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Title: The determination of metal speciation in natural waters by electrochemical techniques


1
The determination of metal speciation in natural
waters by electrochemical techniques
  • Øyvind Mikkelsen

Mikkelsen 2003
2
Overview
  • Theoretical aspects
  • - Natural water
  • - Speciation, and importance of speciation
    studies
  • - Available techniques for speciation studies
  • - Electrochemical techniques
  • Some practical examples
  • - Cu, Cd, Pb and Zn speciation in natural water
  • - Fe(II) and Fe(III) speciation in seawater
  • - Al(III) speciation in natural water
  • Conclusions

Mikkelsen 2003
3
Theoretical considerations
Mikkelsen 2003
4
Natural water
  • Natural water includes e.g. rivers, lakes, ground
    water, wells, seawater,.

Mikkelsen 2003
5
What is speciation?
In water trace metals are present in a wide range
of chemical forms, in both the particulate and
dissolved phases.
The dissolved phase comprises the hydrated ions,
inorganic and organic complexes, together with
species associated with heterogeneous colloidal
dispersion and organometallic compounds.
In some instances these metals are present in
more than one valency state.
6
Possible forms of trace elemements
Simple ionic species Zn(H2O)62
Valency states As(III), As(V), Cr(III),
Cr(IV)
Weak complexes Cu-fulvic acid
Adsorbed on colloidal particles
Cu-Fe(OH)3-humic acid
Lipid-soluble complexes CH3HgCl
Organometallic species CH3AsO(OH2), Bu3SnCl
Particulate Metals adsorbed onto or
contained within clay particles
7
Interactions affecting trace metal speciation
G.E Batley, Trace element speciation analytical
methods and problems, CRC Press, Inc., 1989
8
An example, lead.
Free metal ? Pb2 Ion pair ?
PbHCO3 Complexes with organic pollutants ?
Pb2/EDTA
SOLUTION
Complexes with natural acids ? Pb2/fulvic acid
SUSPENSION
Ion adsorbed onto colloids ? Pb2/Fe(OH)3
COLLODIAL
Metal within decomposing ? Pb in organic
soils organic material Ionic solids ? Pb2
held with the clay structure, PbCO3
SOLID
9
Why speciation studies?
Generally basic reasons for speciation
measurements
Study transport and biogeochemical cycling
processes
Predict biological impact (identify those metal
species which are likely to have adverse effects
on biota and includes measurements both of
bioavailability and toxicity)
10
Toxicity
In general, the toxicity of metals stems from the
fact that they are biological non-degradable and
have a tendency to accumulate in vital organs,
e.g. brain, liver, etc. and their accumulation
become progressively more toxic
11
Toxicity, some examples.
Ionic copper are fare more toxic towards aquatic
organisms than organically-bounded copper, and
that more stable the copper complex, the lower
is its toxicity.
Alkyl compounds of mercury and lead are
especially toxic because they are lipid-soluble
As(III) is fare more toxic than As(V)
Ni, Cr, Cu and Se are known to display
carcinogenic effects due to their interactions
with nucleic acids e.g. whereas Cr(VI) is
anionic and highly toxic Cr(III) is nontoxic,
this because negative charge on CrO4- makes it
able to pass the cell membrane
12
Detection of trace metal speciation?
Lipid soluble forms
Information of speciation can be obtained even
near the total limit of detection because
separation methods can be used prior to the
measurements of the actual species
Particle bond forms
Ionic forms and labile complexes
These species are in principle more difficult to
measure because any separation methods or
attempts of pre- concentration will shift the
distribution of the species.
Molecular spectroscopy ? ? Fails due to
detection limit
Potentiometry ? ? Fails due to
detection limit ?
Voltammetry ? ? ?
ICP-MS ? ? ?
13
Detection of trace metal speciation?
Technique Response
Atomic spectrometry Flame AAS, Flameless AAS All the metal species in the sample, i.e. the total metal determined
Visible absorption spectrometry Free metal ions plus ions released from complexes by the color forming reagent
ICP-MS Total and isotopes
Voltammetry Free metal ions plus any ions released from complexes during analyses. Total electrochem. cont.
Chromatography Non-labile species can sometime be determined separately
14
Detection of trace metal speciation?
15
Metals of common enviromental concern
16
Electrochemical methods
  • Principle information about the analyte is
    achieved from measurements of e.g.
  • potential, current, resistance or conductance.
    There at several methods available
  • Coulometry (measurements of current and time)
  • Conductometry (measurements of conductance)
  • Potentiometry (measurements of potential at zero
    current)
  • Polarography / Voltammetry (measurements of
    current as function of an applied potential)

