Monitoring Trace Amounts of Lead and Arsenic Adsorbed under Environmental Conditions by XAFS Combined with Fluorescence Spectrometry The risk of trace amounts of metals in environmental water to human health has been pointed out [1]. The regulations of metal concentration in water are becoming more severe: e.g., 100 ppb for lead and 10 ppb for arsenic. Co-precipitation and adsorption are promising methods for the removal of trace amounts of metals in water. Information on the removed metal site structure and the chemical state is crucial in the assessment of the removal effectiveness for the most hazardous valence state of metals. X-ray absorption fine structure (XAFS) has become widely used with the state-of-the-art technology of synchrotron radiation and advanced X-ray detectors to monitor the local structure of noncrystalline and heterogeneous/hybrid materials. However, there are still experimental difficulties in measuring at high- energy resolution the absorption edge of the trace amount of an element in samples containing a high concentration of heavy element(s). In this account, 0.12 wt% Pb adsorbed from a 100 ppb aqueous solution and 0.20 wt% As adsorbed from a 200 ppb arsenite solution were monitored by the XAFS combined with a high-energy-resolution f l u o r e s c e n c e s p e c t r o m e t e r [ 2 ] . T h e e n e r g y resolution s were 0.3 eV at 5 keV and 1.1 eV at 8 keV. The latter value includes the contribution of the beamline [3]. These values are smaller than the core- hole lifetime widths of Pb L 3 (5.81 eV) and As K (2.1 4 eV). Thus, the advantages of the method applied in this account are (i) the removal of lifetime broadening [ 4 ], (ii) reasonable signal/background ratio, and that (iii) the method is free from the problems associated with photon-counting losses. The X-ray fluorescence from Pb adsorbed on M g 6 Fe 2 ( O H) 1 6 (C O 3 ) • 3H 2 O (1) was analyzed by a R o w l a n d - t y p e s p e c t r o m e t e r ( r a d i u s 2 2 0 m m) equipped with a J ohansson-type cylindrically bent G e(555) crystal and a scintillation counter at beamline BL10XU [5]. The spectrometer was tuned at 10551.5 eV (Pb L α 1 emission), and the obtained Pb L 3 -edge X- ray absorption near-edge structure (XA NE S) spectra are shown in Figs. 1(a-c). B ased on the comparison of the rising edge and post-edge peak energies, spectrum a for Pd adsorbed from the 1.0 ppm solution resembled only that for 2PbC O 3 • Pb( O H) 2 (Fig. 1(g)). Figures 1(b,c) for Pb adsorbed from a 100 ppb Fig. 1. Normalized Pb L 3 -edge XANES spectra for Pb 2+ on 1 measured utilizing a secondary fluorescence spectrometer (a – c) . The Pb content was 1.0 wt% adsorbed from a 1.0 ppm Pb 2+ solution (a) and Pb 0.30 wt% (b) and 0.12 wt% (c) adsorbed from a 100 ppb solution. XANES spectra for PbY zeolite (d) , PbO (e) , Pb (NO 3 ) 2 (f) , 2PbCO 3 •Pb(OH) 2 (g) , Pb 6 O 4 (OH) 4 (h) , and PbCO 3 (i) were measured in the conventional transmission mode. solution resembled each other. The rising edge position shifted from – 1.1 to – 1.3 eV compared to Fig. 1(a). Figures 1(b,c) resembled only those of Pb Y zeolite (Fig. 1(d)), Pb mordenite or Pb- Z S M -5. Pb 2 + ions replace the protons or N a + sites of zeolites. An unresolved shoulder peak was observed at 130 49 eV in Figs. 1(a-c). A similar peak was also observed for 2PbC O 3 • Pb( O H) 2 (Fig. 1(g)). N o shoulder peak appeared in this region for Pb Y (Fig. 1(d)). In summary, most of the Pb 2 + coagulated as a eutectic mixture of PbC O 3 and Pb( O H) 2 on 1 in the adsorption from a 