Ch.5 Coordination Chemistry

• Complex formation is important in the chemistry of natural waters and wastewater.

• Modify metal species in solution, generally reducing the free metal concentration so that effects and properties which depend on free metal ion concentration.

Ex: soulbility, toxicity, surface properties, adsorption • Complex formation is used in water analysis such as

determination of hardness, Ca2+, Mg2+

concentration, EDTA etc.

S/J p.197-223, S/J p.323-331, 363-370

Toxicity of Copper

(Das et al. 1997)

• Cu2+

concentration vs. toxicity

• Higher concentration →more toxic

Complex formation

Analogy with Acid/BaseBrφnsted acid : donates a proton to water

Definition of Lewis

acid/baseLewis acid : accepts an electron pair ex) metalsLewis base : donates an electron pair ex) ligand

ex) NH3

≡

1 pairs

H2

O ≡

2 pairs

Cl-

≡

3 pairs

Nomenclature

① Metal ⇒ Lewis acids (electron pair acceptor)

Coordination Number (CN) = # of electron pairs Lewis acid can acceptCoordination number is usually 2, 4, 6, or 8

CN is 6 for Fe3+, Cu2+, Zn2+,Al3+

Fe(H2

O)63+

+ H2

O = [Fe(H2

O)OH]2+

+ 3H3

O+

Ka

(Lewis base)

1 for H+ ; the predominant metal cations in typical fresh and ocean water~ Na+, Ca2+, Mg2+

② Ligand ⇒ Lewis base (electron pair donor)~not strong complex formersmost common O -

OH-, H2

O, COO-

i)

Monodentate ligandsN -

NH3

, organic-N, CN-

X -

Cl-, Br-, I-

(other -

C, S, P)HCO3

-, SO4-2

ii) Multidentate -

donate more than one pair , CO32-

Carbonato Sulfato

• Attach to a central metal ion at only one point are called monodentate ligands• Attach at two or more sites are called multidentate ligands or chelating agents

Inorganic complexes

① Aquo (H2

O) -

Fe(H2

O)62+

= Fe2+

Al(H2

O)63+ = Al3+

② Hydroxo (OH-) -

Fe(OH)+, Fe(OH)2+, NaOH, CdOH+

③ Oxo (O) -

MnO4-

permanganate (oxidant)

④ Carbonato (HCO3-,CO3

-2) -

MgHCO3+, SO4

2-

Sulfato

MgCO30 ← ZERO CHARGE SOLUBLE

⑤ Chloro (Cl-, Br-) -

HgCl+

⑥ NH3

- Ammine

p 215.In general, hydroxo, carbonato, and sulfato complexes of metals tend to sorb more strongly at clay surfaces than do the free metal ions (Fig 5-4)

Coordination Number

Species Formula CN

Ni(Ⅱ)Ni(CO)4

2+ 4

Ni(phenothroline)32+ 6

Fe(Ⅲ)

Fe(H2

O)63+ 6

Fe(CN)63- 6

FeCl4- 4

cf.

Formula Name

Fe(CN)64- hexacyanoferrate(Ⅱ) ion

Al(H2

O)63+ hexaaquoaluminum(Ⅲ) ion

Cu(NH3

)42+ tetraammine copper(Ⅱ) ion

MgCO30 carbonatomagnesium(Ⅱ) ion

CaSO40 sulfatocalcium(Ⅱ) ion

The adsorption of cobalt on silica surfaces as a function of the

ionic state of the cobalt

• What’s the dominant species in

• pH<6• 6<pH<8.5

N N

Fe2+

N

N N

N

1.10 phenathroline

red color

Fe2+ color complex –

an indicator for Fe2+ cf: mononuclear complex ~ complexes containing one central ion