In particular voltammetry is suitable for
analyses of trace metal and speciation studies.
Detection limit for the most common heavy metals
are in the range of 10-6 to 10-12 M.
17
Electrochemical detection of trace metals
Voltammetry
Anodic stripping voltammetry
Adsorptive cathodic stripping voltammetry
Square wave stripping voltammetry
Potentiometry
Ion selective electrodes
18
Some advantages for el.chem. techniques
Electroanalysis is a powerful technique for the
study of trace element speciation, and has been
applied to over 30 elements
Four to six metals of prime environmental
concern Cu, Pb, Cd, Ni, Zn an Co can be detected
simultaneously and with a sensitivity in the
range of ng/L
Study of the kinetics of metal complex
dissociation at en electrode is supported by
well-established theory
Electrochemical techniques requires minor sample
pretreatment, resulting in fewer potential
sources for contaminations
Speciation study can be performed in the field
within minutes, with low-cost equipment
19
Range of applicability, el.chem. speciation
methods
Direct applications, determination of
labile and inert metal fraction
redox state
half wave or peak potential shifts
Indirect applications, determination of
fraction bound in inert organic complexes or to
organic colloids, by measurements before and
after UV irradiation after UV irradiation and
lipid soluble complexes, after extraction of
water samples with e.g. n-octanol or 20
n-butanol in hexane
size distribution after ultra filtration
Pre concentration prior to e.g. Carbon Furnaces
AAS
20
Range of applicability labile/inert metal
fraction
Discrimination between labile and inert metal
fraction in the sample
  • Labile metal compromise free metal ion and metal
    that can dissociate in the double
  • layer (near electrode surface) from complexes
    or colloidal particles

- For natural waters the most used techniques are
ASV, AdCSV and SWV
- Applied to e.g. Cu, Pb, Cd, Zn, Mn, Cr, Tl, Sb
and Bi
  • Often the labile metals have been found to
    correlate well with the toxic fraction of
  • the metal

21
Range of applicability redox state
Determination of the redox state of an element in
solution is very important because it can
drastically affect the toxicity, adsorptive
behavior, and metal transport
Applied to distinguish between e.g.
Fe(III)/Fe(II), Cr(VI)/Cr(III), Tl(III)/Tl(I),
Sn(IV)/Sn(II), Mn(IV)/Mn(II), Sb(V)/Sb(III),
As(V)/As(III), Se(VI)/Se(IV), V(V)/(IV),
Eu(III)/Eu(II), U(VI)/U(IV)
22
Redox state, toxicity vs. el.chem. lability
Species Toxicity Electrochemical lability
Arsenic (III) HIGH HIGH
Arsenic (V) LOW LOW
Chromium (III) LOW LOW
Chromium (IV) HIGH HIGH
Thallium (I) HIGH HIGH
Thallium (III) LOW LOW
Cu2 HIGH HIGH
CuCl2 HIGH HIGH
CuCO3 HIGH HIGH
Cu2 -fulvic acid LOW LOW
Cu2 /humic-Fe2O3 MEDIUM MEDIUM
Cu2 -DMP HIGH LOW
23
Range of applicability half wave potential shifts
Shift in the polarographic half wave potential or
ASV peak potential of metal ions in presence of
complexing agents can provide information about
the thermodynamic stability of complexes in
solution.
Quantitative deductions may be difficult due to
the high number of possible present ligands and
metals in natural or polluted water
Sometime however it is possible to do such
quantitative deductions (e.g. the ASV peak for
copper(I)-chloro complex in seawater)
24
Limitations of el. chem. speciation techniques
Unable to measure the concentration of individual
ionic species
E.g. one peak only will appear for a mixture of
Cd2, CdSO4, CdCl, and CdCO3 (which all may
coexist in a river water sample)
Also, electrochemical techniques like
polarography and ASV are dynamic systems which
draw current through the solution and disturb
ionic equilibrium. However with microelectrodes
the current flowing is reduced to nA or pA
Ion-selective electrode potentiometry is the only
method that can measure the activity of a
individual ion but the sensitivity has up to
now been poor
25
Limitations, however.
Other speciation methods, including ion exchange
chromatography, Solvent extraction, dialysis,
and ultrafiltration also disturb the natural
ionic equilibrium in water samples during the
speciation process
In addition often the question is only the
discrimination between to species, where one is
electrochemically active (labile) and the other
species inert
26
Some practical examples
27
Measurements of Cu, Pb, Cd and Zn in waters
28
Measurements of Cu, Pb, Cd and Zn in waters
Heavy metals have a influence on the biological
life, and may cause serious damage due to
toxicity effects
  • free and weakly complexed metals are transported
    across the cell membrane and provide
  • bioconcentration factors between 102 and 105

