Introduction

Environmental Chemistry is learning of chemical procedures occurring in the environment that are impacted via humankind's activities. Such impacts might be felt on a local scale, through the presence of urban air pollutants or toxic substances arising from a chemical waste site, or on a global scale, through exhaustion of stratospheric ozone or global warming. The focus in our courses and research activities is upon expanding a fundamental understanding of the nature of such chemical processes, so that humankind's activities can be precisely evaluated.

The field of environmental chemistry is both extremely broad and extremely interdisciplinary.  Inside the Department of Chemistry we have a core group of faculty whose research interests are in atmospheric and aquatic chemistry, photochemistry, and the chemistry and transport of long- lived pollutants. We interact through other chemists in the department, with numerous other researchers at the university who have related interests, and with nearby government agencies. Indeed, the setting for the study of environmental chemistry is ideal.

Our graduate programme consists of graduate courses which stress the fundamental photochemical, kinetic, and analytical and transport aspects of environmental phenomena, regular seminars, and close interactions between the different research groups. We emphasize the need for students to be able to put their own research into a global context. Environmental Chemistry is quickly expanding, and excellent employment opportunities survive in the academic, government, industrial and public policy sectors. Cycles are sequences of events that repeat themselves in a particular pattern.  In environmental chemistry, our concern is basically with biogeochemical cycles. Precisely, biogeochemical cycles are interconnected  complex  processes  via which matter or elements that make up the biotic and abiotic systems are used over and over again between the living and non-living things.

Several of the extremely significant natural biogeochemical cycles comprise: Carbon Cycle, Nitrogen Cycle, Sulphure Cycle, and Hydrological (Water) Cycle.

Some biogeochemical cycles

The carbon cycle

The carbon cycle is the series of interconnected changes via that carbon is being continuously circulated among the natural compartments of atmosphere, biosphere, hydrosphere, geosphere and pedosphere. There are 4 main reservoirs included in the carbon cycle. Such are the plants, the terrestrial biosphere (fresh water systems and non-living organic material), the oceans (where we have dissolved inorganic carbon and living and non-living marine biota), and the sediments (as well as fossil fuels). Living organisms are mainly composed of water and diverse carbon compounds therefore the cycling of carbon is of prime significance to the support of life. The concentration of carbon in living matter (18per cent) is about 100 times greater than its concentration in the earth (0.19per cent). Carbon exists as carbon dioxide (CO₂) in the atmosphere, bicarbonate ion (HCO) in water and calcium carbonate (CaCO₃) in carbonate rocks (limestone, chalk, coral). It is the main part of hydrocarbon molecules in petroleum and natural gas; and the major constituent of coal and dead organic matters.

In the Earth's atmosphere, carbon exists as carbon dioxide in 0.03per cent level via volume.  This level is now being shifted towards excess due to imbalance of anthropogenic activities (human-induced) such as excessive combustion of fossil fuels and deforestation. In the year 1850, atmospheric CO₂ was about 280 ppm and via 2007, it had enhanced to about 383 ppm.

Ways by which CO₂ is released into the Atmosphere

Several of the ways via which CO₂ is liberated into the atmosphere are:

Respiration of plants and animals: This is an exothermic reaction including the breaking down of organic molecules, for example glucose, into carbon dioxide and water

C₆H₁₂O₆ + 6O₂ 6CO₂+ 6H₂O + energy.

Decay of plants and animals: Fungi and bacteria breakdown the carbon compounds for example carbohydrates, proteins and lipids in lifeless plants and animals, and transfers the carbon to carbon dioxide in the occurrence of oxygen or carbon dioxide and methane (CH₄) in the absence of oxygen for example.  Methanogens

C₆H₁₂O₆    3CO₂(g) +3CH₄(g)

Fermentation of carbohydrates: The enzymatic decomposition of carbohydrates generates CO₂ as a by-product

C₆H₁₂O₆(aq)  Zymase 2C₂H₅OH(aq) + 2CO₂(g)

Burning of fossil and agro fuels: Combustion of fossil fuels like petroleum products, coal, and natural gas and agro fuels liberates CO₂ (and water vapour) into the atmosphere.

