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Sediment and Contaminant Transport in Surface Waters - Chapter 7

Để hiểu và dự đoán việc vận chuyển và số phận của các hóa chất hữu cơ kị nước (HOCs) trong nước bề mặt và các trầm tích đáy, kiến thức về tỷ lệ sorption và từ các hạt rắn trầm tích và phân vùng của các hóa chất này giữa các hạt rắn và nước là cần thiết. Trong tác phẩm đầu, nó thường được giả định rằng hấp thụ và giải hấp xảy ra nhanh chóng và rằng trạng thái cân bằng hóa học giữa chất rắn và nước đã đạt được trong một thời gian rất ngắn. | 7 The Sorption and Partitioning of Hydrophobic Organic Chemicals To understand and predict the transport and fate of hydrophobic organic chemicals HOCs in surface waters and bottom sediments knowledge of the rates of sorption to and from solid sedimentary particles and the partitioning of these chemicals between solid particles and water is necessary. In early work it was often assumed that adsorption and desorption occurred rapidly and that chemical equilibrium between the solids and water was attained in a very short time e.g. see reviews by Sawhney and Brown 1989 DiToro et al. 1991 Baker 1991 . This equilibrium was quantified by means of a partition coefficient Kp L kg defined as C Kp C 7.1 Cw where Cs kg kg is the mass of HOC sorbed to the sediment divided by the mass of the sediment and Cw kg L is the mass of HOC dissolved in the water divided by the volume of water. However early sorption experiments were generally short term hours to a few days and were misleading long-term experiments later demonstrated that both adsorption and desorption processes are often quite slow with time scales of days to months or even longer before equilibrium is attained e.g. Karickhoff and Morris 1985 Coates and Elzerman 1986 . By comparison the time of transport of a sediment particle in a river or lake may be as short as minutes to a few hours. Because of this the assumption of chemical equilibrium in surface waters may not be a good approximation in many real situations and therefore time-dependent sorption processes must be considered in detail. In the first section of this chapter experiments that illustrate basic and important characteristics of this time-dependent sorption as well as steady-state partitioning are presented and qualitatively analyzed. These experiments as well as others demonstrate that sorption times are long that sorption processes depend on the HOC and that these processes are significantly modified by colloids from the water colloids from the sediments