Chem. 250 – 12/02 Lecture

Announcements - I A.Solutions to Furlough Questions and Homework will be posted soon B.New Homework Set (Ch. 13: 1, 2, 5, 12, 19, 23, 27, 33, 34) C.Topics 1.Water Composition (finish up) 2.Complexation 3.Water Pollution Problems 4.Toxic Metals

Water Chemistry A Left-Over Problem 1.A water sample has a measured alkalinity of 0.4 mM and a pH of 6.7. Determine the concentration of [OH - ], [HCO 3 - ], [CO 3 2- ], and [CO 2 ].

Water Chemistry Chemical Reactions Solubility of other metals: - Na and K are soluble but usually not present in soils at high concentrations. - Many other metals that are present in soils in reasonable concentrations (e.g. Al, Fe) are not very soluble at “normal” pH values. Example: Al(OH) 3 (s) ↔ Al OH - K sp = [Al 3+ ][OH - ] 3 = At pH = 8, [Al 3+ ] = M, at pH = 6, [Al 3+ ] = M

Water Chemistry Chemical Reactions Complexation reactions: –Many metals exist in water with total dissolved concentrations greater than equilibrium. This can be cause by formation of metal-ligand complexes. –Examples (for Al 3+ ) Al OH - ↔ Al(OH) 4 - K = 2.0 x Al C 2 O 4 2- ↔ Al(C 2 O 4 ) 3 3- K = 4.0 x 10 15

Water Chemistry Chemical Reactions

Redox Reactions: –Under conditions with plentiful oxygen, most elements are present in oxidized forms examples: SO 4 2-, CO 3 2-, colloidal Fe(OH) 3, NO 3 - (most oxidized forms) –Under conditions with depletion of oxygen, decomposition products tend to be in reduced forms: examples: FeS, CH 4, NH 3

Water Chemistry Water Pollution Issues Acid Rain - Man is a source of both acidic precursor gases (SO 2 and NO X ) and basic precursors (NH 3 and soil dust) - In regions downwind of large SO 2 and NO X sources (industrial areas), acidic rain is expected from oxidation products (H 2 SO 4 and HNO 3 ) - Besides acid rain, harm can come from acidic snow and fog

Water Chemistry Water Pollution Issues Acid Rain – continued Damage from acid rain depends on: - amount of excess acid (over natural) - regional extent of pollutions - ability of soils to buffer acid Carbonate containing soils tend to reduce damage from acid rain while granite or basaltic soils are carbonate poor and suffer more damage from acidification

Water Chemistry Water Pollution Issues

Acid Rain – continued In poorly buffered soils, pH drop is also accompanied by increased solubility of metals. In pH = 5, [Al 3+ ] = M => toxic effects start (e.g. precipitation of Al(OH) 3 in fish gills)

Water Chemistry Water Pollution Issues Acidic Mine Runoff In many mining regions, rock is rich in sulfides (e.g. FeS 2 ) As this rock is exposed, the sulfides can be oxidized (e.g. 2FeS 2 + 7O 2 + 2H 2 O ↔ 2Fe SO H + ) Where sulfides are high enough in concentration, water runoff can have a pH less than zero Both the low pH and dissolution of heavier metals lead to increased toxicity

Water Chemistry Water Pollution Issues Oxygen Depletion Depletion of oxygen in the ocean and lakes (usually seasonally) is a natural process Pollution is a larger problem in bodies of water that are thermally stratified (it is harder to restore oxygen levels) Water depleted of oxygen will support fewer fish species, and the chemistry changes in anoxic waters

Water Chemistry Water Pollution Issues Oxygen Depletion – cont. Pollution can deplete oxygen content in the following ways 1) Thermal pollution (e.g. power plant outflow) 2) Organic compounds that consume oxygen 3) Nutrients (e.g. N or P containing fertilizers) which promote algae growth (but lead to oxygen consumption when algae peak)

Water Chemistry Water Pollution Issues Oxygen Depletion – cont. The ability of polluted water to consume oxygen can be measured through two tests (biological oxygen demand or BOD and chemical oxygen demand or COD). This will depend on the concentrations of organic carbon and the forms present.

