Environmental Chemistry Spring 2005 Chapter 5

Transcription

1 Environmental Chemistry Spring 2005 Chapter 5 Ji Yang, Ph.D. Associate Professor School of Environmental Science and Engineering Shanghai Jiao Tong University

2 Metal Ions Produce Acid Aqueous Environments Metal ions can also shift equilibrium and produce acidic aqueous environments such as in the case of Fe. The following is the reaction that occurs in the situation known as acid mine drainage. Fe(H 2 O) 6 3+ <---> Fe(OH) 3 (s) + 3H + + 3H 2 O Why is this significant? Hint: Fe(OH) 3 (s) + 3H + + 3H 2 O <---> Fe(H 2 O) 6 3+ Ksp = 4 x10-38

3 Metal Ions Produce Acid Aqueous Environments Example: When water at low ph from acid mine drainage, (say ph 2.5 to 3) flows into an aerated stream at a higher ph, (say ph 6 to 8.2) then the iron that was complexed as a hydrate precipitates out as a hydroxide. Such as "yellow breaches creek" or "Yellow Creek".

4 Calcium, Water "Hardness" Calcium Deposits, and Carbonates related theme Minerals containing Ca and Carbonate in contact with water CaCO 3 (calcite, limestone, shells, coral...), CaSO 4, CaMg(CO 3 ) 2, CaCO 3 (s) + CO 2 (aq) + H 2 O <---> Ca 2+ + H 2 CO 3 Water with high CO 2 levels dissolve higher quantities of Ca minerals from geological materials in contact with the water. (formation of caves) When the equilibrium is reversed large quantities of CaCO 3 (s) are deposited thus giving rise to the term hardness. Example? (heating of water eliminating CO 2 deposits Calcium carbonate in pipes and boilers)

5 Species in water Many different species of the same element can exist in solution. Different species have different chemical reactivity. What is a species? Why are they important in aqueous chemistry? Generally -- species are different forms of an element that are chemically distinguishable or that can react in a different chemical manner. These include different oxidation states, metal complexes, organometallic compounds, hydrates, etc.

6 Species in water Think also about Fe(II), Fe(III), Cr(III), Cr(VI), these are examples of oxidation state species. Different ligand or complexed species exist as well as different organic and organometallic species. We will discuss this topic more extensively as a separate topic also and expand on this theme.

7 Complexation and Chelation 82% of the 108 known elements are metals. 30 elements are of key importance and are usually evaluated. 100,000 different organic molecules are produced by industrialized countries. Each may have several environmentally derived conversions into metal binding compounds. Thus several million species formed as complex ligands with other elements, ions, and organic ligands may result form the combination of these two sources.

8 Complexation and Chelation Competing reactions from precipitation, multiple complexation and ligand formation and molecular bond formation are all being applied simultaneously. Solvent and transport medium - All of the fundamental environmental chemistry is water related as water forms the universal solvent for the planet and is the transport medium as well. It also enables most species formation.

9 Complexation, Ligands, and Complexometric Reactions Chelation is based on the formation of complex ligands Metal Ions react with electron-pair donors to form: 1. Complex Ions. (example Cu(NH 3 ) 4 2+ (example Cd 2+ + CN - CdCN + or 2. Coordination Compounds (example Cu(NH 2 CH 2 COO) 2

10 Complexation, Ligands, and Complexometric Reactions Ligands are ions or molecules that form covalent bonds with cations by donating a pair of electrons which are then shared by the ligand and the cation. The number of bonds formed per molecule is referred to a "Dentate" meaning tooth-like. Unidentate reagents form a single bond per molecule.

11 Examples of Ligands Neutral Anionic H 2 O * F -, Cl - *, Br -, I - NH 3 * SCN - RNH 2 (aliphatic amines) CN - HCO 3- * CO 3 2- * SO 4 2- * OH - RN(COOH) 2 RCO 2- (Carboxylate) S 2- * Major forms in natural waters

12 Complexation, Ligands, and Complexometric Reactions Some chelates are built up in a stepwise manner with successive additions of ligand bonds and successive stability constants that generally decrease in stability with the number of ligands. Cu 2+, Cu(NH 3 ) 2+, Cu(NH 3 ) 2 2+, Cu(NH 3 ) 3 2+, Cu(NH 3 ) 4 2+, and Cu(NH 3 ) 5 2+ Example - Zn 2+ + NH 3 <---- ZnNH 3 2+ K 1 = = 3.9 x 10 2 ZnNH NH 3 <---- Zn(NH 3 ) 2 2+ K 2 = = 2.1 x 10 2

13 Complexation, Ligands, and Complexometric Reactions Overall formation constant - Zn NH 3 <---- Zn(NH 3 ) 2 2+ B 2 = = K 1 K 2 = 8.2 x 10 4 For: Zn(NH 3 ) 3 2+ B 3 = K 1 K 2 K 3 AND Zn(NH 3 ) 4 2+ B 4 = K 1 K 2 K 3 K 4

14 Complexation, Ligands, and Complexometric Reactions Rule of thumb - Take note. - Complexes with monodentate ligands are usually less stable than those with multidentate ligands. The formation of complexes occur when electron pair donors contribute electrons to a metal ion through the ligand bond filling empty orbitals of the metal ion.

