Urea In Seawater Research Articles

Bimetallic NiCo Metal-Organic Frameworks with High Stability and Performance Toward Electrocatalytic Oxidation of Urea in Seawater.

The urea oxidation reaction (UOR) is an alternative anodic reaction for hydrogen generation via water splitting. The significance of UOR lies in both H2 production and the decontamination of urea-containing wastewater. Commercial electrocatalysts in this field are generally based on noble metals and show several limitations. Bimetal-organic frameworks (BMOFs) can be excellent candidates for the replacement of noble-metal-based catalysts beacuse of their promising features, such as a tunable structure, high surface area, and abundant sites for electrocatalysis. In this study, a series of nickel-cobalt BMOFs (Nix-Coy-BMOFs: x and y refer to a molar fraction of Ni and Co) were synthesized and applied as electrocatalysts in UOR. In particular, a Ni0.15Co0.85-MOF material with a structure similar to that of its parent Co-MOF, revealed exceptional electrocatalytic performance, as evidenced by low values of overpotential (1.33 V vs RHE at 10 mA cm-2), TOF (0.47 s-1), and Tafel slope (125 mV dec-1). At a 40 mA cm-2 current density, Ni0.15Co0.85-MOF also showed excellent stability during the 72 h tests. This performance of NiCo-BMOF can be assigned to the synergistic effect between Co and Ni, abundant active sites, porosity, and a tunable structure, all of which result in an increased reaction rate due to the acceleration of charge and mass transfers. Thus, the present work introduces an efficient noble-metal-free UOR for energy generation from urea-based wastewater.

Comparison of Salinity Measurement Based on Optical Refractometer and Electric Conductivity: A Case Study of Urea in Seawater

Comparison of Salinity Measurement Based on Optical Refractometer and Electric Conductivity: A Case Study of Urea in Seawater

Validation of Diacetyl Monoxime Method for Quantification of Urea in Sea Water

Urea is widely used in agriculture and aquaculture as a major source of nitrogenous fertilizer. Therefore, urea exists in dissolved form in the marine/freshwater ecosystem. However, if the concentration in waterbodies is higher than a threshold limit, it can be toxic to fish and prawns. Therefore, accurate measurement of urea is a must even in marine water systems. Estimation of urea from freshwater samples using colorimetric method is well established. The well-known diacetyl monoxime method is widely used for accurate estimation of urea from fresh water as well as various biological samples. The presence of various interfering ions in sea water affects the estimation and limits the adaptability of any new methods directly into sea water. The accurate and precise estimation of urea are essential to optimize the nutrient addition. In this study, the diacetyl monoxime method was considered for urea estimation in sea water (3.5% salinity) and the method was validated for specificity, linearity, robustness, ruggedness and sensitivity as per International Council for Harmonization guidelines. During this study, urea showed maximum absorbance at 524 nm wavelength in both the samples. This method was found linear up to 100 ppm of urea in fresh and sea water samples. The variations of wavelength by 5-10%, reagent volumes by 2.5-5%, sample storage, analysis did not show any significant impact on the estimation of urea from both the samples. However, the method was found more sensitive and showed the limit of detection of 0.5 ppm in fresh water and 1.5 ppm in sea water. The overall results indicate that this method is accurate and robust for sea water samples and therefore can be used for estimation of urea in sea water samples.

Automated colorimetric determination of nanomolar urea in seawater by gas-segmented continuous flow analysis using a liquid waveguide capillary cell

A sensitive automated colorimetric method for the determination of nanomolar concentration of urea in seawater is presented. The colorimetry was based on the diacetyl monoxime method and the automated system was constructed by a gas-segmented continuous flow analyzer equipped with a 100 cm path length of liquid waveguide capillary cell. The most suitable surfactant for the system was hexadecyl-trimethyl-ammonium-bromide as compared to Brij-35, sodium dodecyl sulfate, and Triton X-100. Blank selection using a 3% NaCl solution, filtered subtropical surface seawater, and filtered deep seawaters with and without urease treatment showed that the filtered deep seawater with urease treatment had the lowest absorbance and was appropriate. The sample volume needed for the analysis was 1.8 mL, and the analysis rate was 10 samples h−1. Calibration curves showed good linearity for 0–1000 nM urea-N concentration range (r2 = 0.999). The detection limit was 5 nM urea-N. The coefficient of variation for the analysis of seawater samples collected in the subtropical North Pacific was <5% at 30 and 29 nM urea-N. The proposed analytical system was applicable to vertical observation in the subtropical North Pacific.

