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

Marius Gheju,Ionel Balcu

doi:10.3390/chemengineering1020009

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.

Highlights

Metallic elements are widespread components of the rocks and minerals of the earth’s crust
Water treatment technologies should be efficient, and 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
The reuse of waste materials resulting from water treatment technologies, as reactive materials for other water treatment processes, can become a crucial component, especially in developing nations, in their efforts to address pollution caused by industrial activities

Summary

Introduction

Metallic elements are widespread components of the rocks and minerals of the earth’s crust. Cr(III) has a very low mobility in the environment and is 500 to 1000 times less toxic to a living cell than Cr(VI) [9]. It is important to reduce the concentrations of Cr(VI) in wastewaters below the permissible limits, before being discharged into the environment Iron is another important heavy metal with numerous industrial applications, which can be found in natural aqueous environments, mainly in two oxidation states: Fe(II) and Fe(III) [10]. Clay minerals are classified as promising cheap adsorbents for heavy metal removal because they offer several advantages, including that they: (1) are readily available, (2) are inexpensive, (3) have a layered structure and large specific surface area, (4) have a good chemical and mechanical stability, and (5) have a high cation exchange capacity [20,21]. The mechanism of Cr(VI) removal with Be-Fe(0) was evaluated

Methods

Results

Conclusion