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Evaluating the Oxidative Resistance of Co-disposed Coal Waste
Beds Augmented via Microbially-Induced Calcite Precipitation
I. Hajee, S. T. L. Harrison, A. Kotsiopoulos
Centre for Bioprocess Engineering Research, Department of Chemical Engineering,
University of Cape Town, Cape Town, South Africa
ABSTRACT: Acid rock drainage (ARD) is recognized as a serious environmental concern affecting the
mining and mineral industry that deleteriously affects underground water resources and surrounding terrestrial
ecosystems. Limiting the access of natural oxidants to the sulphide-containing mineral is therefore paramount
to prevent the oxidation reactions that promote ARD. Co-disposal techniques, which aim to reduce the
permeation of oxidants to acid-generating fractions by increasing the physical and chemical stability of wastes,
involve the co-mingling of complementary mine wastes. Although this is a successful and promising approach
in the short-term, the loss of structural stability over longer periods leads to a permeable bed that is susceptible
to oxidation. As a counter measure, the co-disposal technique was combined with microbially-induced calcite
precipitation (MICP), where 12 bioreactors were setup with different packing configurations, inoculated with
ureolytic bacteria S. pasteurii, and irrigated with a CaCl2-rich cementation solution. Building on a previous
investigation where MICP formation in co-disposed coal waste was demonstrated, the robustness of the hybrid
MICP-co-disposal method was evaluated. The microbially-stabilised beds were subjected to acidified water at
a pH comparable to acid rain for 180 days, where their structural stability and neutralisation capacity were
constantly monitored. The cemented bioreactors maintained their neutralizing capacity for the entire irrigation
period and contrasted the behaviour of the non-cemented controls, which ultimately yielded an acidic leachate.
The results suggested that the hybrid MICP-co-disposal system is promising for long-term ARD prevention,
even under highly acidic conditions.
INTRODUCTION
Acid rock drainage ARD is an acidic run-off generated from
mining activities when sulfide-bearing minerals are exposed
to aqueous and gaseous oxidants. The presence of various
acidophilic bacteria present in the acidic leachate promotes
ARD generation and accelerates the rate of generation often
in orders of magnitude exceeding 106. ARD is detrimental to
ecosystems via chemical, physical, and ecological pathways
where it leads to species elimination thereby simplifying
the food chain and ultimately reducing ecological stabil-
ity (Gray 1997). Soils contaminated with ARD negatively
impact the soil macro- and micro-biome and crop produc-
tivity mechanisms, including carbon mineralisation, nitro-
gen transformation, and soil enzymatic activity (Munyai
et al. 2021). A significant increase in metal levels may be
observed in contaminated soil, which often exceed critical
limits that affect vegetative growth rates. For example, up
to a 62% reduction in the yield of rice from fields contami-
nated with ARD was reported to have occurred compared
to crops grown on uncontaminated fields (Choudhury et
al. 2017). Additionally, potential health risks are associated
with the ingestion of agricultural soils impacted by ARD
Evaluating the Oxidative Resistance of Co-disposed Coal Waste
Beds Augmented via Microbially-Induced Calcite Precipitation
I. Hajee, S. T. L. Harrison, A. Kotsiopoulos
Centre for Bioprocess Engineering Research, Department of Chemical Engineering,
University of Cape Town, Cape Town, South Africa
ABSTRACT: Acid rock drainage (ARD) is recognized as a serious environmental concern affecting the
mining and mineral industry that deleteriously affects underground water resources and surrounding terrestrial
ecosystems. Limiting the access of natural oxidants to the sulphide-containing mineral is therefore paramount
to prevent the oxidation reactions that promote ARD. Co-disposal techniques, which aim to reduce the
permeation of oxidants to acid-generating fractions by increasing the physical and chemical stability of wastes,
involve the co-mingling of complementary mine wastes. Although this is a successful and promising approach
in the short-term, the loss of structural stability over longer periods leads to a permeable bed that is susceptible
to oxidation. As a counter measure, the co-disposal technique was combined with microbially-induced calcite
precipitation (MICP), where 12 bioreactors were setup with different packing configurations, inoculated with
ureolytic bacteria S. pasteurii, and irrigated with a CaCl2-rich cementation solution. Building on a previous
investigation where MICP formation in co-disposed coal waste was demonstrated, the robustness of the hybrid
MICP-co-disposal method was evaluated. The microbially-stabilised beds were subjected to acidified water at
a pH comparable to acid rain for 180 days, where their structural stability and neutralisation capacity were
constantly monitored. The cemented bioreactors maintained their neutralizing capacity for the entire irrigation
period and contrasted the behaviour of the non-cemented controls, which ultimately yielded an acidic leachate.
The results suggested that the hybrid MICP-co-disposal system is promising for long-term ARD prevention,
even under highly acidic conditions.
INTRODUCTION
Acid rock drainage ARD is an acidic run-off generated from
mining activities when sulfide-bearing minerals are exposed
to aqueous and gaseous oxidants. The presence of various
acidophilic bacteria present in the acidic leachate promotes
ARD generation and accelerates the rate of generation often
in orders of magnitude exceeding 106. ARD is detrimental to
ecosystems via chemical, physical, and ecological pathways
where it leads to species elimination thereby simplifying
the food chain and ultimately reducing ecological stabil-
ity (Gray 1997). Soils contaminated with ARD negatively
impact the soil macro- and micro-biome and crop produc-
tivity mechanisms, including carbon mineralisation, nitro-
gen transformation, and soil enzymatic activity (Munyai
et al. 2021). A significant increase in metal levels may be
observed in contaminated soil, which often exceed critical
limits that affect vegetative growth rates. For example, up
to a 62% reduction in the yield of rice from fields contami-
nated with ARD was reported to have occurred compared
to crops grown on uncontaminated fields (Choudhury et
al. 2017). Additionally, potential health risks are associated
with the ingestion of agricultural soils impacted by ARD