Effect of pH and ionic strength on the interaction of humic acid with aluminium oxide |
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Authors: | E Tombácz Á Dobos M Szekeres H D Narres E Klumpp I Dékány |
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Institution: | (1) Department of Colloid Chemistry Attila József University Aradi Vt. 1., H-6720 Szeged Hungary e-mail: tombacz@chem.u-szeged.hu Tel.: +36-62-544212 Fax: +36-62-426221, HU;(2) Department of Applied Physical Chemistry Research Center Jülich, D-52425 Jülich, Germany, DE |
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Abstract: | The effect of pH and neutral electrolyte on the interaction between humic acid/humate and γ-AlOOH (boehmite) was investigated.
The quantitative characterization of surface charging for both partners was performed by means of potentiometric acid–base
titration. The intrinsic equilibrium constants for surface charge formation were logK
a,1
int=6.7±0.2 and logK
a,2
int = 10.6±0.2 and the point of zero charge was 8.7±0.1 for aluminium oxide. The pH-dependent solubility and the speciation of
dissolved aluminium was calculated (MINTEQA2). The fitted (FITEQL) pK values for dissociation 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 the 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. Adsorption isotherms were determined at pH ∼ 5.5, ∼8.5 and ∼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 isotherms are of the Langmuir type, except that measured at pH ∼ 5.5 in 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 charged interface. With increasing ionic strength the surface complexation
takes place with more and more compressed humic macroions. The contribution of Coulombic interaction of oppositely charged
partners is significant at acidic pH. We suppose heterocoagulation of humic acid and aluminium oxide particles at pH ∼ 5.5
and higher salt content to explain the unusual increase in the apparent amount of humic acid adsorbed.
Received: 20 July 1999 /Accepted in revised form: 20 October 1999 |
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Keywords: | Humic acid Aluminium oxide Adsorption Heterocoagulation Surface complexation |
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