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Aqueous Speciation in Hydrochemistry

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Application of a speciation model to a water sample (water analysis)

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Aqueous Speciation in Hydrochemistry

  1. 1. Aqueous Speciation in Hydrochemistry aqion.de
  2. 2. Parameter Value T °C 25 pH 7.0 Ca mM 3.818 Mg mM 0.864 Na mM 0.957 K mM 0.072 Alkalinity mM 4.960 Chloride mM 1.805 Sulfate mM 1.520 Nitrate mM 1.100 Start: Measured data (water analysis)  cations  anions Example
  3. 3. The Naïve Approach (no speciation) each measured element/parameter  one ion in aqueous solution Element Ion z Ca Ca+2 2 Mg Mg+2 2 Na Na+ 1 K K+ 1 Alkalinity HCO3 - -1 Chloride Cl- -1 Sulfate SO4 -2 -2 Nitrate NO3 - -1 1-to-1 correspondence measured charge
  4. 4. But, in reality: Each measured concentration comprises “free ions” and complexes. Ca = Ca+2 + CaHCO3 + + CaCO3(aq) + CaSO4(aq) + ... total concentration = free ion + complexes in the solutionmeasured Example: naïve approach
  5. 5. The naïve approach ignores at least three things: formation of aqueous complexes the role of alkalinity (which is not an element) activity corrections (for non-ideal solutions) 2 3 1
  6. 6. Aqueous Complexation is described by the Law-of-Mass Action (LMA): ba ba }B{}A{ }BA{ K aA + bB = AaBb equilibrium constant curly braces denote activities as “effective concentrations” 1
  7. 7. Alkalinity should be replaced by DIC as dissolved inorganic carbon: DIC = CO2 + HCO3 - + CO3 -2 + ... The partitioning is described by LMA (plus 3 equilibrium constants). (bi)carbonate complexes 2
  8. 8. Ib IBa1 IzA lg i i 2 i i     ]i[z 2 1 I i 2 iionic strength activity = γ ∙ concentration Activity Correction (DEBYE-HÜCKEL) ion concentration iterative 3
  9. 9. Speciation calculations can be performed by PhreeqC or any other hydrochemistry program (aqion, ...) Results for the example water
  10. 10. Speciation model no yes Ca+2 3.818 3.388 Mg+2 0.864 0.758 Na+ 0.975 0.952 K+ 0.072 0.072 Fe+2 0.052 0.037 Mn+2 0.019 0.013 CaHCO3 + 0.128 MgHCO3 + 0.027 MnHCO3 + 0.004 HCO3 - 4.960 4.762 SO4 -2 1.520 1.139 Cl- 1.805 1.805 NO3 - 1.100 1.100 CO3 -2 0.003 NaSO4 - 0.003 KSO4 - 0.0004 Speciation model no yes CO2 0.948 CaCO3 0.007 CaSO4 0.295 MgCO3 0.001 MgSO4 0.078 NaHCO3 0.002 MnCO3 0.001 MnSO4 0.001 FeCO3 0.001 FeSO4 0.003cations Aqueous Species [mM] anions neutralspecies the Naïve Approach
  11. 11. [mM] free ions HCO3 - CO3 -2 SO4 -2 total free ions HCO3 - CO3 -2 SO4 -2 5.904 4.762 0.003 1.139 Na Na+ NaHCO3 NaSO4 - 0.957 0.952 0.002 0.003 K K+ KSO4 - 0.072 0.072 0.0004 Ca Ca+2 CaHCO3 + CaCO3 CaSO4 3.818 3.388 0.128 0.007 0.295 Mg Mg+2 MgHCO3 + MgCO3 MgSO4 0.864 0.758 0.027 0.001 0.078 Mn Mn+2 MnHCO3 + MnCO3 MnSO4 0.019 0.013 0.004 0.001 0.001 Fe Fe+2 FeHCO3 + FeCO3 FeSO4 0.052 0.037 0.011 0.001 0.003 total 5.220 4.933 0.013 1.520 11.686 Aqueous Speciation [mM] aqueous complexes
  12. 12. [mM] free ions HCO3 - CO3 -2 SO4 -2 total free ions HCO3 - CO3 -2 SO4 -2 5.904 4.762 0.003 1.139 Na Na+ NaHCO3 NaSO4 - 0.957 0.952 0.002 0.003 K K+ KSO4 - 0.072 0.072 0.0004 Ca Ca+2 CaHCO3 + CaCO3 CaSO4 3.818 3.388 0.128 0.007 0.295 Mg Mg+2 MgHCO3 + MgCO3 MgSO4 0.864 0.758 0.027 0.001 0.078 Mn Mn+2 MnHCO3 + MnCO3 MnSO4 0.019 0.013 0.004 0.001 0.001 Fe Fe+2 FeHCO3 + FeCO3 FeSO4 0.052 0.037 0.011 0.001 0.003 total 5.220 4.933 0.013 1.520 11.686 Mass Balance DIC = CO2 + 4.933 + 0.013 = 5.894 mM
  13. 13. DIC = CO2 + HCO3 - + CO3 -2 + ... Note: CO2 = CO2(aq) + H2CO3 Dissolved Inorganic Carbon CO2 0.948 mM HCO3 - 4.762 mM CO3 -2 0.003 mM C complexes 0.181 mM DIC 5.894 mM
  14. 14. Cation Speciation model no yes Ca+2 7.636 6.776 Mg+2 1.728 1.516 Na+ 0.975 0.952 K+ 0.072 0.072 Fe+2 0.104 0.074 Mn+2 0.038 0.026 CaHCO3 + 0.128 MgHCO3 + 0.027 MnHCO3 + 0.004 Total 10.553 9.575 Aqueous Species [meq/L] Anion Speciation model no yes HCO3 - 4.960 4.762 SO4 -2 3.040 2.278 Cl- 1.805 1.805 NO3 - 1.100 1.100 CO3 -2 0.006 NaSO4 - 0.003 KSO4 - 0.0004 Total 10.905 9.954 %94.1100 L/meq529.19 L/meq379.0 100 anionscations anionscations [%]CBE         Charge Balance Error

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