Concentration-dependent kinetics of pollutant desorption from soils.

Area of Science:

• Environmental Science
• Soil Science
• Chemical Engineering

Background:

• Sorption-desorption kinetics are crucial for understanding pollutant transport and bioavailability in soils.
• Contaminant concentration is a key factor influencing these kinetics.
• Previous research examined concentration effects on uptake rates.

Purpose of the Study:

• To investigate the impact of initial phenanthrene sorbed concentration (q(0)) on desorption rates from six soils.
• To analyze the formation and characteristics of a highly resistant fraction during desorption.
• To evaluate the applicability of the dual-mode diffusion model (DMDM) to desorption kinetics.

Main Methods:

• Soils were pre-equilibrated with phenanthrene for 180 days.
• Desorption rates were measured using a polymer adsorbent (Tenax) as a third-phase sink.
• Empirical curve fitting and the dual-mode diffusion model (DMDM) were applied to analyze kinetic data.

Main Results:

• Fractional desorption rates increased with initial sorbed concentration (q(0)) for nonlinear isotherms.
• A highly resistant fraction (4-31% of q(0)) persisted after long-term desorption.
• The resistant fraction decreased with increasing q(0) for nonlinear isotherms but increased for linear isotherms.
• The DMDM provided good fits to the nonresistant fraction, linking diffusivity to immobile sites.

Conclusions:

• Contaminant concentration significantly affects soil desorption rates and the persistence of resistant fractions.
• The dual-mode diffusion model explains concentration-dependent diffusivity through changes in sorbate site occupancy.
• Findings support heterogeneous soil organic matter models and emphasize the need to account for concentration effects in desorption studies.