TiO₂ Nanoparticles in Soil: Adsorption, Transformation, and Environmental Risks

Abstract

Titanium-containing nanoparticles have emerged as materials of significant technological importance due to their multifunctional properties and excellent performance. With their expanding applications, the amount of TiO₂ nanoparticles (TNPs) being released into the soil environment has increased significantly. This review addresses the gap in current research, which has predominantly focused on the environmental behavior of TNPs in aquatic systems while lacking systematic integration of the synergetic mechanism of adsorption–transformation–ecological effects in soil systems and its guiding value for practical applications. It deeply reveals the interaction mechanisms between TNPs and environmental pollutants. TNPs exhibit outstanding adsorption performance towards environmental pollutants such as heavy metals and organic compounds. Specifically, the maximum adsorption capacities of titanate nanowhiskers for the heavy metal ions Cu(II), Pb(II), and Cr(III) are 143.9 mg·g⁻¹, 384.6 mg·g⁻¹, and 190.8 mg·g⁻¹, respectively. Additionally, 1-hydroxydinaphthoic acid surface-modified nano-TiO₂ exhibits an adsorption rate of up to 98.6% for p-nitrophenol, with an enrichment factor of 50-fold. The transformation process of TNPs after pollutant adsorption profoundly affects their environmental fate, among which pH is a critical controlling factor: when the environmental pH is close to the point of zero charge (pHpzc = 5.88), TNPs exhibit significant aggregation behavior and macroscopic sedimentation. Meanwhile, factors such as soil solution chemistry, dissolved organic matter, and microbial activities collectively regulate the aggregation, aging, and chemical/biological transformation of TNPs. In the soil ecosystem, TNPs can exert both beneficial and detrimental impacts on various soil organisms, including bacteria, plants, nematodes, and earthworms. The beneficial effects include alleviating heavy metal stress, serving as a nano-fertilizer to supply titanium elements, and acting as a nano-pesticide to enhance plants’ antiviral capabilities. However, excessively high concentrations of TiO₂ can stimulate plants, induce oxidative stress damage, and impair plant growth. This review also highlights promising research directions for future studies, including the development of safer-by-design TNPs, strategic surface modifications to enhance functionality and reduce risks, and a deeper understanding of TNP–soil microbiome interactions. These avenues are crucial for guiding the sustainable application of TNPs in soil environments.

1. Introduction

Titanium-containing nanoparticles, predominantly composed of TiO₂, have garnered substantial attention in various scientific and technological arenas. The unique properties of TiO₂, including its photogenic activity [1], high specific surface area, remarkable chemical stability, and high reactivity, endow TiO₂ nanoparticles (TNPs) with significant application value, particularly in environmental protection. For instance, TNPs have been utilized in soil remediation [2], in soil-based catalytic oxidation for pollutant removal, as components in fertilizers, and in antiviral applications. Over the past few decades, the scientific community has conducted extensive research on the environmental behavior of TNPs. In aquatic environments, a systematic understanding has been established regarding the adsorption kinetics, aggregation patterns, and biotoxicity of TNPs. For instance, in aquatic environments, a systematic understanding has been established regarding the photocatalytic performance, aggregation patterns, and biotoxicity of TNPs [3,4,5]. Another study, which exposed organisms to 5 µg/L TNPs for 14 days, found that the proportion of mature oocytes in the ovaries decreased by 42%, the sperm malformation rate increased from 5% to 22%, and the fertilization rate dropped by 35%—these results revealed the cumulative toxic effects of TNPs on aquatic organisms [6]. In soil environments, however, most studies in recent years have been single-aspect investigations, focusing on isolated processes such as adsorption, transformation, or toxicity. Taking toxicity research as an example, some studies adjusted the concentration of nano-TiO₂ within the range of 0.1–1000 mg/kg in sandy loam and clay loam soils to observe its effects on the reproduction of Eisenia fetida. The limitation of such single-aspect studies lies in their focus solely on toxicity at the individual or community level, without linking the toxicity to the adsorption/transformation processes of TNPs. This constitutes a research gap in recent years. Nevertheless, soil, as a critical environmental medium, plays a central role in the entire ecosystem. Research on TNPs within the soil environment is not only scientifically interesting but also holds great practical significance. To ensure the sustainable environmental application of these nanoparticles, it is imperative to closely monitor the fate and potential environmental risks of TNPs in the soil ecosystem.

This contamination poses considerable health hazards to humans, potentially giving rise to a range of severe consequences, such as liver and kidney damage, and even an elevated risk of cancer. A comprehensive understanding of the environmental behavior of TNPs is crucial for developing effective strategies to mitigate these detrimental effects. In contaminated soil environments, TNPs initially engage in chemical reactions with heavy metal ions and organic pollutants. Following these interactions and adsorption processes, aggregation of TNPs occurs. This aggregation phenomenon can significantly alter the optical, electrical, magnetic, and catalytic properties of TNPs. Moreover, it may have a profound impact on the efficiency of pollutant removal from the soil [7]. These environmental risks and implications associated with the behavior of TNPs in soil demand thorough investigation and cannot be overlooked, as they are integral to assessing the overall ecological and human health impacts of these nanoparticles in contaminated soil systems.

Moreover, existing research consistently demonstrates that the impacts of TNPs on soil organisms are complex, exhibiting contradictory and multifaceted characteristics. For instance, numerous investigations have indicated that high concentrations of TNPs tend to suppress the activity of soil microorganisms. Conversely, low concentrations of TNPs may stimulate the metabolic activity of soil microorganisms and increase the concentration of extracellular enzymes secreted by them. In general, the influence of TNPs on soil organisms encompasses a spectrum of outcomes, encompassing both beneficial promoting effects and potential adverse consequences [8]. Given this intricate nature, additional in-depth research is imperative to comprehensively evaluate the application prospects of TNPs, aiming to optimize their utilization while minimizing potential ecological risks.

At present, significant uncertainties and knowledge gaps directly impede the widespread application of TNP products in diverse environmental and agricultural contexts, resulting in the commercialization of TNP-based products that are still in an embryonic stage and lack full establishment. In light of these challenges, this review aims to carry out the following: (1) Address the research gaps in recent years by systematically integrating the synergistic mechanism of adsorption–transformation–ecological effect, including summarizing the adsorption behavior of TNPs toward various environmental pollutants, delving into the underlying adsorption mechanisms, analyzing the influence of environmental factors on the adsorption process, and developing strategies to optimize the performance of TNPs. (2) Elaborate on the transformation processes of TNPs in soil ecosystems, innovatively integrate multi-factor coupled regulation of TNP transformation, and detail how these transformations affect the physicochemical and functional properties of TNPs, thereby filling the existing knowledge gaps. (3) Compensate for the gaps in single-aspect toxicity studies in recent years by summarizing the dual (beneficial and detrimental) impacts of TNPs on soil organisms and comparing the toxicity profiles of TNPs when present alone, in coexistence with environmental pollutants, and in their pristine form. This review aims to conduct a comprehensive and rational assessment of the environmental risks posed by TNPs. Overall, this review is dedicated to providing valuable insights and data-driven knowledge, so as to promote the advancement of TNPs toward large-scale and sustainable applications.

2. Adsorption of TNPs in the Soil Environment

2.1. Interaction Between TNPs and Heavy Metals

TNPs exhibit notable potential as soil purifiers, with adsorption capacities for heavy metal ions such as Cd²⁺, Cr⁶⁺, Pb²⁺, and Cu²⁺ that rival or surpass conventional adsorbents. For instance, in a comparative experimental study, it was observed that the application of copper-coated TNPs in chromium-contaminated soil led to a significant reduction in Cr content. The removal rate achieved in this treatment group was substantially higher than those of the control groups lacking TNPs or employing only common adsorbents, underscoring the high efficacy of TNPs in heavy metal remediation.

