Atmospheric mercury and its deposition during the phasing out of an amalgam electrolysis plant: temporal, seasonal, and spatial patterns

Abstract

Large amounts of mercury (Hg) were consumed and emitted into the atmosphere during the process of amalgam electrolysis used to produce chlorine and caustic soda since the nineteenth century. In Europe, amalgam electrolysis has been gradually replaced by advanced Hg-free technologies. In this work, we describe changes in atmospheric Hg and bulk Hg wet deposition during the phasing out of an amalgam electrolytic production line of a chlor-alkali plant in Neratovice, Czech Republic, central Europe. Bulk wet deposition Hg near the chlor-alkali plant was low at 3.6 ± 0.8 μg m^-2 year^-1 due in part to low annual precipitation amounts (486 ± 97 mm) in the period 2015-2021. Nevertheless, Hg deposition was elevated relative to a nearby reference site both before and after decommissioning of the electrolytic line. Switching off the amalgam electrolytic line did not notably affect bulk wet deposition Hg near the chlor-alkali plant. Levels of gaseous elemental Hg (GEM) and particle-bound Hg (PBM) monitored seasonally four times per year over 24-h time periods indicated rapid declines in four nearby settlements set in cardinal directions from the Hg emission source. Mean atmospheric GEM and PBM concentrations decreased rapidly from 9.0 ± 2.1 ng m^-3 and 243 ± 255 pg m^-3 in the period 2013-2017 when amalgam electrolysis was operating to 3.3 ± 0.4 ng m^-3 and 32 ± 6 pg m^-3 in the period 2018-2021 after its decommissioning in November 2017. Seasonal changes of GEM coincided with changes in temperature with the highest concentrations in summer, while PBM air levels were lowest in summer due to the highest seasonal precipitation amount. GEM concentrations at the four monitored settlements at Neratovice remained elevated at 2.8 ng m^-3 with respect to regional background, but PBM levels decreased to background levels.

Keywords: Central Europe; Chlor-alkali plant; Czech Republic; Gaseous elemental mercury; Hg emissions; Legacy mercury; Particle-bound mercury; Spolana Neratovice.