Presented paper

Geochemical Characterization Of Two-Filtered Fractions From Natural Waters Of The Iberian Pyrite Belt: Inferences For The Mobility Of Potentially Toxic Elements In Acid Mine Drainage Systems

Moreno, Filipa (1); Valente, Teresa (1); Gomes, Patrícia (1); Fonseca, Rita (2); Costa, Maria Rosário (3); Costa, Ana (1)
1: Universidade do Minho, Portugal; 2: Universidade de Évora, Portugal; 3: Universidade de Trás-os-Montes e Alto-Douro, Portugal

This work provides insight on the extent to which larger colloidal particles (sized between 0.45 mm and 0.20 mm) may be influencing the mobility of potentially toxic elements (PTE) in superficial and groundwaters from the Iberian Pyrite Belt (IPB). IPB is one of the most significant volcanogenic massive sulphide districts in the world, also representing a paradigmatic example of environmental impact triggered by intensive and long-lasting (more than 5000 years) mining activities. Mining has been risking water-quality through acid mine drainage (AMD), both in the Portuguese and Spanish sectors of the Pyrite Belt. The value of water resources is enhanced by the semi-arid climate of the region with increasing frequency (and magnitude) of droughts, responsible for water scarcity. In this context, the affinity of PTE for certain colloidal sizes, providing a fast transport pathway, is a factor that needs to be accounted for defining reactive mine waste-dumps remediation and/or risk assessment strategies associated to AMD.

Samples of different water types have been collected in 2018 along the Portuguese part of the IPB. Sampled waters have been filtered in the field with syringe filters of 0.45 mm and 0.20 mm pore sizes and acidified to pH <2.0. Both fractions were then analysed by inductively coupled plasma optical emission spectroscopy for the determination of metals (Al, Fe, Cd, Co, Cu, Mn, Ni, Pb and Zn) and arsenic (As) contents. Blanks, replicates and stock solutions were used to assess quality control.

The analyses revealed no (significant) differences between the two-filtered fractions, implying that PTE are in general attached to colloids smaller than 0.20 mm, nanoparticles (< 100 nm), offering attractive surfaces for extensive sorption of PTE, and/or in truly dissolved phases, all somewhat represented in this fraction. A greater contribution of the larger (0.45–0.20 mm) colloidal particles is observed first for Fe and then for Al, and more meaningful in alkaline waters. This probably reflects the (pH-related) abundance of Fe- and Al-oxyhydroxide rich-colloids within the 0.45–0.20 mm range. As regards Zn, it is hypothesized that the typically higher suspended load of the alkaline waters (relative to the more acidic ones) is affecting sorption-desorption reactions in larger colloids, resulting in even larger Zn loads in the <0.20 mm fraction.

The results achieved may thus have several implications (to be discussed) both on human-health or in environmental technology.

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