Trace element accumulation in bryophytes from Allchar (North Macedonia): contamination or genuine hyperaccumulation?

Genuine accumulation of metals/metalloids in bryophytes is limited and highly susceptible to surficial contamination with soil particles. Washing with an apolar solvent removes most surficial contamination prior to elemental analysis. Bryophytes are often the first colonisers of soils toxic from metals/metalloids derived from natural mineralisation or mining wastes. They are ostensibly highly tolerant to the prevailing high concentrations of metals and metalloids in the substrate, but little is known about their ability to (hyper)accumulate these metals/metalloids. Terrestrially growing bryophytes were collected from arsenic-thallium mineralised soils at the Allchar site in North Macedonia. Samples were analysed for elemental concentrations using monochromatic X-ray fluorescence analysis (MXRF) after stringent washing with an apolar solvent and subjected to synchrotron micro-X-ray fluorescence (µXRF) elemental imaging. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were additionally used to assess extraneous contamination and test the efficiency of the washing procedure. The results show that surficial contamination with soil particles is a major challenge for assessing metal and metalloid concentrations in (terrestrial) bryophytes from metalliferous soils. Washing with an apolar solvent (hexane) removes most surficial contamination prior to elemental analysis, indicating some potential for elemental accumulation, as found in Rhynchostegium megapolitanum for thallium. Genuine accumulation of arsenic and thallium is relatively low despite the ability to grow on extremely arsenic-thallium enriched soils. Measured elemental concentrations in bryophyte samples are strongly affected by the washing procedure, highlighting the importance of appropiate sample preparation.