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Title: Proton-arsenic adsorption ratios and zeta potential measurements: Implications for protonation of hydroxyls on the goethite surface
Authors: Zhang, J.S.
Stanforth, R. 
Pehkonen, S.O. 
Keywords: Goethite
Proton-arsenic adsorption ratio
Protonation degree
Reactive hydroxyls
Zeta potential
Issue Date: 1-Nov-2007
Citation: Zhang, J.S., Stanforth, R., Pehkonen, S.O. (2007-11-01). Proton-arsenic adsorption ratios and zeta potential measurements: Implications for protonation of hydroxyls on the goethite surface. Journal of Colloid and Interface Science 315 (1) : 13-20. ScholarBank@NUS Repository.
Abstract: Successfully modeling the surface charge of goethite and anion adsorption on goethite using a surface complexation model (SCM) alone cannot verify the assumptions of this model. In this study, the assumptions of 2-pK triple layer model (TLM) and two-site 1-pK basic stern model (BSM) were assessed with respect to their ability to interpret both the proton-anion adsorption ratios of dimethylarsinate (DMA), monomethylarsonate (MMA), and arsenate and their effect on the ζ-potential. The proton-DMA adsorption ratio is around 0.9 at pH 4.25 and 1.1 at pH 6.75 at DMA surface coverage ranging from 0 to 2 μmol m-2, and the ζ-potential is independent of DMA adsorption at these two pH values. The proton-MMA adsorption ratio increases to 1.5 at pH 4 and 2.1 at pH 6.75 as the MMA surface coverage decreases to 0.5 μmol m-2. The ζ-potential is less dependent on MMA adsorption at a surface coverage range of 0 to 1.8 μmol m-2, and it then decreases with a further increase in the MMA surface coverage at pH 4 and 6.75. The proton-arsenate adsorption ratio decreases to 2 as the arsenate surface coverage approaches zero, and the ζ-potential decreases linearly with the increasing arsenate surface coverage at pH 4 and 6.75. Neither the 2-pK TLM nor the 1-pK BSM give a consistent interpretation of both the proton-arsenic adsorption ratio and the effect of arsenic on the ζ-potential. The results suggest that the 1-pK MUSIC model in which each type of surface hydroxyls has its own intrinsic proton-affinity constant and only one type of surface hydroxyls is involved in DMA, MMA, and arsenate adsorption is preferably pursued. The protonation degree of reactive hydroxyls estimated from proton-arsenic adsorption ratios is 0.2 at pH 4 and 0 at pH 6.75 in 0.001 M NaNO3. © 2007 Elsevier Inc. All rights reserved.
Source Title: Journal of Colloid and Interface Science
ISSN: 00219797
DOI: 10.1016/j.jcis.2007.06.042
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