Estimating the impact of acid pH solution on the metal desorption from lichen Evernia prunastri (L.) Ach

Evernia prunastri is lichen used as bioindicator of air pollutions for decades, moreover numerous studies showed that lichens have the possibility to accumulate metals from air in concentrations that are higher than their metabolic demand. Metals are absorbed directly through the surface of the lichen thallus. The aim of this study was to estimate the desorption of eight heavy metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) from Evernia prunastri with acid pH solutions (4.33, and 5.12) and to determine could the lichens become secondary environment pollution source. The metal analysis was performed by flame atomic absorption spectrometry (FAAS). Solutions used for desorption were prepared by mixing different amounts of H₂SO₄, HNO₃, (NH₄)₂SO₄ and NH₄NO₃. Lichen before desorption were washed in demineralized water and air dried. Three consecutive desorption were performed with each solution, during each of the desorption lichens were in contact with the solution for 24 hours. Results showed that using the solution with pH 4.33, Mn was desorbed in the highest mass percentage (92.7%) from the lichen and desorption mass percentage of Mn and Zn from the lichen by using the solution of pH 5.12 was 88% and 67.2%, respectively. The desorption mass percentage of other investigated metals were lower than 30% except for Cr whose mass percentage was lower than 1% with solution pH 4.33 and below the limit of detection for the applied method by using the solution with pH 5.12. Results indicate that lichens can become secondary environment pollutant and that mass percentage of desorbed metals is depending on strength linked of the metal for the components of the lichen tissues. Low mass percentage of metal desorption could indicate that the metal is tightly bonded inside the lichen cell itself, either they build highly insoluble compounds or complex compounds with lower or higher stability constants. High mass percentage of metal desorption can indicate that metals are linked with an ionic bond to the corresponding chemical groups in the lichens. (author)