cf. polynuclear

complexes

Al(H2

O)63+

+ H2

O ⇄

Al(H2

O)5

OH+

+ H2

OMonohydroxo pentaaquo aluminum(Ⅲ) ion

Ion pairs -

CaCO3

, CaHCO3+, CaSO4

0, CaOH+, MgCO30, MgSO4

0

• the bond strength of metal ion complexes varies considerably• weakly associated complexes• one or more layers of water between the ligand

and the central

ion

5.3. Reaction Rate : the rate of coordination reaction

Labile ~ very fast reactions, Inert ~ very slow reactions

M(H2

O)n

+ L ⇄

ML(H2

O)n-1

+ H2

O~ is virtually complete within seconds to minutes at typical

natural water concentrations (103-M).Metal ion Ligand

(page 201)

Ni2+ CH3COO-

chelate

if ligand

multidentate

Complex -

metal + ligand

(coordination compound)Lewis acid/Lewis base

Mixed ligand

complex -

Fe(H2

O)5

OH2+

-

more than one kind of ligand

5-4 Complex stability and Equilibrium calculations

Ligand

+ central metal ion ⇄

complex

NH3(aq) + Cu2+

⇄

Cu(NH3

)2+

constantstability K10NHCu

)Cu(NH 4

3(aq)2

23

===+

+

General tendency of Complexation① A-metal cations

;

Na+, K+ (Alkali metals) Mg2+, Ca2+

(Alkalinity earth metals)Al3+, Si4+

→ preferentially coordinate with ligands

containing oxygen ligands, such as CO32-, OH-,

borate (B(OH)4-),.....

② B-metals ; Ag+, Zn2+, Hg2+, Pb2+, Sn2+

-

S, P, or N containing ligands

(NH3

, S2-, phosphite) rather than oxygen containing ligands

③ Transition metal ions- Cr2+, Cr3+, Fe2+, Fe3+, Ni2+

~ strong tendency to form complexes with a wide variety of ligands

④ ClO4-

(perchlorate): very little tendency to form complexesNO3

-

: as swamping electrolytes to have a constant ionic strength

Phosphate(PO43-), hydroxide(OH-), carbonate(CO32-) ; potent complex formers (page 202)

Caution! So many exceptions; experimental results can be different with theories

Chelate Effect

the stability of a chelate

generally increases with an increase in the number of points of attachment between the chelating agent and the central

metal ion.

ethylemediamine

diethylenetriamine

triethylenetetramine

Complex stability and Equilibrium calculations

EDTA(Ethylene Diamine TetraAcetic acid)

gand][metal][licomplex const.stability K ==

Ca2+

+ EDTA4-

⇄

(EDTA-Ca)2-

K = 1010.7

Mg2+

+ EDTA4-

⇄

(EDTA-Mg)2-

K = 108.7

If K value were large ⇒ stable complexes.The reaction between EDTA and Ca2+ is complete prior to any reaction EDTA and Mg2+

EBT(blue)

+ Mg2+(red) ⇄

EBT-Mg (red) K = 107

; titration point!

EBT-Mg complex is weaker than EDTA-Mg complex

EDTA is added to the solution containing Ca2+

+ EBT-Mg (red)

Total hardness analysis (Ca2+ Mg2+) (p.204)

Two ways of writing stability consts.

(1) Stepwise formation constants (Central metal ion consecutively adds one ligand)

Hg2+

+ Cl-

⇄

HgCl+ logK1

= 7.15

HgCl+

+ Cl-

⇄

HgCl2

0 logK2

= 6.9

HgCl20

+ Cl-

⇄

HgCl3

-

logK3

= 2.0

HgCl3-

+ Cl-

⇄

HgCl4

2-

logK4

= 0.7

(2) Overall formation constants (Central metal ion combines with

all of the ligands necessary to form a specific complex)

Hg2+

+ Cl-

⇄

HgCl+ logβ1

= 7.15

Hg2+

+ 2Cl-

⇄

HgCl2

0 logβ2

= 14.05

Hg2+

+ 3Cl-

⇄

HgCl3

-

logβ3

= 16.05

Hg2+

+ 4Cl-

⇄

HgCl4

2-

logβ4

= 16.75

Example 5-1 (p 206) COD analysisCOD analysis employs mercuric sulfate, HgSO4, as a source of Hg2+, to complex Cl-

ion, so prevent its oxidation to Cl2(ag) by K2

Cr2

O7

.What will be the concentration of each chloromecury complex (II)

and of the free Cl-

ion in the solution?

