[pt] Esta dissertação apresenta os resultados obtidos no processo de remediação de uma área contaminada por cromo hexavalente por meio da técnica de remediação química in situ (ISCR). A área de estudo localiza-se na
cidade do Rio de Janeiro e foi ocupada por uma fábrica de vidros por cerca de quarenta anos e comprada para construção de condomínios residenciais. A área de estudo passou um processo de gerenciamento ambiental que contemplou diversos estudos ambientais para aquisição de dados e delimitar a contaminação, horizontal e verticalmente. O teste de bancada realizado com o reagente químico polissulfeto de cálcio demonstrou ser eficiente para reduzir as concentrações de cromo hexavalente em solo e água subterrânea. Com base nos
dados adquiridos neste teste foi possível calcular a dose de injeção do polissulfeto de cálcio. Foram realizados setenta pontos de sondagem, pela técnica direct push, para injetar duzentos e cinquenta e dois mil e trinta litros de solução de polissulfeto de cálcio e água, sendo aproximadamente três mil e quinhentos litros por ponto de injeção. Os resultados após a injeção demonstraram que o polissulfeto de cálcio conseguiu remover o cromo hexavalente que estava adsorvido ao solo e reduziu a concentração de cromo hexavalente na água subterrânea entre quarenta e seis e sessenta e sete e noventa e nove e noventa e cinco por cento , após dezenove meses da injeção. Dessa forma, comprovou a eficiência deste reagente químico para remediação de áreas contaminadas por cromo hexavalente, assim como foi observado nos artigos técnicos de estudos de casos nos Estados Unidos e Europa. / [en] This dissertation presents the results obtained in the remediation process
remediation of a contaminated area by hexavalent chromium applying the
technology in situ chemical remediation (ISCR). The study area is located in
the Rio de Janeiro city and was occupied by a glass factory for forty years and
purchased for construction of residential condominiums.
This dissertation presents the results obtained in the remediation process
remediation of a contaminated area by hexavalent chromium applying the
technology in situ chemical remediation (ISCR). The study area is located in
the Rio de Janeiro city and was occupied by a glass factory for forty years and
purchased for construction of residential condominiums.
Chromium is an important metal for the industry and is used in various
products and processes, such as electroplating, leather treatment, pulp, wood
preservation, and refractory manufacturing. The trivalent chromium is essential
from a nutritional point of view, non-toxic and poorly absorbed in the body,
acting in the maintenance of some functions. Cr(III) is the most common being
found and occurs naturally, since the element Cr(VI) can occur naturally, but in
low concentrations, if the groundwater has geochemical conditions the Cr (III)
can be oxidize to Cr (VI). The hexavalent chromium is the most dangerous
valence state and, according to ATSDR (two thousand and twelve), have greater mobility in the
groundwater, being carcinogenic by inhalation of high doses of soluble
chromate salts. The mobility of hexavalent chromium is high in soil and
groundwater because it is not adsorbed by the soil in that valence state, on the
other hand when it is in trivalent form is strongly adsorbed by the soil, forming
insoluble precipitates, having low mobility in soil and groundwater.
The hexavalent chromium remediation by in situ chemical reduction
using calcium polysulfide has been the subject of several field studies
documented in the literature, both for soil and groundwater from the Chromite
Ore Processing Residue (COPR) (Storch, et al., two thousand and two; Graham, et al., two thousand and six ;
Charboneau, et al., two thousand and six ; Wazne, et al., two thousand seven a; Wazne, et al., two thousand seven b;
Chrysochoou, et al., two thousand and ten ; Chrysochoou & Ting, two thousand and eleven ; Pakzadeh & Batista,
two thousand and eleven ; Chrysochoou, et al., two thousand and twelve ).
Calcium polysulfide is a fertilizer to soil and commercially available and
has been used in some remediation studies cases for reducing hexavalent
chromium in soil and groundwater. Being commercially available and used as
fertilizer, it is a relatively cheap chemical reagent in comparison with other
chemical compounds exclusively developed for this purpose.
The stoichiometric demand and the chemical kinetics of the reduction of
Cr (VI) by the calcium polysulfide in aqueous solution was studied by Graham
et al. (two thousand and six) from the chromite ore processing residue (COPR). With this study
it was reported that a molar ratio of a point sixty-six is required (close to the theoretical
value of one point five) and a first-order kinetics with an initial concentration of twenty-six eight point
mg/L and pH of the CPS solution around eleven point five, with the presence of oxygen.
