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Field experiment pitting magnetite nanoparticles against microparticles: Effect of size in the rehabilitation of metalcontaminated soil

ABSTRACT

A significant portion of the current knowledge regarding the use of iron nanoparticles for remediating metal-contaminated soils is derived from laboratory experiments, leaving several unanswered questions. This article presents a field experiment comparing the efficacy of magnetite nanoparticles and microparticles for the immobilization of metals and the growth of plants in metal-contaminated soils. This study aimed to investigate the effects of magnetite particle size on metal immobilization and plant growth in soils exposed to airborne pollution from the Middle-Urals Copper Smelter in the southern taiga subzone near Revda, Russia, 50 km from Ekaterinburg. Magnetite nano- and microparticles were added to forest litter at a 4 % w/w dose. The total metal contents in litter from the study plots were 1-2 orders of magnitude higher than background metal concentrations. The magnetite nanoparticle treatment was found to decrease the concentration of exchangeable copper in soil and improve the growth of red fescue (Festuca rubra L.) on polluted soil compared to the control. In contrast, magnetite microparticles did not show any statistically significant effects. These findings are in line with laboratory results that demonstrated the superior metal adsorption properties of magnetite nanoparticles compared to microparticles. However, this study was limited in duration (2 months), and longer field studies would be necessary to confirm the role of iron particle size in the rehabilitation of metal-contaminated soils.

Keywords
heavy metals; iron oxides; nanomaterials; nanotechnology; nano; micro

INTRODUCTION

The term “metal immobilization in soils” refers to the reduction of the bioavailable metals1 1 For brevity, metalloids (e.g., arsenic) will be referred to as “metals” in the following discussion. in the soil solution by adding amendments (Mench et al., 2000Mench M, Vangronsveld J, Clijsters H, Lepp NW, Edwards R. In situ metal immobilization and phytostabilization of contaminated soils. In: Terry N, Bañuelos G, editors. Phytoremediation of contaminated soil and water. Boca Raton: CRC Press; 2000. p. 323-58.). In other words, the amendments do not extract the metals from the soil but convert them into less soluble or insoluble forms via mechanisms such as adsorption, complexation, or co-precipitation (Kumpiene et al., 2008Kumpiene J, Lagerkvist A, Maurice C. Stabilization of As, Cr, Cu, Pb and Zn in soil using amendments: A review. Waste Manage. 2008;28:215-25. https://doi.org/10.1016/j.wasman.2006.12.012
https://doi.org/10.1016/j.wasman.2006.12...
). As the solubility of the metals decreases, they become less accessible to plants and soil organisms.

Iron oxides have been found to be effective amendments for immobilizing metals in contaminated soils (Komárek et al., 2013Komárek M, Vaněk A, Ettler V. Chemical stabilization of metals and arsenic in contaminated soils using oxides - A review. Environ Pollut. 2013;172:9-22. https://doi.org/10.1016/j.envpol.2012.07.045
https://doi.org/10.1016/j.envpol.2012.07...
) due to their high adsorption capacity for toxic metals like Cu, Zn, Pb, and Cd (Neaman et al., 2008Neaman A, Martinez CE, Trolard F, Bourrie G. Trace element associations with Fe- and Mn-oxides in soil nodules: Comparison of selective dissolution with electron probe microanalysis. Appl Geochem. 2008;23:778-82. https://doi.org/10.1016/j.apgeochem.2007.12.025
https://doi.org/10.1016/j.apgeochem.2007...
, Neaman et al., 2004Neaman A, Mouélé F, Trolard F, Bourrié G. Improved methods for selective dissolution of Mn oxides: Applications for studying trace element associations. Appl Geochem. 2004;19:973-9. https://doi.org/10.1016/j.apgeochem.2003.12.002
https://doi.org/10.1016/j.apgeochem.2003...
). Some researchers prefer to use iron oxide precursors, such as zerovalent iron particles (e.g., iron powder or grit), which convert to iron oxides2 2 For brevity, oxyhydroxides (e.g., goethite) will be referred to as “oxides” in the following discussion. upon corrosion in soil (Kumpiene et al., 2019Kumpiene J, Antelo J, Brannvall E, Carabante I, Ek K, Komárek M, Soderberg C, Warell L. In situ chemical stabilization of trace element-contaminated soil - Field demonstrations and barriers to transition from laboratory to the field - A review. Appl Geochem. 2019;100:335-51. https://doi.org/10.1016/j.apgeochem.2018.12.003
https://doi.org/10.1016/j.apgeochem.2018...
).

A thorough review of the literature suggests that both iron oxides and their precursors, such as zerovalent iron particles, are valuable materials for in situ metal immobilization in soils. Notably, in a study by Dovletyarova et al. (2022b)Dovletyarova EA, Fareeva OS, Brykova RA, Karpukhin MM, Smorkalov IA, Brykov VA, Gabechaya VV, Vidal K, Komárek M, Neaman A. Challenges in reducing phytotoxicity of metals in soils affected by non-ferrous smelter operations. Geogr Environ Sustain. 2022b;15:112-21. https://doi.org/10.24057/2071-9388-2021-141
https://doi.org/10.24057/2071-9388-2021-...
it was discovered that iron oxides formed through the corrosion of zerovalent iron particles and the natural iron oxides originating from ferromanganese nodules exhibited comparable metal adsorption properties in soils polluted by a copper smelter. Similarly, Zhang et al. (2010)Zhang MY, Wang Y, Zhao DY, Pan G. Immobilization of arsenic in soils by stabilized nanoscale zero-valent iron, iron sulfide (FeS), and magnetite (Fe3O4) particles. Chinese Sci Bull. 2010;55:365-72. https://doi.org/10.1007/s11434-009-0703-4
https://doi.org/10.1007/s11434-009-0703-...
demonstrated that zerovalent iron nanoparticles did not present any discernable benefits over magnetite nanoparticles in terms of immobilizing arsenic in soils.

