Aquatic Chemistry (10 hrs)

Transcription

1 Aquatic Chemistry (10 hrs)

2 Water -The quality and quantity of water available to human have been vital factors in determining their well-being. -More then 70% of the earth is covered by water. Living cells are 70% - 95% water - In nature, water naturally exists in all three physical states of matter -- solid, liquid, and gas -Life absolutely depends on the properties of water. -The structure of water is the basis for its unique properties. 2

3 Molecular Structure of Water - Water molecule is polar + H O H +

4 Ability to form hydrogen bonds -Hydrogen bonds are weak attractions between the partially negative oxygen of one water molecule and the partially positive hydrogen of a different water molecule.

5 Hydrogen bonds can form between the water molecules and fluorine, nitrogen, or oxygen atoms on the solute molecule Hydrogen bonds also help to hold some solute molecules in solution.

6 What are the properties of water?

7 Properties of Water Excellent solvent Cohesion Adhesion High Specific Heat High Heat of Vaporization Maximum density as a liquid at 4 C

8 Important properties of water Excellent solvent -Transport of nutrients and waste products - Making biological processes possible in an aqueous medium Ionic and polar compounds dissolve in water

9 Water is a good solvent due to its polarity Interacts with other polar compounds and ionic compounds Polar water Molecules surrounding Na + ions and Cl - ions

10 THE CHARACTERISTICS OF BODIES OF WATER

11 Various bodies of water Surface Rivers, lakes, reservoirs, etc Underground water- wells, etc Sea water Other sources of water- ice and snow, water vapor in the atmosphere rain The water that humans use is mainly, - fresh surface water - ground water

12 Thermal stratification in a non flowing water body Water s unique temperature-density relationship results in the formation of distinct layers within non flowing bodies of water

13 Thermal stratification in a non flowing water body

14 Thermal Stratification in a non-flowing water body

15 The disappearance of thermal stratification causes the entire body of water to behave as a hydrological unit, and the resultant mixing is known as overturn.

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17 Aquatic life (biota) Autotrophic Heterotrophic Produces complex organic compounds from simple inorganic molecules using energy from light or inorganic chemical reactions Utilizes the organic substances produced by autotrophic organisms as energy sources Eg. Algae Eg.Decomposes/reducers (bacteria, fungi)

18 Productivity of a water body The ability of a body of water to produce living material is known as its productivity.

19 Eutrophication A process where water bodies receive excess nutrients that stimulate excessive plant growth

20 Eutrophication of Lakes --nutrient enrichment of lakes mostly from runoff of plant nutrients (nitrates and phosphates) -During hot dry weather can lead to algae blooms -Aquatic plants begin to die -Drops DO levels -Fish die, bad odor

21 PHASE INTERACTIONS CHEMICAL INTERACTIONS INVOLVING SOLIDS, GASES, AND WATER

22 CHEMICAL INTERACTIONS INVOLVING SOLIDS, GASES, AND WATER Some of these important interactions are; * Production of biomass through photosynthesis in algal cell. During this process dissolved solids and gases exchange between cell walls and the water in contact with the cells. Similar exchanges occur when bacteria degrades organic matter. * Processes that occur between sediment and waters.

23 Gases in water

24 Gases in water Dissolved gases O 2 for fish and CO 2 for photosynthetic algae are crucial to the welfare of living species in water. The solubilities of gases in water are The solubilities of gases in water are described by Henry s Law

25 Henry s Law At constant temperature the solubility of a gas in a liquid is proportional to the partial pressure of the gas in contact with the liquid. For a gas, X, this law applies to equilibrium of the type X(g) X(aq) William Henry

26 Mathematically, Henry s Law is expressed as Where, [X(aq)] = aqueous concentration of the gas, P X = partial pressure of the gas, K = Henry s Law constant (applicable to a particular gas at a specified temperature)

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28 Carbon Dioxide in Water -Carbon dioxide is only about 0.038% by volume of normal dry air. -The concentration of gaseous CO 2 in the atmosphere varies with location and season. - It is increasing by about one part per million (ppm) by volume per year. -Rainfall from even an absolutely unpolluted atmosphere is slightly acidic due to the presence of dissolved CO 2.

29 Carbon Dioxide in Water -Carbon dioxide, and its ionization products, bicarbonate ion (HCO 3- ), and carbonate ion (CO 3 2- ) have an extremely important influence upon the chemistry of water. -The carbonate chemistry controls the ph of water which is a master variable for many geochemical processes. The carbonate system is the major source of buffering in the ocean

30 The CO 2 /HCO 3 /CO 3 2- system in water The predominant species formed by CO 2 dissolved in water depends upon ph

31 Substitution of the expressions for Ka 1 and Ka 2 into the a expressions gives the fractions of species as a function of acid dissociation constants and hydrogen ion concentration:

32 The distribution of carbonate species as a fraction of total dissolved carbonate in relation to solution ph HCO 3- is the predominant species in the ph range found in most waters, with CO 2 predominating in more acidic waters.

33 With this graph you can locate the fraction of any of the required carbonate species at any ph.

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35 The CO 2 /HCO 3 /CO 3 2- system in water Determination of total dissolved inorganic carbon (DIC) in a water body C T Aquatic scientists measure DIC most often by acidification of water samples and subsequent quantification of the extracted CO 2 gas

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37 Sediments

38 What are sediments? Sediments are the layers of relatively finely divided matter covering the bottoms of rivers, streams, lakes, reservoirs, bays, estuaries, and oceans. Sediments typically consist of mixtures of fine-, medium-, and coarse-grained minerals, including clay, silt, and sand, mixed with organic matter.

