Synthesis and characterization of sulfur-functionalized silica materials towards developing adsorbents for mercury removal from aqueous solutions Norasikin Saman a , Khairiraihanna Johari a , Hanapi Mat a,b,⇑ a Advanced Materials and Process Engineering (AMPEN) Laboratory, Faculty of Chemical Engineering, 81310 UTM Skudai, Johor, Malaysia b Novel Materials Research Group, Nanotechnology Research Alliance, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia article info Article history: Received 11 June 2013 Received in revised form 2 March 2014 Accepted 25 March 2014 Available online 2 April 2014 Keywords: Adsorbents Silica Sulfur Mercury ion Adsorption abstract Three methods were used to synthesize sulfur-functionalized silica materials with spherical morphology: (a) impregnation of CS 2 and S 8 onto preformed silica support (MCM-TEOS); (b) co-condensation of TEOS and BTESPT/MPTMS; and (c) direct use of BTESPT and MPTMS as precursors. The presence of sulfur functional groups in the synthesized adsorbents was confirmed by the FTIR and EDX analyses. Lower pore arrangements and pore characteristics of sulfur-functionalized silicas were found which were observed from the XRD and nitrogen adsorption desorption analysis. However, these samples showed higher adsorption capacity than silica without sulfur. The highest Hg(II) adsorption capacity (47.50 mg/g) was obtained by the MPTMS-SMs(I) adsorbent synthesized through the co-condensation method. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction The past decade has seen extensive studies in developing functionalized silica as alternative materials for catalysis [1,2], separation medium [3,4], membranes [5,6], drug delivery systems [7,8], immobilization host for enzymes [9] and for environmental technology applications [10]. These are due to the physical and chemical characteristics of the silica materials such as good mechanical strength, do not swell and can sustain high tempera- tures; thus they become the most researched heavy metal adsor- bent [11–16]. Another advantage of using silica materials is that they are easy to modify and functioned in many ways due to the existence of free hydroxyl groups (O–H) on their surfaces, which can be utilized as points of attachment with functional agents. The functionalization of silica materials could be done using various strategies to improve the availability of functional groups in the silica matrices namely impregnation [17,18], post-synthesis grafting [11,12,16,19–25], co-condensation of alkoxysilane and organoalkoxysilanes [13–15,26–31] and direct use of organoalk- oxysilanes as precursors [32–34]. At the beginning, post-synthesis grafting method is widely used. It can produce materials with organic residues anchored covalently onto the pore walls or to the mat rice surfaces. This method gives better control of the modification process and can avoid unnecessary pore blockage [25]. However, a large excess of functionalization agent is needed in order to obtain a high density of the functional group. In comparison, co-condensation provides shorter synthesis time, and high content and good dispersion of the functional groups compared to the grafting method [31]. The introduction of sulfur containing organoalkoxysilanes (e.g. MPTMS and BTESPT) [13,14,16,22,24,26–28] and organosulfurs (e.g. mercaptobenzimidazole and dithiocarbamate) [15,19–21,25] into silica frameworks has been found to result in high adsorption capacity and selectivity towards mercury ions. Thiol compounds such as MPTMS, mercaptobenzimidazole and dithiocarbamate are the most widely used functionalization agents. The availability of terminal sulfur groups in its molecular structure is the main reason for the high Hg(II) adsorption. Other types of sulfur such as ele- mental sulfur (S 8 ), carbon disulfide (CS 2 ) and hydrogen sulfide (H 2 S) are normally functionalized through impregnation. These types of sulfur are commonly used to functionalize onto carbon materials, leading to higher adsorption performance for elemental mercury [35–37] and mercuric ion [38,39]. So far, there is no report on the impregnation of mesoporous silica using these sulfur compounds. The synthesis of sulfur-functionalized silicas using direct use of organoalkoxysilanes as precursors had been reported previously http://dx.doi.org/10.1016/j.micromeso.2014.03.036 1387-1811/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author at: Advanced Materials and Process Engineering (AMPEN) Laboratory, Faculty of Chemical Engineering, 81310 UTM Skudai, Johor, Malaysia. Tel.: +60 7 5535590. E-mail address: hbmat@cheme.utm.my (H. Mat). Microporous and Mesoporous Materials 194 (2014) 38–45 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso