Ureas hydrolysis

In this method, urea is added to the aqueous solution of the metal cations [115]. Decomposition of urea upon heating forms ammonium and carbonate ions, the pH increasing homogeneously in the whole volume of the solution, giving rise to a so-called homogeneous precipitation of the LDH in its carbonate form [116, 117]. Insertion of CNO anions (an intermediate species in the transformation of urea into carbonate) has been also observed in some cases. 

---

For most crops, other than rice, urea in the soil must first undergo hydrolysis to ammonia and then nitrification to nitrate before it can be absorbed by plant roots. One problem is that in relatively cool climates these processes are slow thus plants may be slow to respond to urea fertilization. Another problem, more likely in warmer climates, is that ammonia formed in the soil hydrolysis step may be lost as vapor. This problem is particularly likely when surface appHcation is used, but can be avoided by incorporation of the urea under the soil surface. Another problem that has been encountered with urea is phytotoxicity, the poisoning of seed by contact with the ammonia released during urea hydrolysis in the soil. Placement of urea away from the seed is a solution to this problem. In view of the growing popularity of urea, it appears that its favorable characteristics outweigh the extra care requited in its use. 

Urea hydrolysis is frequently observed as the initial reaction for pesticides having urea bonds, such as linuron, diuron, and chlorsulfuron (10) (eq. 14)... 

Carboxyhc acid ester, carbamate, organophosphate, and urea hydrolysis are important acid/base-catalyzed reactions. Typically, pesticides that are susceptible to chemical hydrolysis are also susceptible to biological hydrolysis the products of chemical vs biological hydrolysis are generally identical (see eqs. 8, 11, 13, and 14). Consequentiy, the two types of reactions can only be distinguished based on sterile controls or kinetic studies. As a general rule, carboxyhc acid esters, carbamates, and organophosphates are more susceptible to alkaline hydrolysis (24), whereas sulfonylureas are more susceptible to acid hydrolysis (25). 

Determination of iron as iron (III) oxide by initial formation of basic iron (III) formate Discussion. The precipitation of iron as iron(III) hydroxide by ammonia solution yields a gelatinous precipitate which is rather difficult to wash and to filter. Iron(III) can, however, be precipitated from homogeneous solution as a dense basic formate by the urea hydrolysis method. The precipitate obtained is more readily filtered and washed and adsorbs fewer impurities than that formed by other hydrolytic procedures. Ignition yields iron(III) oxide. 

The pH at which basic iron(III) formate begins to precipitate depends upon several factors, which include the initial iron and chloride concentration a high concentration of ammonium chloride is essential to prevent colloid formation. It is important to use an optimum initial pH to avoid a large excess of free acid, which would have to be neutralised by urea hydrolysis, and yet there must be present sufficient acid to prevent the formation of a gelatinous precipitate prior to boiling the solution ideally, a turbidity should appear about 5-10 minutes... 

A recently reported synthesis involves refluxing a mixture of IrCl3 and ethanoic acid with urea hydrolysis of the urea proceeds steadily generating the ammonia ligands. This resulting mixture of the penta-, tetra- and tri-ammines can be separated chromatographically [149],... 

The activity of enzymes in the film was estimated in the following way In order to test the activity of urease, we utilized a calorimetric assay based on urea hydrolysis the enzymatic reaction was followed at 590 nm, the suitable wavelength for bromcresol purple (Chandler 1982). Urea concentration was 1.67 ts 10 M. 

Metals have been shown to negatively affect nitrogen transformation and urea hydrolysis. For example, Antil et al.69 discovered that the microbial biomass in a soil receiving sewer water or industrial wastewater decreased with increasing concentrations of cadmium and nickel. The rate of urea hydrolysis was 1.6 times greater in an uncontaminated soil than in a soil containing 0.026 mg Ni g 1. Ammonium concentrations increased for up to 14 days in a soil containing over... 

Antil, R., Mahata, M., and Narwal, R., Urea hydrolysis and nitrogen transformations in soils containing different lead contents, Arch Agron Soil Sci, 52 (1), 69-77, 2006. 

Different effects of formaldehyde on the hydrolysis of urea are reported. On the one hand, Garrido and colleagues,3 applying anoxic conditions, observed that an inhibitory effect started at 50 mg/L formaldehyde and the levels of inhibition were 50% and 90% for concentrations of formaldehyde of 100 mg/L and 300 mg/L, respectively. Similar effects were found by Campos and colleagues,33 working with an anoxic USB, who observed that formaldehyde concentrations in the reactor of 250 to 300 mg/L caused an inhibition of around 53%. This inhibition on the ureolytic activity was also reported by Walker.36 On the other hand, Eiroa and colleagues37 carried out batch assays at different initial urea concentrations from 90 to 370mg/L N-urea in the presence of 430 mg/L formaldehyde. They observed that a complete hydrolysis was achieved and initial urea hydrolysis rates remained constant. 