15 Chelating Agents A chelate is a cyclic complex formed by a cation bonded by two or more donor groups contained in a single ligand. One of the most important group of complexometric agents are the Aminopolycarboxylic and Polyprotic Acids These are large organic compounds that posses amines and carboxylic acid groups. They tend to form 1:1 complexes with metal ions and usually form 4 or 6 ligand bonds (tetra- or hexa- dentates). Example of the most studied of these is EDTA or Ethylenediaminetetraacetic acid. It is a Hexadentate ligand with Six potential bonding sites two amino groups and four carboxyl groups.

16 Chelating Agents EDTA is water soluble and has four ionizable groups It is a zwitterion of the form H 4 Y It is usually used in the salt form: Na 2 H 2 Y The Ionization of EDTA is ph dependent The dissociation constants are: K1 = 1.02 x 10-2, K2 = 2.14 x 10-3, K3 = 6.92 x 10-7, K4 = 5.50 x 10-11

17 Chelating Agents EDTA combines in a 1:1 ratio with all metals regardless of the charge of the particular cation. What about Na and H then (with 2 each)? Note: chelators can also be ion exchangers It forms a stable complex with almost all metal cations (group or= II) The steps in its progressive ionization are illustrated in Figure 13-2, 3, 4. Fundamentals of Analytical Chemistry, Skoog, West, Holler, Saunders College Publishing, NY, NY, pg , 1992.

18 Complexes of EDTA and Metal Ions EDTA forma 1:1 complex with metal ions regardless of the charge on the cation. M n+ + Y 4- <---- MY (n-4)+ The Formation Constants K MY are given for many metal ions. For K MY = Table of Formation Constants for EDTA Complexes

19 Cation K MY Log K MY Ag x Mg x Ca x Sr x Ba x Mn x Fe x Co x

20 Ni x Cu x Zn x Cd x Hg x Pb x Al x Fe x V x Th x

21 Example calculations involving EDTA 1. Example of titration curve using Ca 2+ General rule of thumb for the calculation of concentration pm n+ for a complexometric titration. Region Major Constituent Major supplier of Ca Before the Eq. Pt. Ca 2+ + CaY 2- Ca At Eq. Pt. CaY 2- CaY 2-3. After the Eq. Pt. CaY 2- + Y 4- CaY 2-

22 Example calculations involving EDTA Ca has a larger Formation Constant than Mg and this shows up in the titration curve just as the titration of a weaker acid would in an acid base titration. This illustrates the selectivity of a complexing agent like EDTA for specific ions in solution. The variability of the complex with different metal ions at different ph values results in a different formation constant at different ph values. A minimum ph needed for an acceptable complex of various metal ions is in the range of ph 6-8. Besides competing for different ions other competing reactions must also be considered in environmental complexation reactions.

23 Competing reactions and the complexities of real environmental situations Multiple competing reactions are occurring. Hydroxides of iron and aluminum start forming at a ph of 3.5. Multiple complexing agents such as monodentate Cl -, F -, OH -, NH 3, or H 2 O Chelating agents are found in natural waters, as part of industrial waste waters and sewage. They are ubiquitous in nature. What are some of them? Humic acids, Fulvic acids, Phosphates, anions, biological waste products (NH 3 ), Industrial molecules, etc, and water itself.

24 Competing reactions and the complexities of real environmental situations Example - Case study of EDTA at Oak Ridge National Laboratory - Oak Ridge National Laboratory in Oak Ridge, TN used EDTA to chelate metals to make them more soluble in the clean-up and disposal process and pumped them into disposal pits and disposal wells at concentrations of 10-7 M.

25 Competing reactions and the complexities of real environmental situations Some years after burial EDTA is making metals mobile in the environment that would not be mobile. This case study shows the slow degradation rate of EDTA and other similar chelators and the role they play in transport of metals in the environment. (The use of specific isotopes permitted the positive identification of these metal ions in the environment and identified them as anthropogenic and coming from the study) {pg. 66 text, 6th ed. Env. Chem., by Manahan} Others involving detergents (water softeners), Natural materials - later.

