Passage V

In a lake, water leaches (dissolves out) soluble organic compounds from decaying tree leaves, producing dissolved organic carbon (DOC). DOC is subsequently removed from the water if it is adsorbed by (becomes adhered to the surface of) clay mineral particles that are suspended in the water. Three studies done at a lake exam­ined DOC adsorption by 3 clay minerals—CM1, CM2, and CM3—found in the lake's sediment.

Green leaves were collected from 5 ypes of trees around the lake (maple, oak, pine, magnolia, and rhodo­dendron). A 5 L volume of lake water was filtered to remove all solid particles. The following procedures were performed for each type of leaf: A 100 g sample of the leaves was mixed with a 1 L volume of the filtered lake water. The mixture was then placed in the dark for 10 weeks at 4°C while leaching occurred. At 10 weeks, the mixture was filtered to remove all solid particles. The resulting liquid (the leachate) was analyzed for DOC.

Study 1

The following procedures were performed for each leachate: A 100 mL volume of the leachate was mixed with 10 g of CMl. The mixture was stirred continuously for 2 hr, then filtered to remove all solid particles. The result­ing liquid (the filtrate) was analyzed for DOC. The percent of the leachate DOC that had been adsorbed by CM1 was calculated (see Figure 1).

Study 2

Study 1 was repeated, substituting CM2 for CM1 (see Figure 2).

Study 3

Study 1 was repeated, substituting CM3 for CM1 (see Figure 3).

Figures and table adapted from Todd Tietjen, Anssi Vahatalo, and Robert Wetzel, Effects of Clay Mineral Turbidity on Dissolved Organic Carbon for and Bacterial Production." ©2005 by the Swiss Federal institute for Environmental Science and Technology.