Agriculture Reference
In-Depth Information
sorption and desorption of nutrients (Yumei et al.
2009
; Zhou et al.
2014
). Further-
more, depending on the amount of clay mineral added to hydrogels, the cost of the
material can be substantially reduced, making it competitive in the trading market.
Obtaining hydrogels modified by clay minerals results in materials called com-
posite, defined as a material obtained from two or more constituents, with different
physical and chemical characteristics, and that remain separated at a microscopic
scale (Mitchell
2004
), but the combination ensures the material properties associ-
ated to the interaction of its components (Lan and Pinnavaia
1994
).
Clay minerals-based nanocomposite hydrogels have been studied for various
applications, particularly in controlled sorption and desorption, since their presence
causes more interaction of the gel with cationic groups into the swelling medium.
The formation of nanostructured hydrogels with clay minerals has been studied by
some authors, although they are still focused on modifying hydrogel properties,
especially concerning increments on mechanical strength (Wu et al.
2000
; Liu
et al.
2006
). However, some authors (Kasgoz and Durmus
2008
; Yi and Zhang
2008
; Li et al.
2009
) have observed that the obtained composite hydrogels could
have interesting ions adsorption properties and extended release. Recent studies
show the properties of this class of nanocomposites for capturing ions in solution,
especially for the removal of heavy metals and cationic dyes (Yi and Zhang
2008
;
Li et al.
2009
; Zhang et al.
2014
). However, these papers cited above involved small
amounts of clay (up to 2.0 % by mass of hydrogel).
The choice of using hydrogels as a base of composites, as supporting material for
controlled/slow release in agricultural applications, is because they are
multifunctional materials, i.e., they contribute to keep both water and nutrient
release rates in optimum ranges for development of crops. Therefore, modified
hydrogels are suitable carrier systems for applications in controlled or slow release
of agricultural inputs (Khare and Peppas
1995
).
Mikkelsen et al.
(
1993
) evaluated the effect of hydrogel with MnO,
MnSO
4
4H
2
O, and MnCl
2
in a soybean crop and observed an increase of 89 % in
the accumulation of the micronutrient Mn in the air portion of the crop. Ni
et al. (
2011
) developed a superabsorbent material for slow/controlled release of
urea, and it presented a good release behavior: after 30 days in soil, the material
released about 70.0 % of the incorporated urea. However, the authors incorporated
urea during the material synthesis in the proportion of urea-hydrogel (wt.%) of
3 g urea/g of hydrogel, but water absorption rate was relatively low (50-70 g/g
product), and the high cost hampers current application of the material.
Bortolin et al. (
2013
) synthesized a novel series of hydrogels, composed of
polyacrylamide, methylcellulose, and montmorillonite, in which the presence of
the clay mineral resulted in some improvements of the materials
properties.
Release tests of urea (adsorption-desorption) showed that the inclusion of mont-
morillonite in the hydrogel allowed the composite to release urea in a more
controlled rate than the pure hydrogel and almost 200 times slower than conven-
tional pure urea (Fig.
11.5
). These data demonstrate the great potential of clay
minerals, particularly montmorillonite, as the basis for new slower- or controlled-
release fertilizers.
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