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
INVERSE GEOCHEMICAL MODELING OF
GROUNDWATER WITH SPECIAL EMPHASIS ON
ARSENICSharanya Shanbhogue
Geochemistry 428/62812/09/2010
Overview
• Case Study
• Scope
• Inverse Geochemical Modeling (PHREEQC- GEOL 628)
• Common Ion Effect
• Iron-Arsenic Model
• Conclusions
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.
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
“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
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-
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.
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.
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
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
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
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.
Input Data
1.Input data in PHREEQc
1.PHREEQC –WATEQ4F. dat doesn’t know what Arsenopyrite is!
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.
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)
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
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: ?
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!
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
THANK YOU!