The high efficiency of TNPs in heavy metal removal is primarily attributed to their large specific surface area (SSA), which provides abundant active sites to facilitate metal ion adsorption. In a specific experiment, when the adsorption equilibrium was attained, the maximum adsorption capacities of {101} crystal plane TiO₂, {001} crystal plane TiO₂, and {100} crystal plane TiO₂ for As(Ⅲ) solution were measured to be 148.3 mg/g, 53.7 mg/g, and 32.9 mg/g, respectively [9]. Evidently, the abundance of adsorption sites and the effective bonding between these sites and metal ions are pivotal factors in enhancing the reduction efficiency of heavy metals. Apart from the intrinsic SSA of TNPs, the improvement of adsorption performance can also be accomplished by regulating parameters such as the dosage of TNPs, the initial concentration of heavy metal ions, and the reaction time. Regarding the reaction time, research has demonstrated that the removal of Cu(II) by titanate nanofibers increased rapidly within the initial 180 min and subsequently reached a plateau after 180 min [10]. This is due to the saturation of available adsorption sites.

However, merely controlling the reaction time, dosage, and initial concentration is insufficient for optimizing the removal efficiency; modification with other substances or combination with different materials can also significantly enhance the efficiency. For instance, under the same catalytic conditions, the catalytic efficiency of the ZIF-8/TiO₂ composite is higher than that of pure TiO₂. The results exhibit that the average nitrogen fixation rate of ZIF-8/TiO₂ is approximately 742 μmol·L⁻¹·g⁻¹·h⁻¹, which is 3.78 and 3.71 times higher than those of TiO₂ and ZIF-8, respectively. This enhancement is mainly attributed to the significant increase in the specific surface area of the material after the combination of TiO₂ and ZIF-8. The improved specific surface area addresses the issue of limited reaction sites in TiO₂, while ZIF-8 provides abundant photoreaction sites for the photocatalytic process, thereby improving the photocatalytic efficiency. Given that the improvement in photocatalytic efficiency is an indicator of enhanced heavy metal ion removal capacity, this performance optimization indirectly reflects an increase in the heavy metal ion removal efficiency.

The interaction mechanism between TNPs and heavy metals has emerged as a focal point of extensive research interest. The adsorption of heavy metals by TNPs is dominated by the synergy of multiple mechanisms, among which electrostatic adsorption serves as the key to initial binding. The core regulatory factor of electrostatic interaction is the isoelectric point (pHpzc) of TNPs. Taking anatase TiO₂ (pHpzc ≈ 5.88), which is commonly used in soil remediation, as an example, hydroxyl functional groups are widely present on its surface: when the environmental pH is lower than pHpzc (e.g., pH 4–5), Ti-OH undergoes a protonation reaction (Ti-OH + H⁺ → Ti-OH₂⁺), rendering the TNP surface positively charged and enabling efficient adsorption of anionic heavy metals such as CrO₄²⁻; when the pH is higher than pHpzc, Ti-OH undergoes deprotonation (Ti-OH → Ti-O⁻ + H⁺), and the surface becomes negatively charged, thereby capturing cations such as Pb²⁺ and Cu²⁺. Besides electrostatic adsorption, complexation/chelation and cation exchange are also key mechanisms, each with distinct application scenarios. Among them, complexation mainly relies on the surface hydroxyl groups of TNPs, and enhances the selectivity towards target heavy metals through the dual effects of electrostatic adsorption and hydroxyl complexation. The binding of surface hydroxyl groups is more pronounced under acidic conditions (pH 4–6): under this pH range, the enhanced protonation of Ti-OH promotes electron transfer with heavy metal cations, enabling the formation of stable Ti-O-Cu covalent bonds with Cu²⁺. However, when pH > 7, heavy metals tend to form hydroxide precipitates, which cannot coordinate with Ti-OH, resulting in reduced adsorption capacity. Cation exchange is a unique mechanism exclusive to layered TNPs, where interlayer free Na⁺ can undergo equivalent charge exchange with heavy metal cations. A study demonstrated that the adsorption capacity of titanate nanotubes for Cu²⁺ increases with the increase in Na⁺ content in TNTs. Specifically, at pH 5, the adsorption capacity of TNTs with a Na⁺ mass fraction of 1.21% for Cu²⁺ is approximately 40 mg/g; when the Na⁺ mass fraction increases to 7.35%, the adsorption capacity for Cu²⁺ can reach 120 mg/g [11]. Thus, based on these interaction mechanisms, methods to improve adsorption efficiency are feasible, and this warrants in-depth investigation.

2.2. Interaction Between TNPs and Organic Compounds

TNPs achieve high organic pollutant degradation efficiency via their unique electronic structure: their band gap generates photogenerated electron–hole pairs under UV light or modified visible light. Holes produce highly oxidizing OH to degrade organics, while electrons inhibit organic pollutant desorption and enhance adsorption stability. Abundant surface -OH on TNPs forms strong interactions (hydrogen/coordination bonds) with organic pollutants. Notably, anatase TiO₂ degraded biological methylene blue with 95.27% efficiency under UV for 120 min [12].

In organic pollutant remediation, TNPs have stable crystal structure resisting photocorrosion, dissolution, or collapse. They can be recovered/reused via simple methods, addressing inefficient recovery to improve utilization.

TNPs possess remarkable adsorption capabilities and recyclability, and have thus found applications in the remediation of diverse organic pollutants, including pharmaceuticals, flame retardants, and organic dyes. Additionally, they can serve as catalysts for the synergistic treatment of organic pollution in soil in combination with other substances. Distinct from their interaction with heavy metals, the interaction between TNPs and organic compounds is predominantly governed by redox reactions. In detail, TNPs have the ability to absorb light energy, leading to the generation of excited free electrons and holes. The free electrons are capable of reducing specific electron acceptors, whereas the holes can oxidize electron-donating species (Figure 1).

Polycyclic aromatic hydrocarbons (PAHs), as the most frequently detected organic contaminants in polluted sites, have been the subject of extensive research regarding their degradation by TNPs. For instance, n-doped TiO₂ exhibits enhanced photocatalytic activity towards phenanthrene (a typical PAH), with its degradation rate constant significantly higher than that of pristine TiO₂—this is attributed to the modified band structure of doped TiO₂, which promotes the separation of photogenerated electron–hole pairs [13]. Wen Sheng conducted in-depth investigations into the photocatalytic degradation capacity of TiO₂ towards PAHs in aqueous systems, meticulously examining the degradation rates of PAHs under diverse conditions including light intensity, reaction time, and catalyst dosage, thereby elucidating the feasibility of utilizing TiO₂ for the photocatalytic degradation of PAHs [14]. For instance, the photocatalytic degradation rate constant of phenanthrene is significantly higher when both TiO₂ and O₂ are present—its value reaches 0.1646 h⁻¹-whereas the rate constant is only 0.0514 h⁻¹ when O₂ is present alone [15]. This enhancement is primarily due to the rapid redox reactions driven by photogenerated electrons and holes. The introduction of cerium (Ce) gives rise to the formation of a TiO₂-Ce synergistic system, which effectively promotes the electron transfer of active metals and facilitates the generation of additional reactive oxygen species at oxygen vacancies; due to the presence of variable valence ions and oxygen vacancies in TiO₂, it exhibits Fenton-like reaction activity and undergoes Fenton-like reactions, for instance, through the interaction between the TiO₂/SiO₂ composite oxide and H₂O₂, Ti-O-O-Si peroxide groups are formed, which, upon ultraviolet light irradiation, disintegrate to directly form O₂⁻ radicals, endowing the catalyst with a rapid and highly efficient photocatalytic degradation capacity for organic compounds. Under the synergistic action of the TiO₂/SiO₂ composite oxide and H₂O₂, excellent degradation and decolorization effects have been achieved in the photocatalytic treatment of the organic dye eosin B, and the Fenton-like reaction mediated by TiO₂ broadens the applicable pH range, rendering it particularly suitable for the in situ remediation of contaminated soil where pH adjustment is not practicable.