Cr2

O72-

+ 14H+

+ 6Cl-

→ 2Cr3+

+3Cl2(aq)

+ 7H2

O4H+ + 4Cl-

+ O2

→ 2Cl2

+2H2

O

Hg2+

Hg(OH)+ HgCl+

(Majority of chloride)

hydroxomercury HgCl20(aq)

complexes HgCl3-

not important HgCl42-

Principles of COD analysis

Organics + Cr2 O72- + H+ = Cr3+ + CO2 + H2 O

Organics + O2 = CO2 + H20

Typical Procedure for COD analysis (can be found in the analytical manual)

1. 50 ml of sample2. Add (1 g HgSO4

+ 5ml of H2

SO4

)3. 25 ml of 0.0417 K2

Cr2

O74. Add 70 ml of sulfuric acid reagent

Example 5.1 0.4 g H2

SO4

→ 60 ml (20 ml sample (containing 1000 mg Cl-/L) + 40 ml other reagents)

M102.24SOH 297g

mole 11L

1000ml60ml0.4g C 2

42HgT,

−×==

M109.3935500mg

1mole60ml20ml(Sample)

1LCl 1000mg C 3

-

ClT,−×=××=

The unknowns are ; [Hg2+], [Cl-], [HgCl+], [HgCl2

0(aq)

], [HgCl3-], [HgCl4

2-]~six equations required~pH < 1, hydroxo mercury (Ⅱ)complexes are not important.

(6)-]][Cl[Hg][HgCl =10 = β4

(5)-]][Cl[Hg

][HgCl = 10 = β3

(4)-]][Cl[Hg][HgCl = 10 = β2

(3)- ]][Cl[Hg

][HgCl = 10 = β1

(2)- M10×9.39 =

]4[HgCl + ]3[HgCl + ]2[HgCl + ][HgCl + ][Cl = C

(1)- M10×2.24 =

][HgCl + ][HgCl + ][HgCl + ][HgCl + ][Hg = C

42

24 16.75

32

1316.05

22(aq)

0214.05

27.15

3-

-24

-3(aq)

02

+-ClT,

2-

-24

-3(aq)2

++2HgT,

0

−+

−

−+

−

−+

−+

+

[Cl-] = 3×10-8M ~ 0.001 mg/L

[Hg2+] = 1.42×102-

M

[HgCl+] = 6×103-

M

[HgCl20(aq)

] = 1.59×10-3 M from [Cl-]T

= 9.39×103-M

[HgCl3-] = 4.3×10-9 M

[HgCl42-] = 6×10-16 M

The implication for this result;

4Cl-

→ 2Cl2(aq)

+ 4e-

4e-

+ 4H+

+ O2

→ 2H2

O4H+

+ 4Cl-

+ O2

→ 2Cl2

+ H2

O

32 mg of O2

will oxidize 4×35.5(=142)mg of Cl-

225 mg of O2

← 1000 mg of Cl-

An empirical " chloride correction factor“

•COD due to Cl-

oxidation

Assuming

0.00041×40 (mgCl-/L) × 2 hr ≅

0.0328 mg O2

/L

Brine water (19,000 mg/L) ≅

16 mg O2

/L (16 mg COD/L)

