Effect of biostimulation and bioaugmentation on degradation of polyurethane buried in soil

Abstract

This work investigated biostimulation and bioaugmentation as strategies for removing polyurethane (PU) waste in soil. Soil microcosms were biostimulated with the PU dispersion agent "Impranil" and/or yeast extract or were bioaugmented with PU-degrading fungi, and the degradation of subsequently buried PU was determined. Fungal communities in the soil and colonizing buried PU were enumerated on solid media and were analyzed using denaturing gradient gel electrophoresis (DGGE). Biostimulation with yeast extract alone or in conjunction with Impranil increased PU degradation 62% compared to the degradation in untreated control soil and was associated with a 45% increase in putative PU degraders colonizing PU. Specific fungi were enriched in soil following biostimulation; however, few of these fungi colonized the surface of buried PU. Fungi used for soil bioaugmentation were cultivated on the surface of sterile wheat to form a mycelium-rich inoculum. Wheat, when added alone to soil, increased PU degradation by 28%, suggesting that wheat biomass had a biostimulating effect. Addition of wheat colonized with Nectria haematococca, Penicillium viridicatum, Penicillium ochrochloron, or an unidentified Mucormycotina sp. increased PU degradation a further 30 to 70%, suggesting that biostimulation and bioaugmentation were operating in concert to enhance PU degradation. Interestingly, few of the inoculated fungi could be detected by DGGE in the soil or on the surface of the PU 4 weeks after inoculation. Bioaugmentation did, however, increase the numbers of indigenous PU-degrading fungi and caused an inoculum-dependent change in the composition of the native fungal populations, which may explain the increased degradation observed. These results demonstrate that both biostimulation and bioaugmentation may be viable tools for the remediation of environments contaminated with polyurethane waste.

FIG. 1.

DGGE profiles of fungal communities in soil and on PU after treatment of soil microcosms with sterile distilled water (lanes CS and CP), yeast extract (lanes YS and YP), Impranil (lanes IS and IP), or both Impranil and YE (lanes IYS and IYP). The numbers indicate bands that were sequenced and identified.

FIG. 2.

Phylogenetic analysis of sequences cloned from bands in the DGGE profiles of fungal communities in the soil and on the surface of buried PU whose intensities increased following Impranil treatment. ITS1-5.8S-ITS2 sequences were compared to sequences of putatively closely related species by constructing a neighbor-joining phylogenetic tree (bootstrap corrected with 1,000 samples). Isolates obtained in this study are indicated by bold type, and the numbers in parentheses indicate the corresponding bands in Fig. 1.

FIG. 3.

Tensile strength of PU after 12 weeks of burial in soil treated with YE and/or Impranil. The tensile strength of unburied PU is also shown. The inset shows the values minus the value for the unburied control (n = 15).

FIG. 4.

DGGE of fungal soil communities 4 weeks after inoculation of soil with wheat colonized by PU-degrading fungal isolates. The marker lanes (lanes 3 and 8) contained DGGE products from all eight isolates. The arrows in lanes 4, 6, and 9 indicate bands that comigrated with bands produced by Mucormycotina sp., P. viridicatum, and N. haematococca, respectively.

FIG. 5.

DGGE of fungal communities colonizing PU after 4 weeks of burial in soil inoculated with wheat colonized by PU-degrading fungal isolates. Marker lane 5 contained DGGE products from all eight isolates. The arrows in lanes 3 and 6 indicate bands that comigrated with bands produced by the Mucormycotina sp. and P. viridicatum, respectively.

FIG. 6.

Tensile strength of PU after 4 weeks of burial in soil inoculated with wheat colonized by putative PU-degrading fungi. Unburied PU, PU buried in soil containing sterile wheat, and PU buried in soil with no wheat were included as controls. The error bars indicate standard errors of the means (n = 15).