Role of competing ions in the mobilization of arsenic in groundwater of Bengal Basin: Insight from surface complexation modeling

Arsenic in Porewaters of the Unsaturated Zone of an Argentinean Watershed: Adsorption and Competition with Carbonate as Important Processes that Regulate its Concentration

“…The value used for K CO3 was the one reported by Stachowicz et al (2007) for carbonate adsorption on goethite because no complexation Table 2 constant was found in the literature for the case of ferrihydrite in calculations with the CD-MUSIC model. Biswas et al (2014), Jessen et al (2012) and Charlet et a1. (2007), for example, used the carbonate adsorption constants on ferrihydrite informed by Appelo et al (2002), but they correspond to the 2-pK diffuse layer model; thus, they cannot be applied here.…”

“…Under oxidizing conditions, the iron ''oxides'' group (including ferrihydrite, goethite, nanogoethite, hematite, Fe(III) hydroxides coating clay surfaces) is believed to be the main responsible in regulating arsenic concentration in groundwaters through adsorption-desorption process (Smedley and Kinniburgh 2002;Swartz et al 2004;Biswas et al 2014). The most reactive oxide fraction is generally assumed to be ferrihydrite, because of its high specific surface area and high adsorption capacity Biswas et al 2014).…”

“…Under oxidizing conditions, the iron ''oxides'' group (including ferrihydrite, goethite, nanogoethite, hematite, Fe(III) hydroxides coating clay surfaces) is believed to be the main responsible in regulating arsenic concentration in groundwaters through adsorption-desorption process (Smedley and Kinniburgh 2002;Swartz et al 2004;Biswas et al 2014). The most reactive oxide fraction is generally assumed to be ferrihydrite, because of its high specific surface area and high adsorption capacity Biswas et al 2014). In fact, the arsenate adsorption capacity of ferrihydrite (about 700 lmol g -1 ) is much higher than that of clay minerals such as kaolinite, montmorillonite and illite (\10 lmol g -1 in both cases) (Luengo et al 2011), meaning that minimum contents of ferrihydrite or similar hydrous Stachowicz et al (2007) Aquat Geochem ferric oxides will strongly affect or even control arsenate adsorption.…”

“…There are also regions with high As contents that are not associated with mining processes or geothermal activity. If sediments are subject to reducing conditions, As occurrence in groundwater is normally due to reductive dissolution of Fe (oxy)hydroxides (Nickson et al 2000); if they are subject to oxidizing conditions, where Fe (oxy)hydroxides do not dissolve appreciably, the control of As concentration is usually attributed to adsorption-desorption processes on mineral surfaces Biswas et al 2014). In cases where the environment is not iron rich, but other metal cations (e.g., Ca 2?…”

“…Smedley et al (2005) used also the diffuse layer surface complexation model to calculate distribution coefficients (K d ) of As in the presence and the absence of competitors and could observe low K d values (weak binding) in the second case. More recently, Biswas et al (2014) explored the ability of two different surface complexation models, the mentioned diffuse layer model and the 3-plane CD-MUSIC model van Riemsdijk 1996, 2006), to describe As adsorption and competition on groundwaters and sediments of the Bengal Basin. They found that reductive dissolution of Fe oxyhydroxides and competitive adsorption reactions are responsible for As enrichment in groundwaters.…”