To remove urea and formaldehyde from synthetic wastewater, Campos and colleagues33 operated a coupled system consisting of a biofilm airlift suspension (BAS) reactor to carry out nitrification and an anoxic USB reactor to carry out the denitrification and urea hydrolysis (Figure 19.8). 

Garrido, J.M., Mendez, R., and Lema, J.M., Simultaneous urea hydrolysis, formaldehyde removal and denitrification in a multifed upflow filter under anoxic and anaerobic conditions, Water Res., 35, 691-698, 2001. 

Industrial liquid urea is known as Adblue . This industrial liquid [chemical formula (NH2)2CO] is an aqueous solution of 32.5% wt urea solution (see Figure 7.14). Adblue is corrosive and requires stable materials for the components like tank, pipes, injector, etc. Moreover, this solution is not suitable for year-round use in the Northern countries the freezing point is at —11°C. Urea hydrolysis requires at least 180°C. 

It is worth noting that the partide sizes of samples prepared by the urea hydrolysis method are larger than other LDHs. Such a finding can be rationalized by considering the slow hydrolysis of urea [61], since it can be classified as a very weak Bronsted base (pfCb= 13.8). Its hydrolysis mechanism consists of the formation of ammonium cyanate as the rate-determining step, followed by fast hydrolysis of the cyanate to ammonium carbonate ... 

However, rates of precipitation in soil systems may be quite different from those in solutions because precipitation is catalysed by adsorption of the reacting solutes onto soil surfaces the nature of the solid phases formed may be different and sorption may also alter the effects of inhibitors. There are very few data in the literature on these effects actually measured in soils. Figure 3.15 shows data of Huang (1990) for calcite precipitation in three soils incubated with urea. Precipitation was induced as the pH increased during urea hydrolysis ... 

The removal of fertilizer N in the crop as NH4+ does not lead to acidification. Hydrolysis of urea fertilizer—by far the main form of N fertilizer used in wetland rice, together with ammonium bicarbonate in some countries—consumes 1 mol of H+ per mol of NH4+ formed (Table 7.1, Process 1). So although absorption of N as NH4+ leads to a net export of H+ from the roots to balance the resulting excess intake of cations over anions (Table 7.1, Process 5), this acidity is matched by the H+ consumed in urea hydrolysis. Likewise there is no net generation of acidity as a result of NH3 volatilization, although 1 mol of H+ is left behind per mol of NH4+ converted to NH3 (Table 7.1, Process 3). 

H—N=C=0 is the more stable isomer of cyanic acid.) This reaction represents the slow step of the urea hydrolysis. 

Fig. 9. A possible mechanism for urea hydrolysis at the catalytic site of urease.

Huang, T.C. and Chen, D.H., Kinetic studies on urea hydrolysis by immobilized urease in a batch squeezer and flow reactor, Biotechnol. Bioeng., (1992) 40,10,1203-09. Storey, K.B., Duncan, J.A., and Chakrabarti, A., Immobilization of amyloglucosidase using two forms of polyurethane polymer, Appl. Biochem. Biotechnol., (1990) 23, 3, 221-36. 

In another kinetics study, Huang and Chen immobilized jack bean urease in the form of a thin film on the surface of a reticulated polyurethane foam. The residual apparent activity of the urease after immobilization was about 50%. The good hydrodynamic properties and flexibility of the support were retained in solution after immobilization. Urea hydrolysis was examined in both a batch squeezer and circulated flow reactor. The results suggest potential for practical applications in various reactors. 

Rate Enhancement by Urease The enzyme urease enhances the rate of urea hydrolysis at pH 8.0 and 20 °C by a factor of 1014. If a given quantity of urease can completely hydrolyze a given quantity of urea in 5.0 min at 20 °C and pH 8.0, how long would it take for this amount of urea to be hydrolyzed under the same conditions in the absence of urease Assume that both reactions take place in sterile systems so that bacteria cannot attack the urea. 

Thin-layer chromatography has also been employed in the analysis of these compounds (9,38,150,155-159). Several developers, such as silver nitrate (150,158), / -dimethyl-aminobenz-aldehyde (38), or UV light (150,158) have been used. Derivatization techniques, based on the urea hydrolysis to the corresponding aniline and ulterior reaction with dansyl chloride, have also been carried out (9,159). Moreover, TLC is frequently employed in studies of metabolization with radiolabeled compounds (155-157). 

---