•
Introduction
Chromium
mobility and bio-availability in soils and natural waters are linked
to transfers of electrons between this metal and other constituents
in these environments, where biological and chemical processes control
oxidation and reduction reactions. Gains of electrons (reduction)
by electron-poor, hexavalent chromium (Cr (VI)) convert this toxic,
soluble anion [negatively-charged species e.g. CrO42-]
to electron-rich, trivalent chromium (Cr (III)), a form that is non-toxic,
essential for human health, cationic (positively-charged, e.g. Cr3+),
and only sparingly-soluble in most natural environments. Losses of
electrons (oxidation) by Cr (III) reverse this oxidation state change
and increase the solubility of Cr, thereby increasing the potential
movement of soil-borne Cr with percolating water. Purposeful reduction
of Cr (VI) to Cr (III) can be used in "remediation-by-reduction" schemes
in soils to lower the hazards associated with high levels of Cr (VI)
without changing the total Cr content of a soil, and in so doing,
the potentially-harmful ecological and human health effects of Cr
(VI)-enriched soils are minimized.
New developments in understanding chemical and biological processes
that govern the oxidation-reduction chemistry of Cr in soils and natural
waters are discussed in this review, with emphasis on the differences
between Cr (III) and Cr (VI). These differences are central to concerns
about the leaching of Cr (VI) from waste-enriched soils into groundwater,
the uptake of Cr by plants into the food chain, the exposure of humans
to soil-borne Cr (VI) via skin contact and the development of clean-up
strategies for Cr-contaminated land using chemically- or biologically-mediated
reduction reactions that minimise the potential for oxidation of Cr
(III). The differences between Cr (III) and Cr (VI) are central to
innovative approaches to the regulation of Cr-contaminated land and
they are pertinent to the formulation of health-based, clean-up standards
for Cr-enriched soils (20,27,30,18).
New technologies and modelling approaches are being developed that
use knowledge of Cr speciation and oxidation state differences in
the analysis, characterization and remediation of contaminated land
and waters. The theme of the 18th
Conference of the European Society for Environmental Geochemistry
and Health (September 2000) focused on new ideas and research germane
to environmental chromium contamination and remediation and extended
abstracts of the oral presentations have been published in a special
edition Of Environmental Geochemistry and Health (9).
•
Mobility of Cr (III) and Cr (VI) in Soils
As a trivalent cation, Cr (III) has a strong affinity for negatively-charged
ions and colloids in soils, plants, micro-organisms and animals and
as a result, it is relatively immobile and non-toxic in these environments
and organisms (10,27).
It has a strong affinity for oxygen-containing binding groups, such
as hydroxyl ions and it forms nearly insoluble compounds, such as
Cr(OH)3. Soluble chromium (III)
concentrations in equilibrium with this compound are less than 0.05
parts per billion (<10-9 M) in water
at pH 6 to less than 10-15 M at pH 8(49).
Its solubility increases significantly at pH values less than 5.5
(as Cr3+) and slightly at pH values
greater than 8 (as Cr(OH)4). The
incorporation of iron (II) or (III) into the Cr (III)-containing compound
lowers the solubility of Cr considerably (48),
such as in the inert, highly crystalline chromite ore (FeO•Cr2O3)(3).
Other industrial products, wastes and compounds with low potential
for movement of Cr (III) in natural soil environments are chromite
ore processing residue (19),
chromium-copper-arsenic treated lumber (16),
tannery wastes and chrome-tanned leather, sewage sludge (38,37),
stainless steel, and the emerald-green Cr2O3-containing
material produced upon reduction and precipitation of wastewater-borne
Cr (VI) from chromic acid plating baths (3).
Organic acids, especially those containing carboxyl groups (e.g. oxalic,
citric, and tartaric acids) can co-ordinate with Cr (III) cations
to form organic acid-metal complexes or chelates and some of these
complexes may be soluble from days to months, depending on pH, light,
organic acid concentration and molecular weight and microbial activity
(23,17,36,59).
In peaty soils, wetlands and soils containing high levels of native
or added organic matter, such organic complexes with Cr (III) may
become insoluble as high molecular weight precipitates with humic
acids and other carbon-based compounds (28).
Organic complexation may increase the solubility and bioavailability
of Cr (III) such as in nutritionally available Cr (III)-picolinate
(1) and
in the root zone of plants following reduction of Cr (VI)(26).
In contrast to the commonly observed and predicted low mobility of
Cr (III) in soils, plants, animals and natural waters, Cr (VI) is
more soluble, mobile and bio-available (42,20).
