RESUMO
Long-term stability of arsenic residues is investigated by determining arsenic phases remaining in gold mining residues after two decades of impoundment. The residues, generated by arsenic coprecipitation with iron and lime, were disposed of in-lined sites for 9-16 years (pit C) and 16-23 years (pits A and B). Arsenic is present in the residues as As(V) species, predominantly in the form of amorphous iron arsenate (55-75% As(total,) pits A and B; 55-70% As(total), pit C) and sorbed onto amorphous iron-oxyhydroxides (20-33% As(total), pits A and B; 22-37% As(total), pit C). The presence of minor Ca-arsenate phases (undefined composition) and Al-arsenate coprecipitates is also indicated. The passive enrichment of iron in pits A and B, and the relative low concentration of calcium, sulfur and arsenic if compared to those of pit C, suggest that a soluble Ca-arsenate phase (e.g. CaHAsO(4).H(2)O), a fraction of gypsum and As(III) were dissolved along 16-23 years of residue disposal. The presence of As(V) only and excess iron demonstrates the importance of the oxidation state and high Fe/As ratio on long-term stability of arsenic residues.
Assuntos
Arsênio/análise , Mineração/métodos , Resíduos/análise , Arsênio/química , Cálcio/análise , Precipitação Química , Ouro , Compostos de Ferro/análise , Cinética , Microscopia Eletrônica de Varredura , Oxirredução , Espectrometria por Raios X , Espectrofotometria Atômica , Enxofre/análise , Espectroscopia por Absorção de Raios X , Difração de Raios XRESUMO
A fast and accurate microwave-assisted digestion method for arsenic determination by flame atomic absorption spectrometry (FAAS) in typical, complex residues from gold mining is presented. Three digestion methods were evaluated: an open vessel digestion using a mixture of HCl:HNO(3):HF acids (Method A) and two microwave digestion methods using a mixture of HCl:H(2)O(2):HNO(3) in high (Method B) and medium-pressure (Method C) vessels. The matrix effect was also investigated. Arsenic concentration from external and standard addition calibration curves (at a 95% confidence level) were statistically equal (p-value=0.122) using microwave digestion in high-pressure vessel. The results from the open vessel digestion were statistically different (p-value=0.007) whereas in the microwave digestion in medium-pressure vessel (Method C) the dissolution of the samples was incomplete.