Trace metal/metalloid concentrations in waste rock, soils and spontaneous plants in the surroundings of an abandoned mine in semi-arid NE-Brazil

Abstract

This study aims to determine the concentrations of trace metals/metalloid in waste rock, soils and plants in three different environments of an abandoned mine in Brazil. Waste rocks (X-ray fluorescence, X-ray diffractometry and SEM–EDS), soils (general characterization and potentially bioavailable nutrients and metals) and plants (total metal contents in shoots) were analyzed. Studied plant species included Bidens pilosa, Pityrogramma calomelanos, Ruellia paniculata, Combretum leprosum, Ziziphus joazeiro, Psidium guajava and Mangifera indica. The results showed that copper is the unique potentially toxic element in waste rocks, with concentrations up to 350,000 mg kg−1 in some rocks. The higher proportion of carbonates relative to sulphides naturally attenuates acidification due to sulphide oxidation, preventing acid drainage and soil acidification. Total Cu concentration in soils exceeded the reference values for soil quality in Brazil by 185, 78 and 18 times in the ore processing, waste rock and border areas, respectively, indicating a high contamination of soils even after 25 years of mine abandonment. Bioavailable Cu was positively correlated with inorganic carbon and phosphorus and negatively correlated with organic carbon in soils. Despite high Cu concentration in soils, the Cu levels in plant shoots were within the usual range, except for Bidens pilosa and Pityrogramma calomelanos, which showed 267 and 46 mg kg−1, respectively. Nevertheless, these concentrations are below the levels required to be considered hyperaccumulator species. This indicates that the ability to immobilize Cu in the roots/rhizosphere should be further studied due to its potential to remediate by immobilization and/or revegetate Cu-contaminated soils.