Acid Mine Drainage (AMD) and the pollution associated with it are among the greatest environmental issues faced by mining operations globally [1
]. When oxygenated water accumulates in active or abandoned mines and comes into contact with sulphide-rich minerals from exposed rock faces, the water is converted to a low pH leachate which dissolves heavy metals and other toxic contaminants from the surrounding environment [2
]. The resulting stream is typically characterised by low pH, high dissolved metals and high dissolved sulphate (SO4
) concentrations and is referred to as AMD [4
]. The AMD can continue to decant for centuries after commercial mining operations have ceased and thus poses long-term environmental damage when left untreated [6
]. The development of a low cost, low maintenance and effective AMD treatment process is one of the most important solutions required for the sustainable future of sulphide-rich mineral mining [1
Permeable Reactive Barriers (PRBs) are porous mediums with reactive material specifically designed to react with passing fluids through chemical or biochemical processes [9
]. Since the 1970s, PRBs have been installed in the flow path of groundwater and used as an in situ remediation method for various water contaminants including heavy metals, organohalogen compounds, nitrate and SO4
, among others [9
]. The major advantages of using PRBs for groundwater remediation include the low operational and maintenance costs, minimal operational supervision required and the potential for in situ installation, which limits land usage [12
]. The major limitations of PRBs include the depletion of the reactive chemical compounds over time, the potential for armouring of the reactive surface and the gradual clogging from precipitant build-up in the flow path, all of which can necessitate the need for PRB replacement or the use of sequenced multi-barrier techniques [12
In recent studies [9
], pervious concrete has been investigated as a PRB for the remediation of inorganic pollutants in AMD. When AMD flows through the pervious concrete’s porous network, it reacts with the highly alkaline surface which has a pH typically between 12 and 13 [17
]. The resulting reaction leads to an increase in the AMD pH which reduces the solubility of dissolved metals, leading to their precipitation as metal hydroxides. The dissolution of calcium compounds, mainly portlandite (Ca(OH)2
) and lime (CaO), from pervious concrete is attributed to three main chemical reactions which result in pH correction, metal removal as metal hydroxide precipitants as a result of increased pH and sulphate removal through gypsum precipitation.
The material composition design for pervious concrete PRBs is well publicised [9
]. Studies conducted on pervious concrete material selection found that a water to cement ratio of between 0.25 and 0.3 is most optimal [16
], granite aggregates can achieve marginally improved treated AMD quality over dolomite aggregates [16
] and aggregate size of 9.5 mm achieves marginally improved treated AMD quality over larger 13.2 mm stone, with the drawback of lower porosity and higher risk of clogging [18
The geometric and flow design parameters of pervious concrete PRBs play an important role on the overall performance of the chemical processes involved in AMD remediation as well as the lifecycle cost of the system. Limited literature is available to inform the optimisation of these parameters towards achieving efficient AMD treatment. The main criteria for the geometric and flow design parameters of PRBs are the liquid-concrete contact time and the flow configuration, respectively. The contact time can directly influence pH correction, metal hydroxide precipitation and sulphate removal. The contact time can have a significant influence on the lifecycle cost as higher contact times require greater volumes of pervious concrete. Flow configuration, on the other hand affects, the distribution of the AMD across the PRB porous network, which can influence the chemical reactions taking place during remediation and clogging of the porous network. Flow configuration also has an influence on the concrete volume required and therefore also affects the economics of the system. The two flow configurations available for PRB treatment are column flow and gravity flow. At the time of this study, limited literature was available on the comparative AMD remediation performance of these flow configurations.
This study was conducted to evaluate the effects of flow configuration on the AMD remediation capabilities of pozzolanic pervious concrete using primary experimental data. The study further aimed to evaluate the relationship between contact time and the AMD remediation capabilities for the tested PRB flow configurations. For the column flow experiments, the contact time was adjusted by varying the flow rate of the AMD feed pump. For the gravity flow experiments, the contact time was adjusted by varying the height of the pervious concrete PRB.
Research on pervious concrete and other PRBs for AMD remediation has primarily been focused on material composition and PRB design [15
]. The influence of design factors such as flow configuration on the AMD remediation capabilities and the lifecycle cost of PRBs is largely not understood. This study was undertaken to test the influence of flow configuration and contact time and thereby contribute towards the engineering development of efficient PRB systems for AMD remediation.
This study’s results show that flow configuration has a significant influence on AMD remediation efficiency when using pozzolanic pervious concrete. Experiments conducted in this study found that gravity flow configurations achieved equivalent treated AMD quality to the column flow configuration with two orders of magnitude less liquid-concrete contact time. This finding can have a significant impact on implementation costs. Installation costs are the greatest cost driver for PRBs and it has been estimated that the trenching for in situ PRBs accounts for 70% of the total capital investment cost while the reactive material makes up 10–15% of the total cost [15
]. The gravity flow configuration at a 450 mm barrier height with 1 m/min flow velocity and the column flow configuration at 90 min HRT achieved very similar results. Table 3
presents the volumetric treatment rate of the flow configurations at these duty points where remediation results were most similar. Under unsaturated flow, the gravity configuration achieved a volumetric treatment rate five times higher than the column flow configuration. The finding suggests that significant capital investment cost savings could be gained through the design of pervious concrete PRBs under the gravity flow configuration as a result of the lower concrete volumes required per litre of AMD treated, leading to less reactive material usage and smaller trenching footprints. Gravity flow designs with uniform distribution of AMD across the surface area of the concrete can promote greater utilisation of the concrete’s reactive volume as a result of increased flow saturation, which may reduce the required concrete volume and further improve capital cost savings. One such design is discussed in the following section.
The study results further show that flow configuration has an influence on the degree of suspended solid carryover to the treated AMD. Under the column flow configuration, a greater degree of the precipitated solids was captured on the pervious concrete’s porous surface due to the solids settling inside the PRB in a direction opposing the flow and solids adhering to the concrete through adsorption mechanisms. Under the gravity flow configuration, 8% more suspended solids were carried over into the treated AMD stream due to the flow being in the same direction as the settling suspended solids, allowing sweeping of the solids. The described flow conditions are graphically illustrated in Figure 12
. The reduced solid capturing under the gravity flow configuration is advantageous as slower build-up of solids in the flow path prolongs pore clogging and increases the productive lifespan of PRBs [40
]. Therefore, the research findings indicate that PRBs under gravity flow configurations may have longer productive lifespans than PRBs under column flow. Further research into this finding is recommended to assess the solid retention rate over longer operational timespans.
The results achieved in this study as well as in literature [16
] show that pervious concrete PRBs can be effective at raising pH and at removing contaminants from the AMD. Under both configurations, however, the removed contaminants formed suspended solids, some of which were carried over into the treated AMD. Should pervious concrete PRBs be implemented as an in situ remediation technique, some of the precipitated contaminants can be carried in suspension and still contaminate other water bodies, causing environmental damage. The study found that the suspended solids are highly settleable with settling velocity or 0.67 mm/s, allowing for a fully passive treatment process. The implementation of a sedimentation or filtration process is an essential secondary treatment process after PRB treatment for optimum remediation performance.
The presented study findings suggest that a suitably sized pervious concrete PRB under gravity flow configuration followed by a suitably sized sedimentation basin may provide the most optimal pervious concrete PRB process when optimising for remediation efficiency and capital costs. However, site-specific conditions such as aquafer depth and AMD flow rates may present challenges for a scalable design of the described process. One potential scalable design solution for gravity flow configurations which maximises flow saturation follows the principles of a rotary arm trickling filter where a manifold feeds a rotary distribution system which discharges AMD over the surface area of the pervious concrete medium. The design can operate passively using differential head between the dispersion level of the trickling filter and the static water level of the mine tailings. Figure 13
illustrates the described solution. Design constraints will need to be taken into account when further developing gravity flow solutions.
This study investigated the influence of flow configuration and liquid-concrete contact time on the AMD remediation capabilities of pozzolanic pervious concrete PRBs. Laboratory experiments were conducted on the remediation capabilities of the PRBs under column flow and gravity flow with increasing liquid-concrete contact times. The research found that flow configuration plays a significant role in the remediation efficiency of pervious concrete PRBs. The research found that the two flow configurations achieved equivalent quality of treated AMD while the gravity flow configuration was operated with two orders of magnitude less liquid-concrete contact time than the high retention column flow configuration. The research found that flow configuration also influences the rate at which PRBs captured the precipitated solids in the porous network, leading to pore clogging. This study’s findings suggest that the lifecycle costs of pervious concrete PRBs can be minimised through design of PRBs under the gravity flow configuration due to the lower volume of concrete required per unit of treated AMD and potentially longer productive lifespans due to prolonged clogging. The specific findings of this study are as follows.
The research found a statistically significant (p < 0.05) correlation relationship between the liquid-concrete contact time and pH correction under the column flow configuration. The increase in pH is attributed to the precipitation of most metals from AMD and therefore, the correct sizing of PRBs to satisfy required contact times is a critical design parameter for PRB performance.
Initial concentrations of Fe were reduced to concentrations of less than 1 mg/L at pH values of less than 4 under both configurations, suggesting that the remediation mechanisms of pozzolanic pervious concrete PRB involve the oxidation of Fe2+ to Fe3+ leading to precipitation as Fe(OH)3.
Both gravity flow and column flow configurations at the tested contact times are highly effective for heavy metals Fe and Al removal, with 99% and over 80% removal efficiency, respectively; are limited for SO4, Mg and Na removal with up to 17%, 22% and 20% removal rates, respectively; and are ineffective at reducing Mn concentrations.
The dissolved concentrations of Ca and K under gravity flow and column flow configuration increased by up to 16% and more than threefold, respectively, due to the leaching of the minerals from the pervious concrete resulting in alkalinity and raising the pH.
The pervious concrete PRBs under the gravity flow configuration captured 8% fewer precipitated solids on the reactive surface when compared to the column flow configuration. This finding suggests that slower clogging of the PRB could be attained when operating under the gravity flow configuration leading to longer productive lifespan.