Original article Fabrication of NiO coated SiO 2 and SiO 2 coated NiO for the removal of Pb 2+ ions Saba Noor a , Muhammad Waseem b, *, Umer Rashid a , Muhammad Anis-ur-Rehman b , Wajid Rehman a , Khalid Mahmood a a Department of Chemistry, Hazara University, Mansehra 21300, Pakistan b Department of Physics, COMSATS Institute of Information Technology, Islamabad 44000, Pakistan 1. Introduction Lead is a one of the metals that have many industrial applications. However, its presence in ionic form in industrial wastewater is extremely toxic to both biota and abiota. From the industrial effluents, its concentration has been reported in the range of 40–100 mg/L [1–3]. In the last few decades the pollution of water resources due to indiscriminate disposal of lead has become a worldwide threat. Lead is non-biodegradable and can be readily absorbed into the human body after accumulated in living tissues. The presence of lead in drinking water may cause diseases such as anemia, encephalopathy, hepatitis and nephritic syndrome [4,5]. Different physiochemical methods such as chemical precipitation, filtration, reverse osmosis, electrochemical treatment membrane technologies and ion exchange have been widely applied for the removal of toxic metals. However, sometimes these processes are ineffective and expensive [6]. Adsorption is considered to be one of the most effective physiochemical methods for the removal of heavy metal ions from the aqueous solutions. It is preferred over the other removal techniques because of its ease of operation and low cost [7,8]. The sol–gel technique has several advantages over other methods for the preparation of various adsorbents [9]. It can be used to control the shape, morphology and textual properties of composite materials, in which the mixing of two or more metal oxide phases can be controlled even at nanometer scale level [10]. In literature, the sol–gel method has been widely used for coating silica over a large number of metal ions [11]. SiO 2 has diverse applications in numerous processes such as manufacture of glass, electronic substrates, thin film substrates, electrical insulators, and thermal insulators and humidity sensors. It has interesting physical properties that have been found to have wide applications in various antireflective and protective coating [12]. In the present work, NiO coated silica and silica coated NiO are given preference because the surface groups contributed by the parent materials collectively provide the active sites for the removal of toxic metal ions. In literature limited data are available regarding the physical properties and Pb 2+ adsorption studies toward NiO coated silica or silica coated NiO. In this study, we will focus on the synthesis and physical characteristics of silica coated NiO and NiO coated silica by the sol–gel method. To explore the physical properties of the Chinese Chemical Letters 25 (2014) 819–822 A R T I C L E I N F O Article history: Received 29 July 2013 Received in revised form 17 November 2013 Accepted 16 January 2014 Available online 31 January 2014 Keywords: Characterization Coated materials Lead removal Sol–gel A B S T R A C T We have prepared silica, SiO 2 coated NiO and NiO coated SiO 2 by sol-gel method. The physicochemical properties of the desired materials were investigated by surface charge properties, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, surface area measurements and X-ray diffraction (XRD) analyses. The point of zero charge (PZC) of the solid was determined by the salt addition method. In coated materials, two PZC values were noted representing the surface charge of their counterparts. The SEM image of SiO 2 coated NiO displays a uniform coating of silica on the surface of NiO, whereas in case of NiO coated SiO 2 , a honeycomb like appearance was observed with highly porous structures. In the diffractograms of NiO, the characteristic peaks were suppressed in NiO coated silica, however, no diffraction peak could be seen in SiO 2 coated NiO. Batch adsorption technique was applied for the removal of Pb 2+ ions from aqueous solution. The sorption trend for Pb 2+ ions was observed in the order of NiO coated SiO 2 > SiO 2 coated NiO > NiO > SiO 2 . This trend confirms that the coated materials have more sorption capacities than their parent counterparts. ß 2014 Muhammad Waseem. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. * Corresponding author. E-mail addresses: waseem_atd@yahoo.com, m.waseem@comsats.edu.pk (M. Waseem). Contents lists available at ScienceDirect Chinese Chemical Letters jo u rn al h om epag e: ww w.els evier.c o m/lo cat e/cc let 1001-8417/$ – see front matter ß 2014 Muhammad Waseem. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2014.01.040