B.2.1.1 EFECTOS COMUNES

1_{Institute of Physical Chemistry, University of Stuttgart, Stuttgart, Germany}

The risk assessment of a nanoparticle release into the environment is dependent on a suitable method for tracking said particles. We here report the possible route of radioiodination using the one-pot Iodogen method for the radiolabeling of carbon nanotubes.

Carbon nanotubes (CNT), cylinders of sp2_-hybridized carbon, show intriguing physical properties. Since their discovery in 1991 [1], a substantial amount of research was put into their application. A critical assessment of their mobility and fate in the geosphere and their inges- tion by living organisms is mandatory. However, the monitoring of nanoparticles in complex natural systems like geological formations, ground water or organisms is nearly impossible using conventional methods, especially in the low concentrations which are environmentally rele- vant. In order to investigate these issues, a rapid, repro- ducible way of radiolabeling is highly desirable. Most ra- diolabeling processes for CNT, like binding radiolabeled chemicals on the CNT sidewall [2] or synthesizing them using radiolabelled precursors [3], are in need of elaborate chemistry and often also greatly influence the properties of the CNT due to large substituents. The radioiodination of CNT is a versatile method minimizing the intrusion.

EXPERIMENTAL. The carbon nanotubes were radio-

labeled using the Iodogen (1, 3, 4, 6-tetrachloro-3α‐6α‐

diphenylglycouril) method adopted from protein labeling [4]. Iodogen (25 µL of a 2 mg/mL solution in chloroform) was coated onto the inner wall of an Eppendorf tube by evaporation of the chloroform. A dispersion of 200 µg of CNT in 200 µL of phosphate buffered water as well as typically 200 kBq of [125/131_{I]NaI was added into the tube.} The tube was put into an ultrasonic bath to keep the CNT in dispersion during the 4 h labeling process. Transferring the reaction mixture into another uncoated tube finished the reaction. Remaining unreacted iodine was washed away in 2–4 washing steps until the radiolabeled particles remained stable in the absence of light. 

RESULTS. It was possible to stable label carbon nano-

tubes using the procedure outlined above. In general, the Iodogen acts as oxidizing agent which converts the unre- active iodide I− _{into the reactive species I2 or I}+ _{[5] which} then perform an electrophilic attack onto the CNT side wall forming a covalent carbon-iodine bond [6]. Single wall CNT proved to be more reactive than multi wall CNT, as their higher curvature renders them more reac- tive. Corresponding to that the yield further decreases for graphite. The yield was independent of the pH value in case of 125_{I in the tested pH range of 6–8.}

No change of the ζ potential or the D : G Raman band ra- tio could be observed confirming the non-intrusiveness of the method.

CONCLUSION. A versatile method for CNT radiolabel-

ing was developed without the setback of largely chang- ing the CNT properties. The possible use of 124_{I enables} PET-studies.

ACKNOWLEDGEMENTS. Financial support by the Deutsche

Forschungsgemeinschaft is gratefully acknowledged (support code: FR1643/3-1).

[1] Iijima, S. (1991) Nature 345, 56–58.

[2] Singh, R. et al. (2006) Proc. Natl. Acad. Sci. U.S.A.. 103, 3357– 3362.

[3] Petersen, E.J. et al. (2008) Environ. Sci. Technol. 42, 3090–3095. [4] Fraker, P.J. et al. (1978) Biochem. Biophys. Res. Comm. 80, 849–

857.

[5] Preedy, V.R. et al. (2009) Comprehensive Handbook of Iodine, Aca- demic Press, London.

[6] Deng, X. et al. (2008) Nanotechnology 19, 075101.

Fig. 1: Radiolabeling yields for 200 µg carbon nanoparticles reacted with 200 kBq [131_{I]NaI in distilled water.}

HZDR | Institute of Resource Ecology | Annual Report 2012

44

Interfacial reactions of Sn(II) with anatase (TiO

): EXAFS and surface complexation

modeling

S. Dulnee, B. Merkel,1 A. C. Scheinost

1_{Department for Geology, TU Bergakademie Freiberg, Freiberg, Germany}

Sn(II) is a contaminant of general environmental in- terest, but little is known about its interaction and re- tention by mineral surfaces. We have selected the re-

dox-inactive mineral anatase (TiO2) to study the sur-

face complexation mechanism by combining EXAFS- derived surface structures with surface complexation modeling to model the macroscopic sorption behavior. Three adjustable parameters required for diffuse double layer model were determined in this study: the

surface site density (16 sites/nm2_{), and two log K}+ _val-

ues (4.76 ± 0.36 and −9.90 ± 0.42) of the protolysis re- actions. EXAFS spectra show Sn(II)–Ti distances at 3.26 and 3.60 Å indicating the formation of monoden- tate and bidentate inner-sphere sorption complexes.

