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Arsenic fractionation in estuarine sediments: Does coastal eutrophication influence As behavior?
Marine Pollution Bulletin 96 (2015) 496–501
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Baseline
Arsenic fractionation in estuarine sediments: Does coastal eutrophication influence As behavior? Fabian Sá a, Christian J. Sanders b,⇑, Sambasiva Rao Patchineelam a, Eunice da Costa Machado c, Ana Teresa Lombardi d a
Universidade Federal Fluminense, Departamento de Geoquímica, Outeiro São João Batista, s/n°, Centro, Niterói, RJ, Brazil National Marine Science Centre, School of Environment, Science and Engineering, Southern Cross University, Coffs Harbour, New South Wales, Australia Laboratório de Hidroquímica, Instituto de Oceanografia, Fundação Universidade do Rio Grande, Av. Itália Km 8, Rio Grande, RS, Brazil d Universidade Federal de São Carlos, Departamento Botânica, Rodovia Washington Luis Km 235, CP 676 São Paulo, Brazil b c
a r t i c l e
i n f o
Article history: Available online 27 April 2015 Keywords: Pyrite Mangrove Sea grass Estuary Arsenic
a b s t r a c t The Paranaguá Estuarine Complex (PEC) includes the naturally oligotrophic (NO) Mel Island which is surrounded by sea grasses, a naturally eutrophic (NE) Benito Inlet adjacent to mangrove wetlands and the highly impacted eutrophic (IE) Paranaguá Bay, home of one of Brazil’s largest ports. The results from this study indicate that reactive As and pyrite increase with sediment depth near Paranaguá port in the IE region. At the NE region, near a mangrove fringe, the reactive As, Fe, Mn and pyrite remained relatively high along the sediment column while near the sea grasses at NO the As contents were low. The degree of trace metal pyritization (DTMP) and the degree of pyritization (DOP) was highest at the IE site, slightly increasing with depth. These baseline results indicate that influence of trophic conditions and presence of marine vegetation may be directly related to As behavior in coastal systems. Ó 2015 Elsevier Ltd. All rights reserved.
Diagenetic and geochemical processes are directly related to trace metal behavior in estuarine sediments (Huerta-Diaz and Reimer, 2010; Ye et al., 2011). In particular, Fe supplied minerals are important ligands for trace metal mobility (Machado et al., 2014). The incorporation of As in pyrite (FeS2) and acid volatile sulfides (AVS) are commonly proposed as early diagenetic mechanisms for the removal of As in pore water (Huerta-Diaz and Morse, 1990, 1992; Mucci, 2004). Typically, As supplied formation occurs near the redox boundary layer (sulfidic zone). Most reaction mechanisms suggest that the formation of pyrite occurs in two steps (Berner, 1984). First, hydrogen sulfide is produced during sulfate reduction, reacting with dissolved iron and Fe-minerals to form reactive iron sulfide meta-stable compounds. Second, metastable iron sulfides, mainly amorphous iron sulfides, react with elemental sulfur or polysulfides to form pyrite. This transformation occurs by the addition of sulfur, and not by the removal of iron (Berner, 1984). The direct precipitation of pyrite without intermediate Fe sulfide precursors has been described in salt marsh sediments, where pore waters are supersaturated in amorphous Fe sulfides (Giblin
⇑ Corresponding author. Tel.: +61 66483917. E-mail addresses: [email protected] (F. Sá), [email protected] (C.J. Sanders). http://dx.doi.org/10.1016/j.marpolbul.2015.04.037 0025-326X/Ó 2015 Elsevier Ltd. All rights reserved.
and Howarth, 1984; Huerta-Diaz and Reimer, 2010). This direct reaction of elemental sulfur and poly-sulfides with ferrous Fe (Fe2+) results in the formation of small single crystals of pyrite (Luther et al., 1992) and concomitantly, intense precipitation of As sulfides. The incorporation of As in the pyrite fraction is influenced by the presence of other mineral phases, including Mn oxy-hydroxides and clays (Lowers et al., 2007). To quantify these phases’ the degree of trace metal pyritization (DTMP) may be used (Huerta-Diaz and Morse, 1992). In this work, the DOP was determined in three differing trophic areas within the Paranaguá Estuarine Complex (PEC) (25° 160 and 25° 340 S and 48° 170 and 48° 420 W); (Fig. 1): Naturally oligotrophic (NO), naturally eutrophic (NE) and impacted eutrophic (IE). The middle and inner regions of the PEC are classified as eutrophic while the outer region is oligotrophic (Lana et al., 1997). The general sediment profile in the PEC coastal system are characterized as fine silt to fine sand, organic matter content is between 0% and 30% and biodetritic carbonate between 0% and 20% (Sanders et al., 2012; Lamour et al., 2004). Recent studies near the IE region suggests waste discharge from one of Brazil’s busiest ports, contain sewage and agricultural contamination (Martins et al., 2010) as well as trace metal enrichment (Martins et al., 2012). Martins et al. (2012) found As contents that exceeded 20 mg/kg in surface sediments near the highly populated
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F. Sá et al. / Marine Pollution Bulletin 96 (2015) 496–501
Fig. 1. Location of study area in the Estuarine Complex of Paranaguá (PEC) and sampling stations; naturally oligotrophic (NO), naturally eutrophic (NE) and impacted eutrophic (IE).
