XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3539
sealing of the bioreactors is microbially facilitated by the
ureolytic bacterium S. Pasteurii. Rod and round-shaped
bacteria were observed in the UNIOC-BLND column,
which hinted at the involvement of native urease-produc-
ing bacteria. Although these native bacteria lead to calcite
formation and slightly reduced effluent rates, it was not suf-
ficient to cause complete sealing of the reactor.
Similar to the effluent volumes, the average pH of the
inoculated bioreactors were 8,25 and 8,51 for IRR-BLND
and AGG-BLND, respectively (Table 1). The increase to
above pH 8 indicated that calcite was successfully formed
via MICP in these bioreactors, which was visually con-
firmed by the formation of opaque crystals at the surface
and throughout the length of the co-disposed beds. The pH
of the UNINOC-BLND bioreactors increased from pH
4,72 on Day 1 to an average of pH 7,13, which suggested
that minor calcite formation evolved within this system.
Similar pH values were obtained in a previous study (Hajee
et al. 2023) that later confirmed the presence of calcite
via acid wash tests, which implied that calcite was indeed
formed in the IRR-BLND and AGG-BLND bioreactors
in this investigation. However, since further investigations
were carried out in these bioreactors, acid wash tests could
not be performed in this study.
Synthetic Acid Rain Stress Tests
Effluent Flow Under Synthetic Acid Rain Irrigation
After the 60-day cementation and 60-day air-drying period,
an uninoculated, untreated reactor was packed as a con-
trol (CONTROL-BLND) to comparatively determine the
ARD mitigation potential of the hybrid MICP-co-disposal
system. After irrigation with SAR, any effluent observed
was collected after 24 h and analysed. Table 3 shows the
pH and total effluent volume obtained over the 90 days of
SAR treatment.
The IRR-BLND bioreactor, which sealed on Day 28,
showed a gradual increase in effluent flow over the 90-day
SAR treatment. However, the AGG-BLND column did not
show any effluent throughout the 90-day period, emphasis-
ing the benefit of inoculation via agglomeration in forming
a structurally stable bed that was impermeable to oxidants.
Since the same blended packing configuration was used,
the difference in volume and pH was attributed to the inoc-
ulation method. It was previously noted that bioreactors
inoculated via agglomeration show a higher calcite yield in
the top half of the bioreactor (170,98 mg/g) compared to
bioreactors inoculated via irrigation (156,96 mg/g Hajee
et al. 2023). This manifested to a robust and acid-resistant
bed, with restricted effluent discharge, which prevented the
exposure of the coal waste to oxidants, thus, resulting in
unfavourable conditions for ARD generation.
All bioreactors that received cementing treatment out-
performed the CONTROL-BNLD bioreactor both in
terms of effluent volume and average pH. Interestingly,
the UNINOC-BLND bioreactor exhibited similar efflu-
ent volume and pH profiles to the CONTROL-BLND.
However, in terms of pH, UNINOC-BLND showed a far
superior average pH of 8,55 compared to pH 5,69 of the
CONTROL-BLND bioreactor. This demonstrated that
the cementing period, and likely native urease-producing
bacteria, resulted in the formation of calcite that neutral-
ised the SAR from pH 4,5 to pH 8,55. Even though the
UNINOC-BLND exhibited higher pH levels compared to
the IRR-BLND, it did not serve as a confirmation that the
UNNOC-BLND outperformed the IRR-BLND system as
calcite dissolution may have contributed to the increase in
pH in parallel with the alkaline nature of the cementation
induced by the native bacteria. It is therefore likely that the
high pH resulting from a high dissolution rate may lead to
a shorter neutralization lifespan (Mjonono 2019 Maliela et
al. 2020). However, further studies are necessary to deter-
mine the mechanistic evolution of this system.
Iron Content of Effluent
While access to oxidants and pH are key indicators in terms
of ARD management, another important consideration is
the leachate quality and metal content. Since MICP has been
shown to immobilise heavy metals, an improved leachate
quality was expected in the cementation-treated bioreactors
compared to the control. This can occur via precipitation
Table 2. Sealing day and average pH of the co-disposed
bioreactors
Bioreactor Sealing Day Average pH
IRR-BLND 28 8,25
AGG-BLND 32 8,51
UNINOC-BLND N/A 7,13
Table 3. Total effluent volume and average pH obtained over
the 90-day SAR stress testing period for both treated (IRR-
BLND, AGG-BLND, and UNINOC-BLND) and untreated
(CONTROL-BLD) blended, co-disposed coal waste in a
3WR:2FW configuration
Bioreactor Total effluent, mL Average pH
IRR-BLND 142,95 7,99
AGG-BLND 0 N/A
UNINOC-BLND 231,94 8,55
CONTROL-BLND* 237,62 5,69
*Negative control
sealing of the bioreactors is microbially facilitated by the
ureolytic bacterium S. Pasteurii. Rod and round-shaped
bacteria were observed in the UNIOC-BLND column,
which hinted at the involvement of native urease-produc-
ing bacteria. Although these native bacteria lead to calcite
formation and slightly reduced effluent rates, it was not suf-
ficient to cause complete sealing of the reactor.