Reduction reactions also serve as a vital pathway for TNPs to degrade organic pollutants. The photocatalytic reaction of TiO₂ capitalizes on the band-to-band transition of semiconductors when exposed to ultraviolet light, leading to the generation of conduction band electrons with high reduction activity, which are capable of initiating the reduction reactions of pollutants. These photogenerated conduction band electrons possess a robust reducing ability [ECB (e⁻) = −0.6 V SCE, at pH = 7] and can effectively reduce heavy metal ions such as Cr (VI) [16,17] and degrade halogen-containing compounds like CCl₄. A particular research group demonstrated that under deoxygenated conditions, the photogenerated conduction band electrons can induce rapid reductive debromination of decabromodiphenyl ether (BDE209), with its half-life being merely 2.1 min. Typically, deoxygenation and the adsorption of the substrate onto the surface of the catalyst TiO₂ are essential prerequisites for the reductive degradation of organic compounds by conduction band electrons. However, through N-F co-doping to modulate the energy band structure of TiO₂, the conduction band electrons are able to photocatalytically reduce metal ions such as Cr(VI), Fe(Ⅲ), and Ag(I) even under aerobic conditions. This conclusion not only highlights the potent reducing capability of the photogenerated electrons of TiO₂ but also suggests that by doping metal ions or other elements, it is possible to alter the energy band structure of titanium nanoparticles, expand the light absorption range into the visible light region, and further enhance the photocatalytic performance. These modification strategies not only enhance performance but also pave the way for the safer-by-design of TNPs by controlling their reactivity and stability in soil.

2.3. Environmental Impacts on the Adsorption of TNPs

Generally, the interaction processes between heavy metal ions or organic compounds and TiO₂ ionic nanoparticles are governed by a multitude of conditions, encompassing environmental factors such as pH value, temperature, and coexisting ions, as well as non-environmental factors including contact time, dosage, and initial concentration. Elucidating the influence of these environmental factors on the adsorption behavior of TiO₂ and optimizing the treatment conditions are of paramount importance for the successful implementation of in situ remediation strategies, as they directly impact the efficiency and effectiveness of pollutant removal and the overall performance of TiO₂-based remediation technologies.

The pH value has the potential to influence the surface charge of ionic nanoparticles (NTO). As the pH value increases to a certain extent, the adsorption capacity of TiO₂ or NTO for heavy metals and organic compounds initially rises, subsequently reaches a plateau, and ultimately declines. For instance, within the pH range of 2–5, the adsorption concentration of NTO for Cr (VI) escalates from 21 mg/g to 25 mg/g; it tends to stabilize in the pH range of 5–6; and finally drops to 16 mg/g within the pH range of 6–8 [18]. Likewise, under the conditions of a 1.5 g/L addition amount of the TiO₂ catalyst and an initial azo carmine solution concentration of 40 mg/L, it was discovered that the catalytic effect was most pronounced at pH = 8.2, whereas the catalytic efficiencies at pH = 2.2 and pH = 11.2 were lower than that at pH = 8.2 [19].

In addition, the interaction between heavy metal ions and TNPs is predominantly characterized by physical adsorption and is intricately associated with temperature. In an endothermic adsorption process, an elevation in temperature is conducive to the reaction, whereas for an exothermic adsorption process, the relationship is reversed. For example, in the case of the exothermic adsorption of Ni (II) by supported TNPs, a lower temperature is more favorable. The data indicates that the adsorption capacity at 20 °C is 9.3 mg/g, which is significantly higher than the 7.6 mg/g observed at 80 °C. Conversely, endothermic reactions tend to proceed more favorably at higher temperatures [20]. Regarding organic compounds, an increase in temperature can enhance the Brownian motion of small organic molecules, thereby facilitating the adsorption of organic pollutants by TNPs.

The removal efficiency can also be ascribed to other physical and chemical properties of the soil, including coexisting ions, organic matter, and soil porosity. Research has demonstrated that in the presence of competitive anions, the promoting effect of certain ions on the removal of heavy metals by TNPs may be suppressed. For instance, in the investigation of the phosphate-mediated adsorption of heavy metal ions by TNPs, it was revealed that coexisting competitive ions such as Cl⁻, NO₃⁻, and SO₄²⁻ could weaken the promoting effect of phosphate on TNPs, with the inhibition order being SO₄²⁻ > NO₃⁻ > Cl⁻, which is inversely proportional to the order of their ionic radii. This is because, on one hand, anions with larger ionic radii can occupy more active sites on the surface of TiO₂; on the other hand, competitive adsorption is also related to the valence state of anions, as the electrostatic force between monovalent anions and heavy metal ions is weaker than that between divalent anions [21]. Generally, organic matter, such as humic acid and fulvic acid, is widely distributed in soil. As a result, organic matter may accumulate at the TiO₂ interface, blocking the active surface sites and impeding the contact between TiO₂ and organic compounds. Studies have found that the competitive adsorption between organic pollutants and natural organic matter can lead to a decrease in the photocatalytic activity of titanium-containing compounds, indicating that when the organic matter content in soil is high, it will compete with pollutants for the active sites of TNPs, thereby affecting the pollutant removal efficiency [22]. Meanwhile, soil porosity also exerts a certain influence on the removal efficiency. On one hand, a large porosity enables TNPs to penetrate deep into the soil, increasing the probability of contact with pollutants and facilitating pollutant removal; conversely, a small porosity will restrict the movement of nanoparticles and reduce the removal efficiency. On the other hand, Increasing the porosity can improve the soil structure, promote air circulation, and support microbial activities and pollutant degradation.

TiO₂ provides a promising, low-cost, and environmentally friendly soil remediation method. To design and optimize functional TiO₂ in practical applications, it is necessary to comprehensively consider the interaction mechanism between TiO₂ and heavy metals and organic compounds and the environmental impacts on the removal efficiency. In addition, considering the complexity and variability of the TiO₂.

3. Transformation of Titanium Dioxide Nanoparticles in the Soil Environment

After the application of TiO₂ for the remediation of contaminated soil, the factor exerting the most significant influence on their removal efficiency and availability is their stability or transformation within the soil environment. In an attempt to understand the behavior of TiO₂ in the soil environment, numerous studies have revealed that soil, as a relatively intricate medium, is fundamentally composed of three components: soil particles, soil water, and soil gas. The behavior of TiO₂ in soil can thus be considered as the outcome of its interaction with these various soil components. Nevertheless, the existing issue lies in the absence of suitable characterization procedures for assessing the morphology and behavior of TiO₂ in soil. Instead, through the use of scanning electron microscopy and atomic force microscopy for observation, its behavior, such as aggregation, can be characterized in the aqueous phase. This limitation, namely that the characterization is only applicable to the liquid phase, has consequently led to a large number of studies on the behavior of TiO₂ being conducted in an aqueous medium. Therefore, the research in this section makes use of the transformation process of TiO₂. The behavior of TiO₂ in the soil aqueous phase is likely to have a substantial impact on its ecotoxicity and bioavailability, and the pattern of its behavior in the aqueous phase may offer a valuable reference for its performance within the soil matrix itself.

