Pilot Plant for the Separation of Radium-226, Lead, and Mercury from Process Waters of Natural Gas Production

Deep subsurface waters often contain naturally occurring radionuclides such as radium and lead isotopes at elevated concentrations. When these substances reach the surface during crude oil and natural gas production or in the use of deep geothermal energy, special precautions are required in order to prevent environmental contamination. Under normal conditions, these waters are reinjected into the production wells. However, so-called radioactive scale accumulates on material surfaces. These deposits have to be removed during maintenance work or decommissioning using high-pressure water jets at pressures of up to 3000 bar, for the purpose of metal recycling, for example. The resulting waters are themselves radioactive and additionally contain other toxic metals such as mercury, cadmium, and lead.

Up until a few years ago, such residues were a welcome source of mercury, but this option is no longer available due to regulatory requirements. As a result, disposal of these waters is now very costly.

As an initial step, a drastic reduction in the volume of radioactive process waters at acceptable cost would provide significant relief. The challenge here lies in treating the purified water to such an extent that radioisotopes are no longer detectable. Only then does the water meet the requirements for discharge into municipal wastewater treatment plants or for subsurface infiltration. This is a highly demanding task that cannot be achieved using conventional water treatment technologies. Even processes such as nanofiltration, ultrafiltration, and reverse osmosis reach their limits here: there are 1016 sodium ions for every radium ion to be separated, for example.

More extensive tests were initially carried out using filtration and reverse osmosis. These showed that mercury (Hg) can be separated well or very well using these methods (Table 1). But filtration alone is by no means sufficient to achieve complete separation of radium-226 and lead (Pb-210). A special type of electrodialysis was developed at Fraunhofer IKTS to enable a higher degree of separation. This process makes it possible to meet the defined objective without significant technical effort and with a much better split ratio than reverse osmosis. The split ratio describes the ratio of the purified water stream to the concentrate stream. This determines the remaining volume of concentrates requiring disposal. While reverse osmosis typically achieves ratios of 5:1 to 10:1, electrodialysis enables ratios of 50:1 to 100:1.

Following positive preliminary investigations in the laboratory, the process is now being tested at pilot scale in a so-called field trial at the customer’s site (Fig. 1). The system essentially consists of a simple filtration stage using a filter sand/barite mixture and an electrodialysis unit with a capacity of up to 1 m³/d. Integrated remote monitoring and control also enable sampling operations to be carried out under remote supervision. On-site visits are required only once a week for maintenance purposes.

Initial results obtained with the pilot plant show that the process also performs reliably under harsh real-world conditions. However, further adjustments are still required in the case of large fluctuations in the hydrochemical influent parameters. Subject to successful field trials, the client has expressed strong interest in technical implementation.

In addition, recent developments in cancer therapy indicate a growing demand for radium, as it is required for the production of so-called theranostics. Here, too, the electrodialysis process described above offers potential future applications.