26 Environmental Examples of Applications of EDTA A Common Water Hardness Testing Calcium Carbonate equivalency is calculated by adding drops of EDTA solution to a measured Buffered solution of the water to be tested. The standard usually used is 1 drop per g (1 grain) of CaCO 3. Several decades ago the titration of common metal ions was performed using EDTA on a routine basis. Many of the complexometric procedures are 30 to 100 years old. In the past decades most of the tests have been transferred to spectroscopic methods.

27 Environmental Examples of Applications of EDTA Another form of complex formation - Many metal ions form polymeric species with OH- as a bridging group. This is also another form of a complex formed with the hydroxide ions - 2(Fe(III) and 2(OH-) for a net charge of 4+.

28 A man made chelator in natural water (NTA) Case Study - The sodium salt of NTA is used as a detergent or detergent additive and is a substitute for phosphate detergents. It is present in natural waters as a contaminant in mg and greater quantities. It has a relatively long environmental lifetime in the environment being broken down by a dominantly bacterial mechanism almost exclusively. The text goes through the ionization of Nitrilotriacetic acid (NTA). NTA has 3 hydrogens to dissociate with Kas and pk as as follows. Ka 1 = 2.18 x 10-2 and pka 1 = 1.66 Ka 2 = 1.12 x 10-3 and pka 2 = 2.95 Ka 3 = 5.25 x and pka 3 = 10.28

29 A man made chelator in natural water (NTA) Case Study - Ka 2 dominates the species range for all normal natural waters due to its ph range. Figure 3.4 (Manahan) demonstrates this with the display of HT 2- as the predominant species from ph 3 to ph 10. Example from the book of a series of complex interactions of - Ksp, Ka 3, K f and K w. Hydroxides such as Pb(OH) 2 are solids and form with many metals at natural water ph's (usually at relatively low phs). Many metal ions are suspended in water or precipitated from water as hydroxides. An examination of the relationship of a chelate and a hydroxide. Which are dominant at ph 8.0 in natural water?

30 A man made chelator in natural water (NTA) Case Study - Pb(OH) 2 is a species in suspension in a fresh natural water at ph 8.00 NTA is present and at ph 8 is predominantly HT 2- ion. Pb(OH) 2 (s) + HT 2- PbT - + OH - + H 2 O Equations describing the solubilization are: adding them algebraically Pb(OH) 2 (s) Pb OH - K sp = [Pb 2+ ][OH - ] 2 = 1.61 x HT 2- H + + T 3-

31 A man made chelator in natural water (NTA) Case Study - K a3 = = 5.25 x (not adjusted for alpha [H + ]) Pb 2+ + T 3- PbT - K f = = 2.45 x H + + OH - H 2 O

32 A man made chelator in natural water (NTA) Case Study - Pb(OH) 2 (s) + HT 2- <---- PbT - + OH - + H 2 O A question - which is the predominant species? - complexed lead as PbT - or uncomplexed chelating agent NTA as HT 2-.

33 A man made chelator in natural water (NTA) Case Study - Thus = K/[OH-] = (2.07 x 10-5 )/(1.00 x 10-6 )

34 A man made chelator in natural water (NTA) Case Study - Thus is 20/1 in favor of the complex over the free form of the chelating agent. Also note that the hydroxide form of the solid precipitate was solubilized. What does this tell you about the interaction of chelation in natural waters? The solubility of this complex decreases with increasing ph

35 A man made chelator in natural water (NTA) Case Study - Apply Le Chatelier's principles to the overall reaction - Pb(OH) 2 (s) + HT 2- PbT - + OH - + H 2 O What would increasing [OH - ] do to the [PbT - ]? What would increasing [HT 2- ] do to the [PbT - ]? What would decreasing [OH-] do to the [PbT - ]?

36 A man made chelator in natural water (NTA) Case Study - A statement of Le Chatelier's Principle If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

37 Example: What about for a Carbonate and NTA? This is another very important ubiquitous component in natural waters. A second Example with the chelator NTA. Now let s do the same examination using lead carbonate (PbCO 3 ). PbCO 3 is in equilibrium with the CO 2, HCO 3, CO 3 2- system and the NAT species. At ph 7.0 the HT 2- species is again the dominant species. CO 2 (aq) + H 2 O HCO H + K eq = = 4.45 x 10-7 pk a1 =[-LogK a ]= 6.35 HCO 3- CO H + K eq = = 4.69 x pk a2 =[-LogK a ]= 10.33

38 Example: What about for a Carbonate and NTA? PbCO 3 (s) + HT 2- PbT - + HCO 3 - Components of equilibrium PbCO 3 (s) Pb 2+ + CO 3 2- Ksp = [Pb 2+ ][CO 3 2- ] = 1.48 x Pb 2+ + T - PbT - Kf = = 2.45 x HT 2- H + + T 3- K a3 = = 5.25 x CO H + HCO 3 -

39 Example: What about for a Carbonate and NTA? PbCO 3 (s) + HT 2- PbT - + HCO 3 - Given: In natural waters HCO 3- frequently has a concentration of 1.0 x 10-3 M/L Thus: Thus is thus a 41 to 1 lead complexed as (PbT - ) to uncomplexed free NTA from of (HT 2- ).

