Use of biogenic and abiotic elemental selenium nanospheres to sequester elemental mercury released from mercury contaminated museum specimens

Mercuric chloride solutions have historically been used as pesticides to prevent bacterial, fungal and insect degradation of herbarium specimens. The University of Manchester museum herbarium contains over a million specimens from numerous collections, many preserved using HgCl2 and its transformation to Hgv0 represents a health risk to herbarium staff. Elevated mercury concentrations in work areas (∼1.7 μg m−3) are below advised safe levels (<25 μg m−3) but up to 90 μg m−3 mercury vapour was measured in specimen boxes, representing a risk when accessing the samples. Mercury vapour release correlated strongly with temperature. Mercury salts were observed on botanical specimens at concentrations up to 2.85 wt% (bulk); XPS, SEM-EDS and XANES suggest the presence of residual HgCl2 as well as cubic HgS and HgO. Bacterially derived, amorphous nanospheres of elemental selenium effectively sequestered the mercury vapour in the specimen boxes (up to 19 wt%), and analysis demonstrated that the Hgv0 was oxidised by the selenium to form stable HgSe on the surface of the nanospheres. Biogenic Se0 can be used to reduce Hgv0 in long term, slow release environments.