1. IntroductionChromium(Cr) toxicity is one of the major causes of environmental pollution emanating from tannery effluents. This metal is used in the tanning of hides and leather, the manufacture of stainless steel, electroplating, textile dyeing and as a biocide in the cooling waters of nuclear power plants, resulting chromium discharges causing environmental concerns [1]. Cr exists in nine valence states ranging from –2 to +6. From these, only the hexavalent [Cr(VI)] and trivalent chromium [Cr(III)] have primary environmental significance due they are the most stable oxidated forms in the environment [2]. Both are found in various bodies of water and wastewaters [3]. Cr(VI) typically exists in one of these two forms: chromate (
) or dichromate (
), depending on the pH of the solution [3]. These two divalent oxyanions are very water soluble and poorly adsorbed by soil and organic matter, making them mobile in soil and groundwater [2]. Both chromate anions represent acute and chronic risks to animals and human health, since they are extremely toxic, mutagenic, carcinogenic and teratogenic [4]. In contrast to Cr(VI) forms, the Cr(III) species: predominantly hydroxides, oxides or sulphates, are less water soluble, mobile (100 times less toxic) [5] and (1000 times less) mutagenic [6]. The principal techniques for recovering or removing Cr(VI), from wastewater are: chemical reduction and precipitation, adsorption on activated carbon, ion exchange and reverse osmosis in a basic medium [7]. However, these methods have certain drawbacks, namely high cost, low efficiency, generation of toxic sludge or other wastes that require disposal and imply operational complexity [8].
In this context, considerable attention has been focused in recent years upon the field of biosorption for the removal of heavy metal ions from aqueous effluents [9]. The process of heavy metal removal by biological materials is known as biosorption. Biomass viability does not affect the metal uptake. Therefore any active metabolic uptake process is currently considered to be a negligible part of biosorption. Various biosorbents have been tried, which include seaweeds, moulds, yeast, bacteria, crab shells, agricultural products such modified corn stalks, [10], hazelnut shell [11], orange shell [12] tamarind shell [13]. It has also been reported that some of these biomass can reduce chromium(VI) to chromium(III), like tea fungal biomass [14]; Mesquite [15], Eucalyptus bark [16], red roses waste biomass [17], and Yohimbe bark [18]. The present study is undertaken with following objective: Investigate the use of Citrus limonium shell for the biosorption of Chromium(VI) in aqueous solution.
2. Experimental2.1. Biosorbent Used: Citrus limonium ShellThe shell was obtained from the fruits harvested and offered in the marketplace Republic, between the months of June 2010 to September 2010, of the capital city of San Luis Potosi, SLP. Mexico. To obtain the biomass, lemon rind washed with water trideionized 72 hours under constant stirring, with water changes every 12 hours. Subsequently, boiled 1 hour to remove traces of the fruit was dried at 80˚C for 12 hours in the oven, ground in blender and stored in amber vials until use.
2.2. Determination of Hexavalent, Trivalent, and Total CrHexavalent chromium and trivalent chromium were quantified by a spectrophotometric method employing diphenylcarbazide and chromazurol S, respectively [19,20], total Cr was determined by electrothermal atomic absorption spectroscopy [19].
The values shown in the results section are the mean from three experiments carried out by triplicate.
3. Results and Discussion3.1. Effect of Incubation Time and pHFigure 1 shows the effect of the incubation time and pH. The optimum time and pH for Cr(VI) removal was 100 min and pH 1.0, at constant values of biosorbent dosage (1 g/100 mL), initial metal concentration (50 mg/L), and temperature (30˚C). The literature [12], report a optimum time of 60 min, for the remotion of lead by orange Shell, 30 min and 2 hours for the remotion of Cr(VI) by the tamarind shell and eucalyptus bark [13,16]. Changes in the permeability of unknown origin, could partly explain the differences found in the incubation time, providing greater or lesser exposure of the functional groups of the cell wall of biomass analyzed. Adsorption efficiency of Cr(VI) was observed maximum at pH 1.0 with Citrus limonium waste biomass. This was due to the dominant species (
and
) of Cr ions in solution expected to interact more strongly with the ligands carrying positive charges [21]. These results are like for tamarind shell [13], but the most of authors report an optimum pH of 2.0 like Tamarind shell [22], eucalyptus bark [16], bagassa and sugarcane pulp, coconut fibers and wool, [23], for the tamarind shell treated with oxalic acid [24], at pH of 2.0 and 5.0 for the mandarin bagassa [25], and almond green hull [26].