the potency of adhesion of the majority of ENPs to both and
other particles, by which ENPs frequently agglomerates to
form particle clusters (Cornelis et al. 2014). Aggregation
reduces the specific exterior area of particles and interfacial
energy. In aggregation presence of the natural organic matter
commonly limits their movement in the soil. The actions of
soil organisms manipulate carbon maintenance time and
return in soil, which revolves change carbon stabilization,
aggregation, and yield. Aggregation favors the movement
and deposition of ENPs in soil solution. It can be homoaggregation (between ENPs) or hetroaggregation (between
ENPs and other soil components such as clay, minerals, or
oxides). The collision among the ENPs resulted in the formation of aggregates and thus the establishment of weaker
van der Walls forces or strong chemical bonds takes place.
The presence of natural organic nanoparticles in the soil
porous media further facilitates the hetero-aggregation
among them, thus, severely affects their bioavailability,
toxicity, and transport across porous media. Aggregation of
metal-based ENPs (Ti, Cu, Au, Ag, Ni) was already noted in
soil (Cornelis et al. 2014). Hetero-aggregation most likely
occurs than homoaggregation rates in soil pores and appears
to vary depending on the soil colloid and ENPs nature. The
formation of large hetero-aggregates may also reduce the
translocation and uptake of ENPs through plant cells and
membranes, and thus decreasing their biological availability
(Gogos 2015). The loss of ENPs polymeric coatings under
sunlight catalyzed redox reactions may stimulate instability
and favors hetero-aggregation. The soil pH, clay content,
organic matter, and particle characteristics play an important
role in ENPs transport and retention (Abbas et al. 2020). The
movement of ENPs across the soil solution was likely to be
influenced by zeta potential, surface coating of ENPs,
however, the use of emulsifier during processing helps in
stabilizing while capping agents prevent degradation and
transformation (Sajid et al. 2015).
4.3 Deposition
Engineered nanoparticles (ENPs) undergo deposition in the
soil as a result of collisions and bonding with the surface.
The Brownian diffusion, interception, or gravitational settling inside soil pore wall are responsible and thus deposition
took at a place (Cornelis et al. 2014), while hydrodynamic
drag forces further allow their travel to the collector sites
(Torkzaban et al. 2007). The absence of repulsion due to the
presence of similar charges on the surface and high collision
efficiency always promotes their deposition. The large aspect
ratio of CNT has resulted in higher deposition than other
colloids or ENPs (Lin et al. 2010), due to the coiling features
of CNT around soil particles (Canady and Kuhlbusch 2014).
The deposition can be considered as analogous to the
aggregation of relatively small ENPs with a much larger
colloid, and as dominates as aggregation in the soil matrix.
Li et al. (2020) have demonstrated relatively high mobility
of silver nanoparticles (Ag NPs) in the loamy sand than in
silty soil under low ionic strength and higher flow rates.
Further, the transport of the Ag NPs in loamy sand was
slowed at a low flow rate, due to the dominance of diffusion
and depositions after compression of the electrical double
layer of Ag NPs and soil surface (Braun et al. 2015). The
effect of input concentration, size, and surface coating of Ag
NPs for the transport was also studied and it was stated that
migration was less in ultisols due to high surface area and
retention sites. The increased concentration, lower particle
size, and surface coating of Ag NPs have promoted the
transport (He et al. 2019). However, the transport and
deposition of ENPs are a complex process that is jointly
affected by several factors such as physico-chemical properties of soil, pore-water solution, ENPs features as well as
hydrodynamic behavior. The transport and retention of CuO
nanoparticles in soil subsurface environment was also
affected by soil pH, ionic strength, and humic acid (Fig. 1).
However, the establishment of van der Walls forces
between nanoparticles and collector surface was repulsive,
promoting an unfavorable deposition due to interaction
energy, collision, and aggregation in soil (Ma et al. 2018;
Wu et al. 2020).
4.4 Oxidation/dissolution
The ENPs generally undergoes different oxidation process,
followed by their complexation with organic matter and
chelating agents and finally adsorbs on the colloidal surface.
The dissolution, oxidation–reduction reactions largely
depend upon the structure of ENPs such as soft metal cations
Mg, Ag, Zn, and Cu are susceptible to these reactions
(Boxall et al. 2007). However, the reason for the increasing
trend of ENPs use has exposed the soil with a rising concentration of ENPs. The speed of dissolution of ENPs in
soils can be explained by the type, texture, and source
material of ENPs (Rodrigues et al. 2016). The dissolution of
metal-based ENPs is affected by their chemical properties
and soil conditions like pH, organic carbon, texture, and size
(Arora et al. 2012). Still, the kinetics of oxidation of the
different ENPs in a complex soil matrix, and the relevant
controlling factors are unexplained. According to the reports
in soil systems, the dissolution of ENPs allows the liberation
of free ions in soil solution which further transforms by
reacting with organic matter, soil chelators, or adsorb on soil
particles/minerals followed by precipitation of non-soluble
reactive counterparts. In non-saturated aerated soil, i.e., low
pH and high oxygen contents and the rate of oxidation
enhances. In contrast, the presence of organic coatings
Engineered Nanoparticles in Agro-ecosystems: Implications …
107
other particles, by which ENPs frequently agglomerates to
form particle clusters (Cornelis et al. 2014). Aggregation
reduces the specific exterior area of particles and interfacial
energy. In aggregation presence of the natural organic matter
commonly limits their movement in the soil. The actions of
soil organisms manipulate carbon maintenance time and
return in soil, which revolves change carbon stabilization,
aggregation, and yield. Aggregation favors the movement
and deposition of ENPs in soil solution. It can be homoaggregation (between ENPs) or hetroaggregation (between
ENPs and other soil components such as clay, minerals, or
oxides). The collision among the ENPs resulted in the formation of aggregates and thus the establishment of weaker
van der Walls forces or strong chemical bonds takes place.
