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 H 3BO 3 and CrO 4 2- onto Fe(O)OH at various ionic strengths (I = 0.0, 0.1 and 1.0 M NaClO 4), under normal atmospheric conditions, at 22 ± 3°C and pH 8. Additionally, competitions studies were carried out to investigate the ion-exchange mechanism and compare the individual affinities of H 3BO 3 and CrO 4 2- for Fe(O)OH. The concentration of H 3BO 3 and CrO 4 2- 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 logß Cr= 7.9 ± 0.2. © 2012 Global NEST Printed in Greece. All rights reserved.