Water Chemistry Water Pollution Issues Sewage Problems: Raw sewage is a big source of oxygen demand Sewage Treatment: - Primary Treatment: removal of most suspended organic material (physically based) - Secondary Treatment: decomposition of remaining organic compounds (biologically based) - Tertiary Treatment: removal of other chemicals (nitrates and phosphates) plus disinfection (most common with chlorine gas)

Water Chemistry Water Pollution Issues Addition of chlorine for disinfection results in other problems – creation of disinfectant byproducts (DBPs) DBPs include trihalomethanes which are toxic

Water Chemistry Second Set of Questions 1.The K sp for Fe(OH) 2 and Fe(OH) 3 are 7.9 x and 1.6 x What are saturated concentrations of Fe 2+ and Fe 3+ at a pH of 5 and 7? What will happen as hot spring water containing high concentrations of Fe 2+ is introduced to a river with pH of 8 and with oxygen present? If Fe is measured and known to be in the form Fe 3+ with a conc. of 1.3 μM at pH of 7.0, what might this suggest? 2.Why do the concentrations of some elements in water reflect concentrations in soils while not for other elements?

Water Chemistry Second Set II 1.Acidic rain will cause greater or lesser damage when a water basin has the following types of rocks? a) graniteb) limestonec) basaltd) marble 2.Statues made from the above types of rock will be affected most from acidic precipitation? 3.As run-off from a mine is introduced into a river, what transformations occur to dissolved metals? 4.Lakes under which of the following conditions are more susceptible to oxygen depletion a)Summer (vs. winter)b) cold climate c)High altituded) lakes (vs. rivers)

Toxic Metals (Chapter 13) Four metals covered in detail: Pb, Hg, Cd, and As Behavior is somewhat similar (can bioaccumulate, difficult to remove or “destroy”, can form different species, density is high) These metals all have industrial sources (although natural As is very important also)

Toxic Metals Bioaccumulation Bioaccumulation –When humans are exposed to constant concentrations (e.g. Pb in air or Hg in fish), and elimination rates are slow, concentrations can become much higher than in the environment –Example: A 0.50 kg rat injests 0.20 mg of Pb per day Half-life in rat is 7 days (originally Lifetime) What is steady-state amount/concentration?

Toxic Metals Bioaccumulation Bioaccumulation –If other animals have slow excretion rates, animals at the top of the food chain typically will have higher metal concentrations (e.g. tuna fish, swordfish, sharks in ocean; polar bears)

Toxic Metals Analysis Methods Atomic Spectroscopy (and Mass Spectrometry) Most Commonly Used Variety of Possible Methods: –Atomic Absorption Spectroscopy (AAS), –Atomic Emission Spectroscopy (AES) –Mass Spectrometry (MS) –X-Ray Fluorescence (XRF) X-Ray Fluorescence has the advantage of often needing no sample preparation (can analyze solid or liquid samples) For other methods, the first step involves atomization (conversion to “free” atoms in gas phase or simple ions for atomic mass spectrometers)

Toxic Metals Analysis Methods Atomic Spectroscopy tends to have very good sensitivity (ability to detect small quantities) due to the narrow spectral lines The first step (and often difficult part) involves atomization (conversion from ions in liquid to atoms in gas phase) Three atomization methods are common: –flame (mostly with AAS) –graphite furnace (only with AAS) –inductively coupled plasma (ICP) – with AES or MS

Toxic Metals Analysis Methods Flame Atomization –used for liquid samples –liquid pulled by action of nebulizer –nebulizer produces spray of sample liquid –droplets evaporate in spray chamber leaving particles –fuel added and ignited in flame –atomization of remaining particles and spray droplets occurs in flame –optical beam through region of best atomization sample in fuel (HCCH) oxidant (air or N 2 O) burner head spray chamber nebulizer light beam

Toxic Metals Analysis Methods Electrothermal Atomization (Process) –Sample is placed through hole onto L ’ vov platform –Graphite tube is heated by resistive heating –This occurs in steps (dry, char, atomize, clean) Graphite Tube in Chamber (not shown) L’vov Platform Sample in T time drychar atomize Clean + cool down Ar in chamber flow stops and optical measurements made