3.1. Aggregation of TNPs in the Soil Environment

The aggregation of TiO₂ at the microscopic level hinges on their interactions, and this combination is highly susceptible to a multitude of factors, including environmental factors like pH value, Brownian motion, gravity, and fluid movement, as well as the properties of TiO₂ itself, such as particle size, surface modification, and conductivity. Drawing on the existing literature, a systematic discussion and analysis of the factors influencing their aggregation will be carried out.

Concentration and pH value are indeed pivotal factors influencing the aggregation of nanomaterials. At elevated concentrations, owing to Brownian motion, the collision frequency of TiO₂ rises, thereby augmenting the likelihood of aggregation. For instance, the particle size of TiO₂ was measured using dynamic light scattering (DLS), and the findings indicated that when the concentration of TNPs was below 20.1 mg/L, it exhibited excellent dispersibility and stability. However, once the concentration exceeded 20.1 mg/L, concentration-dependent aggregation phenomena emerged. By adjusting the pH value, it is possible to control the surface charge of nanoparticles, thus generating an electrostatic repulsion force, which serves as the primary driving force to impede nanoparticle aggregation. A particular study employed Brownian dynamics simulation to explore the deposition process of TNPs on the SiO₂ deposition surface under varying pH conditions (5.5, 7, 9, and 11) when the particle diameter D = 20 nm and the ionic strength was 0.01 M. As the simulation progressed, the number of deposited particles displayed an upward trend. At the conclusion of the simulation, the numbers of deposited TNPs at pH values of 5.5, 7.0, 9.0, and 11.0 were 89, 75, 72, and 72, respectively. Evidently, as the pH value increases, the deposition process is somewhat restricted. The underlying reason is that within the investigated pH range, both the surface charges of TNPs and the SiO₂ deposition surface are negative, and as the pH value rises, their negative charges become more pronounced, leading to an increase in the repulsive force and consequently hindering the deposition process [23]. Hence, it is clear that the pH value exerts a certain influence on the aggregation of nanoparticles.

DOM is highly prevalent in soil and can readily interact with TiO₂, and these interactions govern the behavior of TiO₂ within the soil system. Besides these natural interactions, artificial surface modification can also have an impact on the behavior of TiO₂. Fundamentally, both of these approaches alter the surface charge of TiO₂, thereby influencing its aggregation. At varying concentrations, humic acid, a common component of DOM, exerts different degrees of influence on the surface charge of TiO₂, which may lead to changes in its isoelectric point and aggregation stability. Research has revealed that the Zeta potential of TNPs varies significantly with changes in pH, and there is an inverse relationship between the particle size of its aggregates and the absolute value of the potential. That is, the greater the charge carried by the nanoparticles, the smaller the particle size of the aggregates, resulting in higher dispersibility and stronger stability. In the presence of humic acid, the negative charge of TNPs is substantially reduced. Due to the combined effects of electrostatic repulsion and ligand exchange, the particle size of TNP aggregates will decrease, albeit not significantly. Meanwhile, the particle dispersibility increases, and the overall stability of the TNP suspension is enhanced [24].

3.2. Aging Process of TNPs in the Soil Environment

The complexity inherent in the soil environment gives rise to the dynamic alterations of TiO₂. As previously elaborated, aged TiO₂ is systematically characterized as a substance where TiO₂ itself or a variety of substances coat the surface of TiO₂ and undergo redox processes.

TiO₂ exists widely in nature. Its common forms include rutile and anatase, which exhibit unique physical and chemical properties under various environmental conditions. The meteorological transformation products of TiO₂ can include various surface-modified or crystal-transformed forms, and these changes are influenced by atmospheric conditions and environmental factors. In the soil–water system, the aging process of TNPs has also been observed. They may undergo surface redox reactions during the interaction with the environmental medium, affecting their adsorption and photocatalytic degradation abilities of pollutants. Similarly, when environmental conditions such as pH value, temperature, and contact medium change, TNPs will also undergo phase transitions in the laboratory, such as the transformation from an amorphous state to a rutile or anatase crystal form. These morphological changes not only alter the physical and chemical properties of TiO₂ but also affect its stability, mobility, and ecological effects in the environment. Relevant studies have shown that these changes have significant differences.

Compared to unaged TNPs, TNPs that have been treated through natural and artificial aging processes exhibit distinct differences in terms of morphology and properties, and these disparities can be identified via detailed observation and characteristic analysis techniques. Under natural circumstances, the aging of TNPs may result in the formation of specific mineral phases on their surface, such as the rutile phase, and this alteration can be detected by observing changes in soil color characteristics or the emergence of chemical signals. On the contrary, under artificially imposed aging conditions, the morphological changes of TiO₂ are often more pronounced [25]. In a high-temperature and high-humidity environment, the surface properties and aggregation behavior of TNPs will undergo significant changes. Their morphology evolves from regular spheres to irregular aggregates, and the surface roughness increases [26]. Notably, in real soil, TNP aging may be regulated by dissolved organic matter and microorganisms. DOM can coat TNPs via hydroxyl and carboxyl groups to slow the anatase-to-rutile phase transition; organic acids secreted by microorganisms also alter microenvironmental pH, further delaying the transition. This matrix-biology buffering makes the previously observed rapid aging unreflective of real soil conditions, potentially overestimating TNPs’ aging rate. Furthermore, natural soils exhibit seasonal fluctuations, which can disrupt large TNP aggregates formed under artificial aging, increasing TNPs’ mobility and bioavailability; meanwhile, soil particle reorganization may change TNPs’ binding state with the matrix. These dynamic effects impact TNPs’ environmental risks and remediation stability. However, aging is prevalent in soil and environmental systems containing TiO₂ and still significantly affects TNPs’ morphological characteristics. These findings suggest that aging indeed impacts the surface properties of TNPs and their interaction with the matrix.

In addition to the morphological alterations, aging also exerts an influence on the surface characteristics of TiO₂. When TiO₂ undergoes phase transformation or surface reconstruction, a defective surface structure will come into being. Simultaneously, owing to the continuous oxidation during the aging process, the number of surface reaction sites will increase, which is advantageous for the combination of more ligands with the surface of TiO₂ [27]. For those TiO₂ particles with surface functionalization, aging will not only impact their internal crystal structure but also bring about remarkable changes in the chemical properties of the external functional groups. Compared with unmodified TiO₂ particles, specific surface modifications (such as TiO₂ modified by a certain organic acid) will display different environmental behavior patterns after aging. In such a situation, aging may lead to the substitution of the original modifier by other compounds (such as the degradation products of the organic acid), and this substitution will further affect the stability of the modified TiO₂. Making rational use of the aging process of TiO₂ is beneficial for reducing its migration into the environment at the end of the application cycle, thereby facilitating effective recycling. Moreover, by reasonably regulating the aging process of TiO₂, reducing its potential harm to organisms and the soil ecosystem will be a viable strategy to mitigate the environmental risks posed by TiO₂. Nevertheless, it should be noted that there remains uncertainty in the research regarding the ecological effects of TiO₂ aging, and different studies have arrived at opposing conclusions. The specific details will be elaborated in greater depth in the subsequent section. This modulation of the aging process by soil constituents, particularly DOM and microbiological secretions, underscores the critical role of TNP–soil microbiome interactions and suggests that natural surface coatings could be mimicked or enhanced through artificial surface modification strategies to achieve desired environmental stability.

