Polymeric nanofibers for the removal of Cr(III) from tannery waste water

Highlights

• Cysteine-modified polymeric nanofibers were fabricated.

• The modified nanofibers were used to remove Cr(III) from tannery waste water.

• The optimized removal capacity was 1.75 g of Cr(III)/g of polymeric material.

• The removal capacity is far greater than previously reported materials.

Abstract

We demonstrate the use of cysteine-modified polymer nanofibers for the rapid and efficient removal of Cr(III) from real tannery waste water samples. Various parameters such as pH, load of nanofibers and time of exposure were optimized to achieve maximum removal. The optimum parameters were found to be 0.1 mg of nanofibers per mL of tannery waste water with a pH of 5.5 and an exposure time of 45 min. Almost 99% Cr(III) was removed at these ideal conditions thus demonstrating the efficacy of our material. The maximum removal capacity at these ideal conditions was estimated to be approximately 1.75 g of chromium/gram of polymeric material. This is probably due to a variety of factors including the apparent high surface to volume ratio exhibited by these nanofibers and also due to the availability of numerous cysteine groups that are known to have high binding affinities with heavy metal ions. These nanoscale polymeric materials show great potential towards the removal of heavy metal cations from waste waters.

Introduction

Environmental pollution due to industrial effluents is of major concern because of their toxicity and threat to human life and the environment. The discharge of heavy metals effluents in the environment has raised much concern because of potential health hazards associated with the entry of toxic components into the food chain. It is well known that small amounts of some of these ions can cause severe physiological or neurological damage.

Various sources of heavy metal ions in water come from leather tanning, battery manufacturing, basic steel, paper and pulp, metal plating, agrochemicals, petrochemicals and fertilizer industries. Higher concentrations of heavy metals ions in water and soil may increase the uptake of these metals by crops and potentially affect human health via food chains.

Chromium and its compounds are widely used not only in leather tanning but also in many industries such as plating, cement, dye, photography industries, etc. producing large quantities of toxic heavy metal ions that can cause severe environmental and public health problems. The tannery industry, which commonly uses Cr(III) for tanning processes is a major cause for high influx of Cr in the biosphere, accounting for 40% of the total industrial use (Barnhart, 1997). Chromium exists naturally in two forms of Cr(III) and Cr(VI). Factors such as solubility, permeability through biological membranes and subsequent interactions with biological molecules such as proteins and nucleic acids makes Cr(VI) to be more toxic than Cr(III) (Dayan and Paine, 2001, Katz and Salem, 1993). However, it has been shown that Cr(III) can be oxidized to Cr(VI) by organic compounds in hide and also by inorganic species in tannery sludge (Apte et al., 2005). Furthermore, recent research suggests that chlorination of water oxidizes Cr(III) to Cr(VI) in a matter of hours after which the oxidation process plateaus into a steady state equilibrium (Lindsay et al., 2012). Therefore we believe that the removal of Cr(III) from tannery waster is very important.

Conventional methods for the removal of heavy metals from waste water include reduction, ion exchange, evaporation, chemical precipitation and adsorption on activated carbon. In terms of nanoscale material applications, magnetite (Fe₃O₄) nanoparticles have attracted increasing research interest in the fields of catalysis and environmental remediation in recent years (Hsing et al., 2007, Ozgunay et al., 2007). Magnetite nanoparticles possess not only strong adsorption/reduction activities, but also the property of being easily separated and collected by an external magnetic field (Booker et al., 1991, Ngomsik et al., 2005). The good adsorption activities of magnetite nanoparticles for many heavy metal ions have been reported in the literature (Hu et al., 2004, Oliveira et al., 2003, Orbell et al., 1997). As is well known, the co-aggregation problem often constitutes a challenge that nanoparticles have to be confronted with, since the co-aggregation decreases the effective surface area of nanoparticles and thus reduces their reaction activities.

We recently reported the development of a novel and highly effective nanoscale polymeric material for rapid and efficient removal of heavy metal ions from aqueous solutions (Tolani et al., 2010). The polymeric material was fabricated in the form of nanofibers that were subsequently modified by cysteine, a nonessential amino acid with a very high binding constant for some toxic heavy metal ions like As(III), Cd(II), Pb(II) and Cu(II). We demonstrated the removal of these metal ions from aqueous solutions using these modified polymer nanofibers. In this article, we extend the applicability of this material to the removal of Cr(III) and Cr(VI) from real tannery waste water. The tanning industry forms the backbone of the Egyptian leather industry much of which adopts the chromium tanning process because of its processing speed, low costs, and light color of leather and greater stability of the resulting leather. In general, tanning process using chromium compounds is among the most common methods for processing of hides (Sreeram and Ramasami, 2003). In this process about 60–70% of chromium reacts with the hides resulting in about 30–40% of the chromium to remain in solid and liquid. In most cases, the waste water of tanning process is usually discharged, without proper treatment, into the sewerage system causing serious environmental impact. Hence, there is need to minimize the generation of hazardous chemicals, while increasing the treatment efficiencies of the waste water generated. Therefore, investigation of alternative and appropriate materials and technologies for the removal of heavy metals ions from waste waters is of utmost importance.