GEOCHEMICAL METHODS FOR THE DETERMINATION OF DIFFERENT METAL

SPECIES (cont’d)

PART -III

Analytical Methods for Metal Speciation in Water and Solids

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)

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

2. SOLID SAMPLES IN AQUATIC SYSTEMS: SUSPENDED SOLIDS AND SEDIMENTS

2.1. MAIN COMPONENTS:

2.1.1. SAND: weathered grain of SiO2 (quartz). Large particle size – small specific surface area – low to no binding capacity for metals

2.1.2. SILT: Medium size particles – broken down minerals such as quartz, feldspar, etc. – specific surface area remains rather small with low binding capacity

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 hydroxylsaround a large cation that is commonly Al3+

Example clay mineral - 1Kaolinite: a 1:1-layer clay mineral

General chemical formula: Al2Si2O5(OH)4

Alumina octahedra

Silica tetrahedra

Alumina octahedra

Silica tetrahedra

OH

OH

OH

OH

OH

OH

-OH -O- + H+

OH

Example clay mineral - 2Montmorillonite: 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

2.1.4. ORGANIC MATTER

Model Humic Acid

Dimethylsulfoniopropionate

2.2. APPROACH FOR METAL SPECIATION IN PARTICULATE MATTER

2.2.1. Identification of the different metal fractionsEasily exchangeable fractionFraction bound to carbonate mineralsFraction bound Fe- and Mn-oxide mineralsFraction bound to organic matterRefractory/Residual fraction

2.2.2. Development of extraction/solubilization methodsChoice of chemical agents to specifically target one of the above fractions at a time

2.2.3. AnalysisAnalysis as total concentration in each extract

EXAMPLE: Sequential Extraction Procedure for Particulate Trace Metals

(Tessier et al., 1979)

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

FRACTION 1 :Easily exchangeable fraction

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

FRACTION 2

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

FRACTION 3

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

FRACTION 4

Refractory/Residual fraction

Requires hot digestion with strong acid mixtures (e.g. HF-HClO4)

FRACTION 5

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

*Environmental-Relevance

*Metal bio-availability vs. fractions

*Toxicity