In recent years, the possibility of using iron-containing nanoparticles, which are less than 100 nm in size, for the remediation of metal-contaminated soils has generated a surge of research interest (Liang et al., 2022Liang WY, Wang GH, Peng C, Tan JQ, Wan J, Sun PF, Li QN, Ji XW, Zhang Q, Wu YH, Zhang W. Recent advances of carbon-based nano zero valent iron for heavy metals remediation in soil and water: A critical review. J Hazard Mater. 2022;426:127993. https://doi.org/10.1016/j.jhazmat.2021.127993
https://doi.org/10.1016/j.jhazmat.2021.1...
). Iron-containing nanoparticles are believed to possess superior adsorption characteristics compared to microparticles due to their smaller size and larger surface area (Mueller and Nowack, 2010Mueller NC, Nowack B. Nanoparticles for remediation: Solving big problems with little particles. Elements. 2010;6:395-400. https://doi.org/10.2113/gselements.6.6.395
https://doi.org/10.2113/gselements.6.6.3...
). However, our scrutiny of the literature on this topic did not completely resolve concerns about the advantages of nanoparticles over microparticles in immobilizing soil metals. For instance, nanoparticles can be harmful to organisms by inducing oxidative stress (Xue et al., 2018Xue WJ, Huang DL, Zeng GM, Wan J, Cheng M, Zhang C, Hu CJ, Li J. Performance and toxicity assessment of nanoscale zero valent iron particles in the remediation of contaminated soil: A review. Chemosphere. 2018;210:1145-56. https://doi.org/10.1016/j.chemosphere.2018.07.118
https://doi.org/10.1016/j.chemosphere.20...
).

Given the limited number of studies that have directly compared iron micro- and nanoparticles (Danila et al., 2020Danila V, Kumpiene J, Kasiuliene A, Vasarevicius S. Immobilisation of metal(loid)s in two contaminated soils using micro and nano zerovalent iron particles: Evaluating the long-term stability. Chemosphere. 2020;248:126054. https://doi.org/10.1016/j.chemosphere.2020.126054
https://doi.org/10.1016/j.chemosphere.20...
), it is pertinent to inquire whether particle size contributes to the efficient remediation of metal-contaminated soils. In light of this, the question arises: what role does the size of iron-containing particles play in the remediation of metal-contaminated soils?

Our study focused on a specific type of iron-containing particles to answer our research question through a field experiment. We selected magnetite (Fe3O4), a mixed oxide composed of both Fe(III) and Fe(II), for several reasons. Firstly, magnetite has an advantage over other iron oxides as it is a magnetically separable adsorbent. Magnetic separation is significant in the field of water treatment (Linley et al., 2013Linley S, Leshuk T, Gu FX. Magnetically separable water treatment technologies and their role in future advanced water treatment: A patent review. Clean-Soil Air Water. 2013;41:1152-6. https://doi.org/10.1002/clen.201100261
https://doi.org/10.1002/clen.201100261...
). Some authors have suggested that magnetite with adsorbed metals can be extracted from the soil by magnets (Duan et al., 2022Duan LC, Wang QH, Li JN, Wang FH, Yang H, Guo BL, Hashimoto Y. Zero valent iron or Fe3O4-loaded biochar for remediation of Pb contaminated sandy soil: Sequential extraction, magnetic separation, XAFS and ryegrass growth. Environ Pollut. 2022;308:119702. https://doi.org/10.1016/j.envpol.2022.119702
https://doi.org/10.1016/j.envpol.2022.11...
; Fu et al., 2021Fu HY, He HF, Zhu RL, Ling L, Zhang WX, Chen QZ. Phosphate modified magnetite@ferrihydrite as an magnetic adsorbent for Cd(II) removal from water, soil, and sediment. Sci Total Environ. 2021;764:142846. https://doi.org/10.1016/j.scitotenv.2020.142846
https://doi.org/10.1016/j.scitotenv.2020...
; Ji et al., 2022Ji X, Zhou CY, Chen LX, Li YZ, Hua TC, Li Y, Wang CQ, Jin S, Ding HR, Lu AH. Reduction, mineralization, and magnetic removal of chromium from soil by using a natural mineral composite. Environ Sci Ecotechnol. 2022;11:100181. https://doi.org/10.1016/j.ese.2022.100181
https://doi.org/10.1016/j.ese.2022.10018...
). Secondly, the synthesis of magnetite is straightforward (Pomogailo et al., 2011Pomogailo AD, Kydralieva KA, Zaripova AA, Muratov VS, Dzhardimalieva GI, Pomogailo SI, Golubeva ND, Jorobekova SJ. Magnetoactive humic‐based nanocomposites. Macromol Symp. 2011;304:18-23. https://doi.org/10.1002/masy.201150603
https://doi.org/10.1002/masy.201150603...
), requiring only one step, which enables the preparation of large quantities of magnetite for a field experiment. Lastly, commercial magnetite nanoparticle products are widely available (Huang et al., 2018Huang Q, Zhou SW, Lin LN, Huang YC, Li FJ, Song ZG. Effect of nanomaterials on arsenic volatilization and extraction from flooded soils. Environ Pollut. 2018;239:118-28. https://doi.org/10.1016/j.envpol.2018.03.091
https://doi.org/10.1016/j.envpol.2018.03...
).