40 Example: What about for a Carbonate and NTA? As can be seen from Le Chatelier's principle as applied to the overall equation PbCO 3 2- (s) + HT 2- PbT - + HCO 3 - the higher the HCO 3- concentration the less of the complexed lead (PbT - ) and the lower the HCO 3- the greater the complexed lead (PbT - ). Thus in this case the complex depends on the HCO 3 - concentration but both are dependent on the ph. Go back to the alpha table and adjust the ph and look at the effect of ph on complexation of NTA and see that the lower the ph the more the solubility and the higher the concentration of the complex PbT -.

41 Competing Metal Ions in Complexation Example using Ca, Pb and (NTA) Since competing complexes are always present, let s examine one of the most common ions, calcium, and its influence on the NTA complex of lead. Calcium also forms the NTA complex from the dominant ionic form of NTA (HT 2- ). (given a ph of 7.0) Ca 2+ + T 3- CaT - K f = = 1.48 x 10 8 and HT 2- H + + T 3- K a3 = = 5.25 x 10-11

42 Competing Metal Ions in Complexation Ca 2+ + HT 2- CaT - + H + K' = = 1.48 x 10 8 x 5.25 x = 7.75 x 10-3 K' is the product of K f for CaT - and the dissociation of NTA K a3 as before in the Pb example. Again in water Ca 2+ is typically in natural fresh water at a concentration 1.00 x 10-3 M (remember ph at 7.0).

43 Competing Metal Ions in Complexation Thus [CaT - ]/[HT 2- ] is 76/1 thus a 76 to 1 calcium complexed as (CaT - ) to uncomplexed free NTA from of (HT 2- ). What about the competition the carbonate? Combining equilibria: K" = Remember - PbCO 3 2- (s) + HT 2- PbT - + HCO 3 - and the reverse of -(Ca 2+ + HT 2- CaT - + H + ) or CaT - + H + Ca 2+ + HT 2- K' = = 1.48 x 10 8 x 5.25 x = 7.75 x 10-3

44 Competing Metal Ions in Complexation PbCO 3 2- (s) + CaT - + H + Ca +2 + HCO 3- + PbT - K" = K/K' = (4.06 x 10-2 )/(7.75 x 10-3 ) = 5.24 Thus the distribution between the Pb complex and the Ca complex is described by So twice the Ca chelate is present as Pb chelate. Thus the amount of Ca will influence the amount of PbCO 3 solubilized form the solid and we have seen that many other equilibrium effect the amount of an element mobilized.

45 Competing Metal Ions in Complexation In summary influences from (a.) Carbonate and (b.) Carbon dioxide, (c.) Other metal ions, (d.) ph (amount of H+ and OH- ), ionization of NTA, (e) extent of hydroxide Thus many thing effect the complexation and equilibrium in water.

46 Competing Metal Ions in Complexation One final example of competing complexes, solubility's, metal chelates, ph and other equilibrium - Graphical representation of the developed example - This is one reason why we will need mathematical models to assist in combining all these competing equilibrium simultaneously.

47 Complexation by Humic Acids and Other Organic Matter One of the most important types of complexing agents that are naturally occurring are the humic acid and fulvic acids which are a class of compounds of similar origin and similar structural design. These acids are not single molecules but a Class of Molecules. Fulvic acids is generally accepted to be the soluble form of a large complex molecule class of 500 to 2,000 molecular weight. Humic acids are generally accepted to be less soluble and exist in solid or particulate form in water and soil.

48 Complexation by Humic Acids and Other Organic Matter They have been recognized since 1800 but have only been recently studied extensively (since ~ 1970), due to there role in the formation of trihalomethanes {THMs} during chlorinating of water in purification. Both forms ion exchange and complex metal ions accumulating and releasing these metals in complex interactions with ph, ionic strength, temperature, specific ionic content and other interactions. Routine composition overviews suggest compositions of % C; 30-45% O; 3-6% H; 1-5% N; and 0-1% S Examples of these ions show their very complex structure. You can recognize the usual chelating functional groups in abundance.