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 (CH(3)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 (Hg(2+)) or methylmercury chloride (CH(3)HgCl) were agitated by shaking or exposing to ultrasounds in dilute hydrochloric acid (HCl) or nitric acid (HNO(3)) solutions at room temperature. Extractions in HCl (5 or 10% v/v) resulted in substantial leaching of Hg(2+) from soils, whereas 5% (v/v) HNO(3) provided selectivity for quantitative extraction of CH(3)Hg(+) with minimum Hg(2+) leaching. Agitation with ultrasounds in 5% (v/v) HNO(3) for about 3 min was sufficient for extraction of all CH(3)Hg(+) from soils. Coprecipitations with Fe(OH)(3), Bi(OH)(3) and HgS were investigated for removal of residual Hg(2+) in soil extracts. Hydroxide precipitations were not effective. Thiourea or L-cysteine added to soil extracts prior to hydroxide precipitation improved precipitation of Hg(2+), but also resulted in removal of CH(3)Hg(+). HgS precipitation was made with dilute ammonium sulfide solution, (NH(4))(2)S. Adding 30 µL of 0.35 mole L(−1) to soil extracts in 5% (v/v) HNO(3) resulted in removal of all residual Hg(2+) without impacting CH(3)Hg(+) levels. Vapor generation was carried out by reacting Hg(2+)-free soil extracts with 1% (m/v) NaBH(4). No significant interferences were observed from (NH(4))(2)S on the vapor generation from CH(3)Hg(+). The slopes of the calibration curves for CH(3)HgCl standard solutions in 5% (v/v) HNO(3) with and without (NH(4))(2)S were similar. Limits of detection (LOD, 3s method) were around 0.08 µg L(−1) for 5% (v/v) HNO(3) blanks (n = 10) and 0.10 µg L(−1) for 5% (v/v) HNO(3) + 0.005 mol L(−1) (NH(4))(2)S blanks (n = 10). Percent relative standard deviation (%RSD) for five replicate measurements varied between 3.1% and 6.4% at 1.0 CH(3)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) CH(3)Hg(+)) and Hg-contaminated Estuarine sediment (ERM – CC580, 75 ± 4 ng g(−1) CH(3)Hg(+) and 132 ± 3 µg g(−1) total Hg). CH(3)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 to 96 μg g(−1) total Hg) collected from the floodplains of Oak Ridge, TN were analyzed for CH(3)Hg(+) using the procedure by CVG-ICPMS. CH(3)Hg(+) levels ranged from 30 to 51 ng g(−1) and did not correlate with total Hg levels (R(2) =0.01).