C₅H₁₂ + 8O₂         6H₂O + 5CO₂

Thermal decomposition of carbonate rocks or limestone: When limestone soils are heated up or during the production of cement, CO₂ is liberated into the atmosphere.

                    heat

CaCO_3(s)        CaO(s) + CO_2(g)

Warming of surface waters: This leads to the releasing of dissolved CO₂ back into the atmosphere.

Volcanic eruptions: During volcanic eruptions, the volcanic gases liberated into the atmosphere include water vapour, CO₂ and SO₂.

Ways by which CO2 is removed from the Atmosphere

Photosynthesis: Primarily, photoautotrophs (plants and algae) utilize light energy to convert CO₂ and water to organic molecules as glucose and other carbohydrates. To a less extent, chemoautotrophs (bacteria and archaea) convert CO₂ and water to organic matter using energy derived from the oxidation of molecules of their substrates.

6CO₂ + 6H₂O         C₆H₁₂O₆ + 6O₂

Formation of carbonic acid: Carbon dioxide dissolves in rain water and droplets pass through the atmosphere. As well, at the surface of the oceans towards the poles where sea water becomes cooler, carbon dioxide dissolves in water to from carbonic acid. Carbonic acid reacts through weathered silicate rocks to generate bicarbonate ions that are utilized to make marine carbonates.

Conversion of carbon to tissues and shells: Organisms in upper ocean areas of elevated biological productivity transfer reduced carbon to tissues or shells.  

In the oceans, the major carbon reservoir is the inorganic carbon: Whenever CO₂ dissolves in water, a hydrated CO₂ molecule is produced that then forms an equilibrium mixture enclosing bicarbonate (2HCO₃) and carbonate (CO₃) ions. At pH lower than those originate in sea water, carbonic acid (H₂CO₃) will as well be present. This can be summarized as:

• CO_2(g) = CO_2(aq)
• H₂O (l) + CO₂(aq) = H₂CO_3(aq)
• H2CO_{3 (aq)} = H⁺_(aq) + HCO_{3( aq )}

      HCO_3(aq)   = H⁺_(aq) + CO²_3(aq)

Though, due to the reactions

HCO_3(aq) = CO_2(aq) + OH_(aq) and CO₂ + H₂O _(l) = HCO_3(aq) + OH_(aq)  

Most ocean waters have a pH in the range 8 to 8.3 as they enclose more 0H^- ions than H⁺ ions. The overall reaction that takes place when CO₂ dissolves in sea water can be summarized as:

 •   CO_{2(aq) +} H₂O(l) + C₂O_3(aq)  =  2HCO₃    

The carbon cycle revealed in Fig summarizes all the processes so far itemized.

Fig: The Carbon Cycle

 The nitrogen cycle

The nitrogen cycle is the biogeochemical cycle that explains the gradual transformation of nitrogen and nitrogen-enclosing compounds in nature. It is the means via that the supply of nitrogen is dispensed in nature. The Earth's atmosphere, enclosing about 79 percent nitrogen, constitutes the largest pool of nitrogen. Nitrogen is crucial to all life procedures on earth. It is present in all amino acids, proteins and nucleic acids (RNA and DNA). Even though nitrogen is abundant in the atmosphere and the majority of the air we breathe in is nitrogen (oxygen constitutes only 21 percent of the air we breathe in), nitrogen isn't eagerly available for cellular utilization. This is since the strong triple covalent bonds between the N atoms in N₂ molecules make it comparatively inert. By implication, biochemically available nitrogen is in short providing in natural ecosystems.  Hence, plant growth and biomass accumulation are bounded.

In order for plants and animals  to utilize nitrogen for their metabolic processes, N₂ gas must be transferred to a chemically available form such as ammonium (NH⁺), nitrate (NO) or organic nitrogen such as urea,4 3 (NH₂)CO. The nitrogen cycle shown in Figure explains the movement of nitrogen among the atmosphere, biosphere and geosphere in dissimilar forms.

