Perspectives for Photocatalytic Decomposition of Environmental Pollutants on Photoactive Particles of Soil Minerals

The literature shows that both in laboratory and in industrial conditions, the photocatalytic oxidation method copes quite well with degradation of most environmental toxins and pathogenic microorganisms. However, the effective utilization of photocatalytic processes for environmental decontamination and disinfection requires significant technological advancement in both the area of semiconductor material synthesis and its application. Here, we focused on the presence and “photocatalytic capability” of photocatalysts among soil minerals and their potential contributions to the environmental decontamination in vitro and in vivo. Reactions caused by sunlight on the soil surface are involved in its normal redox activity, taking part also in the soil decontamination. However, their importance for decontamination in vivo cannot be overstated, due to the diversity of soils on the Earth, which is caused by the environmental conditions, such as climate, parent material, relief, vegetation, etc. The sunlight-induced reactions are just a part of complicated soil chemistry processes dependent on a plethora of environmental determinates. The multiplicity of affecting factors, which we tried to sketch from the perspective of chemists and environmental scientists, makes us rather skeptical about the effectiveness of the photocatalytic decontamination in vivo. On the other hand, there is a huge potential of the soils as the alternative and probably cheaper source of useful photocatalytic materials of unique properties. In our opinion, establishing collaboration between experts from different disciplines is the most crucial opportunity, as well as a challenge, for the advancement of photocatalysis.


Introduction: The Basics of Photocatalysis and Strategies to Enhance Its Effectiveness
The subject of relationships between pollution and infectious disease has become a hot topic among scientists even prior to the recent COVID-19 pandemic years [1].The pandemic itself has sparked a surge in interest in implementing measures to eliminate toxins, viruses and other pathogens from the environment.Since the advent of the photocatalysis, the method has been proposed for "self-cleaning" solutions, maintenance of clean surfaces, and depolluting applications allowing the removal of inorganic and organic pollutants present in heavily polluted environments.The decomposition and destruction of pollutants are caused by processes involving highly reactive oxidative species (ROS) generated on the surface of semiconductors when exposed to light [2][3][4][5][6][7][8][9].
For the record, the semiconductors are characterized by a filled valence band (VB) and an empty conduction band (CB) [10,11].Thus, upon light illumination by photons of energy higher than the band gap (E BG ) [12], electrons are excited and promoted into about fifty kinds of semiconducting metal oxides and sulphide minerals, as shown in Table 1 [26].
Table 1.The E BG , E ⊖ VB , and E ⊖ CB energy positions at the zero point of charge (pH zpc ) a for oxides and sulphides based on the Xu and Schoonen [26] data.The E ⊖ VB and E ⊖ CB values were recalculated to the electrochemical scale E (vs.SHE) = −E(AVS) − 4.44 V [77,78].-------------------a pH zpc for which the net adsorbed charge within the Helmholtz double layer [79,80] is equal to zero.
The data gathered above (Table 1) can be discussed in a very simplified and condensed way: Minerals of E BG ≤ ca.3.3 eV (380 nm) possess VLD.The oxide minerals are strong photo-oxidation catalysts in aqueous solutions but are limited in their reducing power.The majority of metal oxide semiconductors have valence band edges (E VB ) in the range 1 to 3 V above the reduction potential of H 2 O (relative to the electrochemical SHE scale [77,78]), and energies for conduction band edges (E CB ) are close to or less negative than the reduction potential of H 2 O.More specifically, the electron generated in CB can reduce the substance if E CB is more negative than the reduction potential of the substance (reactant) (Er) (i.e., E CB < Er).Similarly, the h + VB generated in the valence band can oxidize a substance if its reduction potential is lower than the E VB of the semiconductor (i.e., E VB > E r ).One should note that none of the minerals (which are presented in Table 1 Additionally, one has to bear in mind that both E CB and E VB are pH-dependent since the ion balance on the mineral's surface is affected by the pH.Thus, the oxidizing power of h + VB and the reducing power of e − CB will also depend on the pH.For semiconducting metal oxides, the E CB and E VB vary with pH, following the Nernstian relation (1) [26,83] where E ⊖ CB(orVB) is the potential at the pH of the zero point of charge (pH zpc ), and the net adsorbed charge within the Helmholtz double layer [80] is zero.Thus, the pH has to be taken into account since its increase not only results in a lower concentration of H + , which is the major electron scavenger, but also shifts the E CB of minerals toward more negative values.