Much of the knowledge on using iron-containing nanoparticles in soil remediation is derived from laboratory experiments, which leaves numerous questions unanswered. To clarify the role of iron particle size in the rehabilitation of metal-contaminated soils, it is imperative to compare the adsorption properties of iron-containing micro- and nanoparticles in field studies. Thus, our study was designed to focus on a specific location to address our research question. We selected the area surrounding the Middle-Urals Copper Smelter (56° 51’ 0.8” N, 59° 54’ 25.6” E), situated in the southern taiga subzone near Revda, Russia, which has been exposed to air-borne pollution since 1940 (Prudnikova et al., 2020Prudnikova EV, Neaman A, Terekhova VA, Karpukhin MM, Vorobeichik EL, Smorkalov IA, Dovletyarova EA, Navarro-Villarroel C, Ginocchio R, Peñaloza P. Root elongation method for the quality assessment of metal-polluted soils: Whole soil or soil-water extract? J Soil Sci Plant Nutr. 2020;20:2294-303. https://doi.org/10.1007/s42729-020-00295-x
https://doi.org/10.1007/s42729-020-00295...
). The smelter emissions did not affect the acidity of the litter; the pH values (pH <5.0) were typical for both polluted and unpolluted areas surrounding the smelter (Prudnikova et al., 2020Prudnikova EV, Neaman A, Terekhova VA, Karpukhin MM, Vorobeichik EL, Smorkalov IA, Dovletyarova EA, Navarro-Villarroel C, Ginocchio R, Peñaloza P. Root elongation method for the quality assessment of metal-polluted soils: Whole soil or soil-water extract? J Soil Sci Plant Nutr. 2020;20:2294-303. https://doi.org/10.1007/s42729-020-00295-x
https://doi.org/10.1007/s42729-020-00295...
). In acidic conditions, metals tend to be more soluble (Sposito, 2016Sposito G. The chemistry of soils. 3rd ed. New York: Oxford University Press; 2016.), thereby inducing high phytotoxicity (Adriano, 2001Adriano DC. Trace elements in terrestrial environments: biogeochemistry, bioavailability, and risk of metals. 2nd ed. New York, NY: Springer-Verlag; 2001.). As a result, the area under investigation had sparse herbaceous vegetation (Vorobeichik et al., 2014Vorobeichik EL, Trubina MR, Khantemirova EV, Bergman IE. Long-term dynamic of forest vegetation after reduction of copper smelter emissions. Russ J Ecol. 2014;45:498-507. https://doi.org/10.1134/s1067413614060150
https://doi.org/10.1134/s106741361406015...
).

Plants are commonly employed as bioindicators to assess the reduction of metal toxicity in polluted soils following the addition of amendments (Lwin et al., 2018Lwin CS, Seo BH, Kim HU, Owens G, Kim KR. Application of soil amendments to contaminated soils for heavy metal immobilization and improved soil quality-a critical review. Soil Sci Plant Nutr. 2018;64:156-67. https://doi.org/10.1080/00380768.2018.1440938
https://doi.org/10.1080/00380768.2018.14...
). The current study aimed to assess the impact of magnetite particle size on both metal immobilization and plant growth in soils contaminated by a copper smelter.

MATERIALS AND METHODS

Magnetite synthesis and characterization

Magnetite nanoparticles were synthesized via a stoichiometric 1:2 coprecipitation reaction of iron salts (II and III) in the presence of alkali (Pomogailo et al., 2011Pomogailo AD, Kydralieva KA, Zaripova AA, Muratov VS, Dzhardimalieva GI, Pomogailo SI, Golubeva ND, Jorobekova SJ. Magnetoactive humic‐based nanocomposites. Macromol Symp. 2011;304:18-23. https://doi.org/10.1002/masy.201150603
https://doi.org/10.1002/masy.201150603...
). The resulting paste contained 25 % magnetite nanoparticles. To produce magnetite microparticles, the solution was cooled to 4 °C, wrapped in aluminum foil to maintain a constant temperature, and agitated continuously (100 rpm) for 2 h after adding 10 % NaOH. The resulting particles were then dried at 40 °C, resulting in a powder form.

The phase composition of the particles was determined using a Mössbauer spectrometer MS1104EM with 57Co in Rh matrix, ensuring a noise/signal ratio of no more than 2 %. A detailed procedure for this can be found in our previous study (Yurkov et al., 2022Yurkov GY, Pankratov DA, Koksharov YA, Ovtchenkov YA, Semenov AV, Korokhin RA, Solodilov VI. Composite materials based on a ceramic matrix of polycarbosilane and iron-containing nanoparticles. Ceram Int. 2022;48:37410-22. https://doi.org/10.1016/j.ceramint.2022.09.096
https://doi.org/10.1016/j.ceramint.2022....
). Additionally, X-ray diffraction diagrams were recorded using an X-ray diffractometer Diffray-401 (Scientific Instruments, Russia) with Bragg-Brentano focusing and Cr-K? radiation (wavelength 0.22909 nm) at room temperature. Diffraction images were identified using the PDF-2 database of the International Center for Diffraction Data. The coherent scattering domain size was calculated using the method of Waseda et al. (2011)Waseda Y, Matsubara E, Shinoda K. X-ray diffraction crystallography: Introduction, examples and solved problems. Berlin: Springer Science & Business Media; 2011..

A JEM 2100 transmission electron microscope (Jeol, Japan) was utilized with an acceleration voltage of 200 kV to estimate the crystallite size of nanoparticles. The powder was diluted with isopropyl alcohol and dispersed in an ultrasonic bath for 5 min. The resulting solution was then dropped onto a copper mesh coated with an amorphous carbon film. Conversely, for microparticles, a JSM-6700F scanning electron microscope (Jeol, Japan) was employed.

