Determination of metals by total reflection X-ray fluorescence and evaluation of toxicity of a river impacted by coal mining in the south of Brazil
Introduction
Acid mine drainage (AMD) is one of the most serious environmental pollution problems in the mining industry. It is currently a global problem. All over the world, studies have been conducted in order to remediate or to monitor effluents contaminated with AMD [1], [2], [3], [4], [5], [6]. Acid water loaded with toxic heavy metals is generated by the weathering of sulfide minerals (such as pyrite, FeS). Oxidation engenders a series of reactions (S−2 is converted to SO4−2) that are responsible for acids and metals leaching into the environment, which continue releasing acidic water long after the mine has closed. The effects of AMD on bodies of water can be perceived as far away as 23,000 km [7] and can persist for more than a century [8], [9]. Furthermore, the typically low pH values found in AMD can cause metals to be very soluble in water. For instance, concentrations as high as >10 mg L−1 of Fe and up to 50 mg L−1 of Mn have already been reported for AMD [10].
In the south of Brazil (Santa Catharina's department), there are over 1000 abandoned mines. Furthermore, some of the active mines do not have treatment plants for their effluent. Rivers in this region are considered to be dead, since in addition to receiving AMD, domestic effluent and agricultural discharge also increase the toxicity level, causing serious problems for aquatic ecosystems. When the fact that this pollution has occurred over the last 30 years is accounted for, the rivers’ sediments might also have suffered important alterations.
In exploratory studies, the metal concentrations in effluents from this region were measured using atomic absorption spectroscopy. These water samples came from AMD, which had both mining and benefiting processes, and were collected before and after a treatment system. It is worth mentioning that benefiting process of coal has steps of crushing and washing, the latter aiming at removing the coal fine before heating the coal to transform it into coke. Data regarding the pH values, metal concentrations, and sulfate concentrations are shown in Table 1.
According to Table 1, the pH was very low (2.7–3.5). Sulfate levels (1587–8412 mg L−1) and metal concentrations, specifically Fe (69–700 mg L−1), Mn (7.4–47 mg L−1), and Zn concentrations (1.6–5.4 mg L−1), were very high. These results alert researchers to the risk of potential impacts on the aquatic ecosystem in this region's streams. In order to evaluate the degree of contamination in this region, the metal concentration (Fe, Ni, Cd, Mn, Zn, Cr, and Pb) and ecotoxicity of water and sediments collected from the Mãe Luzia River were investigated. Samples were collected at five different stations, from the source to the river mouth of the Ararangua estuary (Atlantic Ocean), as shown in Fig. 1. The flow rates roughly vary between 13 (Mãe Luzia River) to 142 m3 s−1 (Fiorita River). The source (P1) was considered as control sampling point.
Metal contents were determined by total reflection X-ray spectroscopy (TXRF), while ecotoxicity tests were conducted using Daphnia magna. This bioindicator has been used to determine the ecotoxicological level of sewage sludge [11], textile [12], and AMD effluents [13].
Section snippets
Material cleaning procedures
Cleaning solutions were prepared with bi-distilled HNO3 (Merck). All materials were washed with Milliq® water, followed by washing with bi-distilled acid cleaning solution (5%) and rinsing three times with Milliq® water. Metal solutions for the calibration and dilution of samples were prepared using a Titrisol® stock solution (1000 mg L−1) purchased from Merck Co., using the Milliq® water purification system (R = 18.2 MΩ cm−1 at 25 °C). Polyethylene bottles were decontaminated with HNO3 solution for 24
Results and discussion
Table 2 shows the metal concentrations of the six metals evaluated in water samples collected at the five points, as represented in Fig. 1.
According to Table 2, Cr and Pb were not detected in the river. In P1, a high concentration of Fe and Ni were detected (3.1 and 2.2 mg L−1). Mn, Zn, and Cd concentrations in P1 were much lower: 0.17, 0.14, and 0.16 mg L−1, respectively. The Fe concentration increased up to P4, the station at which all the metals showed the highest concentrations. Mn was 1.8 mg L−1
Conclusions
Water and sediment collected from the Mãe Luzia River (south Brazil) at five different stations, passing by a region impacted by coal mining processing, had different metal concentrations, depending on the proximity to the source, the AMD discharge regions, or the river mouth. In the samples evaluated, sediments affected by AMD discharge, which had a high concentration of metals, were shown to be toxic to the bioindicator. A combination of TXRF analysis and ecotoxicity tests with D. magna was
Acknowledgments
This work was partially financed by CNPq and FAPERGS/PRONEX. The authors would like to thank the LNLS (Project D09B-XRF # 4615/05).
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