Competitive Adsorption of Aqueous Carbonic and Acetic Acids by an Aluminum Oxide

Abstract

AbstractThe prominence of carbonates in nature and their potential competitiveness with organic acids for sorption sites in soils may be underestimated by researchers who study the mobility of natural organic matter. The pH‐dependent adsorption envelopes of Na salts of acetic acid (AcOH) and carbonates (H2CO3, HCO−3, CO2−3) on an Al oxide were studied to elucidate the effect of carbonates on the adsorption behavior of organic acids. Using 10−3.48 and 10−3 M concentrations of the carbonate salt reduced the amount of acetate adsorbed by 29 and 48%, respectively. Using 10−3.48 M concentrations of acetate, however, did not affect the carbonate adsorption at equimolar concentrations. The relative affinity of the aqueous compounds for adsorption by the Al oxide was determined based on the pH value of their adsorption edges (i.e., based on their adsorption competitiveness relative to the aqueous OH anions). The relative affinity for adsorption by the Al oxide was OH > carbonate > acetate > Cl. The amount of both the acetate and carbonate adsorbed by Al oxide are reduced in the presence of competitive OH anions at high pH and high concentrations of Cl anions at low pH. The adsorption mechanisms of the carbonate and acetate compounds are suspected to be similar based on the chemical structure of these compounds. Batch adsorption studies, however, cannot unequivocally identify a bonding mechanism, which may include ligand exchange or H bonding. We additionally stress that both the affinities and concentrations of the aqueous compounds play an important role in competitive adsorption behavior.