-may replace Mg at sulfhydryl binding sites
-possible intracellular reaction between Cu and
reduced glutathione which defend the cell against
peroxide damage
-loss of lysosomal membrane stability, which may
lead to a leakage of hydrolytic enzymes into the
cytosol and catabolic breakdown of the cell
-when the capacity of a cell to detoxify
accumulated metal is exceeded, damage to
cyroplasmic constituents will occur, e.g.
ultrastructural deformities, as well as reduction
of cell division rate, respiration,
photosynthesis, motility, electron transport
activity, and ATP production
29
Cu, Pb, Cd and Zn
The surface area of an organism is critical to
the passive metal diffusion process into cell,
therefore bacteria and algal communities
frequently have the highest metal concentrations
in the food web. There are a magnifying through
the food web
E.g. Periphyton has been found to contain up to
1g/kg Cd, while the normal Concentration are 22
mg/kg indigenous bryophyte populations in rivers
draining old metal mines have been shown to
contain up to16 mg/g Pb and 7 mg/g Zn
30
Cu, Pb, Cd and Zn
In fresh water - inorganic fraction computed to
be present mainly as CuCO3 (over 90),
colloidal particles and hydrated iron oxides
In seawater - dominant inorganic species
computed to be carbonato and hydroxy complexes
(CuCO3 up to 80), in addition CuOH and Cu(OH)20
(approx. 6,5), Cu(OH)(CO3)- (approx. 6,5),
CuHCO3 (approx. 1) and Cu
Coastal surface seawater usually has 40 to 60 of
total copper present as inert organic complexes.
In unpolluted seawater ASV-labile copper is
usually less than 50 of dissolved copper, even
at pH as low as 4,7. Most freshwater streams also
has little ASV-labile copper (organically bound)
31
Cu, Pb, Cd and Zn
In fresh water - Computed to exist as PbCO3 and
Pb2(OH)2CO3 (often gt 90 of the inorganic
lead species) - in general lead has a stronger
affinity for some inorganic adsorbents,
especially iron oxide (pH 7), than for organic
ligands, - at pH 6.0 or lower most lead is
found as electro inactive Pb2(OH)2CO3
In seawater - Pb is found as carbonato complexes
(83) and chloro species (11) - 40 to 80 of
dissolved lead is found in the inorganic colloid
fraction
Alkyllead in natural waters may be determined by
ASV after selective organic phase extraction
32
Cu, Pb, Cd and Zn
In fresh water - Dominant form is computed to be
Cd2 and CdCO3 depending on pH - Cd adsorbs to
colloidal particles only at relatively high pH
values, so very little Cd is present as
pseudocolloids
In seawater - Cd is computed to exist as CdCl
and CdCl20 complexes (92)
A high portion (over 70) of Cd is found to be
ASV labile both in seawater and freshwater. In
anoxic water Cd may exist as no-labile CdHS
33
Cu, Pb, Cd and Zn
In fresh water - dominant inorganic forms are
computed to be Zn2 (50) and ZnCO3 (38)
In seawater - main species are computed to be
Zn2 (27), chloro complexes (47), and ZnCO3
(17) - open ocean waters contains as little as
10 ng/L Zn at the surface
The carbonato complexes of Zn, especial the
basic carbonates, may have low ASV lability.
About 59 of the total zinc in seawater and river
water is ASV labile.
34
Cu, Pb, Cd and Zn
Most suitable techniques are ASV and AdCSV
AdCSV
ASV
1. Step The electrode are set to a potential
about 300 mV more negative than the
first expected metal peak
Mn ne- ? M (deposited on the electrode) 2.
Step Cd is than stripped of by reverse the
potential over the electrode
towards more positive value M ? Mn
ne-
1. Step Cations are complexed with surface active
complexing agents (L)
Mn xL ? MLxn 2. Step Metal-complex adsorbs
to the electrode surface MLxn
Met ? MLxn,ads(Met) 3. Step The cation is
released from complex by
reduction MLxn,ads(Met) me- ?
M(n-m) xL Met
35
Speciation scheme for Cu, Pb, Cd and Zn in waters
Aliquot No. Operation Interpretation
1. (20 mL) Acidify to 0,05 M HNO3, add 0.1 H2O2 and UV irradiation for 8 h, than ASV a Total metal
2. (20 mL) ASV at natural pH for seawater add 0.025 M acetate buffer, pH 4,7 for freshwaters ASV-labile metal
3. (20 mL) UV irradiate with 0,1 H2O2 at natural pH, than ASV b (3)-(2)organically bound labile metal
4. (20 mL) Pass through small column of Chelex? 100 resin, ASV on effluent c Very strongly bound metal
5. (20 mL) Extract with 5 mL of hexane-20 n-butanol, ASV on acidified, UV- irrad. aqueous phase d (1)-(5)lipid soluble metal
Sample (unacidified), filter through a 0.45-mm
membrane filter, reject particulates and store
filtrate unacidified at 4?C

