ASIAN JOURNAL OF CHEMISTRY ASIAN JOURNAL OF CHEMISTRY https://doi.org/10.14233/ajchem.2017.20264 INTRODUCTION Wastewater which contains highly toxic and hazardous heavy metal ions that exceed the permissible limit could harmful to humans and the environment. Consequently proper treatment of wastewater contaminated by heavy metals should be carried out before discharge into receiving waters to avoid accumulation of heavy metals in the food chain and persistence in the ecosystem [1]. Cadmium, chromium, copper, nickel, mercury, lead and zinc are among the heavy metals, which are frequently released from industrial activities and should become a major concern to the environment [2]. Nickel is usually released from industrial processes such as electroplating, plastics manufacturing, metal finishing, nickel- cadmium batteries [3] and its maximum permissible con- centration of nickel in drinking water recommended by U.S. Environmental Protection Agency (EPA), Agency for Toxic Substances and Disease Registry (ATSDR) and World Health Organization (WHO) is 0.02 mg/L [4]. In fact, nickel is vital to humans but it could cause a few health problems such as skin irritation, dermatitis and gastrointestinal distress if consumed excessively [5]. Isotherm Study of Nickel(II) Adsorption from Aqueous Solution Using Thermally Treated Rice Husk A.T.A. FARHAN 1 , K.K. ONG 2,* , W.M.Z. WAN YUNUS 1 , M.L. JABIT 3 , A. FITRIANTO 4 , A.G.A. HUSSIN 1 and C.C. TEOH 5 1 Department of Defence Science, Faculty of Science and Defence Technology, Universiti Pertahanan Nasional Malaysia, Kem Sungai Besi, 57000 Kuala Lumpur, Malaysia 2 Department of Chemistry and Biology, Centre for Defence Foundation Studies, Universiti Pertahanan Nasional Malaysia, Kem Sungai Besi, 57000 Kuala Lumpur, Malaysia 3 Technical Services Centre, Malaysian Agricultural Research and Development Institute (MARDI) Headquarter, G.P.O. Box 12301, 50774 Kuala Lumpur, Malaysia 4 Department of Mathematics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia 5 Engineering Research Centre, MARDI Headquarter, G. P.O. Box 12301, 50774 Kuala Lumpur, Malaysia *Corresponding author: Fax: +603 9054291; Tel: +603 90513400; E-mail: ongkhim@upnm.edu.my Received: 24 August 2016; Accepted: 17 November 2016; Published online: 30 December 2016; AJC-18206 The adsorption of heavy metals using low cost adsorbents is being widely investigated. Various modifications and treatments of adsorbents either chemically or physically have been applied to enhance the adsorption capacity. Thermal treatment is one of the approach of improving the porosity of the adsorbent. In the present study, the adsorption of nickel(II) ions using thermally treated rice husk is reported. The adsorption data were evaluated with Langmuir and Freundlich adsorption models. It was found that the equilibrium data fitted well with Langmuir isotherm as indicated with high coefficient of determination (R 2 = 0.9947). The adsorption capacity of the thermally treated rice husk was found to be 1.68 mg/g. It can be seen that nickel(II) ions are considerably adsorbed on thermally treated rice husk and the method used in this study could be an economic approach for removal of nickel(II) ions from aqueous solutions. Keywords: Nickel(II) ions, Adsorption isotherm, Thermally treated rice husk. Asian Journal of Chemistry; Vol. 29, No. 3 (2017), 589-591 Various technologies have been developed to reduce heavy metals including chemical precipitation, ion exchange, chemical coagulation, extraction and membrane separation [6]. However, some drawbacks were reported, for instance formation of secondary pollution, high cost, high energy input, large quantities of chemical reagents and poor treatment effi- ciency at low metal concentration [7]. To overcome these problems, adsorption by various low cost adsorbents derived from industrial by-product, natural material, modified bio- polymers and agricultural waste were developed [8]. According to Mohammed et al. [9], the application of agricultural wastes such as palm oil fuel ash, coconut shell, mangosteen peel, rice husk, durian peel and corncob is environmental friendly and able to compete with activated carbon in terms of adsorption capacity, effectiveness and economic value. Hence, a simple thermal treatment on rice husk was conducted in this study in order to develop a low cost and environmental friendly adsorbent for removal of nickel(II) ions from the aqueous solution. The main objective of this study is to determine the adsorption isotherm of nickel(II) ions onto the thermally treated rice husk by evaluating the experimental data using well-known isotherm models namely Langmuir and