Speciation and quantification of Hg in sediments ...

Environ Monit Assess (2018) 190:49 https://doi.org/10.1007/s10661-017-6394-4

Speciation and quantification of Hg in sediments contaminated by artisanal gold mining in the Gualaxo do Norte River, Minas Gerais, SE, Brazil Valdilene da Penha Rhodes & Jorge Carvalho de Lena & Camila Vidal Alves Santolin & Thais da Silva Pinto & Louise Aparecida Mendes & Cláudia Carvalhinho Windmöller

Received: 13 June 2017 / Accepted: 5 December 2017 # Springer International Publishing AG, part of Springer Nature 2017

Abstract The Iron Quadrangle in SE Brazil was, in the eighteenth century, one of the most important Au producing regions of Brazil. In this region, gold is produced, even today, by artisanal methods that use Hg to increase the extraction efficiency with no control of Hg release to water systems and the atmosphere. In this context, the Gualaxo do Norte River is of particular

V. da Penha Rhodes : J. C. de Lena (*) Departamento de Geologia, Escola de Minas, Campus Morro do Cruzeiro, Universidade Federal de Ouro Preto, Ouro Preto, MG 35400-000, Brazil e-mail: [email protected] V. da Penha Rhodes e-mail: [email protected] C. V. A. Santolin : T. da Silva Pinto : L. A. Mendes : C. C. Windmöller Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, Av. Antônio Carlos, 6627, Belo Horizonte, MG 31270-901, Brazil

C. V. A. Santolin e-mail: [email protected] T. da Silva Pinto e-mail: [email protected] L. A. Mendes e-mail: [email protected] C. C. Windmöller e-mail: [email protected]

interest; its springs are located in the Doce River basin, an important Brazilian basin that supplies water for 3.5 million people. The main goal of this work was to quantify and speciate the Hg in the sediments of the Gualaxo do Norte River using a direct mercury analyzer and gas chromatography-pyrolysis-atomic fluorescence detection system. Statistical analyses consisted of principal component analysis, aiming to assess interactions among elements and species and to group the variables in factors affecting the properties of sediment. The results show that total Hg (THg) and methylmercury (CH3Hg+) concentrations in samples ranged from 209 to 1207 μg kg−1 and from 0.07 to 1.00 μg kg−1, respectively (methylation percentages from 0.01 to 0.27%). Thermal desorption analysis showed that mercury is mainly present in the oxidized form, and correlation analyses pointed to a relationship between THg and MnO, indicating that manganese can oxidize and/or adsorb Hg. Together, MO and CH3Hg+ are important parameters in the third principal component, indicating the influence of OM on the methylation process. This first investigation on Hg methylation in this small-scale gold mining area points to the possibility of Hg bioaccumulation and to the need of better understanding the biogeochemical cycle of Hg in this area. Samples were collected in 2012, prior to the 2015 Fundão Dam disaster. The results are also a record of the characteristics of the sediment prior to that event. Keywords Mercury speciation . Gualaxo do Norte River . Doce River basin . Sediment . Small-scale gold mining activities

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Introduction Mercury has been known to mankind since ancient times. The first detailed description of Hg hazard was given by Stock (1926), an inorganic chemist who worked with boron and silicon chemistry and used mercury in vacuum equipment. Only in the 1960s, after the accident in Minamata Bay, Japan, the risks posed by Hg contamination and its consequences were understood (Lacerda and Solomons 1998). Hg may be considered a significant public health problem due to its effects on the nervous system, which include mental retardation, cerebral palsy, deafness, blindness, and dysarthria. Once released into the environment, metallic mercury undergoes chemical transformations (Barkay et al. 2003; Tomiyasu et al. 2000). In sediment, by the action of bacteria and/or abiotic agents, it is transformed into its cationic species (Hg2+ and Hg22+) and into methylated species (Hg(CH3)2 and CH3Hg+). The latter may also be transformed back to Hg0. In water, CH3Hg+ enters the food chain being absorbed by the biota. Finally, Hg(CH3)2 and Hg0 are the dominant volatile species in the atmosphere (Fitzgerald and Lamborg 2005). From the beginning of the sixteenth century to the end of the eighteenth, Brazil was a major global producer of gold. This metal was discovered in the central part of the country (Minas Gerais) and its production did not require, at that time, the amalgamation step. With the exhaustion of the mines in the nineteenth century, this process, and hence Hg, was introduced to explore lowgrade deposits. Currently, gold exploitation is carried out by small entrepreneurs in the Amazonian basin on a large scale (Villas Bôas et al. 2001; Malm et al. 1995; Lechler et al. 2000; Lacerda and Solomons 1998) and also in the state of Minas Gerais (Windmöller et al. 2007; FEAM 2005), particularly in the Doce River basin, which is the subject of the present study. Artisanal small-scale gold mining was reported by the United Nations Environment Programme, in 2013, as the anthropogenic activity that most contributes to Hg emission to the atmosphere (United Nations Environment Programme 2013). The amalgam Au-Hg is burned near river mining, generally in open air, to obtain gold separated from the ore, releasing large amounts of gaseous elemental Hg into the atmosphere. During the process, considerable amounts of mercury are lost in metallic form also into rivers and soil (Lacerda 1997a, b; Lacerda et al. 1995). These extractive activities have negative impacts on environmental conditions. The risks