•The longer the oxidation was allowed to proceed, the greater the

extent of chloromerciry(Ⅱ) complex dissociation and the greater was the COD due

to Cl-

oxidation

(hr) timeLCl00041.0

-

××mg

LCl40 -mg

5.5 Hydrolysis of Metal Ions - H2 O and OH- as Ligands

Free metal ions are complex with water ~ should be hydratedThe interaction of these hydrated metal ions with acids and bases ~ligand exchange reaction, hydrolysis

Al(H2

O)63+

+ H2

O ⇄

Al(H2

O)5

OH2+

+ H3

O+

Al(H2

O)5

OH2+

+ H2

O ⇄

Al(H2

O)4

(OH)2+

+ H3

O+

Al(H2

O)4

(OH)2+

+ H2

O ⇄

Al(H2

O)3

(OH)3(S)

+ H3

O+

Al(H2

O)3

(OH)3(S)

+ H2

O ⇄

Al(H2

O)2

(OH)4-

+ H3

O+

~the transfer of protons to water molecules to form hydronium ion → hydrated metal cations proton donors

acid-base reactions (proton transfer reactions)→ rewrite the replacement of a water of hydration by a hydroxyl ion

• Fe(III) and many other multivalent ions behave similarly to the aquoaluminum(III) ion

5-13

2522

362

14w

-32

9w2

252

-362

10 K OH OHO)Al(H OH O)Al(H

10 K OH OH O2H

10 K OH OHO)Al(H OH O)Al(H

=+⇔+

=+⇔

=+⇔+

+++

+

++

• the equilibrium constants are different but that each define the

relationship between Al(H2

O)63+

and Al(H2

O)5

OH2+.

• In general, the % of the hydrolyzed species increases as the pH increases.

Metal Hydrolysis (S&J 209-217)

AgCl ⇄

Ag+

+ Cl-

KSO

CaCO3 ⇄

Ca2+

+ CO32-

KSO

total soluble Ca = [Ca]T

= [Ca2+]

Hydrolyzing ZnCO3(S)

= Zn2+

+ CO32-

KSO⇅

react with H2

O

∴ Total soluble zinc = ZnT ≠ [Zn2+]

Acidity

Zn2+

+ H2

O = ZnOH+

+ H+

HA + H2

O = A-

+ H3

O+ Ka

(Brɸnsted acid)

Zn2+, Al3+, Fe3+ - Brɸnsted acid

Zn(H2

O)62+

= Zn2+

(aqua zinc ion)

Zn(H2

O)62+

+ H2

O ⇄

Zn(H2

O)5

(OH)+

+ H3

O+(Ka

)

H2O

H2O

H2O H2O

H2O

H2O

Example 5-2. Calculate the pH of a 10-5 M Hg(ClO4 )2 solution at 25°C

The property of weak acid of the same strength as acetic acid For example, the pH of equimolar H3PO4 and Fe3+ solutions are similar

Example 5-3. What is the dose of NaOH to maintain the alkalinity of the water during alum coagulation/flocculation (Al2(SO4)3)? Assuming that only hydroxoaluminum (III) species is Al(OH)3(s).

pH 8.0, AlK = 80 mg/L + 25 mg/L of Alum(Al2

(SO4

)3

14H2

OMW of

Al2

(SO4

)3

14H2

O : 594

Amount of NaOH required in order to maintain AlkAlK = CB - CA

= constant

Assume a stoichiometric reaction with the hydorxide to form aluminum hydorxideAl2

(SO4

)3

․14H2

O + 6 NaOH = 2 Al(OH)3(S)

+ 6 Na+

+ 3 SO42-

(25/594)×6 ×10-3 = 0.25 eq/L ≡

10.1 mg NaOH/L ≡

12.5 mg/L AlK

The amount of Alk lost as a result of alum addition

However, it does not accurately represent the detailed equilibrium picture.

