Chromium In the Aquatic Environment
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Transcript Chromium In the Aquatic Environment
Chromium In the Aquatic
Environment
Polina Liberman
Sarah Schmidt
June 7, 2002
Outline
Introduction
Chemistry of Chromium
• Cr(III)
• Cr(VI)
• Precipitation and Dissolution
Speciation Analysis
• On-line Methods
• Off-line Methods
Chromium in the Environment
• Sources of Chromium
• Environmental/Health Impacts
• Example: The Great Lakes
Conclusion
Questions?
Introduction
In fresh waters, trace metals may exist in
various physicochemical forms. This phenomenon,
also called speciation, refers to the partitioning of
trace metals among solids, colloids, surfaces,
dissolved free ions, complexed with inorganic
ligands in the dissolved phase, and complexed with
organics in the dissolved phase.
Chromium speciation is of particular interest in
the environment due to the existence of two major
chromium species that have significantly different
environmental implications.
Chemistry of Chromium
Two common, stable oxidation states: Cr(III) & Cr(VI)
Factors that control interconversion between species:
• concentration of Cr species
• oxidizing or reducing species
• electrochemical potentials of redox reactions
• ambient temperature
• light
• acid-base reactions
• complexing agents
• precipitation reactions
Don’t exist as free ions Cr6+ or Cr3+
Cr(III)
Cr(III) characteristics
• harmless trace element essential for life
• micronutrient in an organic form
• most thermodynamically stable Cr oxidation state
• hard acid
In absence of complexing agents Cr(III) exists as hexaaquachromium(3+) Cr(H2O)63+, a moderately strong acid, and
its deprotonated forms
CrOH2+ and Cr(OH)3(aq) are dominant forms in environment
Forms complexes with water, ammonia, urea, ethylenediamine,
and other organic ligands containing oxygen, nitrogen or
sulphur donor atoms
Cr(III)
Cr(VI)/Cr(III) redox potential is high so oxidation of Cr(III) is
negligible without mediate species
Sources of Oxygen needed for oxidation of Cr(III) to Cr(VI).
Most of these are not present in high enough concentrations in
natural waters to accomplish the transition.
• water (most important)
• ozone
• hydrogen Peroxide
• manganese dioxide
• lead dioxide
Inverse relationship between Eh and pH thus Cr(III) is more
easily oxidized at higher pH
Cr(VI)
Cr(VI) characteristics
• powerful epithelial irritant
• confirmed human carcinogen
• toxic to many plants, aquatic animals, and bacteria
Exists as chromate(CrO42-) (pH>7), HCrO4-(1<pH<7),
dichromate(Cr2O72-), or chromium trioxide(CrO3)
In acidic solution it has a very high positive redox potential,
therefore strongly oxidizing and unstable in presence of e- donors
HCrO4- + 7H+ + 3e- = Cr3+ + 4H2O
In basic solution reduction of CrO42- occurs
CrO42- + 4H2O + 3e- = Cr(OH)3 + 5OH-
Precipitation and Dissolution
Solubility of Cr(III) and Cr(VI) vary over many orders of
magnitude
Cr(VI) ions are soluble at all pHs but chromate (CrO42-) can exist
as insoluble salt of a variety of divalent cations such as Ba2+, Sr2+,
Pb2+, Zn2+, and Cu2+ whose rates of precipitation vary and are pH
dependent
Most Cr(III) water soluble species don’t occur naturally and are
unstable in the environment
• hydroxylation, which is pH dependent, is the principle
reaction of Cr(III) with the trihydroxide, Cr(OH)3, being the
least soluble.
