Fixation and stabilization of chromium in contaminated materials

A highly flexible multi-step treatment technology for chemical fixation and stabilization of leachable chromium, particularly hexavalent chrome, in contaminated soils, solid wastes, concrete, sludge, sand and gravel and waste waters is disclosed. The process comprises reducing hexavalent chromium to chromous (Cr.sup.2+) and chromic (Cr.sup.3+) forms in the presence of water under alkaline conditions and fixing the reduced chromium forms with phosphate. The process reduces Toxicity Characteristic Leaching Procedure chromium levels below the regulatory threshold of 5 mg/l as required by the USEPA.

The present invention is directed to a process for the fixation and 
stabilization of leachable and soluble chromium (chrome) in contaminated 
soil, solid wastes, waste waters and the waste products of industrial 
processes. 
BACKGROUND OF THE INVENTION AND PRIOR ART 
The growing concern for the protection of the environment has led federal, 
state and local governments to enact a series of laws and regulations 
placing strict standards on the permissible percentages of heavy metals in 
waste waters, solids and solid wastes. Prior to the enactment of these 
laws, industries that generated solid and liquid wastes containing heavy 
metals, such as chromium, were not regulated and they disposed of these 
waste materials with little or no regard for the environmental 
consequences. Chromium contaminated wastes are generated by several 
industries, including metal finishing or plating operations, mining 
operations, milling operations, tanneries and operations using bichromates 
for processing organic products. Careless handling of such chromium 
containing materials and wastes has in many cases led to the contamination 
of the soil in the vicinity of these facilities and elsewhere. 
Chromium exists in various oxidation states ranging from valences of 2- to 
6+. Hexavalent chrome has a 6+ valence state and forms divalent anions 
like CrO.sup.4.sup.2- (Chromate) and Cr.sub.2 O.sub.7.sup.2- (dichromate) 
under acidic conditions. Both chromate and dichromate anions are highly 
reactive and extremely mobile in soils and waste materials. Chromates and 
dichromates are strong oxidizing agents that are extremely toxic and 
hazardous to living organisms. For example, chromic acid and sodium 
chromates are used in laboratory for the oxidation of organics compounds. 
Therefore, hexavalent chromium that is discovered in soils or exists as an 
industrial waste material must be destroyed, contained and/or stabilized 
to reduce the risks to the environment caused by its rapid migration and 
highly toxic characteristics. 
The Resource Conservation and Recovery Act of 1976, commonly known as the 
RCRA, provided for federal classification of hazardous waste. The 
statutory language defines "hazardous waste" as solid waste or 
combinations of solid waste which pose a "substantial present or potential 
hazard . . . when improperly treated, stored, transported, or disposed of, 
or otherwise mismanaged." Any solid waste that exhibits one of the hazard 
characteristics defined in subpart C of Part 261, Volume 40, Code of 
Federal Regulations is, by definition, a hazardous waste. 
A solid waste is considered to be a hazardous waste if it is listed by the 
Environmental Protection Agency (EPA), or it exhibits characteristics of 
either ignitability, corrosivity, reactivity, or toxicity as determined by 
the Toxicity Characteristic Leaching Procedure (TCLP) (USEPA Method 1311). 
Historically, toxicity characteristic regulations had been based on the 
Extraction Procedure (EP) Toxicity Test (USEPA Method 1310), which 
specified laboratory steps to be followed in analyzing samples. The test 
was aimed at identifying the tendency of wastes to generate a leachate 
with concentrations of contaminants greater than the values listed in 
Appendix II of the code of Federal Regulations, Part 261.24, page 406, 
revised Jul. 1, 1988. If concentrations of leachable, mobile chromium were 
found to be greater than 5 milligrams per liter, the material was 
considered characteristically toxic for chromium and hence hazardous with 
respect to chromium content. Such characteristically toxic wastes required 
treatment to comply with the USEPA regulations defining the treatment 
standards for chromium and other parameters of concern. This EP Toxicity 
Test is now obsolete, and has been replaced by the TCLP test for 39 
different parameters including chromium. 
