European Geothermal Workshop 2024

Mitigation of Pb scale deposits in geothermal installations by Pb sorption onto natural clinoptilolite: the effect of acetate and Cl ions

Not scheduled

20m

Oral presentation

Speaker

Mr Val Maverick Abecia (Belgian Nuclear Research Centre (SCK CEN))

Description

Mineral scale deposits and other amorphous solids generated by geothermal installations can contain elevated activities of Naturally Occurring Radionuclides (NORs) originating from ²³⁸U and ²³²Th and their progeny (e.g., ^226/228Ra, ²¹⁰Pb, and ²¹⁰Po) in the reservoir rocks. The NORs-containing deposits can be classified as Naturally Occurring Radioactive Material (NORM) Waste posing a health and environmental hazard. Anti-scalants/inhibitors are used to mitigate the scale deposition, which comprise organics adding to natural organic matter coming from the reservoir rocks. Little is known about the role of organic matter in NOR fractionation and mobility and its possible contribution to NORM Waste generation. As a possible alternative to chemical inhibition, the feasibility of using natural clinoptilolite as a sorbent material for NORs such as ²¹⁰Pb is being assessed as part of the PERFORM II Project (Horizon 2020 Europe). This study investigates the effect of organic matter on the Pb sorption efficiency onto natural clinoptilolite, using acetate (OAc^-) and stable Pb as proxies for organics and NORs, respectively. The experiments were conducted at room temperature, a fixed pH of 5.0 in a background electrolyte of 0.1 M NaCl, a fixed solid-to-liquid ratio of 5 g clinoptilolite/L solution, and a 14-day sorption time. The results reveal that at baseline conditions (no OAc^-), the adsorbed Pb fraction (%Pb_ads) is between ~49~100%. With a fixed amount of OAc^- added, a minimal decrease of 2-9 %Pb_ads is observed, which can be associated with the effect of Pb-OAc complexation. In comparison, chloride complexation results in a more drastic decrease in Pb sorption by ~20%. Based on sorption modeling in PHREEQC using a modified version of the ThermoChimie 12A database and the extended Debye-Hueckel activity model, sorption is revealed to be partly linked to surface complexation onto the amphoteric basal sites of clinoptilolite and partly to cation exchange. The performed sorption tests demonstrate the feasibility of using natural clinoptilolite as a sorbent material for NORs removal, but further tests using real geothermal brine and simulated real conditions in geothermal installations should be conducted to fully evaluate its performance as a viable alternative to chemical inhibition.