3.3. Chemical/Biological Transformation of TNPs in the Soil Environment

In addition to the phenomena of agglomeration and aging, TiO₂ will also experience chemical and biological transformations within the soil environment, and these processes have an impact on its stability and bioavailability (Figure 2). When compared with chemical transformation, biological transformation is regarded as a secondary route for the degradation of TNPs in the soil system. In numerous instances, the biologically mediated transformation in soil can be broadly categorized into two types: biological oxidation and biological reduction. Owing to the current limitations in research, the number of known species capable of degrading TNPs is relatively small. However, these species can still interact with TiO₂, leading to alterations in its surface properties or aggregation state.

Among these transformations, chemical transformation is generally closely associated with the surrounding environment. And in comparison, with biological transformation, chemical transformation serves as the primary mode of transformation. Chemical transformation can be classified into four aspects: adsorption and desorption, redox reaction, photocatalytic reaction, and the generation of radicals. TNPs engage in chemical reactions with soil components like minerals and organic matter. These reactions include processes such as adsorption, desorption, and oxidation-reduction. TNPs can be adsorbed onto the surface of soil particles, participate in exchange reactions with metal ions present in the soil, and under light conditions, they may also undergo photocatalytic reactions to produce active substances such as radicals. These reactions thus have an impact on the chemical properties of TNPs themselves as well as their migration and transformation processes within the soil.

Compared with chemical transformation, biotransformation is a secondary pathway for TNP degradation in soil, mainly because few known microbes can directly degrade TNPs. However, TNPs notably affect their environmental behavior and ecological effects via microbial interactions, mainly in two ways: (1) their impact on microbial community structure and function; (2) microbes and their metabolites may alter TNPs’ surface properties or aggregation state. Extensive studies show TNPs inhibit microbes in key biogeochemical cycles, with concentration-dependent effects. For example, [28] nano-TiO₂ inhibits soil ammonifying, nitrifying, and autotrophic nitrogen-fixing bacteria; antibacterial rate rises with concentration and varies by soil type (sandy soil > black soil > peat soil) [29] further noted TNPs at 10 mg L⁻¹ (low concentration) may temporarily stimulate some microbial activity, but high concentrations inhibit significantly. Moreover, ammonia-oxidizing bacteria are more sensitive to TNPs than nitrogen-fixing bacteria, highlighting potential interference with nitrogen cycling.

In the soil environment, the stability of TNPs cannot be determined in a simplistic manner merely by considering its aggregation state, the extent to which complex pollutants or environmental substances cover its surface, and the ensuing redox reactions. Notably, the accumulation and transformation of TNPs in soil is not an isolated event; this process ultimately exerts broader ecological effects by influencing soil microbial community functions, plant uptake, and other pathways. For instance, studies have shown that both TNPs and ZnO NPs can reduce wheat biomass and inhibit the activities of soil protease, catalase, and peroxidase, which raises potential concerns regarding the quality and safety of agricultural products as well as their transmission in the food chain [30]. Hence, we have systematically organized and analyzed the aggregation phenomena, the aging processes, as well as the chemical and biological transformations of TiO₂ in the soil. Through this analysis, we have discovered that as these processes occur, the properties of TiO₂ change correspondingly. The aforementioned changes are of utmost significance in relation to the environmental risks that TiO₂ poses within the soil ecosystem, and they are directly linked to the environmental safety of TiO₂. In the subsequent part, our focus will be on the environmental risk issues that TiO₂ encounters when it is applied on a large scale and over an extended period in soil remediation and agricultural practices.

4. Environmental Risks of TNPs in Soil

Given the exceptional performance of TNPs in eliminating soil pollutants, they hold great promise to supplant traditional materials in the realm of soil remediation. Moreover, TiO₂ exerts a beneficial influence on soil organisms and is employed as a nanomaterial-based pesticide in agricultural production. Nevertheless, it is imperative to acknowledge that TiO₂ may also impose negative impacts on soil organisms and ecosystems, and these detrimental effects could potentially nullify the advantages it confers. Consequently, we systematically summarize the impacts of TiO₂ on soil organisms and elucidate the underlying action pathways, with the aim of evaluating its environmental risks comprehensively (Table 1). A comparative analysis of the toxicity of fresh and aged TiO₂ was also carried out, and the cytotoxic mechanisms at the cellular level were explored in depth. These efforts are intended to conduct a thorough and in-depth environmental risk assessment, thereby effectively ensuring the safety and sustainability of TiO₂ in practical applications.

4.1. Positive Effects on Soil Organisms

TNPs exert a positive influence on crops. They can reduce the stress caused by heavy metals, serve as nano-fertilizers by supplying titanium elements, and function as nano-pesticides to enhance the antiviral capabilities of plants. Thanks to the excellent adsorption ability of TiO₂, the TiO₂-mediated removal of heavy metals from soil has yielded remarkable outcomes. It has alleviated the toxicity of heavy metals and facilitated plant growth. For instance, the combined application of TNPs, cadmium-tolerant Bacillus, and zinc oxide nanoparticles demonstrates a synergistic promoting effect on the growth of maize in cadmium-contaminated soil. This combination can reduce the bioaccumulation of cadmium and regulate physiological mechanisms to mitigate cadmium toxicity, thus offering a novel strategy and perspective for the remediation of cadmium-contaminated soil and the safe production of crops. Specifically, the root length and plant height of maize in the treatment group of 100 μmol L⁻¹ CdCl₂ + 200 mg L⁻¹ TiO₂ are lower than those in the group with 200 mg L⁻¹ TiO₂ alone. Moreover, the maize in the group with 100 μmol L⁻¹ CdCl₂ is much smaller compared to the previous two groups. Under the co-exposure of cadmium and TNPs, as the concentration of TNPs increases, the activities of superoxide dismutase, ascorbate peroxidase, and peroxidase gradually rise. This indicates that the application of titanium dioxide nanoparticles can alleviate the phytotoxicity of cadmium to maize. A possible mechanism is that these nanoparticles activate the antioxidant enzyme system of maize, thereby reducing the oxidative stress response triggered by heavy metals [31]. Apart from alleviating heavy metal stress, TNPs can also be utilized as nano-pesticides to boost plant immunity and prevent diseases. Different dosages (0, 0.01, 0.05, and 0.1 mg mL⁻¹) of TNPs were sprayed onto 9-week-old tomato plants, respectively. After 24 h, the attachment of TNPs on the surface of tomato plants was observed through scanning electron microscopy and contact angle tests, and the quantitative distribution of TNPs within the plants was measured by inductively coupled plasma mass spectrometry. Simultaneously, in vitro leaf and pot experiments on tomatoes were conducted to observe the disease resistance of TNPs against tomato wilt and its effects on the contents of peroxidase and catalase [32]. The in vitro antibacterial effect is quite remarkable: 0.01–0.1 mg mL⁻¹ TNPs exhibit high biological activity against Ralstonia solanacearum at various time intervals, and the inhibitory activity increases with the rise in concentration. They work by inducing the disintegration of the cell membrane, damaging the integrity of the cell membrane, causing severe genomic DNA damage to bacteria, reducing the expression of pathogenic genes, and decreasing the levels of cellulose and pectinase. These phenomena may be attributed to the stimulation of TNPs, which leads to the production of reactive oxygen species in cells. Additionally, it enhances plant disease resistance: The in vitro leaf and pot experiments on tomatoes reveal that TNPs can enhance the plants’ resistance to tomato wilt, significantly increase the contents of peroxidase and catalase, and have no obvious negative impact on plant growth. Therefore, TNPs can be employed as an environmentally friendly, biocompatible, and efficient nano-pesticide for controlling tomato wilt, providing a new approach to reducing the use of chemical pesticides. In the in vitro antibacterial experiment, 0.01–0.1 mg mL⁻¹ TNPs showed high biological activity against Ralstonia solanacearum (a plant pathogenic fungus) at 6 h, 12 h, and 24 h and other different time intervals. And this inhibitory activity increases with the increase in concentration. They function by inducing cell membrane disintegration, damaging cell membrane integrity, causing serious genomic DNA damage to bacteria, reducing the expression of pathogenic genes, and decreasing the levels of cellulose and pectin enzymes to inhibit the growth of Ralstonia solanacearum. This clearly shows that TNPs have a significant inhibitory effect on the growth of plant pathogenic fungi. Another study involved adding different concentrations of TNPs to the soil used for ginseng cultivation. A control group and treatment groups were set up, and ginseng was planted in a greenhouse for 90 days. Regarding the effect on ginseng physiological indexes, an appropriate concentration of TNPs can significantly enhance the root activity of ginseng and has a notable impact on the antioxidant enzyme system. In terms of the effect on soil physical and chemical properties, the application of TNPs significantly increases the soil organic matter content in both the rhizosphere and the soil for ginseng cultivation [33].

