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In Situ, High-Resolution Profiles of Labile Metals in Sediments of Lake Taihu

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ABSTRACT

Characterizing labile metal distribution and biogeochemical behavior in sediments is crucial for understanding their contamination characteristics in lakes, for which in situ, high-resolution data is scare. The diffusive gradient in thin films (DGT) technique was used in-situ at five sites across Lake Taihu in the Yangtze River delta in China to characterize the distribution and mobility of eight labile metals (Fe, Mn, Zn, Ni, Cu, Pb, Co and Cd) in sediments at a 3 mm spatial resolution. The results showed a great spatial heterogeneity in the distributions of redox-sensitive labile Fe, Mn and Co in sediments, while other metals had much less marked structure, except for downward decreases of labile Pb, Ni, Zn and Cu in the surface sediment layers. Similar distributions were found between labile Mn and Co and among labile Ni, Cu and Zn, reflecting a close link between their geochemical behaviors. The relative mobility, defined as the ratio of metals accumulated by DGT to the total contents in a volume of sediments with a thickness of 10 mm close to the surface of DGT probe, was the greatest for Mn and Cd, followed by Zn, Ni, Cu and Co, while Pb and Fe had the lowest mobility; this order generally agreed with that defined by the modified BCR approach. Further analyses showed that the downward increases of pH values in surface sediment layer may decrease the lability of Pb, Ni, Zn and Cu as detected by DGT, while the remobilization of redox-insensitive metals in deep sediment layer may relate to Mn cycling through sulphide coprecipitation, reflected by several corresponding minima between these metals and Mn. These in situ data provided the possibility for a deep insight into the mechanisms involved in the remobilization of metals in freshwater sediments.

No MeSH data available.


Correlation analysis between DGT-labile Co and Mn in sediments (a) and the changes of their ratio with sediment depth (b).
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ijerph-13-00884-f006: Correlation analysis between DGT-labile Co and Mn in sediments (a) and the changes of their ratio with sediment depth (b).

Mentions: The distributions of labile Co were very similar to those of labile Mn at the five sites (Figure 5). This was further verified by positively significant correlations at very significant levels (p < 0.01) between them (Figure 6). Their precise correspondence in profiles has been reported in both freshwater [54,59] and marine systems [33,35,60,61,62]. Three mechanisms have been developed to explain their similarity in remobilization. (1) The reductive remobilization of Co and Mn required very similar redox conditions and they tended to occur simultaneously but independently; (2) Co may released from organic matter decomposition which supply electrons to cause the reduction of Mn(IV); (3) Co may be incorporated in Mn oxyhydroxides, and released with the reductive remobilization of Mn [35,61,63]. The last one should be the reason responsible for the similarity between the distributions of labile Co and Mn. It has been shown that Mn oxides had much greater preference than Fe oxides in trapping Co; Enrichment of Co within Mn oxides was 9 times that of Fe (oxyhydr)oxide from an investigation in marine sediment [62]. The presence of Co in individual Mn-oxide particles has also been verified by elemental analysis with electron microscopy [64]. And their association can be through sorption of Co(II) by Mn oxides [65]. Co(II) adsorbed to the oxide can be oxidized to Co(III) and subsequently incorporated into Mn oxide structure [66]. Co is then released from reductive dissolution of particulate Mn oxides, as verified in the water column of two freshwater lakes [63,65].


In Situ, High-Resolution Profiles of Labile Metals in Sediments of Lake Taihu
Correlation analysis between DGT-labile Co and Mn in sediments (a) and the changes of their ratio with sediment depth (b).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5036717&req=5

ijerph-13-00884-f006: Correlation analysis between DGT-labile Co and Mn in sediments (a) and the changes of their ratio with sediment depth (b).
Mentions: The distributions of labile Co were very similar to those of labile Mn at the five sites (Figure 5). This was further verified by positively significant correlations at very significant levels (p < 0.01) between them (Figure 6). Their precise correspondence in profiles has been reported in both freshwater [54,59] and marine systems [33,35,60,61,62]. Three mechanisms have been developed to explain their similarity in remobilization. (1) The reductive remobilization of Co and Mn required very similar redox conditions and they tended to occur simultaneously but independently; (2) Co may released from organic matter decomposition which supply electrons to cause the reduction of Mn(IV); (3) Co may be incorporated in Mn oxyhydroxides, and released with the reductive remobilization of Mn [35,61,63]. The last one should be the reason responsible for the similarity between the distributions of labile Co and Mn. It has been shown that Mn oxides had much greater preference than Fe oxides in trapping Co; Enrichment of Co within Mn oxides was 9 times that of Fe (oxyhydr)oxide from an investigation in marine sediment [62]. The presence of Co in individual Mn-oxide particles has also been verified by elemental analysis with electron microscopy [64]. And their association can be through sorption of Co(II) by Mn oxides [65]. Co(II) adsorbed to the oxide can be oxidized to Co(III) and subsequently incorporated into Mn oxide structure [66]. Co is then released from reductive dissolution of particulate Mn oxides, as verified in the water column of two freshwater lakes [63,65].

View Article: PubMed Central - PubMed

ABSTRACT

Characterizing labile metal distribution and biogeochemical behavior in sediments is crucial for understanding their contamination characteristics in lakes, for which in situ, high-resolution data is scare. The diffusive gradient in thin films (DGT) technique was used in-situ at five sites across Lake Taihu in the Yangtze River delta in China to characterize the distribution and mobility of eight labile metals (Fe, Mn, Zn, Ni, Cu, Pb, Co and Cd) in sediments at a 3 mm spatial resolution. The results showed a great spatial heterogeneity in the distributions of redox-sensitive labile Fe, Mn and Co in sediments, while other metals had much less marked structure, except for downward decreases of labile Pb, Ni, Zn and Cu in the surface sediment layers. Similar distributions were found between labile Mn and Co and among labile Ni, Cu and Zn, reflecting a close link between their geochemical behaviors. The relative mobility, defined as the ratio of metals accumulated by DGT to the total contents in a volume of sediments with a thickness of 10 mm close to the surface of DGT probe, was the greatest for Mn and Cd, followed by Zn, Ni, Cu and Co, while Pb and Fe had the lowest mobility; this order generally agreed with that defined by the modified BCR approach. Further analyses showed that the downward increases of pH values in surface sediment layer may decrease the lability of Pb, Ni, Zn and Cu as detected by DGT, while the remobilization of redox-insensitive metals in deep sediment layer may relate to Mn cycling through sulphide coprecipitation, reflected by several corresponding minima between these metals and Mn. These in situ data provided the possibility for a deep insight into the mechanisms involved in the remobilization of metals in freshwater sediments.

No MeSH data available.