A
lthough not as common as solvent or fuel
products, chromium contamination of
groundwater is relatively widespread. The
U.S. EPA has estimated that as many as
1300 sites in the United States may have
groundwater contaminated with chromium—some
of which date to World War II.
As with most groundwater contaminants, chromi-
um contamination is traditionally remediated with an
extraction and treatment system—“pump and treat”.
Groundwater is extracted from the aquifer through a
well network and conveyed to an aboveground treat-
ment plant, where the chromium is removed using
anion exchange or precipitated with various treatments.
In many ways, chromium contamination has been
an ideal candidate for pump and treat, because the
most common chemical form, hexavalent chromium
[Cr(VI)], is highly soluble and not readily adsorbed
onto sediment surfaces. However, because of well-
known limitations inherent to methods that require
groundwater extraction, such as exponentially de-
creasing response to treatment and diffusion-limited
464 A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / DECEMBER 1, 2002 © 2002 American Chemical Society
DECEMBER 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 465 A
rates of extraction, pump-and-treat remediation of
chromium-contaminated groundwater is not always
satisfactory (1–3). Thus, alternative treatment meth-
ods have been developed. Most are in situ methods
that treat the impacted groundwater in the aquifer
and eliminate the extraction step. This article pro-
vides an overview of this technology.
Chromium primer
Chromium has three relatively stable valence states:
Cr(0) (the metallic state), Cr(III), and Cr(VI). Other va-
lence states are possible, but are not very stable. Cr(III)
is relatively insoluble in water under common envi-
ronmental conditions (pH 6–9), forming hydroxides
and oxyhydroxides alone, and a solid solution with
iron (4, 5). However, Cr(VI) is quite soluble and mo-
bile in the environment.
Chromium has various industrial uses, including
chrome plating, steel making, corrosion inhibition,
wood preservation, well drilling (as a fluid additive),
biocides, and paint and primer pigments. Smelters
that produce chromium can also be a source of con-
In Situ Treatment of
CHROMIUM-
CONTAMINATED
GROUNDWATER
New technologies
show promise for
removing chromium(VI)
pollution at lower cost.
J O N A T H A N F R U C H T E R
U.
S.
D
EP
AR
TM
EN
T
OF
E
N
ER
GY
IN
SE
TS
: P
AC
IF
IC
N
OR
TH
W
ES
T
N
AT
IO
N
AL
L
AB
OR
AT
OR
Y
tamination, because the recovery of chromium from
chromite ores requires oxidation to Cr(VI) before con-
version to other forms.
Cr(VI) is usually shipped commercially in the
dichromate form (Cr2O7
2–), because the chromium is
more concentrated. However, in dilute, near-neutral
pH aqueous solution, Cr(VI) commonly forms hy-
drochromate (HCrO4
–) and chromate (CrO4
2–) anions.
These anions are generally poorly adsorbed on
soils and sediments because they already have nu-
merous uncharged or negatively charged surface sites
at near-neutral pHs. Because they are sparingly sol-
uble, barium chromate precipitates may form under
some circumstances. However, the hydrochromate
and chromate ions generally are not retarded but flow
unimpeded in groundwater aquifers (6).
Cr(VI) concentrations in groundwater in the
United States are federally regulated under the Safe
Drinking Water Act, with a maximum contaminant
level of 0.100 mg/L as total chromium, and under the
Clean Water Act with an ambient water quality crite-
ria of 0.011 mg/L as chromium. Some U.S. states and
other countries have implemented more stringent
standards than the U.S. government for chromium
in groundwater. There is continued uncertainty as to
the appropriate drinking water standard for both
hexavalent and total chromium.
The in situ approach
In situ remediation of chromium-contaminated
groundwater involves chemically or biologically re-
ducing Cr(VI) to Cr(III), which is less toxic, less solu-
ble, and less mobile than Cr(VI). In addition, Cr(III)
can then precipitate as a hydroxide, usually as a solid
solution with ferric iron hydroxide, and will be effec-
tively immobilized (7, 8). This reduction is usually a
permanent solution, because Cr(III) is not easily re-
oxidized to Cr(VI) under conditions that occur in most
natural groundwater environments.
Cr(III) oxides and hydroxides occur naturally in
small concentrations in the sediment and soil in many
groundwater aquifers (crustal average concentration
= 102 ppm) (9). Although manganese oxides could re-
oxidize Cr(III), Cr(VI) is rarely detected in these
aquifers. In many instances, the chromium oxidation
by manganese oxides in soils and sediments appears
to be limited by surface alteration effects (10).
Various approaches to reducing Cr(VI) in situ have
been developed and tested. These methods usually in-
volve adding some already reduced compound to act
as a source of electrons.