Optimization of a colorimetric method to determine trace urea in seawater

AbstractThis study optimized the experimental conditions for the analysis of dissolved urea in seawater. The kinetics of the colorimetric reaction of urea with diacetyl monoxime were studied under different conditions: reagent stability, pH, and reaction temperature and time, based on which robust procedures were developed for both normal (&gt; 100 nmol L−1, using a five centimeter cuvette) and trace (&lt;100 nmol L−1, using a one meter path length capillary cell) levels of dissolved urea. Our trace level method showed very high sensitivity (detection limit of 1.2 nmol L−1) and high reproducibility (relative standard deviations were 0.29–1.09%), which was a significant improvement as compared to what was reported previously. The current method afforded a more stable colorimetric product, which provided better reproducibility when a large number of samples were analyzed. The normal‐level method was applied to studying the urea distribution in estuarine surface water, and the trace‐level method was applied to studying the vertical distribution of urea in the South China Sea basin (&gt; 4000 m).

Urea uptake by the scleractinian coral Stylophora pistillata

Urea can be one of the major sources of nitrogen for phytoplankton, but little is known about its importance for corals. Experiments were therefore designed to assess the uptake rates of urea by the scleractinian coral Stylophora pistillata; 15N-urea was used to follow the incorporation of nitrogen into the zooxanthellae and animal tissue. The uptake kinetics of urea in the tissue of S. pistillata showed that there is a concentration-dependent uptake of urea. The transport of urea was composed of a linear component (diffusion) at concentrations higher than 6 μmol N-urea l − 1 and an active carrier-mediated component, at lower concentrations. The value of the carrier affinity ( K m = 1.05 μmol urea l − 1 ) indicates a good adaptation of the corals to low levels of urea in seawater. At the in situ concentration of ca. 0.2 μmol N-urea l − 1 , the uptake rate was equal to 0.1 nmol N h − 1 cm − 2 . Urea uptake was at least four times higher in the animal than in the algal fraction, and five times higher when corals were incubated in the light than in the dark. These results could be explained by the involvement of urea in the calcification process, which is also enhanced by light. Comparison of urea uptake rates with nitrate or ammonium uptake rates for the same S. pistillata species, at in situ concentrations, showed that urea is preferred to nitrate and may therefore be an important source of nitrogen for scleractinian corals.

A new application of the diacetyl monoxime method to the automated determination of dissolved urea in seawater

A new application of the diacetyl monoxime method for the determination of dissolved urea in seawater is presented, based on the Alpkem autoanalyzer (OI, Analytical). This system has several advantages over automated methods previously described in the literature. The urea cartridge can be easily built using nut/ferrule connections, and the external thermostatic bath is not required. The system is set up to operate at low urea concentrations (0–3 µM urea), but it is linear up to 100 µM urea. The sample volume needed for analysis is 0.4 ml, and the rate of analysis is 25 samples h−1. The precision of analysis (CV) of seawater samples is 2.1% at 0.50 µM urea, and the detection limit is 0.02 µM urea. The interference on the colorimetric measurement due to formation of Brij-35 aggregate in strongly acidic media was eliminated by substituting this wetting agent with hexadecyl–trimethyl–ammonium bromide; thus, this method is suitable for other autoanalyzer technologies that use small-diameter transmission tubes and small-sized spectrophotometric flowcells.

Assessment of a trap for measuring larval supply of intertidal barnacles on wave-swept, semi-exposed shores

Larval supply and settlement of Semibalanus balanoides (L.) on a semi-exposed wave-swept shore in E Scotland were assessed by means of a new larval trap design. Two midshore sites were utilised throughout the peak of the 2001 settlement season (May 11–June 3). Larval supply was quantified by 176 ml baffled cylinder traps filled with a 4-M killing solution of urea in seawater. Larval trap washout of urea typically ranged from ∼25% per day under minimal wave action to a maximum of 39% under heavy wave action during an onshore gale. The maximum capture for a single trap was 334 cyprids over one tide. Significant and consistent positional effects, both on urea retention and larval capture, were found for replicate traps separated by 9 cm. These indicated fine-scale variations in hydrodynamic flow and larval supply over the substratum which will have implications for the quantification of spatial heterogeneity of larval input to the benthos. The retention of urea and larval capture efficiency generally were not compromised by traps being serviced daily, as opposed to tidally, except perhaps for captures at very low larval densities. Larval supply measured on a tidal basis was strongly correlated between sites ( r=0.975), but supply at Site C was ∼5× that at Site T: the reduction at Site T could not be explained by its slightly shorter immersion time and hence ‘availability’ of cyprids alone. At Site C settlement was quantified for 5×5 cm quadrats of natural substratum cleared daily. Settlement on these clearances (subject to possible grazing by limpets) was quantified only from May 20–June 3, after the peak of larval supply on May 17, and the variation in larval supply explained 65% of the variance in settlement. The larval supply/settlement relationship at Site T was quantified using grooved acrylic panels. A high correlation coefficient (0.961) was obtained for the daily supply/settlement relationship for these panels over the period May 15–June 3, which included the peak of supply. Larval supply varied up to 15-fold within a site on consecutive tides and up to 100-fold between sites on the same tide. Although further improvements to both the larval trap and the settlement panels can be made, this larval trap does appear capable of providing high-resolution data on tidal or daily larval supply over a wide range of wave conditions and larval concentrations.

