Summary Band 22

Thiele, S. (1997): PAHs in polluted soils: Investigations on the characterization, optimization and prognosis of the microbial degradation of PAHs as well as the estimation of the PAH-fraction available to plants. Bonner Bodenkundl. Abh. 22 (1997), 318 pp.

 

To investigate the microbial PAH decomposition pot experiments were conducted under field conditions completed by laboratory studies. Easily degradable organic substances, mineral nutrients, in part lime and synthetic surfactants were added in varied combinations to ten different soil samples from polluted industrial sites. In preliminary experiments 20 surfactants were tested. Four nonionic products were selected according to their good PAH solubilization properties and low toxicity for microorganisms. Soil samples sterilized by HgCl2 were set up to quantify abiotic processes of the PAH decrease. 15 out of 16 PAHs listed on the US EPA list (1979; without Acenaphthiylene) were determined.

Depending on the kind and quantity of the amendments the biological activity of the soil sampIes could considerably be increased. Thus an essential decrease of the contaminants' toxicity for soil microorganisms and cultivated plants was achieved. By increasing the (co-)metabolic decomposition activity of the microorganisms and depending on the different treatments, a decrease of the PAH contents between 4.6 and 90.9% was achieved within 74 weeks. Still after 121 weeks, an in parts remarkable further decrease of the PAH contents could be observed.

Above all, the decrease of the PAHs is based on the microbial decomposition and bonding and for the most part can be fitted by two coupled exponental functions. Effects like the removal by plants or leaching by percolating water are not relevant. Abiotic processes make up less than about 30% of the PAH decrease. On the other hand mobilizations of PAHs could also be observed. Apart from solubilizing agents of pedo-, and biogenic as well as technical origin, this is caused by the fluctuating water content of the soils.

Higher PAH contents lead to higher PAH decreases. However, they end at higher remainder contents. The persistence of PAHs is mainly determined by their availability, which in turn can approxirnately be described by their KF values as a determinant of their adsorption. Therefore lower condensed PAHs with low KF values are more availabIe and are decomposed more strongly. In contrast the contents of the higher condensed 5- and 6-ring PAHs are primarily influenced by fixation and remobilization processes with the quantity of the soil exchange sites being less important than their quality.

The PAH degradation is increased by the soil treatments. However, the effects are only in parts statistically significant. The addition of easily degradable organic matter (C/N < 20) in a higher quantity (3%w/w) combined with the addition of nitrate and a surfactant most effectively increases the degradation of the PAHs. Raising the pH of a very strong acid soil sampIe revealed an optimum PAH decomposition at pH 6.

Valid data on the microbial PAH degradation were availabIe from the pot experiments. The PAH uptake from soil by food plants was investigated by lysimeter trials of the LUA NRW (DELSCHEN et al. 1996). It therefore was possibIe to use the appropriate soil sampIes for the development of laboratory methods for the determination of the degradable as well as the plant availabIe PAH portions. Screening tests were carried out with different extractants: H2O (bidest.), and salt solutions, 0.02 - 5% concentrated solutions of nonionic surfactants, organic solvents and solvent mixtures. The most important selection criterion was the correlation between the results of the extractions and the target parameters. In addition different variations of the extraction procedure were tested.

More detailed experiments on the determination of the degradable PAH contents were conducted with three variants of each of the ten pot trial sampIes. On the basis of the screening tests toluene/acetone (3:2) and the single as well as the twice repeated extraction with 0.5% concentrated surfactant solutions (Genapol UDD 88 or Synperonic LF/RA 30, resp.) were chosen. The results of these extractions describe the contents of the degradabIe PAHs weIl with r = 0.615 - 0.878. For that purpose the PAHs were classified in groups depending on the degree of condensation and an equation of regression was calculated for each group. However, cIoser correlations to the soil characteristics or physicochernical parameters of the PAHs are not existant. Nine soil sampIes from the lysimeter experiments were used to characterize the PAH portion in soils available to plants. The uptake of PAHs by plants was measured on the basis of their contents in spinach surveyed with the soils. The PAH contents extractable with ethanol/water (1:4v/v) and IM NH4NO3 correlate cIosely with the contents of the different PAH groups in the spinach with r = 0.697 - 0.930. The tested extraction procedures can be recommended for application.

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