The WSC-g-PAA/PVA/NP contains amounts of carboxyl groups, the
protonation extent of which mainly rely on the changes of pHs of
soaking medium. Therefore, the water absorbency is closely linked to
the pH values of the medium. The equilibrium water absorbency kept
raising until pH reached 7, and then a decline appeared when pH was
higher than 7 (Fig. 7a); this demonstrated that WSC-g-PAA/PVA/NP
was stimuli-responsive to the pH. This pH stimuli-responsive property
was mainly attributed to the protonation of carboxylate anions
under an acidic circumstance, which could lead to the disappearance
of anion-anion repulsion. Furthermore, the hydrogen-bonding interaction
among hydrophilic groups was also reinforced because of the
protonation. At the same time the physical crosslinking degree of
skeleton network was also heightened, resulting from the relatively
lower water holding capacities. With the sustained increasing pH,
both the negative effect of H+ on electrostatic repulsive force and
the hydrogen-bonding interaction among carboxylate groups were
weakened by degrees and as a result more water could be absorbed
by WSC-g-PAA/PVA/NP. However, excessive Na+ in the immersion solution
could screen the charges of −COO− groups (so called “charge
shielding effect”). This process would weaken the electrostatic repulsion,
consequently, the swelling capacities declined with increasing
pH from 7 to 12 [34].
From the curves displayed in Fig. 7(b) and (c), it can be seen that
the release curves of phosphorus and nitrogen fertilizers displayed
totally opposite tendencies. With the rising of pH from 2 to 12, the
release amount of phosphorus firstly increased and then decreased,
instead of which the release amount of nitrogen firstly decreased
and then increased. Under strongly acidic circumstances, the groups
of -COO− on WSC-g-PAA/PVA/NP transformed into -COOH, leading
to the decreased anion–anion repulsion between carboxyl and PO3−
4 .
Besides, the change from −COO− to −COOH would also bring about
the reinforcing of hydrogen-bonding interaction, which was not conducive
to the release of phosphate. On the contrary, the acrylamide
in WSC-g-PAA/PVA/NP was prone to react with H+ at low pH, and
then produce NH4Cl. Thereby, the amount of nitrogen in solution was
increased. When the solution was in an alkaline conditions, the react
between sodium ions and carboxyl groups would be induced, resulting
in the reduce of electrostatic repulsion between carboxyl and
PO3−
4 . This would lead to the lower release amount of phosphorus.
Instead, the total release amount of nitrogen at high pH was higher,
mainly due to the yielding of NH3 by the react between acrylamide
and OH—.