System selection and design
The goal of any in situ aquifer treatment method is
to deliver an appropriate reagent to the contamina-
tion in the aquifer. Therefore, one way to classify the
different treatment options is by reagent and deliv-
ery system. The correct option is then a matter of
choosing the right combination for the site condi-
tions. To chemically reduce Cr(VI), reagents usually
consist of reduced forms of three elements: carbon for
most biological remediation systems and iron or sul-
fur for abiotic approaches and certain specialized bi-
ological systems. Delivery systems include trenches,
infiltration galleries, groundwater wells, and direct-
push injection.
Choosing the correct approach begins with char-
acterizing the site by geochemical, hydrological, and
geological means, along with describing the nature
and distribution of the contaminants. These charac-
teristics are tied together with a site conceptual
model. Frequently, a numerical model is used to
make the site description more quantitative.
Treatability studies in the lab and sometimes at the
field scale also help facilitate the selection process.
The following are examples of successful remediation
systems.
In situ treatment methods
These methods involve abiotic approaches, usually
involving reduced iron or sulfur compounds as the
electron donor, and sometimes both.
Permeable reactive barriers. Permeable reactive
barriers, or treatment walls, treat groundwater as it
flows away from the source and through the aquifer.
The permeable barrier cuts off only the flow of con-
taminants, but not the groundwater. In the trench-
and-fill barrier configuration, a trench is excavated
and filled with a chemically reactive medium. Care
must be taken to ensure that the hydraulic conduc-
tivity is equal to or higher than that of the surround-
ing aquifer to allow groundwater to flow through the
treatment zone. As the water flows through the zone,
the chromium is reduced by the reactive medium and
subsequently precipitates as a Cr(III) hydroxide.
Because permeable reactive barriers are passive,
their operation and maintenance costs are low.
Another benefit is that the contaminants that require
treatment and disposal are not brought to the sur-
face. Barriers can be used even if the contaminant
source has not been identified or well characterized,
and the natural groundwater flow pattern is unaltered.
However, this approach cannot treat contaminant
sources and is not suitable for all geologic or hydro-
logic regimes. For example, hydraulic conductivities
466 A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / DECEMBER 1, 2002
FIGURE 1
Cross-sectional schematic of a trench-
and-fill permeable reactive barrier
The barrier, which uses metallic iron to reduce
contaminants, has found some use in treating
chromium.
Fill
Reactive cell
Treated
groundwater
Aquitard
Contaminant
plume
DECEMBER 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 467 A
that are low may not permit sufficient groundwater
flow to treat significant volumes of contaminated
water. Groundwaters that are very high in dissolved
constituents, such as calcium, may form precipitates
that clog the barrier’s pores. Another limitation is that
the barrier must eventually be replaced. If this hap-
pens too often, the cost may be higher than alterna-
tive approaches.
Iron particle barriers. These barriers are the most
common form of permeable systems. They rely on
metallic iron [Fe(0)] for chemically reducing contam-
inants (Figure 1). Although originally developed to
treat chlorinated organic solvents, they have found
some use in treating chromium and other metallic
contaminants (11, 12). Particulate iron has the ad-
vantage that each atom can donate up to three elec-
trons, giving the barrier significant redox (reduction/
oxidation) capacity.
When iron is present, Cr(III) can precipitate as a
mixed iron–chromium hydroxide, which has a lower
solubility than pure chromium hydroxide. This type of
barrier is most frequently emplaced using trenching
techniques, although other methods have been used.
Often sheet piles are used to facilitate the installation
of the barrier. Barriers emplaced by trenching have
most commonly been restricted to depths of 10 m or
less below the surface (13). Various methods for in-
stallation at deeper depths have been investigated, in-
cluding vibrating beams and jet grouting, but these
become increasingly difficult at greater depths. In ad-
dition, the high pHs that form in these barriers may
lead to precipitation of various minerals, with subse-
quent plugging, so Fe(0) barriers rarely use their full
redox capacity (14). Nevertheless, iron particle barri-
ers for Cr(VI) reduction appear to be operating suc-
cessfully in the United States (15). Some difficulties
with passivation of the iron particles in a Cr(VI) bar-
rier have been reported by Danish scientists (16).
Other permeable reactive barriers. Other forms of
permeable reactive barriers have been used to treat
chromium contamination. One particularly inexpen-
sive alternative uses sawdust, compost, and limestone
(17 ). This barrier is actually a cross between chemi-
cal and biological methods, because the compost acts
as a carbon source for microbial populations, which
are active in the reduction of the chromium. Naturally
occurring zeolite coated with cationic surfactants has
also been investigated as an adsorbant for chromium
in permeable reactive barriers (18).