In addition, dynamic light scattering analyses were conducted using a NanoBrook Omni particle analyzer equipped with a solid-state He-Ne laser at 633 nm and scattering angles of 15 and 25°. To achieve a concentration of 0.1 g L-1, the samples were appropriately diluted and adjusted to a pH of 6.9 before analysis. All measurements were taken at consistent intervals. Following dispersion in an ultrasonic bath for 10 s, a period of 100 s was allowed for the system to reach equilibrium.

Specific surface areas of the samples was determined by measuring them at a temperature of liquid nitrogen (77 K) using a Sorbtometr-M (Katakon, Russia). The surface area measurements were based on adsorption isotherms and were calculated using the well-established Brunauer-Emmett-Teller (BET) method, following a standard procedure (Neaman et al., 2003Neaman A, Pelletier M, Villieras F. The effects of exchanged cation, compression, heating and hydration on textural properties of bulk bentonite and its corresponding purified montmorillonite. Appl Clay Sci. 2003;22:153-68. https://doi.org/10.1016/s0169-1317(02)00146-1
https://doi.org/10.1016/s0169-1317(02)00...
).

Experimental field conditions

In this study, forest litter was chosen as the experimental substrate over mineral soil, as metal concentrations in the litter in the study area are one order of magnitude higher than in mineral soil (Prudnikova et al., 2020Prudnikova EV, Neaman A, Terekhova VA, Karpukhin MM, Vorobeichik EL, Smorkalov IA, Dovletyarova EA, Navarro-Villarroel C, Ginocchio R, Peñaloza P. Root elongation method for the quality assessment of metal-polluted soils: Whole soil or soil-water extract? J Soil Sci Plant Nutr. 2020;20:2294-303. https://doi.org/10.1007/s42729-020-00295-x
https://doi.org/10.1007/s42729-020-00295...
). This results in higher toxicity in the litter, which inhibits the development of herbaceous vegetation in the study area (Vorobeichik et al., 2014Vorobeichik EL, Trubina MR, Khantemirova EV, Bergman IE. Long-term dynamic of forest vegetation after reduction of copper smelter emissions. Russ J Ecol. 2014;45:498-507. https://doi.org/10.1134/s1067413614060150
https://doi.org/10.1134/s106741361406015...
).

Soil pH impacts metal immobilization by iron oxides (O’Reilly and Hochella, 2003O'Reilly SE, Hochella MF. Lead sorption efficiencies of natural and synthetic Mn and Fe-oxides. Geochim Cosmochim Ac. 2003;67:4471-87. https://doi.org/10.1016/s0016-7037(03)00413-7
https://doi.org/10.1016/s0016-7037(03)00...
; Tiberg et al., 2016Tiberg C, Kumpiene J, Gustafsson JP, Marsz A, Persson I, Mench M, Kleja DB. Immobilization of Cu and As in two contaminated soils with zero-valent iron - Long-term performance and mechanisms. Appl Geochem. 2016;67:144-52. https://doi.org/10.1016/j.apgeochem.2016.02.009
https://doi.org/10.1016/j.apgeochem.2016...
). Changes in the surface chemistry of iron oxides are pH-dependent (Kosmulski, 2001Kosmulski M. Chemical properties of material surfaces. New York: Marcel Dekker; 2001.), with higher pH values resulting in a higher negative surface charge. However, at higher pH values, metals may precipitate as insoluble compounds, masking the effects of iron oxides. In preliminary laboratory experiments using soils from the study area, the effect of zerovalent iron microparticles was not evident in limed soils (Dovletyarova et al., 2022aDovletyarova EA, Fareeva OS, Zhikharev AP, Brykova RA, Brykov VA, Vorobeichik EL, Slukovskaya MV, Vítková M, Ettler V, Yáñez C, Neaman A. Choose your amendment wisely: Zero-valent iron nanoparticles offered no advantage over microparticles in a laboratory study on metal immobilization in a contaminated soil. Appl Geochem. 2022a:143:105369. https://doi.org/10.1016/j.apgeochem.2022.105369
https://doi.org/10.1016/j.apgeochem.2022...
). Therefore, lime was deliberately not added to the litter in the present study to isolate the effects of iron oxides on metal immobilization.

Experimental design

The experiment was conducted in a native spruce-fir forest, located approximately 3 km from the copper smelter (56° 49’ 49.9” N, 59° 52’ 02.9” E). The soils in the study area were classified as silt loam Retisols (IUSS Working Group WRB, 2015IUSS Working Group WRB. World reference base for soil resources 2014, update 2015: International soil classification system for naming soils and creating legends for soil maps. Rome: Food and Agriculture Organization of the United Nations; 2015. (World Soil Resources Reports, 106).). Fifteen 0.50 × 0.50 m test plots were allocated on June 22-23, 2022, with three treatments (control, magnetite-micro, magnetite-nano) distributed across five blocks. The plots were spaced 1-3 m apart within each block and located no closer than 1 m from large trees (>0.30 m in diameter). The blocks were separated by distances of 3-5 m. The plots were established in a relatively uniform forest patch, free from dense vegetation or visible signs of soil disturbance, and with a mean litter thickness of 5.2±0.9 cm (mean ± standard deviation, n = 15). The mean dry weight of the litter was 1.05 ± 0.22 kg plot-1, and the mean bulk density was 83 ± 20 kg m-3.

A composite litter sample weighing approximately 50 g (fresh weight) was collected from each plot. The sample was air-dried at room temperature for approximately three days and subsequently ground for chemical analysis. Carbon content of the litter was determined to be 38 ± 2.6 % using the 2100 CN analyzer (Analytik Jena).