Similar to the effluent volumes, the average pH of the
inoculated bioreactors were 8,25 and 8,51 for IRR-BLND
and AGG-BLND, respectively (Table 1). The increase to
above pH 8 indicated that calcite was successfully formed
via MICP in these bioreactors, which was visually con-
firmed by the formation of opaque crystals at the surface
and throughout the length of the co-disposed beds. The pH
of the UNINOC-BLND bioreactors increased from pH
4,72 on Day 1 to an average of pH 7,13, which suggested
that minor calcite formation evolved within this system.
Similar pH values were obtained in a previous study (Hajee
et al. 2023) that later confirmed the presence of calcite
via acid wash tests, which implied that calcite was indeed
formed in the IRR-BLND and AGG-BLND bioreactors
in this investigation. However, since further investigations
were carried out in these bioreactors, acid wash tests could
not be performed in this study.
Synthetic Acid Rain Stress Tests
Effluent Flow Under Synthetic Acid Rain Irrigation
After the 60-day cementation and 60-day air-drying period,
an uninoculated, untreated reactor was packed as a con-
trol (CONTROL-BLND) to comparatively determine the
ARD mitigation potential of the hybrid MICP-co-disposal
system. After irrigation with SAR, any effluent observed
was collected after 24 h and analysed. Table 3 shows the
pH and total effluent volume obtained over the 90 days of
SAR treatment.
The IRR-BLND bioreactor, which sealed on Day 28,
showed a gradual increase in effluent flow over the 90-day
SAR treatment. However, the AGG-BLND column did not
show any effluent throughout the 90-day period, emphasis-
ing the benefit of inoculation via agglomeration in forming
a structurally stable bed that was impermeable to oxidants.
Since the same blended packing configuration was used,
the difference in volume and pH was attributed to the inoc-
ulation method. It was previously noted that bioreactors
inoculated via agglomeration show a higher calcite yield in
the top half of the bioreactor (170,98 mg/g) compared to
bioreactors inoculated via irrigation (156,96 mg/g Hajee
et al. 2023). This manifested to a robust and acid-resistant
bed, with restricted effluent discharge, which prevented the
exposure of the coal waste to oxidants, thus, resulting in
unfavourable conditions for ARD generation.
All bioreactors that received cementing treatment out-
performed the CONTROL-BNLD bioreactor both in
terms of effluent volume and average pH. Interestingly,
the UNINOC-BLND bioreactor exhibited similar efflu-
ent volume and pH profiles to the CONTROL-BLND.
However, in terms of pH, UNINOC-BLND showed a far
superior average pH of 8,55 compared to pH 5,69 of the
CONTROL-BLND bioreactor. This demonstrated that
the cementing period, and likely native urease-producing
bacteria, resulted in the formation of calcite that neutral-
ised the SAR from pH 4,5 to pH 8,55. Even though the
UNINOC-BLND exhibited higher pH levels compared to
the IRR-BLND, it did not serve as a confirmation that the
UNNOC-BLND outperformed the IRR-BLND system as
calcite dissolution may have contributed to the increase in
pH in parallel with the alkaline nature of the cementation
induced by the native bacteria. It is therefore likely that the
high pH resulting from a high dissolution rate may lead to
a shorter neutralization lifespan (Mjonono 2019 Maliela et
al. 2020). However, further studies are necessary to deter-
mine the mechanistic evolution of this system.
Iron Content of Effluent
While access to oxidants and pH are key indicators in terms
of ARD management, another important consideration is
the leachate quality and metal content. Since MICP has been
shown to immobilise heavy metals, an improved leachate
quality was expected in the cementation-treated bioreactors
compared to the control. This can occur via precipitation
Table 2. Sealing day and average pH of the co-disposed
bioreactors
Bioreactor Sealing Day Average pH
IRR-BLND 28 8,25
AGG-BLND 32 8,51
UNINOC-BLND N/A 7,13
Table 3. Total effluent volume and average pH obtained over
the 90-day SAR stress testing period for both treated (IRR-
BLND, AGG-BLND, and UNINOC-BLND) and untreated
(CONTROL-BLD) blended, co-disposed coal waste in a
3WR:2FW configuration
Bioreactor Total effluent, mL Average pH
IRR-BLND 142,95 7,99
AGG-BLND 0 N/A
UNINOC-BLND 231,94 8,55
CONTROL-BLND* 237,62 5,69
*Negative control