Thus, the hexavalent chromium is reduced to chromium hydroxide, slightly
water soluble compound which is precipitated to the soil. The trivalent
chromium has low solubility, toxicity, mobility, reactivity and is considered
stable.
There are various application techniques of chemical reagents in the
underground environment, and choosing the most appropriate method for each
area depends on the type of contaminant, geological environment, groundwater
and surface interference, depth, thickness and size of the contaminated area. As
described by Suthersan (mil novecentos ninety-six), the injection of chemical reagents has to
achieve two objectives: (one) creating and maintaining an ideal redox
environment and other parameters such as pH, presence or absence of
dissolved oxygen, etc.; and (two) the delivery and distribution of the necessary
reagents for a homogeneous way throughout the injection area, both
horizontally and vertically. Thus, it is essential that the conceptual model of the
study area is very detailed, so there is no doubt in the choice of chemical
reagent application methodology.
Although there are numerous laboratory studies on hexavalent chromium
remediation using calcium polysulfide, there are few reports in the literature on
the field application, especially case studies in Brazil, therefore, this case study
becomes a demonstration applying calcium polysulfide as a remediation
technique for hexavalent chrome, with geochemical data, which are important
for monitoring chemical reduction. This case study shows the effectiveness,
dosage and concentration of the study area, and may apply to other hexavalent
chromium remediation projects.
Materials and Methods
A former glass factory (the Site ) operated in the North Zone of Rio de
Janeiro / RJ, Brazil from the mid-thousand nine hundred and fifty s to two thousand and five. A portion of the facility was
used to store raw material to produce glass, including arsenic oxide, and
another portion of the Site was used to conduct industrial plating using
hexavalent chromium (Cr(VI)) in the glass molds. In two thousand and nine, the Site was
purchased for mixed use redevelopment, demanding an environmental
assessment and subsequent remediation.
Between two thousand and nine and two thousand and twelve several phases of site investigation was
conducted. The results of the investigation indicated that Cr(VI) was present in
soil at concentrations up to approximately twenty one mg/kg and in groundwater at
concentrations up to approximately thirty mg/L. These concentrations exceeded
regulatory criteria of three hundred mg/Kg for soil and zero point zero five mg/L for groundwater. A
phased remedial approach was developed consisting of the following: (a)
excavation and off-site disposal of two and four hundred ton of Cr(VI) impacted soil from the
source area, performed in the unsaturated and saturated zone soils in the Cr(VI)
source area; (b) post-excavation monitoring of the groundwater conditions; and
(c) groundwater treatment following the excavation program. Hexavalent
chromium concentrations in groundwater decreased significantly following the
excavation, however, additional reduction of concentrations of Cr(VI)
contaminant in groundwater was required.
Then was designed and implemented a set of bench-scale treatability
tests in order to evaluate groundwater remediation alternatives. Several
proprietary and non-proprietary reductants for co-treatment of Cr(VI) were
evaluated. Calcium polysulfide were selected to treat Cr(VI).
To reduce residual Cr(VI) concentrations in the groundwater plume
located downgradient of the former excavated source area, dois e seven hundred cubic meters
were targeted for active treatment. The groundwater remediation approach
consisted of the injection of thirty liters of CPS (twenty nine percent) diluted in two hundred and twenty
liters of water, yielding a total of two hundred and fifty liters of solution injected using direct
push technology into seventy two locations.
Groundwater Monitoring
As part of chemical reagent injection stage was performed the baseline
monitoring with collection of soil and groundwater samples. The soil sampling
was performed by direct push technique using PVC liner with two inches in
diameter, to analyze the total and hexavalent chromium concentrations.
Six months after the injection were installed sixteen monitoring wells, eight
shallow wells (five meters) and eight deep wells (nine meters) spread upstream, side,
middle and downstream of the injection area.
Groundwater geochemical parameters (i.e., temperature, total solids
dissolved, specific conductance, pH, oxidation-reduction potential, and
temperature) were measured at the time groundwater samples were collected.
Groundwater samples were collected and analyzed for total and dissolved
chromium, hexavalent chromium, iron, arsenic, manganese, calcium, sulfate,
and sulfide. Samples were field-filtered with disposable zero point forty-five μm polyethylene
filter capsules prior to collecting samples for dissolved metals.