What about other aluminum species?~ a variety of mononuclear hydroxo aluminum(Ⅲ)complexes

polynuclear hydroxoaluminum(Ⅱ)Al2

(OH)24+, Al7

(OH)174+, Al13

(OH)345+

; Fig 5.11mg/L Alum (0.01 eq/L) 0.5 mg/L AlK reduction

Characteristics

; the polynuclear complexes have higher OH-

and less H2O content per mole of aluminum than the mononuclear hydroxo aluminum(Ⅲ)complexesThese might be a more effective coagulant.

~ The stability region of the aluminum hydroxide solid is more confined when polynuclear hydroxo aluminum(Ⅲ) complexes are disregarded.

“Residual turbidity” verses “Alum dose” plot

• How alum fractions as a coagulant for a water of moderate alkalinity-> At low alum doses there is no reduction in turbidity

4. ① Ca2+,Mg2+→ analysis of hardness

EDTA(ethylenediaminetetraacetic acid)......Chelating agent

② the titrimetric finish of the COD test1, 10-phenanthroline complexing agent to detect Fe2+

③ Chloride analysis by the mercurimetric acid -

HgCl20

complex

Zinc SpeciationTotal soluble Zn = [Zn(Ⅱ)]T

in equilibrium with ZnO(S)

Step1 Write equilibria

Zn2+

+ H2

O = Zn(OH)+

+ H+ K1 = 10-8.96

Zn2+

+ 3H2

O = Zn(OH)3-

+ 3H+ K2 = 10-28.4

Zn2+

+ 4H2

O = Zn(OH)42-

+ 4H+ K3 = 10-41.2

ZnO(S)

+ 2H+

= Zn2+

+ H2

O KS

= 1011.14

Table 5.1 metal more soluble at low pH

Step2 List species in [Zn(Ⅱ)]T[Zn(Ⅱ)]T = [Zn2+] + [Zn(OH)-] + [Zn(OH)3

-] + [Zn(OH)42-]

Step 3 Write solubility constraint

][][

][H[ZnO]O]][H[Zn K

2

2s

22

s +

+

+

+

==H

Zn

Step 4 Write [species] as a function of [H+]

2s3

4

232-

4

s23

22-

3

21

21

2s

2

][HKK

][H][ZnK ][Zn(OH) -

][HKK

][H][ZnK ][Zn(OH) -

][HKK ][H

][ZnK [Zn(OH)] -

][HK ][Zn-

++

+

++

+

++

++

++

==

==

==

=①

②

③

④

Substitute solubility product restraint to equilibrium constant equations

(1~4).][Zn

][H][Zn(OH) K3

][Zn][H][Zn(OH) K2

][Zn][H[Zn(OH)] K1

2

424

2

33

2

+

+−

+

+−

+

++

=

=

=

⇒

Step 5 Take log[ ] as a function of pH

+ log[Zn2+] = logKs

+2log[H+]= 11.14 -

2pH

log[Zn(OH)]+

= logK1

+ log(Ks

) + log[H+]= 2.18 -

pH

log[Zn(OH)3

]-

= logK2

+ log(Ks

) -

log[H+]= -17.26 + pH

log[Zn2+] = 11.14 -

2pH -

①

log[Zn(OH)+] = 2.18 -

pH -

②

log[Zn(OH)3-] = -17.26 + pH -

③

log[Zn(OH)42-] = -30.56 + 2pH -

④

Zn2+

+ H2

O = Zn(OH)+

+ H+K1

= 10-8.96 -

①

Zn2+

+ 3H2

O = Zn(OH)3-

+ 3H+K2

= 10-28.4 -

②

Zn2+

+ 4H2

O = Zn(OH)42-

+ 4H+K3

= 10-41.2 -

③

ZnO(s)