In situ redox manipulation (ISRM). ISRM tech-
nology creates a permeable subsurface treatment
zone to reduce mobile chromium in groundwater to
an insoluble form. The permeable treatment zone is
created by reducing Fe(III), which is present as sur-
face oxides, to Fe(II) within the aquifer sediments
(19). Some of the Fe(III) in 2:1 smectite clays is also
reduced by injecting sodium dithionite (Na2S2O4) into
the aquifer (Figure 2) (20). Sodium dithionite is a
FIGURE 2
In situ redox manipulation (ISRM) process
This technology creates a permeable subsurface treatment zone in aquifer sediments, where mobile chromium in
groundwater is reduced to a less soluble and mobile form. RM-X are monitoring wells.
Mobile field lab
Injection solution
RM-2
RM-5
RM-7
RM-6
RM-8 RM-1a
RM-1b RM-4
RM-3
RM-9
Injection
well
Static viewer level
Vadose zone
Office/storage/trailer
Low-
permeability unit
Permeable
treatment zone
Groundwater flow
Contaminant
plume from
upgradient
source
High-
permeability unit
Low-
permeability unit
strong reducing agent that has several desirable char-
acteristics for this type of application, including in-
stability in the natural environment and reaction and
degradation products that ultimately oxidize to sul-
fate. This instability is beneficial because it means
that the reaction period is rapid, and that after a
period of several days, no dithionite remains in the
aquifer. Potassium carbonate/bicarbonate is added
to the injection solution as a pH buffer to enhance
the stability of dithionite during the reduction of avail-
able iron (by buffering H+ generated during iron
reduction).
As with permeable reactive barriers, an ISRM treat-
ment zone is placed perpendicular to the groundwa-
ter flow to intercept the contaminant plume. This
geometry is created by a series of overlapping injec-
tion/withdrawal wells. Advantages include the use of
conventional groundwater wells, which leads to eas-
ier installation at greater depths, and the ability of a
single injection of dithionite to create a treatment
zone that will last for many years. The technology is
limited to clastic (silt, sand, and gravel) aquifers that
have sufficient hydraulic conductivity to allow the
reagent injection (>10–2 cm/s). The aquifer materials
must also contain at least small amounts of reactive
iron compounds (0.01–0.1%), although this is not usu-
ally a concern. Most aquifers contain considerably
more total iron than this, but only a fraction of it is
reactive. The longevity of the barrier depends on sev-
eral factors, including the amount of reactive iron,
the concentration of oxygen and contaminant—both
of which will reoxidize the barrier—in the ground-
water, and the groundwater flow velocity.
Chemically enhanced pump and treat. Also called
geochemical fixation, chemically enhanced pump
and treat adds a chemical reducing agent to the treat-
ed groundwater before it is reinjected into the aquifer.
In this way, the residual Cr(VI) that is not actually re-
moved during the groundwater extraction phase can
be treated in situ, alleviating some of the problems
of conventional pump and treat (21).
The reagent of choice, usually sodium metabisul-
fite, Na2S2O5 , or calcium polysulfide, CaSx, is a func-
tion of site geochemistry. Ferrous sulfate and sodium
bisulfide have also been used. However, ferrous sul-
fate injection can lower pH as a result of reactions
that are similar to acid mine drainage, and sulfides
can cause precipitation, which may clog the aquifer.
Chemically enhanced pump and treat can be used
to treat source areas. Another advantage is that treat-
ed water does not need to be discharged to the sur-
face. However, the approach needs a sufficiently
permeable aquifer, removal of reduction reaction
products if they become too concentrated, and reg-
ulatory approval to reinject treated water.
Electrochemical methods
Electrochemical or electokinetic remediation places
a series of electrodes into the contaminated zone, to
which a low-voltage (50–150 V), direct current charge
is applied (22). Contaminant ions in the water will
migrate toward the electrode of opposite charge,
which is called electromigration. Because hydrogen
ions will migrate, the pH will decrease at the anode
and increase at the cathode.
For groundwater remediation, the electrodes can
simply be placed in slotted nonmetallic wells, such
as those made of polyvinyl chloride. The drift veloc-
ities of the contaminant ions are relatively slow,
around 1 cm per day, so that the electrokinetic
method is not applicable to fast-moving groundwa-
ters. The slow drift velocity also requires relatively
close well spacing, another potential limitation.
Attempts to increase the drift velocity using higher
voltages can lead to problems with soil heating.
However, it can be useful for treating unsaturated
soils and slow-moving groundwater in tight aquifers
in which the permeability is too low to permit other
types of in situ remediation. Highly mobile anions
such as chromium are good candidates for electroki-
netic remediation, because they drift through the
aquifer with little or no adsorption. A successful pilot-
scale demonstration of the electrokinetics technolo-
gy for vadose-zone Cr(VI) contamination has been
conducted, but there have been no reported deploy-
ments for groundwater chromium contamination to
date (23).