Environ Monit Assess (2018) 190:49

are not always restricted to the limits of the extraction area. They are significantly higher when this mining activity is inserted in populated areas. It is also known that Hg is a global pollutant, i.e., it is transported over long distances in the atmosphere. Even in areas where there is no natural or anthropogenic source, as in the Arctic and Antarctic, the presence of the metal is observed (Lehnherr 2014; Ferro et al. 2014). The Gualaxo do Norte River, the sampling area, is located in the Doce River basin; with an area of 86,700 km2, it supplies water to 3.5 million people in 228 municipalities (CBRDH 2016). Its springs are located near the Antonio Pereira and Bento Rodrigues districts (Fig. 1), Northern/Northwestern Mariana, and Southeastern in the Iron Quadrangle, Minas Gerais. The Fundão Dam was built up on the springs of the Santarém stream, which is a tributary of the Gualaxo do Norte River. With the rupture of this dam, in 2015, the river was strongly affected. The actual composition of the sediments has changed and also the Hg species content and certainly its spatial distribution. This area was chosen because of the intense mining activity observed there. A characteristic of the artisanal mining is the constant change of work area by the prospectors. However, in this area, a continuity of the mining activities has been observed for at least the last 15 years. Prospectors in the area uses luices or rock boxes (cradles) to separate heavier minerals—among them gold. Following this step, Hg is mixed in a slurry in a water suspension and strongly stirred for the amalgamation process. After decantation of the amalgam, the supernatant is poured in the river and the solid is finally burned in open air, volatizing Hg and recovering gold. Mercury contamination in this region has been studied in the past two decades. Buscher (1992) evaluated the distribution of Hg in the Carmo River, Tripuí, and Água Suja streams (tributary of the North Gualaxo Rio). This author reported total Hg concentrations in the silt/clay granulometric fraction of sediment between 0.11 and 22.6 mg kg−1. Zeferino (1997), working on a similar theme, determined total Hg in sediments from Carmo, Gualaxo do Norte, Piracicaba, and Santa Barbara Rivers and ÁguaSuja stream. Sediment from Gualaxo do Norte River showed mercury concentrations between 0.3 and 1.6 mg kg−1, with an outlier of 23.6 mg kg−1. In Água Suja stream sediment, the Hg content values varied between 0.2 and 54.8 mg kg−1. Cursino et al. (1999), studying bacterial resistance to Hg,


determined concentrations in Carmo River sediment between 0.10 and 0.55 mg kg−1. Ramos (2005), also working in the Carmo River, reported total Hg concentrations between 0.23 and 1.73 mg kg−1. Windmöller et al. (2007) also working in Carmo River reported Hg concentrations between 0.03 and 0.47 mg kg−1. Finally, Varejão et al. (2009), studying the fractionation of Hg in stream sediment in the same region, recorded total Hg concentrations from 0.18 to 0.69 mg kg−1. Although the total Hg concentrations reported in the most recent publications have pointed to lower values, the importance of constant updating of the contamination situation is clear as well as a better understanding of the geochemical cycle of the metal in this region. There is little information in these research works regarding Hg speciation. Windmöller et al. (2007) reported the presence predominantly oxidized Hg in sediment samples, and Varejão et al. (2009) accounted 42 to 56% of the total Hg as Hg0. There is no information in the literature on methylation, bioaccumulation, or biomagnification of mercury in this region.