+ 2H+

= Zn2+

+ H2

OK4

= 1011.14 -

④

⇊①+④

ZnO(S) + 2H+

+ Zn2+

+ H2

O = Zn2+

+ H2

O + Zn(OH)+

+ H+

ZnO(S) + H+

= Zn(OH)+

K4

= 2.18

②+④

ZnO(S) + 2H+

+ Zn2+

+ 3H2

O = Zn2+

+ H2

O + Zn(OH)3-

+ 3H+

ZnO(S) + 2H2

O = Zn(OH)3-

+ H+

K5

= -17.26

③+④

ZnO(S) + 2H+

+ Zn2+

+ 4H2

O = Zn2+

+ H2

O + Zn(OH)42-

+ 4H+

ZnO(S) + 3H2

O = Zn(OH)42-+ 2H+ K6

= -30.56

log[Zn(OH)42-] = logK3

+ logKS

-

2log[H+]= -30.56 + 2pH

ZnO(s) solid exists → straight line

• Note the stability region of ZnO(s) solid, outside the region -

no ZnO(s),

inside region -

ZnO(s) forms

Find minimum metal solubility from metal hydrolysis stability diagramMetal Removal –

Industry waste water

treatment

p. 262 [Cd(OH)2(s)

] Raise pH ⇒ Precipitation, but don't raise pH too much.

Coagulation -

particle removalpH & pC -

Al(OH)3(S)

(alum), alum sulfate, Al2

(SO4

)3Fe(OH)3(s)

Toxicity; metal toxicity depends on speciationCu2+, CuOH+, CuCO3

, toxicity of Cu depends on pH

Fig 5-2 4Pb2+

+ 4H2

O ⇄

Pb4

(OH)44+

+ 4H+

logK = -19.9

3Pb2+

+ 4H2

O ⇄

Pb3

(OH)42+

+ 4H+

logK = -23.2

6Pb2+

+ 8H2

O ⇄

Pb6

(OH)84+

+ 8H+

logK = -42.7

In pH<6,Pb2+

is the major species Pb2+

→ Pb4

(OH)44+

→ Pb6

(OH)84+

, Pb3

(OH)42+

• Table 5-1, at pH 2, no Fe2(OH)24+ in the 10-4 M whereas a significant amount existat 10-2

M

Table 5-2, Fig 5-4

•The bulk of the total metal content of natural waters and wastewaters is usually associated with particulates either as solid precipitate or adsorbed on particle surfaces such as clays or organic detritus.

•In general, hydroxo, carbonato, and sulfato complexes of metals tends to sorb more strongly at clay surfaces than do the free metal ions.

below pH 6, entirely Co2+

⇒ very little adsorption

6<pH<8.5 cobalt removal by adsorption

CoOH+,Co(OH)20

5.6. Complexes with other inorganic ligands

Natural waters contain significant concentration of inorganic and organic ligands other than H2O and OH-

The predominant metal cations in fresh and ocean watersThe major cations ~ Na+, Ca2+, Mg2+

~ not strong complex former 6.5<pH<8.5The major ligands : HCO3

-, Cl-, SO42-

cf. Fe3+,Al3+

⇓Table 5-6 (p. 219)⇓

do not consider complexation w/ OH-,HCO3-,Cl-,SO4

2-,CO32-

when Ca2+, Mg2+, K+, Na+ are being treated.

The chemical composition for ocean and river water

Consider the following species (p 217)

• The results show that only in ocean water are any of the metals in this model complexed to any significant extent by the inorganic ligands examined

Ignoring the complexes of Ca2+, Mg2+, Na+, and K+

with OH-, HCO3-, Cl-,

SO42-, and CO3

2-, when dealing with fresh waters of neutral pH values

High pH, high concentration of sulfate!

Heavy metals are much less toxic to fish in hard water that in soft water

Fig 5.5 Copper species distribution in a water containing total inorganic carbon. Caution; If solution were not in equilibrium with the solid, the species distribution would not be quite the same.

