Adsorption of Adenine on Bentonite and Montmorillonite with and without Preadsorbed Sulfide

摘要

Bernal 1951 was the first that suggested that minerals could be played an important role in the adsorption of key monomers from dilute solution and their subsequent condensation to form biopolymers. Besides some doubts about the role of minerals in the processes of selection of monomers (Zaia et al. 2002, Zaia 2004, Benetoli et al. 2007, Benetoli et al. 2008), adsorption of biomolecules on minerals is an important issue in prebiotic chemistry (Lambert, 2008). In the present work, the adsorption of adenine on bentonite and montmorillonite with and without preadsorbed sulfide was studied at different pH (2.00, 7.00). The adenine was dissolved in seawater at concentrations of 600, 1,200, 2,400 and 3,600 μg 5 mL?1. All clays were processed as follow: to five different sets of four tubes (15 mL) containing 500 mg of clay (with or without sulfide preadsorbed) were added: (a) 5.00 mL of seawater, (b) 5.00 mL of seawater with 120 μg mL?1, (c) 5.00 mL of seawater with 240 μg mL?1, (d) 5.00 mL of seawater with 480 μg mL?1 and (e) 5.00 mL of seawater with 720 μg mL?1. The pH was adjusted to 2.00 or 7.00 with HCl or NaOH. The tubes were mixed for 4 h, after they were spun for 15 min at 2,000 rpm; the aqueous phase was used for the adenine analysis (UV 260 nm). All results are presented as mean ± standard error of mean, and the number of experiments was always five with four sets each. For montmorillonite the following results of adenine adsorbed were obtained: pH 2.00 [without sulfide 291.0±10.6, 821.0±4.0, 1382.6± 10.1, 1600.5±16.6; with sulfide 379.5±11.4, 929.5±19.9, 1625.0±31.5, 1890.2±31.1] and pH 7.00 [without sulfide 269.9±12.9, 583.6±14.5, 911.3±9.0, 1048.5±18.3; with sulfide 143.5±15.6, 224.6±29.8, 434.2±14.9, 612.5±20.4]. For bentonite the following results of adenine adsorbed were obtained: pH 2.00 [without sulfide 411.2±14.7, 773.8±24.1, 1,108.8± 6.5, 1,387.9±17.4; with sulfide 405.7±17.4, 808.5±19.5, 1,149.4±19.3, 1,402.8±25.2] and pH 7.00 [without sulfide 174.6±7.2, 296.2±7.3, 459.7±10.7, 548.9±16.9; with sulfide 62.7± 10.7, 103.6±10.1, 120.6±20.0, 247.2±8.3]. For all samples adenine was more adsorbed at pH 2.00 than pH 7.00. At pH 2.00 bentonite and montmorillonite are negatively charged and adenine is positively charged and at pH 7.00 adenine is neutral (Benetoli et al. 2008). Thus the difference of charges clays/adenine could explain why adenine is more adsorbed at pH 2.00 than at pH 7.00. Sulfide increased the adsorption of adenine at pH 2.00 when compared to the samples without it, by the other hand decreased the adsorption at pH 7.00. These results are now under analysis by FT-IR and M?ssbauer spectroscopy.