Biodegradation of pyrene in sand, silt and clay fractions of sediment

Abstract

Microbial degradation is the dominant pathway for natural attenuation of PAHs in environmental compartments such as sediments, which in turn depends on the bioavailability of PAHs. The bioavailability of PAHs has seldom been studied at the sediment particle size scale. We evaluated biodegradation of pyrene by Mycobacterium vanbaalenii PYR-1 as a function of sediment particle sizes, and investigated the relationship between the rate of degradation on sand, silt and clay particles with their individual desorption kinetics measured with the Tenax extraction method. Regression analysis showed that the total organic carbon (TOC), black carbon (BC), and specific surface area (SSA) of the specific particle size fractions, instead of the particle size scale itself, were closely related (P<0.01) with the mineralization rate. While the fraction in the rapid desorption pool (F (rapid)) ranged from 0.11 to 0.38 for the whole sediments and different size groups, the fractions mineralized after 336-h incubation (0.52 to 0.72) greatly surpassed the F (rapid) values, suggesting utilization of pyrene in the slow desorption pool (F (slow)). A biodegradation model was modified by imbedding a two-phase desorption relationship describing sequential Tenax extractions. Model analysis showed that pyrene sorbed on silt and clay aggregates was directly utilized by the degrading bacteria. The enhanced bioavailability may be attributed to the higher chemical concentration, higher TOC or larger SSA in the silt and clay fractions, which appeared to overcome the reduced bioavailability of pyrene due to sorption, making pyrene on the silt and clay particles readily available to degrading microbes. This conjecture merits further investigation.

Fig. 1

Distribution of particle mass, total organic carbon content, and pyrene concentration among sand-, silt-, and clay-sized aggregates in Salinas Potrero (SP) and San Diego Creek (SDC) sediments (mass fractions: filled bars; TOC fractions: blank bars; pyrene concentration fractions: stripped bars)

Fig. 2

Pyrene degradation kinetics up to 336 h by the PYR-1 strain in SP sediment. a Mineralization in the sediment size aggregates: sand-(filled square), silt-(filled circle), and clay-sized (filled triangle) aggregates. b Observed mineralization values in the whole sediment particles (filled square) and the calculated values from the three sized fractions (filled circle) Bars represent standard deviations (n = 3)

Fig. 3

Pyrene degradation kinetics up to 336 h by the PYR-1 strain in SDC sediment. a Mineralization in the sediment size aggregates: sand-(filled square), silt-(filled circle), and clay-sized (filled triangle) aggregates. b Observed mineralization values in the whole sediment particles (filled square) and the calculated values from the three sized fractions (filled circle) Bars represent standard deviations (n = 3)

Fig. 4

Desorption kinetics of pyrene from SP (a) and SDC (b) sediment (filled circle), and different size fractions, sand (open circle), silt (filled inverted triangle), and clay (open triangle). S/S ₀ is the ratio of chemical concentration remaining in the sediment after desorption time t to that present in the sediment before desorption, and t is the cumulative desorption time

Fig. 5

Microbial mineralization of pyrene (filled square: observed values) on various sized particles in SP sediment. The solid lines represent the model 1 predicted values, and the dash lines represent the model 2 predicted values

Fig. 6

Microbial mineralization of pyrene (filled square: observed values) on various sized particles in SDC sediment. The solid lines represent the model 1 predicted values, and the dash lines represent the model 2 predicted values