Diagram is based on the presence of CuO(S)

pH < 6.5

: mainly, Cu2+

At higher pH(pH > 10) : CuCO30

,Cu(OH)3-, Cu(OH)4

2-

finally, pH span of natural waters,CuCO30

Cu2+

+ CO32-

⇄

CuCO3

0(aq)

logK = -6.8 (6.5 < pH < 9.5)

AlK & pH

• Only at below pH 6.5 is free copper ion, Cu2+, the predominant copper-

containing species• At higher pH values the

carbonatecopper (II) complex, CuCO3

o, and the hydroxocopper (II) complexes, Cu(CH)3

-

and Cu(OH)4

-2, are the major copper-containing species

• Alkalinity has a profound effect on the free copper ion concentrationCopper is more toxic to fish in soft water than in hard water High hardness is usually accompanied by high alkalinity

• Besides complexes with inorganic ligands, the river waters contained significant amounts of copper complexed by organic ligands, that is, in the categories of “amino acid complex”

and “inert humic complex”.

The toxicity of "cyanide" to fishtoxic component of cyanide solution ⇒ HCNHCN ⇄

H+

+ CN-

Ka

= 6.17×10-10

pKa

= 9.3Ni2+

+ 4CN-

⇄

Ni(CN)4

2-

β4

= 1030

pH 7.5no nickel

HCN ~ 1.1 mg/L

with nickel ~ 0.08 mg/L

5.7 Complexes with Organic Ligands

1. Water Analysis Application

① EDTA → for hardness

analysis

② 1.10 phenathroline → Fe2+

quantitative analysis, an indicator in COD test

③ CH3

- CNOH∣

→

Ni

quantitative analysis red complex

CH3

-

CNOH(Dimethylglyoxime)

2. Natural Organic Ligands

5.7.1 The Nature of Copper in Water and Wastewater

Copper forms complexes with numerous ligandsInorganic ~ carbonate, cyanideorganic ~ amino acid complex

inert humic complex

COOH

OH

OH + Cu2+ ↔ + 2H+ β1=1012.8

C

O OCu

O

OH

(Table 5-8)Cu2+ ~ not a major complex~ The distribution of copper between solution and particulates phase

More complicated!Stiff (1971, England) 48~88 (average 69)% of total copper ~ in the particulate phase

Ocean water vs. River water

Organic Ligand

~ donate electron pairs N, O, S, Cl

ex) EDTA ethylenediamine tetraacetic acid

HOO2

HC CH2

COOH∖

∕

N-CH2

-CH2

-N∕

∖

HOOCHC

CH2

COOH (hexaqdentate)1:1 complexes -

1 mole ligand/mole metal

divalent metal Ca2+, Mg2+, Fe2+, Cu2+,~application for hardness analysis (p.225, Table 5-8)

Organic Ligands

1.Naturally occurringa. Natural waters; humic & fulvic acids

Cu2+, Fe2+, Fe3+

b. Wastewaters amino acids-N, -

COOH

For amino acids β1 7 to 9, β2 14 to 16, cysteine –SH, β1 19.5

c. Surfacesmetal " surface complexation"

d. Stiff (1971, England) 48~88% of total copper heme group bonds Fe3+~in the particle phase

2. Synthetic Ligands

"builder"NTA -

nitrilotriactic acid,

laundry detergents

~ forms complexes with Ca2+, Mg2+ to improve the efficiency

pKa

of NTA; 1.8, 2.9, 10.4

pH=7, HL2-

Function to increase efficiency of detergent to form complex with Ca2+, Mg2+

Mn+

+ L3-

= MLn-3

K1

~sequestering agentscf. corrosion control

~ Negative effect: forming complex with heavy metals, it is not treated but move far way to offer heavy metals by being bio-degradable itself⇒ can play detergent role even in the ‘hard water’

Table 5-10

107-M NTA ~ too small whole concentration.CuNTA-

4%PbNTA 2%NiNTA 1%

Fig 5-8, Table 5-11

Mn+ logK1

Ca2+ 7.61

Cu2+ 12.7

Fe3+ 15.87

Fe2+ 8.83

(Ca2+,Mg2+)

