Assessment of metal distribution in different Fe precipitates related to Acid Mine Drainage through two sequential extraction procedures

In the last decades, the contamination by metals from mining areas, especially that due to Acid Mine Drainage (AMD), has become a hot topic for the scientific community. The precipitation of secondary minerals related to AMD is the most important natural method of scavenging elements from such solutions. These minerals are a dynamic system, in which mineralogical and chemical transformations could be triggered by sudden changes of the chemical-physical conditions. Therefore, minerals related to AMD could merely act as temporary sink for inorganic pollutants, and understanding the stability over time of metals in these phases is very important. This paper aims to evaluate metal lability in 4 different natural precipitates related to AMD by using two different sequential extraction procedures. Two samples were composed of goethite and jarosite, one sample of schwertmannite, and the last sample of an amorphous Fe phase. The results obtained allowed concluding that the amorphous Fe phase was able to scavenge a high amount of metals from the flowing solutions, but it was also the most reactive compound. Therefore, this phase represented a major threat for the environment. The results obtained showed that the two sequential extraction procedures were not able to dissolve completely the Fe phases, probably because the sample-extractant volume ratio and the time and nature of contact was not optimised for this kind of samples. For this reason, the results of sequential extraction procedures on AMD impacted materials could be unreliable in case of Fe oxyhydroxide very rich samples