Surface modified silica gel for extraction of metal ions: an environment friendly method for waste treatment R. K. Sharma* 1 , S. Mittal 1 , S. Azami 1 and A. Adholeya 2 A surface modified silica gel chelating resin was synthesised by immobilising tetracycline (TCN) on the surface of modified silica gel to produce a granular sorbent for the adsorption and estimation of copper, nickel and cadmium. Metal ions were quantitatively retained on the column packed with immobilised silica gel in the pH ranges of 5 . 0–6 . 5 for Cu, 6 . 8–7 . 8 for Ni and 6 . 6–7 . 8 for Cd. The distribution coefficient D determined for each metal ion was as follows (mL g 21 ): Cu, 2 . 45610 2 ; Ni, 1 . 84610 2 ; Cd, 2 . 33610 2 . Methods have been developed to determine copper, nickel and cadmium in steel, medicine tablets, alloys and river water samples. Molecular modelling studies were also performed on TCN-imobilised silica gel for gaining an insight into the active structure of TCN on the silica surface. Keywords: Chelating resin, Immobilised silica gel, Adsorption, Column, Molecular modelling Introduction Copper, nickel, cadmium are important constituents of modern alloys and these elements are quite abundant in untreated waste water of iron and steel, leather tanning, metal plating, textile, battery, electrowinning and metal finishing industries. These elements pose a contamina- tion risk to the natural environment 1 because at elevated concentrations they are highly toxic. Selective separation of metal ions at trace level from complex matrixes before their determination continues to be a challenging problem. It is very important in the context of environmental monitoring, geochemical explorations and high purity material designing 2–3 and becomes a necessity many times. Applications of chelating resins can be easily envisaged for such separations. 4 The extraction of metal ions using chelat- ing resins is a green analytical method because it does not involve the use of toxic chlorinated organic solvents, which are very frequently used in conventional liquid– liquid extraction technique or other methods. 5 Chelating solid phase can be tailor made by immo- bilising appropriate chelating agents on the support matrixes for selective trace metal analysis. Many solid supports are available for this purpose, 6–7 but silica based chelating resins are most commonly used because immobilisation reactions on silica surface are relatively simple and reproducible, 8 especially when compared with the immobilisation on organic polymers, which involve complicated multistep reactions. Silica based chelating resins show greater thermal stability, fast metal ion exchange kinetics, the lack of which precludes the use of many organic based polymers. 9–11 Silica has good mechanical strength and swelling stability required for its use in solid phase extraction. 12–13 Tetracycline is of wide spread applications in biological and medicinal fields. 14–17 It shows strong chelation behaviour with various metal ions. 15–21 In the present work a new chelating resin has been synthesised and applied for the separation and estimation of trace amount of copper, nickel and cadmium. Experimental Instruments A Shimdazu AA-640-13 atomic adsorption spectro- photometer was used for atomic absorption spectro- metric measurements. A Perkin Elmer Spectrum 2000 FTIR spectrometer was used to record IR spectra (in KBr) in the range of 400–4000 cm 21 . The pH measurements were made by ECIL (India) digital pH Meter, model 56 51A. The sorption–desorption studies of the metal on the chelating matrix were generally carried out on the column of 100 mm length and 7 m i.d.. Reagents and solutions Tetracycline (TCN) was procured from CDH (India) and used as obtained. Column chromatographic silica gel (G.S. Chemicals, India), 60–120 mesh was activated by heating it in a vacuum oven for 12 h at 393 K. Organic solvents were dried using the methods described in the literature. 22 Metal salts used were of AR grade and double distilled water has been used throughout the study. pH in the range of 3 . 5–6 . 0 was maintained using 1 Department of Chemistry, University of Delhi, Delhi-110007, India 2 Tata Energy Research Institute, India Habitat Place, Lodhi Road, New Delhi, India *Corresponding author, email rksharma@vsnl.com ß 2005 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 20 January 2005; accepted 20 April 2005 232 Surface Engineering 2005 VOL 21 NO 3 DOI 10.1179/174329405X50082