Direct Evidence of Arsenic(III)-Carbonate Complexes Obtained Using Electrochemical Scanning Tunneling Microscopy Mei-Juan Han, † Jumin Hao, † Christos Christodoulatos, † George P. Korfiatis, † Li-Jun Wan, ‡ and Xiaoguang Meng* ,† Center for Environmental Systems, Stevens Institute of Technology, Hoboken, New Jersey 07030, and Institute of Chemistry, Beijing National Laboratory for Molecular Sciences, Chinese Academy of Sciences (CAS), Beijing 100080, China Electrochemical scanning tunneling microscopy (ECSTM), ion chromatography (IC), and electrospray ionization- mass spectrometry/mass spectrometry were applied to investigate the interactions between arsenite [As(III)] and carbonate and arsenate [As(V)] and carbonate. The chemi- cal species in the single and binary component solutions of As(III), As(V), and carbonate were attached to a Au- (111) surface and then imaged in a 0.1 M NaClO 4 solution at the molecular level by ECSTM. The molecules formed highly ordered adlayers on the Au(111) surface. High-resolution STM images revealed the orientation and packing arrangement of the molecular adlayers. Matching the STM images with the molecular models constructed using the Hyperchem software package indicated that As- (III) formed two types of complexes with carbonate, including As(OH) 2 CO 3 - and As(OH) 3 (HCO 3 - ) 2 . No com- plexes were formed between As(V) and carbonate. IC chromatograms of the solutions revealed the emergence of the new peak only in the aged As(III)-carbonate solution. MS spectra showed the presence of a new peak at m/z 187 in the aged As(III)-carbonate solution. The results obtained with the three independent methods confirmed the formation of As(OH) 2 CO 3 - . The results also indicated that As(OH) 3 could be associated with HCO 3 - through a hydrogen bond. The knowledge of the formation of the As(III) and carbonate complexes will improve the understanding of As(III) mobility in the environment and removal of As(III) in water treatment systems. Arsenic is a common contaminant in groundwater worldwide. Long-term exposure to arsenic can cause skin, lung, urinary bladder, liver, and kidney cancer in humans. 1-5 The most common arsenic species in natural water and sediment are As(III) and As- (V). 6 As(III) is more toxic to humans and has higher mobility in the environment than As(V). The mobility of the arsenic species and their adsorption by metal oxides and hydroxides are influ- enced by common anions such as phosphate, silicate, and carbonate. 7,8 This effect is usually attributed to competitive adsorption of the anions and arsenic species on the solid surface. Recently, Kim et al. 9 and Lee and Nriagu 10 proposed the formation of As(III)-carbonate complexes, which increases the mobility of arsenic in groundwater aquifers. Neuberger and Helz 11 confirmed this hypothesis by measuring the solubility of As 2 O 3 in concentrated carbonate solutions. However, quantum chemical calculations indicate that As(III) carbonato complexes predicted from the lanthanide correlation are very unstable relative to arsenious acid H 3 AsO 3 . 12 Since carbonate is the most abundant anion in groundwater and surface water, further studies are necessary to determine the interactions between As(III) and carbonate. The invention of scanning tunneling microscopy (STM) in the early 1980s 13-15 enabled imaging of chemical species at molecular and atomic resolution. STM is a surface analysis technique that probes the electronic properties of surfaces. 16 It can provide atomic resolution micrographs of individual molecules deposited on a solid surface in solution. 17 STM has been used to distinguish chiral molecules 18-19 and investigate the reaction mechanisms of * To whom correspondence should be addressed. E-mail: xmeng@ stevens.edu. † Stevens Institute of Technology. ‡ Beijing National Laboratory for Molecular Sciences. (1) Cullen, W. R.; Reimar, K. J. Chem. Rev. 1989, 89, 713. (2) WHO. Arsenic, Environmental Health Criteria 18; World Health Organiza- tion: Geneva, 1983. (3) Evaluation of Carcinogenic Risks to Humans, Overall Evaluations of Carci- nogenicity: An Updating of IARC Monographs, Vols. 1-42; Volume Supple- ment 7; International Agency for Research on Cancer: Lyon, 1987; p 100. (4) Wu, M. M.; Kuo, T. L.; Hwang, Y. H.; Chen, C. J. Am. J. Epidemiol. 1989, 130, 1123. (5) Bates, M. N.; Smith, A. H.; Hopenhayn-Rich, C. Am. J. Epidemiol. 1992, 135, 462. (6) Smith, A. H.; Hopenhayn-Rich, C.; Bates, M. N.; Goeden, H. M.; Hertz-, Picciotto, I.; Duggan, H. M.; Wood, R.; Kosnett, M. J.; Smith, M. T. Environ. Health Perspect. 1992, 97, 259. (7) Meng, X.; Bang, S.; Korflatis, G. P. Water Res. 2000, 134, 1255. (8) Meng, X. G.; Korfiatis, G. P.; Christodoulatos, C.; Bang, S. B. Water Res. 2001, 35, 2805. (9) Kim, M.-J.; Nriagu, J.; Haack, S. Environ. Sci. Technol. 2000, 34, 3094. (10) Lee, J. S.; Nriagu, J. O. Am. Chem. Soc., Symp. Ser. 2003, 835, 33. (11) Neuberger, C. S.; Helz, G. R. Appl. Geochem. 2005, 20, 1218. (12) Tossell, J. A. Am. Chem. Soc., Symp. Ser. 2004, 915, 118. (13) Binnig, G.; Rohrer, H. Helv. Phys. Acta 1982, 55, 726. (14) Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Appl. Phys. Lett. 1982, 40, 178. (15) Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Phys. Rev. Lett. 1982, 49, 57. (16) Wiesendanger, R.; Bode, M.; Dombrowski, R.; Getzlaff, M.; Morgenstern, M.; Wittneven, C. Jpn. J. Appl. Phys. 1998, 37, 3769. (17) Weiss, P. S.; Eigler, D. M. Phys. Rev. Lett. 1993, 71, 3139. (18) Xu, Q. M.; Wang, D.; Wan, L. J.; Wang, C.; Bai, C. L.; Feng, G. Q.; Wang, M. X. Angew. Chem., Int. Ed. 2002, 41, 3408. Anal. Chem. 2007, 79, 3615-3622 10.1021/ac062244t CCC: $37.00 © 2007 American Chemical Society Analytical Chemistry, Vol. 79, No. 10, May 15, 2007 3615 Published on Web 04/19/2007