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Challenges with simultaneous equilibrium

Challenges with simultaneous equilibrium. Speciation programs (MINEQL). Compilation, calculations and QC of data. After the analysis the data must be: compiled, reckoned in terms of equivalent charge and

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Challenges with simultaneous equilibrium

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  1. Challenges with simultaneous equilibrium Speciation programs (MINEQL)

  2. Compilation, calculations and QC of data • After the analysis the data must be: • compiled, • reckoned in terms of equivalent charge and • quality controlled by ion balance and agreement between measured and calculated conductivity

  3. Speciation with different ligands present • In aqueous solution, containing both metal and ligand, the distribution of species will depend on factors such as concentrations, stoichiometry, pH and ionic strength • The calculations become very complex where a metal cation have the opportunity to bind to more than one type of ligands • Multiple iterations of the calculations are necessary • In natural water systems • Basic: CO32-, OH-, Org-, Cl- • Acid: F-, SO42-, Org-, Cl- • For such calculations we apply computer programs as MINEQL and ALCHEMI

  4. Relevant balanced equilibrium reactions Definition of unknown Equilibrium expressions Mass balance Charge balance No of unknown < No of equations No Stop Yes Approximations Solve equations No Are assumptions justified? Yes Scheme for chemical equilibrium calculations 1 2 and 3 • Approximations are commonly done by assuming the concentration of specific species are 0 Molar • Only mass balance and charge balance equations can be simplified

  5. Equilibrium expressions • KW, KSP, KA, KB, bn, KREDOX, Kd • Mass(read: concentration) balance • Set the equilibrium molarities (MX) up against each other (MX vs. MY) and against the analytical molarity (MX vs. cX) • Analytical concentration is the concentration of a substance dumped into a solution. It includes all the forms of that substance in the solution. • Charge balance •  eqv./L positive charge =  eqv./L negative charge Set of expressions

  6. Equilibrium expressions • KW, KSP, KA, KB, bn, KREDOX, Kd

  7. Ex.1: BaSO4 in HCl solution • We see from the molecular formula that: • So that: • The hydroniumion (H+) has two sources: HCl (=cHCL) and the auto-proteolysis of water (=[OH¯]): • Ex.2: Ag2CrO4 solution • We see from the molecular formula that: • So that: 2. Mass balance

  8. The law of physics demand that • Number of positive charge is equal to number of negative charge • Charge contribution of a specie = Valens · Molar concentration Ex. 1: • In neutral pHsolutions one can disregard the H+ and OH- ions Ex. 2: • No new information 3. Charge balance

  9. Metal hydrolysis • The hydrolysis is described by a set of equilibrium reactions • {Fe3+} is determined by replacing each of the other parts of the mass equation with their equilibrium expression: • Then the other species can be determined from the {Fe3+} and bE.g.;

  10. Speciation; Shortcomings • The equilibrium model is based on a choice of complexes and their stability constants, which makes the results questionable

  11. Speciation programmes • MINEQL+ is a chemical equilibrium model capable of calculating • aqueous speciation • solid phase saturation • precipitation-dissolution • adsorption. • An extensive thermodynamic database is included in the model

  12. Tutorial • Start out by choosing components that define your system • Find thermodynamic constants in database in ”Scan Thermo”

  13. Tutorial • The Calculation Wizards Tool is a collection of 5 input options to describe the chemistry of the system • Running the calculation

  14. Tutorial • Multirun manager • Titration • 2 way analysis • Field data • Output manager • Types of Output • The Header • The Log • The MultiRun Table • Component Groups • Special Reports

  15. Tutorial • Graphics manager • Bar and X-Y plots • Run through the 4 problems

  16. Assignment • Run a complete speciation analysis on the samples and describe the chemical composition of the samples

  17. Proposed issues Soil – soil water interactions • Describe the changes in water chemistry through the catchment • Why does the chemistry change is such a manner? What are the main processes controlling the water chemistry through the watershed? Temporal variation • Compare the generated data, representing a snap shot in time, with the volume weighted average concentrations found in the literature (http://www.nilu.no/projects/ccc/onlinedata/intro.html and http://folk.uio.no/rvogt/KJM_MEF_4010/Data%20from%20Skogforsk/ • Why are the data different?

  18. Proposed issues Heavy metal mobility and toxicity • Make a risk assessment of heavy metal contaminants at the site. Base the discussion on your data of heavy metal content and speciation results • Are there any significant amount of heavy metals in the water samples? Is the environmental conditions favourable for heavy metal mobility? Are the heavy metals in a toxic form?

  19. Report • The report (~ 4p) should include the following paragraphs • Abstract • Introduction • Material and methods • Results • Discussion • Conclusion

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