Fig 1.
Comparison of predicted results for Eqs (6), (7) and (8).
Total phthalate concentration present = 1.0 mmol L–1; solid phase adsorption maximum = 2.0 μmol m–2 (equivalent to 0.2 mmol L–1) for Eqs (6) and (7), or 1.0 μmol m–2 (equivalent to 0.1 mmol L–1) for Eq (8); pK6 = –7.51, pK7 = –18.72 and pK8 = –15.02. The curves shown and the corresponding pK values were optimized using IExFit to fit a single datum point at pH = 6 and phthalate adsorption = 0.5 μmol m–2. No aqueous phase protonation reactions were included in this simulation.
Fig 2.
Comparison of predicted results for Eqs (2) and (7).
Total phthalate concentration present = 1.0 mmol L–1, solid phase adsorption maximum = 2.0 μmol m–2 (equivalent to 0.2 mmol L–1), pK2 = –8 and pK7 = –18.7. The initial chloride concentrations were either zero, fixed value (4.54 mM), or pH dependent variable values. For the pH dependent variable values, the Cl− values were fixed at 1.27 mM above pH 3.18, but increased due to addition of HCl acid below pH 3.18 (total Cl− input at pH 2.33 was 4.54 mM). Resultant curves were generated by IExFit. No aqueous phase protonation reactions were included in this simulation.
Fig 3.
Structure of goethite, α-FeOOH.
The double chains of octahedra run parallel to c. Note four possible adsorption sites on the {110} face consisting of singly, doubly and triply coordinated hydroxyl groups, plus a fourth consisting of two hydroxyl groups on one Fe atom.
Fig 4.
Adsorption of H+, Na+ and Cl− ions on goethite using the 4-site model described in Table 1.
Symbols: solid = H+, circles = Cl−. The curves shown were generated and optimized using IExFit software.
Table 1.
Adsorption parameters for retention of Cl− ions on goethite.
Results for this four-site model are shown in Fig 4. This ion-exchange model assumes a 4.4 μmol m–2 total number of surface site. For each surface site (Sa, Sb, Sc and Sd) the adsorption max = total number of sites = {SOH} + {SOH2Cl} + {SONa}.
Fig 5.
Adsorption of H+, Na+, Cl− and phthalate (L2–) ions on goethite using the competitive ion-exchange model described in Table 2.
The aqueous phase protonation reactions of phthalate ions were also included in the model. Symbols: solid = H+, circles = Cl−, triangles = L2–. The curves shown were generated and optimized using IExFit software.
Table 2.
Adsorption parameters for retention of phthalate (L2–) ions on goethite.
Results for this model are shown in Fig 5. This ion-exchange model assumes a 4.4 μmol m–2 total number of surface site, with the same adsorption maxima values used in Table 1. For each surface site (Sa, Sb, Sc and Sd) the adsorption max = total number of sites = {SOH} + {SOH2Cl} + {SONa} + 2{(SOH2)2L}.