To implement the treatment, the litter layer was removed from each plot, and cones, large branches, and roots were extracted. The litter was mixed thoroughly to ensure homogeneity and a portion of the litter was collected for chemical analysis. Additionally, a 6 mm thick polypropylene mesh was positioned over the mineral soil horizon to ensure that only the litter was sampled at the end of the growing season, with no inclusion of the mineral soil horizon.

The treatments used in the experiment were prepared as follows:

  1. Control treatment consisted of 300 mL distilled water and 0.8 g fertilizer.

  2. Magnetite micro treatment consisted of 40 g magnetite powder, 300 mL distilled water, and 0.8 g fertilizer.

  3. Magnetite nano treatment consisted of 160 g magnetite paste (equivalent to 40 g magnetite powder since paste contained 25 % magnetite nanoparticles), 180 mL distilled water (equivalent to 300 mL of water since paste contained 75 % water), and 0.8 g fertilizer.

Magnetite nanoparticles and microparticles were added to the forest litter at an equal dose of 4 % w/w, considering the mean dry weight of the litter (1.05 ± 0.22 kg plot-1). Furthermore, equal amounts of water were added to all the treatments. Fertilizer composition for all treatments comprised N (12 %), P2O5 (8 %), K2O (14 %), MgO (2 %), S (8 %), B (0.1 %), Cu (0.1 %), Fe (0.1 %), Mn (0.2 %), Mo (0.01 %), and Zn (0.1 %). To avoid cross-contamination, separate containers were utilized for each treatment, and the litter was hand-mixed with different gloves for each sample before returning it to the plot, where it was placed on top of the mesh.

On June 28, five days after the treatments were applied, 3 g of red fescue (Festuca rubra L.) seeds were sown per plot. This species was chosen for the experiment as it is a common native species in the study area (Rafikova and Veselkin, 2022Rafikova OS, Veselkin DV. Leaf water extracts from invasive Acer negundo do not inhibit seed germination more than leaf extracts from native species. Manage Biol Invasions. 2022;13:705-23. https://doi.org/10.3391/mbi.2022.13.4.08
https://doi.org/10.3391/mbi.2022.13.4.08...
) and is frequently used for remediating metal-contaminated soils (Winterhalder, 1996Winterhalder K. Environmental degradation and rehabilitation of the landscape around Sudbury, a major mining and smelting area. Environ Rev. 1996;4:185-224. https://doi.org/10.1139/a96-011
https://doi.org/10.1139/a96-011...
; Slukovskaya et al., 2019Slukovskaya MV, Vasenev VI, Ivashchenko KV, Morev DV, Drogobuzhskaya SV, Ivanova LA, Kremenetskaya IP. Technosols on mining wastes in the subarctic: Efficiency of remediation under Cu-Ni atmospheric pollution. Int Soil Water Conserv Res. 2019;7:297-307. https://doi.org/10.1016/j.iswcr.2019.04.002
https://doi.org/10.1016/j.iswcr.2019.04....
).

Laboratory analysis indicated a high seed germination rate of 92 %. However, due to dry weather conditions, each plot was irrigated with 2 L of water on August 16 and 23, 2022. On September 1-2, 2022, composite litter samples (~50 g fresh weight) were also collected from each plot. The entire aboveground biomass of fescue was collected from each plot using scissors, which corresponded to a growth duration of approximately two months. Notably, the duration of this experiment, which spanned two months, is consistent with that of a previous laboratory study conducted by Danila et al. (2020)Danila V, Kumpiene J, Kasiuliene A, Vasarevicius S. Immobilisation of metal(loid)s in two contaminated soils using micro and nano zerovalent iron particles: Evaluating the long-term stability. Chemosphere. 2020;248:126054. https://doi.org/10.1016/j.chemosphere.2020.126054
https://doi.org/10.1016/j.chemosphere.20...
.

Laboratory studies and statistical analysis

Total metal concentrations in the pre-treatment litter samples were determined by atomic absorption spectroscopy using the ContrAA 700 instrument from Analytik Jena. Prior to the analysis, a microwave digestion process was carried out with concentrated HNO3 in accordance with the manufacturer’s instructions for the Berghof microwave system. Standard reference materials, including CRM-482 lichen and CRM-100 beech leaves obtained from the Community Bureau of Reference (BCR) of the Commission of the European Communities, were used throughout the analysis. The experimental values for the target metals were within 100 ± 20 % of the certified values. The litter pH was determined for both pre-treatment and end-of-season samples using a KNO3 0.01 mol L-1 solution with a litter/solution ratio of 1/25.

In recent decades, numerous studies have attempted to predict the “phytoavailable” metal fraction by correlating plant responses with various soil metal pools. It is generally believed that metal soluble fractions extracted by chemically non-aggressive neutral salts are useful in assessing metal phytotoxicity in contaminated soils (Kabata-Pendias, 2004Kabata-Pendias A. Soil-plant transfer of trace elements - An environmental issue. Geoderma. 2004;122:143-9. https://doi.org/10.1016/j.geoderma.2004.01.004
https://doi.org/10.1016/j.geoderma.2004....
; McBride et al., 2009McBride MB, Pitiranggon M, Kim B. A comparison of tests for extractable copper and zinc in metal-spiked and field-contaminated soil. Soil Sci. 2009;174:439-44. https://doi.org/10.1097/SS.0b013e3181b66856
https://doi.org/10.1097/SS.0b013e3181b66...
). Our recent study with soils near a copper smelter in central Chile (Lillo-Robles et al., 2020Lillo-Robles F, Tapia-Gatica J, Díaz-Siefer P, Moya H, Celis-Diez JL, Santa Cruz J, Ginocchio R, Sauvé S, Brykov VA, Neaman A. Which soil Cu pool governs phytotoxicity in field-collected soils contaminated by copper smelting activities in central Chile? Chemosphere. 2020;242:125176. https://doi.org/10.1016/j.chemosphere.2019.125176
https://doi.org/10.1016/j.chemosphere.20...
) suggests that the exchangeable copper fraction was the best indicator of metal phytotoxicity in the soil. Therefore, we determined the exchangeable forms of metals in the end-of-season litter samples using a 0.01 mol L-1 KNO3 extract at a litter/solution ratio of 1/20. Atomic absorption spectroscopy was employed for metal determinations.

