CD-enhanced soil remediation by ‘sugar flushing’

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There are several soil remediation technologies utilizing cyclodextrins as solubilizers of organic contaminants (Gruiz et al. 2019). Soil washing/flushing with cyclodextrin solutions proved to be efficient both in laboratory and in the field.
Soil flushing with HPBCD solution (‘sugar flushing’) (Boving & Brusseau, 2000) is a potential technology for the removal of crude oil containing normal alkanes (nC15–nC35) and polyaromatic hydrocarbons (PAHs) from porous media (Gao et al., 2012). The beneficial effect of CD extraction decreases with increasing n-alkane chain length. CD significantly enhanced PAH extraction from sand, and the enhancement effect increased in the order of parent compounds < C-1 substituted < C-2 substituted < C-3 substituted for most PAHs tested (Gao et al., 2012).
Carboxymethyl BCD (CMBCD) was found to be effective for washing soils containing mixed (organic and inorganic) contaminants (Brusseau et al., 1997; Chatain et al., 2004, Skold et al., 2009). The mechanism includes salt formation with metals and inclusion complex formation with organic pollutants. Especially good results were obtained when RAMEB and EDTA were applied together to a soil contaminated with metals and PCBs. By repeating the extraction three times, as much as 76% and almost 100% of the PCB and mobile-metal content (Cd, Cu, Mn and Pb), respectively, could be removed (Ehsan et al., 2007). Good removal efficiencies (>80%) for PAHs, toxic metals and fluorine were obtained by applying CMBCD and carboxymethyl chitosan (CMC) solution for washing a soil from an abandoned metallurgic plant (Ye et al., 2014).
The efficiency of the ‘sugar flushing’ technology using aqueous HPBCD solution has been demonstrated in various field experiments on a waste dump site (McCray & Brusseau, 1998, 1999) and on military sites (Tick et al., 2003; Blanford et al., 2000) contaminated with PCE or TCE as DNAPLs. The push-pull flushing system (applying the same well periodically for injection and extraction) proved to be more efficient than the line-drive system (separate injection and extraction wells) (Figure above shoes the scheme of technology) (Boving et al., 2008). The HPBCD solution injected to the well was extracted after a reaction time and the contaminant was separated by stripping, distillation or sorption on activated carbon. The regenerated HPBCD solution was pumped back for the next cycle of soil flushing.

Cost efficiency analysis showed that the total implementation costs for the CD-enhanced and surfactant-enhanced soil flushing technologies (CDEF and SEF) are comparable. The time needed to complete remediation can be reduced significantly by both CDEF and SEF technologies (Blanford et al., 2006). CDEF also offers the ecological benefit of only introducing a non-toxic and degradable material into the subsurface. Regeneration of the CD solution makes the technology more economical (Boving et al., 2007). Recent studies have revealed that the HPBCD remained in the subsurface after cessation of the active remediation was utilized as co-substrate in anaerobic degradation of the chlorinated hydrocarbons (Hinrichs, 2004, Blanford et al., 2018). In this spontaneous process also nitrate and sulfate concentrations were reduced due to their role as terminal electron acceptors in the anaerobic biodegradation, while no significant change was observed in the concentration of dissolved oxygen and total iron at the site.
Soil flushing with CD solution can be combined with in situ bioremediation utilizing the benefits of the enhanced bioavailability of the CD-solubilized contaminants (Gruiz et al., 2011; Leitgib et al., 2007; Molnár et al. 2005).
The soil effluents deriving either from soil washing or soil flushing can be decontaminated in various ways, but the effects of the complex-forming agents or surfactants on the selected technology should be considered:

  • Stripping (the residence time in the stripper should be increased as the volatility of the contaminants is usually decreased in the presence of both CDs and surfactants) (Kashiyama & Boving, 2004);
  • Sorption on activated carbon (the sorbent should be carefully selected to avoid the sorption of the additives without reducing the sorption of the organic pollutants) (Sniegowski et al., 2014);
  • Advanced oxidation processes (AOPs);
  • Liquid/liquid extraction by using vegetable oil (colza oil) (Petitgirard et al., 2009);
  • Electrochemical treatment (Gomez et al., 2010);
  • Biological treatments (the most economical technologies for regeneration of the CD solutions containing the soil contaminants using either the indigenous microflora or activated sludge) (Berselli et al., 2004; Yoshii et al., 2001; Gruiz et al., 2008). CDs are nontoxic to the microbes, they catalyze the decomposition processes and are eventually biodegraded themselves (Fenyvesi et al., 2005, Molnár et al., 2005; Verstichel et al., 2004).

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