a) Bring to pH 4.7 with acetate
buffer, b) Not valid if Fe gt 100 mg/L, c)
Optional step,

d) Solvent
dissolved in aqueous phase must be removed first
36
Measurements of Fe in seawater
37
Fe in seawater
Iron is one of the most important bioactive trace
metal in the oceans. The first-row transition
metal plays a key role in the biochemistry and
physiology of oceanic phytoplankton. Low iron
concentrations are suggested to limit
phytoplankton growth and biomass in certain
oceanic regions
38
Fe in seawater
The oceanic chemistry is highly complicated, and
still not fully understood. Dissolved iron can
exist in two different oxidation states, Fe(III)
and Fe(II).
Thermodynamically Fe(III) is the stable form in
oxygenated water, however several processes
reduce Fe(III) to Fe(II). Fe(II) may exist
for several minutes in surface water(pH 8) before
it is oxidized back to Fe(III).
Presence of Fe2 may cause an increase in the
dissolved iron fraction making more iron
available for use by biota.
39
Fe in seawater
Inorganic speciation of dissolved Fe(III) and
Fe(II) differ considerably. Inorganic Fe(III)
species are dominated by hydrolysis products,
Fe(OH)2, Fe(OH)30, and Fe(OH)4-. Free hydrated
Fe3 ion is extremely rare.
Inorganic Fe(II) however exists in primarily as
Fe2 ion.
Evidence is also found for complexing of Fe(III)
and Fe(II) with organic ligands.
40
Fe in seawater
Since total dissolved iron in oceanic surface
waters can be very low (down to a few pM), there
is a need for highly sensitive techniques.
Iron(II) at nanomolar levels has been determine
by e.g. and colorimetry preceded by
preconcentration of iron(II) using octadecyl
silica as stationary phase
However the most suitable technique is AdCSV
41
Fe in seawater
Fe(III) complexed with 1-nitroso-2-napthol is
preconcentrated onto a hanging mercury drop
electrode (adsorption). (Addition of H2O2 secures
that all iron is oxidized to Fe(III)
Concentration of Fe(II) is calculated from the
difference between analyses with and without
added 2,2-dipyridyl, which masks Iron(II).
42
Fe in seawater
Recent results from our laboratory has shown a
new ASV technique that can be used for detection
of Fe(II) down to 50 ng/L on solid dental
amalgam electrode.
Analyses can be performed directly in the sample
with only the additions of citrate or oxalate
43
Fe in seawater
Detection of iron (II) with DPASV in tri-sodium
citrate-5,5-hydrate (0.02M) solution. Addition of
iron (II) standard to solutions of 1,67 ppb, 3,34
ppb, 5 ppb, 15 ppb, 25 ppb, 50 ppb,
pre-deposition time 180 s.
44
Measurements of Hg in water
45
Hg in water
Mercury has no known essential functions, though
it has been used to treat syphilis, actually with
some success.
Mercury probably affects the inherent protein
structure which may interfere with functions
relating to protein production. Mercury has a
strong affinity for sulfhydryl, amine,
phosphoryl, and carboxyl groups, and inactivates
a wide range of enzyme systems, as well as
causing injury to cell membranes.
Main problems seem to result from its attack on
the nervous system. Mercury may also interfere
with some functions of selenium, and can be an
immunosuppressant
46
Hg in water
Mercury dissolved in water is present in many
forms, including organomercurials, such as
methylmercuric chloride, phenylmercuric chloride
and other alkyl- and arylmercury compounds.
Among the co-existing forms of mercury in natural
water the most toxic to man and biota are
organomercurials (up to 46 of the total mercury
content has been found in this form in river
water samples, and up to 63 in unfiltered
samples)
47
Hg in water
Organomercurials as methyl-mercury has high lipid
solubility, something that makes bioaccumulation
a serious problem. Bioaccumulation up to 103 to
104 have been reported for mercury in fish .
48
Hg in water
LD50 of different organomercuric compounds
Compound Anions LD50 rat (mg/kg) Vapour pressure 20?C (mg/L)
HgCl 210
HgCl2 37
Methylmercury Br- 94000
Cl- 10 94000
I- 90000
Acetate 75000
Hydroxide 10000
Ethylmercury I- 9000
Cl- 40 8000
Methoxyethylemercury Cl- 2600
Acetate 2
Phenylmercury Acetate 17
Cl- 60 5
49
Hg in water
Organic and inorganic mercury can be detected
with a glassy carbon electrode modified with
thiolic resin. Detection limits in low ?g/L
50
Hg in water, detection of Hg2, MeHg, EtHg,
PhHg
not treated
treated
E2 -1,0V
Hg2 MeHg EtHg PhHg
R. Agraz et al.
51
Hg in water
Some advantages,
- good sensitivity
- good selectivity
- pH changes in the sample is unnecessary
- may be performed in presence of high conc. of a
varity of anions and cations
- possible use also for salt or brackish water
Some disadvantage
  • Fe or Mn particulate in suspension can interact
    (0,06 mg/L Fe and 0,12 mg/L Mn)

