Both of the produced species are reducing agents and tend to release electrons.
Therefore, in order to increase the reducing properties, the pH is, during the
application, maintained to be >11 with a buffer solution of potassium carbonate
and bicarbonate (Fruchter et al. 2000).
Sodium dithionite can react with hexavalent chromium either directly or indirectly. The direct reaction relies on the direct oxidation of S(+III) to S(+VI) and
simultaneous reduction of Cr(+VI) to Cr(+III):
2CrO
2À
4 þ S 2 O
2À
4 þ 2H 2 O þ 2H
þ
! 2Cr OH
ð Þ 3 s
ð Þ þ 2SO
2À
4
ð3:5Þ
Indirect reaction relies on the reduction of Cr(+VI) by Fe(+II), which is generated
from naturally occurred Fe(+III) by the dithionite reduction (US EPA 2000):
S 2 O
2À
4 þ 2Fe
3þ s
ð Þ þ 2H 2 O ! 2Fe
2þ s
ð Þ þ 2SO
2À
3 þ 4H
þ
ð3:6Þ
HCrO
À
4 þ 7H
þ
þ 3Fe
2þ s
ð Þ ! Cr
3þ s
ð Þ þ 4H 2 O þ 3Fe
3þ s
ð Þ
ð3:7Þ
Chromium hydroxide Cr(OH) 3 is formed during the direct reduction; however, a
permeable reactive zone with Fe(+II) is formed during the indirect reduction, where
residual Cr(VI) content in groundwater flowing into it is reduced and Cr(OH) 3 or the
mixed mineral Cr (1-x) Fe x (OH) 3 are formed (Palmer and Puls 1994; Henderson
1994).
Sulfites, sulfates, and thiosulfates created by the decomposition of dithionite in
the environment may further react to form a substance similar to gypsum:
CaSO 3 Á½H 2 O, CaSO 4 Á2H 2 O, and CaS 2 O 3 Á6H 2 O (Amonette et al. 1994). This
leads to the stabilization of the produced sulfates; therefore, they do not need to be
pumped out in cases where high groundwater salinity is not acceptable because of
increased sensitivity of the aquifer. On the other hand, insoluble precipitates can
decrease the permeability around the application wells, which represents a disadvantage of the precipitation of these substances.
3.2.1 Site Description
The method of in situ reduction with sodium dithionite was applied to remediating
land contaminated by an accidental release of wastewater from a chromium plating
factory. The bedrock formed by the granite massif is in its upper part, weathered and
overlapped by sandy eluvium, clayey loams, and backfill. The contaminated area is
in a saturated zone of sandy eluvium and weathered granite and in a fracture system
of bedrock. The groundwater is partially drained by a surface watercourse, which
flows approximately 150 m away from the source of the contamination, and rises
seasonally at the base of the slope above the watercourse, and part of the groundwater is drained by an unsealed sewerage collector. The maximum concentration of
Cr(VI) in groundwater (2700 mg/L) was found in well HS-1, which is located 5 m
3 Other Chemical Reductive Methods
55
Therefore, in order to increase the reducing properties, the pH is, during the
application, maintained to be >11 with a buffer solution of potassium carbonate
and bicarbonate (Fruchter et al. 2000).
Sodium dithionite can react with hexavalent chromium either directly or indirectly. The direct reaction relies on the direct oxidation of S(+III) to S(+VI) and
simultaneous reduction of Cr(+VI) to Cr(+III):
2CrO
2À
4 þ S 2 O
2À
4 þ 2H 2 O þ 2H
þ
! 2Cr OH
ð Þ 3 s
ð Þ þ 2SO
2À
4
ð3:5Þ
Indirect reaction relies on the reduction of Cr(+VI) by Fe(+II), which is generated
from naturally occurred Fe(+III) by the dithionite reduction (US EPA 2000):
S 2 O
2À
4 þ 2Fe
3þ s
ð Þ þ 2H 2 O ! 2Fe
2þ s
ð Þ þ 2SO
2À
3 þ 4H
þ
ð3:6Þ
HCrO
À
4 þ 7H
þ
þ 3Fe
2þ s
ð Þ ! Cr
3þ s
ð Þ þ 4H 2 O þ 3Fe
3þ s
ð Þ
ð3:7Þ
Chromium hydroxide Cr(OH) 3 is formed during the direct reduction; however, a
permeable reactive zone with Fe(+II) is formed during the indirect reduction, where
residual Cr(VI) content in groundwater flowing into it is reduced and Cr(OH) 3 or the
mixed mineral Cr (1-x) Fe x (OH) 3 are formed (Palmer and Puls 1994; Henderson
1994).
Sulfites, sulfates, and thiosulfates created by the decomposition of dithionite in
the environment may further react to form a substance similar to gypsum:
CaSO 3 Á½H 2 O, CaSO 4 Á2H 2 O, and CaS 2 O 3 Á6H 2 O (Amonette et al. 1994). This
leads to the stabilization of the produced sulfates; therefore, they do not need to be
pumped out in cases where high groundwater salinity is not acceptable because of
increased sensitivity of the aquifer. On the other hand, insoluble precipitates can
decrease the permeability around the application wells, which represents a disadvantage of the precipitation of these substances.
3.2.1 Site Description
The method of in situ reduction with sodium dithionite was applied to remediating
land contaminated by an accidental release of wastewater from a chromium plating
factory. The bedrock formed by the granite massif is in its upper part, weathered and
overlapped by sandy eluvium, clayey loams, and backfill. The contaminated area is
in a saturated zone of sandy eluvium and weathered granite and in a fracture system
of bedrock. The groundwater is partially drained by a surface watercourse, which
flows approximately 150 m away from the source of the contamination, and rises
seasonally at the base of the slope above the watercourse, and part of the groundwater is drained by an unsealed sewerage collector. The maximum concentration of
Cr(VI) in groundwater (2700 mg/L) was found in well HS-1, which is located 5 m
3 Other Chemical Reductive Methods
55