Since Cr (VI) is also toxic (principally as an oxidizing agent in
living cells) scientific research, industrial remediation efforts,
public concern and government regulatory activity have focused on
minimizing the mobility of this form of Cr in contaminated soils.
Chromium (VI) is an anion in most natural environments, principally
as bichromate (HCrO4) between pH
1 and 6.4 and as chromate (CrO42-)
at higher pH values. In concentrated, acidic solutions, dichromate
(Cr2O72-)
may exist, but it reverts rapidly to bichromate or chromate upon dilution
or neutralization. While sodium and potassium salts of chromates are
freely-soluble over the pH range 1-14, calcium chromate (CaCrO4)
is only moderately soluble (0.14 M) and lead and barium chromates
(PbCrO4 and BaCrO4)
are only sparingly-soluble (10-3
to 10-6 M) at near-neutral pH values.
They become more soluble under acidic and strongly-alkaline conditions
(42).
Under strongly acid conditions, such as in Cr plating bath wastes,
certain Fe (III) - Cr (VI) precipitates may form, e.g. KFe(CrO4)2(4).
In addition to solubility controls on Cr (VI) concentrations in soils
and natural waters, the oxidation of certain forms of Cr (III) to
Cr (VI) may occur via reactions with naturally-occurring, manganese
oxides and hydroxides [Mn (III), (IV) (hydr) oxides] or with hydrogen
peroxide added to soils (6,5,59,31,50).
The extent of oxidation of Cr (III) in soils is strongly dependent
on the form of Cr (III), the oxidizing potential of the soil for Cr
(III), its reduction potential for Cr (VI) and on pH as a modifying
master variable. These four parameters can be quantified, or ranked,
and then added to obtain a Potential Chromium Oxidizing Score (PCOS)
for a particular soil-waste Cr combination (27).
This approach to assessing potential Cr (III) oxidation of different
forms of Cr (III) in individual soils under specified conditions invites
discussion and refinement and it is intended to lead to accurate predictions
of the levels of mobile Cr (VI) in soils enriched with Cr (III). The
PCOS approach may also assist in establishing health-based limits
and clean-up standards for Cr (VI) at specific sites that contain
different forms of waste Cr and that have different potentials for
oxidation of Cr (III) and reduction of Cr (VI).
The oxidation-reduction reactions represented and quantified by PCOS
may be visualized as balanced on a seesaw, with soil pH acting as
a control on its position. See Fig. 1 below (20).
Soluble
and freshly precipitated forms of Cr (III) (e.g. CrCl3
and Cr(OH)3) added to near-neutral
soils with high levels of Mn (III), (IV) (hydr) oxides may oxidize
up to 15% of the added Cr (III) under laboratory conditions of optimum
aeration and soil mixing (6).
Aged, less soluble and more crystalline forms of Cr (III) (e.g. Cr2O3)
are much less prone to oxidation (23).
The presence of reducing agents for Cr (VI) (e.g. Fe (0), Fe (II),
sulfides and organic matter) tips the seesaw toward reduction and
lowers the PCOS. The control of oxidation and reduction reactions
of Cr in soils by pH is complex and it is expressed through effects
on the solubility and form of Cr (III), the reactivity of oxidizing
and reducing agents, the charge on soil colloids and the thermodynamics
and kinetics of the oxidation-reduction and of competing reactions
that result in precipitation of Cr (III).
In soils containing Cr (VI) and having a low potential for its reduction,
the dissolution of Cr (VI) precipitates and adsorption-desorption
reactions govern its solubility and potential mobility (25,19).
Since most alkali and alkaline earth salts of Cr (VI) are freely-
or moderately soluble and since Pb2+
and Ba2+ are uncommon constituents
in most wastes and materials added to soils, adsorption reactions
may retain Cr (VI) on soil colloid surfaces under some conditions.
The retention of Cr (VI) anions in soils is similar to that of sulfate:
it is favored by low pH, high levels of mineral colloids that develop
positive charge at low pH (e.g. Fe and Al oxides) and low levels of
competing anions (e.g. phosphate, hydroxide, fluoride, or organic
matter)(25).
Chromium (VI) retention in soils and sediments is greater than that
of nitrate, but less than that of phosphate. Under alkaline conditions,
CrO42-
is quite soluble and mobile. Under this condition, it may become a
contaminant of groundwater in the presence of percolating water from
rain, melting snow or irrigation water unless it is reduced to an
insoluble form of Cr (III).
Natural soil environments are characterized by pH values from approximately
3 to 10 and by a range of oxidation-reduction potentials (Eh) from
-300 to +1000 mV, depending on pH(7).