The Indicators of Soil Redox Activity: Redox Activity of Soil
Oxidation and reduction reactions occur instantly in soils.Soil redox abilities depend on the properties and concentration of substances contained in the so-called "soil solution".
The indicators of soil redox activity: A straightforward indicator characterizing the oxidation and reduction ratios in soil is its reduction potential E h , which can be calculated based on redox half-reactions with the Equation (2) [83-86]: where E ⊖ h [V vs. SHE] is the reduction potential under standard conditions (all activities = 1, m is the number of exchanged protons, n is the number of exchanged electrons, F is the Faraday constant [C mol −1 ], and {Ox}/{Red} is the ratio of the activities of oxidized to reduced species. In addition to E h , in the "soil chemistry", the reader may encounter two more indicators of soil redox activity, which are less known to general chemists, i.e., rH and pe.The redox potential of soil solutions depends on the degree of saturation with molecular hydrogen (H 2 ) and the pH of the environment.The higher H 2 concentration causes the greater reduction capacity of the solution and vice versa.The rH indicator is a negative logarithm of the hydrogen pressure in the soil-"soil solution" system, which demonstrates the relationship between redox and soil pH [87].
Because many redox-active elements (mainly metals) are involved in soil redox processes, pE (or pe) equal to −log{activity of electron} (Equation ( 4)) seems to be a more universal redox indicator [71,76,86,88]. where Redox activity of soil: In all systems, E h , rH, and pE are governed by the pH and the activities of oxidized {Ox} and reduced species {Red}.(For example, E h higher than 200 mV is usually associated with the dominance of electron acceptors in the soil, e.g., O 2 , NO 3 − , MnO 2 , and Fe 2 O 3 .)For complex systems such as soil, where the coverage of all components in the equations can be extremely complicated, the solution is to use the professional geochemical software [89,90].
Normal limits of pH in the environment are 4 and 9, and lower are found in acid sulfate soils, while the upper end of the pH limits is associated with water in contact with carbonate rocks.Whereas the theoretical limits of E h are determined by water instability and the release of gases, O 2 upon oxidation, and H 2 upon reduction.The upper limit of E h is defined by the oxidation of H 2 O ( 2H 2 O ⇌ O 2 + 4H + + 4e − , E ⊖ = 1.23 V), whereas its lower limit is defined by the reduction in H + ( H 2 ⇌ 2H + + 2e − , E ⊖ = 0.00 V).The potentials of the above half-reaction depend on the pH and follow the Nernstian equations.(Importantly, major photoactive minerals (see above) remain stable in these pH and E h ranges [71,84,86,91].) From an agricultural point of view, E h values within 200-750 mV are beneficial for normal plant development.The E h value of 750 mV for soil is associated with full aerobiosis, at which there is already a violation of the correctness in plant nutrition, while E h lower than 200 mV is associated with reducing processes harmful for plants.The potential E h is mainly influenced by the soil moisture, pH, and microbiological reactions.Increasing soil moisture reduces the value of E h , and drying has the opposite effect.E h fluctuates depending on the hydrologic regimes [92] and the season.For instance, in the temperate climate zone (of middle latitudes 23.5 • to 66.5 • N/S of the equator), it is the lowest in spring and increases in summer and autumn [87].
It seems obvious that sunlight has its share of daily and seasonal fluctuations of E h since the photochemical redox processes occur upon sunlight.Interestingly, even the effects of soil drying by sunlight are different from those by drying in the dark [71].Although light will generally not penetrate the soil surface deeper than 2 mm, on light-exposed soil, this depth will be sufficient to create a redox interface, especially since upward diffusion may extend the effective depth of the sunlight.The redox balance in the soil is affected by exposure to sunlight.Numerous soil components are photoactive, and their chemistry will vary significantly in sunlight compared with darkness.They include Fe(III) species, polycarboxylates, humic acids, and MnO 2 .Probably the most prevalent reactions in soil are photoredox transformations of Fe(III) and associated organic ligands [93][94][95].
