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Low Cost Solution for Heavy
Metals Contamination Removal
Guest article by Doug Austin PE, ADT Environmental Solutions
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“Battery breaking” plants were once
common in the U.S. Operators took in spent batteries from cars and
other vehicles and literally smashed them to facilitate the recovery
and recycling of lead and other materials. Such low technology
operations often produced sites contaminated by high concentrations
of lead. Making things worse, the battery (sulfuric) acid that often
spilled during operations tended to accelerate the migration of lead
Dozens of such plants in the U.S.
were closed and abandoned by their owners during the 4-5 years
following 11/84, when the U.S. Congress passed tough, enforceable
environmental protection laws (HSWA Amendments to the Resources
Conservation and Recovery Act). Many of these properties
subsequently became Superfund sites.
The ‘Blackhawk site’ was located in a
remote area outside of Des Moines, Iowa. It operated profitably
until early 1985 then sold the property to real estate speculators.
During the early 1990s the City of Des Moines was expanding toward
the site and the city’s boundaries were extended to include the
region around it. Developers began construction of a large housing
development with a 10-acre central area that would become a park,
complete with community center, swimming pool, children’s play area,
picnic benches, barbeques, etc.
Development of the park was scheduled
to take place after most of the homes were constructed and sold.
When contractors and heavy equipment moved in to begin excavation of
the swimming pool, they immediately uncovered substantial volumes of
battery debris. Lead concentrations ranged upwards of 50,000 ppm in
some areas, with TCLP leachability of lead as high as 750 mg/L. The
total volume of soil with leachable lead exceeding the regulatory
maximum of 5 mg/L TCLP was estimated at 35,000 yd3. Construction
operations were halted as the worried developer considered his
options, and as surrounding residents became increasingly aware and
The chosen solution centered on
implementation of a process of ‘mineral synthesis’. That is,
chemical reagents introduced to affected media initiate the
formation of stable, lead bearing mineral forms. Candidate minerals
included species such as anglesite (lead sulfate, Pb(SO4)2 and
galena (lead sulfide, PbS – the most common lead ore).
However, the preferred mineral
option, from the standpoints of minimum cost, ease of
implementation, best overall results, and longevity was ‘lead
substituted, calcium hydroxyapatite. This mineral has the
following generic chemical formula: Ca5-nPbn(PO4)3OH (where n=5)
This solution has several advantages, including the following:
- Treated media is very stable and
secure over a wide range of environmental conditions (2.5=pH=13
- Lead bearing mineral forms have
low solubility in water (<5 parts per billion).
- Apatite mineral forms are hard
(Mhos hardness = 5.0) and resist physical degradation.
- Treatment results are effectively
permanent (stable over a wide range of environmental conditions).
- In-situ application. Site
operations involve minimal noise and there is no dust generation.
Typically, mineral forming reagents are liquids or slurries
applied directly to impacted media and permitted to soak in
(typically 0.5 to 0.7 meters). ‘In-situ’ operations are inherently
less noisy than ex-situ treatment systems involving pugmills or
similar mixing systems and there is significantly less likelihood
of transient dust generation.
- Speed. 50,000 cubic meters of
impacted media could be treated in less than two months.
- Low Cost. Significantly less than
US$50 per cubic meter, inclusive of all site operations and
subsequent disposition of treated media.
- Regulatory acceptance. USEPA and
State of Iowa regulators were familiar with results from a
difficult site in neighboring Missouri and were very supportive.
Minerals of the apatite group are
characterized by a phenomenon known as isomorphic substitution.
For example, hydroxyapatite has the following chemical formula:
This common mineral will “scavenge”
heavy metals (incl. some radionuclides) into positions within its
crystalline structure that might normally be occupied by other ions.
This ‘substitution’ results in little or no change to the
crystalline structure and physical properties of the mineral. For
example, under the right conditions lead, nickel, strontium, and
several other ionic metals will preferentially replace calcium,
taking up that ion’s position in the apatite crystal structure.
Similarly, arsenic, chromium and other anionic species can
substitute for phosphate (PO4).
