Quantification of sedimentary black carbon using the chemothermal oxidation method:: an evaluation of ex situ pretreatments and standard additions approaches

The highly condensed products and residues of incomplete combustion of biomass and fossil fuel termed black carbon ( BC) partake in a multitude of important geochemical processes. However, ambiguity persists because different quantification methods give highly variable results, leaving it unclear whether this reflects method difficulties or that different methods simply mirror different parts of the BC spectrum. Introducing additional wet chemical ex situ pretreatment steps (Gelinas et al. 2001b), to a commonly applied chemothermal oxidation (CTO) method led to large handling and transfer losses of hydrophobic diesel-soot BC (NIST SRM-2975). The resulting yield of soot BC spiked to a coastal marine and a freshwater sediment was only 6 +/- 1% and 70 +/- 4%, respectively, for the entire ex situ procedure. Instead, a standard addition approach yielded statistically significant linear returns of incremental soot BC additions. The slopes suggested that 51 +/- 6% and 78 +/- 6% of the added soot BC was accounted for in the marine and freshwater sediment, respectively. The lower recovery of soot BC in the marine sediment is likely caused by chloride enhancing the contact between BC surfaces and mineral oxides, catalyzing the BC oxidation. A consequential lowering of the oxidation energy from 375 degreesC to 360 degreesC resulted in increased soot BC yields of 75 +/- 7% and 97 +/- 3% in the marine and freshwater sediments, respectively. The standard addition approach suggests a native "soot BC-like" concentration of 0.48 mg BC/g dry weight ( soot BC/total organic carbon = 0.03) in the bioturbated marine sediment from Kosterfjord, Skagerrak and 6.2 mg BC/g dry weight ( soot BC/TOC = 0.10) in the heavily polluted Rhine River sediment.