While 0.01 mol L-1 CaCl2 is often considered the most appropriate agent for extracting exchangeable metals from metal-polluted soils, it should be noted that divalent cations, such as Ca2+, can promote the flocculation of dissolved organic carbon in the soil solution (Sauvé, 2002Sauvé S. Speciation of metals in soils. In: HE Allen, editor. Book Ttle Pensacola, Florida. Society of Environmental Toxicology and Chemistry; 2002. p. 7-37.). This mechanism may lead to underestimating the exchangeable metal fraction when using 0.01 mol L-1 CaCl2, as metals with a strong affinity for dissolved organic carbon can precipitate out (Neaman et al., 2009Neaman A, Reyes L, Trolard F, Bourrie G, Sauvé S. Copper mobility in contaminated soils of the Puchuncavi valley, central Chile. Geoderma. 2009;150:359-66. https://doi.org/10.1016/j.geoderma.2009.02.017
https://doi.org/10.1016/j.geoderma.2009....
). For this reason, we prefer to use 0.01 mol L-1 KNO3 for exchangeable metal extraction, which is also widely used for exchangeable metal extraction from metal-contaminated soils (Almas et al., 2000Almas AR, McBride MB, Singh BR. Solubility and lability of cadmium and zinc in two soils treated with organic matter. Soil Sci. 2000;165:250-9. https://doi.org/10.1097/00010694-200003000-00007
https://doi.org/10.1097/00010694-2000030...
; Luo et al., 2006Luo X-S, Zhou D-M, Liu X-H, Wang Y-J. Solid/solution partitioning and activity of heavy metals in the contaminated agricultural soils around a copper mine in eastern Nanjing city, China. J Hazard Mater. 2006;A131:19-27. https://doi.org/10.1016/j.jhazmat.2005.09.033
https://doi.org/10.1016/j.jhazmat.2005.0...
; Moreno-Caselles et al., 2000Moreno-Caselles J, Moral R, Perez-Espinosa A, Perez-Murcia MD. Cadmium accumulation and distribution in cucumber plant. J Plant Nutr. 2000;23:243-50. https://doi.org/10.1080/01904160009382011
https://doi.org/10.1080/0190416000938201...
; Perez-Esteban et al., 2013Perez-Esteban J, Escolastico C, Moliner A, Masaguer A. Chemical speciation and mobilization of copper and zinc in naturally contaminated mine soils with citric and tartaric acids. Chemosphere. 2013;90:276-83. https://doi.org/10.1016/j.chemosphere.2012.06.065
https://doi.org/10.1016/j.chemosphere.20...
).

All aboveground fescue biomass was meticulously washed using a sequential process comprising tap water, 0.1 mol L-1 HCl, distilled water, 0.05 mol L-1 EDTA, and distilled water (twice). Subsequently, the fescue biomass was dried at 70 °C and weighed using an analytical balance with an accuracy of 0.0001 g. The metal concentrations in the fescue samples were determined as previously described for the litter samples.

Statistical analyses were performed using R, version 4.1.2. The statistical unit was the sample area (plot) in all cases. Prior to analysis, metal contents were log-transformed. A one-way analysis of variance was conducted, and differences among treatments were assessed using Tukey’s test with the multcomp package (Hothorn et al., 2008Hothorn T, Bretz F, Westfall P. Simultaneous inference in general parametric models. Biometrical J. 2008;50:346-63. https://doi.org/10.1002/bimj.200810425
https://doi.org/10.1002/bimj.200810425...
).

RESULTS AND DISCUSSION

Magnetite characterization

The data obtained from Mössbauer spectroscopy revealed that the samples of magnetite nano- and microparticles were composed solely of Fe3O4. Similarly, X-ray diffraction analysis confirmed the presence of only the magnetite phase, Fe3O4, in both samples of magnetite nano- and microparticles. In addition, Mössbauer spectroscopy data indicated that the magnetic domain size was smaller in the nanoparticles than in the microparticles. Furthermore, the diffraction maxima of the X-ray diffraction pattern of the nano sample were notably broader, indicating that the size of Fe3O4 crystallites in the nano sample was smaller than that in the micro-sample.

