Mercury accumulation in suspended matter of foam and water of the Black Sea

Abstract

The ability of suspended matter to concentrate mercury may be the prevailing factor in Black Sea purification. As a result of sedimentation, suspended particles transport pollution from the surface layer of the water column and, as a consequence, can deposit them in bottom sediments, thus participating in self-purification of marine area. Suspended matter, as a dispersed phase of an aqueous medium, considered as a heterogeneous dispersed system, can be more saturated with mercury than water itself, as a dispersion medium. In this work, contribution of dissolved and suspended forms of mercury to its total content was determined, and concentrating ability of suspended matter relative to mercury, which affects biogeochemical self-purification of waters from mercury, was estimated. All water samples were separated into filtrate and suspension by filtration through nucleopore filters with a pore diameter of 0.45 μm. Measurements of mercury concentration were carried out using a Hiranuma-1 analyzer by the method of atomic absorption spectrophotometry. Concentration of dissolved mercury in water was determined per liter, while in suspended matter – per liter and per gram of dry weight. Prevalence of dissolved form of mercury was revealed regardless of the season, with its percentage varying from 66.3 to 85.8 % of total mercury concentration. Average content of suspended form varied in the range of 14.2–33.7 % of its total form. Values of the dry weight of suspended matter (mss) varied from 0.1 to 15.0 mg·L−1 over the entire period studied, and an accumulation coefficient of mercury in suspended matter (Kss) varied from n·10³ to n·107. Significant contribution of suspended form of mercury in sea foam to its total content in stormy weather was established. With dry weight of suspended matter in seawater reaching 9.6 mg·L−1, the concentration of dissolved form of mercury reached 55 ng·L−1, and the concentration of suspended one reached 20 ng·L−1. In sea foam, the concentration of suspended sedimentary matter was of 895.2 mg·L−1; mercury concentration reached 200 ng·L−1 in dissolved form and 260 ng·L−1 in suspended one. Total mercury concentration in sea foam in this case exceeded the threshold limit value (100 ng·L−1) for seawater. The accumulation coefficient of mercury in suspended matter (Kss) was 3.8·104 for seawater and 1.5·103 for foam. Such distribution of mercury in sea suspension, foam, and water, as well as Kss values obtained, may indicate high significance of suspended matter in self-purification of marine area. At a low mercury content in water, the concentrating ability of suspended matter, characterized by relatively high values of its mercury accumulation coefficient, becomes a very significant factor in the sedimentation self-purification of waters from mercury; however, with an increase in water pollution with mercury, the effect of this factor decreases.