Oxygen Transport through Selected Aquatic Macrophytes

The extent of O2 transport from aerial plant tissue into the root zone was evaluated for several floating and emergent aquatic macrophytes that have characteristics favorable for wastewater treatment. The highest Oi transport rates from aerial tissue into the root zone were associated with plants having a small root mass. As root mass increased, the rate of O, transport decreased for aquatic macrophytes evaluated. Pennywort (Hydrocotyle umbellata L.) had the highest O2 transport rate of all aquatic macrophytes with an overall rate of 3.49 g O2 kg ' dry root mass h '. Pickerelweed (Pontederia cordata L.) had the highest O2 transport capacity of emergent plants with a rate of 1.54 g O2 kg' h '. Waterhyacinth [Eichhornia crassipes (Mart.) Sollms], an important floating aquatic plant in wastewater treatment, had a transport rate of 1.24 g Oj kg' h". Nitrification in a waterhyacinth-based water treatment system due to O: transport was calculated to vary from 6 to 22 kg ha" d". Additional Index Words: Emergent aquatic plants, Floating aquatic plants, Wastewater treatment, Nitrification, BOD, reduction. The use of aquatic macrophytes for wastewater renovation has become an important consideration for communities with limited financial resources (Duffer, 1982; Univ. of Florida, Inst. of Food and Agric. Sci., Soil Sci. Dep., Gainesville, FL 32611. Florida Agric. Exp. Stn. Journal Series no. 8438. Received 25 Feb. 1987. * Corresponding author. Published in J. Environ. Qual. 17:138-142 (1988). Reddy and Smith, 1987). Pollutant removal in an aquatic plant-based wastewater treatment system is attributed to plant assimilation and biochemical or physical processes in the root zone, water column, and underlying sediment (Reddy, 1984; Good and Patrick, 1987). Both plant assimilation of pollutants and microbially mediated processes are influenced by the capacity of aquatic plants to transport O2 into the root zone. An anatomical adaptation of aquatic macrophytes is the development of aerenchyma cell structure, which facilitates the exchange of O2 from aerial tissue into the root zone (Dacey, 1980). Oxygen transport through plant tissue has been established for several aquatic macrophytes (Coult and Vallance, 1958; Armstrong, 1964; Teal and Kanwisher, 1966; Armstrong, 1967). The O2 is used for root respiration and prevents the root zone from becoming anoxic. If the amount of O2 transported from aerial tissue into the root zone exceeds the plant demands, diffusion may occur into the surrounding aqueous media. The O2 can then be consumed by aerobic bacteria and result in decomposition of organic matter or nitrification. Both of these transformations are critical for wastewater renovation. The objective of this study was to determine the extent of O2 transport from aerial plant tissue into the root zone of selected aquatic macrophytes that have characteristics favorable for wastewater treatment. 138 J. Environ. Qua!., Vol. 17, no. 1, 1988 MATERIALS AND METHODS The apparatus used to measure O2 transport consisted of an O2 electrode (Yellow Springs Instrument Co., Yellow Springs, OH) connected to a PVC column sealed at one end to a PVC plate (Fig. 1). The electrode was held in place with silicon glue. The columns were 10 to 30 cm in length with a 5.1 or 7.6 cm i.d. Four columns were built for each column length and diameter combination for replication. The columns were of different length and diameter to accommodate differences in root morphology. Two barriers were used to restrict O2 exchange between atmosphere and water. Each barrier was evaluated for its effectiveness in preventing O5 leakage from atmosphere into the water column. The procedure to evaluate O5 leakage through each barrier was similar to the following procedures described for a barrier with plant. Oxygen transport was calculated from the net change in dissolved O5 concentration with time. Results were expressed as g O2 kg-~ dry root mass. Oxygen transport of plants were corrected for O2 leakage through each barrier with no plant. Aquatic plants used in this study were collected from the St. Johns River marsh. They were cultured in nutrient medium containing NH,-N = 10.5 mg L-’; NO3-N = 10.5 mg L-~; PO~-P = 3.1 mg L-~; K = 23.0 mg L-’; Ca = 20.0 mg L-~; Mg = 5.0 mg L-’; Fe = 0.6 mg L-t; and micronutrients. Micronutrients were applied through liquid fertilizer (Nutrispray-Sunniland, Chase and Co., Sanford, FL) to obtain concentrations of 4 mg Fe L-~; 0.2 mg Cu L-’; 1.5 mg Mn L-~; 0.04 mg B L-~; 0.02 mg Mo L-~; and 3 mg S L -’. Paraffin Oil Barrier Water in the PVC column was pruged with N2 gas until the dissolved O5 concentrations were less than 0.3 mg O5 L-’. A waterhyacinth [Eichhornia crassipes (Mart.) Solms] plant was placed in each container and the magnetic stirrer was turned on. Nitrogen gas was bubbled through the water column for an additional 5 min. At the end of 5 min, the aquarium stone was removed from the water column and a l-cm layer of heavy paraffin oil was poured around the plant. The plant roots were kept below the paraffin oil layer. The dissolved O2 concentration was recorded immediately and represented time 0 and the magnetic stirrer was turned off. At 30-min intervals, the dissolved O5 concentration was recorded after a 30-s stir. at the end of 120 min, the paraffin oil was discarded and the plant was washed thoroughly with tap water. The relationship between O2 transport and time was linear for 120 min and steady-state conditions were reached at 180 or 210 min. Split-rubber Stopper Barrier A waterhyacinth plant was placed in a no. 11 or no. 14 splitrubber stopper having a center hole of 1.7 to 2.0 cm in diam. The excess space surrounding the plant was plugged with glazing seal (used for setting glass in greenhouses). Water in the PVC columns was purged with N2 gas for several minutes until the dissolved O2 concentration was less than 0.3 mg O2 L-~. A waterhyacinth plant/rubber stopper was positioned on top of the PVC column so that the roots were submerged in the water. The magnetic stirrer was turned on and N gas was bubbled for an additional 5 min. The aquarium stone was removed from the water column and the split-rubber stopper was forced into the column. The outside connection between the split-rubber stopper and the PVC column was sealed with glazing seal. The dissolved O5 concentration was recorded immediately and represented time 0 and the magnetic stirrer was turned off. The dissolved O5 concentration was measured at 30-min intervals after a 30-s stir. At the end of 120 min, the plant was removed from the split-rubber stopper and washed thoroughly. Aquatic Plant Glazing Seal Split-rubber Stopper 02 Electrode PVC Column