A Room Temperature Procedure for the Manual Determination of Urea in Seawater

Several earlier studies underpin the important role of dissolved organic matter and more particularly urea in phytoplanktonic nitrogen uptake fluxes. Generally, the determination of urea concentrations relies on the formation of an imidazolone-thiosemicarbizide complex, a complexation which requires very accurate temperature control when carried out at high temperature. It is also possible, however, to obtain reliable results with a room temperature procedure. The measured abundances for both complexation at high temperature (85°C, 20min) and at ambient temperature (22°C, 72h) are closely comparable. Lower values are observed for temperatures <10°C though. Moreover, a comparison of both techniques reveals similar precision (coefficient of variation: 2%), sensitivity (slope of calibration line: 0·2) and detection limit (0·14mM). The room temperature alternative to the earlier described method is therefore a handy tool for urea analyses, when a strict temperature control is difficult or impossible.

A modified manual method for the determination of urea in seawater using diacetylmonoxime reagent

A manual method for the quantitative determination of urea in seawater was developed based on the reaction of urea with diacetylmonoxime. Critical factors include the treatment of collection bottles, sample storage, pre-filtration techniques, water purification, reaction temperature and reproducible cooling of samples during analysis. The limit of detection was 0·14 μg-at urea-N 1 −1 with Beer's Law being obeyed in the range tested of 0–15 μg-at urea-N 1 −1 . The precision (±1 SD) of replicate samples of 1,2 and 15 μg-at urea-N 1 -1 was 0·024, 0·019 and 0·03 μg-at urea-N 1 −1 ( n = 10) respectively.

A Method for Estimating Uptake and Production Rates for Urea in Seawater using [14C] Urea and [15N] Urea

Improved estimates of the rates of urea production and uptake by natural populations of phytoplankton were made after determining the change in 15N-atom% enrichment of urea during incubations. A [14C] urea method is described by which the change in enrichment is measured. Estimates of uptake rates are increased (relative to uptake rates determined without correction for isotope dilution) by up to 83% using a 15N accumulation model and by &gt;210% using a 15N disappearance model. A discrepancy exists between [15N] urea removed from the aqueous phase and 15N accumulated in the particulate phase at stations occupied in the northeastern Bering Sea. The ability to find in the particulate fraction the 15N removed from solution as [15N] urea was improved by 72% following removal of the &gt;20-μm particulate fraction. This corresponded to only a 4% reduction in the concentration of chlorophyll a and a 37% reduction in the concentration of particulate N. Removal of microzooplankton may have improved the efficiency of urea-N retention by phytoplankton.

Comparison of methods for the analysis of dissolved urea in seawater

A comparison between the diacetyl monoxime and urease methods for measuring dissolved concentrations of urea in seawater was conducted in artificial seawater, phytoplankton-culture filtrate and both natural and ureaspiked field samples from coastal and oceanic enviroments during 1984. The urease technique underestimated urea concentrations in unbuffered photoplankton-culture filtrate as a result of the inhibition of the urease enzyme, causing the incomplete hydrolysis of urea in these samples. Factors responsible for inhibiting urease included pH, seawater ions, and possibly extracellular metabolites produced in unialgal cultures. Seawater type and time of sample collection were important variables affecting urea measurement by the urease method, and recovery of internal standards ranged from 40 to 100%. Increasing the heating time of the urease assay, or the concentration of urease added to the seawater samples increased the amount of urea determined by the urease method. However, measured values were still less than the concentration of the urea internal standards. The diacetyl monoxime method was suitable for urea determinations in all the seawater samples we examined; it was easily automated, and the results were accurate and reproducible. This modified technique is recommended for measuring disolved concentrations of urea in seawater.

Comparison of methods for the analysis of dissolved urea in seawater : Price, N.M. and P.J. Harrison, 1987. Mar. Biol., 94(2):307–317.

Comparison of methods for the analysis of dissolved urea in seawater : Price, N.M. and P.J. Harrison, 1987. Mar. Biol., 94(2):307–317.

