Direct evidence of arsenic(III)-carbonate complexes obtained using electrochemical scanning tunneling microscopy

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 chemical 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 NaClO4 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)2CO3- and As(OH)3(HCO3-)2. No complexes 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)2CO3-. The results also indicated that As(OH)3 could be associated with HCO3- 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.