"Mesocosms" were used to determine the kinetics of zinc removal in the Galkeno pilot wetland (See photograph, 77 Kb). These were translucent cylinders, sealed at the bottom, initially containing wetland sediments and plants, but no mine water. Mine water which has been chemically characterized is added to start the test. Samples are taken periodically thereafter to determine changes in water chemistry.
There are two chief advantages in using these mesocosms. One is that changes in water chemistry result only from wetland-related processes, not dilution or other confounding factors. The other advantage is that mine water can be diluted for a test, say by ½ or ¼, so that the response of a mesocosm to various metal loadings can be assessed. In the above study, six mesocosms were prepared: three filled with full-strength mine water, and three with mine water diluted by two-thirds. An example of the results produced by this test are shown in the following figure.
Zinc is removed quite well in these mesocosms, to less than 0.3 mg/L (ppm). Although initial zinc removal appears to follow a linear rate, it tapers off at low concentrations. For design purposes, where we design to achieve low zinc concentrations, the kinetics of exponential decay might be better predictors of performance. In the above example, the zinc removal data fits very well an exponential decay curve, with r²= 0.93.
The mesocosms were also helpful in determining how metals are removed in the wetland. Previous studies indicated that wetland sediments might harbour microbes which reduce sulphate (SO42-) to hydrogen sulphide (H2S). This reaction is depicted below:
CH3COOH + SO42- ® H2S + 2 HCO3-
This was of interest because many dissolved metals, including zinc, react with sulphide to form highly insoluble compounds. Such compounds should be retained permanently - and harmlessly - in the wetland sediments.
According to the above reaction, one sulphate should disappear for each sulphide formed. Therefore, one way to determine if sulphide is formed is to measure the disappearance of sulphate in mesocosms. The following figure shows that sulphate concentrations indeed decreased in three mesocosms where it was measured. This coincided (essentially) with the decrease in zinc noted in the previous figure, suggesting that zinc was removed by forming insoluble sulphide compounds.
These and other data were used to establish the rate of zinc removal in wetlands. Based on these data, we could determine how long it will take to reduce zinc to the desired concentration. Finally, this rate is used in conjunction with flow rates to determine the size (as a surface area) required to provide the necessary treatment.
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