The balance of redox in soil changes under the influence of light in numerous, often competing processes.These may be reversible processes such as the Fe 2+ /Fe 3+ redox transformations, as well as irreversible, after which reaction products such as CO 2 leave the soil environment.Importantly, environmental toxins may also participate in photochemical processes, which may lead to their degradation.Examples include the degradation of aromatic and polyaromatic hydrocarbons, aryl ketones and dioxins caused by • OH radicals produced in the Haber-Weiss reaction [71,96], and photocatalytic nitrogen-oxide conversion in red soil [97,98].

Susceptibility of Organic Pollutants and Pathogenic Microorganisms to Oxidativeand Bio-Degradation: The Persistent Micro-Pollutants as a Main Challenge for AOPs
Here, we should emphasize that research on the decontamination of waters containing persistent organic pollutants has been the core of photocatalytic research for years (see, for example, a review [99]).
Numerous experimental and review papers have been published.Particularly worth recommending are the relatively recent reviews on the application of photocatalysis for toxicity reduction in real wastewaters [120] and the elimination of viruses and other pathogens from recreational waters, food packaging, hospital surfaces, etc. [121][122][123][124].
The recent reviews of Valeriani et al. [125,126] are particularly noteworthy.Both works were carried out according to a rigorous protocol of meta-analysis [127], demonstrating the most current trends in the antimicrobial effectiveness of innovative photocatalysts.
The persistent micro-pollutants as a main challenge for AOPs: With few exceptions, like perfluorinated and polyfluorinated substances (PFASs) [82,128], the ROS generation in photocatalysis very effectively induces oxidative decomposition of pollutants, which can even lead to their complete mineralization [8,15].Therefore, one can hypothesize that the photocatalysis processes on the soil surface will take part in natural oxidative reduction processes occurring in soil [88] and as such may contribute to its decontamination.However, a complete mineralization (i.e., the formation of CO 2 , H 2 O, and NH 3 , exclusively) of the pollutant seems essential in many cases since degradation products have the potential to be as harmful or even more harmful toxins than the parent compounds.
The persistent micro-pollutants are frequently eliminated from the waste applying a multi-stage process where so-called advanced oxidation processes (AOPs) [2] are usually used prior to the biological stage to initially decompose the pollutants [129,130].The micro-pollutant leftovers can appear incidentally even in the municipal waste, where the AOP pre-treatment may potentially worsen the situation due to the formation of novel toxins upon oxidation.For instance, this appears to be the case for phenylurea-derived compounds like herbicides such as Linuron, Diuron, and Metobromuron or the antimicrobial additive of personal care products such as Triclocarban [131][132][133][134][135][136][137][138][139].The phenylurea herbicides' production and use on beans, soybeans, tomatoes, tobacco, potatoes, flax, and sunflowers will result in their release to the environment through various waste streams.If released to the soil, the phenylurea herbicides will have moderate mobility.Volatilization of them should not be important; thus, the herbicides may be degraded on soil surfaces.Research shows that some wastewater bacteria are able to hydrolyze the urea bridge in phenylurea herbicides producing monochloroanilines and dichloroanilines [139][140][141][142].The fate of these metabolites is not certain; however, they may slowly decompose, as well as bioaccumulate or bind to soil particles and undergo auto-oxidation [143].This is of special importance since chloroanilines have been named "probable carcinogens" by the U.S. EPA due to their association with bladder cancer [27][28][29][144][145][146].The environmental toxicity of Linuron and its metabolites had been partially eliminated with its replacement by Metobromuron.However, our laboratory study and computational predictions for both herbicides (Linuron and Metobromuron as well) foresees the formation of similar hazardous products upon the AOP treatment [134,[147][148][149].Among them are cyanates, e.g., isocyanatomethane (methyl isocyanate, MIC, CH 3 -N=C=O)-the toxin accused of causing nearly 3800 deaths in the Bhopal disaster [35,150,151].Therefore, the suspicion that trace amounts of MIC could be formed during incomplete degradation of linuron-like pesticides ought to raise legitimate concerns.
Even when the situation is not particularly dramatic, peculiar products of the AOP reactions can avoid subsequent biodegradation.Moreover, such contamination may be significantly harmful or even destroy successive biological stages of waste decontamination [130,148,152].