The presence of natural organic nanoparticles in the soil
porous media further facilitates the hetero-aggregation
among them, thus, severely affects their bioavailability,
toxicity, and transport across porous media. Aggregation of
metal-based ENPs (Ti, Cu, Au, Ag, Ni) was already noted in
soil (Cornelis et al. 2014). Hetero-aggregation most likely
occurs than homoaggregation rates in soil pores and appears
to vary depending on the soil colloid and ENPs nature. The
formation of large hetero-aggregates may also reduce the
translocation and uptake of ENPs through plant cells and
membranes, and thus decreasing their biological availability
(Gogos 2015). The loss of ENPs polymeric coatings under
sunlight catalyzed redox reactions may stimulate instability
and favors hetero-aggregation. The soil pH, clay content,
organic matter, and particle characteristics play an important
role in ENPs transport and retention (Abbas et al. 2020). The
movement of ENPs across the soil solution was likely to be
influenced by zeta potential, surface coating of ENPs,
however, the use of emulsifier during processing helps in
stabilizing while capping agents prevent degradation and
transformation (Sajid et al. 2015).
4.3 Deposition
Engineered nanoparticles (ENPs) undergo deposition in the
soil as a result of collisions and bonding with the surface.
The Brownian diffusion, interception, or gravitational settling inside soil pore wall are responsible and thus deposition
took at a place (Cornelis et al. 2014), while hydrodynamic
drag forces further allow their travel to the collector sites
(Torkzaban et al. 2007). The absence of repulsion due to the
presence of similar charges on the surface and high collision
efficiency always promotes their deposition. The large aspect
ratio of CNT has resulted in higher deposition than other
colloids or ENPs (Lin et al. 2010), due to the coiling features
of CNT around soil particles (Canady and Kuhlbusch 2014).
The deposition can be considered as analogous to the
aggregation of relatively small ENPs with a much larger
colloid, and as dominates as aggregation in the soil matrix.
Li et al. (2020) have demonstrated relatively high mobility
of silver nanoparticles (Ag NPs) in the loamy sand than in
silty soil under low ionic strength and higher flow rates.
Further, the transport of the Ag NPs in loamy sand was
slowed at a low flow rate, due to the dominance of diffusion
and depositions after compression of the electrical double
layer of Ag NPs and soil surface (Braun et al. 2015). The
effect of input concentration, size, and surface coating of Ag
NPs for the transport was also studied and it was stated that
migration was less in ultisols due to high surface area and
retention sites. The increased concentration, lower particle
size, and surface coating of Ag NPs have promoted the
transport (He et al. 2019). However, the transport and
deposition of ENPs are a complex process that is jointly
affected by several factors such as physico-chemical properties of soil, pore-water solution, ENPs features as well as
hydrodynamic behavior. The transport and retention of CuO
nanoparticles in soil subsurface environment was also
affected by soil pH, ionic strength, and humic acid (Fig. 1).
However, the establishment of van der Walls forces
between nanoparticles and collector surface was repulsive,
promoting an unfavorable deposition due to interaction
energy, collision, and aggregation in soil (Ma et al. 2018;
Wu et al. 2020).
4.4 Oxidation/dissolution
The ENPs generally undergoes different oxidation process,
followed by their complexation with organic matter and
chelating agents and finally adsorbs on the colloidal surface.
The dissolution, oxidation–reduction reactions largely
depend upon the structure of ENPs such as soft metal cations
Mg, Ag, Zn, and Cu are susceptible to these reactions
(Boxall et al. 2007). However, the reason for the increasing
trend of ENPs use has exposed the soil with a rising concentration of ENPs. The speed of dissolution of ENPs in
soils can be explained by the type, texture, and source
material of ENPs (Rodrigues et al. 2016). The dissolution of
metal-based ENPs is affected by their chemical properties
and soil conditions like pH, organic carbon, texture, and size
(Arora et al. 2012). Still, the kinetics of oxidation of the
different ENPs in a complex soil matrix, and the relevant
controlling factors are unexplained. According to the reports
in soil systems, the dissolution of ENPs allows the liberation
of free ions in soil solution which further transforms by
reacting with organic matter, soil chelators, or adsorb on soil
particles/minerals followed by precipitation of non-soluble
reactive counterparts. In non-saturated aerated soil, i.e., low
pH and high oxygen contents and the rate of oxidation
enhances. In contrast, the presence of organic coatings
Engineered Nanoparticles in Agro-ecosystems: Implications …
107