Mitigation of Cr(VI) Aqueous Pollution by the Reuse of Iron-Contaminated Water Treatment Residues

Abstract

Reducing the levels of heavy metals in wastewaters below the permissible limits is imperative before they are discharged into the environment. At the same time, water treatment technologies should be not only efficient, but also affordable. In accordance with these principles, this study assessed the possibility of recovering iron-contaminated residues, resulting from the treatment of synthetic acid mine drainage, for the subsequent remediation of Cr(VI) polluted aqueous solutions. Bentonite, an inexpensive and available natural material, was used as an adsorbent for the removal of Fe(II) from synthetic acid mine drainage (AMD). Then, Fe(II)-contaminated bentonite, the residue generated during the treatment of AMD, was recovered and activated with sodium borohydride in order to convert the adsorbed Fe(II) to Fe(0). Subsequently, the Fe(0)-containing bentonite (Be-Fe(0)) was further used for the treatment of Cr(VI) contaminated aqueous solutions. Reactive materials investigated in this work were characterized by means of scanning electron microscopy-energy dispersive angle X-ray spectrometry (SEM-EDX), X-ray diffraction spectroscopy (XRD), point of zero charge, and image photographs. The effect of several important parameters (pH, temperature, metal concentration, and ionic strength) on both treatment processes was investigated and discussed. It was shown that the efficiency of Cr(VI) removal with Be-Fe(0) was much higher than with bentonite. On the basis of the present study it can be concluded that residues generated during the treatment of AMD with bentonite can be used as a cheap precursor for the production an Fe(0)-based reagent, with good Cr(VI) removal potential.

Sequential Abatement of FeII and CrVI Water Pollution by Use of Walnut Shell-Based Adsorbents

“…In this previous study, bentonite was used for sequential adsorption of heavy metals from aqueous solutions, proving not only that reusability of exhausted adsorbents is possible, but also that some adsorbed metals may have a beneficial role in the subsequent adsorption process. However, this research also revealed that using bentonite as adsorbent suffers from multiple drawbacks, including low adsorption capacity and leaching of structural iron at strong acidic pH [25].…”

“…from aqueous solutions, has been reported in the last years by numerous studies [21][22][23][24]. Instead, to our knowledge, few researchers have addressed the issue of reusability of cheap exhausted adsorbents, resulted from the removal of a particular heavy metal, in treatment processes of aqueous effluents polluted with a different type of heavy metal [25]. In this previous study, bentonite was used for sequential adsorption of heavy metals from aqueous solutions, proving not only that reusability of exhausted adsorbents is possible, but also that some adsorbed metals may have a beneficial role in the subsequent adsorption process.…”