Organic matter contents vary widely depending on soil texture, soil
moisture, rainfall, temperature, vegetation, all integrated by the
relative rates of plant growth and organic matter decomposition. In
the presence of mineral acids, such as sulfuric acid formed from the
oxidation of sulfides in aerated soils, soil pH values less than 3
may be measured. In the presence of calcium carbonate, pH values will
often be 7.5 to 8 in calcareous soils, while in the presence of soda
ash (sodium carbonate), soil pH may exceed 10. These extremes in pH
are sometimes encountered in soils to which strongly acid or strongly
alkaline wastes have been added. The speciation, solubility, mobility
and bioavailability of Cr. are all influenced by pH and Eh as master
variables controlling chemical reactions and biological activity.
Therefore, relevant environmental conditions chosen to study and predict
the behaviour of Cr in soils may cover a wide range in acidity, redox
status, organic matter content, clay content and other properties.
•
Remediation-by-Reduction of Cr (VI) in Soils
Cleaning
up Cr (VI) contamination of soils using reduction reactions is possible
in "remediation-by-reduction" schemes employing microbiological and
chemical processes. Plant-based, microbe-induced, or abiotic chemical
reduction reactions for Cr (VI) can lower the solubility and potential
mobility of Cr and thereby decrease potential effects of Cr (VI) on
human health and ecosystem integrity without removal of Cr (III) from
the soil.
Microorganisms may reduce Cr (VI) to Cr (III) intracellularly or by
making the extra-cellular environment more reducing (through exudation
of reducing agents or by lowering pH to favour Cr (VI) reduction)(2,11).
Under anaerobic soil conditions with high electron pressure (as reflected
in low Eh readings), anaerobic micro-organisms may play little direct
role in the chemical reduction of Cr (VI) by Fe (II) or sulfides;
but under aerobic conditions where reducing agent concentrations are
low and Eh conditions are not favourable for Cr (VI) reduction, micro-organisms
may play a more important role (13,11).
Iron cycling between Fe (II) and Fe (III) may couple with microbial
reduction of Cr (VI) to detoxify anaerobic soils and natural waters
(58).
Various kinds of microorganisms can reduce Cr (VI) in soils and bioreactors.
Sulfate-reducing, mixed cultures in bio-films (52)
were effective and immobilized bioreactors using Bacillus coagulants
have been tested for Cr (VI) reduction after isolation of adapted
bacterial strains from electroplating wastewater and Cr-polluted soil
(44).
Malate as a carbon source was found to yield maximum Cr (VI) reduction
rates and soluble enzymes catalyzed the reaction, indicating that
metabolic control was responsible for the reduction reaction. Bacillus
subtilis, a common soil bacterium, reduced Cr (VI) under aerobic conditions
and appeared to detoxify the Cr (VI) rather than by using it in dissimilatory
electron transport (13).
Recent studies have focused on enrichment methods for mixed cultures
that could be used for Cr (VI) reduction in contaminated soils (56,57,51).
Many new techniques and chemical reactions have been developed and
tested for the remediation of Cr (VI)-contaminated soils and groundwater,
including those using carbon-based materials, zero- and divalent-Fe,
reduced sulfur-containing compounds and hydrogen (H2)
gas. The rates and extent of reduction of Cr (VI) by each of these
are dependent on pH, aeration status and the concentration and reactivity
of the reducing agent.
Condensed tannin gels have been used to esterify and reduce Cr (VI),
followed by complexation of Cr (III) by modified, oxidized tannins
with increased levels of carboxylic acid groups (39).
Manures have been used successfully to reduce Cr (VI) in chromite
ore processing residue-enriched soils (15).
Patents have been issued in the United States for the ascorbic acid-reduction
of Cr (VI) in chromite ore processing residue (21,22).
Sorption of p-methoxyphenol by soil colloids enhanced the reduction
of Cr (VI) relative to the reaction rate in solution, even under alkaline
pH conditions (60).
Precursors of humic acids in soils, such as tannic and Gallic acids,
were more effective than humic acid in reducing Cr (VI), but the oxidation-reduction
reaction resulted in polymerized organic polymers that complexed Cr
(III) and that were poorer reducing agents than the precursors (40).
Zero-valent Fe has been used to reduce Cr (VI) in groundwater, especially
with reactive permeable barrier walls (47)
and nanoscale particles of zero-valent Fe have been used successfully
for soluble Cr (VI) reduction. These so-called "Ferragels" are 21
times more effective on a per mole basis than commercial Fe filings
(45).
