**7. Fate of nanomaterials in soil**

In assumption, soil is meant to be the biggest receiver of NPs. When nanoparticles or nanoformulations are given to plants, the substance eventually makes its way to the soil, where it may be used. Because of its proximity to plant roots and microbes, the nanomaterial establishes a unique connection with them once it is in the soil. In addition to soil, which is the most abundant source of natural nanoparticles, both as primary particles and aggregates, it is also regarded an externally significant environmental matrix. Dissolution, transformation, and aggregation/ disaggregation are among of the mechanisms that regulate the fate of NMs in soils. In soils, several of the mechanisms that determine NM fate and behavior, such as straining, deposition/mobilization and diffusive transport are substantially different (**Figure 1**). The significance of these factors varies depending on the NM and soil conditions [92, 93]. Because dissolution destroys them and aligns their destiny and bioavailability with the soluble components, dissolution may be critical for some NMs. Examples include the fast dissolving of ZnO, which is likely to be ephemeral in soil unless coated with compounds that prevent dissolution. When it comes to dissolving, pH is the most essential factor to take into consideration.

For example, researchers [92] used a worldwide database to compare soil saturation extract pH and ionic strength to NMs critical coagulation concentrations. Since the pH and Ionic Strength of most soil solutions are below the critical coagulation concentration of most nanomaterials, homoaggregation would be sluggish in most soils. As in aquatic contexts, heteroaggregation is anticipated to play a major role in soils because soil porewaters frequently include larger quantities of natural colloids in suspension. In most soils, NM condensation will occur in the topsoil with limited transport to the depths, resulting in increased straining (**Figure 1**). In soil porewaters, NOM has been reported to stabilize NMs and prevent both homo- and heteroaggregation, according to several studies [94]. NM movement in soils has been studied in a limited number of researches, and this is likely the most important gap in understanding the true danger of NM transport. NM transport researches in soils have evolved from utilizing inert stationary phases (e.g., quartz beads) in columns to employing real soils in the recent decade. It appears that the CNTs are retained in soils due to their large aspect ratio, resulting in considerable straining. There is a high concentration of fullerenes in soils due to interactions with soil organic matter.

#### **Figure 1.**

*The key mechanisms that determine the fate of natural colloids and nanomaterials in soils. 1, colloid generation; 2, engineered nanomaterial leaching from biosolids; 3, homoaggregation; 4, fragmentation; 5, sedimentation; 6, heteroaggregation; 7, size exclusion; 8, straining; 9, deposition; 10, convective transport [92].*