Effective Nov. 8, 1990, the USEPA established the treatment standard for 
chromium wastes (DOO7), and particularly for chromium contaminated soils 
and solid wastes, at a toxicity characteristic level of 5 milligrams per 
liter in the extraction fluid according to the TCLP test. The TCLP test is 
much more rigorous--and is more uniformly applicable to a larger number of 
parameters--than the EP Toxicity Test. It replaced the EP toxicity method 
for RCRA waste determination. The TCLP test requires sizing of waste 
material to less than 3/8 inches or 9.5 mm and agitation of a 100 g waste 
sample in 2 liters of specified extraction fluid for 18 hours on a 
rotating agitator at a speed of about 30 revolutions per minute. The 
chromium concentration is determined in the extraction fluid after 
filtration under pressure, and expressed in units of milligrams per liter 
(mg/l). 
Chromium occurs in aqueous systems as both the trivalent (Cr.sup.+3) and 
the hexavalent (Cr.sup.+6) ions. Chromium is present in industrial wastes 
primarily in the hexavalent form, as chromate (CrO.sub.4.sup.-2) and 
dichromate (Cr.sub.2 O.sub.7.sup.-2). Chromium compounds are added to 
cooling water to inhibit corrosion. They are employed in the manufacture 
of inks and industrial dyes and paint pigments, as well as in chrome 
tanning, aluminum anodizing, and other metal cleaning, preplating, and 
electroplating operations. Chromates are also contained in some 
preservatives and fire-retardant chemicals used in wood preservative 
treatments. Automobile parts manufacturers are one of the largest 
producers of chromium-plated metal parts. Frequently the major source of 
waste chromium is the chromic acid bath and rinsewater used in such 
metal-plating operations. Reduction of hexavalent chromium from a valence 
state of plus six to plus three, and subsequent hydroxide precipitation of 
the trivalent chromic ion, is the most common method of hexavalent 
chromium control. To meet increasingly stringent effluent standards, some 
industries have turned to ion exchange to treat chromate and chromic acid 
wastes. Evaporative recovery of concentrated chromate and chromic acid 
wastes has also proved technically and economically feasible as a 
pollution abatement alternative. The application of other processes, such 
as electrochemical and activated-carbon adsorption techniques, is 
receiving increasing attention. 
U.S. Pat. No. 5,009,793, by Muller relates to the process of metal 
separation by adjustment of the pH in the range of 3.5 to 11 so that the 
dissolved metal salts are precipitated as metal hydroxides. The pH 
adjustment is initially made with an acid and then adjusted with the milk 
of lime in the presence or absence of hydrogen peroxide as an aid to 
oxidation and hydroxylation. Under natural conditions, this process is 
reversible and the precipitated chromium can convert back to hexavalent 
chrome. In U.S. Pat. No. 5,000,858, Manning and Wells discuss a process 
for removing hexavalent chromium from water. The process comprises the 
steps of: decreasing the pH value to below 3, adding a reducing agent to 
convert hexavalent chromium to chromic ion, increasing the pH in order to 
precipitate the metal hydroxide, adding an anionic polymer as a flocculant 
to settle the trivalent precipitate, and removing the solids to achieve a 
treatment goal of 0.05 mg/l chromium in the treated water. This process is 
cumbersome and generates a hazardous solid waste by the TCLP test 
criteria. 
U.S. Pat. No. 4,684,472 by Abbe and Cole relates to the reduction of 
soluble chromium with sulfide salts to generate a solid precipitate and an 
aqueous fraction. This method generates a chromium sulfide sludge which 
under natural environmental conditions due to biological activity can 
oxidize into chromates and sulfates. In U.S. Pat. No. 3,981,965, Gancy and 
Wamser discuss a method of treating solid waste material with sulfide ion 
to convert soluble chromium to an insoluble state. The treatment additives 
include calcium sulfide, sodium hydrosulfite, dithionates, dithionites, 
thiourea, thioglycolic acid and sodium xanthates. These reducing compounds 
convert hexavalent chrome (Cr.sup.6+) into trivalent chromium (Cr.sup.3+) 
and divalent chromium (Cr.sup.2+) forms which under oxidative conditions 
may revert back to hexavalent chromium. 
U.S. Pat. No. 4,678,584, by Elfine relates to the use of trithiocarbonates, 
especially Na.sub.2 CS.sub.3 and CaCS.sub.3 to precipitate metals as 
insoluble sulfides. A mole of trithiocarbonate removes a mole of heavy 
metal from waste water and forms a metal bearing solid waste and an 
effluent relatively free of metals. However, under natural environmental 
conditions these metal sulfides may oxidize, transforming chromates of 
various metal species into soluble and mobile chromium. 
