3540 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
between CO32– and Ca2+ or other metals present or by sub-
stitution into the calcite crystal if the metal ion radius is
close to Ca2+ (Zhu and Dittrich, 2016 Chen and Achal,
2020). However, the bioreactors that received cementation
solution were exposed to aqueous oxidants for 60 days lon-
ger than the control bioreactors, which was only exposed to
aqueous oxidants during the SAR stress test. As such, the
iron content in the effluent during the 60-day cementation
period and SAR testing period was combined to assess the
mobilisation of iron cations in the treated bioreactors.
Effluent collected for each phase of treatment and test
phase were analysed for ferric and ferrous iron concentra-
tion and total iron concentration. In Table 4, the Fe2+, Fe3+,
and Total Fe content in the effluent of each bioreactor over
the cementation period is reported with Table 5 showing
the resulting Fe2+, Fe3+, and Total Fe content in the efflu-
ent of each bioreactor following the SAR testing period.
Table 6 subsequently shows the collective total Fe2+, Fe3+,
and Total Fe content in the effluent of each bioreactor over
the entire treatment and SAR testing regime.
In terms of the total Fe, as expected, the CONTROL-
BLND showed the highest effluent iron content
(393,69 mg/L), followed by the UNINOC-BLND, IRR-
BLND, and AGG-BLND (Table 6). Since no effluent was
observed for AGG-BLND during SAR testing, the lowest
total Fe content was obtained from this run. Additionally,
the IRR-BLND column showed four-times lower total
Fe (101,56 mg/L) compared to the CONTROL-BLND
(393,69 mg/L), signifying that even though effluent was
observed, MICP limited the amount of Fe available for
leaching. Even though the UNINOC-BLND showed
250,23 mg/L total Fe leached (Figure 2), only approxi-
mately 0,5% (1,32 mg/L) was leached during the SAR test-
ing period. This indicated that the majority of the accessible
iron leached during the cementation period (248,91 mg/L).
Once the treatment phase was completed, the microbially
generated calcite was sufficient to prevent Fe leaching -even
under aggressive SAR conditions. This further demonstrated
the importance of inoculation. In accordance with a previ-
ous study, more calcite formed in the bottom half of unin-
oculated bioreactors compared to inoculated ones (Hajee
et al. 2023). Since uninoculated bioreactors form calcite at
a slower rate, they allow for the cementation solution to
freely collect at the bottom of the bioreactors, leading to
calcite formation in a bottom-up manner. Interestingly, the
UNINOC-BLND leached significantly less Fe than IRR-
BLND during the SAR testing period. This is not due to
the fact that all the Fe present in the coal waste was leached
out (as evidenced by the high Fe content observed in the
control), but rather that calcite limits the amount of Fe
available to be leached, either by co-precipitation or by pre-
cipitating the Fe during crystal formation. This also shows
that the calcite penetration depth is important to improve
metal immobilisation, since both the AGG-BLND and
the UNINOC-BLND bioreactors exhibited a deeper cal-
cite penetration depth. As with all bioreactors, it is likely
that the CONTROL-BLND still contained a significant
amount of Fe since co-disposed beds play a significant role
in limiting ARD generation and metal leaching (Donald
Mjonono 2019). Further studies are therefore required to
understand the full lifecycle and lifespan of these systems,
to elucidate the robustness of other packing configurations
and their hydrodynamic implications.
CONCLUSION
In this investigation, the robustness of the MICP-co-disposal
beds as an ARD prevention strategy was evaluated using pH
4,5 synthetic acid rain. Bioreactors that received cementing
solution maintained neutral conditions over 90 days of SAR
treatment, with the bioreactor inoculated via agglomeration
remaining sealed throughout the testing period. This led to
a reduction in oxidant ingress, and therefore, a reduced risk
of ARD generation. Additionally, the cementation-treated
bioreactors outperformed the untreated control system
Table 4. The Fe2+, Fe3+, and Total Fe effluent content over the
cementation period
Bioreactor Fe2+, mg/L Fe3+, mg/L Fe Tot, mg/L
IRR-BLND 17,84 19,94 37,77
AGG-BLND 19,24 11,76 31,00
UNINOC-BLND 246,32 2,59 248,91
Table 5. The Fe2+, Fe3+, and Total Fe effluent content over the
90-day SAR period
Bioreactor Fe2+, mg/L Fe3+, mg/L Fe Tot, mg/L
IRR-BLND 39,98 23,80 63,79
AGG-BLND 0,00 0,00 0,00
UNINOC-BLND 1,27 0,06 1,32
CONTROL-BLND 378,31 15,39 393,69
Table 6. The Fe2+, Fe3+, and Total Fe effluent content over the
cementation and 90-day SAR period
Bioreactor Fe2+, mg/L Fe3+, mg/L Fe Tot, mg/L
IRR-BLND 57,82 43,74 101,56
AGG-BLND 19,24 11,76 31,00
UNINOC-BLND 247,59 3,91 250,23
CONTROL-BLND 378,31 15,39 393,69
between CO32– and Ca2+ or other metals present or by sub-
stitution into the calcite crystal if the metal ion radius is
close to Ca2+ (Zhu and Dittrich, 2016 Chen and Achal,
2020). However, the bioreactors that received cementation
solution were exposed to aqueous oxidants for 60 days lon-
ger than the control bioreactors, which was only exposed to
aqueous oxidants during the SAR stress test. As such, the
iron content in the effluent during the 60-day cementation
period and SAR testing period was combined to assess the
mobilisation of iron cations in the treated bioreactors.