Basic Processes of the Nitrogen Cycle

(a) Nitrogen Fixation: This is the procedure via which the atmospheric nitrogen is converted into a form that is readily available to plants and subsequently to animals and humans. There are four methods of converting atmospheric nitrogen (N₂) into more biochemically available forms.

(i) Biological Fixation: Symbiotic bacteria, for instance Rhizobium, connected through the root nodules of leguminous plants and several free-living bacteria, for example Azotobacter, are able to covert (fix) free nitrogen to organic nitrogen.

(ii) Industrial Fixation: In the industrial Haber-Bosch process, atmospheric nitrogen and hydrogen (obtained from natural gas or petroleum) are joined to form ammonia, NH₃

 N₂(g) + 3H₂(g)     2NH₃(g)

The ammonia produced can be utilized to build fertilizers and explosives.

 2NH₃(g) + H₂SO₄(aq)  Iron catalyst 450-600^oC 200atm (NH₄)₂ SO₄ (s)

(iii) Combustion of fossil fuels: The exhaust fumes from internal combustion engines are made up of volatile matters together with oxides of nitrogen.

(iv) Electrical storms (lightning) and photolysis: During electrical storms, nitrogen is oxidized to NO, which is oxidized via ozone in the atmosphere to form NO₂. NO₂ in turn is reduced back to NO by photolysis. Such reactions are significant aspects of atmospheric chemistry, but they are inadequate for both terrestrial and aquatic nitrogen turnover.  

(a) Assimilation: Plants can absorb NO or NH⁺ ions from the soil (Nitrogen uptake) through their roots. Absorbed nitrate is 1^st decreased to nitrite ions and then ammonium ions for subsequent incorporation into amino acids, nucleic acids and chlorophyll. In leguminous plants through root nodules, nitrogen in the shape of ammonium ions can eagerly be assimilated. Animals and human beings are incapable of utilizing nitrogen from the atmosphere or inorganic compounds therefore; they based on plants or other animals (except ruminants) that feed on plants, for their protein.

(b) Ammonification: At death, the proteins stored in the body of plants and animals become waste substances. Urine encloses the nitrogen consequential from the metabolic breakdown of proteins in form of urea, (NH₂)₂CO. Urea is speedily hydrolyzed via the enzyme to ammonium carbonate, (NH₄)₂CO₃.

NH₂C NH₂ = O + 2H₂O

Urease

(NH₄)₂CO₃.

Nitrification:  The excess ammonia liberated via bacterial action on urea and proteins that aren't utilized through plants is oxidized via the autotrophic nitrifying bacteria-Nitrosomonas and Nitrobacter. Under aerobic conditions, Nitrosomonas convert ammonia to nitrite while nitrite is further oxidized to nitrate via Nitrobacter.

2NH₃ + 3O₂ Nitrobacter 2NO + 2H⁺ + 2H₂O

2NO₂ + O₂  Nitrosomonas 2NO₃

The bacteria derive energy from the oxidation processes. Several of the nitrate shaped is utilized via plants while the excess is carried away in water percolating through the soil since the soil doesn't have the ability to hold nitrate for long. It is significant for the nitrite ions to be converted to nitrate ions since accumulated nitrites are toxic to plant life.

(d) Denitrification:  Nitrate and nitrite are reduced under anaerobic conditions by pseudomonas and clostridium bacteria. Nitrate is reduced to nitrite while nitrite is reduced to ammonia. Most of the nitrate is later reduced to nitrogen thus, completing the nitrogen cycle. This constitutes a serious loss of fertilizing matter in soil when anaerobic conditions develop. As well several denitrifying bacteria produce N₂O from nitrate reduction. The N₂O produced enters the atmosphere and is reduced through photolysis to produce N₂ and an excited state of oxygen, that oxidizes N₂O to NO.

