inverse geochemical modeling of groundwater with special emphasis on arsenic
DESCRIPTION
Sharanya Shanbhogue. Inverse Geochemical modeling of groundwater with special emphasis on arsenic. Geochemistry 428/628 12/09/2010. Overview. Case Study Scope Inverse Geochemical Modeling (PHREEQC- GEOL 628) Common Ion Effect Iron-Arsenic Model Conclusions. - PowerPoint PPT PresentationTRANSCRIPT
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INVERSE GEOCHEMICAL MODELING OF
GROUNDWATER WITH SPECIAL EMPHASIS ON
ARSENICSharanya Shanbhogue
Geochemistry 428/62812/09/2010
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Overview
• Case Study
• Scope
• Inverse Geochemical Modeling (PHREEQC- GEOL 628)
• Common Ion Effect
• Iron-Arsenic Model
• Conclusions
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Case Study –Zimapan Valley, Mexico
Location of Study Area What’s going on?• High Concentrations of
Arsenic (As) in groundwater.
• Possible reasons:1. Leaching of mine tailings.2. Dissolution of As rich
smelter and subsequent infiltration.
3. Interaction of Groundwater with As-bearing rocks.
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Groundwater Chemistry• Concentrations of species
obtained from Detzani-Muhi wells
• Modeling suggests presence of As in samples.
• Origin of As: Aresenopyrite, scorodite, and tennantite minerals.
Concentration Input(mmol / L)
Detzanf Muhi
Alkalinity 4.296 4.337
As 6.994*10-3 13.35*10-3
Ca 3.023 1.737
Fe 3.224*10-3 3.9408*10-3
Mg 0.4033 0.555
SO4 1.494 0.9102
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“Common I(r)on Effect”• Iron(Fe) may effect Arsenic reaction.
• Reactions:
FeS2+ 3.5O2+ H2O = Fe2+ + 2SO42-+ 2H+
FeAsS + 3.25O2+ H2O = Fe2+ + SO42- + H3AsO4
• Another groundwater example:
Ca+2 release---> gypsum(CaS04)dissolution
Calcite(CaC03) precipitation
Common ion: Ca
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As in GroundwaterEh-pH Diagram for As-Fe-O-H-S system
•This graph shows that the As minerals present in the well are “NOT STABLE” as a result they will dissolve.
•Rationale:
As is supposedly originating from Arsenopyrite/Scorodite
Stable forms: HAsO42-
and
H2AsO4-
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Impact
• As concentration in municipal water was 0.3 mg /L
• El-Muhi deep well 1 mg/L
• WHO standard 0.01 mg/L
• People consumed water directly from As polluted wells.
• High As concentrations in their drinking water in India and Bangladesh.
• The interaction of the underlying As-rich aquifers with organic material creates reducing conditions and mobilizes As by a complex sequence of reactions.
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SCOPE
• Inverse geochemical modeling of water data to establish a suitable rationale for interaction between As-bearing rocks and groundwater.
• Effect of other species on Arsenic release.
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Inverse ModelingInverse modeling attempts to determine sets of mole transfers of phases that account for changes in water chemistry between one or a mixture of initial water compositions and a final water composition.
Solution to Solid (precipitation, exchange)
Solid to Solution(dissolution, exchange)
gases, water
Need to KnowInitial SolutionFinal Solution
Reacting Phases
Initial Solution Final Solution
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Example
2% CO2
atm CO2
How much calcite precipitates?
Initial Solution
Final Solution (mg/kg) (mg/kg)
Na 12 4
Ca 49 11
Mg 3 3
Cl 12 17
HCO3- 104 15
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ReactionsFeS2+ 3.5O2+ H2O = Fe2+ + 2SO4
2-+ 2H+
(pyrite)∆H =-294 kcal/mollog k =208.46
FeAsS + 3.25O2+ H2O = Fe2+ + SO42- + H3AsO4
(Arsenopyrite)∆H –324 kcal/mollog k = 198.17
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PHREEQC Modeling
1. Open PHREEQCi
2. Right Click on the Screen
Properties tab will pop up
1.Go to the database
scroll down and choose
the required database.
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Input Data
1.Input data in PHREEQc
1.PHREEQC –WATEQ4F. dat doesn’t know what Arsenopyrite is!
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Modifying the database1. Go to the database
(WATEQF.dat).
2. Access the text file.
3. Under phases: Add the
Arsenopyrite reaction.
4. Save the file as GEOL628.dat.
5. Now this database will
understand Arsenopyrite and
its related species.
6. Use GEOL628.dat for further
modeling.
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Arsenolite, Arsenopyrite, Ca3(AsO4)2:4w, Fe(OH)3(a), Fe3(OH)8, Goethite, Hematite, Maghemite, Magnetite, Scorodite, Siderite, Siderite
Anhydrite, Aragonite, Artinite, As2O5(cr), As2S3(am), As_native, Brucite, Calcite, CH4(g), Claudetite, CO2(g), Dolomite,Dolomite(d), Epsomite, FeS(ppt), Greigite, Gypsum, H2(g), H2O(g), H2S(g), Huntite, Hydromagnesite, JarositeH, Mackinawite, Magnesite, Melanterite, Nesquehonite, O2(g), Orpiment, Portlandite, Pyrite, Realgar, Sulfur
Saturation Indices(SI’s)
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Iron and Arsenic• 3Fe2++ 2HAsO4
2− = Fe3(AsO4)2+2H+
• log_k= −15.9
• Fe3++HAsO42− = FeAsO4+H+
• log_k= −11.7 • Hypothesis:
Fe AsLenoble et al, (2005), Journal of Hazardous Materials, 123: 31
Ramos at al., (2009), J. Phys. Chem. C, 113 (33), 14591–14594
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Iron and Arsenic & PHREEQC
• Idea : To model addition of Fe in the well to see the changes that occur.
• PHREEQC Modeling: Add Fe as new phase using the modified database (GEOL 628).
• Output Status: Failed – Errors
• The Problem: ?
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Conclusions
• As can naturally occur in groundwater.
• Inverse Modeling results suggest that most of the saturated minerals are those containing Fe.
• Literature suggested that Fe is used to immobilize As.
• My attempts to model the addition of NZVI (Fe0 )to groundwater for As remediation FAILED!
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References• Ramos at al., (2009), J. Phys. Chem. C, 33:14591–14594
• Lenoble et al, (2005), Journal of Hazardous Materials, 123: 262-268.
• Sharif et al., (2008), Journal of hydrology, 350: 41-55
• Kim et al., (2000), Environ. Sci. Technol, 34: 3094-3100
• Armienta et al., (2001), Environmental Geology, 40: 571-581
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THANK YOU!
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