic cycles - predominantly involve subsurface rocks of various kinds.2. Exogenic cycles - occurs largely on earth’s surface and usually have an atmospheric component.

In general, sediment and soil can be viewed as being shared between the two cycles and constitute the predominant interface between them.

Cycles of matter:

1. Carbon cycleCarbon may be present as gaseous atmospheric CO2, relatively small but significant portion of global carbon. Some of the carbon is dissolved in surface and ground water as HCO3

- . A very large amount is present in minerals, particularly calcium and magnesium carbonate. Photosynthesis fixes inorganic carbon as biological carbon, represented as {CH2O}, which is a constituent of all life molecules. Another fraction is fixed as petroleum and natural gas, with a much larger amount as hydrocarbonaceous kerogen (the organic matter in oil shale), coal, and lignite, represented as CxH2x. Manufacturing processes are used to convert hydrocarbons to xenobiotic compounds with functional groups containing halogen, oxygen, nitrogen, phosphorous or sulfur. These compounds are particularly significant because of their toxicological chemical effects.

2. Nitrogen cycleIt occurs in all the spheres of the environment. The N2 is very stable so that breaking it down to atoms that can be incorporated with inorganic and organic chemical forms of nitrogen is the limiting step in this cycle. This occurs by highly energetic processes in lightning discharges that produce nitrogen oxides. Elementary nitrogen is also incorporated into chemically bound forms, or fixed by biochemical processes mediated by microorganisms. The biological nitrogen is mineralized to the inorganic form during the decay of the biomass. Large quantity of nitrogen is fixed synthetically under high temperature and high pressure condition according to the following overall reaction: N2 + 3H2 --- 2NH3 . The production of gaseous N2 and N2O by microorganisms and the evolution of these gases to the atmosphere complete the nitrogen cycle through a process called denitrification.

3. Oxygen cycleIt involves the interchange of oxygen between the elemental form of gaseous O2, contained in a huge reservoir in the atmosphere, and chemically bound O in CO2, H2O and organic matter. It is tied with other cycles particularly the carbon cycle. Elementary oxygen becomes chemically bound by combustion and metabolic processes in organisms. This element readily combines with, and oxidizes other species, such as carbon in aerobic respiration, or carbon and hydrogen in the combustion of fossil fuels, such as methane:

CH4 + 2O2 ------- CO2 + 2H2O

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Elemental oxygen also oxidizes inorganic substances, such as iron (II) in minerals:

4FeO + O2 ----- 2Fe2O3

A particularly important aspect of the oxygen cycle is stratospheric ozone. The oxygen cycle is completed when elemental oxygen is returned to the atmosphere. This is done through photosynthesis mediated by plants.

4. Phosphorus cycleP is usually the limiting nutrient in ecosystem. There are no common stable gaseous forms of P, so the phosphorus cycle is endogenic. In the geosphere, P is held largely in poorly soluble minerals, such as hydroxyapatite, a calcium salt, deposits of which constitute the major reservoir of environmental phosphate. Soluble P from phosphate mineral and other sources such as fertilizers, is taken up by plants and incorporated into nucleic acids, which make up the genetic material of organisms. Mineralization of biomass by microbial decay returns P to the salt solution from which it may precipitate as mineral matter. The anthrosphere is an important reservoir of P in the environment. Large quantities of phosphate are extracted from phosphate minerals for fertilizer, industrial chemicals, and food additives. P is a constituent of some extremely toxic compounds, especially organophosphate insecticides and military poison nerve gas.

5. Sulfur cycleThis cycle involves several gaseous species, poorly soluble minerals and several species in solution. It is tied with the oxygen cycle in that sulfur combines with oxygen to form sulfur dioxide and sulfate ion. Among the significant species involved in the sulfur cycle are H2S; mineral sulfides as PbS; H2SO4, the main constituent of acid rain; and biologically bound sulfur in sulfur-containing proteins. Sulfur dioxide gas is somewhat toxic air pollutant involved in the sulfur-containing fossil fuels. The major detrimental effect of sulfur dioxide in the atmosphere is its tendency to oxidize in the atmosphere to produce sulfuric acid, the species responsible for acidic precipitation called acid rain.

Energy Source and Cycles of energy

All processes such as biogeochemical cycles on Earth are driven by energy from the sun. Surface temperature of the sun is approximately 5,780 K. One square meter area perpendicular to the line of the solar flux at the top of the atmosphere receives energy at a rate of 1,340 watts, sufficient to power an electric iron. Energy in natural system is transferred by heat or work. Chemical energies in the food ingested by organism are converted to work or heat by metabolic processes.

Sun transmits energy to Earth as electromagnetic radiation. Energy can be carried thru space at the speed of light, 3x108 by electromagnetic radiation (VIS. IR, UV, Microwave, Radio wave, Gamma Ray, and X- ray). Solar energy absorbed in evaporation of ocean water is carried as Latent Heat and released inland. Release of latent heat provides the energy that largely carries heat from equatorial regions towards the Earth’s pole, and powers massive storms.

Solar energy captured by green plants energizes chlorophyll which in turn powers metabolic processes to produce carbohydrates from H2O and CO2. Carbohydrates are repositories for chemical energy that can be converted by organisms to heat and work through metabolic reactions with oxygen gas. Ultimately most of energy is converted to low-grade heat which is eventually reradiated away from the earth by IR.

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