Biosorption of Cr(VI) onto the Litter of Natural Trembling Poplar Forest Murat Dundar, a Cigdem Nuhoglu, b and Yasar Nuhoglu c a Department of Environmental Engineering, Bartın University, Bartın 74100, Turkey b Department of Physics, Atatu¨rk University, Erzurum 25240, Turkey c Department of Environmental Engineering, Atatu¨rk University, Erzurum 25240, Turkey; ynuhoglu@atauni.edu.tr (for correspondence) Published online 3 December 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.10520 Batch biosorption experiments onto the litter of natural trembling poplar (Populus tremula) forest (LNTPF) were investigated for the biosorption of chro- mium(VI) ions from aqueous solutions. The influence of different experimental parameters such as initial pH, particle size, agitating speed, initial Cr(VI) con- centration (C 0 ), adsorbent concentration (m), and temperature on chromium(VI) ion removal were eval- uated. The efficiency of chromium(VI) removal increases with a rise of adsorbent concentration, agi- tating speed, temperature, and with a fall of solution pH, particle size, and initial Cr(VI) concentration. The optimum biosorption conditions were determined as pH 5 2.0, particle size 0.15 mm, agitating speed 5 300 rpm, C 0 5 30 mg L 21 ,m 5 10 g L 21 , and T 5 458C. The equilibrium was reached at around 150 min. The biosorption kinetics showed the pseudo-sec- ond-order model better than the pseudo-first-order model. Kinetic studies showed that the biosorption process was an endothermic process. The experimen- tal biosorption data were fitted to the Freundlich bio- sorption model. The maximum biosorption capacity was found to be 16.97 mg g 21 removed by raw ad- sorbent. Electron paramagnetic resonance studies combined with FTIR spectroscopy were used to repre- sent the biosorption mechanism. These studies display new remarkable findings. Ó 2010 American Institute of Chemical Engineers Environ Prog, 30: 599–608, 2011 Keywords: biosorption, forest litter, biosorption mechanism INTRODUCTION Heavy metals make a significant contribution to environmental pollution as a result of human activ- ities such as mining, smelting, electroplating, energy and fuel production, dye industry, power transmis- sion, sludge dumping, and military industries. As heavy metals are nonbiodegradable materials and hence are accumulated in living organisms by food chains, they are accepted as most dangerous contami- nants that are accumulated by the living beings and, up to now, there are no widely accepted methods to have them removed. Therefore, the best solution is preventing the entrance of heavy metals in the efflu- ents [1]. Chromium is essential for plants and animals in little quantities, but when present in excess con- centrations above certain limits can be very harmful to living organisms [2]. Recent researches indicate that the Cr(IV) oxidation state of chromium, which is formed during disproportionation of the Cr(V) spe- cies, causes oxidative damage to DNA. Industrial sources of Cr(VI) include leather tanning, cooling tower blowdown, plating, electroplating, anodizing baths, rinse waters, etc [3]. The most common method applied for chromate control is reduction of Cr(VI) to its trivalent form in acidic medium and subsequent hydroxide precipita- tion of Cr(III) by increasing the pH. These methods are energy consuming and expensive, and therefore, biosorption/bioremediation techniques involving the use of living and dead biomasses are more favorable. As an alternative to the existing methods, an inexpen- sive, naturally available plant biomass can be used as an adsorptive material for reduction of Cr(VI) to the Ó 2010 American Institute of Chemical Engineers Environmental Progress & Sustainable Energy (Vol.30, No.4) DOI 10.1002/ep December 2011 599