Chromium In the Aquatic Chromium In the Aquatic EnvironmentEnvironment

Polina LibermanSarah SchmidtJune 7, 2002

OutlineOutline 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?

IntroductionIntroductionIn 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 ChromiumChemistry 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) 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)6

3+, 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(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) 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(Cr2O7

2-), or chromium trioxide(CrO3) In acidic solution it has a very high positive redox potential,

therefore strongly oxidizing and unstable in presence of e- donorsHCrO4

- + 7H+ + 3e- = Cr3+ + 4H2O In basic solution reduction of CrO4

2- occurs

CrO42- + 4H2O + 3e- = Cr(OH)3 + 5OH-

Precipitation and DissolutionPrecipitation 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 (CrO4

2-) 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 Eh-pH diagram

Speciation AnalysisSpeciation 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 AnalysisSpeciation 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 AnalysisSpeciation 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 AnalysisSpeciation 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 ChromiumSources 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 ChromiumSources 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 ChromiumHealth 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 Environmental Impact of ChromiumChromium

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 Environmental Impact of ChromiumChromium

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 Environmental Impact of ChromiumChromium

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 LakesNatural 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)

ConclusionConclusionThe 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 isimportant 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 SpeciationChromium Speciation

Questions??