The occurrence of alkanes in fulvic acid, a soil humic fraction

Abstract

Normal plus branched-cyclic alkanes were found to account for a maximal 0.16 per cent of the weight of a water-soluble soil fulvic acid. Only 3 per cent of the total alkanes could be extracted by hexane, benzene and ethyl acetate from untreated solid fulvic acid. Nheptane extracted approximately 12 per cent of the total alkanes from a solution of fulvic acid in methanol, and also from a suspension of untreated soil in methanol. Most of the alkanes were extractable only after methylation of the fulvic acid and adsorption on aluminum oxide. Reduction of hydrogen bonding by methylation appeared to be related to the release of alkanes by the fulvic acid. About one third of the total alkanes consisted of n-alkanes, the remainder were branchedeyclic hydrocarbons. Normal alkanes ranged from C 14 to C 36. Weight distribution plots of nalkanes exhibited two well denned curves. The first consisted of C 14 to C 23 n-alkanes, which accounted for 60 per cent of the mixture and had a C-odd to C-even carbon atom ratio of 1.00. The second curve was formed by C 24 to C 36 n-alkanes, which constituted the remaining 40 per cent of the mixture and had a C-odd to C-even ratio of 1.10. The shapes of the two distribution curves, and the C-odd to C-even carbon atom ratios of the n-alkanes which constitute these curves, suggest that C 14 to C 23 n-alkanes may be of microbiological origin whereas the higher n-alkanes may have originated from plants. Normal alkanes extracted from untreated fulvic acid contained considerably greater proportions of C 24 to C 34 hydrocarbons than of those of lower molecular weight. This indicates that the fulvic acid, possibly because of steric considerations, retains higher molecular weight alkanes less tightly than hydrocarbons of lower molecular weight.