Soil microorganisms are also profoundly influenced by TNPs, with some microorganisms potentially experiencing positive effects. For instance, in research focusing on anaerobic microorganisms, titanate nanofibers were synthesized through the hydrothermal method and subsequently utilized for the photocatalytic degradation of organic pollutants in the anaerobic digestion liquid of poultry manure. The findings revealed that this nanomaterial could effectively decompose organic waste, significantly decrease the chemical oxygen demand and volatile fatty acid content in the digestion products, and simultaneously achieve decolorization. Although this process does not directly confer benefits to anaerobic microorganisms, it notably improves their living environment [34]. TNPs play a crucial role in creating a more suitable growth environment for microorganisms. They achieve this by increasing the soil organic matter content and regulating soil enzyme activity. In such an optimized environment, microorganisms can more readily access carbon sources, nitrogen sources, and other essential nutrients, thereby promoting their growth and reproduction. Moreover, this favorable environment also contributes to maintaining the stability of the microbial community structure. The stability of the microbial community structure is of paramount importance for the proper functioning of the soil ecosystem. It is intricately involved in numerous ecological processes, including the formation of soil aggregates, the resistance against pathogenic bacteria invasion, and many other vital functions.

In addition to plants and microorganisms, many invertebrates (such as nematodes and earthworms) live in the soil. A study showed that with the increase in the concentration of TNPs and the extension of the exposure time, the mortality rate of earthworms gradually increases. In the high-concentration TNP treatment group, earthworms reveal obvious poisoning symptoms in a short time and eventually die. This may be because TNPs enters the body of earthworms, destroys the normal structure and function of cells, and interferes with the physiological metabolism process of earthworms [35]. Given that soil invertebrates have little demand for TiO₂, it is crucial to regulate the content of TiO₂ in the soil. An appropriate amount of TNPs can ensure the safety of soil invertebrates and maintain the balance and stability of the soil ecosystem. However, once the content of TiO₂ exceeds the appropriate range, it may have a significant negative impact on the soil biota, and this impact is not only limited to invertebrates but also affects plants and microorganisms. The following part will deeply explore the various adverse effects brought by excessive TiO₂ in the soil.

4.2. Negative Effects on Soil Organisms

The detrimental effects of TNPs on soil organisms are not to be ignored. Regarding plants, excessively high concentrations of TiO₂ can exert negative impacts similar to those of heavy metals. Such high concentrations stimulate the copious production of reactive oxygen species in plants, thereby inducing oxidative stress damage and ultimately inhibiting plant growth. This assertion has been corroborated by a body of research. Both high concentrations of TNPs and Cd are capable of inducing oxidative stress responses in maize seedlings. Cd, a quintessential heavy metal pollutant, can trigger the extensive production of ROS upon entering plants. Similarly, at elevated concentrations, TNPs can also modify the antioxidant enzyme system in plants, including altering the activities of superoxide dismutase, catalase, and peroxidase [36]. Furthermore, TiO₂ not only retards plant growth, resulting in stunted plant stature, but also interferes with chlorophyll synthesis during the process of photosynthesis, leading to a reduction in chlorophyll content. Studies have demonstrated that the chloroplast structure of plants treated with 0.1 g/L TNPs remains intact. For plants treated with 0.5 and 1 g/L TNPs, the overall chloroplast structure and membrane remain unchanged, yet the chloroplasts exhibit slight swelling. In contrast, for plants treated with 2 and 3 g/L TNPs, significant alterations occur in the chloroplast ultrastructure. These changes include a distortion of the chloroplast shape, disintegration of the grana lamella stacks and stroma lamella stacks, detachment of chloroplasts from the plasma membrane, and their dispersion within the cytoplasm. Given that the chloroplast serves as the primary site for chlorophyll synthesis and photosynthesis, such ultrastructural changes, particularly the disintegration of the grana lamella stacks and stroma lamella stacks, disrupt the normal environment and conditions essential for chlorophyll synthesis. This interference not only hinders the chlorophyll synthesis process but may also impinge upon the activities and functions of enzymes associated with chlorophyll synthesis. Consequently, it can be reasonably postulated that high concentrations of TNPs have an adverse impact on chlorophyll synthesis and content. In conclusion, the negative effects of TiO₂ on plants are multi-faceted, encompassing growth inhibition, induction of oxidative stress, and reduction in chlorophyll content.

Soil microorganisms exhibit high sensitivity to TNPs, and TiO₂ can significantly influence microbial biomass and activity. Different dosages of TNPs (0 mg/kg, 200 mg/kg, 500 mg/kg, 1000 mg/kg, 2000 mg/kg) were uniformly incorporated into cultivated red soil to establish a soil–TNP–microorganism system. As the dosage of TNPs increased, the colony numbers of bacteria, fungi, and actinomycetes in the soil gradually declined. Compared with the blank control, when the dosage of TNPs reached 2000 mg/kg, the number of bacterial colonies decreased by approximately 60%, the number of fungal colonies decreased by around 45%, and the number of actinomycete colonies decreased by about 50%. However, the experiment only lasted 30 days, corresponding to a short-term exposure scenario. However, TNPs can accumulate year by year in soil. This long-term accumulation effect is absent in the short-term experimental design, which may lead to the underestimation of ecological risks. Furthermore, this experiment only refers to colony count and does not explore changes in functional genes—yet variations in such functional genes are likely to be more ecologically meaningful. This indicates that TNPs has a pronounced inhibitory effect on the growth and reproduction of soil microorganisms [37]. Furthermore, the minimum inhibitory concentration (MIC) of TNPs for various soil microorganisms was determined. For instance, the MIC of TNPs for nitrifying bacteria is 80 mg/L, and for ammonifying bacteria, it is 100 mg/L. When the concentration of TNPs reaches or exceeds the MIC, the growth of microorganisms is significantly inhibited, as evidenced by a reduction in colony numbers. In the plate-counting experiment, when the concentration of TNPs was 100 mg/L, the number of nitrifying bacteria colonies decreased by 70% compared to the control group, and the number of ammonifying bacteria colonies decreased by 55%. The change in the community structure was analyzed using high-throughput sequencing technology. The results revealed that after the addition of TNPs, the diversity index of the soil microbial community altered. In the treatment group with a TNP concentration of 200 mg/kg, the Shannon diversity index decreased from 4.5 in the control group to 3.8, while the Simpson index increased from 0.08 to 0.15, suggesting a reduction in microbial community diversity and a transformation in the community structure. Simultaneously, the relative abundances of dominant flora also underwent changes. For example, the relative abundance of Proteobacteria decreased from 35% in the control group to 25%, whereas the relative abundance of Firmicutes increased from 20% to 30%. These alterations may have a substantial impact on the energy flow, nutrient cycling, decomposition processes, biodiversity, and stability of the soil ecosystem.

