Removal of inorganic mercury by selective extraction and coprecipitation for determination of methylmercury in mercury-contaminated soils by chemical vapor generation inductively coupled plasma mass spectrometry (CVG-ICP-MS).

A procedure is developed for selective extraction of methylmercury (CH3 Hg+ ) 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 μg g-1 inorganic mercury (Hg2+ ) or methylmercury chloride (CH3 HgCl) 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 CH3 Hg+ with minimum Hg2+ leaching. Agitation with ultrasounds in 5% (v/v) HNO3 for about 3 min was sufficient for extraction of all CH3 Hg+ 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 CH3 Hg+ . HgS precipitation was made with dilute ammonium sulfide solution, (NH4 )2 S. Adding 30 μL of 0.35 mol L-1 (NH4 )2 S to soil extracts in 5% (v/v) HNO3 resulted in removal of all residual Hg2+ without impacting CH3 Hg+ levels. Vapor generation was carried out by reacting Hg2+ -free soil extracts with 1% (m/v) NaBH4 . No significant interferences were observed from (NH4 )2 S on the vapor generation from CH3 Hg+ . The slopes of the calibration curves for CH3 HgCl standard solutions in 5% (v/v) HNO3 with and without (NH4 )2 S were similar. Limits of detection (LOD, 3s method) were around 0.08 μg L-1 for 5% (v/v) HNO3 blanks (n = 10) and 0.10 μg L-1 for 5% (v/v) HNO3  + 0.005 mol L-1  (NH4 )2 S blanks (n = 10). Percent relative standard deviation (%RSD) for five replicate measurements varied between 3.1% and 6.4% at 1.0 CH3 HgCl level. The method is validated by analysis of two certified reference materials (CRM); purely Methylmercury sediment (SQC1238, 10.00 ± 0.291 ng g-1  CH3 Hg+ ) and Hg-contaminated Estuarine sediment (ERM - CC580, 75 ± 4 ng g-1  CH3 Hg+ and 132 ± 3 μg g-1 total Hg). CH3 Hg+ values for SQC1238 were between 13.0 and 13.2 ng g-1 , and 79 and 81 ng g-1 for ERM - CC580. Hg-contaminated soils (57-96 μg g-1 total Hg) collected from the floodplains of Oak Ridge, TN were analyzed for CH3 Hg+ using the procedure by CVG-ICPMS. CH3 Hg+ levels ranged from 30 to 51 ng g-1 and did not correlate with total Hg levels (R2  = 0.01).