Cr3+ + 3OH- = Cr(OH)3 logK=30
• also precipitates as (Cr,Fe)(OH)3 which has lower solubility
than Cr(OH)3 and rapid precipitation/dissolution kinetics
Eh-pH diagram
Speciation Analysis
Speciation is an analytical process consisting of identification
and quantification of various forms of a given element present
in analyzed samples
Typically includes
• sampling
• sample storage
• sample pre-treatment
• instrumental analysis
Distinction between determination of total Cr, which is less
complex and determination of Cr(VI)
There is a lack of reliable analytical procedures to extract
Cr(VI) from environmental samples without altering its
oxidation state
Cr is present in the environment at trace or ultra trace levels and
is hard to detect
Speciation Analysis
Off-line methods
• Separation and pre-concentration of Cr species are carried
out before the insertion into the detection instrument
• Spectroscopic methods are generally used for detection
UV-Vis spectrometry
Atomic Absorption Spectrometry (AAS)
Electrothermal atomic absorption spectrometry (ETAAS)
Inductively coupled plasma atomic emission
spectrometry (ICP-AES)
• Have many disadvantages
complicated
time consuming
affects Cr speciation
results often in losses of the analyte
Speciation Analysis
On-line methods
• Separation, identification, and quantification of Cr are carried
out in one-step analytical process
• Separation techniques:
Flow-Injection Analysis (FIA)
High performance liquid chromatography (HPLC)
• Detection techniques:
Flame atomic absorption spectrometry
ETAAS
Direct current plasma atomic emission spectrometry
(DCP-AES)
ICP-AES or ICP-MS (mass spectrometry)
Speciation Analysis
Despite advances in past 25 years much remains to be done
• need for routine Cr species analysis
• need simplification of the speciation schemes
• need to minimize perturbation of the systems
• need for accurate analysis of complexed,
protonated/deprotonated, monomeric/polymeric and
adsorbed/dissolved forms
• need for development of Cr isotope speciation
Sources of Chromium
Natural Sources
• Weathering of rock constituents
• Wet precipitation
• Dry fallout from the atmosphere
• Runoff from terrestrial systems
Industrial use – begins with the mining of chromite, typically ferrous
chromite (FeO·Cr3O3)
• Vast majority of the ore is oxidized or reduced and used in other
forms
Oxidizing agents – Sodium carbonate, Calcium oxide
Reducing agents – Aluminum, Silicon, Carbon
• Production of metal alloys makes up 70% of U.S. chromium usage
Sources of Chromium
Examples of chromium chemicals used in industry
• Cr(VI) chemicals – Chromium trichloride (CrCl3),
Chromium nitrate (Cr(NO3)3)
• Cr(II) and Cr(III): small amounts compared with Cr(VI)
Industrial wastewater discharge from:
• Metallurgical industries
• Electroplating/Tanning industries
• Sanitary landfill leaching
Health Impacts of Chromium
EPA Max. Contaminant Level : 0.1mg/L (total Cr)
Routes of human exposure
• Dermal absorption
• Ingestion
• Inhalation
Health effects of exposure include:
• Irritation of the skin
Dermatosis (skin ulceration)
Dermatisis (allgeric sensitization)
• Respiratory problems
Respiratory cancers (caused by CaCrO4)
Ulceration/perforation of nasal septum
Irritation of upper airways
Environmental Impact of
Chromium
Chemical speciation greatly affects chromium transport within
land and water systems
• Efficient adsorption of metals by soils limits Cr input to the
atmosphere
• Cr(VI) is the most mobile form of Cr in soil and water
systems
• Redox conversion from Cr(III) to Cr(VI) increases Cr
dislocation from soil to water systems
Transport of Cr in various types of natural water systems is
controlled by specific conditions pertaining to each system.
Such conditions are temperature, depth, degree of mixing,
amount or organic matter present
Environmental Impact of
Chromium
Differences in transport mechanisms for various natural water
systems
• OCEANS
Oceans receive Cr from two sources – rivers, atmosphere
Precipitated and dissolved Cr exist in equilibrium
Dissolved Cr is lost from oceanic water via incorporation
into biologic material
Dissolved Cr also lost through adsorption onto sediment
particles
Dissolution of this incorporated Cr occurs both in the
water column, and the sediment-water interface
Environmental Impact of
Chromium
Differences in transport mechanisms for various natural water systems
• LAKES
Higher biological activity, greater ratio of sediment-to-water
surface area
High organic matter supports a reductive and complexing
environment, favoring Cr(VI)
Very transient mixing/transport features compared to oceans
Lower dissolved solids; higher particulate loads
More influenced by river and industrial inputs than oceans
In anoxic lakes, both concentration and speciation vary with
depth and season. Sunlight affects the redox reactions of
chromium. Specifically, sunlight degrades organic chromium
and releases inorganic chromium
Natural Example: Great Lakes
A 1993 study examined chromium concentrations in Lake
Superior, Lake Erie, and Lake Ontario
Shows that Cr(VI) is the dominant species (75%-85% of the
total chromium concentration)
Particulate Cr and Cr(III) concentrations below detection
Only under strongly reducing conditions was there a significant
formation of Cr(III)
Concentration of colloidal/organic Cr is approximately 10% of
the total dissolved Cr in the lake water
As expected, high Cr concentrations in the lakes occur at key
locations where industrial discharge is high (Thunder Bay and
Sault Ste. Marie in Lake Superior, Cleveland and
Detroit/Windsor for Lake Erie)
Conclusion
The chemistry of chromium yields two common forms
stable in the environment, Cr(VI) and Cr(III). Chromium
speciation plays a significant role from an environmental
standpoint, because these two forms have considerably
different environmental impacts. Analyzing chromium
speciation and its behavior in various aquatic systems is
important in order to study the effect the metal has on the
natural environment and on human health. Monitoring any
deleterious effects of industrial discharge is of particular
significance because it is a source of chromium that can be
controlled with regulations. With further technological
developments, Cr species analysis will be improved so that
speciation can be determined with more accuracy.
Chromium Speciation
Questions??