In addition to the studies on plants and microorganisms, it is equally essential to investigate the impacts of TNPs on various soil indicator species, such as nematodes and earthworms. This investigation is crucial for comprehensively evaluating whether the migration of TiO₂ to the surrounding soil system will lead to negative consequences, thereby facilitating a more thorough assessment of the potential adverse effects of TiO₂ migration (Figure 3). In an experiment, Caenorhabditis elegans were exposed to three different particle sizes of titanium dioxide (5, 35, 300 nm) at three distinct concentrations (0.1, 1.0, 10.0 g/L) for 24 h. The results demonstrated that nematodes exposed to 5 nm TiO₂ in the medium-and high-concentration exposure groups exhibited abnormalities in multiple systems, including reproduction, movement, and development. For nematodes exposed to 35 nm TiO₂, abnormalities in indicators related to reproduction and movement were only observed in the high-concentration exposure group. In the case of nematodes exposed to 300 nm TiO₂, a decrease in the body bending frequency was noted in the high-concentration exposure group. These findings indicate that exposure to TNPs can affect the movement, reproduction, and development of Caenorhabditis elegans, and such effects are correlated with the particle size and exposure concentration. Meanwhile, as a key invertebrate in the soil ecosystem, the tolerance of earthworms to TiO₂ warrants further exploration. A study has shown that as the concentration of TNPs increases and the exposure time prolongs, the mortality rate of earthworms gradually rises. In the high-concentration TNP treatment group, earthworms display obvious poisoning symptoms within a short period and ultimately die. This phenomenon is likely attributed to the entry of TNPs into the earthworms’ bodies, which disrupts the normal structure and function of cells and interferes with the physiological metabolism processes of earthworms. Notably, Current research predominantly focuses on the negative impacts of TiO₂ on individual soil organisms while overlooking the interactions between different species. Therefore, when conducting risk assessments of TiO₂, it is imperative to comprehensively consider the inter-species interactions to obtain more accurate and reliable research results.

Notably, the negative effects of TNPs on soil organisms may be further exacerbated when coexisting with emerging pollutants such as nanoplastics—common contaminants in soil derived from agricultural film degradation and wastewater irrigation. A study on freshwater algae Scenedesmus obliquus demonstrated that fluorescent polystyrene nanoplastics significantly enhanced the toxicity of TNPs: when 1 mg/L fluorescent polystyrene nanoplastics were mixed with nTiO₂ (0.025, 0.25, 2.5 mg/L), the total ROS, superoxide anion, and hydroxyl radical generation in algal cells increased by 1.8–2.5 times compared to single nTiO₂ exposure, while lipid peroxidation levels rose by 40–60% and esterase activity (an indicator of cell metabolic function) decreased by 30–50% [38]. Dynamic light scattering analysis confirmed that fluorescent polystyrene nanoplastics and nTiO₂ formed hetero-aggregates in the medium (with mean hydrodynamic diameter increased by 20–35% compared to single nTiO₂), which enhanced particle adhesion to algal cells and promoted toxic exposure. This mechanism can be extrapolated to soil systems: soil nanoplastics may form similar aggregates with TNPs, thereby increasing the accumulation of TNPs on the surface of soil organisms (e.g., the cuticle of earthworms or the cell membrane of soil microalgae) and exacerbating oxidative stress damage—consistent with the ROS-induced toxicity mechanism of TNPs described earlier. However, current research on such combined toxicity is limited to aquatic organisms, and quantitative data on the interaction between TNPs and nanoplastics in soil (e.g., the effect of soil DOM on aggregate stability or the dose–response relationship of combined toxicity to soil organisms) remain lacking, which needs to be addressed in future studies.

Table 1. Summary of Ecological Effects and Key Influencing Factors of TNP Exposure on Different Soil Organism Groups.

Soil Organism Group	Negative Ecological Effects	Positive Ecological Effects (Under Specific Conditions)	Key Influencing Factors	Reference Sources
Soil Plants	1. High concentration (≥2 g/L) induces ROS outburst, abnormal SOD/CAT/POD activities, and chloroplast structure disintegration; 2. Inhibits chlorophyll synthesis and causes stunted plants; 3. Toxicity is more significant under combined Cd exposure than single TNP exposure	1. 200 mg/L TNPs can activate the antioxidant enzyme system of maize and alleviate Cd stress; 2. 0.01–0.1 mg·mL−1 TNPs enhance the resistance of tomatoes to bacterial wilt	TNP concentration, coexisting heavy metals, exposure duration	[32,37]
Soil Microorganisms	1. 2000 mg/kg TNPs reduces the number of bacterial/fungal/actinomycete colonies by 45–60%; 2. 200 mg/kg TNPs decreases the Shannon diversity index (from 4.5 to 3.8) and the abundance of Proteobacteria; 3. Ammonifying bacteria/nitrifying bacteria are more sensitive to TNPs (MIC: 80–100 mg/L)	1. Low concentration (10 mg·L−1) temporarily stimulates the metabolic activity of some microorganisms; 2. TNPs improve the living environment of microorganisms after degrading organic pollutants	TNP dosage, soil type, differences in functional genes	[35,39]
Soil Invertebrates	1. Nematodes: 5 nm TNPs cause abnormalities in reproduction/movement/development; 300 nm TNPs only reduce body bending frequency; 2. Earthworms: high-concentration TNPs cause intestinal cell damage; mortality increases with concentration/duration	Soil invertebrates have extremely low demand for TiO2	TNP particle size, exposure concentration, sensitivity of organism groups	[36]

Table 1. Summary of Ecological Effects and Key Influencing Factors of TNP Exposure on Different Soil Organism Groups.

Soil Organism Group	Negative Ecological Effects	Positive Ecological Effects (Under Specific Conditions)	Key Influencing Factors	Reference Sources
Soil Plants	1. High concentration (≥2 g/L) induces ROS outburst, abnormal SOD/CAT/POD activities, and chloroplast structure disintegration; 2. Inhibits chlorophyll synthesis and causes stunted plants; 3. Toxicity is more significant under combined Cd exposure than single TNP exposure	1. 200 mg/L TNPs can activate the antioxidant enzyme system of maize and alleviate Cd stress; 2. 0.01–0.1 mg·mL−1 TNPs enhance the resistance of tomatoes to bacterial wilt	TNP concentration, coexisting heavy metals, exposure duration	[32,37]
Soil Microorganisms	1. 2000 mg/kg TNPs reduces the number of bacterial/fungal/actinomycete colonies by 45–60%; 2. 200 mg/kg TNPs decreases the Shannon diversity index (from 4.5 to 3.8) and the abundance of Proteobacteria; 3. Ammonifying bacteria/nitrifying bacteria are more sensitive to TNPs (MIC: 80–100 mg/L)	1. Low concentration (10 mg·L−1) temporarily stimulates the metabolic activity of some microorganisms; 2. TNPs improve the living environment of microorganisms after degrading organic pollutants	TNP dosage, soil type, differences in functional genes	[35,39]
Soil Invertebrates	1. Nematodes: 5 nm TNPs cause abnormalities in reproduction/movement/development; 300 nm TNPs only reduce body bending frequency; 2. Earthworms: high-concentration TNPs cause intestinal cell damage; mortality increases with concentration/duration	Soil invertebrates have extremely low demand for TiO2	TNP particle size, exposure concentration, sensitivity of organism groups	[36]

Given that soil invertebrates have little demand for TiO₂, it is crucial to regulate the content of TiO₂ in the soil. An appropriate amount of TNPs can ensure the safety of soil invertebrates and maintain the balance and stability of the soil ecosystem. However, once the content of TiO₂ exceeds the appropriate range, it may have a significant negative impact on the soil biota, and this impact is not only limited to invertebrates but also affects plants and microorganisms.

