6.4 Injection of Reductants
In situ chemical reduction efficiency is highly dependent on the intimate contact
between contaminants and reductant (Noubactep et al. 2012). The injection method
of reductant is therefore a critical factor influencing treatment efficiency (Comba
et al. 2011b). The selection of injection method depends on the selected reductant.
Indeed, reductant can be in liquid (sodium dithionite), pure phase (edible oils),
emulsion and foam (calcium polysulfide), gas (hydrogen), or particulate form
(zero-valent iron). Thus, injection methods are fundamentally different (Tratnyek
et al. 2014).
6.4.1 Injection of Dissolved Reductants
For moderately permeable phreatic zone, dissolved reductant injection can be
performed without major difficulties via conventional injection wells, involving
the extraction of contaminated water and the replacement by clean water (McCarty
2010). Flow rates and injection pressures are calculated from the hydraulic flow rate
of groundwater, the concentrations to be achieved (determined by the stoichiometry), natural reductant demand (NRD), and contact time (determined by degradation
kinetics).
For high hydraulic conductivity, contact time between pollutant and reductant
may not be sufficient. In this case, it is possible to pump the water downstream and
reinject it upstream (recirculation or injection/extraction “push–pull” (Hyman and
Dupont 2001; FRTR 2007; Colombano et al. 2010).
Injection of reducing agents (Fig. 6.5) may be improved by hydraulic and
pneumatic fracturing or by in situ soil mixing in the case of permeable zone
(Brown 2010). Fracturing consists of injecting air or water at high pressure in
order to improve permeability (Tratnyek et al. 2014). Deep soil mixing involves
reductant injections via hollow augers for an in situ mechanical mixing with soil,
thereby generating attrition, expansion, and unclogging phenomena (Olson et al.
2012).
6.4.2 Injection of Gases
Gas injection can be accomplished by in situ sparging. It is recommended to use
hollow-fiber membranes to reduce as much as possible the size of the bubbles and
the contact surfaces, hence promoting dissolution of reductive gases (Leeson et al.
2002; Johnson and Johnson 2012).
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R. Rodrigues et al.
In situ chemical reduction efficiency is highly dependent on the intimate contact
between contaminants and reductant (Noubactep et al. 2012). The injection method
of reductant is therefore a critical factor influencing treatment efficiency (Comba
et al. 2011b). The selection of injection method depends on the selected reductant.
Indeed, reductant can be in liquid (sodium dithionite), pure phase (edible oils),
emulsion and foam (calcium polysulfide), gas (hydrogen), or particulate form
(zero-valent iron). Thus, injection methods are fundamentally different (Tratnyek
et al. 2014).
6.4.1 Injection of Dissolved Reductants
For moderately permeable phreatic zone, dissolved reductant injection can be
performed without major difficulties via conventional injection wells, involving
the extraction of contaminated water and the replacement by clean water (McCarty
2010). Flow rates and injection pressures are calculated from the hydraulic flow rate
of groundwater, the concentrations to be achieved (determined by the stoichiometry), natural reductant demand (NRD), and contact time (determined by degradation
kinetics).
For high hydraulic conductivity, contact time between pollutant and reductant
may not be sufficient. In this case, it is possible to pump the water downstream and
reinject it upstream (recirculation or injection/extraction “push–pull” (Hyman and
Dupont 2001; FRTR 2007; Colombano et al. 2010).
Injection of reducing agents (Fig. 6.5) may be improved by hydraulic and
pneumatic fracturing or by in situ soil mixing in the case of permeable zone
(Brown 2010). Fracturing consists of injecting air or water at high pressure in
order to improve permeability (Tratnyek et al. 2014). Deep soil mixing involves
reductant injections via hollow augers for an in situ mechanical mixing with soil,
thereby generating attrition, expansion, and unclogging phenomena (Olson et al.
2012).
6.4.2 Injection of Gases
Gas injection can be accomplished by in situ sparging. It is recommended to use
hollow-fiber membranes to reduce as much as possible the size of the bubbles and
the contact surfaces, hence promoting dissolution of reductive gases (Leeson et al.
2002; Johnson and Johnson 2012).
338
R. Rodrigues et al.