PEC port, Paranaguá Bay and highly impacted Laranjeiras Bay. Furthermore, the highest concentrations of total dissolved As (TDAs) were found in the IE site (17.1–20.1 lg L1) and NE (9.28–12.8 lg L1) compared to the much lower values at the NO site (5.50–8.21 lg L1). Organic compounds of As (Asorg) content were also found in higher concentrations in the eutrophic regions (IE and NE) (dos Anjos et al., 2012). In this work, six sediment cores, 20 cm in length (Diameter (Ø) = 7 cm), were collected by a diver at the three study regions (Fig. 1). Two sediment cores were collected at each site, one for pH and Eh measurements and the other for arsenic (As), iron (Fe), manganese (Mn) and copper (Cu) analyses as well as carbonate, organic matter, and grain size measurements. The pH and Eh were measured using in situ electrodes (Analion), every 2 cm through holes in the coring devise. The sediment cores were then sectioned in inert conditions (N2) glove bags, from 0 to 2; 2 to 4; 4 to 8; 8 to 10; 10 to 15 and 15 to 20 cm depth intervals, immediately after extraction. The As sequential extraction analyses followed the methods described by (Huerta-Diaz and Morse, 1990) using the following sequences: (1) HCl fraction – Samples were digested with 1 M HCl for 16 h. This fraction contains monosulfides, Fe oxyhidroxides, amorphus and crystalline, hydrated aluminum silicates and carbonates. (2) Silicate fraction – Determined through two leaching cycles with 10 M HF for 16 h, respectively. The elements associated with silicate fraction are nonreactive during early diagenesis toward pyrite formation but were not used for the calculations of DOP and DTMP for the reasons outlined in (Huerta-Diaz and Morse, 1990).
(3) Pyrite fraction – Determined through digestion in concentrated HNO3 for 2 h then extracted. This fraction contains pyrite and associated trace metals. The DOP and DTMP were determined using the following equations:
DOPð%Þ ¼ FePyrite
þFePyrite FeHCl 100
ð1Þ
ð2Þ
DTMPð%Þ ¼ MePyrite
þMePyrite MeHCl 100
Arsenic was analyzed by atomic absorption spectrometry with a graphite furnace (GFAA) using 10 mg L1 of palladium as a matrix modifier. Iron and Mn was measured using an atomic absorption spectrophotometer (AAS). For quality assurance, certified reference standard EnviroMat Contaminated Soil SS-2 (MCR SS2) was analyzed during the entire experiment. The recovery percentage for each sequence was near 95%, 98% and 96% for Mn, Fe and As, respectively. Each sample was run in triplicates and the relative standard deviation was less than 10%. The granulometric composition for each sediment interval was determined by sieving and pipetting, according to the methods described in Suguio (1973) (Table 1). The statistical parameters of the sediments were obtained by SysGran 3.0 software for Windows. The organic matter was estimated through the loss of ignition (LOI) method, using 5 g of homogenized sediment placed in crucibles then heated at 450 °C for 5 h. The carbonate content was determined by fumigating samples with 10% hydrochloric acid (HCl). The difference in weight, before and after fumigation, was determined to be carbonate material. The sediments from the NO site are classified as fine sand with an average organic matter and carbonate content of 0.9% and 1.7% respectively (Table 1). The particle size varied little along the entire
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F. Sá et al. / Marine Pollution Bulletin 96 (2015) 496–501
Table 1 Depth variation in the percentage of carbonate, organic matter (O.M.), sand, silt and clay, followed by average and standard derivations (SD). Depth (cm)
Carbonate (%)
O.M. (%)
Sand (%)
Silt (%)
Clay (%)
NO (oligotrophic non-impacted) 1 1.2 3 2.6 6 1.1 9 1.2 13 0.9 18 3.1
0.8 1.4 0.5 0.7 0.7 1.5
94.8 96.3 94.6 96.8 94.5 94.1
1.0 2.1 3.1 3.1 4.1 4.1
3.0 1.0 2.1 0.0 1.0 0.0
Average SD
0.9 0.4
95.2 1.1
2.9 1.2
1.2 1.2
IE (eutrophic impacted) 1 1.4 3 2.3 6 5.0 9 3.3 13 2.1 18 2.4
1.5 1.9 4.3 6.1 4.2 5.1
95.7 98.0 96.0 72.5 81.0 84.6
2.1 2.1 3.0 20.4 10.0 8.2
2.1 0.0 1.0 7.1 9.0 7.2
Average SD
3.8 1.8
88.0 10.2
7.6 7.1
4.4 3.8
NE (eutrophic non-impacted) 1 9.1 3 10.1 6 11.4 9 11.7 13 11.1 18 11.1
17.3 17.4 17.4 17.5 17.9 17.7
8.5 2.9 3.1 12.8 6.4 5.5
56.5 85.0 91.1 83.1 84.6 85.5
35.0 12.1 5.9 4.2 9.0 8.2
Average SD
17.5 0.2
6.5 3.7
81.0 12.3
12.4 11.4
1.7 0.9
2.7 1.3
10.7 1.0
sediment core, approximately 95% fine sand (Table 1). The pH (7.9 ± 0.1) and Eh (60 ± 7.8 mV) values in the surface sediment layers were the highest among the sampled sites (Fig. 2). The NE region had the highest organic matter (17.5%) and carbonates (10.7%) of the three sites (Table 1). Sediments were predominantly silt with clay (Table 1). The general particle size of this site may be classified as silt (80.1%). The low hydrodynamics and high organic material content (17.5%) is likely influenced by the mangrove forests adjacent to the study region. The pH values (7.1 ± 0.1) which was constant down the sediment column and the redox potential (Eh) values (199.3 ± 28.6 mV) indicate reducing conditions down core (Fig. 2). The NE region has the highest average reactive and pyrite concentrations relative to the other distinct study regions (Fig. 2). The IE site had lower organic matter (3.8 ± 1.8) and carbonate (2.7 ± 1.3%) content than in the NE site (Table 1) but the organic matter was higher and the carbonates lower than the NO region. The IE site contained fine sand at surficial layers to very fine sand in the deeper layers (mean values of sand 84.6%) (Table 1). The pH values (7.1 ± 0.1) at IE site was lowest amongst the sampled regions, and the Eh (322.7 ± 23.8 mV) exceeded the values from the NE site (Fig. 2). The eutrophic sites (NE and IE) with anoxic sediments, contained a high fraction of As–pyrite (NE 4.3 ± 0.5 mg/kg and IE 2.2 ± 1.5 mg/kg), slightly higher than the reactive (As–HCl) fraction (NE = 2.2 ± 1.0 mg/kg and IE = 0.8 ± 0.6 mg/kg). The concentrations of the As–HCl fraction increase with depth at the IE site while the As–HCl fraction decreases with depth at NE site (Fig. 3). The increasing As–HCl fraction with depth at the IE site may be associated to the sulfide content (Krumbein, 1983). At the NO site, As– HCl and As pyrite fraction are highest at the surface, then dropping to the lowest value found at any site. Indeed, the top layer of the NO sediments was noticeably higher at the surface layer, which is likely related to the precipitation of Fe and Mn (oxy)hydroxides at a redox boundary, then subsequently decreasing.
