Fases portadoras do arsénio em solos da área mineira de São Domingos e em solos não contaminados do Pomarão e Serra do Caldeirão Arsenic containing phases in soils from São Domingos mining area and in non-contaminated soils from Pomarão and Serra do Caldeirão

Abstract

A mina de São Domingos situa-se a SE de Portugal na Faixa Piritosa Ibérica. No tempo dos romanos foi feita a exploração intensiva de ouro, cobre e prata a partir do gossan e, nos tempos modernos fez-se também a exploração dos sulfuretos maciços de cobre com teores elevados de arsénio, zinco e chumbo. Para avaliar a perigosidade que representa o arsénio para o meio determinou-se a sua distribuição pelas várias fases suporte nos solos desenvolvidos sobre as escombreiras de gossan, tendo como referência solos não contaminados do Pomarão (SE Portugal) e da Serra do Caldeirão (S Portugal). Os solos (fracção São Domingos mining area is located in the Iberian Pyrite Belt, in south-east Portugal. Intensive gossan gold, copper and silver mining was done in roman times. More recently, mining activities were concentrated in the copper, zinc, arsenic and lead rich sulfide orebodies. To evaluate arsenic hazard, soils developed on the gossan mine waste piles were analysed, using as reference non-contaminated soils from Pomarão (southeast Portugal) and Serra do Caldeirão (south of Portugal). Soils were characterized by the classical methodologies for pH (H2O), particle size distribution, cation exchange capacity, Fe and Mn oxides, and mineralogy. The soils (fraction <2 mm) were analysed, for total concentrations of As, Cu, Fe, Mn, Pb and Zn by instrumental neutron activation analysis after acid digestion and, for the concentration of As associated with the different soil phases (exchangeable complex, organic matter, total and non-crystalline Fe oxides and Mn oxides), by hydride generation atomic absorption spectrometric after selective chemical extractions with appropriate reagents using the parallel method. São Domingos soils contain high levels of total As (1940-3030 mg kg-1) but the available fraction is very low (<0,02% from total) being lower than Pomarão and Serra do Caldeirão available fractions (respectively, 0,13 and 1% from total). Arsenic is mainly in the residual fraction of the soils of São Domingos mining area (91-94% of total) while in the Pomarão and Serra do Caldeirão reference soils arsenic is mainly associated with crystalline Fe oxides (respectively, 24 and 37% of total). The arsenates: carminite (Fe2Pb(AsO4)2(OH)2), mimetite (Pb5(AsO4)3Cl), segnitite (Fe3Pb(AsO4)(HAsO4)(OH)6), and seldom kankite (FeAsO4•3.5H2O) were identified in São Domingos soils. The low solubility of these solid phases can explain the low availability of arsenic present in these soils in spite of the high concentrations of total arsenic. The results show that, under the present physical-chemical conditions, arsenic in the soils of the São Domingos mine presents no environmental hazard.