Arsenic (As) is a toxic and highly mobile contaminant, which forms dangerous watersoluble compounds for human health . Anthropogenic sources and aquatic organisms allow the methylation of inorganic arsenicals to monomethylated (MMA), dimethylated (DMA), and trimethylated (TMA) arsenic species , which usually have a lesser extent than inorganic ones; however, increasing herbicides and pesticides worldwide makes them a dangerous water and groundwater pollutant (Scheme 1a for molecular structures). Moreover, trivalent and pentavalent methylarsenicals promote diabetes via distortion of glucose metabolism . Due to the latter, the World Health Organization (WHO) has adopted regulatory standards to limit the arsenic concentration in drinking water, imposing a limit value of 10 μg/L . Additionally, the toxicity of methylarsenicals depends on their methylation degree and pH ; pentavalent TMA V , MMA V, and DMA V species are less toxic than inorganic pentavalent arsenicals (iAs V ); while the trivalent MMA III , DMA III, and TMA III species are much more toxic than inorganic trivalent forms (iAs III ). Thus, the toxicity depends on the uptake rate of each arsenic compound where less toxic species (pentavalent forms) can be converted to the more toxic species (trivalent forms) increasing in the following order; TMA V < MMA V ≈ DMA V < iAs V < <iAs III < MMA III < DMA III < TMA III .