Solidification methods based on Portland cement (see U.S. Pat. No. 
4,741,776 to Bye), pozzolans, lime kiln dust, calcium carbonate and 
powdered lime for chromium fixation are temporary solutions. Furthermore, 
these methods increase waste volume and mass, and therefore, only dilute 
the chromium in the final waste matrix. In U.S. Pat. No. 3,201,268, 
Hemwall discusses the stabilization of soils having lead salts by the 
addition of phosphoric acid. The use of phosphoric acid alone for chromium 
fixation of waste media and process materials containing hexavalent 
chromium does not work because the treated waste fails to pass the TCLP 
test criteria. 
The prior art methods are temporary solutions, generally applicable to 
waste waters. The need exists for a cost-effective permanent treatment 
technology which can be applied to solid wastes, precipitated solids, 
soils, sand, gravel, sludges, waste waters and other waste media 
containing hexavalent chrome. 
SUMMARY OF THE INVENTION 
The present invention is a process for the fixation and stabilization of 
chromium in waste materials comprising the steps of: (a) contacting said 
waste material in an alkaline aqueous media with a water soluble reducing 
agent capable of donating electrons in an amount and for a time sufficient 
to convert a substantial amount of the hexavalent chromium to divalent and 
trivalent chromium compounds, and (b) contacting the thus treated waste 
material with a water soluble phosphate source capable of reacting with 
the divalent and trivalent chromium compounds formed in step (a) in an 
amount and for a time sufficient to convert a substantial amount of said 
compounds to non-leachable and stable minerals species, wherein the 
TCLP-chromium levels are decreased below 5 mg/l. 
In some applications an alkalizing agent is added to the aqueous media in 
step (a) to maintain a pH of at least 7. The quantities of the additives 
(that is reducing agent, alkalizing agent and phosphate source) vary over 
a wide range based on several parameters, including: the chemical and 
mineral composition of the waste material, the amount of hexavalent 
chromium and other chromium forms in the waste material, the compositions 
and concentrations of the additives and the specific additives that are 
used. 
The addition of reducing agents like dithionites and ferrous sulfates in an 
alkaline environment to drive the hexavalent chromium reduction process to 
completion is an essential prerequisite in the present invention. It 
results in an end product that passes the TCLP test criteria and the paint 
filter test used for solid wastes. The waste material generated by this 
process is readily accepted for disposal by licensed landfills. 
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The chrome fixation and stabilization process disclosed herein consists of 
a series of steps for the treatment of liquid and non-liquid chromium 
contaminated materials. In the initial step, a soluble reducing agent is 
used to convert the chromium to divalent and trivalent forms and produce a 
first waste mixture. The reducing agent in either solution or a powder or 
crystal form is added in an amount of up to about 30% by weight of the 
contaminated material and preferably less than about 10% by weight. The 
amount of reducing agent used depends on the amount and type of chromium, 
the composition and structure of the waste material and the reactivity of 
the specific reducing agent selected. A suitable reducing agent is chosen 
from a group consisting of sodium dithionite or sodium hydrosulfite, 
ferrous sulfate, sodium sulfite, sodium bisulfite, sodium metabisulfite, 
sulfur dioxide, sodium sulfide, hydrazine, and mixture of two or more of 
these reducing agents or other compounds capable of transforming 
hexavalent chromium into a divalent or trivalent chromic form by donating 
one or more electrons. Sodium dithionite is the preferred reducing agent 
for the present process. In order to allow contacting of the reducing 
agent with the chromium bearing solid or waste material, water may have to 
be added to waste materials having a low moisture content. For some 
applications, an amount of water up to about 50% by weight of the waste 
material being treated is added. 
Since the reduction/conversion process is fully reversible and depends to a 
large extent on the pH and oxidation reduction conditions, it is essential 
that favorable conditions are created to drive the reduction reaction to 
completion. This is accomplished by controlling the pH at above 7 by 
adding an alkalizing agent (base). 