Divalent-Fe has been used for reduction of Cr (VI) in soils and aqueous
systems, with Fe (II) in soluble and insoluble forms and with and
without light-induced reduction of Fe (III)(17,8,32,33,12).
Reduced sulfur-containing compounds (e.g. iron (II) sulfides, dithionite,
thiols and hydrogen sulfide) have been used to reduce Cr (VI) directly
or to create reduced colloids or conditions in soils and groundwater
(61,55,29,54).
Hydrogen gas has also been used to reduce Cr (VI) under methanogenic
conditions (35).
•
Bio-availability of Cr in Soils
Differences
in the toxicity, solubility and mobility of Cr (III) and Cr (VI) have
led to regulatory dilemmas surrounding the establishment of allowed
maximum Cr levels in soils. One of the ways that Cr may affect humans
is via skin contact. Depending on Cr (VI) concentration and exposure
time, skin contact with Cr (VI) in dust from Cr-contaminated soils
may cause allergic contact dermatitis in sensitized individuals (43,41).
The release of soluble, adsorbed and precipitated forms of Cr (VI)
from soil particles to the skin may cause eczema-like rashes, especially
if the Cr (VI) is not reduced to Cr (III) by sweat and oils in the
skin and if the concentration is above a certain threshold value.
Nethercott et al. (41)
showed that the 10% minimum elicitation threshold was 0.089g Cr (VI)
per cm2 of skin, equivalent to 450
mg Cr (VI)/kg soil. The threshold value for Cr (III) was 165,000 mg/kg,
reflecting the much lower toxicity of Cr (III) than Cr (VI). In general,
Cr (VI) is reduced and detoxified in most body fluids, including sweat,
saliva, gastric juices, epithelial-lining fluid and blood. If the
reduction capacity of these natural defenses is exceeded, then human
health effects may be expressed. Human health effects were demonstrated
in industrial settings before strict occupational health standards
were implemented for Cr (VI) in ambient workplace air, especially
in chromite ore refineries where alkaline, Cr (VI)-rich mists and
dust were common.
The interaction of plant roots growing in soil enriched with Cr compounds
and waste materials has been a concern with respect to potential impacts
on ecological plant community structure, uptake of Cr into food crops
and reduction of Cr (VI) or oxidation of Cr (III) in the soil proximate
to roots (26).
The scrub and small-stature forest on serpentine "barrens" has been
hypothesized to be caused by the high Cr content of the serpentine
soils, but the soil Cr content is not correlated with plant-available
Cr levels, presumably because the principal form of Cr in serpentinite-rich
soils is insoluble Cr (III) similar to Cr2O3
(14,46).
The low solubility of Cr (III) in tannery wastes has also limited
plant-availability of Cr. James and Bartlett (24)
did, however, observe the oxidation of small fractions (up to 1%)
of tannery-borne Cr (III) in a high-Mn soil.
Root-soil interactions due to pH changes and root exudates may influence
the oxidation state of Cr and thereby increase or decrease the pool
of available Cr for absorption by plants. James and Bartlett (26)
observed that reduction of Cr (VI) in the root zone, followed by complexation
of newly formed Cr (III), may enhance uptake of Cr by bean (Phaseolus
sp.) and maize (Zea Mays L.) into both roots and shoots. Lytle et
al. (34)
observed detoxification via reduction of Cr (VI) by fine lateral roots
of water hyacinth (Eichornia crassipes) growing in nutrient culture.
Oxalate-bound Cr (III) was found in the shoots, following translocation
from the site of reduction in the roots. Carboxylic acids have also
been shown to increase uptake of Cr (III) by tomato (Lycopersicum
esculentum) and maize (53).
Plant uptake of Cr is typically low due to the binding of Cr (III)
in roots and soils, thereby minimizing translocation to the leaves
and fruit. However, the coupling of reduction of Cr (VI) and organic
complexation of Cr (III) is a possible explanation for elevated levels
of Cr (III) in plants grown in Cr (VI)-enriched soils.
•
Conclusion
The
oxidation states of Cr in natural waters and soils (Cr (III) and Cr
(VI)) and the inter-conversions between the two are keys to understanding
the mobility, toxicity, bioavailability and remediation of environments
enriched with Cr-containing wastes. Almost all concerns and investigations
surrounding the environmental fate of Cr focus on the oxidation-reduction
chemistry and biology of this important, fascinating metal, including
solubility, adsorption, movement, plant uptake, root-soil interactions,
metal-microbe interactions, human health effects, clean-up technologies,
risk assessment and regulation.
•
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