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Naphthalene Sorption to Organic Soil Materials Studied with Continuous Stirred Flow Experiments

Danish Inst. of Agricultural Sciences, Dep. of Crop Physiology and Soil Science, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark

Timo J. Heimovaara and J. M. Verstraten

Dep. of Physical Geography and Soil Science, Univ. of Amsterdam, Nw. Prinsengracht 130, 1018 VZ Amsterdam, The Netherlands

*Corresponding author (Hubert.deJonge{at}agrsci.dk).

ABSTRACT

Estimation of sorption-desorption kinetics of hydrophobic contaminants in soils and sediments is a prerequisite for assessing the risk of hazardous compounds and for studying the feasibility of bioremediation treatments. Naphthalene sorption studies were carried out with four organic soil materials, using a batch sorption technique and a continuously stirred flow (CSF) cell. Reproducibility of the CSF experiments was tested (root mean squared error = 0.074), and an experiment with different inputs showed that experimental results were independent of input pulse length (RMSE = 0.090). Single-particle and multi-particle linear driving force models and bicontinuum models were tested. When the sorption coefficient K_om was fixed at the values obtained from the batch experiments, the RMSE modeling error increased with increasing N₂ surface area, S_N2, of the soil materials. The high RMSE for soil materials with a high N₂ surface area was the result of strong sorption-desorption non-singularity, most probably due to a larger fraction of the applied naphthalene diffusing to "slow" sorption sites. A dual resistance sorption model was able to accurately describe the data (average RMSE = 0.143) with two free parameters. However, parameter uncertainty resulted from the simultaneous optimization of the rate parameter, , and K_om. The combination of batch sorption experiments, input-response experiments, and model exercises give supporting evidence that sorption kinetics of hydrophobic organic compounds to soil OM are controlled by (i) rapid pore diffusion toward S_N2 ( 0.1 h^-1) and (ii) slow diffusion into the soil organic matter structure ( 0.01–0.001 h^-1).

Received for publication February 18, 1997.