Vibrational spectroscopic characterization of some environmentally important organoarsenicals — A guide for understanding the nature of their surface complexes

Abstract

Organoarsenicals are found in the environment from the biomethylation of inorganic arsenic compounds and from anthropogenic sources. It is clear that organoarsenicals pose a health and an environmental risk due to their potential cycling to the most toxic forms of arsenic as a result of redox activity in soils and natural waters. The environmental fate of arsenic compounds depends to a large extent on the surface interactions with geosorbents, mainly minerals and organic matter. Hence, elucidating the nature of surface complexes is important in understanding binding mechanisms and thermodynamics. In this paper, we report the vibrational spectra of a number of organoarsenicals in the aqueous and solid phases using attenuated total internal reflectance Fourier transform infrared (ATR-FTIR), transmission FTIR, and Raman spectroscopies. Analysis of the aqueous phase spectra revealed that for completely deprotonated anions, increasing the organic substituents on the AsOx moiety results in increasing the frequency of v(AsOx), whereas the opposite trend is observed for completely protonated molecules. Analysis of solid phase spectra showed that incorporation of water molecules in the crystalline structure and extensive hydrogen bonding with neighboring molecules significantly affect As–O bond lengths and hence frequencies of v(AsOx). Results are discussed in the context of identifying geometry of organoarsenicals surface complexes in situ using the ATR-FTIR technique.Key words: ATR-FTIR, organoarsenicals; oxyanion adsorption, arsenate, in situ spectroscopy.

In situ ATR–FTIR and surface complexation modeling studies on the adsorption of dimethylarsinic acid and p-arsanilic acid on iron-(oxyhydr)oxides

“…1 has been discussed in detail in our recent publication reporting adsorption mechanism of DMA [34]. Briefly, the assignment of the observed frequencies was based on comparison with the spectra of bulk DMA in the liquid (pK a = 6.1) and solid acid and salt phases [37], desorption kinetics due to chloride and phosphate anions, and aided by frequency calculations on geometry-optimized clusters of DMA-iron oxide clusters. Our data suggest the simultaneous formation of inner-and outer-sphere DMA(ads), which gives rise to spectral components in the range 700-880 cm À1 .…”

“…Relative to arsenate, these two organoarsenicals replace hydroxyl groups with two methyl and one aromatic groups, respectively. We examined earlier the effect of organic substitution and protonation on the stretching frequency of As-O groups, m(As-O) [37,38]. Here, spectroscopic investigations of the adsorption isotherms of DMA on hematite and goethite were carried out -for the first time using ATR-FTIR-using solution concentrations as low as 1 lM at pH 7 and I = 0.01 M. The adsorption isotherms of p-AsA from ATR-FTIR measurements were reported earlier [35] at pH 7, where values of binding constants were extracted from Langmuir model fits.…”

Arsenic contamination of natural waters in San Juan and La Pampa, Argentina

“…), which was measured in up to 97 % of water samples from both San Juan and La Pampa. Organoarsenicals can arise through anthropogenic activity (Cowen et al 2008;Vaclavikova et al 2008;McSheehy et al 2003). The community of Encon is principally an arid semi-desert rural community with local goat farming being the main agricultural activity.…”

Synthesis and characterization of cellulose-goethite composites and their adsorption properties with roxarsone

“…Raman spectroscopy after and before sorption phase of roxarsone has a Raman band at 636 cm -1 assigned to the As-C stretching and a band at 809 cm -1 due to As-O stretching (Cowen, Duggal, Hoang & Al-Abadleh, 2008). The bands at 1152…”