Toxic Metals Analysis Methods Inductively Coupled Plasma (ICP) –A plasma is induced by radio frequency currents in surrounding coil –Once a spark occurs in Ar gas, some electrons leave Ar producing Ar + + e - –The accelerations of Ar + and e - induce further production of ions and great heat production –The sample is introduced by nebulization in the Ar stream –Much higher temperatures are created (6,000 K to 10,000 K vs. <3500 K in flames) ICP Torch Quartz tube Argon + Sample RF Coil

Toxic Metals Analysis Methods The lamp is a hollow cathode lamp containing the element(s) of interest in cathode. The lamp must be changed to analyze a different element. A very narrow band of light emitted from hollow cathode lamps are needed so that for absorption by atoms in flame mostly follows Beer ’ s law The monochromator serves as a coarse filter to remove other wavelength bands from light and light emitted from flames Lamp source Flame or graphite tube monochromatorLight detector AA Spectrometers

Toxic Metals Analysis Methods Emission Spectrometers In emission measurements, the plasma (or flame) is the light source A monochromator or polychromator is the means of wavelength discrimination ICP-AES is faster than AAS because switching monochromator settings can be done faster than switching lamp plus flame conditions In ICP-MS, a mass spectrometer replaces the monochromator Plasma (light source + sample) Monochromator or Polychromator Light detector or detector array Liquid sample, nebulizer, Ar source

Toxic Metals Analysis Methods X-Ray Fluorescence X-Rays interact with matter by causing transitions with inner shell electrons An incoming X-ray will cause an inner shell electron to be removed An outer shell electron will replace the inner shell electron causing an X-ray emission The emitted X-ray will have a wavelength dependent upon the element X-Ray Fluorescence is not as sensitive or accurate as other methods Pb e-e- e-e- inner shell outer shell

Toxic Metals Analysis Methods Comparison of Instruments InstrumentCostSpeedSensitivity Flame-AA Low (~$10- 15K) SlowModerate (~0.01 ppm) GF-AA Moderate (~$40K) SlowestVery Good Sequential ICP-AES Moderate MediumModerate Simultaneous ICP-AES High FastGood ICP-MS Highest (~$200K) FastExcellent

Toxic Metals Lead Sources: –Ingestion from lead pipes, lead containing glass or ceramics, lead paint –Inhalation (Was a problem in past with leaded gasoline) Toxicity: –Affects normal development (more of a problem with children) –Slow excretion rates Analysis: Can use most of methods

Toxic Metals Mercury Sources: –Industrial Sources (in various products) –Coal Combustion –Food Chain Bioaccumulation Forms: –Reduced Hg (Hg o ) (vapor or liquid) –Inorganic Hg (e.g. Hg 2+ ) –Organic (CH 3 Hg + ) – much more toxic because of slower excretion rate and stability Toxicity: Neurological Problems Analysis: Cold vapor Atomization Methods (High T methods are not efficient)

Toxic Metals Cadmium Sources: –Common industrial pollutant – released in industrial processes –Also in cigarette smoke Toxicity: –Cd can replace Ca in bones Analysis: Can use most of methods

Toxic Metals Arsenic Sources: –Industrial sources (e.g. pesticides –Naturally present in water in certain regions Toxicity: –Accute effects –Carcinogen Analysis: Difficult to atomize As (most common using hydride technique)

Toxic Metals Questions 1.Why do animals higher on the food chain have higher concentrations of Hg? 2.List 3 methods for analyzing metals 3.Which method is best for a fast survey of soil samples to find soils with high metal concentrations? 4.Which method is best for determining multiple elements at low concentrations? 5.Why is graphite-furnace AA a slow technique? 6.Which metal has a significant natural source?

Toxic Metals Questions 1.An organic form of Hg is found in river sediments. Bottom feeding fish have an uptake rate of 70 μ g Hg per day. If the half-life of this form of Hg is 78 days, what would be the steady state mass of Hg in the fish?