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

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.