NO₃ N₂ + N₂O

NO₃ NO₂  NO    N₂O   N2

Fig: The Nitrogen Cycle

The sulphur cycle

On the earth surface sulphur exists as elemental sulphur, sulphur dioxide, sulphuric acid, salts of sulphate, hydrogen sulphide, sulphur trioxide, organic sulphur compounds (such as dimethylsulphide) and amino acids (cystein and methionine). The biogeochemical transformations of such sulphur species among the atmosphere, biosphere, hydrosphere and geosphere is termed the sulphur cycle. Most on the sulphur earth is tied up in salts and rocks or buried deep in the ocean in oceanic sediments. Sulphur enters the atmosphere through sources that are both natural (for instance volcanic eruptions, bacterial processes, evaporation from water or decaying organisms) and human (for example wide-scale industrial emission of SO₂ and H₂S).

The major reservoir for sulphur is the crust, with a small, but potentially damaging proportion in the atmosphere. In the air, sulphur is usually oxidized from organic sulphur or elemental sulphur to SO₂ and SO₃ ending up as sulphate (SO₄^2-) in sulphate salts or sulphuric acid. The sulphate compounds dissolve in rain water and obtain precipitated (as rainfall) either as salts or acid rain. In the atmosphere, the oxidation of reduced forms of sulphur via O₂ occurs O₂ biological control; but can as well take place through the actions of microorganisms in the soil, sediment and water column.

H₂S (S) SO₂  SO₃ SO₄^2-

Microorganisms in the soil or water act upon the SO₄^2- in the presence of carbohydrates to finally liberate H₂S

SO₄^2- + H⁺ + 2CH₂O HS^- + 2H₂O + 2CO₂ HS^- + H⁺ H₂S

The formation of H₂S is a characteristics feature of anaerobic marine sediments. In the oceanic surface waters, dimethyl sulphide is formed much more commonly than H₂S because of the presence of the compound dimethyl sulphonopropionate produced via several species of phytoplankton.

 SO₄^2-     (organic hydrosulphide) H₂S + [S] organic sulphides organic compound

H₂S + (CH₃)₂ S.

When organic sulphur compounds are decayed via bacteria, the initial sulphur product is generally H₂S. 

R- SH  (bacteria) H₂S + RH

The hydrogen  sulphide  produced  might  be  liberated  as  a  gas  to  the atmosphere, where it is oxidized, or it may react through metal ions in the sediments or water columns to form insoluble sulphides.

H₂S + [S] Fe²⁺ FeS + FeS₂

or 2Fe(OH)₃  + 3H₂S 2FeS + S + 6H₂O

FeS + S (Fe³⁺) + FeS₂

Many marine phytoplankton generate compounds that breakdown to produce dimethylsulphide, (CH₃)₂S, a compound thought to be the main biogenically produced sulphur compound liberated from oceans. Dimethyl sulphide is rapidly oxidized to form SO₂ and ultimately, SO₂. Several microorganisms in muds can generate elemental sulphur from sulphur compounds. The black colour of many kinds of sediment is partially due to the presence of iron sulphides in addition to organic matter. Oxidation of the sulphides when exposed to the surface leads to the formation of sulphuric acid:

2FeS₂ + 2H₂O + 7O₂   2FeSO₄ + 2H₂SO₄.

This redox reaction occurs speedily in the presence of water and dioxygen particularly when microorganisms are included.

Fig: The Sulphur Cycle

The water cycle

Movement of water in its physical states through and around our planet is accomplished by the water cycle. Through evaporation, water from the ocean and soil goes as water vapour into the atmosphere; and via transpiration, plants liberate water into the atmosphere.  

In the atmosphere, as the temperature reduces, water vapour condenses into water droplets to form the clouds that precipitate as rainfall. A huge proportion of this goes straight into the ocean or indirectly through run- off. Part of the water on the soil sinks down via percolation to form groundwater. The groundwater is pulled through the roots of plants and is transpired back into the atmosphere. Most of the rain and snow that fall on the continents come from plant transpiration than from ocean evaporation. The water cycle transfers sun's heat energy to various places of the Earth; It as well moves contamination or pollution round the planet. Every living thing is straight connected to the water cycle.

Fig: The Water Cycle

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