UREA MONITOR FOR MEASURING HABs
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Anthropogenic nutrient enrichment represents an enduring and increasing worldwide threat to estuaries and coastal ecosystems. Recent changes in agricultural practices have increased urea loading to coastal waters. It is now the preferred synthetic nitrogen fertilizer for many farmers including corn and soybean farmers of the United States Midwest and East Coast due to its relatively high nitrogen density, low fire and explosion hazard , and high soil retention. In addition, poultry waste commonly applied to fields of the mid-Atlantic and Delmarva Peninsula (USA) as fertilizer is another significant source of urea to coastal waters via the decomposition of uric acid. Recent measurements of urea in the tributaries of the Chesapeake and Maryland’s Coastal Bays found individual concentrations ≤ 24µg at N/L and annual mean concentrations ≤ 2.6µg at N/L in areas with intensive agriculture and poultry operations. Such unnatural concentrations contribute to general eutrophication, and may promote harmful algal blooms (HABs).
An improved understanding of the role of urea in the formation of Harmful Algal Blooms will require urea measurements at appropriate time scales (~hourly) before and throughout the lifecycle of the bloom. Thus, we are developing an autonomous, in-situ urea monitor. We intend to deploy the instrument in the spring of 2005 in the Choptank River, MD and the spring of 2006 in a Coastal Bay of Maryland where elevated urea concentrations and harmful blooms of Prorocentrum minimum and Aureococcus anophagefferens occur respectively.
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Fig. 1 NAS-3X and Heater Core |
Figs. 2a: Narrow bore FEP tubing is tightly coiled to maximize turbulent flow and reduce sample spreading. 2b: Conductive copper sheets and two heaters are bonded to both sides of FEP coil with thermally conductive epoxy to form the heater core. 2c: The potted heater core (Fig. 1) is surrounded on all sides by approximately 2 in of water resistant foam insulation inside a PVC box forming final heater. At equilibrium, the heater draws 0.5 amps at 24 volts while submerged in 50 C water. This suggests that when deployed with deep cycle batteries and solar chargers it is unlikely that the power demands of the heater will limit the deployment length or sampling frequency.
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We are adapting an existing EnviroTech, LLC NAS-3X autonomous, in-situ nutrient monitor (Fig. 1) for direct measurements of urea using a modification of the colormetric diacetyl monoxime method developed by Rahmatullah and Boyde with sample and reagent heating at 900 C. To reduce analysis time to approximately one half hour, we designed and built a submersible, flow through heater capable of heating 3ml to 900 C (Fig. 2). The heater consists of a single layer coil of fluorinated ethylene propylene (FEP) tubing 1/16 in ID x 1/8 in OD x 1/32 in wall thickness (Fig. 2a) sandwiched between two 0.0135 in thick copper sheets cut to the shape of the FEP tubing coil. The copper sheets were bonded to the FEP tubing with thermally conductive epoxy (Epic Resins 0126 epoxy) such that the tubing was uniformly coated and all voids were filled with the epoxy. A Fluke 80PK1 thermocouple probe accurate to ±20 C was embedded in the center of the coil before potting. The same epoxy was used to bond 2 Hytek Microsystems, Inc HY-7110 proportionally controlled micro heaters to each side of the potted coil (Fig. 2b). These heaters use an external resistor to set the final temperature (500 C to 1000 C ±0.2). The resultant heater core and required wiring were potted in a small PVC box with thermally insulating epoxy (Epic Resins R3500/H5064) (Fig 1). The potted heater core was then insulated with approximately 2 in of low density, water resistant foam inside a larger PVC box with a lid (Fig. 2c)
A 4cm pathlength colorimeter with a light emitting diode (LED) light source centered at 525 nm (peak light absorbance of the final recation product) and an appropriate photodiode detector was purchased from EnviroTech, LLC. Initial test with this detector produced very “noisy” results due to a failure to prevent bubbles formed during heating from interfering with light transmission in the relatively wide bore flow cell. Data presented here were collected using an EnviroTech Nitrate detector with a narrow bore, 2cm pathlength flow cell and optical components broadly centered around 545 nm. |
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Fig 3: Urea Calibration - All Standards were prepared in low nutrient seawater (salinity 15) and run on EnviroTech LLC NAS-3X nutrient monitor. The 0.99 然 and 9.98 然 standards were triplicates and all other standards were duplicates. All calibration results are presented.
- Limit of detection (3 sigma) = 0.19 然
- RSD = 3.3% at 0.99 然, 0.87% at 9.98 然 |
Table 1: Light transmission (EnviroTech LLC nitrate colorimeter) through colored water collected from Island Creek (ICW) in Oxford, MD increases with acidification, but less so when the acidified sample is heated under analytical conditions (900 C for 15 minutes). These results indicate an average offset of 0.73 然 when this ICW sample was acidified and heated. Flexible programming of the analytical process with the NAS-3X allows for optical blank correction of each sample so that variations in salinity, turbidity, and dissolved organic matter will not influence the urea signal.
*Catalytic reagent: 86 然 FeCl3 in 5 M H2SO4 |
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