4.3. Cytotoxic Mechanism at the Cellular Level

Artificially aged TiO₂ is more manageable than its naturally aged counterpart and poses a lower risk when introduced artificially. To meet the requirements of agricultural applications, the design of more environmentally friendly TiO₂ products is essential. This demand underscores the importance of delving into the cytotoxic mechanisms of TiO₂ at the cellular level, which represents a crucial current research direction. Numerous studies have already elucidated the toxic manifestations of TiO₂ at the physical and biological levels, such as the inhibition of plant root growth, alterations in the soil microbial community structure, and suppression of biological reproductive functions. However, to effectively mitigate these negative impacts in future practical applications, a thorough analysis of the cytotoxic mechanisms of TiO₂ at the cellular level is imperative. Drawing on existing research findings, this section focuses on several cellular-level cytotoxic mechanisms of TiO₂. The primary cytotoxic mechanisms involve the oxidative stress response mediated by reactive oxygen species and the adverse effects on normal physiological functions that occur following excessive intracellular accumulation of TiO₂. The following figure illustrates the corresponding processes.

Owing to the minuscule size of TNPs, the process by which they adhere to the cell membrane and penetrate into the cell can lead to alterations in the physiological morphology and function of the cell. The surface charge characteristics of TNPs are influenced by various factors, with the pH value being a notable one. In an acidic environment, the surface of TNPs can acquire a positive charge, enabling them to bind electrostatically to the numerous negatively charged groups present on the cell membrane.

For plants, the exposure and accumulation of TiO₂ ionic nanoparticles on the root surface exert multiple effects. Firstly, they can disrupt the orderly arrangement of cells, directly inflicting damage on the plants [38]. Secondly, TiO₂ impedes the absorption and transportation of nutrients and water by plants. The deposition of TiO₂ on the root surface blocks the pores of the cell membrane, thereby hindering these essential physiological processes. Notably, certain plants have developed detoxification mechanisms against TiO₂. Upon the entry of TiO₂ into plants, some species secrete organic substances, while others produce antioxidant enzymes to counteract the toxicity of TiO₂. Given these findings, prior to the large-scale application of TiO₂ in agriculture, the artificial sulfurization of TiO₂ could be considered as a design strategy. This approach has the potential to mitigate the cytotoxic impact of TiO₂ on crops, ensuring safer and more sustainable agricultural practices.

For microorganisms, the attachment of TiO₂ to the cell membrane can induce membrane damage, subsequently leading to alterations in membrane potential, membrane depolarization, loss of membrane integrity, and membrane rupture [39]. This membrane damage disrupts the cell membrane’s barrier function, enhances cellular permeability, and causes the leakage of intracellular substances, including small organic molecules and ions, thereby impairing the normal physiological functions of the cell. Beyond its physical impacts, TiO₂ exhibits photocatalytic activity, generating reactive oxygen species (ROS) such as hydroxyl radicals under light exposure. These ROS attack lipid, protein, and other components within the cell membrane, initiating lipid peroxidation reactions [40]. Once TiO₂ enters the cell, it may damage various biomacromolecules and organelles through its physical properties or the ROS it generates, interfering with the cell’s normal metabolic processes and physiological functions.

For invertebrates, TiO₂ located on the cell surface can enter the cell through clathrin-mediated endocytosis and accumulate, thus causing other harms to reproduction. In addition to surface exposure, the ingestion of TiO₂ by earthworms will also cause damage to their intestines. So far, the information on the cytotoxic mechanism of invertebrates is limited. Unlike the fluid environment where TiO₂ can be evenly distributed, the ideal conditions in the laboratory may be very different from the real soil environment. Further research is needed to record and clarify the cytotoxic mechanism of invertebrates in this regard.

The aforementioned cytotoxic mechanisms at the cellular level are the cornerstone for understanding the ecological risks of TNPs, yet their true environmental significance must be elucidated through studies on trophic transfer and in vivo exposure. Cell membrane damage, oxidative stress, and intracellular metabolic disturbance not only lead to the decline of an organism’s physiological functions but also affect the entire soil food web in a “bottom-up” manner. For instance, earthworms or nematodes subjected to TNP-induced toxic stress exhibit inhibited growth, reproductive capacity, and locomotor behavior [41]. This not only reduces their population size but also renders them more vulnerable to predators, thereby transferring TNPs and their associated toxic effects to higher trophic levels.

Current research is mostly limited to in vitro or short-term in vivo experiments on single species and lacks a systematic evaluation of the migration, transformation, and biomagnification effects of TNPs in the complete “soil–plant–invertebrate–vertebrate” food chain. Therefore, future studies must integrate multitrophic microcosm models and tracking techniques to conduct long-term in vivo research, so as to quantitatively assess how cytotoxicity ultimately translates into ecosystem-level risks.

5. Summary and Perspective

This paper provides a comprehensive review of the adsorption, transformation, and environmental risks of TNPs within the soil ecosystem (Figure 4). A majority of studies have indicated that TiO₂ holds considerable potential in a variety of soil ecological remediation efforts and agricultural production activities. Nevertheless, in order to obtain long-term and stable benefits and circumvent the limitations of nanoparticles in environmental applications, the factors affecting its remediation efficiency and stability, as well as the associated environmental risks, cannot be ignored.

Fully utilizing the positive effects of titanium dioxide on soil and avoiding its negative impacts are conducive to maintaining the stability of soil ecosystems, promoting agricultural development, further improving crop yields, and ensuring human food supply. By clarifying the duality of TNPs’ ecological effects, it is expected to realize the synergistic remediation of contaminated soil by TiO₂ and soil organisms or to improve the soil ecological environment and enhance soil quality.

Notably, the design of safer-by-generation TNPs through strategic modifications is crucial. Two highly promising avenues include (1) surface coating with inert materials like silica, which can mitigate direct toxicity by reducing reactive oxygen species generation and nanoparticle aggregation; and (2) elemental doping (e.g., with nitrogen or cerium), which tailors the photocatalytic activity of TNPs for enhanced efficiency under visible light, potentially reducing the required dosage and unintended environmental impacts. Prioritizing such engineered designs will be key to developing sustainable and environmentally compatible TNP applications.

To address these gaps and support the sustainable and safe application of TNPs, future research should prioritize the following efforts:

• Establish multitrophic microcosm models such as “soil–earthworm–bird” or “soil–plant–insect” systems to simulate the transfer process of TNPs in real food webs;
• Strengthen in vivo toxicological studies and focus on the multi-dimensional impacts of TNPs on the growth, development, reproduction, and behavior of organisms under long-term, low-dose exposure;
• Explore the connection between transformation and bioaccumulation and investigate how the chemical form transformation of TNPs in organisms affects their bioaccumulation potential and toxicity.