Download Hydrogeochemistry

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project

Document related concepts

Jahn–Teller effect wikipedia , lookup

Hydroformylation wikipedia , lookup

Metal carbonyl wikipedia , lookup

Metalloprotein wikipedia , lookup

Evolution of metal ions in biological systems wikipedia , lookup

Spin crossover wikipedia , lookup

Ligand wikipedia , lookup

Stability constants of complexes wikipedia , lookup

Coordination complex wikipedia , lookup

Transcript
Complexes

Complex – Association of a cation and an
anion or neutral molecule



All associated species are dissolved
None remain electrostatically effective
Ligand – the anion or neutral molecule
that combines with a cation to form a
complex


Can be various species
E.g., H2O, OH-, NH3, Cl-, F-, NH2CH2CH2NH2
Importance of complexes

Complexing can increase solubility of
minerals if ions involved in reactions are
complexed


Total concentration of species (e.g.,
complexed plus dissolved) will be higher in
solution at equilibrium with mineral
E.g., Solution at equilibrium with calcite will
have higher SCa2+ if there is also SO42present because of CaSO4o complex

Some elements more common as
complexes



Particularly true of metals
Cu2+, Hg2+, Pb2+, Fe3+, U4+ usually found as
complexes rather than free ions
Their chemical behavior (i.e. mobility, toxicity,
etc) are properties of complex, not the ion

Adsorption affected by complex



E.g., Hydroxide complexes of uranyl (UO22+)
readily adsorbed by oxide and hydroxide
minerals
OH- and PO4- complexes readily adsorbed
Carbonate, sulfate, fluoride complexes rarely
adsorbed to mineral surfaces

Toxicity and bioavailability depends on
complexes




Toxicity – e.g. Cu2+, Cd2+, Zn2+, Ni2+, Hg2+,
Pb2+
Toxicity depends on activity and complexes
not total concentrations
E.g., CH3Hg+ and Cu2+ are toxic to fish
other complexes, e.g., CuCO3o are not

Bioavailability – some metals are essential
nutrients: Fe, Mn, Zn, Cu

Their uptake depends on forming complexes
General observations

Complex stability increases with increasing
charge and/or decreasing radius of cation


Space issue – length of interactions
Strong complexes form minerals with low
solubilities

Corollary – Minerals with low solubilities form
strong complexes

High salinity increases complexing


More ligands in water to complex
High salinity water increases solubility
because of complexing
Complexes – two types

Outer Sphere complexes


AKA – “ion Pair”
Inner Sphere complexes

AKA – “coordination compounds”
Outer Sphere Complexes

Associated hydrated cation and anion



Metal ion and ligand still separated by
water




Held by long range electrostatic forces
No longer electrostatically effective
Association is transient
Not strong enough to displace water
surrounding ion
Typically smaller ions – Na, K, Ca, Mg, Sr
Larger ions have low charge density
Outer Sphere complexes

Metal ion and ligand still separated by
water




Association is transient
Not strong enough to displace water
surrounding ion
Typically smaller ions – Na, K, Ca, Mg, Sr
Larger ions have low charge density


Relatively unhydrated
Tend to form “contact complexes”

Larger ions have low charge density


Relatively unhydrated
Tend to form “contact” ion pairs – with little
water in between
Inner Sphere Complexes

More stable than ion pairs




Metal and ligands immediately adjacent
Metal cations generally smaller than ligands
Largely covalent bonds between metal ion
and electron-donating ligand
Charge of metal cations exceeds
coordinating ligands

May be one or more coordinating ligands
An Aquocomplex – H2O is ligand
Outer sphere – partly
oriented water
Coordinating cation
Inner sphere – completely
oriented water, typically 4
or 6 fold coordination

For ligand, L to form inner-sphere complex



Must displace one or more coordinating
waters
Bond usually covalent nature
E.g.:
M(H2O)n + L = ML(H2O)n-1 + H2O

Size and charge important to number of
coordinating ligands:



Commonly metal cations smaller than ligands
Commonly metal cation charge exceed charge
on ligands
These differences mean cations typically
surrounded by several large coordinating
ligands

E.g., aquocomplex



Maximum number of ligands depends on
coordination number (CN)
Most common CN are 4 and 6, although 2,
3, 5, 6, 8 and 12 are possible
CN depends on radius ratio (RR):
RR =
Radius Coordinating Cation
Radius Ligand

Maximum number of coordinating ligands


Depends on radius ratio
Generates coordination polyhedron

All coordination sites rarely filled



Only in aquo-cation complexes (hydration
complexes)
Highest number of coordination sites is
typically 3 to 4
The open complexation sites results from
dilute concentration of ligands

Concentrations of solution



Water concentrations – 55.6 moles/kg
Ligand concentrations 0.001 to 0.0001 mol/kg
5 to 6 orders of magnitude lower


Ligands can bond with metals at one or
several sites
Unidentate ligand – contains only one site


E.g., NH3, Cl- F- H2O, OH-
Bidentate

Two sites to bind: oxalate, ethylenediamine
Various
types of
ligands

Multidentate – several sites for complexing

Hexedentate – ethylenediaminetetraacetic
acid (EDTA)
Additional
multidentate
ligands
Thermodynamics of complexes

Strength of the complex represented by
stability constant


Kstab also called Kassociation
An equilibrium constant for formation of
complex

Typical metals can form multiple complexes
in water with constant composition



Al3+, AlF2+, AlF2+, AlF3
SAl = Al3+ + AlF2+ + AlF2+ + AlF3
Example:
Kstab =
Al3+ + 4F- = AlF4aAlF4(aAl3+)(aF-)4


Complexation changes “effective
concentrations” of solution
Another example:
Ca2+ + SO42- = CaSO4o



Here the o indicates no charge – a
complex
This is not solid anhydrite – only a single
molecule
Still dissolved
Kstab =


aCaSO4o
(aCa2+)(aSO42-)
aCaSO4o is included in the Kstab calculations
It is a dissolved form

Examples of Kstab calculations and effects
of complexing on concentrations