The particle size variation may influences the differences in concentrations between the Mn and Fe content in the pyrite fraction between the NO and IE region. However, this variation is not noted in the As content in the same region of the pyrite fraction. Variation in the As content may be explained by the presence of a potential pollutant sources in the IE region, such pollutant source may originate from port activities, including dredging and an active fertilizer industry. These and other industrial activities may be responsible for the As enrichment at the IE region, though further studies should be conducted to determine if point pollutant sources exist. The species of seagrass in NE region is Halodule wrightii, is the most common seagrass in Brazil (Sordo et al., 2011). It grows in coastal, shallow water, and is known to have a role in cycling trace metals (Prange and Dennison, 2000). The content of metals in Halodule wrightii can be quite high, with different patterns of internal distribution of metals, such as low concentrations of Fe. The unimpacted area of this estuarine system is bound in mangrove forests. It is important to note that this vegetation may affect the physical and biogeochemical properties of coastal soils (Andrade et al., 2012; Sanders et al., 2008). Based on the trace metal content in both the HCl and pyrite fractions, it is possible to calculate the degree of pyritization (DOP) and the degree of trace metal pyritization (DTMP). The DOP and DTMP calculations are based on Eqs. (1) and (2) and the results are shown on Figs. 4 and 5. Huerta-Diaz and Morse (1990) and Huerta-Diaz and Morse (1992) classified he trace metals into three groups according to their behavior DTMP–DOP: Group 1 – Presents complete transition from reactive fraction to the pyrite fraction even at low levels PDO and independent of the type of sedimentary environment. Group 2 – Metal exhibit a gradual increase in the level of DTMP with increasing DOP concentrations with shared equally between the pyrite and reactive stages. Small variations may occur as analyzed metal. Group 3 – Impoverished in metals typically pyrite fraction consisting of transition metals which do not form complexes with sulfides. In this study, similar behaviors in DOP and DTMP–As were observed in the eutrophic sites, NE and IE. This can be verified by the maximum DOP values at the IE and NE, of 68.9% and 68.1% respectively. The highest values of DTMP–As (99.1–99.5%) in the NE, is likely related to the higher concentrations of organic matter (5 fold higher), fine-grained sediments and low hydrodynamics, all of which are favorable conditions for pyritization. The high metal pyritization at the NE site is indicative of sulfur, organic matter and Fe availability, because the absence of these factors limits the production of pyrite (Bollhöfer and Rosman, 1988) An incomplete transformation of iron sulfides to pyrite was observed in anoxic sediments of Kau Bay in Indonesia (Middelburg et al., 1989) and in areas of the Amazon (Kasten et al., 1998). The incomplete pyrite transformation in these areas may be due to a decrease in zero-valent sulfur during the pyrite formation. These processes may explain the different behavior of reactive and pyrite fractions between differing regions of the PEC. The NE site presented DTMP–As values that follow the DOP trend, remaining constant with depth, near 40% DOP–As (%) and near 100% DTMP–As (%) (Figs. 4 and 5). Furthermore, the DOP–As (%) varied little and DTMP–As (%) remained almost constant, at 90% down the sediment column similar to the NE site. In contrast, the NO site contained significantly lower DTMP–As and DOP–As (%) values. These results demonstrate the differing pyritization processes in specific areas of the PEC. Eutrophic areas show increasing in DTMP–As along with an increase in DOP in relation
F. Sá et al. / Marine Pollution Bulletin 96 (2015) 496–501
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Fig. 2. Depth variations in pH and Eh (mV) in the sediment cores from this study.
Fig. 3. Depth variations of As (mg/kg), Fe (g/kg) and manganese (g/kg) content in the naturally oligotrophic (NO), naturally eutrophic (NE) and impacted eutrophic (IE).
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Fig. 4. Depth profiles of DOP As (%), Fe (%), Mn (%) and (DTMP) As (%), Fe (%), Mn (%) in the naturally oligotrophic (NO), naturally eutrophic (NE) and impacted eutrophic (IE).
