Health Scope. 2015 February; 4(1): e19892. Published online 2015 February 10. Research Article Hexavalent Chromium Removal From Aqueous Solution Using Functionalized Multi-Walled Carbon Nanotube: Optimization of Parameters by Response Surface Methodology Hooshyar Hossini 1 ; Abbas Rezaee 1,* ; Golamreza Mohamadiyan 1 1 Department of Environmental Health, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, IR Iran *Corresponding author: Abbas Rezaee, Department of Environmental Health, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, IR Iran. Tel: +98-2182883575, Fax: +98- 2182883575, E-mail: rezaee@modares.ac.ir Received: April 30, 2014; Revised: July 16, 2014; Accepted: July 26, 2014 Background: Chromium is one of the most commonly used heavy metals in industry. It is known as a pollutant that its discharge into the environment needs special care. Objectives: The current study aimed to evaluate the functionalized multi-walled carbon nanotubes (f-MWCNTs) as a novel adsorbent of chromium in aqueous solutions. Materials and Methods: The virgin MWCNT was protonated using sulfuric acid as a strong oxidant. The operating conditions such as initial chromium concentration, contact time, adsorbent dosage and pH were evaluated in the adsorption process. A systematic model for chromium adsorption was presented based on the experimental design. Response surface method (RSM), as a mathematical technique, was used for modeling and optimizing considerable parameters. All experiments were performed according to the Standard Methods for the Examination of Water and Wastewater Results: According to the statistical analysis, R 2 (0.998), adjusted R 2 (0.996), prediction R2 (0.906), coefficient of variation (1.38%), and the mean removal efficiency (54.76%) were calculated. Conclusions: The f-MWCNTs can be applied as an adsorbent with excellent properties in the adsorption process. The Efficiency of 61.75% was predicted by the model in the optimum conditions. In the experimental condition an adsorption percentage of 59.44% was obtained. Keywords: Chromium; Adsorption; Wastewater Copyright © 2015, Health Promotion Research Center. This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommer- cial 4.0 International License (http://creativecommons.org/licenses/by-nc/4.0/) which permits copy and redistribute the material just in noncommercial usages, provided the original work is properly cited. 1. Background Chromium is a heavy metal that is discharged into the environment as industrial wastes, causing environmental problems. Electroplating, fertilizers, pigments, tanning, mining and metallurgyare the main industrial sources of chromium (1). Generally, chromium exists in the environ- ment in two forms: trivalent Cr (III) and hexavalent Cr (VI) (2). According to toxicological studies, Cr (VI) is considered 1000 times more toxic than Cr (III) (3). The Cr (VI) can make some diseases like liver, kidney, lung, and gastrointestinal cancer (4). In aqueous solutions, Cr (VI) is present as di- chromate (Cr 2 O 2 -7) and chromate (CrO-4), in acidic and al- kaline conditions, respectively (5). Chemical precipitation, membrane filtration, liquid extractions, ion exchange, and adsorption are proposed as the available technologies for chromium removal (3, 6, 7). Among the techniques, absorption process is considered more seriously since it is cost-effective in operation and investment, and easy to use. The main problems with the other methods are high costs and complicated operation. Carbon nanotubes (CNTs) are new adsorbents in the carbon family and are in- tensively studied to remove various pollutants (3, 4, 8-10). Carbon nanotubes are presented in two forms: single-wall and multi-wall depending on the number of their layers (3). The use of multi-walled carbon nanotubes (MWCNTs) as an adsorbent to treat organic and inorganic pollutants such as dyes, phenanthrene, benzene, and heavy metals is studied intensively, and the results indicate that MWCNTs are suitable candidates for the preconcentration and solid- ification of pollutants from large volumes of wastewater (11). Functionalization of carbon nanotubes is a new ap- proach to improve the applications. Carbon materials are usually functionalized by oxidative processes to change the sidewalls and tube tips (12). Adsorption is one of the applications of functionalized carbon nanotubes. For this purpose, oxidization and carboxylation by acidic and car- boxylic groups is common (13). 2. Objectives One of the techniques to improve the interaction between matrix and pollutants is submitting carbon nanotube to a process called functionalization. Func- tionalization is a chemical process that inserts func- tional materials on the sidewall of carbon nanotube. The technique can be utilized to obtain better interaction