Chromium In the Aquatic Environment

1
Chromium In the Aquatic Environment

• Polina Liberman
• Sarah Schmidt
• June 7, 2002

2
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?

3
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.

4
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
• Dont exist as free ions Cr6 or Cr3

5
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
  hexa-aquachromium(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

6
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

7
Cr(VI)

• Cr(VI) characteristics
• powerful epithelial irritant
• confirmed human carcinogen
• toxic to many plants, aquatic animals, and
  bacteria
• Exists as chromate(CrO42-) (pHgt7),
  HCrO4-(1ltpHlt7), 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-

8
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 dont 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 logK30
• also precipitates as (Cr,Fe)(OH)3 which has lower
  solubility than Cr(OH)3 and rapid
  precipitation/dissolution kinetics

9
Eh-pH diagram

10
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

11
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

12
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)

13
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

14
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 (FeOCr3O3)
• 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

15
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

16
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

17
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

18
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

19
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

20
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)

21
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.

22
Chromium Speciation

• Questions??