Transcript Speciation

Øyvind Mikkelsen
The determination of metal speciation in natural
waters by electrochemical techniques
Mikkelsen 2003
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
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• Conclusions
Theoretical considerations
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Natural water
• Natural water includes e.g. rivers, lakes, ground water, wells,
seawater,….
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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.
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Possible forms of trace elemements
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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
Interactions affecting trace metal speciation
G.E Batley, Trace element speciation; analytical methods and problems, CRC Press, Inc., 1989
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An example, lead.
Free metal
Ion pair
Complexes with
organic pollutants
 Pb2+
 PbHCO3
SOLUTION
 Pb2+/EDTA
Complexes with
natural acids
 Pb2+/fulvic acid
SUSPENSION
Ion adsorbed onto
colloids
 Pb2+/Fe(OH)3
COLLODIAL
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Metal within
decomposing
organic material
Ionic solids
 Pb in organic soils
 Pb2+ held with the
clay structure, PbCO3
SOLID
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)
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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
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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)
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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 CrO4makes it able to pass the cell membrane
Detection of trace metal speciation?
Lipid soluble forms
Particle bond forms
Ionic forms and labile
complexes
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
These species are in principle more difficult to measure
because any separation methods or attempts of preconcentration will shift the distribution of the species.
Molecular spectroscopy ?  Fails due to detection limit
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Potentiometry ?
 Fails due to detection limit ?
Voltammetry ?
ICP-MS ?


Detection of trace metal speciation?
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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
Detection of trace metal speciation?
Flame AAS
Sensitivity
Interference
Speciation
Efficiency
Capacity
Cost
Online
Maintaince
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Linear range
El. term. AAS
ICP/MS
Voltammetri
Metals of common enviromental concern
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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.
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Electrochemical detection of trace metals
Voltammetry
Anodic stripping voltammetry
Adsorptive cathodic stripping
voltammetry
Square wave stripping
voltammetry
Potentiometry
Ion selective electrodes
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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
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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
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Pre concentration prior to e.g. Carbon Furnaces AAS
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
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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)
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Redox state, toxicity vs. el.chem. lability
Species
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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
HIGH
LOW
Cu2+ -DMP
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)
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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
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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
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
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Some practical examples
Measurements of Cu, Pb, Cd and Zn in waters
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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
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-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
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
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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
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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)
Cu, Pb, Cd and Zn
In fresh water
- Computed to exist as PbCO3 and Pb2(OH)2CO3 (often > 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
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Alkyllead in natural waters may be determined by ASV after selective organic phase
extraction
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+
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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.
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Cu, Pb, Cd and Zn
Most suitable techniques are ASV and 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)
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2. Step Cd is than stripped of by reverse the potential
over the electrode towards more positive value
M  Mn+ + ne-
AdCSV
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
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
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Sample (unacidified), filter through a 0.45-mm membrane filter, reject particulates and store filtrate
unacidified at 4C
buffer, b) Not valid if Fe > 100 mg/L, c) Optional step,
d) Solvent dissolved in aqueous phase must be removed first
a) Bring to pH 4.7 with acetate
Measurements of Fe in seawater
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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
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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.
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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.
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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
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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).
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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
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Peak height ( m A)
Fe in seawater
57
52
47
I (m A)
42
40
30
20
R2 = 0.9998
10
0
0
37
20
40
60
Conc (mg/L)
32
27
22
17
12
-1200
-1000
-800
-600
-400
-200
0
E (mV)
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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.
Measurements of Hg in water
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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
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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)
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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 .
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Hg in water
LD50 of different organomercuric compounds
Compound
Anions
LD50 rat Vapour pressure
(mg/kg)
HgCl
210
HgCl2
37
Methylmercury
BrCl-
Ethylmercury
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Phenylmercury
94000
10
94000
I-
90000
Acetate
75000
Hydroxide
10000
I-
9000
ClMethoxyethylemercury
20C (mg/L)
40
8000
Cl-
2600
Acetate
2
Acetate
17
Cl-
60
5
Hg in water
Organic and inorganic mercury can be detected with a glassy carbon
electrode modified with thiolic resin. Detection limits in low mg/L
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2+
+
+
+
Hg in water, detection of Hg , MeHg , EtHg , PhHg
Sample
not treated
E1= -0,5V
Hg2+
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(Hg2+)
E2= -1,0V
Hg2+
MeHg+
(MeHg2+)
treated
E3= -1,35V
Hg2+
MeHg+
EtHg+
PhHg+
(EtHg2+)
(PhHg2+)
Hg2+ total
(TMS)
R. Agraz et al.
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)
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Measurements of AL(III) speciation in water
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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 Driscoll’s
Method is maybe the most used technique.
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- 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
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
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Al(III) speciation in natural water
pH 5,2 and SVRS
SVRS
Al3+, AlOH2+, Al(OH)2+, AlSO4+, AlF2+, AlF2+, AlHPO4+, …
Al-SVRS
pH 8,5 and SVRS
Al3+, AlOH2+, Al(OH)2+, AlSO4+, AlF2+, AlF2+, AlHPO4+, Al-NOM, …
SVRS
Al(OH)4
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Al(III) speciation in natural water
ASV at HMDE, solochrome violet RS
Untreated
Acid reactive Al (Alr)
Acidification to pH 1.0, than
determination at pH 8.5
Total monomeric Al (Ala)
Determinated at pH 8.5
Original water samples
(untreated)
Labile monomeric Al (Ali)
Determination at pH 5.2
Acid soluble aluminium
(Als)
Als = Alr - Ala
Non-labile monomeric
Al (Alo)
Alo = Ala - Ali
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Filtered
0.45 mm
X. Wang et. al
Conclusions
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.
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Low cost instruments, good detection limits (ng/L), may be used online in
field
Thank you for your attention!