A Review on Cr(VI) Adsorption Using Inorganic Materials

Chromium compounds are widely used in electroplating, metal finishing, magnetic tapes, pigments, leather tanning, wood protection and electronic and electrical equipment. Hexavalent chromium is a highly toxic metal and produces health damages. The most soluble, mobile and toxic forms of hexavalent chromium in soils are chromate and dichromate and the chromium distribution is also controlled by redox processes, its adsorption decreases with increasing pH and when competing dissolved anions are present. Several techniques have been developed to remove Cr(VI) from wastewater but these techniques have disadvantages such as high cost, non-selective, pH dependence, etc. The use of low cost sorbents has been investigated as a replacement for current costly methods; natural materials with a high adsorption capacity for heavy metals can be obtained. Modification of the sorbents can improve adsorption capacity. This paper includes some techniques for remove Cr(VI) with clays, silica and zeolites from aqueous solutions, some of the treated adsorbents show good adsorption capacities.

• Sorption; Hexavalent Chromium; Clay; Silica; Zeolites

1. Introduction

Hexavalent chromium is a strong oxidizing agent that is water-soluble in its anionic form. Due to its solubility, it is highly mobile in soil and aquatic environments and readily penetrates plant and animal epidermis where it irritates the tissues [1,2]. To meet strict environmental regulations, Cr(VI) in wastewater is typically removed by either adsorption or a reduction + precipitation method, both of which have problems. Adsorption of Cr(VI) anions by cellulose-based sorbents is relatively ineffective, while carbon is expensive. Therefore, Cr(VI) anions must be first reduced to Cr(III) cations for effective sorption. Likewise, to precipitate Cr(VI), it must first be reduced to Cr(III). The reduction of Cr(VI) is advantageous in itself because the trivalent form is a thousand times less toxic [3] and can be effectively adsorbed or precipitated.

The reduction of Cr(VI) in wastewater is typically done with a reducing agent like iron or iron(II) ions under acidic conditions. The process usually involves lowering the pH to 1 or 2, adding reducing agent, then raising the pH to the level for adsorption or precipitation [4]. All of these steps require large amounts of reagents, which affect the cost, the sustainability, the overall quality and ionic properties, and the amount of sludge generated. Thus, new ways to remove Cr(VI) from wastewater in a single step would be advantageous.

In this review, we focus attention on new ways to remove Cr(VI) from aqueous solutions using inorganic materials. In this sense we center our study on the use of natural and modified clays, silica and zeolites.

Figure 1 shows a classification of thenatural inorganic materials that have been recently studied for Cr(VI) sorption.

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