<strong>C₁₄-₂₂ n-alkanes in Soil from the Freetown Layered Intrusion, Sierra Leone: Products of Pt Catalytic Breakdown of Natural Longer Chain n-alkanes?</strong>

Abstract

:Soil above a platinum-group element (PGE)-bearing horizon within the Freetown Layered Intrusion, Sierra Leone contains anomalous concentrations of n-alkanes (CnH2n+2) in the range C14 to C22 not readily attributable to an algal or lacustrine origin. Longer chain nalkanes (C23 to C31) in the soil were derived from the breakdown of leaf litter beneath the closed canopy humid tropical forest. Spontaneous breakdown of the longer chain n-alkanes to form C14-22 n-alkanes without biogenic or abiogenic catalysts is unlikely as the n-alkanes are stable. In the Freetown soil, the catalytic properties of the PGE (Pt in particular) may lower the temperature at which oxidation of the longer chain n-alkanes can occur. Reaction between these n-alkanes and Pt species such as Pt 2+ (H2O)2(OH)2 and Pt 4+ (H2O)2(OH)4 can bend and twist the alkanes, and significantly lower the Heat of Formation. Acknowledging the possibility of microbial catalysis and the difficulty of identifying a direct organic geochemical source of the lighter n-alkanes, this paper explores the theoretical potential for abiogenic Pt species catalysis as a mechanism of breakdown of the longer n-alkanes to form C14-22 alkanes. This novel mechanism could substantiate the presence of the PGE in solution predicted by soil geochemistry and illustrate processes involving the PGE. Graphical AbstractThe anomalous proportion of C14 to C22 n-alkanes with a C31 molecule distorted by Pt 4+ (H2O)2(OH)4