Dosage automatique de l'urée dans l'eau de mer: une méthode très sensible à la diacétylmonoxime

The determination of urea in sea water was performed by an automated method based on the reaction of urea with diacetyl-monoxime. The sensitivity, which has been greatly enhanced compared to previous methods, by using reagents containing thiosemicarbazide and iron III, is 0.044 absorbance units/μg-at urea-N/L with the 50 mm flow cell. The precision is ±0.01 μg-at urea-N/L and the limit of detection (2 SD) is 0.02 μg-at urea-N/L. The absorbance–concentration relationship is linear up to 15 μg-at urea-N/L. The analytical blanks have been strictly determined and the interferences studied. Citrulline is the main compound that interferes. However, in attempting to remove this compound from the samples by a chelating resin, we demonstrated its absence from the analyzed coastal waters. The color development with urea does not depend on the salt concentration. The problems of contamination and storage are discussed: great care must be taken in handling the samples; quickly freezing the samples is proved very suitable for storage during a long time. Some examples of application of the method to oceanic, coastal and estuarine waters show concentrations ranging from 0.00 to 3.0 μg-at urea-N/L.

The chemical nature of host hatching factors in the monogenean skin parasites Entobdella soleae and Acanthocotyle lobianchi

Fully-developed eggs of the monogenean Entobdella soleae from the skin of the common sole ( Solea solea) hatch when treated with dilute solutions of urea or ammonium chloride in sea water. There is some evidence that arginine may stimulate hatching but the eggs do not respond when treated with sea water solutions containing trimethylamine oxide or glutamine. Sole skin mucus contains sufficient urea to stimulate hatching but insufficient ammonia. Solutions of urea in sea water stimulate hatching in the monogenean parasite Acanthocotyle lobianchi found on ray skin. Sea water solutions containing ammonium chloride and trimethylamine oxide failed to hatch the eggs of A. lobianchi and the eggs were also insensitive to various amino acids made up at concentrations found in host mucus. Experiments with urease confirmed that urea in ray ventral skin mucus is the host hatching factor for A. lobianchi. Skin mucus from the common sole failed to stimulate hatching in A. lobianchi. The role as hatching factors of excretory products in host gill effluent, skin mucus and urine is discussed.

Decomposition of urea associated with photosynthesis of phytoplankton in coastal waters

Decomposition of urea in seawater was studied in Mikawa Bay, a shallow eutrophic bay on the southern coast of central Japan. The urea concentration in seawater ranged from 1.3 to 5.9 μg-at. N/1 and comprised 12 to 40% of the dissolved organic nitrogen. Using 14C labelled urea, the rate of CO2 liberation from urea and the incorporation rate of urea carbon into the particulate organic matter were determined. For the surface samples, high rates of CO2 liberation from urea as well as the incorporation of urea carbon into the particulate organic matter were observed in the light, while much lower rates were obtained in the dark. Incubation experiments with exposure to different light intensities revealed that the rate of CO2 liberation from urea and the incorporation of urea carbon into particulate organic matter changed with light intensity, showing a pattern similar to that of photosynthesis. The highest liberation and incorporation rates were observed at 12,000 lux. Incubation in light and in dark produced marked decreases and increases, respectively, in urea and ammonia, while no appreciable changes were observed for nitrate and nitrite. It is suggested that urea decomposition associated with photosynthetic activity of phytoplankton is one of the major processes of urea decomposition, and that it plays a significant role in the nitrogen supply for phytoplankton in coastal waters.

An automated analysis for urea in seawater1

An automated method for the determination of urea in seawater, adapted to a Technicon AutoAnalyzer, is sensitive enough to detect urea at the 0.05 µg‐atom urea‐N liter−1 level.

Determination of urea in natural water by an improved Newell method

The method for determination of urea in sea water proposed by Newell et al. was re-examined in detail and partly modified. The following method is recommended.To 25 mlof a sample solution are added 3 g of sodium chloride for fresh water or 2 g for sea water (Fig. 1), 3 mlof concentrated sulfuric acid and 1 mlof the mixed reagent (4.0 g of diacetyl monoxime, 0.05 g of semicarbazide hydrochloride and 15 g of MnCl24H2O in 200 mlof water). The solution is allowed to stand for 90 min at 70°C (Fig. 2). After cooling in running water, the absorbance is measured at 520 mμ against water using a 5-cm cell. The blank is measured by the same procedure with distilled water for fresh water or artificial sea water for sea water. The common ions in natural water and nitrogen-containing compounds do not interfere as seen in Table I. Although citrulline, which is produced by decomposition of arginine, gives a positive error, it is unlikely that such amount of citrulline exsists in natural water. The molar extinction coefficient for urea is 22000. The relative standard deviations were 3 and 5% at the concentration of 86 and 43 μg urea/l, respectively.

A UREASE METHOD FOR UREA IN SEAWATER1

A simple method for the quantitative determination of urea in seawater is described. It involves addition of urease and subsequent measurement of the liberated ammonia. Data are presented for coastal waters near La Jolla, California, and for coastal waters off central Peru. In the marine euphotic zone the cycle of supply and utilization of urea may be similar to that of ammonia.