Fig. 5. Degree of trace metal pyritization (DTMP) (As%) as a function of the degree of pyritization (DOP).
to the NO region, e.g. Huerta-Diaz and Morse (1990). Indeed, low levels of both DTMP–As and DOP–As were noted at the NO site, ranging from 3.51% to 13.43%, and 17.6% to 37.2%, respectively. In contrast, concurrent DTMP–As and DOP behavior was not observed in the NO region. At the NO site the low values of DTMP–As are indicative of limited sulfide availability. Hence, the superficial sediment layer (0–2 cm) of the NO site shows high concentrations of both As–pyrite and As-reactive fractions (2.21 mg/ kg), even higher than those found at the eutrophic sites. At the NO site, large sea grass meadows are present as noted in (Sordo et al., 2011). Indeed, the sea grass roots assimilate sulfur (S), preferring the reduced form (sulfide) to the oxidized sulfate present
in sea water (Pulich and White, 1991). Sea grass roots provide substrate for microbial activity as well as enhancing anaerobic processes (Holmer et al., 2001). Thus, the presence of sea grass and other coastal vegetation may have a significant influence on the sediment geochemistry. It should be pointed out the possible overestimation of the pyrite-bound As fraction, in relation to organic-rich sediments at the NE site as suggested in Huerta-Diaz and Morse (1990); (Ye et al., 2011). However, Huerta-Diaz and Morse (1990) performed extractions with and without the analysis of the organic fraction and the results support the claim that this metalloid are more significantly associated with the pyrite fraction. In sediments with low organic matter content this fraction may be discarded, except for Cu, but should not be ignored in sediments with >3% organic matter (Huerta-Diaz and Morse, 1990). As such, the As pyrite fraction in the NE site, may be influenced by the high levels of organic matter. In this work, the organic fraction was not analyzed and it is assumed that the As–pyrite fraction may have higher values due to the conditions found in NE and IE sites. Therefore, further research should be done on the organic fraction and its origin (terrestrial or marine) in this and other similar study sites (Chaillou et al., 2003; Huerta-Diaz and Morse, 1990, 1992; Keon et al., 2001). Sullivan and Aller (1996) identified hydrological and geochemical processes on early diagenesis related to As along differing estuarine areas. These authors found that the diagenetic cycle of As in Amazon shelf sediments are directly influenced by the flow of the Amazon River, increasing the diffusive processes with increasing river discharge and subsequent resuspension of sediments. Saulnier and Mucci (2000) showed Fe release from anoxic sediment through resuspension in the water column is removed swiftly, generally less than 60 min, while the Mn and As remain in solution for longer than a week. The similarity of the residence
F. Sá et al. / Marine Pollution Bulletin 96 (2015) 496–501
time of the Mn and As suggests that a significant fraction of As (III) is oxidized to As (V) through precipitating Mn oxyhydroxides. However, previous studies have shown that a majority of As (III) released into the solution is adsorbed or co-precipitated by Fe oxy-hydroxides in the first hours of resuspension (Saulnier and Mucci, 2000). Another plausible hypothesis to explain the low concentrations of the As–pyrite and As-reactive fractions at the IE site is dilution through bioturbation and changing redox conditions. There were noticeable fluctuations in particle size within the eutrophic regions, from the 8–11 cm depth. Surface sediments from NE are fine grained, while at the IE site sand is predominated. This grain size difference may explain the increase in the As bound to meta-sulfides while pyrite are mostly found below the 11 cm depth. This stratification may be explained by the transfer of metals from the reactive fraction to the pyrite fraction. In contrast, the reducing conditions, the OM-rich and fine-grained nature of these sediments along the entire sediment column at NE site, provides strong evidence for the high As pyritization. The differing geochemical properties of inundated sediments, including specific trophic conditions, hydrodynamics characterization and the presence of vegetation, such as sea grass and mangrove, may play an important role in influencing pyritization (Huerta-Diaz and Reimer, 2010). This work shows baseline data demonstrating that As behavior is directly related to many factors, including trophic conditions, grain size, organic matter content and coastal vegetation. The As pyritization processes is extremely important when considering the effects and impacts of coastal development, such as coastal eutrophication and diminishing coastal vegetation, including sea grass and mangroves. Acknowledgements This research is a contribution to projects from the Brazilian research agencies CAPES and CNPq. C.J. Sanders is supported by a Southern Cross University fellowship and the Australian Research Council (DP150103286). References Andrade, R.A., Sanders, C.J., Boaventura, G., Patchineelam, S.R., 2012. Pyritization of trace metals in mangrove sediments. Environ. Earth. Sci. 67, 1757–1762. Berner, R.A., 1984. Sedimentary pyrite formation: an update. Geochim. Cosmochim. Acta 48, 605–615. Bollhöfer, A., Rosman, K.J.R., 1988. Isotopic source signatures for atmospheric lead: the Northern Hemisphere. Geochim. Cosmochim. Acta 65, 1727–1740. Chaillou, G., Schäfer, J., Anschutz, P., Lavaux, G., Blanc, G., 2003. The behaviour of arsenic in muddy sediments of the Bay of Biscay (France). Geochim. Cosmochim. Acta 67, 2993–3003. dos Anjos, V.E., Machado, E.D.C., Grassi, M.T., 2012. Biogeochemical Behavior of Arsenic Species at Paranaguá Estuarine Complex, Southern Brazil. Aquat. Geochem. 18, 407–420. Giblin, A.E., Howarth, R.W., 1984. Porewater evidence for a dynamic sedimentary iron cycle in salt marshes. Limnol. Oceanogr. 29, 47–63. Holmer, M., Andersen, F.O., Nielsen, S.L., Boschker, H.T.S., 2001. The importance of mineralization based on sulfate reduction for nutrient regeneration in tropical seagrass sediments. Aquat. Bot. 71, 1–17.