In the next step of the process, an alkaline environment with a negative 
redox potential is created by the addition of an alkalizing agent to form 
a second waste mixture. This step can also be carried out prior to or at 
the same time as the addition of the reducing agent. Acceptable alkalizing 
agents include quicklime, calcium hydroxide, calcium oxide, sodium 
hydroxide, potassium hydroxide, Dolomite, slaked lime, magnesium 
hydroxide, magnesium oxide and mixtures consisting of two or more of these 
alkalizing agents. The alkalizing agent is added to maintain the pH above 
7, with a preferred pH range of 9-12. The reduction reaction should be 
completed within about 72 hours but in some cases it may take up to about 
a week. The length of time will depend on the type and amount of chromium 
in the waste material, the alkalinity and structure of the waste material 
and the amount and concentration of the specific reducing agent used. Lime 
(that is quicklime, powdered lime or slaked lime) is the preferred 
alkalizing agent. When the preferred quantities of reducing and alkalizing 
agents are used, the reduction reaction is completed within about 6 to 12 
hours. The alkalizing agent is added in an amount of up to about 15% by 
weight of the contaminated material and preferably up to about 3% by 
weight. 
The redox reactions are accelerated in the presence of water and alkaline 
conditions with the corresponding and simultaneous oxidation of the 
electron source, that is, the reducing agent. If the chromium contaminated 
soil or waste material is already alkaline, such as concrete, an 
alkalizing agent may not be needed during the reduction reaction step and 
the complete reduction of hexavalent chrome is possible with the addition 
of only a reducing agent. 
During the reduction step, the chromic III form (Cr.sup.3+) is stabilized 
as chromic hydroxide when the reduction process goes to completion, 
leaving non-toxic levels of hexavalent chrome in the alkaline mixture. The 
objective when adding an alkalizing agent is to assure the completion of 
the reduction reaction and to stabilize the chromic III form as 
Cr(OH).sub.3 and the chromous II form as Cr(OH).sub.2. Conditions that 
would reverse this step are those that modify the pH. A pH below 7 or 
above 12 can destabilize chromic hydroxide and chromous hydroxide 
resulting in the formation of Cr.sup.3+, Cr.sup.2+, 
Cr(OH).sup.2+,Cr.sup.0, Cr.sub.2 O.sub.3, Cr(OH)4, Cr(OH).sup.3-.sub.6 and 
other transient species in different proportions. Also, oxidizing agents 
can reverse the reduction reaction and change chromic species into chrome 
under conditions of positive oxidation reduction potential (ORP). Adequate 
control of the reduction reaction and the pH is therefore essential for 
the reactions to completed. Inadequate quantities of the reducing agent or 
the alkalizing agent may increase the time for completion of the reduction 
reaction, prevent complete hexavalent chromium reduction or result in the 
reversal of the reaction. 
Chromium hydroxides in the colloidal and subcolloidal state are produced by 
the reduction reaction. These chromium hydroxides are combined with a 
phosphate source in the final step of the process to fix the chromium in a 
mineral form and produce the final waste material. The phosphate source is 
added in an amount of up to about 30% by weight of the chromium 
contaminated material. In a preferred embodiment, it is added in an amount 
of up to about 15% by weight of the contaminated material. The amount of 
the phosphate source used depends on the amount of chromium, the 
composition and structure of the waste material and the concentration and 
reactivity of the specific phosphate source. The phosphate source can be a 
dry crystal or powder or it can be in solution and it is selected from a 
group consisting of phosphoric acid, trisodium phosphate, triple 
superphosphate, ammonium phosphate, potassium phosphate, condensed 
phosphates and mixtures of two or more of these phosphate sources. 
Phosphoric acid is the preferred phosphate source. The mixture of treated 
material resulting from the reduction reaction is thoroughly mixed in a 
pugmill or mixer with an appropriate phosphate source on a continuous 
basis or in a batch mode. When the waste material being treated is a 
liquid, it is filtered within about 72 hours after the phosphate source is 
added and before it is cured. This removes the excess liquid and separates 
the chromium containing minerals and other solids that have formed. The 
filtrate is then tested and retreated if necessary, recycled or properly 
disposed in an environmentally safe manner. The resulting mixture of 
treated material so created is then staged on a curing pad. The curing 
permits the mixture to solidify and mineral structures to form while it 
dries. The drying of the wet material may take about 2 to 5 days depending 
on weather conditions and other factors that vary from site to site and 
location to location. The drying process is accelerated by mixing, tilling 
or agitating. 