It appears that the disinfection of wastewater is an easier process than the elimination of persistent organic pollutants.On the "molecular level" the processes of disinfection/hygienization of waste leads to decomposition of the natural, organic compounds, which are essential for pathogen survival and multiplication [153].
Individual molecules of proteins, lipids, sugars, and nucleic acids are relatively unstable and quite easily oxidized and hydrolyzed [154][155][156][157][158][159][160].However, living cells are able to regenerate oxidative damage quite efficiently through enzymatic and non-enzymatic repair processes [161].(It should be noted that this could lead to the selection of pathogens with greater resistance.)Therefore, one must keep in mind that the reduction in the pathogen population to a level corresponding to the requirements imposed by regulatory institutions (see [162]) will require an oxidation process of high intensity.It is unlikely that will be achievable in natural conditions because only a small portion of solar energy can be utilized in photocatalytic processes, which is evident from the comparison of E BG values, shown in Table 1, with the well-known spectrum of sunlight (see https://www.astm.org/g0173-03r20.html,accessed 1 July 2024).

Interference of Redox Processes by Soil Organic Matter: Impact of Humic Acids on the Effectiveness of Photocatalysis In Vivo and In Vitro
Soil organic matter (SOM) is one of the key elements of carbon circulation in nature [163,164].SOM seems to be the most valuable part of the soil from an agricultural perspective but also for growth of natural vegetation cover [87,165].It consists of mainly humified organic debris of plants and other organisms and labile organic compounds derived from exudates of soil microorganisms and plants' roots.Numerous functions are performed by SOM in soil, starting from physical functions (the stabilization of soil structure, water retention, and thermal properties) [165][166][167][168][169]; through chemical functions (the retention of cations, buffering capacity and pH effects, chelation of metals, and interactions with xenobiotics); and ending with the biochemical functions such as a reservoir of metabolic energy, a source of macronutrients, ecosystem resilience [165,167], or even allelopathy [170].Interestingly, the reducing environment of humic acids promotes the formation of metallic and oxide nanoparticles in both laboratory and natural conditions [75,171,172].
From the point of view of the environment decontamination, an interesting feature is the immobilization of inorganic substances as a result of the formation of complexes with inorganic cations.For example, humified organic matter and polyvalent metal cation complexes participate in the formation of micro-aggregates with clusters and silt particles, oxides, and aluminosilicates [166].On the other hand, the immobilization of toxic-to-plants noble metals, by humic acids of peat, in the form of metal nanoparticles was observed [171,173] and confirmed in laboratory conditions [75,[174][175][176].Unfortunately, water-soluble humic acid (HA) compounds in the disinfection processes of drinking water and wastewater are considered as precursors of highly toxic, carcinogenic, and mutagenic disinfectant by-products [150,177].The chemistry of the processes leading to the formation of toxic derivatives of HA has been previously extensively studied and described in the basic works on radical chemistry of aromatic compounds [178][179][180].HAs are poly-aromatic compounds that have a variety of components including quinone, phenol, catechol, and sugar moieties [181,182], with significant antioxidant properties and the ability to scavenge free radicals [182][183][184][185][186][187][188][189][190][191][192][193][194][195][196][197][198][199][200][201].
The strong inhibitory effect of natural organic matter is also a major challenge for photocatalytic water purification.This organic matter can scavenge photogenerated h + VB , e − CB , and radicals and occlude ROS generation sites upon adsorption.Additionally, the quantum efficiency of photocatalysis can be reduced due to the absorption of light by organic compounds, when the light quantum has too low energy to cause dissociation of chemical bonds and its absorption causes the solution to only heat up [202].The fact that humic acids scavenge • OH radicals and its precursor h + VB , desired when photocatalysis is used for the degradation of humic acids, causes a decrease in the effectiveness of photocatalysis when its purpose is to degrade other pollutants [189].A large contribution has been made to improve the quality of drinking water, thanks to the development of organic matter removal methods [203][204][205][206][207][208].In this trend, works dedicated to increasing the photocatalysis' efficiency counteracting the inhibitory effect of humic acids by decreasing the HA surface adsorption and mitigation of the e − CB -h + VB recombination were also created [209][210][211].

