Alkalinity content expressed as bicarbonate concentration, already rather high in
2014 (around 600 mg L
À1 ), slightly decreased, except in DCC3.1 H and B, where
the increase was ascribable to the degradation products of dithionite. This result is
also supported by the increase in Cl
À content in DCC3.1. The data also indicated that
dithionite did not reach DCC3.2 and DCC3.3; even if a slight increase in Cl
À content
was observed in DCC3.2, it was not related to an increase in alkalinity. As previously mentioned, dithionite can be consumed by organic matter, clays, and other
trace elements in presence. Here, dithionite can also reduce the passive layer on the
surface of aged nZVI particles, resulting in the rejuvenation of the particles (Xie and
Cwiertny 2010).
Concerning the data acquired between June 2 and July 8, 2015, alkalinity and Cl
À
content remained in the same order of magnitude for DCC3.2 and 3.3. A slight
increase in both parameters is nevertheless of interest in DCC3.2, and in a lesser
extent in DCC3.3. As dithionite did not reach DCC3.2, dechlorination process
downstream from the injection wells can thus be attributed to the previous nZVI
solution injections, natural attenuation or both of them.
In agreement with the presence of iron-reducing bacteria (IRB) and dithionite, the
reduction of Fe(III) in Fe(II) species could have depassivate iron particles (Roden
and Zachara 1996; Gerlach et al. 2000; Williams et al. 2005), resulting in the
reactivation of the particles for reductive dechlorination. This process can be envisaged until the total depletion of nZVI particles.
Evolution of TCE, cis-1,2-DCE, 1,1-DCE, and VC Content
Table 6.23 presents the individual evolution of TCE, cis-1,2-DCE, 1,1-DCE, and
VC content in the control piezometers of line 3.
In DCC3.1, a decrease in all four compounds was observed between June 2 and
July 8. At both levels, TCE reached an abatement in the range from 45 and 70%. It
reached 88–93% for cis-1,2-DCE; 88% for 1,1-DCE; and 90–100% for VC. Results
showed that the use of dithionite in combination with nZVI clearly decreased COCs
content up to 100% in the first cPZ DCC1.1 “H” and “B.” In comparison, the
effective mixture of both reagents resulted in higher reductions at the laboratory
scale. The difficulty and problems encountered for the injection of nZVI particles
clearly highlight the superiority of using dithionite solution for in situ reductive
dechlorination.
The observed behavior for DCC3.1 is clearly different for DCC3.2 and DCC3.3,
evidencing that dithionite did not reach either DCC3.2 or DCC3.3. In DCC3.2, two
different behaviors were observed versus time. Between June 2 and June 22, a slight
but remarkable decrease was observed for TCE content; reduction reached 14–19%,
whereas it reached 9–12% between June 22 and July 8. In the same time, cis-1,2DCE increased in the range from 13 to 25% between June 2 and June 22, and from
8 to 20% between June 22 and July 8. Both results tended to evidence the occurrence
of biological processes occurring in DCC3.2; highlighting a slight increase of the
biostimulation between June 2 and June 22. The same behavior as the one observed
368
R. Rodrigues et al.
2014 (around 600 mg L
À1 ), slightly decreased, except in DCC3.1 H and B, where
the increase was ascribable to the degradation products of dithionite. This result is
also supported by the increase in Cl
À content in DCC3.1. The data also indicated that
dithionite did not reach DCC3.2 and DCC3.3; even if a slight increase in Cl
À content
was observed in DCC3.2, it was not related to an increase in alkalinity. As previously mentioned, dithionite can be consumed by organic matter, clays, and other
trace elements in presence. Here, dithionite can also reduce the passive layer on the
surface of aged nZVI particles, resulting in the rejuvenation of the particles (Xie and
Cwiertny 2010).
Concerning the data acquired between June 2 and July 8, 2015, alkalinity and Cl
À
content remained in the same order of magnitude for DCC3.2 and 3.3. A slight
increase in both parameters is nevertheless of interest in DCC3.2, and in a lesser
extent in DCC3.3. As dithionite did not reach DCC3.2, dechlorination process
downstream from the injection wells can thus be attributed to the previous nZVI
solution injections, natural attenuation or both of them.
In agreement with the presence of iron-reducing bacteria (IRB) and dithionite, the
reduction of Fe(III) in Fe(II) species could have depassivate iron particles (Roden
and Zachara 1996; Gerlach et al. 2000; Williams et al. 2005), resulting in the
reactivation of the particles for reductive dechlorination. This process can be envisaged until the total depletion of nZVI particles.
Evolution of TCE, cis-1,2-DCE, 1,1-DCE, and VC Content
Table 6.23 presents the individual evolution of TCE, cis-1,2-DCE, 1,1-DCE, and
VC content in the control piezometers of line 3.
In DCC3.1, a decrease in all four compounds was observed between June 2 and
July 8. At both levels, TCE reached an abatement in the range from 45 and 70%. It
reached 88–93% for cis-1,2-DCE; 88% for 1,1-DCE; and 90–100% for VC. Results
showed that the use of dithionite in combination with nZVI clearly decreased COCs
content up to 100% in the first cPZ DCC1.1 “H” and “B.” In comparison, the
effective mixture of both reagents resulted in higher reductions at the laboratory
scale. The difficulty and problems encountered for the injection of nZVI particles
clearly highlight the superiority of using dithionite solution for in situ reductive
dechlorination.
The observed behavior for DCC3.1 is clearly different for DCC3.2 and DCC3.3,
evidencing that dithionite did not reach either DCC3.2 or DCC3.3. In DCC3.2, two
different behaviors were observed versus time. Between June 2 and June 22, a slight
but remarkable decrease was observed for TCE content; reduction reached 14–19%,
whereas it reached 9–12% between June 22 and July 8. In the same time, cis-1,2DCE increased in the range from 13 to 25% between June 2 and June 22, and from
8 to 20% between June 22 and July 8. Both results tended to evidence the occurrence
of biological processes occurring in DCC3.2; highlighting a slight increase of the
biostimulation between June 2 and June 22. The same behavior as the one observed
368
R. Rodrigues et al.