Post depositional changes of sedimentary organic matter influence chromium speciation in continental slope sediments - A case study.

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 (Cr) in the slope areas of both the coasts were not elevated and ranged from 56.8 to 163 mg·kg (avg. 97.3 ± 28.3 mg·kg). 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; avg. 89.6 ± 24.3 mg·kg) 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; avg. 6.7 ± 6.3 mg·kg) followed by reducing binding phase (Fe/Mn-oxyhydroxide phase) (0.1-1.7 mg·kg; avg. 0.7 ± 0.3 mg·kg). Concentration of Cr (VI) in the slope sediments, varied from 0.2 to 8.3 mg·kg (avg. 1.2 ± 1.0 mg·kg), was also predominantly associated with the SOM. The δC 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 Cr) increased with increasing SOM concentration by 0.008 mg·kg. One unit increase of sedimentary molar C/N ratio was found to increase normalized Cr (VI) (w.r.t. Cr) by 0.003 mg·kg 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.