COMPORTAMENTO GEOQUÍMICO DO FLÚOR EM ÁGUAS E SEDIMENTOS FLUVIAIS DA REGIÃO DE CERRO AZUL, ESTADO DO PARANÁ, BRASIL

Abstract

Fluorine is an essential element for human health, but consumption of water with F- in excess will cause adverse effects in teeth and bones (fluorosis). According to WHO, a maximum of 1,5 mg/L F- is recommended for drink- ing water, but this level may vary mainly according to local climatic conditions. Some fluorspar deposits are found in the Ribeira Valley (SP-PR) in certain anomalous areas for fluorine as shown in previous geochemical studies using stream sediments and pan concentrates. This work was conceived to assess the quality of surface waters and sediments in the Cerro Azul region, where the Volta Grande and the Mato Preto fluorspar deposits are known. The water quality parameters (pH, Eh, electric conductivity, DO, temperature, turbidity and TDS) as well as alkalinity were measured in situ. Filtered (<0.45 µm) water samples were analyzed by ionic chromatography for anions and by ICP-OES for cations. The fluorine contents were determined by ion selective electrode (ISE) yielding concentrations from 0.07 to 2.54 mg/L F- . The highest F- concentrations coincide with the Mato Preto deposit sur- rounding area and, these samples were also characterized by higher Ca2+ , Sr2+ and Ba2+ contents in relation to those located far from the mine. Nevertheless, the water consumed by local population showed F- concentrations within the recommended limits. The hydrogeochemical results indicated in the area that surrounds the Mato Preto deposit, the surface waters used for human consump- tion, presents some risk for health and sanity. Positive correlations of F with Si, Ca2+ , Li+ and K+ in surface water allowed to infer that F- could be a result from the alteration of silicates such as biotite and hornblende, and even from the dissolution of fluorspar. Additionally, the bulk chemical compositions of river sediments were analyzed in the < 177 μm and < 63 μm fractions by XRF and, for fluorine content by the method of alkaline fusion + ISE. Concentrations of F vary in the interval 330 to 1300 μg/g F and generally the highest values were obtained in the <63 μm fraction. X-ray diffraction and scanning electron microscopy analysis allowed infer some of the possible mineral phases that contribute with fluorine to the fluvial sediments: micas (biotite), hornblende, illite and smectite as well as fluorspar and apatite.