ABSTRACT
Influences of post depositional changes of sedimentary organic matter (SOM) on chromium (Cr) speciation in continental slope sediments from the east and west coast of India are presented in this study. Average concentrations of total sedimentary Cr (CrT) in the slope areas of both the coasts were not elevated and ranged from 56.8 to 163 mg·kg-1 (avg. 97.3 ± 28.3 mg·kg-1). The geology of adjacent land masses influenced sedimentary Cr concentration in the slope areas. Geochemical fractionation study showed that a major fraction (51.6-159 mg·kg-1; avg. 89.6 ± 24.3 mg·kg-1) of the total sedimentary Cr was present in residual part of the sediments. Prime hosting phase for non-residual sedimentary Cr was oxidizable binding phase (SOM) (1.4-23.9 mg·kg-1; avg. 6.7 ± 6.3 mg·kg-1) followed by reducing binding phase (Fe/Mn-oxyhydroxide phase) (0.1-1.7 mg·kg-1; avg. 0.7 ± 0.3 mg·kg-1). Concentration of Cr (VI) in the slope sediments, varied from 0.2 to 8.3 mg·kg-1 (avg. 1.2 ± 1.0 mg·kg-1), was also predominantly associated with the SOM. The δ13Corg values of the SOM (ranged from -19.4 to -21.4; avg. -20.3 ± 0.5) suggests that they were derived from marine sources. Based on molar C/N ratio (ranged from 6.3-12.9; avg. 9.0 ± 2.0), the SOM were classified as labile organic matter. Increasing molar C/N ratio in the studied sediments was used as a proxy to reflect post depositional changes of the SOM. Association of normalized Cr (VI) (with respect to CrT) increased with increasing SOM concentration by 0.008 mg·kg-1. One unit increase of sedimentary molar C/N ratio was found to increase normalized Cr (VI) (w.r.t. CrT) by 0.003 mg·kg-1 in the sediments. This study suggests that post-depositional changes of SOM may increase Cr binding capacity and control its speciation and mobility in marine sediment system.
ABSTRACT
This study investigated the linkage between mercury (Hg) speciation in the surficial sediments from a mangrove ecosystem of the Zuari Estuary, west coast of India, with Hg bioaccumulation in gastropods collected from the same area. Multiple operationally defined protocols and methods were used for determination of Hg speciation study in the mangrove sediments. Moderately low concentrations of Hg were observed in the sediments, ranging from 37.3⯱â¯1.9 to 79.6⯱â¯4.0⯵g/kg. Geochemical fractionation showed that a significant part of sedimentary Hg was present within the structure of the sediment (residual fraction) and not bioavailable. Non-residual Hg was primarily associated with oxidizable (sedimentary organic matter (SOM) or sulfide) binding phase of the sediments, and ranged from 9.2⯱â¯0.3 to 78.5⯱â¯3.9⯵g/kg. Concentration of methylmercury (MeHg) (a neurotoxin) in the sediments varied from 1.7⯱â¯0.1 to 4.4⯱â¯0.1⯵g/kg. l-Cysteine, a suitable complexing ligand, extractable Hg concentration in the sediments ranged from 4.3⯱â¯0.1 to 15.9⯱â¯0.3⯵g/kg. Statistical analysis suggested that MeHg was adsorbed on Fe/Mn oxyhydroxide phases in the sediments. l-Cysteine was found to extract sedimentary MeHg and thermodynamically less stable Hg-SOM complexes from the sediments. Concentrations of bioaccumulated Hg in soft tissues of the gastropod, Pirenella cingulata, ranged from 57.6⯱â¯4.4 to 224.4⯱â¯7.2⯵g/kg. Positive correlations existed between the concentration of bioaccumulated Hg in the gastropods and the concentrations of Hg associated with the oxidizable phase, sedimentary MeHg and l-Cysteine extracted Hg in the sediments. This study indicated that operationally defined Hg species can be useful in estimating bioavailable Hg to obligatory deposit feeder in tropical mangrove systems.
