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