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The aim of this work was to quantify and to speciate Hg in the sediment of the Rio Gualaxo do Norte, an area of artisanal and small-scale gold mining. It is important to point out that the samples were collected during the year 2012, prior to the 2015 Fundão Dam disaster. The results of this work are a record of the characteristics of the sediment prior to that event. They enable any future comparison and assessment of their environmental impact. In this context, it was also aimed to evaluate the mode of transport of the element to the hydrological system and to describe a distribution of the CH3Hg+ species in the sediment, contributing to an understanding of the biogeochemical cycle of the metal.

Materials and methods Sampling and sample preparation Sediment samples were collected from nine sites selected along 26 km of the river in August 2012 (Fig. 1). The

Fig. 1 Location of the Gualaxo do Norte River with sampling points. Map modified from Costa (2001)

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points were chosen to cover the whole course of the river. The sampling was performed at points of local low fluvial energy where the deposition of finer material is favored. Sediment samples of approximately 1 kg were collected at each point according to the recommendations of the USGS (Shelton and Capel 1995). The samples were placed in amber flasks and kept at 4 °C until arrival at the laboratory; part of them was dried at room temperature, disaggregated, and sieved, and the fraction < 0.063 mm (silt-clay) was used for the analytical determinations except for the determination of CH3Hg+; in this case, part of the sample was lyophilized and kept in the freezer until analysis. Chemical characterization of the samples For complete characterization of sediment, bulk chemical composition was obtained by X-ray fluorescence spectrometry using a Panalytical spectrometer model PW2400. Samples (1000 g) were fused with 6 g of a 1:1 LiBO2/Li2B4O7 mixture. Mineralogical analyses by X-ray diffraction were also carried out using a Rigaku D-Max diffractometer, equipped with a copper tube (radiation Cu Kα, λ = 1.5418 Å), operating with a voltage of 40 kV and a 30-mA current. The sweep amplitude was undertaken step by step in the interval from 4° to 50° 2θ, with an increment of 0.05° and time of 1 s in each step. Determination of organic carbon to obtain the organic matter content in the samples was carried out in an elemental analyzer CHNS/O, PerkinElmer, PE2400, Series II. Sediment samples of the air-dried fraction < 2 mm were macerated and analyzed for organic carbon, hydrogen, and nitrogen in a CHN element analyzer instrument. Samples were oxidized with O2 at high temperature, generating CO2, H2O, and N2, which were separated by a column composed of CuO/AgVO3 and determined. The percentage of organic matter was calculated by multiplying the organic C content by a factor of 1.72, which considers that the carbon contributes 58% of the composition of the humic acid fraction. For the evaluation of the degree of humification, the C/N ratio was taken (Radojević and Bashkin 1999). THg determination and speciation by thermo-desorption in DMA-80 The determination of total Hg (THg) was performed using a direct mercury analyzer (DMA-80, Milestone).


In this technique, samples are subjected to a heating program that involves 30 s ramp heating to reach 200 °C (drying step), followed by another 30 s ramp to reach 750 °C. The sample is kept at this temperature for 120 s (pyrolysis step). Absorption measurements were carried out at 253.65 nm. To evaluate the performance of the equipment, a reference material, GBW08301 river sediment, with a THg value of 220 ± 40 μg kg−1, was analyzed. The THg found in the certified reference material, analyzed by DMA, was 261 ± 11 μg kg−1. This value corresponds to 118% recovery, showing therefore good accuracy of the method. The detection limit (LOD) and quantitation limit (LOQ) for this technique were 0.90 and 1.78 μg kg−1, respectively. Speciation by thermo-desorption is based on the principle that different species of Hg are released from a solid matrix at different temperature ranges (Windmöller 1996). Approximately 0.3000 g of sample was weighed and subjected to different temperature programs, each one including a temperature level for 3 min: 50, 100, 150, 200, 250, 300, 400, 500, 600, and 700 °C. At the end of each temperature level, the equipment quantified total Hg. The results were compared with patterns of Hg species analyzed under the same conditions as the samples, such as Hg0, HgCl2, Hg2Cl2, HgSO4, and HgS. These standards were prepared and analyzed by Windmöller and coworkers in 2017 using solid dilution (Windmöller et al. 2017). As there is no reference material related to mercury oxidation state quantification, the same reference material used to validate the total Hg determination of GBW 08301 (river sediment) was used in the analysis. The sum of all quantification steps (from 50 to 700 °C) was compared with the certified total Hg value. The results were good 224 ± 19 μg kg−1 compared to the certified reference value 220 ± 40 μg kg−1. Determination of CH3Hg+ concentration in sediment The determination of CH3Hg+ was performed on a gas chromatographer coupled to a pyrolysis system with an atomic fluorescence detection system (CG-pyro-AFS), MERX, Mark Brooks Rand Labs, USA. The sediment samples were subjected to a distillation system (Brooks Rand Labs) modified from Horvat et al. (1993) as described by Mendes et al. (2016). Samples of 0.5000 g were weighed in a Teflon tube to which 30 mL of ultrapure water was added together with 500 μL H2SO4 8 mol L−1 and 200 μL KCl 20% w/v. Teflon tubes for collection of the distillate containing 5 mL of

