Effect Of Dissolved Organic Matter And UV On Mercury Release From Water/Air Interface

As the important natural organic ligand and adsorptive carrier, Dissolved organic matter (DOM) is a very active chemical component both in terrestrial ecosystems and aquatic ecosystems. DOM exists as a complex mixture of a variety of polyfunctional organic compounds, from low (molecular weight) MW compounds such as aliphatic acid, aromatic moieties, a-amino acid, and monosaccharide, oligosaccharides, to a few macromolecules such as enzymes, polyphenols and humic substance. They are one kind of organic matters which can be dissolved in water, acid or base solution, such as organic matter in natural water, in soil solution and the one which can be extracted from soils with water, acid or base. And DOMs have more active sites than the solid organic matter. Then that can result in the influence of the characteristics of acid and base, the availability of nutrients. Additionally, it may affect the emovement and environmental behavior of pollutants such as the toxicity, transportation, transformation and biodegradability. Not only can the heavy metal be transported into water, but impact their activity through the interaction between DOMs and heavy metal. As a kind of organic ligands, DOM contain a great number of different groups. Those could react with the metal ions, oxides and organic matter in water, soil and sediments, via ion exchange, adsorption, complex, chelation and other reactions.The emission of Hg from water/air interface was investigated in water in laboratory experiment. Three factors were taken into account, including sources of DOM, wavelength of radiation and Hg species. The water was prepared using Hg(NO₃)₂ and HgCl₂. The DOM was obtained from rice straw, humus soil and compost.The results showed the effect of DOM on mercury release varied with the sources of DOM, as well as the common characteristics. The emission from the DOM-added water was apparently low, regardless of the materials for DOM extraction. Among, the strongest inhibition was observed in DOM from the humus soil. From the DOM_R-added and DOM_S-added water, the exchange amount of Hg is 51 ng, which reduced 37% over the control. On the other hand, the release amount was only 20.1 ng for the DOM_H-added treatment, which reduced 75% over the control. It can be deduced the formation of stable Hg-organic complexes in the solution may inhibit the reduction of Hg²⁺, and resulting in the decrease in Hg release. The ligands for complexes mainly include the hydroxyl, carboxyl, amino group and so on. As the type and quantity of functional groups were different for the DOM_H, DOM_R and DOM_S, their binding with heavy metals were consequently different. The highest inhibition of DOM_H on Hg release may be attributed to more organic ligand or higher binding ability.To investigate the effect of UV radiation on mercury release from the water/air interface, two patterns were employed including irradiation and non-irradiation. In the pattern of irradiation, the light intensity is 266.67 W/m² and irradiation distance is about 25 cm. In the steps of S1, S3 and S5 (all non-radiation), the release of mercury gradually inc reased over time and reached stable condition. Otherwise, in the Ultraviolet irradiation steps of S2 and S4, the mercury release reduced to zero around. The mercury emissions reduced to zero in about 60 s in the present of 254nm-UV light, while those reduced to zero in 110 s around in the present of 310 nm- and 365 nm-UV light. It can be concluded that the inhibition of 254 nm-UV light was far greater than the other two on Hg emission. It can be seen the similar trend of Hg release over steps compared the DOM-added treatment and control. The Hg emission decreased in the order: S1>S3>S5. In general, the Hg emissions were less from the DOM-added water than the control. In particular, the emission reduced most when adding the DOM_H, showing its highest inhibition. In S2 and S4 with irradiation, the emission also reduced to zero in decades of seconds in all DOM_H treatments. That indicated the effect of DOM on Hg release was negligible in the present of ultraviolet light.In order to study the effect of coexistent anion on Hg emission, the HgCl₂ and Hg(NO₃)₂ were added in the water and the Hg concentrations were same. The results showed there was the Hg emission of HgCl₂ lower than Hg(NO₃)₂, when added no DOM. However, the dynamic characteristics were different in the present of UV light. The Hg emission gradually increased in the present of Cl^-, while decreased for the NO₃^-.