Effect of pH and ionic strength on the interaction of humic acid with aluminium oxide

The effect of pH and neutral electrolyte on the interaction between humic a cid/humate and gamma-AlOOH (boehmite) was investigated. The quantitative ch aracterization of surface charging for both partners was performed by means of potentiometric acid-base titration. The intrinsic equilibrium constants for surface charge formation were log K-a,1(int) = 6.7 +/- 0.2 and log K-a ,2(int) = 10.6 +/- 0.2 and the point of zero charge was 8.7 +/- 0.1 for alu minium oxide. The pH-dependent solubility and the speciation of dissolved a luminium was calculated (MINT-EQA2). The fitted (FITEQL) pK values for diss ociation of acidic groups of humic acid were pK(1) = 3.7 +/- 0.1 and pK(2) = 6.6 +/- 0.1 and the total acidity was 4.56 mmol g(-1) The pH range for th e adsorption study was limited to between pH 5 and 10, where the amount of the aluminium species in the aqueous phase is negligible (less than 10(-5) mol dm(-3)) and the complicating side equilibria can be neglected. Adsorpti on isotherms were determined at pH similar to 5.5, similar to 8.5 and simil ar to 9.5, where the surface of adsorbent is positive, neutral and negative , respectively, and at 0.001, 0.1, 0.25 and 0.50 mol dm(-3) NaNO3. The isot herms are of the Langmuir type, except that measured at pH similar to 5.5 i n the presence of 0.25 and 0.5 mol dm-3 salt. The interaction between humic acid/humate and aluminium oxide is mainly a ligand-exchange reaction with humic macroions with changing conformation under the influence of the charg ed interface. With increasing ionic strength the surface complexation takes place with more and more compressed humic macroions. The contribution of C oulombic interaction of oppositely charged partners is significant at acidi c pH. We suppose heterocoagulation of humic acid and aluminium oxide partic les at pH similar to 5.5 and higher salt content to explain the unusual inc rease in the apparent amount of humic acid adsorbed.