Effluent collected for each phase of treatment and test
phase were analysed for ferric and ferrous iron concentra-
tion and total iron concentration. In Table 4, the Fe2+, Fe3+,
and Total Fe content in the effluent of each bioreactor over
the cementation period is reported with Table 5 showing
the resulting Fe2+, Fe3+, and Total Fe content in the efflu-
ent of each bioreactor following the SAR testing period.
Table 6 subsequently shows the collective total Fe2+, Fe3+,
and Total Fe content in the effluent of each bioreactor over
the entire treatment and SAR testing regime.
In terms of the total Fe, as expected, the CONTROL-
BLND showed the highest effluent iron content
(393,69 mg/L), followed by the UNINOC-BLND, IRR-
BLND, and AGG-BLND (Table 6). Since no effluent was
observed for AGG-BLND during SAR testing, the lowest
total Fe content was obtained from this run. Additionally,
the IRR-BLND column showed four-times lower total
Fe (101,56 mg/L) compared to the CONTROL-BLND
(393,69 mg/L), signifying that even though effluent was
observed, MICP limited the amount of Fe available for
leaching. Even though the UNINOC-BLND showed
250,23 mg/L total Fe leached (Figure 2), only approxi-
mately 0,5% (1,32 mg/L) was leached during the SAR test-
ing period. This indicated that the majority of the accessible
iron leached during the cementation period (248,91 mg/L).
Once the treatment phase was completed, the microbially
generated calcite was sufficient to prevent Fe leaching -even
under aggressive SAR conditions. This further demonstrated
the importance of inoculation. In accordance with a previ-
ous study, more calcite formed in the bottom half of unin-
oculated bioreactors compared to inoculated ones (Hajee
et al. 2023). Since uninoculated bioreactors form calcite at
a slower rate, they allow for the cementation solution to
freely collect at the bottom of the bioreactors, leading to
calcite formation in a bottom-up manner. Interestingly, the
UNINOC-BLND leached significantly less Fe than IRR-
BLND during the SAR testing period. This is not due to
the fact that all the Fe present in the coal waste was leached
out (as evidenced by the high Fe content observed in the
control), but rather that calcite limits the amount of Fe
available to be leached, either by co-precipitation or by pre-
cipitating the Fe during crystal formation. This also shows
that the calcite penetration depth is important to improve
metal immobilisation, since both the AGG-BLND and
the UNINOC-BLND bioreactors exhibited a deeper cal-
cite penetration depth. As with all bioreactors, it is likely
that the CONTROL-BLND still contained a significant
amount of Fe since co-disposed beds play a significant role
in limiting ARD generation and metal leaching (Donald
Mjonono 2019). Further studies are therefore required to
understand the full lifecycle and lifespan of these systems,
to elucidate the robustness of other packing configurations
and their hydrodynamic implications.
CONCLUSION
In this investigation, the robustness of the MICP-co-disposal
beds as an ARD prevention strategy was evaluated using pH
4,5 synthetic acid rain. Bioreactors that received cementing
solution maintained neutral conditions over 90 days of SAR
treatment, with the bioreactor inoculated via agglomeration
remaining sealed throughout the testing period. This led to
a reduction in oxidant ingress, and therefore, a reduced risk
of ARD generation. Additionally, the cementation-treated
bioreactors outperformed the untreated control system
Table 4. The Fe2+, Fe3+, and Total Fe effluent content over the
cementation period
Bioreactor Fe2+, mg/L Fe3+, mg/L Fe Tot, mg/L
IRR-BLND 17,84 19,94 37,77
AGG-BLND 19,24 11,76 31,00
UNINOC-BLND 246,32 2,59 248,91
Table 5. The Fe2+, Fe3+, and Total Fe effluent content over the
90-day SAR period
Bioreactor Fe2+, mg/L Fe3+, mg/L Fe Tot, mg/L
IRR-BLND 39,98 23,80 63,79
AGG-BLND 0,00 0,00 0,00
UNINOC-BLND 1,27 0,06 1,32
CONTROL-BLND 378,31 15,39 393,69
Table 6. The Fe2+, Fe3+, and Total Fe effluent content over the
cementation and 90-day SAR period
Bioreactor Fe2+, mg/L Fe3+, mg/L Fe Tot, mg/L
IRR-BLND 57,82 43,74 101,56
AGG-BLND 19,24 11,76 31,00
UNINOC-BLND 247,59 3,91 250,23
CONTROL-BLND 378,31 15,39 393,69