RESUMO
A procedure is developed for selective extraction of methylmercury (CH3Hg+) from heavily Hg-contaminated soils and sediments for determination by chemical vapor generation inductively coupled plasma mass spectrometry (CVG-ICP-MS). Soils artificially contaminated with 40⯵gâ¯g-1 inorganic mercury (Hg2+) or methylmercury chloride (CH3HgCl) were agitated by shaking or exposing to ultrasounds in dilute hydrochloric acid (HCl) or nitric acid (HNO3) solutions at room temperature. Extractions in HCl (5 or 10% v/v) resulted in substantial leaching of Hg2+ from soils, whereas 5% (v/v) HNO3 provided selectivity for quantitative extraction of CH3Hg+ with minimum Hg2+ leaching. Agitation with ultrasounds in 5% (v/v) HNO3 for about 3â¯min was sufficient for extraction of all CH3Hg+ from soils. Coprecipitations with Fe(OH)3, Bi(OH)3 and HgS were investigated for removal of residual Hg2+ in soil extracts. Hydroxide precipitations were not effective. Thiourea or l-cysteine added to soil extracts prior to hydroxide precipitation improved precipitation of Hg2+, but also resulted in removal of CH3Hg+. HgS precipitation was made with dilute ammonium sulfide solution, (NH4)2S. Adding 30⯵L of 0.35â¯molâ¯L-1 (NH4)2S to soil extracts in 5% (v/v) HNO3 resulted in removal of all residual Hg2+ without impacting CH3Hg+ levels. Vapor generation was carried out by reacting Hg2+-free soil extracts with 1% (m/v) NaBH4. No significant interferences were observed from (NH4)2S on the vapor generation from CH3Hg+. The slopes of the calibration curves for CH3HgCl standard solutions in 5% (v/v) HNO3 with and without (NH4)2S were similar. Limits of detection (LOD, 3s method) were around 0.08⯵gâ¯L-1 for 5% (v/v) HNO3 blanks (nâ¯=â¯10) and 0.10⯵gâ¯L-1 for 5% (v/v) HNO3 + 0.005 mol L-1â¯(NH4)2S blanks (nâ¯=â¯10). Percent relative standard deviation (%RSD) for five replicate measurements varied between 3.1% and 6.4% at 1.0 CH3HgCl level. The method is validated by analysis of two certified reference materials (CRM); purely Methylmercury sediment (SQC1238, 10.00⯱â¯0.291â¯ngâ¯g-1â¯CH3Hg+) and Hg-contaminated Estuarine sediment (ERM - CC580, 75⯱â¯4â¯ngâ¯g-1â¯CH3Hg+ and 132⯱â¯3⯵gâ¯g-1 total Hg). CH3Hg+ values for SQC1238 were between 13.0 and 13.2â¯ngâ¯g-1, and 79 and 81â¯ngâ¯g-1 for ERM - CC580. Hg-contaminated soils (57-96⯵gâ¯g-1 total Hg) collected from the floodplains of Oak Ridge, TN were analyzed for CH3Hg+ using the procedure by CVG-ICPMS. CH3Hg+ levels ranged from 30 to 51â¯ngâ¯g-1 and did not correlate with total Hg levels (R2â¯=â¯0.01).