The Perspectives of Photocatalysis on Soils Minerals "In Vivo": The Limits Set by Diversity of Soils and the Environmental Conditions on the Earth
Soil is a major component of the Earth's geosystem and constitutes the outer layer of the lithosphere (the continental crust).Soil-forming factors include climate, relief, parent material (bedrock), organisms (plants, animals, fungi, and human being), and ground water; they all interact over time [212][213][214][215].
Soil consists of not only a solid phase (minerals and organic matter) but also a porous phase (gases and water).Usually, the solid phase consists of half of the soil, thus significantly affecting the physical and chemical properties.
The continental crust is characterized by a huge variability of minerals and rocks.Because they are a soil substratum, the elements derived from weathering form soil's chemical composition.The elements found in the continental crust are divided into three groups depending on the average occurrence (Clarke number [215]).The first group consists of elements with a high Clarke number, which constitute the main mass of rocks and soils.This group includes O, Si, Al, Fe, Ca, Na, K, Mg, C, H, S, P, and Cl [216].They determine the geochemical properties of the landscape, mainly the conditions for the migration of other elements [217].The second group is low Clarke number elements.Their migration depends on the conditions created by the elements of the first group.The last group is rare elements, whose content in the continental crust is lower than 0.01%.
Iron as a first group element commonly occurs in the soil.The Fe content in the continental crust is between 4.1 and 5.1%, depending on whether its mass or weight share is calculated [214,218].Iron oxides are considered to be chemical compounds with great potential in photocatalysis.Ti is similar (0.3-0.6%, second group) [219].These two semiconductors are quite common in soils, especially containing Fe, so there is a possibility to use the topsoil in the process of photocatalysis [200,220,221].
In addition to a photocatalyst, which is stable under given pH value and temperature conditions, for oxidation to sunlight to occur, the following are also necessary: sunlight, atmospheric oxygen, and humidity.The soil surface is exposed to sunlight in the ca.280 to 4000 nm range [222,223], so it has the ability to initiate solar energy conversion into chemical energy and therefore to control environmental pollution and decontamination [224,225].
Experimental research on the soils' usage in the decontamination of pollutants has been carried out several times [97,200,226].The obtained results are promising, but are there opportunities to use this phenomenon outside the laboratory?There is no clear answer to this question yet.Under natural conditions, soil, as a complex component affected by many soil-forming factors, is characterized by significant variation of physical and chemical properties and undergoes many transformations.There are a number of crucial issues that would need to be resolved for widespread photocatalytic soil usage in vivo.
The supply of solar energy and oxygen is limited to the topsoil.In most climate zones, radiation is largely absorbed by the vegetation.In woody and shrubby vegetation zones, only a small part of the radiation reaches the soil surface [227].The Earth's areas with sparse vegetation are a different situation; these are primarily the desert, semi-desert, and tundra zones.The supply of solar energy is much higher because of low soil shading [228,229].In the polar climate (tundra zone), the radiation is limited with large seasonal variability, including polar nights [230].In deserts and semi-deserts, the supply of energy is significant and is not disturbed by cloud cover.This is a consequence of low humidity and high atmospheric pressure.In these areas the soil cover is thin or not present at all.On the surface, there are different rocks and minerals (sands, clays, etc.).In the temperate climate zone, the soil surface is covered by vegetation almost all year long.Only agricultural areas before the plant growing season (usually from October until April) are not covered by vegetation.Another issue is the variability of the soil surface.Mostly, relief is not flat, so there is a variation in the supply of energy to the surface.In the Northern Hemisphere, the northern slopes are less exposed to the sunlight than the southern slopes.That affects the soil properties, such as the thickness, moisture, and nutrient content [231].The solar radiation depends on the height of the sun above the horizon, which varies depending on the season.For example, the sun angle for Warsaw (42 • N, Poland) difference between December and June is more than 45 degrees.(To obtain an overall, global picture of the solar energy supplied to the Earth's surface, the reader can go to the handbook [222] and to the online interactive maps on the "World Bank.Global Solar Atlas" web page [230]).
