2.5.1 Effect of Temperature on DNAPL Recovery
Thermal treatments have been used effectively for DNAPL recovery. Considering
the example of coal tar, increasing the temperature can remobilize the residual tar by
decreasing the values of the following parameters: density, interfacial tension with
water, contact angle with water on a solid medium, and viscosity (Huling and
Weaver 1996; Heron et al. 1998; U.S. Army Corps of Engineers 2014). Specifically,
coal tar viscosity is very sensitive to temperature and can vary by one to two orders
of magnitude when the temperature is increased from 20
C to 70
C (Baker et al.
2006; Brown et al. 2006; Birak and Miller 2009; Philippe et al. 2017).
A few cases of thermally enhanced coal tar pumping have been reported in
scientific literature, as they relate to field testing (McLaren et al. 2009). The authors
studied thermal enhancement as a sustainable alternative technique. The coal tars
were heated to 30
C, which reduced their kinematic viscosity by almost one order of
magnitude (100–10 cSt). Globally, 22 m
3 of tar were recovered in 6 months of
pumping, with a 30% reduction in costs compared to conventional pump-and-treat
methods.
Thermal enhancement can produce high recovery yields (90%), and can be
particularly interesting to treat highly contaminated areas with low permeabilities
where flushing is not a suitable remediation technique (Suchomel et al. 2014).
2.5.2 Effect of Surfactant Addition on DNAPL Recovery
The recovery mechanisms during surfactant flushing include two main stages:
(1) decreasing interfacial tension (IFT) and increasing contaminant solubility
(NAPLs); (2) mobilizing the residual contamination (Pennell et al. 2014).
(1) Decreasing IFT and Increasing Contaminant Solubility
At low concentrations, surfactant molecules will mainly accumulate at solid–liquid
or liquid–liquid interfaces (NAPL/water interface in our case, where a pure phase
exists). Surfactant molecules will gradually cover the NAPL/water interface as
surfactant concentration increases.
Increasing surfactant concentration will reduce IFT until all NAPL/water interfaces are covered. At this stage, the increase in surfactant concentrations will no
longer reduce the IFT: the surfactant molecules will agglomerate (forming surfactant
micelles), and will increase the solubility of the NAPL (present in the dissolved
phase). This concentration is called the Critical Micelle Concentration (CMC)
(Vishnyakov et al. 2013). The CMC of a surfactant depends on surfactant structure,
system temperature, ionic strength, and whether or not organic additives are present
in the solution (Laha et al. 2009).
Surfactant performance depends on their working environmental conditions. For
example, system temperature and salinity can influence surfactant effectiveness.
When the temperature increases, the reaction between the hydrophilic component
96
S. Colombano et al.
Thermal treatments have been used effectively for DNAPL recovery. Considering
the example of coal tar, increasing the temperature can remobilize the residual tar by
decreasing the values of the following parameters: density, interfacial tension with
water, contact angle with water on a solid medium, and viscosity (Huling and
Weaver 1996; Heron et al. 1998; U.S. Army Corps of Engineers 2014). Specifically,
coal tar viscosity is very sensitive to temperature and can vary by one to two orders
of magnitude when the temperature is increased from 20
C to 70
C (Baker et al.
2006; Brown et al. 2006; Birak and Miller 2009; Philippe et al. 2017).
A few cases of thermally enhanced coal tar pumping have been reported in
scientific literature, as they relate to field testing (McLaren et al. 2009). The authors
studied thermal enhancement as a sustainable alternative technique. The coal tars
were heated to 30
C, which reduced their kinematic viscosity by almost one order of
magnitude (100–10 cSt). Globally, 22 m
3 of tar were recovered in 6 months of
pumping, with a 30% reduction in costs compared to conventional pump-and-treat
methods.
Thermal enhancement can produce high recovery yields (90%), and can be
particularly interesting to treat highly contaminated areas with low permeabilities
where flushing is not a suitable remediation technique (Suchomel et al. 2014).
2.5.2 Effect of Surfactant Addition on DNAPL Recovery
The recovery mechanisms during surfactant flushing include two main stages:
(1) decreasing interfacial tension (IFT) and increasing contaminant solubility
(NAPLs); (2) mobilizing the residual contamination (Pennell et al. 2014).
(1) Decreasing IFT and Increasing Contaminant Solubility
At low concentrations, surfactant molecules will mainly accumulate at solid–liquid
or liquid–liquid interfaces (NAPL/water interface in our case, where a pure phase
exists). Surfactant molecules will gradually cover the NAPL/water interface as
surfactant concentration increases.
Increasing surfactant concentration will reduce IFT until all NAPL/water interfaces are covered. At this stage, the increase in surfactant concentrations will no
longer reduce the IFT: the surfactant molecules will agglomerate (forming surfactant
micelles), and will increase the solubility of the NAPL (present in the dissolved
phase). This concentration is called the Critical Micelle Concentration (CMC)
(Vishnyakov et al. 2013). The CMC of a surfactant depends on surfactant structure,
system temperature, ionic strength, and whether or not organic additives are present
in the solution (Laha et al. 2009).
Surfactant performance depends on their working environmental conditions. For
example, system temperature and salinity can influence surfactant effectiveness.
When the temperature increases, the reaction between the hydrophilic component
96
S. Colombano et al.