CHEMICAL FRACTIONATION AND ENVIRONMENTAL RISK ASSESSMENT OF POTENTIALLY TOXIC ELEMENTS IN TOPSOILS OF THE CAATINGA BIOME

The distribution and mobility of potentially toxic elements were evaluated in 50 topsoils from the Caatinga biome in Brazil using BCR (Community Bureau of Reference) sequential extraction, the risk assessment code (RAC), and principal component analysis (PCA). The residual fraction was dominant for most elements, with concentrations reaching up to 154 mg kg-1 for Ni and 82 mg kg-1 for Cr. However, significant mobility was observed in the acid-soluble fraction, particularly for Ni (0.16-10 mg kg-1) and Cd (0.12-1.24 mg kg-1), whereas Pb (0.75-15 mg kg-1) and Zn (0.34-4.6 mg kg-1) showed moderate mobility. Consequently, RAC classified Ni (13.3%) and Cd (12.7%) as medium environmental risk, while other metals showed low risk. PCA revealed that soil pH correlated positively with Ni, Cu, and Cr, and negatively with Zn, Cd, and Pb. Organic matter and soil texture were also important in influencing metal mobility. These findings indicate that, despite the geogenic origin of these soils, the high mobility of Cd and Ni in specific pH conditions poses potential ecological risks. This suggests that environmental monitoring in the semi-arid Caatinga ecosystem should focus on bioavailable fractions rather than total content to prevent trophic transfer effectively.