1 Korean J. Chem. Eng., 31(4), 1-10 (2014) DOI: 10.1007/s11814-014-0101-8 INVITED REVIEW PAPER pISSN: 0256-1115 eISSN: 1975-7220 INVITED REVIEW PAPER † To whom correspondence should be addressed. E-mail: mubarak.yaseen@gmail.com, mubarakmujawar@ucsi.edu.my Copyright by The Korean Institute of Chemical Engineers. Adsorption of chromium (VI) on functionalized and non-functionalized carbon nanotubes Nabisab Mujawar Mubarak* , ** ,† , Raj Kogiladas Thines*, Noor Rosyidah Sajuni*, Ezzat Chan Abdullah***, Jaya Narayan Sahu**** , Poobalan Ganesan***** , and Natesan Subramanian Jayakumar** *Department of Chemical and Petroleum Engineering, Faculty of Engineering, UCSI University, Kuala Lumpur 56000, Malaysia **Department of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia ***Malaysia - Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia, Jalan Semarak, Kuala Lumpur 54100, Malaysia ****Department of Petroleum and Chemical Engineering, Faculty of Engineering, Institut Teknologi Brunei, Tungku Gadong, P. O. Box 2909, Brunei Darussalam *****Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia (Received 16 August 2013  accepted 8 April 2014) AbstractWe did a comparative study on the adsorption capacity of Cr (VI) between functionalized carbon nanotubes (CNTs) and non-functionalized CNTs. The statistical analysis reveals that the optimum conditions for the highest removal of Cr (VI) are at pH 9, with dosage 0.1 gram, agitation speed and time of 120 rpm and 120 minutes, respectively. For the initial concentration of 1.0 mg/ l , the removal efficiency of Cr (VI) using functionalized CNTs was 87.6% and 83% of non-functionalized CNTs. The maximum adsorption capacities of functionalized and non–functionalized CNTs were 2.517 and 2.49 mg/g, respectively. Langmuir and Freundlich models were adopted to study the adsorption isotherm, which provided a K L and K F value of 1.217 L/mg and 18.14 mg 1n L n /g functionalized CNT, while 2.365 L/mg and 2.307 mg 1n L n /g for non-functionalized CNTs. This result proves that functionalized CNTs are a better adsorbent with a higher adsorption capacity compared with the non-functionalized CNTs. Keywords: CNT, Chromium, Heavy metal, Adsorption, Separation, Functionalization INTRODUCTION Environmental issues and their consequences affect all living crea- tures and flora on earth, further creating a serious problem in the past few decades. Water pollution has been highlighted as one of the most crucial issues which needs an immediate solution to avoid further impact on the environment. One of the main contributions to water pollution is the emission of heavy metal ions from exten- sive industries such as mining, cement ceramics, glass industries fossil fuel, battery manufacturing industries and production of plas- tics, involving the use of metal compounds [1]. The indirect con- sumption of these heavy metals could lead to several disease problems such as kidney disease, anaemia, cancers of the digestive tract and lungs, nervous system failure [2-4] and eventually to death as well. Hence, to avoid further impact on the environment, these heavy metals have to be eradicated from the wastewater before they are discharged into any type of water source. Many methods and tech- niques have been investigated and applied to remove these heavy metals from the water such as ion exchange, membrane separation, reverse osmosis, electrolysis and adsorption [5,6]. All of these meth- ods have been employed and have flourished in removing an im- pressive percentage of heavy metal ions from the wastewater, but the adsorption method stands out to be one of the most versatile methods due to its flexibility and simplicity. Adsorbents are known to be the foundation of this adsorption technology. A variety of ad- sorbents such as zeolite, activated carbon, egg shell, silica gel and slag [7-11] have already been engaged with the removal of heavy metal applications, but it is more desirable to explore a higher adsorp- tion capability of adsorbent to remove heavy metals from any aque- ous solutions. Carbon nanotubes (CNTs) as a new member in the adsorbent family, which was first reported in 1991 [12], have been proven for their higher efficiency of the removal of heavy metal ions from aqueous solutions due to remarkable mechanical [13], electrical [14], and chemical properties [15]. On the same note, CNTs have large specific surface area, high thermal and chemical stabilities and provision for large scale synthesis, making them a good candi- date for adsorption kinetics [16], and their light mass density [17] have made them one of the successful adsorbents to be integrated with the removal of heavy metal ions from aqueous solutions [18, 19]. Further research has also been carried out to prove that the ad- sorption capacity of CNTs can be increased by altering their chem- ical and physical properties by functionalizing them with acids or alkaline solutions such as nitric acid, sulfuric acid and potassium permanganate [20,21]. In this research, we performed statistical optimization and a com- parative study on the removal of Cr (VI) from aqueous solution by the usage of and non-functionalized (NFCNTs) and functionalized CNTs (FCNTs). The effect of each process parameter pH, dosage and agitation time on the removal of Cr (VI) was investigated using FCNTs and NFCNTs. The equilibrium kinetic and isotherm model