Some investigators have prepared controlled release urea fertilizers
by either blending with superabsorbent materials or polymerization
with a superabsorbent mixture. However, these slow release formulae
have thus far experienced the undesirable “burst effect” that hampers
the controlled release property. Some polymer shells also remain in
the soil for a long time after nutrients have been completely released.
Hence, an approach to enhance their biodegradability to avoid hazardous
emissions and other effects was presented by Ni [54]. He prepared
CRCU with an attapulgite matrix as the fertilizer core with two layers
of coating: ethyl cellulose joined to a plasticizer as the inner coat, and
a sodium carboxymethylcellulose (CMC) plus hydroxyethylcellulose
(HEC) based hydrogel as the outer coat. Attapulgite is a type of octahedral
layeredMg–Al–silicate absorbent mineralwith hydroxyl groups on
its surface. It is almost inert towards salts (like urea), so it is preferred as
a substrate for superabsorbent composite materials [55]. After 24 h of
soil incubation, the urea release rate was 8.7%. During this phase,
water diffused gradually into the granules as slower release was facilitated
by the hydrophobic ethylcellulose coating. During the second
stage, from day two to five, there was consistent release caused by the
diffusion of nutrients outwardly followed by dynamic mass transfer to
the external atmosphere. In the last phase, from day two onwards, the
solution's concentration within was lowered as bulk water was
absorbed. During this stage, attapulgite absorbed the remaining nutrients
which further enhanced the slow release of urea.
Another recent study by Yang et al. [57], addressed the issue of
polymer biodegradability with double coated urea granules produced
with biodegradable biopolyurethane derived fromliquefied corn Stover
as the inner coating, and a superabsorbent material based on chicken
feather meal modified with acrylic acid as the outer coating. For the
inner coating, urea granules were placed in a rotary drum and the
coating solution was poured on rotating granules. Different runs were
made to produce different mass coatings. For the outer layer, the
acrylic-acid-modified-chicken-feather-meal (MCFM-AA) solution was
poured on previously prepared coated granules followed by an adherent
(MCFM-AA powder) to produce the final compact product. Release
kinetics was studied in deionized water as well as in soil. The periodic
increments in themass coating of the inner coating layer caused significant
reductions in release rates. For example, N release slowed from
1.5 days to 13 and then to 57 days as the mass of the inner coating
increased from 3.2% to 5.3% and 8.5% (wt.%), respectively.