52
Measurements of AL(III) speciation in water
53
Al(III) speciation in natural water
Many natural waters are affected of serious
acidification problems due to acid precipitation
and other ecological problems, resulting in Al
mobilization
The impact of Al highly depend on its existing
chemical form, therefore speciation measurements
of Al is very important
Graphite Furnace atomic adsorption spectrometry
involving Driscolls Method is maybe the most
used technique.
- use of hazardous organic solvent methyl
isobutyl ketone - expensive - greater errors
for the indirectly detection of inorganic
monomeric Al at low conc. of total Al
54
Al(III) speciation in natural water
ASV at HMDE, solochrome violet RS (SVRS)
At pH 5,2 citrate, oxalate tartrate, salicylate,
humic an fulvic acids display very strong
complexation ability with Al(III)
Therefore at pH 5,2, SVRS is only able to
sequester sulfato, silicato and fluoro complexes
in addition to a small portion of unstable
organic complexes
At pH 8,5 SVRS shows much stronger complexation
ability than citrate, oxalate..
Therefore at pH 8,5, the total monomeric Al will
be able for electrochemical determination
55
Al(III) speciation in natural water
56
Al(III) speciation in natural water
ASV at HMDE, solochrome violet RS
Acid reactive Al (Alr) Acidification to pH 1.0,
than determination at pH 8.5
Untreated
Acid soluble aluminium (Als) Als Alr - Ala
Total monomeric Al (Ala) Determinated at pH 8.5
Non-labile monomeric Al (Alo) Alo Ala - Ali
Labile monomeric Al (Ali) Determination at pH 5.2
Original water samples (untreated)
Filtered 0.45 mm
X. Wang et. al
57
Conclusions
58
Conclutions
Electrochemical techniques are a important tool
for measuring speciation of metals in e.g.
natural waters
Detection of free metal ions plus any ions
released from complexes during analyses (total
electrochem. cont.)
Can also be used to detect other speciation
forms, e.g. total metal, organically bound
labile metal, strongly bound metal, and lipid
soluble metal after different types of sample
pretreatments.
Low cost instruments, good detection limits
(ng/L), may be used online in field
59
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