118
G. Fijałkowska et al.
The PAM adsorbed amount may also depend on the soil and clay mineralogy [1,
76, 82]. The amount of adsorbed polymer on the surface of soil particles is up to
three orders of magnitude smaller than the amount adsorbed on clay mineral particles. Under neutral or acidic pH, the adsorption of anionic polyacrylamide on the
surface of clay materials such as montmorillonite, kaolinite, and illite is comparable. However, under basic conditions, the amount of adsorbed polymer decreases
in order: illite, kaolinite, and montmorillonite [46, 76]. What is more, under alkaline pH conditions greater adsorption of anionic PAM on the illite surface compared
to montmorillonite one was observed. The presence of polyacrylamide with high
molecular weight contributes to flocculation process. However, the structure of flocs
depends on clay mineral type. These micro-aggregates of soil particles with polyelectrolyte can be formed in both neutral and acidic solutions. Other studies focused
on the stability of smectic soil aggregates in the presence of PAM [74]. It turned
out that the macromolecules that entered the pores of the aggregates did not have a
significant effect on the stability of their structure. On the other hand, this polymer
increased the percentage of stable aggregates in kaolinite soils with different structures [89] and particle size [90]. The formation of aggregates, stability of macro- and
micro-aggregates, their degradation mechanism, and chemical dispersion depend not
only on the amount of clay minerals and organic matter but also on soil mineralogy
[91]. The influence of the mineralogical composition is difficult to determine because
soils usually contain a mixture of clay materials and their properties are modified
by binding with other minerals. Under unstable conditions (such as high sodium
content or low electrolyte concentrations), soils with high montmorillonite content
are unstable, soils with high kaolinite content are relatively stable, and soils rich in
2:1 clay minerals or with low content of montmorillonite show intermediate stability
[92, 93]. The effectiveness of PAM as a soil aggregate stabilizing factor has also
been studied. The addition of polymer had a positive effect on increasing the moisture content of soil aggregates, thanks to which their stability improved (in ascending
order: kaolin < illitic < smectite soils).
The content of clay in the soil and their mineralogy significantly affects the stabilization of aggregates by PAM. The effectiveness of stabilization may result from the
natural stability of aggregates and soil properties affecting polymer adsorption on
the solid surface. The optimal pH value and the presence of easily soluble minerals,
which provide divalent cations to the soil solution, can contribute to increasing
PAM adsorption on the surface of soil particles and the ability to increase aggregate
stability.
Polyacrylamide treatments has significant impact on crop growth and yield as
well as nutrient effect [28, 94–97]. Wallace [98] studied the interaction of polyacrylamide with the following nutrients: P, Zn, Cu, Fe, Mn, and Mo. The concentration of
these elements in the soil solution containing PAM is significantly different than in
systems without macromolecular additive. Moreover, these differences in microelements concentration affect the plant growth—the studies shown that plant growth is
improved in the nutrient solutions with polyacrylamide soil flocculant [98].
G. Fijałkowska et al.
The PAM adsorbed amount may also depend on the soil and clay mineralogy [1,
76, 82]. The amount of adsorbed polymer on the surface of soil particles is up to
three orders of magnitude smaller than the amount adsorbed on clay mineral particles. Under neutral or acidic pH, the adsorption of anionic polyacrylamide on the
surface of clay materials such as montmorillonite, kaolinite, and illite is comparable. However, under basic conditions, the amount of adsorbed polymer decreases
in order: illite, kaolinite, and montmorillonite [46, 76]. What is more, under alkaline pH conditions greater adsorption of anionic PAM on the illite surface compared
to montmorillonite one was observed. The presence of polyacrylamide with high
molecular weight contributes to flocculation process. However, the structure of flocs
depends on clay mineral type. These micro-aggregates of soil particles with polyelectrolyte can be formed in both neutral and acidic solutions. Other studies focused
on the stability of smectic soil aggregates in the presence of PAM [74]. It turned
out that the macromolecules that entered the pores of the aggregates did not have a
significant effect on the stability of their structure. On the other hand, this polymer
increased the percentage of stable aggregates in kaolinite soils with different structures [89] and particle size [90]. The formation of aggregates, stability of macro- and
micro-aggregates, their degradation mechanism, and chemical dispersion depend not
only on the amount of clay minerals and organic matter but also on soil mineralogy
[91]. The influence of the mineralogical composition is difficult to determine because
soils usually contain a mixture of clay materials and their properties are modified
by binding with other minerals. Under unstable conditions (such as high sodium
content or low electrolyte concentrations), soils with high montmorillonite content
are unstable, soils with high kaolinite content are relatively stable, and soils rich in
2:1 clay minerals or with low content of montmorillonite show intermediate stability
[92, 93]. The effectiveness of PAM as a soil aggregate stabilizing factor has also
been studied. The addition of polymer had a positive effect on increasing the moisture content of soil aggregates, thanks to which their stability improved (in ascending
order: kaolin < illitic < smectite soils).
The content of clay in the soil and their mineralogy significantly affects the stabilization of aggregates by PAM. The effectiveness of stabilization may result from the
natural stability of aggregates and soil properties affecting polymer adsorption on
the solid surface. The optimal pH value and the presence of easily soluble minerals,
which provide divalent cations to the soil solution, can contribute to increasing
PAM adsorption on the surface of soil particles and the ability to increase aggregate
stability.
Polyacrylamide treatments has significant impact on crop growth and yield as
well as nutrient effect [28, 94–97]. Wallace [98] studied the interaction of polyacrylamide with the following nutrients: P, Zn, Cu, Fe, Mn, and Mo. The concentration of
these elements in the soil solution containing PAM is significantly different than in
systems without macromolecular additive. Moreover, these differences in microelements concentration affect the plant growth—the studies shown that plant growth is
improved in the nutrient solutions with polyacrylamide soil flocculant [98].