Performance of FeOOH-brick based composite for Fe(II) removal from water in fixed bed column and mechanistic aspects

Abstract

A new composite adsorbent – HCl-activated brick coated with iron oxihydroxide – was employed for the removal of iron(II) from aqueous solution using fixed bed column. The effects of various experimental conditions such as: bed depth, flow rate and column diameter on Fe(II) adsorption were examined. Kinetic Thomas model was used in describing column performance and prediction of breakthrough curves. The bed depth service time (BDST) model was tested in an attempt to obtain a mathematical description of the Fe(II) adsorption on to the modified brick and to assess column capacity. The saturated column was regenerated by a bleach solution and the resulting material could be re-used efficiently for Fe(II) removal. A mechanistic interpretation of iron(II) adsorption on to this composite was also addressed in the present work by investigating the evolution of ions contents versus elution time in column effluents using ICP-AES, and by detecting chemical modifications at the brick surface during the course of the adsorption process by means of scanning electron microscopy and X-ray photoelectron spectroscopy. Our findings showed clearly that the sorption of iron(II) proceeded via an ions-exchange mechanism implicating the Na+ and H+ ions and two-line ferrihydrite (which was identified by X-ray diffraction) present at the brick surface and leading to the generation of a bi-dentate complex, (S)2Fe, with the following stoechiometric balance: [Na+]released+[H+]released≅2[Fe2+]adsorbed. Adsorbed iron was further found to be preferentially associated with iron oxide hydroxide in relatively stable chemical forms which were assigned to solid phases: (Fe2+)x(Fe3+)yOOH.