Chromium removal via coprecipitation with carbonates and iron oxyhydroxides minerals: The effect of organic complexing agents

With the recent review of the European Environmental Regulations for Water Quality, which plan to reduce contaminant concentration limits in drinking and surface waters by 2036, an increasing demand for wastewater depollution arises. Metastable amorphous nanoprecursors of iron oxyhydroxides (IOH) and calcium carbonates (CC) interact with various pollutants in natural systems via processes such as coprecipitation and adsorption. In this research, ferrihydrite (FHY) and calcium carbonates were testes for Cr (VI) retention to cover acidic to neutral environments and neutral to basic media, respectively. An original and novel approach is proposed for investigating Cr uptake capacity and mechanisms as well as minerals transformation in the presence of Cr (VI) and two biodegradable organic compounds (EDTA - ethylenediaminotetraacetic acid and EDDS - ethylenediamine-N,N'-disuccinic acid). Geochemical modelling of Cr coprecipitated species was also considered. TCLP (toxicity characteristic leaching procedure) experiments revealed that Cr remains fixed in FHY structure under near neutral conditions. One year old ferrite's stability in the presence of Cr and organic compounds was examined and showed that the addition of organics and Cr slowed FHY transformation into crystalline polymorphs. Cr (VI) sequestration by ferrites also occurred as reduced Cr (III) - its less toxic specie. Quantitatively, higher amount of Cr (VI) was removed by coprecipitation (up to 97.65 mg/g) then adsorption (ca. 33.76 mg/g). These findings provide insight into the uptake processes that influence Cr (bio)availability and mobility, that affect the elemental cycle, including transition across the trophic chain as well as at contaminated environments (i.e., mining sites and acid mine drainage) were ferrihydrite form naturally.