Removal of hexavalent chromium from aqueous solution by agricultural waste biomass

Abstract

In the present study adsorption of Cr(VI) from aqueous solutions onto different agricultural wastes, viz., sugarcane bagasse, maize corn cob and Jatropha oil cake under various experimental conditions has been studied. Effects of adsorbent dosage, Cr(VI) concentration, pH and contact time on the adsorption of hexavalent chromium were investigated. The concentration of chromium in the test solution was determined spectrophotometrically. FT-IR spectra of the adsorbents (before use and after exhaustion) were recorded to explore number and position of the functional groups available for the binding of chromium ions on to studied adsorbents. SEMs of the adsorbents were recorded to explore the morphology of the studied adsorbents. Maximum adsorption was observed in the acidic medium at pH 2 with a contact time of 60 min at 250 rpm stirring speed. Jatropha oil cake had better adsorption capacity than sugarcane bagasse and maize corn cob under identical experimental conditions. The applicability of the Langmuir and Freundlich adsorption isotherms was tested. The results showed that studied adsorbents can be an attractive low cost alternative for the treatment of wastewaters in batched or stirred mode reactors containing lower concentrations of chromium.

Introduction

Water pollution by chromium is of considerable concern, as this metal is used in a variety of applications including steel production, electroplating, leather tanning, nuclear power plant, textile industries, wood preservation, anodising of aluminium, water-cooling and chromate preparation [1]. Chromium exists in trivalent and hexavalent forms in aqueous systems. The trivalent form is an essential nutrient [2], but hexavalent form is toxic, carcinogenic and mutagenic in nature. It is highly mobile in soil and aquatic system and also is a strong oxidant capable of being adsorbed by the skin [3]. The hexavalent form is 500 times more toxic than the trivalent form [4]. Human toxicity includes lung cancer, as well as kidney, liver, and gastric damage [5], [6]. The tanning process is one of the major sources of chromium pollution at global scale. In the chromium tanning process, the leather takes up only 60–80% of applied chromium, and the rest is usually discharged into the wastewaters causing serious environmental impact. Chromium ion in liquid tanning wastes occurs mainly in trivalent form, which gets further oxidized to hexavalent form. The maximum levels permitted for trivalent chromium in wastewater are 5 mg/l and for hexavalent chromium are 0.05 mg/l [7]. In order to remove Cr(VI) from effluents to the permissible level, chromium is conventionally removed by precipitation [8], ion exchange [9] and adsorption [10] methods.

The commercially available activated carbon in granular or powder form is effective for the removal of various heavy metal ions. However, due to prohibitive cost their use is limited in developing countries like India. So there is a need to develop low cost and easily available adsorbents for the removal of heavy metal ions from the aqueous environment. Several types of biomasses have been investigated for their use in wastewater treatment for heavy metal removal [11], [12], [13]. An abundant source of potentially metal-sorbing biomass is cellulosic agricultural wastes. Although their sorption capacity is usually lesser than activated carbons but these materials could be an inexpensive substitute for the heavy metal laden wastewater's treatment [1]. Biomass is widely available, inexhaustible, and inexpensive material that exhibit significant specificity for the heavy metal ions.

Sugarcane bagasse, a byproduct of cane sugar processing, is generated in large quantities in India. Bagasse is either used as fuel by sugar mills or a raw material for paper manufacturing. Corncobs are highly voluminous, costless agricultural waste of corn milling process. They have a bulk density of 0.320 g/cm³ for a particle size range of 0.85–2.00 mm. Maize corncobs are very rich in cellulose and hemicelluloses which comprises ≈80% of the dry matter. They contain many polymeric materials that possess many functional groups. Jatropha crop has recently been introduced in Northern India for biodiesel recovery from its seeds. There is no reported use of the Jatropha oil cake generated in the process of biodiesel recovery from its seeds. Yet no work has been reported on Jatropha oil cake as an adsorbent for the removal of heavy metal ions from the aqueous solutions.

In the present study an attempt has been made to explore the use of sugarcane bagasse, corncobs and Jatropha oil cake as sustainable adsorbents for chromium removal from aqueous systems under different experimental conditions.