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The Lab for Contaminant Analysis
has a range of capabilities for determining other routine and customized
analtyical services for geologic, aqueous, and environmental samples.
In addition to the instrumentation and services located at the Whitehall
Forest lab facility, the Soil Charactertization Lab located in the Crop
and Soil Sciences Department on campus has a range of instrumentation
for basic characterization of solid materials that can be accessed thru
the Contaminants Lab. Pricing for the various services are given in
the fee schedule.
pH, EC and Eh
Hydrogen ion activity (pH), electrical conductivity (EC, in units of S
[Siemens] per m) and redox potential (Eh, in volts) are important characterisitcs
of both solutions and extracts of solids that can be readily determined
at the Lab. pH is determined with a H-sensitive glass electrode calibrated
against solutions of known (H+). Precision is typically only good to two
decimal places. EC is determined with a conductivity cell monitored using
a resistance/conductivity meter; EC in mS/cm is closely related to total
dissolved salts in a solution. Redox potential is an estimate of the oxidation-reduction
status, indicative of dissolved oxygen and the potential for formation
of reduced gas species such as hydrogen sulfide. It is measured potentiometrically
using a standard platinum electrode, and reported to the nearest mV.
For solids, a paste is typically mixed at a 2.5(water):1(solids) ratio,
equilibrated, and pH and EC determined in the supernatent; pH and Eh do
not vary greatly with the chosen ratio, but EC is sensitive to the volume
of water and is thus a relative or comparative value.
Particle Size Analysis
Particle size distribution of a soil or sediment material is determined
using sedimentation rate of particles in water as described by Stokes'
Law. Materials are disaggregated and suspended in water, and suspension
densities or particle concentrations measured over time of settling to
obtain a particle size distribution. Typically sand (0.05-2 mm), silt
(0.002-0.05 mm) and clay (<0.002 mm) sizes are determined, although
other size limits may be chosen. Sand may be further sub-divided by mechanical
seiving into coarse and fine fractions. Clay or colloidal content is an
important property affecting many other physical and chemical characteristics
of a soil material.
Cation Exchange Capacity
Cation exchange capacity (CEC) refers to the capacity (in meq or cmol
of cation charge) of a mass of soil material to retain cations on charged
surfaces in the colloid fraction of the soil; thus, it describes the number
of negatively charged sites on a soil material. It is typically determined
by displacing adsorbed cations (largely limited to Ca, Mg, Na, K, and
Al) with a neutral salt such as BaCl2, and measuring the displaced cations
using atomic absorption or ICP spectroscopy (the "sum of cations"
method). Alternatively, a cation such as ammonium may be used to fully
saturate the exchange sites of a sample, then dispaced and measured as
an estimate of CEC. Anion exchange capacity (AEC, or the number of positive
sites) can also be measured in this method by determining retention of
an associated anion such as nitrate or chloride. Anion concentrations
are determined by ion chromatography or colorimetry. CEC and AEC are important
surface chemical properties related to nutrient status and contaminant
behavior in soil/sediment systems.
Surface Area
Surface area of a soil or geologic material is closely related to both
particle size distribution and mineralogy; it represents the amount of
potential reactive surface of soil that may be in contact with pore waters,
and therefore reactive with contaminants. Surface areas may be measured
a variety of ways, but nitrogen (N2) gas adsorption is a standard method;
this measures external surface area, excluding very fine pores within
expansible clay minerals. A Micromeretics BET surface area instrument
located in the Soil Characterization Lab automates the process of measuring
the amount of N2 gas adsorbed by a sample, which is used to calculate
surface area in m2 per g of solid, assuming a monolayer coverage of the
surface with adsorbed gas. Adjusting mass of adsorbent allows determination
of areas over the range of 0.05 to 500m2/g.
Mineralogy
Identification of the minerals present in a geologic, soil, or sediment
sample is an advanced characterization technique that may be important
in understanding the physical and chemical behavior of that material in
a wide range of settings. Mineralogy of the clay colloid (<0.002 mm)
fraction is of most importance, since minerals in this size fraction possess
most of the charge and chemical reactivity in a sample from a nutrient
or contaminant point of view. Organic colloids are determined by measurement
of organic carbon in the solid; inorganic colloids can be identified and
semi-quantified using X-ray diffraction analysis. In this analysis, the
<0.002 mm fraction is separated using sedimentation and mounted on
glass slides after saturation with different exchangeable cations. The
slides are placed in a diffractometer located in the Soil Characterization
Lab, which impinges x-rays on the sample over a range of angles of incident
radiation. Diffraction patterns so obtained can be used to identify phyllosilicate
(clay) minerals such as kaolinite and vermiculite present in the sample,
and estimate their abundance.
Improved quantification of clay mineralogy is possible using Differential
scanning calorimetry (DSC). Dried samples of material are heated in a
DuPont DTA unit, which records calorimetric changes corresponding to mineral
phase transitions within the sample. Amounts of heat absorbed or released
at given temperatures can be used to accurately compute quantities of
minerals such as kaolinite, gibbsite, and Fe oxides present in the sample.
Amounts of reactive oxides such as Fe oxides (hematite and goethite),
Al oxides (gibbsite), and Mn oxides are important in specific reactions
related to trace metals and organic contaminants in soils and sediments.
Amounts of "free" (oxidic) oxides can be estimated by reductive
extraction of the soil with dithionite reagent, and measurement of Fe,
Mn, and Al in the extract. Results are reported as % Fe2O3, % Al(OH)3,
and % MnO2 in the sample
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