Mechanism of deleading of silicate glass by 0.5 N HNO 3 S.K. Sadrnezhaad a,c, * , Raﬁ Ali Rahimi a,b , Gholamreza Raisali d , Farhad Foruzanfar b a Materials and Energy Research Center, P.O. Box 14155-4777, Tehran, Iran b Material Research School, Nuclear Science and Technology Research Institute (NSTIR), Atomic Energy Organization of Iran, Rajaei Shahr, Karaj, Iran c Center of Excellence for Production of Advanced Materials, Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Tehran, Iran d Radiation Applications Research School, Nuclear Science and Technology Research Institute, Atomic Energy Organization of Iran, End of Karegare Shomali Street, Tehran, Iran article info Article history: Received 20 November 2008 Received in revised form 9 July 2009 Available online 24 September 2009 PACS: 61.43.Fs 64.70.ph 66.30.h 66.30.Ny 82.30.b 82.39.Wj Keywords: Chemical durability Ion exchange Diffusion and transport Silicates abstract Mechanism of removal of lead from silicate glass containing 68.5 wt% PbO by 0.5 N HNO 3 was investi- gated by incorporation of the chemical-analyses/weight-loss data into shrinking-core model (SCM) and minimization of the difference. Scanning electron microscopy (SEM), emission spectrometry with induc- tively coupled plasma (ICP), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDS) were used to determine the compositional changes of the lead-silicate glass (LSG) samples. Dual inter-diffusion chemical reaction mechanisms having respective activation energies of 83.49 and 47.80 kJ/mol domi- nated the deleading process. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction In spite of toxic nature, lead-silicate glass (LSG) is widely used in ophthalmic, electronic, biotechnology and indoor applications [1–5]. LSG glass surfaces roughen when contacting acidic environ- ments. This deteriorates optical transparency of LSG. In spite of economic susceptibility, very little is known of the property changes due to the deleading episode. Based on the previous re- searches, lead is a network-former at more than 50 mol% and a net- work modiﬁer at less than 50 mol% [6–9]. Silicon atoms are randomly distributed in tetrahedral positions of diamond-like glass lattices with oxygen atoms in middle of Si–O–Si and Si–O– Pb combinations in low-lead-silicate glasses. Deleading is, there- fore, accountant partially of LSG structural destruction which grad- ually occurs when LSG is encountered with acid solutions. Mizuno et al. [10] studied room temperature leaching of 25– 70 mol% PbO LSG glasses in 0.1 N HNO 3 . Ohtake [11] used 35– 60 mol% PbO LSG samples in 10 4 N HNO 3 . PbO content of the sam- ples leached in 0.01 N HNO 3 at 90 °C by Bertoncello et al. [8] was 45.3 wt%; while that of Bonnet et al. [9] in 0.01 N HNO 3 at 90 °C was 66 wt% PbO. Because of dilution of their acid solutions, the above authors did not observe formation of gel-layer on the surface of their samples. A single-step diffusion regime was, therefore, suf- ﬁcient for explaining their leaching results. The LSG samples used in this investigation contained 68.5 wt% lead oxide. This composition indicates superb radiation shielding effects required in hot-cell window glasses. Since a much higher HNO 3 concentration (0.5 N, pH 0.3) was used in this investigation, the formation of a gel-layer of 10–80 lm thickness on the surface of the leached LSG sample was observed. Using model calculations, a dual-stage mechanism could best explain the experimental re- cords obtained. This was performed by incorporation of the frac- tional conversion data into various integrated rate equations via trial/error technique to ﬁnd the governing mechanism. Quantitative weight-loss data combined with EDS, XRD and ICP analyses of the probe surfaces, gel-layer and the acid solutions were used to determine the lead values extracted via the ion-ex- change reaction. The software previously developed and used for simulation of the gas–solid reaction was adopted for the liquid–so- lid reaction that occurs in the present LSG system [12–14]. The best 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.08.019 * Corresponding author. Address: Materials and Energy Research Center, P.O. Box 14155-4777, Tehran, Iran. Tel.: +98 2166165215; fax: +98 2166005717. E-mail addresses: sadrnezh@sharif.edu (S.K. Sadrnezhaad), rrahimi@nrcam.org (R.A. Rahimi), graisali@aeoi.org.ir (G. Raisali), fforoozanfar@nrcam.org (F. Foruzan- far). Journal of Non-Crystalline Solids 355 (2009) 2400–2404 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol