Hardrock Mining: EBR Uranium Removal Case Study
Spent ore and leachate water materials from a gold drilling exploration project in the Yukon Territories, Canada, were assessed for microbial communities and metal transformation capabilities. The spent ore was rinsed with deionized water and the resulting leachate water was treated in a bench-scale operation for a three-month period. This well controlled laboratory-scale test work was used as a proof-of-concept to demonstrate treatment feasibility for obtaining mining operations permits and provided baseline data for process adjustments and future pilot-scale work.
The influent contained high levels of 21 metals as well as several non-metal inorganics. For example, nitrate, a co-contaminant that is a preferred electron acceptor over dissolved uranium species and must be removed before microbial uranium transformation can occur, fluctuated in concentration from 150 mg/L to 250 mg/L, as N.
The influent contained high levels of 21 metals as well as several non-metal inorganics. For example, nitrate, a co-contaminant that is a preferred electron acceptor over dissolved uranium species and must be removed before microbial uranium transformation can occur, fluctuated in concentration from 150 mg/L to 250 mg/L, as N.
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Uranium removal averaged over 99% during the 80-day testing period. Average influent uranium concentrations initially were measured at 36 µg/L and subsequently rose to 125 µg/L, three and a half times the initial amount. Despite these changes in water chemistry, microbial transformation of soluble uranium to insoluble uranium was successful and uranium effluent discharge levels continually were measured below 1 µg/L. Nitrate was removed 1000-fold to below the detection limit of 0.200 mg/L. Although the fluctuating water chemistry observed in this system could inhibit the efficiency of microbial metal reduction, the EBR’s constant supply of electrons helped maintain and stabilize a low oxidation-reduction potential (ORP) and aid the EBR microbe consortia in maintaining the desired uranium contaminant removal. |
Influent total uranium concentration (closed squares), effluent total uranium concentration (closed triangles) and EBR oxidation-reduction potential (open circles).
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Coal Mining: EBR Uranium Removal Case Study
Water from an open pit coal operation located in British Columbia, Canada, was generated via precipitation infiltration through waste coal heaps, collected in a sedimentation pond, and treated in an on-site pilot-scale EBR unit. The operation was conducted during September and October, providing an opportunity for cold weather testing. Uranium was a secondary contaminant of interest. Influent water temperatures ranged from 18.7ºC to below 1ºC and nitrate concentrations ranged from 45 mg/L to 53 mg/L, as N. Uranium was removed from average concentrations of 18.4 µg/L to 0.091 µg/L, a >99% removal efficiency in the first stage EBR. Uranium removal was not affected by the elevated concentration of competing electron acceptors.
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The EBR system was insulated and had a water heater connected to the influent source in order to mediate water temperature variability, however, water temperatures still dropped within the EBR to as low as 4 ºC. Stress in the form of lower temperatures, which can slow down/inhibit microbial enzymatic processes, did not affect the consortia’s ability to transform soluble U(VI) to insoluble U(IV). This was shown by uranium effluent measurements consistently below the detection limit of 0.200 µg/L and further demonstrated by both total and dissolved uranium concentrations following similar removal trends.
In conventional bioreactors, organics metabolism is required during dissimilatory metals reduction processes and can be significantly hindered at lower temperatures. Excess electrons, provided directly via the EBR electrodes, ensured that the treatment efficiency was not affected by metabolic organic electron donors during temperature stress. |
Influent total uranium concentration (closed circles), influent dissolved uranium (closed squares), effluent total uranium concentration (crosses), effluent dissolved uranium (open squares), influent temperature (closed triangles), and EBR temperature (open triangles).
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Flotation-Influenced Waters: EBR Uranium Removal Case Study
Water from an underground mining project producing copper, lead, zinc, silver, using three floatation circuits for metal recovery. Suppressants (e.g., cyanides) and collectors (e.g., xanthates) were added at each step to recover only the desired products. Xanthates and cyanide from this process can be toxic to microbes and inhibit microbial metals reduction. Water and solids from the floatation circuit were discharged to a tailing pond, where solids were allowed to settle and water was re-circulated to the mining metal recovery circuits. This water recirculation practice and the complex floatation process made the water chemistry a moving target, with many difficult to treat constituents. Because of a positive water balance, excess water accumulated in the tailings pond needed to be treated before being discharged to the environment. Tailings dam waters were assessed for contaminant removal in an on-site pilot-scale operation from June to August. Uranium was a secondary contaminant of interest.
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The aggregate concentration of competing electron acceptors was 10,872.7 µg/L, greatly surpassing uranium levels. Despite higher flow rates and complex water chemistry, uranium removal increased from 86.5% within the first thirty days to 95.8% for the last half of testing. Average total uranium influent concentration of 1.99 µg/L was removed to below 0.1 µg/L.
Influent ORP fluctuated rapidly during the testing period from +20 to -360 mV, signifying variable, and sometimes limited, metabolically obtained electron availability to the system. Although influent uranium concentration rose steadily during the testing period, it only constituted 0.018% of the total contaminants competing for electrons in the system. However, the EBR system’s directly supplied electrons helped maintain a stable negative ORP environment throughout the entire testing period (average of -238 mV for the 65-day testing period), providing an electron rich environment for successful microbial uranium reduction. This was confirmed by uranium effluent concentrations continuously measured to below 0.1 µg/L during the testing period, despite elevated aggregate metals and inorganics competing for electrons. |
Influent total uranium concentration (closed squares), effluent total uranium concentration (closed circles), aggregate concentration of competing electron acceptors (open squares), and influent ORP (crosses).
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FGD Waters: EBR Uranium Removal Case Study
A pilot-scale EBR assessment of uranium removal from Flue Gas Desulfurization (FGD) waters containing high levels of eight inorganics and 18 metals, with elevated total hardness averaging 14,100 mg/L, as CaCO3, and reaching as high as nearly 18,000 mg/L, as CaCO3. Hardness cations (Ca2+, Mg2+) readily react with dissolved phosphorous in the system, an essential macro-nutrient needed for microbial growth, function and survival. Because of this, waters with elevated hardness concentrations are historically difficult to treat using conventional biotreatments processes and often require softening precipitation pretreatments. Due to the exceedingly elevated hardness parameters, pH adjustment pre-treatment was implemented in order to lower the pH from average of 8.1 to 6.9 to maintain phosphate solubility in this wastewater. It was projected that low pH conditions would ensure phosphate does not react with hardness cations, and consequently phosphorous should be successfully delivered to the biofilm for microbial processes. One-step pH adjustment is a more economical approach than a water softening precipitation.
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Uranium was removed from an average concentration of 30.3 µg/L to below the detection limit of 2.75 µg/L for the majority of the 120-day testing period. Both total and dissolved uranium followed similar concentrations indicating microbial transformation of soluble to insoluble uranium and its subsequent entrapment within the bioreactor media. Nitrate was removed completely in the system, from average concentrations of 17.72 mg/L to below the detection limit of 0.100 mg/L. Hardness concentrations rose substantially throughout the testing period. However, the pH adjustment to facilitate phosphorous delivery for microbial function, coupled with a metabolic-free available electron environment provided by the EBR, helped maintain a negative ORP environment and provided the electrons required for effective microbial uranium and general metals reduction.
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Influent total uranium concentration (closed squares), influent dissolved uranium (closed circles), effluent total uranium concentration (plus signs), effluent dissolved uranium (crosses), and influent hardness as CaCO3 (open circles).
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EBR Case Studies
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It's not only Selenium!The EBR technology is well suited for removal of variety of oxyanions, such as nitrates, sulfates, and dissolved metals.
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