3.21

4.31 2.83

2.45 1.865

2.505 8.42

3.93 0.077

0.134 1.52

0.829 0.316

0.446 0.37

0.21 0.27

0.60 0.692

0.229 1.05

0.941 24.88

42.65

44.87

42.07

RGN7

RGN8

8.09

40.07 RGN6

16.28

3.63 2.695 2.11 6.32 0.123 1.15 0.336 0.31 0.43 0.828 0.863 37.7

40.33 RGN5

11.54

3.01

3.30 2.625 1.915 6.04 0.132 1.14 0.340 0.31 0.42 0.837 0.878 37.19

3.83

18.00

11.59

2.075 1.75 2.47 0.059 0.465 0.131 0.054 0.201 0.341 1.06 71.27

3.07

RGN4

3.89

2.235

2.025 1.785

2.225 9.75

7.09 0.124

0.167 2.38

2.09 0.584

0.358 0.21

0.27 0.65

0.81 1.38

1.06 0.738

0.735 28.92

25.80

37.64 RGN3

17.38

46.90 RGN2

14.45

3.41 1.945 1.98 6.40 0.169 1.20 0.100 0.13 0.40 1.70 0.641 46.60 34.05

7.74

N (%) C (%) LOI (%) P2O5 (%) K2O (%) Na2O (%) CaO (%) MgO (%) MnO (%) TiO2 (%) Fe2O3 (%)

RGN1

X-ray diffraction study shows that the sediment is basically composed of hematite, magnetite, quartz, kaolinite, and goethite. Table 1 presents the bulk chemical composition of the sediment; Table 2 displays the THg, CH3Hg+, and percentage of CH3Hg+ content; and Table 3 shows the CHN analysis results. The results show that THg is present in the samples in the range of 209 ± 3 to 1207 ± 18 μg kg−1, being clearly higher in the springs of the river, where the gold

Al2O3 (%)

Results and discussion

SiO2 (%)

Statistical analyses were performed with Minitab version 17. It consisted of principal component analysis (Manly, 2004), aiming to assess interactions among elements and analyzed species and to group the variables in factors affecting the behavior of sediment.

Sample

Statistical analyses

Table 1 Bulk chemical composition of Gualaxo do Norte River sediments obtained by X-ray fluorescence analysis, LOI (lost on ignition), and C and N analysis

ultrapure water were placed in an ice bath. The samples were distilled at 125 °C under a nitrogen flow of 61 mL min−1 for approximately 3 h after which 75% of the distillate had been collected. Then, the distillate was diluted to 50 mL and transferred to a glass jar derivatization system. To this system were added 300 μL of sodium acetate buffer and 50 μL of sodium tetraethylborate solution (NaBEt4). The set was left to stand for 17 min for the contact of the reactants (reaction time) and then purged with nitrogen gas at 91 mL min−1 for 25 min. Traps were dried with nitrogen gas for 6 min and analyzed individually by the thermodesorption system coupled to a chromatographic column filled with OV-3. The GC temperature was maintained at 35 °C. Argon gas was used as mobile phase, and the flow rate used was 17 mL min−1. It is worth pointing out that the parameters of the chromatographic column (temperature GC and mobile phase flow rate) and derivatization (flow of the bubbler, trapping time, volume of sodium tetraethylborate, trap drying time, reaction time) were optimized preliminarily for analysis by Mendes et al. (2016). The certified reference material ERM®-CC580 (estuarine sediment) was used to ensure the quality of CH3Hg+ analysis in the sediments. The CH3Hg+ concentration determined in the material was 71.1 ± 2.1 μg kg−1, whose value represents 95% recovery, showing a good precision of the method.