The addition of the phosphate source results in highly stable and insoluble 
mineral species being formed by irreversible geochemical reactions that 
occur fairly rapidly under conditions of ambient temperature, pressure and 
moisture or humidity. In most cases, the final waste material is 
sufficiently cured after 72 hours so that at least about 95% of the 
reactions that form the final mineral structures have been completed. When 
the preferred amounts of additives are used in this process, the final 
waste material is sufficiently cured within about 6 hours so that at least 
about 95% of the reactions have been completed. When the most preferred 
amounts of the additives are used, the final waste material is 
sufficiently cured within about 6 hours so that at least about 99% of the 
reactions have been completed. 
The extent and type of mineral species formed as a result of the treatment 
process vary greatly depending on the composition of the soil or waste 
materials and the matrices of the minerals, the level of chrome 
contamination, and the ingredients and the amounts of the various 
treatment additives used in the different steps of the present process. 
Several synthetic mineral species are formed including kotchubeite, 
euchsite, chromitite and mixed calcochroites, callophanite and 
chromopicotite. Other mixed minerals formed include calcium-chromium 
phosphates, calcium-potassium-chromium phosphates, potassium-chromium 
phosphates, calcium-chromium silicates, chromium oxides and chromium 
hydrates. These final waste materials are geochemically stable and 
environmentally safe.

The following examples are given by way of illustration only and should not 
be considered as limitations on this invention. 
Example I 
Chrome ore tailings were found to exhibit the characteristics of 
corrosivity (pH near 12.0) and toxicity for chromium as tested by the TCLP 
regulatory test criteria. Samples contained nearly 2% total chromium and 
84 to 91 mg/l of TCLP chromium when tested using extraction fluid II as 
described in the latest toxicity test procedure; USEPA Method 1311, 
SW-846. Approximately 500 g of this hazardous waste material was mixed 
with 30 g of sodium dithionite (Na.sub.2 S.sub.2 O.sub.4) and 100 ml of 
water to facilitate the chromate reduction to trivalent chromic form. 
Since the waste material was highly alkaline, no lime was added. Six (6) 
hours were allowed for completion of the reduction process. 
After six (6) hours, 30 ml or 51 g of 85% concentration phosphoric acid 
were added to the treated waste material and mixed thoroughly. The final 
mixture was allowed to stand for 4 hours. After this curing period, the 
treated material was analyzed for TCLP-chromium and no detectable 
leachable chromium (&lt;0.2 mg/l) was found in the treated material. The pH 
of the treated material was neutral (pH of about 7) as compared to the 
highly alkaline untreated sample (pH of about 12). The results are 
presented below in Table I. 
TABLE I 
______________________________________ 
Sample pH Eh Moisture 
Total Cr 
TCLP-Cr 
Description 
(S.U.) (mV) (%) (mg/kg) 
(mg/1) 
______________________________________ 
Untreated 
12 -285 15.7 1.98 90 
Treated 6.9 -0.2 31.5 1.95 ND (&lt;0.2) 
______________________________________ 
The table shows the characteristics of the treated and untreated waste 
material. The treated material had a neutral pH as compared to the highly 
alkaline untreated material, the oxidation reduction potential (Eh) was 
almost zero, the moisture content had increased due to the addition of 
water in the process, the total chromium content was unchanged and the 
TCLP-chromium content was significantly reduced to a non-toxic level of 
less than 2 mg/l. 
EXAMPLE II 
The treatment process was employed on a larger scale in an application of 
the process of the present invention using two one-cubic yard lots of 
highly contaminated waste material. One cubic yard lot weighing 
approximately 1.5 tons was treated with 210 lbs. of sodium dithionite and 
70 gallons of water. The pH did not have to be adjusted because of the 
high alkalinity of the material. After six hours, a test for hexavalent 
chromium was negative. About 22 gallons or 308 lbs. of 85% concentration 
phosphoric acid was then added and thoroughly mixed in a Mini-Maxon Mixer 
to allow the geochemical, irreversible, and permanent fixation of chromium 
as phosphate mineral species which are highly insoluble under natural 
environmental conditions. After a curing time of 4 hours, the treated 
material contained TCLP-chromium below the detection limit of 0.2 mg/l. 
Tests after 49 hours, 1 week and 1 month of the completion of the treatment 
showed that the TCLP-chromium in the treated sample was less than 0.1 mg/l 
as compared to an average of 54 mg/l in two untreated samples. 
