Experimental study on the unintentional abiotic methylation of inorganic mercury during analysis: Part 2: Controlled laboratory experiments to elucidate the mechanism and critical discussion of the species specific isotope addition correction method

The chemical behaviour of inorganic mercury in complex soil and sediment samples, possibly responsible for the methylation of mercury, was further investigated. Following the previous methylation experiments with the mentioned problematic matrices the possible pathways of inorganic mercury methylation aided by humic acid were discussed on a chemical basis. The HPLC-ICP-MS system described in the first part was used as a valuable tool for the investigation of the abiotic mercury methylation with the aid of enriched mercury isotopes. Experiments with different carboxylic acids, which are the major functional groups in a high molecular weight ? humic acid molecules have shown that they are good methyl donors for Hg2+. Organic acids with a-methyl groups showed the highest methylation efficiency. Furthermore thermal experiments have shown that the artifact formation potential of a humic rich sediment sample was lowered by heat treatment and nearly removed by treatment at 200 degrees C-300 degrees C within 3 h. Sandy sediments showed an artifact formation potential as well. In this case a heat treatment at 700 degrees C for 3 h was necessary to remove the abiotic methylation potential. The application of the Species-Specific Isotope Addition method for sediments was discussed critically. Consideration of the methylation due to a humic acid molecule showed that the isotope and the ambient inorganic mercury could undergo different amounts of methylation. Therefore the actual value may deviate from the value obtained from the SSIA method. This means also that it could be possible that the SSIA method could detect only a small amount of an actually considerably higher existing artefact. To be certain about the comparability of the spiked Hg-200(2+) tracer and the ambient inorganic mercury, the exact mechanisms leading to the measured artefact which are responsible and up to now no unidentified have to be clarified. (C) 1999 Elsevier Science Ltd. All rights reserved.