SYSTEMATIC COMPARISON OF ADSORPTION BEHAVIORS OF CARBOXYLATES AND THEIR ANALOGS ONTO HYDROUS IRON(III) OXIDE

To develop a preconcentration method for organic traces occurring and circulating in natural waters, the usefulness of hydrous iron oxide (HIO) as an adsorbent was examined by measuring the percent adsorption of 34 carboxylates and their analogues (spiked to 0.1 mM concentration in a 40 mi-sample solution) onto HIO (a constant amount of 4 mg as iron) as a function of the pH of the solution. The adsorption characteristics were discussed in reference to the chemical formulae of the organic molecules tested. (1) Those molecules with more than two HOOC-residues are adsorbed quantitatively onto HIO from solutions with pH 4.0 similar to 5.5. (2) The presence of a HO-residue with the HOOC-residue facilitated the adsorption, but that of a Cl-residue suppressed the adsorption. (3) The effect of the HO- or Cl-residue was enhanced when it was located adjacent to the HOOC-residue in the molecule. (4) The H2N-residue on aromatic rings had little effect on the adsorption, but that on alkyl chains suppressed the adsorption. (5) As a whole, the pH(1/2) values, read off the respective adsorption curves as the pH where 50% adsorption occurred, were inversely proportional to the pK(2) values for the dissociation of the HOOC-residue. According to these empirically obtained rules, the success of adsorption preconcentration of a family of organic materials by HIO can be anticipated.