Example III 
A sand-like material containing 7287 mg/kg of total chromium and 26.2 mg/l 
of TCLP-chromium was obtained for treatment according to the process of 
the present invention. Approximately 500 g of this hazardous material was 
mixed with 10 g of sodium dithionite and 50 ml of water and then 2 g of 
lime was added and mixed thoroughly. This mixture was allowed to stand and 
cure overnight (approximately 12 hrs.). Twenty-five ml of 25% phosphoric 
acid reagent (that is 6.25 ml of phosphoric acid by volume in 18.75 ml of 
water or 10.6 g of phosphoric acid by weight in 18.75 g of water) was 
added and blended thoroughly. The treated material was cured for four 
hours to irreversibly fix any leachable chromium in the phosphate mineral 
species. The TCLP-chromium in the treated material was 0.3 mg/l, well 
below the regulatory threshold limit of 5 mg/l for materials to be 
classified as non-hazardous waste. 
EXAMPLE IV 
A chromium contaminated clay sample containing 342 mg/l of TCLP-chromium 
and 9850 mg/kg of total chromium was treated as follows: 
Step I: about 20 grams of sodium dithionite, Na.sub.2 S.sub.2 O.sub.4, by 
weight along with 200 ml of water were mixed with 1000 grams of the 
contaminated clay, 
Step II: about 5 grams of lime were added and mixed with the contaminated 
clay, 
Step III: after 12 hours, 34 grams of phosphoric acid by weight was mixed 
with the contaminated clay for permanent fixation and generation of highly 
stable and insoluble phosphate mineral species. 
After curing for 4 hours, the TCLP-chromium in the treated clay sample was 
reduced to 2.4 mg/l which is roughly half the regulatory threshold limit 
of 5 mg/l 
EXAMPLE V 
An in-situ, 12 ft..times.12 ft..times.1 ft. test plot containing clay soil 
and sludge contaminated with 70.4 mg/l of TCLP chromium were treated by 
the process of the present invention. The volume of the test plot was 
approximately 144 cu ft. or 5.33 cu yds. About 114 kg of sodium dithionite 
(Na.sub.2 S.sub.2 O.sub.4) in solution was added and mixed with the help 
of a rototiller. Since the clay soil and sludge were saturated with water 
due to rain, no extra water was needed. After 30 minutes, about 45.5 kg of 
lime was spread over the test plot and mixed thoroughly with the help of 
the rototiller. The treated soil was allowed to cure for 10 to 12 hours in 
order to allow complete reduction of the hexavalent chrome to the divalent 
chromous (Cr.sup.2+) and trivalent chromic (Cr.sup.3+) forms. 
After the treated soil had cured, approximately 41 gallons (or 574 lbs.) of 
an 85% concentration of phosphoric acid was pumped over the test plot and 
blended thoroughly. The treated material was allowed to cure and after 4 
hours, it was tested and found to have TCLP-chromium levels below 0.34 
mg/l. 
Results of five test runs are summarized in Table II below: 
TABLE II 
______________________________________ 
In-situ Application of Treatment Process To Heavy 
Clay Soils For Fixation of Leachable Chromium 
Test TCLP-Chromium 
Run Untreated, mg/l 
Treated, mg/l 
______________________________________ 
I 70.4 0.11 
II 115.9 0.07 
III 161.4 0.2 
IV 110.1 0.3 
V 125.6 0.1 
______________________________________ 
EXAMPLE VI 
An alkaline soil sample (pH of about 11) containing a measured value of 
nearly 4600 mg/kg of total chromium and a TCLP-chromium value of 138 mg/l 
was treated using the process of this invention for chemical fixation and 
stabilization of leachable chromium. Approximately 100 g of the hazardous 
soil sample was treated as follows: 
(a) 3 g sodium dithionite (Na.sub.2 S.sub.2 O.sub.4) in solution were added 
and mixed and allowed to cure for 9 hours (lime was not needed because the 
sample was highly alkaline), and 
(b) 25 ml of 20% phosphoric acid (i.e., 5 ml or 8.5 g phosphoric acid and 
20 ml of water) were added and mixed thoroughly. This final mixture was 
allowed to cure for 3 hours. 
The treated sample was rendered non-hazardous as the TCLP-chromium levels 
decreased to below the detection limit of 0.2 mg/l.