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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Baseline
Arsenic fractionation in estuarine sediments: Does coastal eutrophication influence As behavior? Fabian Sá a, Christian J. Sanders b,⇑, Sambasiva Rao Patchineelam a, Eunice da Costa Machado c, Ana Teresa Lombardi d a
Universidade Federal Fluminense, Departamento de Geoquímica, Outeiro São João Batista, s/n°, Centro, Niterói, RJ, Brazil National Marine Science Centre, School of Environment, Science and Engineering, Southern Cross University, Coffs Harbour, New South Wales, Australia Laboratório de Hidroquímica, Instituto de Oceanografia, Fundação Universidade do Rio Grande, Av. Itália Km 8, Rio Grande, RS, Brazil d Universidade Federal de São Carlos, Departamento Botânica, Rodovia Washington Luis Km 235, CP 676 São Paulo, Brazil b c
a r t i c l e
i n f o
Article history: Available online 27 April 2015 Keywords: Pyrite Mangrove Sea grass Estuary Arsenic
a b s t r a c t The Paranaguá Estuarine Complex (PEC) includes the naturally oligotrophic (NO) Mel Island which is surrounded by sea grasses, a naturally eutrophic (NE) Benito Inlet adjacent to mangrove wetlands and the highly impacted eutrophic (IE) Paranaguá Bay, home of one of Brazil’s largest ports. The results from this study indicate that reactive As and pyrite increase with sediment depth near Paranaguá port in the IE region. At the NE region, near a mangrove fringe, the reactive As, Fe, Mn and pyrite remained relatively high along the sediment column while near the sea grasses at NO the As contents were low. The degree of trace metal pyritization (DTMP) and the degree of pyritization (DOP) was highest at the IE site, slightly increasing with depth. These baseline results indicate that influence of trophic conditions and presence of marine vegetation may be directly related to As behavior in coastal systems. Ó 2015 Elsevier Ltd. All rights reserved.
Diagenetic and geochemical processes are directly related to trace metal behavior in estuarine sediments (Huerta-Diaz and Reimer, 2010; Ye et al., 2011). In particular, Fe supplied minerals are important ligands for trace metal mobility (Machado et al., 2014). The incorporation of As in pyrite (FeS2) and acid volatile sulfides (AVS) are commonly proposed as early diagenetic mechanisms for the removal of As in pore water (Huerta-Diaz and Morse, 1990, 1992; Mucci, 2004). Typically, As supplied formation occurs near the redox boundary layer (sulfidic zone). Most reaction mechanisms suggest that the formation of pyrite occurs in two steps (Berner, 1984). First, hydrogen sulfide is produced during sulfate reduction, reacting with dissolved iron and Fe-minerals to form reactive iron sulfide meta-stable compounds. Second, metastable iron sulfides, mainly amorphous iron sulfides, react with elemental sulfur or polysulfides to form pyrite. This transformation occurs by the addition of sulfur, and not by the removal of iron (Berner, 1984). The direct precipitation of pyrite without intermediate Fe sulfide precursors has been described in salt marsh sediments, where pore waters are supersaturated in amorphous Fe sulfides (Giblin
⇑ Corresponding author. Tel.: +61 66483917. E-mail addresses: [email protected] (F. Sá), [email protected] (C.J. Sanders). http://dx.doi.org/10.1016/j.marpolbul.2015.04.037 0025-326X/Ó 2015 Elsevier Ltd. All rights reserved.
and Howarth, 1984; Huerta-Diaz and Reimer, 2010). This direct reaction of elemental sulfur and poly-sulfides with ferrous Fe (Fe2+) results in the formation of small single crystals of pyrite (Luther et al., 1992) and concomitantly, intense precipitation of As sulfides. The incorporation of As in the pyrite fraction is influenced by the presence of other mineral phases, including Mn oxy-hydroxides and clays (Lowers et al., 2007). To quantify these phases’ the degree of trace metal pyritization (DTMP) may be used (Huerta-Diaz and Morse, 1992). In this work, the DOP was determined in three differing trophic areas within the Paranaguá Estuarine Complex (PEC) (25° 160 and 25° 340 S and 48° 170 and 48° 420 W); (Fig. 1): Naturally oligotrophic (NO), naturally eutrophic (NE) and impacted eutrophic (IE). The middle and inner regions of the PEC are classified as eutrophic while the outer region is oligotrophic (Lana et al., 1997). The general sediment profile in the PEC coastal system are characterized as fine silt to fine sand, organic matter content is between 0% and 30% and biodetritic carbonate between 0% and 20% (Sanders et al., 2012; Lamour et al., 2004). Recent studies near the IE region suggests waste discharge from one of Brazil’s busiest ports, contain sewage and agricultural contamination (Martins et al., 2010) as well as trace metal enrichment (Martins et al., 2012). Martins et al. (2012) found As contents that exceeded 20 mg/kg in surface sediments near the highly populated
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Fig. 1. Location of study area in the Estuarine Complex of Paranaguá (PEC) and sampling stations; naturally oligotrophic (NO), naturally eutrophic (NE) and impacted eutrophic (IE).
