Mobilization of Manufactured Gas Plant Tar with Alkaline Flushing Solutions

Department of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States.
Environmental Science & Technology (Impact Factor: 5.33). 11/2011; 46(1):426-33. DOI: 10.1021/es202278s
Source: PubMed


This experimental study investigates the use of alkaline and alkaline-polymer solutions for the mobilization of former manufactured gas plant (FMGP) tars. Tar-aqueous interfacial tensions (IFTs) and contact angles were measured, and column flushing experiments were conducted. NaOH solutions (0.01-1 wt.%) were found to significantly reduce tar-aqueous IFT. Contact angles indicated a shift to strongly water-wet, then to tar-wet conditions as NaOH concentration increased. Column experiments were conducted with flushing solutions containing 0.2, 0.35, and 0.5% NaOH, both with and without xanthan gum (XG). Between 10 and 44% of the residual tar was removed by solutions containing only NaOH, while solutions containing both NaOH and XG removed 81-93% of the tar with final tar saturations as low as 0.018. The mechanism responsible for the tar removal is likely a combination of reduced IFT, a favorable viscosity ratio, and tar bank formation. Such an approach may have practical applications and would be significantly less expensive than surfactant-based methods.

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    ABSTRACT: Tars at former manufactured gas plants (FMGPs) are a major environmental concern and present a number of challenges to remediators. This experimental study investigates the relationship between composition and tar-water interfacial tension (IFT), a property of primary importance in determining the transport of tar in porous media. Nine field-collected FMGP tars and a commercially available coal tar were characterized by means of fractionation, gas chromatography, Fourier transform infrared (FTIR) spectrometry, and vapor pressure osmometry. The tar-aqueous IFT of the tars, as well as resins and asphaltenes extracted therefrom, were measured over a range of pH. The IFTs were found to be strongly dependent on pH, with the lowest values obtained at high pH. The reduction of IFT at high pH was found to correlate well with the I(C═O) values from the FTIR analysis, which provide an indication of the relative amount of carbonyl groups present. Reductions of IFT at low pH were also observed and found to correlate well with the extractable base concentration. The aromaticity and asphaltene average molar mass are also correlated with IFT reductions at both low and high pH, suggestive of compositional patterns related to the tar source material.
    Environmental Science & Technology 08/2012; 46(18):10214-21. DOI:10.1021/es3024836 · 5.33 Impact Factor
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    ABSTRACT: The remediation of former manufactured gas plant (FMGP) sites contaminated with tar DNAPLs (dense non-aqueous phase liquids) presents a significant challenge. The tars are viscous mixtures of thousands of individual compounds, including known and suspected carcinogens. This work investigates the use of combinations of mobilization, solubilization, and chemical oxidation approaches to remove and degrade tars and tar components in porous medium systems. Column experiments were conducted using several flushing solutions, including an alkaline-polymer (AP) solution containing NaOH and xanthan gum (XG), a surfactant-polymer (SP) solution containing Triton X-100 surfactant (TX100) and XG, an alkaline-surfactant- polymer (ASP) solution containing NaOH, TX100, and XG, and base-activated sodium persulfate both with and without added TX100. The effectiveness of the flushing solutions was assessed based on both removal of polycyclic aromatic hydrocarbon (PAH) mass and on the reduction of dissolved-phase PAH concentrations. SP flushes of 6.6 to 20.9 PV removed over 99% of residual PAH mass and reduced dissolved-phase concentrations by up to two orders of magnitude. ASP flushing efficiently removed 95-96% of residual PAH mass within about 2 PV, and significantly reduced dissolved-phase concentrations of several low molar mass compounds, including naphthalene, acenaphthene, fluorene, and phenanthrene. AP flushing removed a large portion of the residual tar (77%), but was considerably less effective than SP and ASP in terms of the effect on dissolved PAH concentrations. Persulfate was shown to oxidize tar components, primarily those with low molar mass, however, the overall degradation was relatively low (30-50% in columns with low initial tar saturations), and the impact on dissolved-phase concentrations was minimal.
    Journal of Contaminant Hydrology 10/2014; 167:44-60. DOI:10.1016/j.jconhyd.2014.08.002 · 2.20 Impact Factor


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