• The importance of conducting model studies and experiments on complex multi-metal, multi-ligand systems at concentrations of metals and ligands that closely resemble the environment being studied

• The influence of NTA as a complexing agent on the distribution of metal species can be assessed by comparing the concentrations of various metal species both in the presence and absence of NTA

Calculated distribution of some selected metals in a water in the absence of NTA

• NTA at 3 x 10-6

M effectively draws the metals that are complexed by the inorganic ligands present from these complexes into NTA complexes

• The metals that are poorly complexed by inorganic ligands are not significantly affected by the addition of this amount of NTA

5.7.3 Metal Ion Association with Humic Substance

Humic Substance ~pooly biodegradable decomposition products and byproduct of natural organic matter produced by both plants and animals extremely complex and diverse~not well-defined (chemical structure)~as amorphous, brown or black, hydrophilic, acidic polydisperse substances.common functional groups

on the basis of their solubility① Fulvic acids ~ are soluble in both dilute acid(pH1) and dilutebase② Humic acids ~ soluble in dilute base but not soluble in dilute acid③ Humin ~ insoluble in both ~ dilute acid and base Fulvic acids fraction is the predominant group of its in natural waters.

Table 5-12 (p.233)

*Fulvic acids ~ lower molecular weights than humic acids~ contain a higher percentage of oxygen~ appear to be located in a greater percentage of carboxyl groups

• Structure of fulvic acid

• Structure of Humic Acid

5.7. Interaction of Humic Substances with Metals

Schnitzer reported. Fe(Ⅲ) is more than two orders of magnitude greater than expected in a solution in a water at pH~7 in equilibrium with ferric hydroxide.

Example) 100 mg/L of fulvic acid ⇒ 8.4 mg/L Fe(Ⅲ), 4.0 mg/L Al⇒ can be melted over self-solubility

Two modes of binding to be significant(1) The formation of complexes or chelates(2) The association between the humic substances and a colloidal

particle of

metal hydroxide~ the ability to combine with (or bind) considerable quantities of metal ions

Example -

Titrate Lake Ontario with NTA

[Ca2+]T

= 103-

M[Cu2+]T

= 2×106-

M[Fe]T = 2×106-M ≅

[Fe2+]T

most

LT

= [H3

L] + [H2

L-] + [HL2-] + [L3-] + [CaL-] + [CuL-] +[FeOHL-]Low LT

: LT

= CuL-

dominates(because of existing much Fe3+) high LT

: CaL-

dominates

Example -

Many metals can be enriched into peat: 1000 times

higher than the concentration of the metal in water,

Christman (1968), the same color intensity

TOC 10~30mg/L

[Fe3+]T small

Fe3+

+ 3OH-

= Fe(OH)(S)

Speciation : ① Amount of metal ② Amount of ligand ③ K1

3. Analysis

a. EDTA -

Ca2+,Mg2+, ⇒ total hardness titrationsb. Fe2+

: 1-10 ion phenathroline

N N

Fe2+ FeL32+ orange collor

4. Metal bufferingTo maintain regular metal concentraion in medioprovide small equil of metalEDTA -

insure solubility of Fe3+

5. Hazardous waste treatment

Cyanide = CN-

S&J pp223HCN ~ the most toxicHCN = CN-

+ H+ Ka

= 10-9.2

Ni2+

+ 4CN-

= Ni(CN)42-

β4

= 1030

no evidence Ni(CN),......

NiT

= [Ni2+] + [Ni(CN)42-]

pH ignore NiOH+

CNT

= [HCN] + [CN-] + 4[Ni(CN)42-]

[Ni2+] = NiT

- [Ni(CN)42-] = NiT

-

β4

[Ni2+][CN-]4

= NiT

/ [1+β4

(CN-)4+] -

①

4-2

-24

][CN[Ni]Ni(CN) β4 +=