Page 5 of 11 49 OM (%)


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extraction activity is more intense, particularly at sampling points RGN1 and RGN3. As stated before, these locations represent places where the gold extraction activity was intense. Sampling surveys in this region were carried out in sites where there are evidences of present or recent gold mining activities. In general, the THg range is similar to the values found by other authors that studied similar areas in the Iron Quadrangle in the last decade and lower than the concentrations found in previous years. It is possible that this reflects a decrease of mining activities in the region. The values found are above the quality values for sediment, showing that there is contamination of the area. Pinedo-Hernández et al. (2015) also studied the impact of gold mining on sediment in the Mojana region, Colombia, and they found values between 196.2 and 1187.6 μg kg−1, which are very similar to the values shown here. In sediment from swamps from the same mining area in Colombia, Marrugo-Negrete et al. (2015) found 145–1021 μg kg−1. Figure 2 shows the results of Hg speciation using heating in steps. These results were obtained in the same equipment and operational conditions as the standard Hg compound analysis described in Windmöller et al. (2017), whose results are displayed in Fig. 3. Comparing the thermal desorption ranges of Fig. 2 with the Hg standards of Fig. 3, one can observe that the majority of Hg in Fig. 2 is released from the samples at 200 °C, indicating the predominant presence of oxidized form Hg. Only few samples show a small signal at 50 °C, corresponding to Hg0, which is the Hg species utilized in gold mining. These results show that at least part of the residue of Hg from the process of amalgamation with gold that is lost to the water system is oxidized and adsorbed and/or precipitates on the sediment. Hg oxidation was also observed by other authors studying soil +

+

Table 2 THg, CH3Hg concentration, and percentage of CH3Hg content in Gualaxo do Norte River sediment

contaminated by gold mining activity (Durão Jr. et al. 2009; Windmöller et al. 2015). One can observe in Fig. 2 that some samples show the majority of Hg releasing at 400 °C and others at 600 °C. It is not possible to infer the exact Hg compound or interaction with the matrix by comparing the thermal release profile with the standards in Fig. 3 (there are no Hg patterns in Fig. 3 with maximum Hg desorption at 600 °C); one can say only that there are two more important bonds of Hg in these sediment samples: one of them is more weakly bound and the other one is more strongly bound to the matrix. Certainly, the conversions between the oxidation states and the interactions of the oxidized Hg with the sediment components are steps that directly influence the methylation of the metal. To understand how this influence is occurring specifically in the study material, a systematic study doping sediment with Hg and monitoring the formation of CH3Hg+ would be necessary. The content of CH3Hg+ ranged from 0.07 ± 0.01 to 1.00 ± 0.07 μg kg −1. The higher concentrations were observed at stations RGN1, RGN3, and RGN9. At these points, the river loses its velocity leading to poor oxygenation of water, which may favor the methylation process. The studies already cited, carried out by Marrugo-Negrete Table 3 Loading values of each variable in each of the principal components for sediment samples of Gualaxo do Norte River Eigenvalue

7.3663

3.8250

1.2289

Proportion

0.526

0.273

0.088

Cumulative

0.526

0.799

0.887

Variable

PC1

PC2

PC3

SiO2

0.279

−0.171

0.100

Al2O3

0.358

−0.079

−0.013

Fe2O3

−0.338

0.154

−0.162

TiO2

−0.218

−0.366

−0.103

MnO

0.195

0.402

−0.055

MgO

0.348

0.082

0.077

CaO

0.258

−0.322

−0.080

Na2O

0.265

−0.250

0.424

K2O

0.337

0.127

0.197

Sample

THg (μg kg−1)

CH3Hg+ (μg kg−1)

% CH3Hg+

RGN1

1207 ± 18

0.46 ± 0.06

0.04

LOI

0.355

0.083

−0.163

0.093

−0.414

−0.433 −0.614

RGN2

650 ± 12

< 0.07

–

N

RGN4

266 ± 11

< 0.07

–

OM

0.238

−0.102

–

THg

0.176

0.429

−0.139

0.017

0.289

−0.328

RGN5

431 ± 20

< 0.07

RGN6

411 ± 22

< 0.07

–

CH3Hg+

RGN7

544 ± 6

0.07 ± 0.01

0.01

RGN8

212 ± 6

0.58 ± 0.05

0.27

Loadings are written in italics in the component where they have the highest value, i.e., the component in which they have the highest importance