PEC port, Paranaguá Bay and highly impacted Laranjeiras Bay. Furthermore, the highest concentrations of total dissolved As (TDAs) were found in the IE site (17.1–20.1 lg L1) and NE (9.28–12.8 lg L1) compared to the much lower values at the NO site (5.50–8.21 lg L1). Organic compounds of As (Asorg) content were also found in higher concentrations in the eutrophic regions (IE and NE) (dos Anjos et al., 2012). In this work, six sediment cores, 20 cm in length (Diameter (Ø) = 7 cm), were collected by a diver at the three study regions (Fig. 1). Two sediment cores were collected at each site, one for pH and Eh measurements and the other for arsenic (As), iron (Fe), manganese (Mn) and copper (Cu) analyses as well as carbonate, organic matter, and grain size measurements. The pH and Eh were measured using in situ electrodes (Analion), every 2 cm through holes in the coring devise. The sediment cores were then sectioned in inert conditions (N2) glove bags, from 0 to 2; 2 to 4; 4 to 8; 8 to 10; 10 to 15 and 15 to 20 cm depth intervals, immediately after extraction. The As sequential extraction analyses followed the methods described by (Huerta-Diaz and Morse, 1990) using the following sequences: (1) HCl fraction – Samples were digested with 1 M HCl for 16 h. This fraction contains monosulfides, Fe oxyhidroxides, amorphus and crystalline, hydrated aluminum silicates and carbonates. (2) Silicate fraction – Determined through two leaching cycles with 10 M HF for 16 h, respectively. The elements associated with silicate fraction are nonreactive during early diagenesis toward pyrite formation but were not used for the calculations of DOP and DTMP for the reasons outlined in (Huerta-Diaz and Morse, 1990).
(3) Pyrite fraction – Determined through digestion in concentrated HNO3 for 2 h then extracted. This fraction contains pyrite and associated trace metals. The DOP and DTMP were determined using the following equations:
DOPð%Þ ¼ FePyrite
þFePyrite FeHCl 100
ð1Þ
ð2Þ
DTMPð%Þ ¼ MePyrite
þMePyrite MeHCl 100
Arsenic was analyzed by atomic absorption spectrometry with a graphite furnace (GFAA) using 10 mg L1 of palladium as a matrix modifier. Iron and Mn was measured using an atomic absorption spectrophotometer (AAS). For quality assurance, certified reference standard EnviroMat Contaminated Soil SS-2 (MCR SS2) was analyzed during the entire experiment. The recovery percentage for each sequence was near 95%, 98% and 96% for Mn, Fe and As, respectively. Each sample was run in triplicates and the relative standard deviation was less than 10%. The granulometric composition for each sediment interval was determined by sieving and pipetting, according to the methods described in Suguio (1973) (Table 1). The statistical parameters of the sediments were obtained by SysGran 3.0 software for Windows. The organic matter was estimated through the loss of ignition (LOI) method, using 5 g of homogenized sediment placed in crucibles then heated at 450 °C for 5 h. The carbonate content was determined by fumigating samples with 10% hydrochloric acid (HCl). The difference in weight, before and after fumigation, was determined to be carbonate material. The sediments from the NO site are classified as fine sand with an average organic matter and carbonate content of 0.9% and 1.7% respectively (Table 1). The particle size varied little along the entire
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Table 1 Depth variation in the percentage of carbonate, organic matter (O.M.), sand, silt and clay, followed by average and standard derivations (SD). Depth (cm)
Carbonate (%)
O.M. (%)
Sand (%)
Silt (%)
Clay (%)
NO (oligotrophic non-impacted) 1 1.2 3 2.6 6 1.1 9 1.2 13 0.9 18 3.1
0.8 1.4 0.5 0.7 0.7 1.5
94.8 96.3 94.6 96.8 94.5 94.1
1.0 2.1 3.1 3.1 4.1 4.1
3.0 1.0 2.1 0.0 1.0 0.0
Average SD
0.9 0.4
95.2 1.1
2.9 1.2
1.2 1.2
IE (eutrophic impacted) 1 1.4 3 2.3 6 5.0 9 3.3 13 2.1 18 2.4
1.5 1.9 4.3 6.1 4.2 5.1
95.7 98.0 96.0 72.5 81.0 84.6
2.1 2.1 3.0 20.4 10.0 8.2
2.1 0.0 1.0 7.1 9.0 7.2
Average SD
3.8 1.8
88.0 10.2
7.6 7.1
4.4 3.8
NE (eutrophic non-impacted) 1 9.1 3 10.1 6 11.4 9 11.7 13 11.1 18 11.1
17.3 17.4 17.4 17.5 17.9 17.7
8.5 2.9 3.1 12.8 6.4 5.5
56.5 85.0 91.1 83.1 84.6 85.5
35.0 12.1 5.9 4.2 9.0 8.2
Average SD
17.5 0.2
6.5 3.7
81.0 12.3
12.4 11.4
1.7 0.9
2.7 1.3
10.7 1.0
sediment core, approximately 95% fine sand (Table 1). The pH (7.9 ± 0.1) and Eh (60 ± 7.8 mV) values in the surface sediment layers were the highest among the sampled sites (Fig. 2). The NE region had the highest organic matter (17.5%) and carbonates (10.7%) of the three sites (Table 1). Sediments were predominantly silt with clay (Table 1). The general particle size of this site may be classified as silt (80.1%). The low hydrodynamics and high organic material content (17.5%) is likely influenced by the mangrove forests adjacent to the study region. The pH values (7.1 ± 0.1) which was constant down the sediment column and the redox potential (Eh) values (199.3 ± 28.6 mV) indicate reducing conditions down core (Fig. 2). The NE region has the highest average reactive and pyrite concentrations relative to the other distinct study regions (Fig. 2). The IE site had lower organic matter (3.8 ± 1.8) and carbonate (2.7 ± 1.3%) content than in the NE site (Table 1) but the organic matter was higher and the carbonates lower than the NO region. The IE site contained fine sand at surficial layers to very fine sand in the deeper layers (mean values of sand 84.6%) (Table 1). The pH values (7.1 ± 0.1) at IE site was lowest amongst the sampled regions, and the Eh (322.7 ± 23.8 mV) exceeded the values from the NE site (Fig. 2). The eutrophic sites (NE and IE) with anoxic sediments, contained a high fraction of As–pyrite (NE 4.3 ± 0.5 mg/kg and IE 2.2 ± 1.5 mg/kg), slightly higher than the reactive (As–HCl) fraction (NE = 2.2 ± 1.0 mg/kg and IE = 0.8 ± 0.6 mg/kg). The concentrations of the As–HCl fraction increase with depth at the IE site while the As–HCl fraction decreases with depth at NE site (Fig. 3). The increasing As–HCl fraction with depth at the IE site may be associated to the sulfide content (Krumbein, 1983). At the NO site, As– HCl and As pyrite fraction are highest at the surface, then dropping to the lowest value found at any site. Indeed, the top layer of the NO sediments was noticeably higher at the surface layer, which is likely related to the precipitation of Fe and Mn (oxy)hydroxides at a redox boundary, then subsequently decreasing.
