New insights in the mechanisms of arsenic association with iron oxides in the environment

A mechanism responsible for long-term As fixation in the environment is described here for the first time. Arsenic (1.6 ± 0.5 wt%) was unambiguously identified in mesocrystals of Al-hematite with an As/Fe molar ratio of 0.026 ± 0.006 in environmental samples. The As-bearing Al-hematite is interpreted as a secondary phase formed from iron (hydr)oxides. Experimental evidence supports a mechanism whereby arsenic adsorption onto Al-ferrihydrite Nanoparticles (NPs) is followed by Al-ferrihydrite aggregation in a process of self-assembly Oriented Attachment (OA) to produce nanosized Al-hematite mesocrystals, with goethite as a possible intermediate phase. Arsenic species are incorporated and thus irreversibly trapped in the aggregates. Structural Al was identified in all the As-bearing iron (hydr)oxides. Aluminum plays a major role in increasing As adsorption onto ferrihydrite NPs, controlling the NPs size, and favoring crystal aggregation by decreasing the NPs surface energy to promote attachment events