1. PART -III Analytical Methods for Metal Speciation in Water and Solids
GEOCHEMICAL METHODS FOR THE DETERMINATION OF DIFFERENT METAL SPECIES (cont’d)
PART -III Analytical Methods for Metal Speciation in Water and Solids

2. COMMONLY USED ANALYTICAL SPECIATION TECHNIQUES FOR METALS IN ENVIRONMENTAL SAMPLES
Liquid Samples
* Alkylation techniques
* Redox based techniques
Solid samples
* Chemical sequential extraction techniques
* Qualitative physical analytical techniques (minerals)

3. 1. ANALYTICAL APPROACHES FOR METAL SPECIATION IN LIQUID SAMPLES
1.1. Chromatography and metal speciation:
(e.g. chromium +3 and +6, inorganic and organic arsenic compounds, organo-metal species (e.g. methyl-mercury)…)
1.2. Redox chemistry combined with:
* hydride generation and spectrocopy (e.g. arsenic +3 and +5)
* Purge and trap and detection by spectrometry

4. 2. SOLID SAMPLES IN AQUATIC SYSTEMS: SUSPENDED SOLIDS AND SEDIMENTS
2.1. MAIN COMPONENTS:
SAND: weathered grain of SiO2 (quartz). Large particle size – small specific surface area – low to no binding capacity for metals
SILT: Medium size particles – broken down minerals such as quartz, feldspar, etc. – specific surface area remains rather small with low binding capacity

5. 2.1.3. CLAY: Comprised of layer of silicates formed from two basic units:
(1). A tetrahedron of 4 oxygen atoms surrounding a central cation, primarily Si4+
(2). An octahedron of 6 oxygen atoms or hydroxyls
around a large cation that is commonly Al3+

6. Example clay mineral - 1 Kaolinite: a 1:1-layer clay mineral
General chemical formula: Al2Si2O5(OH)4 OH OH
Alumina octahedra OH OH
Silica tetrahedra OH
Alumina octahedra
Silica tetrahedra OH OH
-OH -O- + H+

7. Example clay mineral - 2 Montmorillonite: a 2:1 - layer clay mineral
This is an hydrated sodium calcium aluminium magnesium silicate hydroxide (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O.
Montmorillonite is a 2:1 clay = 2 tetrahedral sheets sandwiching a central octahedral sheet

8. ORGANIC MATTER
Model Humic Acid
Dimethylsulfoniopropionate

9. 2.2. APPROACH FOR METAL SPECIATION IN PARTICULATE MATTER
Identification of the different metal fractions
Easily exchangeable fraction
Fraction bound to carbonate minerals
Fraction bound Fe- and Mn-oxide minerals
Fraction bound to organic matter
Refractory/Residual fraction
Development of extraction/solubilization methods
Choice of chemical agents to specifically target one of the above fractions at a time
Analysis
Analysis as total concentration in each extract

10. FRACTION 1 :Easily exchangeable fraction
EXAMPLE: Sequential Extraction Procedure for Particulate Trace Metals (Tessier et al., 1979)
FRACTION 1 :Easily exchangeable fraction
For metals loosely bound to surfaces and edges of minerals, increasing the ionic strength of the solution in contact with such solids would release previously bound metals by simple exchange between cations
MgCl2 is an example salt used to increase the ionic strength of the solution
A soluble ligand (e.g. CH3COONa) is generally added to make sure that released metals remain in solution

11. FRACTION 2 Fraction bound to carbonate minerals
Dissolution of carbonate minerals with a weak acid (e.g. CH3COOH), and as in step-1, a ligand (e.g. CH3COONa) is also added to make sure that dissolved metals remain in solution

12. FRACTION 3 Fraction bound Fe- and Mn-oxide minerals
A reducing agent (e.g. NH2OH.HCl) is used to convert the solid and oxidized forms of Fe and Mn to reduced and soluble species (e.g. Fe2+ and Mn2+)
CH3COOH is also be added to avoid metal sorption on oxidized solid surfaces

13. FRACTION 4 Fraction bound to organic matter
An oxidizing agent is used (H2O2) under acidic conditions (HNO3) necessary to keep metals in solution
A ligand (CH3COONH4)can also be added to avoid metal sorption on oxidized solid surfaces

14. FRACTION 5 Refractory/Residual fraction
Requires hot digestion with strong acid mixtures (e.g. HF-HClO4)

15. Why this order? 1. Easily exchangeable fraction
2. Fraction bound to carbonate minerals
3. Fraction bound Fe- and Mn-oxide minerals
4. Fraction bound to organic matter
5. Refractory/Residual fraction

16. *Environmental-Relevance *Metal bio-availability vs. fractions *Toxicity