The particle size variation may influences the differences in concentrations between the Mn and Fe content in the pyrite fraction between the NO and IE region. However, this variation is not noted in the As content in the same region of the pyrite fraction. Variation in the As content may be explained by the presence of a potential pollutant sources in the IE region, such pollutant source may originate from port activities, including dredging and an active fertilizer industry. These and other industrial activities may be responsible for the As enrichment at the IE region, though further studies should be conducted to determine if point pollutant sources exist. The species of seagrass in NE region is Halodule wrightii, is the most common seagrass in Brazil (Sordo et al., 2011). It grows in coastal, shallow water, and is known to have a role in cycling trace metals (Prange and Dennison, 2000). The content of metals in Halodule wrightii can be quite high, with different patterns of internal distribution of metals, such as low concentrations of Fe. The unimpacted area of this estuarine system is bound in mangrove forests. It is important to note that this vegetation may affect the physical and biogeochemical properties of coastal soils (Andrade et al., 2012; Sanders et al., 2008). Based on the trace metal content in both the HCl and pyrite fractions, it is possible to calculate the degree of pyritization (DOP) and the degree of trace metal pyritization (DTMP). The DOP and DTMP calculations are based on Eqs. (1) and (2) and the results are shown on Figs. 4 and 5. Huerta-Diaz and Morse (1990) and Huerta-Diaz and Morse (1992) classified he trace metals into three groups according to their behavior DTMP–DOP: Group 1 – Presents complete transition from reactive fraction to the pyrite fraction even at low levels PDO and independent of the type of sedimentary environment. Group 2 – Metal exhibit a gradual increase in the level of DTMP with increasing DOP concentrations with shared equally between the pyrite and reactive stages. Small variations may occur as analyzed metal. Group 3 – Impoverished in metals typically pyrite fraction consisting of transition metals which do not form complexes with sulfides. In this study, similar behaviors in DOP and DTMP–As were observed in the eutrophic sites, NE and IE. This can be verified by the maximum DOP values at the IE and NE, of 68.9% and 68.1% respectively. The highest values of DTMP–As (99.1–99.5%) in the NE, is likely related to the higher concentrations of organic matter (5 fold higher), fine-grained sediments and low hydrodynamics, all of which are favorable conditions for pyritization. The high metal pyritization at the NE site is indicative of sulfur, organic matter and Fe availability, because the absence of these factors limits the production of pyrite (Bollhöfer and Rosman, 1988) An incomplete transformation of iron sulfides to pyrite was observed in anoxic sediments of Kau Bay in Indonesia (Middelburg et al., 1989) and in areas of the Amazon (Kasten et al., 1998). The incomplete pyrite transformation in these areas may be due to a decrease in zero-valent sulfur during the pyrite formation. These processes may explain the different behavior of reactive and pyrite fractions between differing regions of the PEC. The NE site presented DTMP–As values that follow the DOP trend, remaining constant with depth, near 40% DOP–As (%) and near 100% DTMP–As (%) (Figs. 4 and 5). Furthermore, the DOP–As (%) varied little and DTMP–As (%) remained almost constant, at 90% down the sediment column similar to the NE site. In contrast, the NO site contained significantly lower DTMP–As and DOP–As (%) values. These results demonstrate the differing pyritization processes in specific areas of the PEC. Eutrophic areas show increasing in DTMP–As along with an increase in DOP in relation
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Fig. 2. Depth variations in pH and Eh (mV) in the sediment cores from this study.
Fig. 3. Depth variations of As (mg/kg), Fe (g/kg) and manganese (g/kg) content in the naturally oligotrophic (NO), naturally eutrophic (NE) and impacted eutrophic (IE).
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Fig. 4. Depth profiles of DOP As (%), Fe (%), Mn (%) and (DTMP) As (%), Fe (%), Mn (%) in the naturally oligotrophic (NO), naturally eutrophic (NE) and impacted eutrophic (IE).