There is no universally accepted method for determining the particle size of a material (Pankratov and Anuchina, 2019Pankratov DA, Anuchina MM. Nature-inspired synthesis of magnetic non-stoichiometric Fe3O4 nanoparticles by oxidative in situ method in a humic medium. Mater Chem Phys. 2019;231:216-24. https://doi.org/10.1016/j.matchemphys.2019.04.022
https://doi.org/10.1016/j.matchemphys.20...
). However, the use of various techniques yielded comparable results (Table 1). Specifically, the sizes of the magnetic domain, coherent scattering domain, and crystallites were all on the same order of magnitude for the nano sample (Table 1). While the hydrodynamic diameter of the nano sample was an order of magnitude larger, this is typical for magnetite nanoparticles due to aggregation (Bondarenko et al., 2020Bondarenko LS, Kovel ES, Kydralieva KA, Dzhardimalieva GI, Illes E, Tombacz E, Kicheeva AG, Kudryasheva NS. Effects of modified magnetite nanoparticles on bacterial cells and enzyme reactions. Nanomaterials. 2020;10:1499. https://doi.org/10.3390/nano10081499
https://doi.org/10.3390/nano10081499...
). Similarly, the crystallite size and hydrodynamic diameter of the micro-sample were in agreement with each other. It is worth noting that each microparticle contains multiple magnetic and coherent scattering domains (Liu et al., 2010Liu XM, Shaw J, Jiang JZ, Bloemendal J, Hesse P, Rolph T, Mao XG. Analysis on variety and characteristics of maghemite. Sci China Earth Sci. 2010;53:1153-62. https://doi.org/10.1007/s11430-010-0030-2
https://doi.org/10.1007/s11430-010-0030-...
; Waseda et al., 2011Waseda Y, Matsubara E, Shinoda K. X-ray diffraction crystallography: Introduction, examples and solved problems. Berlin: Springer Science & Business Media; 2011.). Nevertheless, all of the methods employed consistently indicated a significantly smaller size for the studied nanoparticles than the microparticles. Correspondingly, the BET surface area was found to be greater for the nanoparticles in comparison to the microparticles (Table 1).

Table 1
Characteristics of magnetite nano- and microparticle samples

Litter characterization

The litter collected from the study plots (Table 2) exhibited total metal contents that were 1-2 orders of magnitude greater than the background metal contents in litter obtained from unpolluted regions located roughly 30 km away from the copper smelter (Prudnikova et al., 2020Prudnikova EV, Neaman A, Terekhova VA, Karpukhin MM, Vorobeichik EL, Smorkalov IA, Dovletyarova EA, Navarro-Villarroel C, Ginocchio R, Peñaloza P. Root elongation method for the quality assessment of metal-polluted soils: Whole soil or soil-water extract? J Soil Sci Plant Nutr. 2020;20:2294-303. https://doi.org/10.1007/s42729-020-00295-x
https://doi.org/10.1007/s42729-020-00295...
). However, such elevated metal contents are typical in soils near metal smelters (Vorobeichik, 2022Vorobeichik EL. Natural recovery of terrestrial ecosystems after the cessation of industrial pollution: 1. A state-of-the-art review. Russ J Ecol. 2022;53:1-39. https://doi.org/10.1134/s1067413622010118
https://doi.org/10.1134/s106741362201011...
).

Table 2
Mean and standard deviation (n = 5) of total metal content and pH in litter samples before amendment application across different treatments. No statistically significant differences were found among the treatments. Additionally, the mean and standard deviation (n = 5) of litter pH at the end of the growing season are presented, and no statistically significant differences were observed among the treatment

Each plot had a high degree of variability in total metal concentrations, which is customary for the area under investigation (Vorobeichik and Pozolotina, 2003Vorobeichik EL, Pozolotina VN. Microscale spatial variation in forest litter phytotoxicity. Russ J Ecol. 2003;34:381-8. https://doi.org/10.1023/a:1027308400182
https://doi.org/10.1023/a:1027308400182...
). Nonetheless, it should be emphasized that no statistically significant differences were found in total metal contents or in the initial pH of the litter (i.e., before the application of the amendment) in the plots designated for different treatments (Table 2). This implies that differences in litter properties in the various plots did not impact plant responses.

Effects of amendments

Application of the amendment did not alter the pH of the litter (Table 2). Nonetheless, compared to the control (Table 3), the amendment of magnetite nanoparticles led to a reduction in the exchangeable copper concentrations in the litter, typically regarded as the most bioavailable (Lillo-Robles et al., 2020Lillo-Robles F, Tapia-Gatica J, Díaz-Siefer P, Moya H, Celis-Diez JL, Santa Cruz J, Ginocchio R, Sauvé S, Brykov VA, Neaman A. Which soil Cu pool governs phytotoxicity in field-collected soils contaminated by copper smelting activities in central Chile? Chemosphere. 2020;242:125176. https://doi.org/10.1016/j.chemosphere.2019.125176
https://doi.org/10.1016/j.chemosphere.20...
). The findings of Tiberg et al. (2016)Tiberg C, Kumpiene J, Gustafsson JP, Marsz A, Persson I, Mench M, Kleja DB. Immobilization of Cu and As in two contaminated soils with zero-valent iron - Long-term performance and mechanisms. Appl Geochem. 2016;67:144-52. https://doi.org/10.1016/j.apgeochem.2016.02.009
https://doi.org/10.1016/j.apgeochem.2016...
suggest that in the soil treated with magnetite, copper was sequestered via bidentate inner-sphere complexes with magnetite. However, further research is warranted to decipher the precise binding mechanisms of copper by magnetite in the context of this field experiment.

Table 3
Mean and standard deviation (n = 5) of exchangeable metal forms in litter samples and metal content in plant biomass across different treatments. Different letters for exchangeable copper indicate significant differences among the treatments (Tukey test, p≤0.05). No statistically significant differences were found among the treatments for all other cases

Due to copper immobilization in the litter, the dry biomass of fescue shoots exhibited a significant increase in plots treated with magnetite nanoparticles compared to the control (Figure 1). Although there was only a marginally significant difference in copper contents in fescue shoots between the plots treated with magnetite nanoparticles and the control (Table 3; p=0.052), the trend indicated a reduction in metal phytoavailability in the litter due to the use of nanoparticles.

Figure 1
Dry shoot biomass of red fescue (shoot DW) under different treatments. 0: control; 1: magnetite microparticles; 2: magnetite nanoparticles. Box plots show the lower, median, and upper quartile, with whiskers indicating the most extreme data points and dots indicating outliers. Significant differences among treatments (Tukey test, p≤0.05) are denoted by different letters.

