Continuous extraction with acidified subcritical water of arsenic, selenium and mercury from coal prior to on-line derivatisation-atomic fluorescence detection

Abstract

Acidified subcritical water is proposed for the continuous extraction of minor pollutants (namely, selenium, arsenic and mercury) from coal prior to continuous derivatisation (by hydride formation for Se and As, and cold vapor formation for Hg) and determination by atomic fluorescence. Coal samples (3 g) were subjected to a 15 min static extraction followed by a 90 min dynamic extraction with water modified with 4% (v/v) HNO₃ for both steps. An in-depth study of the variables affecting the continuous leaching step, as well as those referring both to preconcentration (for mercury), derivatisation and detection (all of them) was performed. The linear ranges of the calibration curves for all analytes were at the ng ml^–1 level, with correlation coefficients, r², better than 0.999 for Se and As and better than 0.99 for Hg. The method was validated by using a bituminous coal reference material (NIST SRM 1635). The good precision of the method [RSDs (n = 6) of 12.0, 4.7 and 6.5% for As, Se and Hg, respectively], together with its safety and rapidity make it a good alternative for the determination of these analytes in coal.