Spectophotometric studies on the competitive adsorption of boric acid (b(iii)) and chromate (cr(vi)) onto iron (oxy) hydroxide (fe(o)oh)

Abstract

Various pollutants (e.g. boron and hexavalent chromium) are introduced into the aquatic
 environment from a variety of industrial operations causing damages to environment and
 affecting human health. Boron in irrigation water is of particular interest because it can have
 beneficial or toxic effect on plants, depending on its concentration. Pollution of the environment
 with hexavalent chromium (Cr(VI)) and associated toxicity to microorganisms, plants, animals
 and humans is of major concern. Indeed, chromium in environmentally significant
 concentrations is found near to tanneries and involves large volumes of wastewater. One of the
 most effective remediation technologies used for the removal of B(III) and Cr(VI) from aquatic
 systems and wastewater is their sorption on metal oxide surfaces. However, in order to
 understand better the mechanisms involved and improve the efficiency of remediation
 technologies further fundamental studies are needed. The present study is focused on the
 adsorption of H3BO3 and CrO42- onto Fe(O)OH at various ionic strengths (I = 0.0, 0.1 and 1.0 M
 NaClO4), under normal atmospheric conditions, at 22 ± 3 oC and pH 8. Additionally,
 competitions studies were carried out to investigate the ion-exchange mechanism and compare
 the individual affinities of H3BO3 and CrO42- for Fe(O)OH. The concentration of H3BO3 and
 CrO42- in solution was determined spectrophotometrically by means of azomethine-H and DPC
 (1,5-diphenylcarbazide), respectively. The results obtained indicate that the release of Cr(VI)
 and therefore its concentration in solution increases as the amount of B(III) is increased in the
 sorption system. This phenomenon is due to the replacement of Cr(VI) ions by ions B(III) on the
 surface of Fe(O)OH. Evaluation of the experimental data results in a value for the competition
 constant which is equal logK= -3.5 ± 0.2, indicating that the adsorbent surface has greater
 affinity for Cr(VI) than for B(III) species. The formation constant for the Cr(VI)-Fe(O)OH surface
 complex is calculated to be logßCr= 7.9 ± 0.2.