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Proton Adsorption on an Aluminum Oxide in the Presence of Bicarbonate

Dep. of Plant Sci., U-67, Univ. of Connecticut, Storrs, CT 06269

K. Swanson

99 Mill St., Glastonbury, CT 06033

*Corresponding author (cschulth{at}canrl.cag.uconn.edu).

ABSTRACT

The aqueous species of CO₂ [CO₂(aq), H₂CO₃, HCO^–₃ and CO^2–₃] are often ignored in adsorption studies despite the fact that CO₂ in soil air can be hundreds of times greater than in the atmosphere. Adsorption studies that do not account for CO₂ species may be under-estimating proton adsorption in natural environments. The objective of this study was to discern the effect CO₂ species have on proton adsorption to an Al oxide. The backtitration method was used to produce proton adsorption curves as a function of pH for samples of Al oxide with and without aqueous bicarbonate. Sodium chloride was used to vary the ionic strength. Proton adsorption increased in the presence of the bicarbonate below pH 9.7. The greater the concentration of bicarbonate, the greater the proton adsorption. The point of zero salt effect (PZSE = 8.30) was unaffected by the bicarbonate. The data presented show an approximate 1:1 stoichiometry for proton removal with inorganic-C (HCO₃ plus all other species) adsorption data. That is, this research directly quantified that every inorganic C adsorbed (expressed as HCO₃ adsorbed) is accompanied by the removal of one proton (more precisely, 1.10 ± 0.28) from the aqueous phase by the Al oxide. Thus, the net removed from the liquid phase is equivalent to one H₂CO₃ in mass-balanced terms. The use of traditional titration methods, rather than the backtitration method, resulted in a questionable pH-dependent pattern that reflects the impact of Al dissolution on these measurements.

Storrs Agric. Exp. Stn. Scientific Contribution no. 1619.

Received for publication July 24, 1996.

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