Mercury removal from solid mixed waste Page: 4 of 6
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matrices. When KI/I2 leaching solutions are applied to a
mercury contaminated solid the solid mercury species are
transformed to the soluble mercury complex Hgl-,
according to the following reactions:
Hg0 + I, === > HgI2 (1)
HgI2 + 2 1- ===> HgI-4 (2)
HgO + H2O + 4 1- ==> HgI4 + 2 OH- (3)
HgS + 12 + 2 1- ==> HgI + S (4)
STUDY OBJECTIVES
The specific objectives of this research project are to:
1)determine if KI/2 leaching can effectively remove Hg
from solid waste streams typical of DOE mixed waste, 2)
determine the fate of radionuclides following KI/I2
leaching, and 3)develop the engineering relationships
necessary to support the design of a full-scale treatment
facility to remove mercury from mixed waste.
EXPERIMENTAL APPROACH
The study objectives will be met by conducting a series of
bench- and pilot-scale experiments as given in Table 2.
During each phase, KI/2 leaching will be evaluated and
treatment efficiency assessed. The phase 1 and 2 studies
should be completed during FY94. After consultation with
research staff from GE, the process factors to be evaluated
in the laboratory studies were selected. The experimental
factors that will be evaluated include reaction temperature
and leaching solution concentration. Factors to be held
constant include liquid to solid ratio (3.0 : 1, wt basis),
reaction time (4 h), and mixing speed (-150 rpm). A
fractional factorial design was used to design optimization
experiments which will allow the determination of the most
important process variables. During the optimization
experiments both the mercury removal from the solid
matrix and the loss of iodine during treatment are measured
and evaluated.
METHODS
The initial laboratory studies were conducted in the
batch mode. Clean synthetic soil was contaminated by
adding measured amounts of Hg0, HgS, or HgO to 10 gins
of soil contained in 125 ml erlenmeyer flask. The pre-
treatment mercury concentration was 20,000 ppm. The
flasks were capped after Hg addition and agitated for a
period of 24 h at 40 *C. KI/2 solutions were prepared
using reagent grade KI and I2 and deionized (DI) water at
least 12 h prior to soil treatment. The KI/l2 leachingsolutions were prepared and stored in dark glass containers
to minimize I2 losses prior to treatment. Thirty mis of
leaching solutions were added to each flask containing soil
to be treated. Control flask had pH adjusted DI water
added to them instead of KI/I2 leaching solution. A blank
using surrogate soil with no added Hg was also treated with
the KI/12 leaching solution. The flasks containing soil and
leaching solution were either capped or covered with
parafilm (depending on treatment temperature) and placed
in pre-heated Lab-line environmental shaking chambers.
After the 4 h reaction time had elapsed, the contents of the
flasks were vacuum filtered through 0.45 a glass fiber
filters. The sediments remaining in the flask and the filter
cake were washed with an additional volume of 30 ml DI
water. The volume of the supernate and wash water were
measured and the liquids were combined in 80 ml glass
sample jars. The filter and filter cake were returned to the
erlenmeyer flask for digestion. All samples were stored at
room temperature in glass containers prior to analysis.
The mercury concentration in the post-treatment
samples was measured using the Cold Vapor Atomic
Adsorption (CVAA) method. A Perkin Elmer model 400
Atomic Absorption Spectrophotometer equipped with an
autosampler (Perkin Elmer model 90) and a Flow Injection
Mercury/Hydride Analyses system (FIAS-100) was used.
In order to overcome iodide interferences, alkaline carrier
and reductant solutions were used as suggested by Munaf
er al . Aqueous samples were diluted to the instrument
range (1-20 ppb) by serial dilutions with a preservative
solution containing 5% (v/v) HNO3 and 0.01% (w/v)
K2Cr207. All samples were analyzed within 24 h of
dilution. The residual soil (plus filter) were prepared for
analysis by first digesting the solids with 40 mis of 12 N
HNO3 for a period of 16 h. The sediments remaining in
the flask were then washed with an additional 40 ml of 12
N HNO3. After filtration the supernate was stored in glass
sample bottles prior to analysis. HNO3 was selected as the
digestion agent since it has been reported in the literature
to be able to solubilize all forms of Hg except HgS and has
less hazards associated with it than other agents such as
Aqua Regia. Alternate digestion methods will be used for
samples that may contain HgS.
The iodine and iodide concentrations in the leaching
and post-treatment solutions was measured using the Leuco
Crystal Violet Method'. A standard curve was made using
standards prepared from reagent grade I, and KI crystals.
After color development the absorption at a wavelength of
592 nm measured with a Bausch and Lomb
spectrophotometer.
DISCUSSIONG o, QS3
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Gates, D.D.; Morrissey, M.; Chava, K.K. & Chao, K. Mercury removal from solid mixed waste, article, December 31, 1994; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc676032/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.