Results and Discussion
Dose calculations for the reduction of hexavalent chromium have been
performed with the data obtained in the bench-scale treatability test and
resulted in a stoichiometric demand of four mlCPS/kg soil to the treatment of the
study area. Therefore thirty liters of solution was used containing twenty-nine percent
calcium polysulfide and approximately two hundred and twenty two liters to perform their mixture,
totaling two hundred and fifty-two thousand and thirty liters of solution.
For solution injection were performed seventy two soil borings with eight point five meters
deep, and the product was injected range between two point five and eightpoint five meters. The
depth of injection was from two point five mbgl covered any change in water level due to
seasonal variation.
In each soil boring was injected chemical reagent solution comprised four hundred and seventeen
liters of calcium polysulfide diluted in tree and eighty-three liters of water for a total volume
of tree and a half liters of solution injected per point.
The comparative analysis results of the third monitoring campaign ( eighteen
months post-injection) with the baseline campaign (september/two thousand and twelve) indicated
reduction of hexavalent chromium concentrations between forty-six point sixty-seven and ninety-nine and ninety-five percent.
Regarding the second monitoring campaign (twelve months post-injection),
the hexavalent chromium concentrations reduced between twenty-three point ninety-nine and ninety nine point seventy-nine percent in
five of the fifteen monitoring wells that were sampled. In three of the fifteen
monitoring wells the hexavalent chromium concentrations remained below the
quantitation limit used by the analytical laboratory method. There was no
increase in hexavalent chromium concentration, compared the results of the
third and second monitoring campaign.
The evaluation of the Eh and pH values measured in the monitoring
campaigns showed that the pH value was in the acidic range (about four ) and after
removal of contaminated soil with hexavalent chromium pH raised to between
five and six, after the chemical reagent injection pH increased to the basic range
(above seven point five). In the second and third campaign the pH reduced to acid range
(below six point five), which can be regarded as the pH value of the area background.
The Eh has inversely proportional behavior, increasing between the first and
third campaign, and in the third campaign the measured values are in the
ranges considered as moderately reducing (hundred to four hundred mV) and oxidizer (> four hundred
mV).
The pH variation also showed a relationship between increased
concentrations of calcium, iron, manganese and sulfate (in some monitoring
wells). There were also reductions in hexavalent chromium concentrations in
monitoring wells.
Conclusion
This case study indicates that the use of different remediation techniques
when applied together (excavation and chemical reduction), reducing the time
required for remediation of a contaminated site without impacting the final cost
of remediation. The chemical reduction of hexavalent chromium using calcium
polysulfide was effective to reduce the concentration to less than the
quantification limit of the analytical method used.
Therefore, as presented it is necessary to carry out several studies to
detail the hexavalent chromium concentration in the site, as well as understand
the geochemistry of groundwater and performing bench-scale tests to evaluate
the effectiveness of the chemical reagent in the site study hydrogeological
environment and calculate the required dose.
The treatability test with calcium polysulfide demonstrated the feasibility
of using this chemical reagent by In Situ Chemical Reduction (ISCR) to reduce
the hexavalent chrome concentration in soil and groundwater. The test resulted
in a stoichiometric demand of four mlCPS /kg soil to the treatment of the study
area.
Soil samples collected six months after injection showed that the calcium
polysulfide could desorb hexavalent chromium from the soil, since,
contaminant concentrations were not detected in the samples.
neteen months after the injection of the chemical reagent the groundwater
concentrations of hexavalent chromium reduced from forty-six point sixty-seven to ninety-nine and ninety-five percent in
relation to baseline campaign. And, of the fifteen monitoring wells in just three
wells hexavalent chromium concentrations were detected.
This demonstrates the effectiveness of using calcium polysulfide to
remediate hexavalent chromium in soil and groundwater, confirming the
studies by Storch et al. (two thousand and two), Graham et al (two thousand and six), Charboneau et al. ( two thousand and six),
Wazne et al. (two thousand and seven a), Wazne et al. (two thousand and seven b), Chrysochoou et al. (two thousand and ten),
Chrysochoou & Ting (two thousand and eleven), Pakzadeh & Batista (two thousand and eleven), Chrysochoou et al
(2012) in several areas in United States and Europe.
| Identifer | oai:union.ndltd.org:puc-rio.br/oai:MAXWELL.puc-rio.br:25019 |
| Date | 10 August 2015 |
| Creators | RAFAEL FERREIRA GODOY |
| Contributors | EURIPEDES DO AMARAL VARGAS JUNIOR |
| Publisher | MAXWELL |
| Source Sets | PUC Rio |
| Language | Portuguese |
| Detected Language | English |
| Type | TEXTO |
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