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Fig. 2 Results of Hg speciation by thermal desorption using a direct Hg analyzer

et al. (2015) and Pinedo-Hernández et al. (2015) in sediments from areas in Colombia impacted by gold mining, showed content of CH3Hg+ (between 8 and 68 μg kg −1) higher than the range found here. Sediment samples from swamps showed a range from 8 to 68 μg kg −1 and those from the river a little lower, from 4.1 to 30.6 μg kg −1. According to the authors, the conditions of marshy environment with high organic content and lower rate of water flow favor the methylation of Hg in the samples from

swamps. Mendes et al. (2016) studied sediment from Descoberto-MG, Brazil, an area contaminated by Hg attributed to gold mining. Concentrations of CH3Hg+ of up to 8.0 μg kg−1 were found in solid material from sedimentation boxes used to contain soil carried by rain. This value is higher than that found in this work, but similar to the quantified CH3Hg+ content (< 0.07 to 1.87 μg kg−1) in sediment from the river that crosses the area. The results of this work are also comparable to the


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b

Hg

800

0

600 400 200 0

100

200

300

Hg2Cl 2

-1

-1

Hg Concentration (µg kg )

a


49

300

400

500

250 200 150 100 50 0

100

c

300

400

500

600

d 120

-1

HgCl 2

140

100 80 60 40 20 0 100

200

300


-1


200

Temperature (°C)

Temperature (°C)

400

500

600

HgSO 4

100 80 60 40 20 0 100

e -1


200

300

400

500

600

Temperature (°C)

Temperature (°C)

HgS

200 180 160 140 120 100 80 60 40 20 0 100

200

300

400

500

600

Temperature (°C) Fig. 3 Graphics of Hg standards analyzed by thermal desorption using a direct Hg analyzer (modified from Windmöller et al. 2017) - (a) Hg0, (b) Hg2Cl2, (c) HgCl2, d) HgSO4, and e) HgS refer to

dessorption patterns for each correspondig Hg species(modified from Windmöller et al. 2017)

CH3Hg+ concentrations found in marine sediments studied by Beldoswisk et al. (2014) and Carrasco and Vassileva (2015), whose values ranged from 0.061 to 0.94 μg kg−1 and from 0.89 to 7.5 μg kg−1, respectively. Many factors influence the Hg methylation and demethylation processes, such as light, quantity, and type of organic matter, and microbial activity, such as sulfatereducing bacteria (Acha et al. 2011) and cyanobacteria,

among others. It is possible that the high incidence of sunlight and the composition of Gualaxo sediment with complexing agents for Hg2+ would not favor a predominance of the methylation processes over the demethylation processes, which would explain the absence of high concentrations of CH3Hg+ found in these samples. However, new studies that generate more CH3Hg+ concentration data from sediments and other biological


matrices from these mining areas of the Iron Quadrangle are necessary to evaluate these processes in a more conclusive way. In any case, it is observed that Hg methylation occurs at least to a small extent, which is very important to better understand the Hg biogeochemical cycle. More studies looking for bioaccumulation of Hg are also necessary. The chemical composition data (Tables 1 and 2) were used in a PCA study. The CH3Hg+ data below the LD (0.07 μg kg−1) have been replaced by one half of that value (0.035 μg kg−1) (Reinmann et al. 2008; Templ et al. 2008). The results are shown in Table 3. The data can be explained in terms of three principal components, which account for 88.7% of the variance of the data. The first component (PC1), representing 52.6% of the variance data, has the predominance of variables SiO2, Al2O3, MgO, CaO, K2O, Fe2O3, and LOI (loss on ignition). This component can be interpreted as the bulk composition of sediment whose main minerals are quartz [SiO2], kaolinite [Al2Si2O5(OH)4], goethite [FeOOH], hematite [Fe2O3], and magnetite [Fe3O4]. It is significant that the loading relative to Fe2O3 has the opposite sign in relation to the loading of other variables prevalent in this component. This means that they contribute in the opposite direction to the score of the component. This may be linked to iron minerals in flux from the mining companies that accumulate in some river sites. The second component, representing 27.3% of the variance of the data, has the predominance of variables TiO2, MnO, and Hg, the last two with loading of the same sign (Table 3). This component may represent the relationship between mercury in the sediment and MnO, which could mean that Hg is adsorbed on Mn bearing minerals, or the oxidation of Hg0 by Mn increases the Hg content in the sediment. Mn and minerals which contain this element play an important role in the transport of Hg. The understanding of this process is not clear yet. Manganese oxides are in fact used as adsorbent material for gaseous elemental Hg because of their oxidizing and adsorbing properties. Miller et al. (2015) showed that manganese oxides are responsible for the oxidation of Hg in soil from a chlor-alkali plant. Some studies have shown similar behavior regarding the correlation between Hg and Mn. Windmöller et al. (2007), also working with sediment samples from the Iron Quadrangle, showed the predominance of oxidized Hg. Feng et al. (2011) reported that Mn in sediment governs the process of inorganic Hg diffusion to the water column. In oxic conditions, Mn is found in the form of MnO2, and Hg ions are strongly adsorbed by

