Environmental Engineering Reference
In-Depth Information
Sulphuric acid
+
U
F
U
M
Sodium hydroxide
Water
+
U
M
F
...............
Monomethylol urea
...............
+
U
M
U
M
F
Methylene diurea
+
...........
U
M
U
M
U
M
Dimethylene triurea
U
M
U
M
U
M
U
M
Trimethylene tetraurea
U
M
U
M
U
M
U
M
U
M
...
Fig. 5.6. The reaction products of urea (U) and formaldehyde (F). M
methyl group.
ΒΌ
formed. As the polymer lengthens and the number of 'chain links' in-
creases, solubility decreases and nitrogen is released more slowly. Urea
formaldehyde is one of the oldest controlled-release forms of nitrogen.
Developed in the 1940s, it is still manufactured and universally used. The
polymerization process, presented in Fig. 5.6, starts with the reaction of
urea and formaldehyde. The product of this reaction, monomethylol urea,
closely resembles urea but releases nitrogen a little more slowly and has
somewhat reduced potential for leaf burning. Addition of another urea
molecule to the chain produces methylene diurea (calledMDU) which has
even slower nitrogen release and less burning potential. Addition of the
next one produces dimethylene triurea (called DMTU). Then comes tri-
methylene tetraurea, and so on. Many of these polymers are mixed with
other forms of nitrogen and sometimes with other nutrients, organic
fertilizers, etc. Long-chain urea formaldehyde polymers are among the
slowest-release forms of nitrogen available.
Isobutylidene diurea (IBDU) is made by reaction of isobutylaldehyde
and urea. The solubility of this product is very low, but in the presence of
soil moisture, IBDU hydrolyses back to urea and butylic acid. The re-
leased urea is converted to ammonia and in this form is available to
plants. Besides level of moisture, the rate of nitrogen release is governed
by the particle size of this product, which is generally available in coarse,
fine and powder grades. Because the ratio of surface area to volume of
small particles is larger than that of large ones, smaller particles hydro-
lyse faster. Even in powder form, IBDU releases nitrogen relatively
