Particulate bound mercury pollution in the Central Italian Herbarium (Firenze, Italy)

The attention devoted to air quality is particularly important in workplaces, such as museums, where the health of visitors and workers must couple with the safeguard of collections. This especially holds for herbaria where, until the middle of the last century, the collections were protected using a solution of mercury dichloride (HgCl₂) to prevent cryptogamic or animal infestations. The decomposition of HgCl₂ causes the Hg reduction through a reaction pathway that is still poorly known, and the consequent release of Hg⁰ in the indoor atmosphere. Besides Hg⁰, Hg in air exists also as bound to particulate (PBM). In the museums’ atmosphere, this fraction may represent a non-negligible proportion of total atmospheric Hg and should be monitored.

This study aims to characterize the PBM in the Central Italian Herbarium of Firenze (University of Firenze, Italy), one of the largest herbaria worldwide. Here recent studies proved high levels of Hg⁰.

PBM sampling has been carried twice (2018 and 2020 soon after the lockdown period caused by the Covid-19), collecting the dust on a SEM-EDS stub from different surfaces (furniture, wall cornice, sample cabinet). Samples were roughly divided according to their deposition time between old (OD), almost-new (AD) and new dust (ND). The samples were analyzed using SEM-EDS to characterize the dimension and the chemical speciation of Hg particulate.

Hg-particles were detected in all the three types of dust collected in both the years: the mean dimension is 0.80±0.01 µm (3σ). The highest number of Hg-particles has been always reached in the AD, i.e. the dust collected directly on the packages containing herbarium specimens, with a strong increase in the 2020 sample. Additionally, the EDS microanalysis revealed that Hg-particles are now mainly associated with S (sometimes with O), suggesting the presence of sulphate or sulphide.

The above evidences show that PBM constitutes a fraction of Hg pollution in the Herbarium that cannot be ignored. The number of particles strongly increased in a period of low attendance of the Herbarium rooms and consequently cleaning, due to the COVID-19 pandemic: despite this, almost all are still classifiable as fine particulate (i.e. ECD< 2.5 µm) particularly harmful for human health. The presence of sulphate/sulphide indicated the change of Hg speciation with time and its reaction with S and O. These compounds, although less bioavailable than Hg⁰, still represent a risk for both herbarium workers and visitors.

The results of the present study offer preliminary information on the abatement system to be installed in the museum halls, which should be supplied with filters to retain very fine particles (< 1 µm).