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this compound and prevented from being released into the interstitial waters. Under reducing conditions, the predominant species is Mn2+, which in turn is soluble in water and releases Hg to interstitial water. Finally, the third component, which represents 8.8% of the variance of the data, has the predominance of Na2O, N, MO, and CH3Hg+. These last three, with loading of the same sign, can be interpreted as the association between organic matter and methylation of Hg. It is important to note that the samples were collected during the year 2012, before the 2015 Fundão Dam disaster. The results also give a picture of the sediment before that event. They make possible any future comparison and assessment of its environmental impact.

Conclusions The results show that Hg from gold mining activity is being transported along the Gualaxo do Norte River (MG), Brazil, which may lead to extended contamination. THg content shows high values at points RGN1 (1207 μg kg−1), RGN3 (940 μg kg−1), and RGN7 (549 μg kg −1), where intense activity is carried out even today. Although these concentrations are lower than those found in long-held research, it is not possible to conclude that the extent of contamination from this activity is declining. Certainly, Hg lost to water systems is spread through the water and sediment, and also the methylation of the metal allows its accumulation in living organisms decreasing its concentration in the sediment. At least part of the Hg released to the water system is being oxidized to Hg2+ and transported through the river adsorbed on manganese oxides. CH3Hg+ is found in relatively high concentrations at points RGN1 (0.46 μg kg −1), RGN3 (1.00 μg kg −1), and RGN9 (0.58 μg kg −1) where the water of the river loses its velocity, which favors the methylation process. Statistical analyses show that the composition of the Gualaxo North River sediment can be understood as the contribution of three components: (1) its general or mineralogical composition, (2) mercury distribution from prospector activity being associated with Mn and transported by Mn minerals, and (3) methylation of Hg. This process is particularly pronounced in the mouth of the river where the oxygenation of the water is poor. After the rupture of the Fundão dam, it is reasonable to suppose that the Hg present in the sediments of the Gualaxo do Norte River has been diluted by the 62

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billion cubic meters of released mud. In fact, the samples collected in 2012 are covered by mud or mixed with it. In 2016, in a field work with undergraduate students, an expedited sampling was made at the mouth of the Gualaxo do Norte River. Hg in that sample was below the LD of the technique. Similar work performed by IGAM (state water agency), in the RGN and along the Rio Doce, assessed the presence of several metals in sediments including Hg. In the same week of dam breaking only at Governador Valadares (a city 331 km away from the Fundão dam), there was Hg in sediments with a content above 490 μg/kg. In the previous points (this includes the Gualaxo do Norte River), the Hg was below the LD. It should be pointed out that on the day of the IGAM sampling survey, the mud had not yet reached the city of Governador Valadares. This first investigation about Hg methylation in this small-scale gold mining area points to the possibility of bioaccumulation of the metal and the need to better understand the biogeochemical cycle of the metal in this area. Since there is no forecast of interruption of this activity in this area in the near future, it is important to carry out new studies to assess the environmental impact caused by the extraction of gold in this region. Funding information The authors thank CNPq and CAPES for granting scholarships to Rhodes (M.Sc.) scholarship, Santolin (Ph.D.), Mendes (Ph.D.), and project 577309/2008-0; the authors also thank FAPEMIG (Project APQ-2728-5.02/07 and APQ03861-09) and PRPq/UFMG (Pró-reitoria de Pesquisa) for the financial support.

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