Four surface complexes, Anat_sOSn+_{, Anat_sSnOH,}

Anat_sOH(Anat_sO)Sn+_{, and (Anat_sO)}

2Sn, with

log K values of 3.66, 3.68, 4.63, and 4.41, respectively, were required to model the sorption data.

EXPERIMENTAL. Anatase was obtained from [1]. It

was equilibrated in 0.01 M NaCl at least 24 h before use in sorption experiments and acid-base titrations.

Sorption experiments were conducted in the pH range 2 to 9 in a Jacomex glove box with O2 level less than 2 ppm. All test tubes were wrapped in aluminum foil to prevent photocatalytic redox reactions. We used 2 g/L anatase and 10 µM Sn(II) initial solution concentration. After equili- bration for 24 h, suspensions were centrifuged and Se(II) in the supernatant was determined by ICP-MS, while the wet-pastes were used to determine the interfacial reaction of Sn(II) with anatase by X-ray absorption spectroscopy (XAS) at The Rossendorf Beamline at ESRF using the Sn K-edge (29,200 eV). Surface complexation modelling and estimation of log k values was carried out by using PHREEQC and PEST codes and the MINTEQ.V4 data- base.

RESULTS.

Interfacial reactions of Sn(II) on anatase. The XANES

edge energy of 29,203 keV and a fit of the EXAFS coor- dination shell with 3 to 4 O atoms at a distance of 2.09 Å confirmed that the divalent oxidation state of Sn(II) was maintained in all samples. Two Sn–Ti distances of about 3.26 and about 3.58 Å were found, indicative of two dif- ferent binding coordinations. For the sample prepared with the shortest reaction time of 4 h under neutral pH, Sn(II) is coordinated to only one Ti atom at 3.60 Å indica- tive for the formation of a monodentate surface complex. A stronger Sn–Ti scattering at 3.26 and 3.58 Å with coor- dination numbers of 2 to 4 indicated formation of biden- tate surface complexes. These surface complexes were found in the samples prepared with longer reaction times (>4 h). The bidentate complexes may form either at two acid surface OH groups (bridging OH groups) or at two basic surface OH groups (terminal OH groups), exchang- ing H+ _{with cations and OH}− _{with anions, respectively.} The possible Sn(II) surface complexation structures are shown in Fig. 1. The first scheme in Fig. 1 depicts the formation of a Sn(II) complex on an O-vacancy of ana- tase.

Surface complexation modeling. The surface site density of anatase as investigated by fast backward titration was determined to be 15.94 sites/nm2_{. The two equilibrium} constants of the protolysis reactions were optimized as 4.76 ± 0.36 and −9.90 ± 0.42, which are in the expected range in line with previous work [2, 3]. Equations (1) to (3) describe the acid-base surface sorption of anatase and equations (4) to (7) represent the surface complexation reactions. Ca – Cb = H+ – OH− + Anat_sOH2+ – Anat_sO− (1) Anat_sOH + H+ _{→ Anat_sOH} 2+ (2) Anat_sOH → Anat_sO− _{+ H}+ ₍₃₎ Anat_sOH + Sn+2 _{→ Anat_sOSn}+ _{+ H}+ ₍₄₎

Anat_sOH + Sn(OH)2 → Anat_sOSnOH + H2O (5)

2Anat_sOH + Sn+2 _{→ Anat_sOH(Anat_sO)Sn}+ _{+ H}+ ₍₆₎

2Anat_sOH + Sn(OH)2 → (Anat_sO)2Sn +2H2O (7)

Note that positive and zero charges of the Sn(II) com- plexes Sn+2 _{and Sn(OH)2 facilitate the formation of inner-} sphere surface complexes. Not only monodentate but also bidentate complexes are feasible. Their optimized loga- rithmic equilibrium constants are 3.66, 3.68, 4.63, and 4.41, which provide a good fitting of the observed data (Fig. 2). Moreover, the formation of a bidentate complex without the release of H+ _{may take place (Eq. (6), as was} also proposed by Gunneriusson [4].

[1] Comarmond, M.J. et al. (2011) Environ. Sci. Technol. 45, 5536– 5542.

[2] Ludwig, C. et al. (1995) J. Colloid Interf. Sci. 169, 284–290. [3] Weng et al. (1997) Wat. Sci. Tech. 35, 55–62.

[4] Gunneriusson , L. (1993) PhD thesis. Fig. 2: Distribution of Sn(II) surface species.

2 3 4 5 6 7 8 9 0 20 40 60 80 100 % S n sorbe d pH TOT Anat_sOH(Anat_sO)Sn+ (Anat_sO)₂Sn Sn Anat_sOSn+ Anat_sOSnOH ~Ti Ti~ O ~Ti Ti~ OH HO OH Sn0,+ Sn+ ₍₂₎ (1) HO OH OH │ │ │ ~Ti─OH─Ti─OH─Ti~ Sn0 _Sn0_OH (3) (4)

HZDR | Institute of Resource Ecology | Annual Report 2012 ₄₅

Sorption of Se(IV) oxyanions onto maghemite