The presence of Fe oxides in the soils is a fact, but their content varies both spatially and on a global scale, as in the soil profile [232][233][234].Fe evolution in soils is controlled both by natural factors (rock weathering, pedogenic processes driven), causing Fe transformation and translocation within and from soil (eluviation illuviation and reduction oxidation processes) [235], and by human impacts (industry and agriculture) [236].
Assuming that the photocatalysis occurs only in the surface, the presence of free oxides in the topsoil (humus horizon) is negligible.Their greatest accumulation occurs in horizons, such as ferralic, nitic, or cambic, which occur deeper in soil profile [237].Hence, soils in which Fe oxides are abundant are mainly tropical and subtropical soils, such as Ferralsols and Nitisols.In the humus horizon, the content of Fe oxides is lower due to accumulation by humus, which constitutes the sorption complex of the soil.
The diversity of soils on the Earth means that the environmental conditions, such as climate, parent material, relief, and vegetation, should be included in experiments of photocatalytic properties and deeply studied in the future.The potential of the soils is huge; they can be used as the basis of more sustainable alternatives, instead of synthetic materials for decontamination of the pollutants.

Challenges and Future Perspectives
Our work on the review has shown the need for cooperation and information exchange between researchers involved in the photocatalysis field and environmental scientists.Personally, we learned a lot from each other during our writing sessions.In fact, the initiative for this review came from chemists involved in photocatalysis research (DP and JK).It happened under the influence of events related to the COVID-19 pandemic: The media reports were full of information about the estimated lifespan of the virus on surfaces made of various materials and the urgent need for systematic sterilization.Our experience with photocatalysis on the particles of iron oxides [38][39][40] was telling us that the wet surface of rusty steel could be photocatalytically active, and the virus' lifespan on such a surface should be shorter than on stainless steel.We thought that the top layers of reach in iron oxide red soils should have similar properties.Thus, for example, the contaminated red soil exposed to sunlight should "purify" itself, and sunlit wasteland would be decontaminated by systematic plowing.These naive ideas were quickly dispelled during our internal discussion.
Therefore, we believe that for scientists who want to further develop photocatalytic decontamination, the main challenges may not be technical problems but rather establishing cooperation between specialists in various fields.It is unfortunate that we often use terms that are not understandable to our potential scientific partners.Without describing and understanding the basic topics, it is difficult to seize the opportunities and implement any applications.We could recommend seeking possibilities to conduct joint research and utilize local sources of natural minerals.This should help overcome the limitations and challenges of using soil minerals as photocatalysts, such as soil composition variability and the potential environmental impacts of large-scale applications.

Summary and Conclusions
The upper layers of the lithosphere can be a good source of unique semi-conducting materials of natural photocatalytic properties.A combination of many factors is required for the photocatalysis process to be effective.In paragraphs 4 to 6 of this work, we tried to show a realistic assessment of the conditions for effective photocatalytic decontamination in vivo and in vitro.Particular requirements such as adequate, intensive, and long-lasting sunlight; the presence of specific minerals; and moderate humidity of the soil solution mean that under natural conditions, photocatalytic processes cannot be fully effective.It cannot be ruled out that decontamination of desert and semi-desert soils can be partly attributed to photocatalysis, which can lead to the mineralization of organic matter.One can put forward the thesis that the probability that in this way, nature, without the support of technology, will cope with the pesticide residues or pathogenic microorganisms is very small.In practice, these processes call for highly advanced technical solutions.In rare circumstances, effective photocatalysis can occur spontaneously without human intervention.A forward-looking idea seems to be usage of natural photoactive minerals in new and existing technologies utilizing the photocatalytic process.The applications that are cited in [121], such as building materials using cement-based products, ceramic tiles, bituminous membranes, etc., can be good examples.
It appears that there are no negative prospects for photocatalysis development.There are also economic aspects behind the use of natural minerals: for many years to come the lower photocatalytic efficiency of natural semiconductors will be offset by their lower price.

Conflicts of Interest:
The authors declare no conflict of interest.

Author Contributions:
Conceptualization, D.P. and A.S.; writing-original draft preparation, D.P., A.S., J.B.K., K.I.H. and M.C.; writing-review and editing, D.P.; supervision, D.P. All authors have read and agreed to the published version of the manuscript.Funding: This research received no external funding.