Effect of acetate and chloride on Pb sorption onto clinoptilolite at geothermal conditions

Abstract

Naturally occurring radioactive materials generated in geothermal installations comprise minerals and amorphous precipitates (scales). Scales like galena (PbS) are often mitigated by anti-scalants, which upon degradation, potentially form organic precipitates and may be linked to elevated ²¹⁰Pb activities. An alternative to anti-scalants is Pb removal by sorption onto natural zeolite (clinoptilolite). In this paper we present the results of sorption experiments to evaluate the effect of acetate (OAc${}^{-}$, proxy for low molecular weight organics in geothermal brine) and salinity on Pb sorption. The results reveal that Pb is efficiently sorbed (47%–99%) onto clinoptilolite across all tested conditions. The presence of OAc${}^{-}$ in solution resulted in a slightly decreased ($\le$5%) Pb sorption suggesting a minimal effect on sorption efficiency, while the presence of Cl${}^{-}$ decreased sorption significantly ($\sim$18%–20%). Empirical sorption isotherm fitting and mechanistic modeling was implemented in PHREEQC to simulate sorption mechanisms for Pb, and to further elucidate the effect of OAc${}^{-}$ on the sorption processes. The outcomes of the present study suggest that natural organic matter and organic-based inhibitors in the geothermal brine do not impact Pb sorption efficiency onto clinoptilolite, but the high salinity of the brine (mainly NaCl) most likely poses an issue.