In some cases, a heavy metal ion
species will not combine into an apatite mineral form. Examples
include cadmium, selenium, antimony, etc. For these heavy metal
species mineral forms such as sulfides and sulfates are preferred
for the purpose of leachability reduction.
The process of isomorphic
substitution is essentially irreversible. The substituting metallic
ion becomes an integral part of a mineral crystal structure which is
physically durable (Mohs>5), chemically stable (2.0<pH<13), and from
which the metal will not leach into the environment. A key aspect of
this phenomenon, for the purposes of use during clean up projects,
is that appropriate minerals are easily synthesized at ambient
conditions in a wide range of solid and liquid media. Even more
important for the owners of contaminated sites, such methods are
readily implemented in the field for significantly lower cost than
all other known methods.
Synthesis at Blackhawk
The Blackhawk Site was a moderately
level property covering roughly 60,000 m2. Roughly 2/3 of the
property was contaminated by lead at levels exceeding regulatory
standards. However, in no case was contamination found at depths
below 2 meters. Soil on the site was a moderately permeable glacial
till. This suggested a strategy involving direct application to the
soil surface of a liquid phosphate reagent. The chosen reagent is
aggressively hygroscopic and rapidly distributes itself through the
top 0.7 meters of the soil. The reagent soaks into soil very
quickly, and there was no concern about runoff.
In areas where contamination was
found at depths greater than 0.7 meters (roughly 70% of the site), a
treated layer or ‘lift’ is removed and staged elsewhere on the site
in order to provide access to lower layers requiring treatment.
Note – Depending upon media
characteristics more than one reagent type may be required to insure
all of the materials necessary to form a target mineral species are
present. At Blackhawk, all requirements were met with a
single, low cost reagent.
The strategy was to remove all
vegetation and debris from the site surface and to stake out plots
measuring roughly 10 meters square. A tank truck carrying the chosen
reagent was connected to a chemical pump with a timer. The pump flow
rate and timer were set to provide the quantity of reagent for a 900
m2 area and 0.7-meter penetration. A site worker wearing appropriate
protective clothing distributes the reagent as uniformly as possible
over the soil surface within the staked area until the pump timed
out (typically 2-3 minutes).
The apatite forming reaction is
complete within seconds. Immediately following reagent application,
excavators remove the top 0.7 meters of treated soil and the
operation is repeated for the next ‘lift’, if required. Treated
materials were staged elsewhere on the site pending confirmation
testing. Testing samples were collected immediately following
treatment. If necessary, staged materials may receive additional
applications using ‘land row’ methods (linear piles measuring 1
meter high and 2-3 meters wide. At Blackhawk, however, none of
the 41,700 m3 of treated material required a second treatment
At the Blackhawk Site no more than
three lifts were required. At other sites where similar strategies
have been employed there have been as many as 12 lifts required to
pursue contamination to its ultimate depth. At sites where
contamination is found at depths greater than 10 meters, or where
excavation in lifts is impractical for some reason, there are
pressure injection techniques for reagents that may be more costly
(50% higher, or less), but are equally effective.
Under U.S. environmental regulations
it is possible to leave previously contaminated materials in place
following treatment. A cosmetic soil cap may assuage individuals who
have residual concerns. At the Blackhawk Site the conservative
developer elected to have treated media hauled to a nearby
construction debris landfill for use as ‘daily cover’, at a cost
less than 10% of disposal at a hazardous waste facility.
The table below randomly lists
typical confirmation test results from treated soil at the Blackhawk
|Total Lead (ppm)
|*Where ND <
ADT has hundreds of formulations for
treating toxic heavy metals in a wide variety of media. This
approach to resolving heavy metal contamination is exceptionally
flexible and cost effective and can be adapted to most site
For more information contact:
Mr. Ken Pepperling
ADT Environmental Solutions
2020-C 8th Avenue #290
West Linn, Oregon 97068
Telephone: 503-638-0459 or 877-265-3395
Web site: http://www.adtenv.com/
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