39 Formation of Sediments and Its Importance Importance: Of particular importance is the transfer of chemical species from sediments into aquatic food chains.

40 Examples for sediment dwelling organisms are various kinds of shellfish (shrimps, crayfish, crab, clams) and a variety of worms, insects that are of particular concern because they are located at the bottom of the food chain.

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42 Sediments may be formed by simple precipitation reactions, several of which are discussed below. When a phosphate-rich wastewater enters a body of water containing a high concentration of calcium ion, the following reaction occurs to produce solid hydroxyapatite:

43 Calcium carbonate sediment may form when water rich in high level of calcium as temporary hardness (loses carbon dioxide to the atmosphere) Oxidation of reduced forms of an element can result in its transformation to an insoluble species,

44 Biological activity is responsible for the formation of some aquatic sediments. Some bacteria, including Ferrobacillus, Gallionella, and some forms of Sphaerotilus, utilize iron compounds in obtaining energy for their metabolic needs. These bacteria catalyze the oxidation of iron(ii) to iron(iii) by molecular oxygen:

45 In anaerobic bottom regions of bodies of water, some bacteria use sulfate ion as an electron receptor, whereas other bacteria reduce iron(iii) to iron(ii): The net result is a precipitation reaction producing a black layer of iron(ii) sulfide sediment:

46 This frequently occurs during the winter, alternating with the production of calcium carbonate by-product from photosynthesis during the summer. Alternate layers of FeS and CaCO 3 in a lake sediment

47 Water Quality Parameters

48 Water quality parameters 1. Chemical parameters -CO 2, Alkalinity, DO, BOD, COD, ph, salinity, other dissolved gases, hardness, etc. 2. Physical parameters -Temperature, transparency, turbulence, conductivity etc. 3. Biological parameters - microorganism content in water, etc.

49 Alkalinity -The capacity of water to accept H + ions (protons) is called alkalinity. (Alkalinity is a measure of the capacity of water to neutralize acids) -Generally, the basic species responsible for alkalinity in water are, bicarbonate ion, carbonate ion, and hydroxide ion

50 -Higher alkalinity is associated with a body of water s ability to "soak up" proton without altering the ph. -Alkalinity prevents sudden changes in the acidity level of water and, hence, is important for fish and other aquatic life. High alkalinity prevents wide ph fluctuations

51 Alkalinity Following equation is the complete equation for alkalinity in a medium where the only contributors to it are HCO 3-, CO 3 2-, and OH - :

52 Method of Analysis Alkalinity by this definition can be determined by HCl titration of the water sample. 1. Phenolphthalein Alkalinity A. Phenolphthalein indicator B. ph 8.3 endpoint of titration C. Measures Carbonates and Hydroxide ions 2. Total Alkalinity A. Methyl Orange indicator B. ph 4.3 endpoint of titration

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54 Hardness Total hardness is primarily the total concentration of metal ions (cations) in water (mg/l), which includes mainly Ca 2+ and Mg 2+ Hard water is generally associated with high alkalinity

55 Alkalinity CaMg(CO 3 ) 2 Dolomite Ca 2+ + Mg CO 2-3 Hardness Hardness and Alkalinity Generally Go Hand- In-Hand but They Are NOT One and the Same -Can have hard water and low alkalinity water high in CaCl 2 and/or MgCl 2

56 Hardness Temporary hardness -Bicarbonates of Calcium and Magnesium - This "temporary" hardness can be reduced either by boiling the water, or by the addition of lime (calcium hydroxide) Permanent hardness -Sulfates, chlorides of calcium and magnesium - Cannot be removed by boiling

57 Effects of hard water 1. With hard water, soap solutions form a white precipitate (soap scum) A major component of such scum is calcium stearate, which arises from sodium stearate, the main component of soap 2 C 17 H 35 COO - + Ca 2+ (C 17 H 35 COO) 2 Ca 2. Hard water also forms deposits that clog plumbing

58 Factors that affect ph --Dissolved CO 2 -Bacterial activity -Chemicals flowing into the water body -Sewage overflows -Acid rains -proteins, and fatty acids -Acidic metal ions, particularly Fe 3+

59 Acid Rain Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions (low ph) Even in the absence of air pollutants, rain-water is slightly acidic (ph 5.6) due to atmospheric carbon dioxide. This is due to air pollution by NOx and SO 2 from vehicles and power plants..

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61 Metal ions in water Metal ions in aqueous solution exist as aqua ions, where water molecules act as ligands, and coordinate to the metal ion via the oxygen donor atoms as shown for the [Al(H 2 O) 6 ] 3+ hexaaqua ion below:

62 Hydrolysis of metal aqua ions The aqua ions undergo hydrolysis, to a greater or lesser extent. The first hydrolysis step is given generically as, M(H 2 O) x n+ + H 2 O [M(H 2 O) x-1 (OH)] (n-1)+ +H 3 O + [ ] [ ] Metal aqua ions can act as Brønsted acids, which means that they can act as proton donors.

63 Factors that control the acidity of metal ions in aqueous solution: -Metal aqua ions display varying pk a values that are dependent on size and charge -The higher the charge, the smaller the metal ion stronger the M-OH 2 interaction.

64 Hydrolysis of metal aqua ions