How crucial is the impact of calcium on the reactivity of iron-organic matter aggregates? Insights from arsenic

nvironmental iron-organic matter (Fe-OM) aggregates play a major role in the dynamic of pollutants. Nowadays, there is a lack of information about the control exerted by their structural organization on their reactivity towards metal(loid)s and in particular, the impact of major ions, such as calcium. The sorption capacity of mimetic environmental Fe-OM-Ca aggregates was investigated relative to the Fe/organic carbon (OC) and Ca/Fe ratios using As as a probe. It was shown that Fe speciation is the key factor controlling the reactivity of Fe-OM-Ca aggregates regarding the high affinity of Fe(III)-oligomers towards As and the high sorption capacity of ferrihydrite-like nanoparticles. Moreover, when it occurs at high concentration, Ca competes with Fe for OM binding leading to an increase in the amount of ferrihydrite-like nanoparticles and binding site availability. As a consequence, Ca not only impacts the ionic strength but it also has a dramatic impact on the structural organization of Fe-OM aggregates at several scales of organization, resulting in an increase of their sorption capacity. In the presence of high amounts of Ca, Fe-OM-Ca aggregates could immobilize pollutants in the soil porous media as they form a micrometric network exhibiting a strong sorption capacity.

Automated summary

The study shows that the Fe speciation is the key factor controlling the reactivity of Fe-OM-Ca aggregates towards As, with high concentrations of Ca leading to an increase in the availability of ferrihydrite-like nanoparticles and binding sites, enhancing their sorption capacity. Additionally, high levels of Ca compete with Fe for OM binding, impacting the structural organization of Fe-OM aggregates and increasing their sorption capacity.