Biological in situ methods
Reduction of Cr(VI) by living organisms either can
occur inside the cell or can be mediated in solution
by extracellular enzymes. It can involve direct reduc-
tion of Cr(VI) or biological reduction of another metal
species, such as iron, followed by abiotic reduction of
chromium by the reduced metal.
Microbial reduction. Microbial reduction of Cr(VI)
has been known for over two decades, with early stud-
ies showing that facultatively anaerobic Pseudomonas
species are capable of catalyzing direct metabolic re-
ductions of Cr(VI) to Cr(III) (24). Since that time, nu-
merous investigators have shown that bacterial
reduction of Cr(VI) is a widespread trait across sev-
eral chemotrophic and phototrophic bacterial genera.
In situ reduction of chromium by bacteria can be
achieved by the introduction of nutrients (electron
donors), microbes (bioaugmentation), or both.
Nutrients may be sugars (e.g., molasses) or organic
acids such as acetate, which can be used by many
microorganisms (25), or lactate, which is metabolized
by a restricted number of organisms. Injection of nu-
468 A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / DECEMBER 1, 2002
PA
CI
FI
C
N
OR
TH
W
ES
T
N
AT
IO
N
AL
L
AB
OR
AT
OR
Y
DECEMBER 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 469 A
trients alone assumes that a suitable population of in-
digenous metal-reducing or metal-accumulating or-
ganisms exists at the site. Recent research has shown
that such organisms occur widely, so bioaugmenta-
tion should not be needed (26).
Several species of bacteria, yeast, and algae cul-
tured in the laboratory are capable of sequestering
metals or changing the redox status of the aquifer so
that the metals precipitate or are more easily adsorbed
(27). The unicellular yeast Saccaromyces cerevisiae
demonstrates the most favorable results for metal
accumulators.
Reduction of Cr(VI) to Cr(III) by microorganisms
can be direct or indirect. Direct enzymatic reduction
can be achieved by two types of bacteria: dissimila-
tory metal-reducing bacteria that can use metals as
electron acceptors for growth, or the fermentative
and other anerobic metabolic groups that reduce met-
als, especially relatively easy-to-reduce metals like
Cr(VI), as a byproduct of their primary metabolic ac-
tivity. An example of a dissimilatory metal-reducing
bacterium is Shewanella oneidensis, strain MR-1 (28).
A fermentative bacterium that has been shown to re-
duce Cr(VI) is Enterobactor cloacae, strain HO1 (29).
Cr(VI) reduction by mixed cultures enriched from soil
samples has also been demonstrated in the labora-
tory (26). Numerous other bacterial genera have also
been shown to reduce Cr(VI).
The indirect approach uses iron-reducing bacte-
ria, such as Shewanella alga, strain BrY, to reduce iron
oxides and iron-containing clay minerals in aquifer
materials to the ferrous state (30). In this way, a re-
ducing barrier of ferrous iron, similar to that described
in the ISRM barrier section, can be created.
Microbial remediation offers relatively low costs
and only uses environmentally benign, carbon-based
reducing agents rather than sulfur or iron. The ulti-
mate result of carbon metabolism is usually CO2 as
opposed to sulfates or ferric iron salts, which may
cause secondary problems. However, there are four
concerns: Nutrients must be injected periodically over
the entire remediation period, which may be years;
sufficient formation permeability is needed to allow
injection of nutrients; achieving microbial growth
where it is needed can be difficult; and unwanted for-
mation plugging can occur as a result of excessive
growth near injection points.
Phytoremediation. Phytoremediation uses plants
to remediate contaminated soil and groundwater
through uptake, accumulation/sequestration, or bio-
chemical degradation. All vascular plants take up met-
als through their root systems, and some can
accumulate and store large amounts. Laboratory stud-
ies and small-scale field studies of the uptake/
sequestration of several metal contaminants in plants,
including chromium, have been conducted. Phyto-
remediation
本文档为【地下水 铬污染修复】,请使用软件OFFICE或WPS软件打开。作品中的文字与图均可以修改和编辑,
图片更改请在作品中右键图片并更换,文字修改请直接点击文字进行修改,也可以新增和删除文档中的内容。
该文档来自用户分享,如有侵权行为请发邮件ishare@vip.sina.com联系网站客服,我们会及时删除。
[版权声明] 本站所有资料为用户分享产生,若发现您的权利被侵害,请联系客服邮件isharekefu@iask.cn,我们尽快处理。
本作品所展示的图片、画像、字体、音乐的版权可能需版权方额外授权,请谨慎使用。
网站提供的党政主题相关内容(国旗、国徽、党徽..)目的在于配合国家政策宣传,仅限个人学习分享使用,禁止用于任何广告和商用目的。