In contrast, applying magnetite microparticles did not elicit any statistically significant effect on the concentrations of exchangeable metals in the litter or the metal concentrations in fescue shoots (Table 3). Of significant note, fescue growth was not improved with the application of magnetite microparticles compared to the control. Our finding is in line with previous laboratory results demonstrating superior metal adsorption properties of magnetite nanoparticles relative to microparticles (Ajmal et al., 2020Ajmal Z, Usman M, Anastopoulos I, Qadeer A, Zhu RL, Wakeel A, Dong RJ. Use of nano-/micro-magnetite for abatement of cadmium and lead contamination. J Environ Manage. 2020;264:110477. https://doi.org/10.1016/j.jenvman.2020.110477
https://doi.org/10.1016/j.jenvman.2020.1...
).

Restoring vegetation cover in metal-polluted areas, as demonstrated in this study, has several practical benefits, such as reducing soil erosion (Bienes et al., 2016Bienes R, Marques MJ, Sastre B, García-Díaz A, Ruiz-Colmenero M. Eleven years after shrub revegetation in semiarid eroded soils. Influence in soil properties. Geoderma. 2016;273:106-14. https://doi.org/10.1016/j.geoderma.2016.03.023
https://doi.org/10.1016/j.geoderma.2016....
), improving air quality (Terzaghi et al., 2017Terzaghi E, Morselli M, Semplice M, Cerabolini BEL, Jones KC, Freppaz M, Di Guardo A. SoilPlusVeg: An integrated air-plant-litter-soil model to predict organic chemical fate and recycling in forests. Sci Total Environ. 2017;595:169-77. https://doi.org/10.1016/j.scitotenv.2017.03.252
https://doi.org/10.1016/j.scitotenv.2017...
), minimizing metals transport (Tordoff et al., 2000Tordoff GM, Baker AJM, Willis AJ. Current approaches to the revegetation and reclamation of metalliferous mine wastes. Chemosphere. 2000;41:219-28. https://doi.org/10.1016/S0045-6535(99)00414-2
https://doi.org/10.1016/S0045-6535(99)00...
), decreasing the impact of metal-rich dust on human health (Bierkens et al., 2011Bierkens J, van Holderbeke M, Cornelis C, Torfs R. Exposure through soil and dust ingestion. In: Swartjes FA, editor. Dealing with contaminated sites. Dordrecht: Springer; 2011. p. 261-86.), and enhancing the aesthetic value of the land (Ivanova et al., 2020Ivanova L, Slukovskaya M, Kremenetskaya I, Alekseeva S, Neaman A. Ornamental plant cultivation using vermiculite-lizardite mining waste in the industrial zone of the Subarctic. In: Vasenev V, Dovletyarova E, Cheng Z, Valentini R, Calfapietra C, editors. Green technologies and infrastructure to enhance urban ecosystem services. Switzerland: Springer; 2020. p. 199-204.).

CONCLUSIONS

Magnetite nanoparticles performed better than microparticles in remediating metal-contaminated soils. Specifically, the application of magnetite nanoparticles resulted in a significant reduction in soil exchangeable copper concentrations and enhanced the growth of red fescue in polluted soils. Conversely, the application of magnetite microparticles did not produce any statistically significant effects. These findings align with previous laboratory studies demonstrating that magnetite nanoparticles possess superior metal adsorption properties compared to their micro-sized counterparts.

However, despite the growing interest in using iron nanoparticles to remediate metal-contaminated soils, much of the available knowledge is derived from laboratory experiments. This study, which is the first to compare the effectiveness of magnetite nanoparticles with microparticles in field conditions, contributes to filling this knowledge gap. Nevertheless, the study is limited by its short duration and drought conditions that affected plant growth. To address these limitations, longer field studies and ongoing monitoring of the effects of iron particles on plant growth in different weather conditions are recommended.

  • 1
    For brevity, metalloids (e.g., arsenic) will be referred to as “metals” in the following discussion.
  • 2
    For brevity, oxyhydroxides (e.g., goethite) will be referred to as “oxides” in the following discussion.

ACKNOWLEDGEMENTS

This study was supported by the Russian Foundation for Basic Research (grant 20-54-26012, fieldwork and analysis, manuscript writing), the FONDECT 1200048 project (granted to Alexander Neaman, manuscript writing), and the RUDN University Strategic Academic Leadership Program (granted to Elvira A. Dovletyarova, fieldwork, and analysis, manuscript writing). D.A. Pankratov’s work was funded by the government allocation project 122040600057-3 (awarded to Lomonosov Moscow State University). The research team acknowledges Kamila A. Kydralieva for magnetite synthesis and Elmira Akhunova for chemical analyses. The authors would like to express their utmost gratitude to Andrei Tchourakov, of Vancouver, BC, Canada, academic editor and translator, for the English editing of this paper.

  • How to cite: Smorkalov IA, Vorobeichik EL, Dzeranov AA, Pankratov DA, Dovletyarova EA, Yáñez C, Neaman A. Field experiment pitting magnetite nanoparticles against microparticles: Effect of size in the rehabilitation of metalcontaminated soil. Rev Bras Cienc Solo. 2023;47:e0230017 https://doi.org/10.36783/18069657rbcs20230017

APPENDIX A. SUPPLEMENTARY DATA

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Edited by

Editors: José Miguel Reichert https://orcid.org/0000-0001-9943-2898and Yuri Jacques Agra Bezerra daSilva https://orcid.org/0000-0001-6865-7146.

Publication Dates

  • Publication in this collection
    20 Oct 2023
  • Date of issue
    2023

History

  • Received
    28 Feb 2023
  • Accepted
    07 June 2023
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