A comparison of two techniques for studying sediment desorption kinetics of hydrophobic pollutants

A comparison of two techniques (gaseous purge and vial desorption) for studying the kinetics of desorption of hydrophobic pollutants from natural sediments was conducted using identical, pre-equilibrated pollutant-sediment suspensions. Desorption profiles for the two techniques [for Lindane, Aldrin, 2,2'-dichlorobiphenyl (2,2'-DCB), 4,4'-dichlorobiphenyl (4,4'-DCB), and 2,2',6,6'-tetrachlorobiphenyl (TCB)] were then compared, based on the distribution of pollutant mass between the labile (fast) and non-labile (slow) desorption phases and the release rate constants for each phase of release. The vial desorption technique shows many practical advantages over the gaseous purge technique, including its more realistic mixing conditions, the use of an independent sample for each data point (as opposed to a calculation of a cumulative mass purged at each time point), the fact that the vials constitute a closed system and are therefore less subject to ambient contamination, and the relatively low demands of time and money for the vial technique. No consistent trends in labile rate constants or in pollutant distribution between the labile and non-labile phase were observed between the two techniques. A comparison of kinetic parameters shows much faster non-labile rate constants for the gaseous purge technique, attributed to the violent, continuous agitation employed, which likely disrupted sediment aggregates and oxidized the natural organic matter associated with the sediment. Non-labile rate constants have implications for the long-term fate of compounds adsorbed to repetitively disturbed sediments. This study suggests that the traditionally less popular vial desorption technique may yield more realistic non-labile desorption rate constants.     

Labile and non-labile desorption rate constants for 33 PCB congeners from lake sediment suspensions

Lake sediments contaminated with polychlorinated biphenyls (PCBs) were purged using a gas stripping technique to estimate desorption rate constants. Desorption profiles and modeling of the data clearly show a two-step release of PCBs from sediment suspensions that can be described as a labile (fast) release followed by a non-labile (slow) release. Data are summarized by labile and non-labile rate constants and by mass in each phase as a function of suspended solids concentration for twelve pure congeners and nine co-eluting data sets (twenty-one chromatograph peaks total). Labile desorption rate constants range from 113 days(-1) to 1.43 days(-1) for the 100 mg/l sediment suspension, from 67.7 days(-1) to 2.45 days(-1) for the 1000 mg/l sediment suspension, and from 8.41 days(-1) to 0.946 days(-1) for the 5000 mg/l sediment suspension. Labile rate constants consistently decreased with increasing suspended solids, and, in general, decreased with increasing degree of chlorination (reflected in increasing retention time in the chromatogram). No consistent trend was observed for the non-labile rate constants with suspended solids concentration or degree of chlorination. The average non-labile rate constant for the PCB congeners studied here was 0.154 days(-1) (s.d.=0.158; n=63). The distribution between the labile and non-labile phases also failed to indicate dependence on suspended solids concentration, chlorine substitution pattern, or molecular weight of the congener, although the data from the 5000 mg/l suspension consistently contained less labile components. The average distributions (n=63) were 60.1% in the labile phase and 39.9% in the non-labile phase.     

1950年至2016年美国西北太平洋煤炭相关污染物的大气沉降

与煤有关的元素是有毒的和持久性的污染物，自从工业革命以来，主要是由点源排放在全球传播。华盛顿州生态部从华盛顿州东北部的深湖采集了一个沉积柱，以评估美国太平洋西北部最近的大气沉积变化。沉积柱被分为深度层段和210Pb测年。对每个截面的样品进行消化并使用ICP-MS进行金属和类金属的分析。数据显示最近砷、钡、硒和汞的浓度的增加。与1993年美国地质调查局怀俄明州弗雷蒙特冰川上部的冰芯数据、亚洲煤炭消耗数据和天气模式进行比较，表明向深湖沉积物输入的污染物是亚太地区燃煤活动的结果。最显著的是，自进行冰芯分析的1993年以来，深湖的汞沉积量从1996年的20 ppb增加到2014年的9470 ppb（增加了400倍）。