Elementos de terras raras em sedimentos dos rios Negro e Solimões: concentrações basais, assinaturas geoquímicas, risco ecológico e sustentabilidade analítica na Amazônia

Abstract

Rare earth elements (REEs) are important tracers of geological, geochemical, and hydrosedimentary processes and have attracted growing environmental interest because of their expanding technological applications and the potential for enrichment associated with anthropogenic activities. This thesis aimed to improve the understanding of the occurrence, distribution, and major geochemical controls of REEs in Amazonian sediments, with emphasis on the Negro and Solimões rivers, by integrating a systematic literature review, geochemical characterization, environmental assessment, establishment of regional geochemical reference values, and evaluation of analytical sustainability. First, a systematic review conducted according to the PRISMA 2020 guidelines identified 12 eligible studies and showed that REE distribution in the Amazon is predominantly controlled by sediment provenance, tropical weathering, grain size, mineralogy, hydrochemical characteristics, and hydrosedimentary dynamics, although localized enrichments may occur in areas affected by mining activities. We then analyzed bottom sediments from 21 sampling sites, including 12 in the Negro River and 9 in the Solimões River, during the low-water season. Samples underwent microwave-assisted acid digestion, followed by multielement determination using ICP-MS. Data interpretation included geochemical normalization, fractionation ratios, Ce and Eu anomalies, environmental indices, and principal component analysis. Mean ΣREE concentrations were 121.28 ± 60.86 mg kg⁻¹ in the Negro River and 129.42 ± 19.01 mg kg⁻¹ in the Solimões River, with no statistically significant difference between the two systems, whereas heavy REEs occurred at significantly higher concentrations in the Solimões River. The Negro River exhibited greater spatial variability and more pronounced geochemical fractionation, whereas the Solimões River showed a more homogeneous signature, consistent with the stronger influence of Andean-derived particulate transport. Normalized REE patterns, fractionation ratios, and Ce and Eu anomalies revealed differences in sediment provenance and geochemical processes between the two river systems. Environmental indices indicated no relevant contamination and low mean ecological risk, supporting the predominance of geogenic controls in the investigated areas. Based on local concentration distributions, we established river-specific geochemical reference values (GRVs) for surface sediments from the Negro and Solimões rivers to support future regional environmental assessments. Evaluation of the microwave-assisted acid digestion and ICP-MS procedure using the AGREE and BAGI metrics indicated analytical robustness, moderate environmental sustainability, and high practical applicability. Overall, the findings show that REE distribution in Amazonian sedimentary systems results from interactions among geological provenance, weathering, river hydrochemistry, and sedimentary dynamics, highlighting the need for regionalized geochemical reference values and analytical approaches that combine metrological reliability, applicability, and sustainability for environmental monitoring in the Amazon.