IOSR Journal of Applied Chemistry (IOSR-JAC) e-ISSN: 2278-5736. Volume 6, Issue 2 (Nov. – Dec. 2013), PP 66-75 www.iosrjournals.org www.iosrjournals.org 66 | Page Removal of chromium (VI) from aqueous solution using chemically modified orange (citrus cinensis) peel Shadreck Mandina, Fidelis Chigondo, Munyaradzi Shumba, Benias Chomunorwa Nyamunda, Edith Sebata Lecturer, Department of Educational Foundations, Management and Curriculum Studies, Midlands State University, Gweru, Zimbabwe Lecturer, Department of Chemical Technology, Midlands State University, Gweru, Zimbabwe: Lecturer, Department of Chemical Technology, Midlands State University, Gweru, Zimbabwe: Lecturer, Department of Chemical Technology, Midlands State University, Gweru, Zimbabwe: Lecturer, Department of Chemical Technology, Midlands State University, Gweru, Zimbabwe: Abstract: The removal of Cr (VI) from aqueous solutions onto chemically modified orange peel was studied at varying initial metal concentrations, adsorbent doses, pH and contact times. Batch experiments were carried out under optimized conditions to evaluate the adsorption capacity of the orange peel chemically modified with sodium hydroxide. The residual Cr (VI) concentrations after biosorption were analyzed by FAAS. The biosorbent was characterized by FT-IR spectroscopy and BET. The characterization of the orange peel biomass suggested the possible contribution of carboxyl and hydroxyl groups in Cr (VI) biosorption. Chemically modified orange peel exhibited more adsorption potential as compared to the raw orange peel. The biosorption efficiency of the orange peel was dependent on the pH of the Cr (VI) solution, with pH 2 being optimal. The removal rate of Cr (VI) ions increased with increase in contact time and remained constant after an equilibrium time of 180 min. The removal of Cr (VI) ions increased with increase in biosorbent concentration with the optimal adsorbent dosage at 4.0 mg/L. The increase in initial Cr (VI) ion concentration led to an increase in the percentage removal of Cr (VI). The adsorption data fitted well with the Freundlich isotherm model with R 2 = 0.987 for the raw orange peel and R 2 = 0.995 for the modified orange peel. The Freundlich constants K f and n were 97.07 [mg/g (L/mg) n ] and 0.79 (g/L) for the raw orange peel and 139.0 [(mg/g)(L/mg) n ] and 0.815 (g/L) for modified orange peel respectively. The present study revealed that orange peel which is a low cost agricultural material could be used as an efficient sorbent for the removal of Cr(VI) from aqueous solutions and that chemical modification of the biosorbent using sodium hydroxide enhanced adsorption capacity. Keywords: Adsorption isotherm, Biosorption, Chemical modification, Chromium (VI), Orange peel I. Introduction The removal of heavy metals from industrial effluents is a field of research that has attracted increasing attention from the scientific community as the quest for green chemistry takes centre stage. Heavy metals in wastewaters are hazardous to the environment and therefore their removal before waste water discharge is apparent [1]. These substances are stable and persistent environmental contaminants since they are non- biodegradable [2]. Chromium is one of the toxic metals often found in effluents discharged from industries involved in paints, pigments, dyes, textiles, leather tanning, electroplating, metal finishing, nuclear power plants and chromate preparation [3]. Chromium can exist in several oxidation numbers but only chromium (III) and chromium (VI) are stable enough to occur in the environment. The hexavalent form is more toxic than the trivalent one [4]. Inhalation and retention of Cr (VI) containing material can cause damage to internal organs [5]. Skin contact of chromium (VI) compounds can lead to skin diseases [6]. The toxicological impact of chromium (VI) originates from its oxidizing ability as well as from the formation of free radicals during the reduction of Cr (VI) to Cr (III) occurring inside the cell [7]. Many physicochemical methods for heavy metal removal from aqueous solution have been developed. These methods include precipitation, resin chelation, electrochemical deposition, reverse osmosis, ion exchange, coagulation and solid-phase extraction [8]. These techniques however, have disadvantages such as incomplete metal removal, high reagent and energy requirements and generation of toxic sludge [9]. Cost effective alternative technologies to conventional methods are essential for the removal of heavy metals from industrial effluent. An innovative technique that is both efficient and economical is termed bioremediation or biosorption [10-12]. This has resulted in the easy and efficient removal of metals that could not be removed by other techniques. The major advantages of biosorption over other conventional treatment methods include low cost, high efficiency, minimization of chemical and biological sludge, and regeneration of biosorbent by desorption