WATER RESOURCES RESEARCH GRANT PROPOSAL
Project ID: 2003LA18B
Title: Metal Speciation in Particulates in the Mississippi River in Louisiana
Project Type: Research
Focus Categories: Hydrogeochemistry, Toxic Substances, Water Quality
Keywords: Metals, Toxic Metals, Particulates, Metal Availability, Speciation
Start Date: 03/01/2003
End Date: 02/29/2004
Federal Funds: $25725.00
Matching Funds: $58856.00
Congressional District: 6
Principal Investigator: Roy, Amitava
Abstract: Metals are transported
in the Mississippi River water principally as dissolved and adsorbed species.
Some amount of transport may also take place as small mineral fragments.
species are those attached to the suspended particulate matter (SPM) in the
water. The SPM includes silt- and clay-sized mineral particles in suspension
in the flowing water. Mineralogically, the SPM consists of quartz, feldspars,
and clay minerals, the stable silicates and alumino-silicates on the Earth’s
surface. The mineral grains are often coated with iron and manganese hydroxides
which aid the adsorption process. The colloidal fraction, extending in size
down to a few nanometers, may also be included in the SPM as it is also important
to transportation by adsorption. Most elements are significantly partitioned
onto the SPM and thus it is the major carrier of metals in the Mississippi.
The distribution of metals between the dissolved and the adsorbed fractions
has mostly been determined by separating water samples into different fractions
based on filter size followed by bulk chemical analysis of each fraction.
The metal adsorption process can be further characterized by sequentially
extracting the metals from the SPM in various (mostly) acidic media. Some
speciation information has lately been obtained using voltammetric stripping
analysis. All these techniques are operationally defined and the correspondence
between the results obtained by them and actual speciation is unclear. The
adsorption processes of various metals are thus not very well understood.
It is also suspected that the dissolved fraction contains metals adsorbed
onto the colloidal fraction which is too small to be retained on filters (<0.45
or 0.4 µm) normally used. X-ray absorption spectroscopy (XAS) is an
element-specific analytical technique which can be used to probe the local
structure around atoms. Speciation information, for example the oxidation
state, and the location of an atom relative to other atoms, i.e., whether
an atom is adsorbed onto a surface or within a crystal structure, can be obtained
by XAS. XAS can be used to study solid, liquid or gaseous samples with practically
no sample preparation. Thus Mississippi River water samples can be directly
studied. The evidence obtained by this method is more direct than most other
techniques. A wide range of elements over a large concentration range can
be studied by XAS. The XAS signal from an element in a sample is also additive,
thus the phases in which the element is present can be identified and quantified.
Systematically sampled Mississippi River water will be studied by XAS at the
J. Bennett Johnston Sr., Center for Advanced Microstructures and Devices.
The adsorption processes of heavy metals such as Cr, Cu, Pb, Zn, etc. on the
SPM will be investigated. The importance of the coarser SPM, particularly
the silicate minerals, the iron and manganese hydroxides, and the colloidal
fraction (<0.45 µm) can be clarified. In addition, the transport
of P, an important nutrient in the River water, can also be investigated
XAS. The study will results in an improved understanding of the metal transport
processes in the Lower Mississippi River. This understanding will result
better models of the metal transport processes and can also help in improving
the quality of the River water.
Progress/Completion Report PDF