1.0 ppm solution. In contrast, in the adsorption from a 100 ppb solution, the ma j or Pb phase was ion-exchanged Pb 2 + via surface reaction A. – O H + Pb 2 + → – O Pb + + H + (A) 0.0 0.4 0.8 1.2 1.6 2.4 2.8 3.2 2.0 13020 13040 13060 13080 13100 13120 94 78 78 104 90 97 56.3 54.4 60.5 a b c d e f g h i 100 81 55.5 105 89 77 76 53.1 60.1 94 94 53.6 54.9 Energy (eV) Normalized Intensity Value in figure = Energy (eV) – 13000 50.4 99 Yasuo Izumi Tokyo Institute of Technology E-mail: yizumi@chemenv.titech.ac.jp Fig. 2. Pb 2+ adsorption mechanism on 1 from 1.0 ppm and 100 ppb aqueous Pb 2+ solutions. References [1] http://www.who.int/water_sanitation_health/en/ [2] Y. Izumi et al. : J . Electron S pectrosc. R elat. P henom. 119 ( 2 00 1 ) 1 93 . [ 3 ] Y. Izumi , F . K iyotaki , T. M inato , D . M asih an d Y. S ei d a: P hys. S cr. - in press. [ 4 ] Y. Izumi an d H . N agamori: B ull. C hem. S oc. J pn. 73 ( 2 000) 1 58 1. [ 5 ] Y. Izumi et al. : A nal. C hem. 74 ( 2 00 2 ) 38 1 9 . A shoul d er peak at 1 3049 e V suggests a minor contri b ution from the coagulate d eutectic mi x ture of PbCO 3 an d Pb(OH) 2 on 1 . The interpretation of this d ifference that d epen d s on the Pb concentration is illustrate d in F ig. 2. The surface of 1 has a b uffering effect an d the p H value in the pro x imity of 1 b ecomes 7 – 8 d ue to the effect of coagulant chemicals r elease d through a slight d issolution of 1 [ 5 ]. In the p H region , a forwar d reaction of B procee d s from a 1. 0 ppm solution. Pb 2 + + n CO 3 2 – + 2 ( 1 – n )OH – → n PbCO 3 •( 1 – n )Pb(OH) 2 (B) In the a d sorption from a 1 00 pp b Pb 2 + solution , e q uili b rium shifte d to the left-han d si d e of e q uation B . The ratio of free Pb 2 + in the solution d ramatically increase d, an d thus , the Fig. 3. Normalized As K -edge XANES spectra for As metal (a) , As 2 O 3 (b) , KH 2 AsO 4 (c) , and As adsorbed on 2 (Fe 15.3 wt%, d – f ) from solutions of 16 ppm of KH 2 AsO 4 (d) , 16 ppm of As 2 O 3 (e) , and 200 ppb of As 2 O 3 (f) . Spectra (d – f) were measured utilizing a high-energy- resolution fluorescence spectrometer. The spectra (a – c) were measured in the conventional transmission mode. ion e x change reaction ( e q uation A) d ominantly procee d e d . H ence , the major species of ion- e x change d Pb 2 + an d the minor phases of the coagulate d eutectic mi x ture of PbCO 3 an d Pb(OH) 2 were d etecte d (F igs. 1 (b, c )) . The A s K -e d ge XAN E S spectra were also measure d s imilarly t o the Pb L 3 -e d ge spectra d escri b e d a b ove. The fluorescence spectrometer was tune d to 1 0544 . 3 e V (A s K α 1 emission ) . S amples of 0 . 48 – 0 .2 0 wt % A s a d sor b e d from 1 6 ppm of arsenate , 1 6 ppm of arsenite , an d 2 00 pp b of arsenite a q ueous solutions were stu d ie d (F igs. 3 ( d - f )) on an F e-impregnate d montmorillonite (N a 1. 5 C a 0 . 096 A l 5 .1 M g 1. 0 F e 0 . 33 S i 12 O 2 7 . 6 (OH) 6 . 4 ) (2) [ 3 ]. The three spectra resem b le d one another. The strong peak positions in the post-e d ge region were very close to that for the spectrum of KH 2 A s V O 4 (F ig. 3( c ) ) rather than that for the spectrum of A s III 2 O 3 (F ig. 3(b)) . Thus , the o b serve d o x i d ation of trace amounts of arsenite upon a d sorption on 2 to arsenate is important in the preservation of the environment b ecause arsenate affects far less human health than arsenite. a b c d e f 0 2 4 6 8 10 11868.6 11860 11870 11880 11890 Energy (eV) 11872.1 Normalized Intensit y 100
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