Laboratory simulation of chemical evaporation from dredge-produced sediment slurries

Louis J. Thibodeaux, Hubert Huls, Raghunathan Ravikrishna, Kalliat T. Valsaraj, Mike Costello, Danny D. Reible

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1 Scopus citations

Abstract

A laboratory-scale apparatus and associated chemodynamic transport model was tested with samples of field-contaminated sediment containing high concentrations of low volatility polyaromatic (PAHs) and high volatility petroleum hydrocarbons (BETXs). The experiments were designed to measure the flux to air while attempting a realistic simulation of the aquatic conditions resulting from the extraction, transport, and treatment of the dredged material (DM) derived from the bed-sediment source material. The test apparatus provided for horizontal air flow over a chamber containing particle-in-water slurries that could also be artificially stirred. This batch desorption design was adapted to mimic both the wind-water enhanced and the dredging operations mechanical turbulence effects on the evaporation process. The air was sampled to quantify the flux of 12 chemicals in experiments using sediment-to-water ratios of 1 and 8%. Naphthalene was the most abundant constituent in the sediment and gave the highest fluxes. Initial values of 86, 130, and 190 mg/m 2·h were observed for the three experiments conducted. The flux of all chemicals in the 1% (quiescent) and 8% (mixed) experiments decreased exponentially with time over the 24-h test period as predicted by the chemodynamic model, which assumed fast particle-to-water chemical release followed by water solution concentration driven evaporation. Both the water-side and air-side mass-transfer resistances were observed to control the evaporation kinetics of the PAHs while the BETXs were only water-side controlled. This confirmed the well-known two-resistance theory for mass transfer by evaporation; however, further data analysis on the applicability of the two-phase equilibrium partitioning theory in predicting concentrations in water was a failure. This approach, uniformly used by practitioners in an a priori fashion to estimate the solute concentrations in sediment-water (s/w) slurries underestimated the PAH concentrations by a factor of 60 (average) and overestimated the BETX concentrations by a factor of 30 (average). The overall simulation tests results indicate that although the evaporation process from dredge related s/w slurries follows well-established evaporation kinetics, the conventional assumed equilibrium release model is inappropriate. This implies that laboratory simulations will be necessary for realistically quantifying the chemical solute concentrations produced by the s/w slurries formed during environmental dredging.

Original languageEnglish
Pages (from-to)730-740
Number of pages11
JournalEnvironmental Engineering Science
Volume21
Issue number6
DOIs
StatePublished - 2004

Keywords

  • Dredge remediation releases
  • Flux to air measurements
  • Solid-water partitioning
  • Transport to air
  • Water-to-air partitioning

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