Fig. 5. Degree of trace metal pyritization (DTMP) (As%) as a function of the degree of pyritization (DOP).
to the NO region, e.g. Huerta-Diaz and Morse (1990). Indeed, low levels of both DTMP–As and DOP–As were noted at the NO site, ranging from 3.51% to 13.43%, and 17.6% to 37.2%, respectively. In contrast, concurrent DTMP–As and DOP behavior was not observed in the NO region. At the NO site the low values of DTMP–As are indicative of limited sulfide availability. Hence, the superficial sediment layer (0–2 cm) of the NO site shows high concentrations of both As–pyrite and As-reactive fractions (2.21 mg/ kg), even higher than those found at the eutrophic sites. At the NO site, large sea grass meadows are present as noted in (Sordo et al., 2011). Indeed, the sea grass roots assimilate sulfur (S), preferring the reduced form (sulfide) to the oxidized sulfate present
in sea water (Pulich and White, 1991). Sea grass roots provide substrate for microbial activity as well as enhancing anaerobic processes (Holmer et al., 2001). Thus, the presence of sea grass and other coastal vegetation may have a significant influence on the sediment geochemistry. It should be pointed out the possible overestimation of the pyrite-bound As fraction, in relation to organic-rich sediments at the NE site as suggested in Huerta-Diaz and Morse (1990); (Ye et al., 2011). However, Huerta-Diaz and Morse (1990) performed extractions with and without the analysis of the organic fraction and the results support the claim that this metalloid are more significantly associated with the pyrite fraction. In sediments with low organic matter content this fraction may be discarded, except for Cu, but should not be ignored in sediments with >3% organic matter (Huerta-Diaz and Morse, 1990). As such, the As pyrite fraction in the NE site, may be influenced by the high levels of organic matter. In this work, the organic fraction was not analyzed and it is assumed that the As–pyrite fraction may have higher values due to the conditions found in NE and IE sites. Therefore, further research should be done on the organic fraction and its origin (terrestrial or marine) in this and other similar study sites (Chaillou et al., 2003; Huerta-Diaz and Morse, 1990, 1992; Keon et al., 2001). Sullivan and Aller (1996) identified hydrological and geochemical processes on early diagenesis related to As along differing estuarine areas. These authors found that the diagenetic cycle of As in Amazon shelf sediments are directly influenced by the flow of the Amazon River, increasing the diffusive processes with increasing river discharge and subsequent resuspension of sediments. Saulnier and Mucci (2000) showed Fe release from anoxic sediment through resuspension in the water column is removed swiftly, generally less than 60 min, while the Mn and As remain in solution for longer than a week. The similarity of the residence
F. Sá et al. / Marine Pollution Bulletin 96 (2015) 496–501
time of the Mn and As suggests that a significant fraction of As (III) is oxidized to As (V) through precipitating Mn oxyhydroxides. However, previous studies have shown that a majority of As (III) released into the solution is adsorbed or co-precipitated by Fe oxy-hydroxides in the first hours of resuspension (Saulnier and Mucci, 2000). Another plausible hypothesis to explain the low concentrations of the As–pyrite and As-reactive fractions at the IE site is dilution through bioturbation and changing redox conditions. There were noticeable fluctuations in particle size within the eutrophic regions, from the 8–11 cm depth. Surface sediments from NE are fine grained, while at the IE site sand is predominated. This grain size difference may explain the increase in the As bound to meta-sulfides while pyrite are mostly found below the 11 cm depth. This stratification may be explained by the transfer of metals from the reactive fraction to the pyrite fraction. In contrast, the reducing conditions, the OM-rich and fine-grained nature of these sediments along the entire sediment column at NE site, provides strong evidence for the high As pyritization. The differing geochemical properties of inundated sediments, including specific trophic conditions, hydrodynamics characterization and the presence of vegetation, such as sea grass and mangrove, may play an important role in influencing pyritization (Huerta-Diaz and Reimer, 2010). This work shows baseline data demonstrating that As behavior is directly related to many factors, including trophic conditions, grain size, organic matter content and coastal vegetation. The As pyritization processes is extremely important when considering the effects and impacts of coastal development, such as coastal eutrophication and diminishing coastal vegetation, including sea grass and mangroves. Acknowledgements This research is a contribution to projects from the Brazilian research agencies CAPES and CNPq. C.J. Sanders is supported by a Southern Cross University fellowship and the Australian Research Council (DP150103286). References Andrade, R.A., Sanders, C.J., Boaventura, G., Patchineelam, S.R., 2012. Pyritization of trace metals in mangrove sediments. Environ. Earth. Sci. 67, 1757–1762. Berner, R.A., 1984. Sedimentary pyrite formation: an update. Geochim. Cosmochim. Acta 48, 605–615. Bollhöfer, A., Rosman, K.J.R., 1988. Isotopic source signatures for atmospheric lead: the Northern Hemisphere. Geochim. Cosmochim. Acta 65, 1727–1740. Chaillou, G., Schäfer, J., Anschutz, P., Lavaux, G., Blanc, G., 2003. The behaviour of arsenic in muddy sediments of the Bay of Biscay (France). Geochim. Cosmochim. Acta 67, 2993–3003. dos Anjos, V.E., Machado, E.D.C., Grassi, M.T., 2012. Biogeochemical Behavior of Arsenic Species at Paranaguá Estuarine Complex, Southern Brazil. Aquat. Geochem. 18, 407–420. Giblin, A.E., Howarth, R.W., 1984. Porewater evidence for a dynamic sedimentary iron cycle in salt marshes. Limnol. Oceanogr. 29, 47–63. Holmer, M., Andersen, F.O., Nielsen, S.L., Boschker, H.T.S., 2001. The importance of mineralization based on sulfate reduction for nutrient regeneration in tropical seagrass sediments. Aquat. Bot. 71, 1–17.
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