1 Introduction

Thallium (Tl) is a hypertoxic heavy element that exhibits extreme toxicity to humans, surpassing the toxicities of many well-known heavy metals, such as lead (Pb) and cadmium (Cd) (David et al. 2019; Genchi et al. 2021; Jiang et al. 2022; Liu et al. 2020; Vaněk et al. 2022). In general, Tl-containing materials can be rapidly absorbed via inhalation, ingestion, as well as dermal contact (Deng et al. 2024). Thallium poisoning, also known as thallotoxicosis, can cause severe destruction to the human body, such as rapid hair loss and even baldness (Adeel et al. 2023; Álvarez-Barrera et al. 2019; Beiyuan et al. 2020; Ma et al. 2020). Acute Tl poisoning may result in coma, paralysis, and, in the gravest instances, fatality (Campanella et al. 2019; Genchi et al. 2021; Liu et al. 2022a, b). In addition, mutagenesis, carcinogenesis and teratogenic effects are also associated with Tl poisoning (Tong et al. 2022). The fatal dose of soluble Tl to human beings is exceptionally low (merely 10–15 mg kg−1), and thus it has been listed as a priority pollutant by numerous agencies such as US Environmental Protection Agency (US EPA 2021; Liu et al. 2023; Zhong et al. 2022). Meanwhile, arsenic (As) and vanadium (V) in the environment have also gained widespread attention due to their acute toxicity and non-biodegradability (Firas et al. 2014; Sharfalddin et al. 2022). On the one hand, ingestion of As is associated with increased risks of cancer in the lung, liver, skin, bladder and kidneys (Jaafarzadeh et al. 2023). Recent studies also suggest a potential association between As exposure and cardiovascular diseases as well as diabetes (Ghosh and Sil 2023). For the relatively less studied V, excessive exposure to this element can lead to active lesions in the kidneys, livers, spleen, bones and nervous system, with emerging evidence that V may cause respiratory problems and developmental toxicity in humans (Genchi et al. 2021; Peng et al. 2022; Zhu et al. 2023; Wang et al. 2023).

In recent years, numerous anthropogenic activities including chemical, pharmaceutical, electronic, optical and aerospace industries have contributed to the escalation of Tl, As and V in agricultural soils, which far exceeded the self-purification capacity of soil (Liu et al. 2018; Morosini et al. 2023; Rader et al. 2019; Vaněk et al. 2018; Zhao et al. 2020). Besides, accumulation of pollutants in agricultural soils continues to escalate in numerous developing countries, leading to pronounced Tl-As-V complex contamination with associated detrimental health implications (Albanese et al. 2023; Wang et al. 2020, 2021). Consequently, the development of strategies for the remediation of soils contaminated with these intricate metal(loid)s is of immediate concern.

Biochar (BC), as an environmentally friendly immobilizer, is of great significance among chemical immobilization methods, which currently has emerged as a research hotspot in the remediation of soils contaminated by toxic metal(loid)s (Liu et al. 2022a, b; Wu et al. 2020; Chen et al. 2022). Biochar can improve soil physicochemical properties, including soil organic matter content, cation exchange capacity, mineral composition, among others, through alterations in soil pH and the facilitation of processes such as ion exchange, surface adsorption and precipitation (Bolan et al. 2022; Rees et al. 2014). These mechanisms, in turn, exert influence on the behavior of metal(loid)s within the soil environment. Furthermore, the highly porous structure of biochar contributes to adsorption of metal(loid)s (Li et al. 2017). The abundant electron-rich functional groups on biochar surface, such as hydroxyl, carboxyl and phenols, can interact with metal(loid)s to form complexes or chelates (Gomez-Eyles et al. 2013; He et al. 2019; Luo et al. 2020), significantly reducing the bioavailability of metal(loid)s in soils. However, the ability of conventional biochar to immobilize toxic metal(loid)s is limited. To further improve the adsorption/immobilization for toxic metal(loid)s of biochar, various substances (e.g., Fe, Mn and their compounds) have been widely added to modify biochar (Beiyuan et al. 2023; Qin et al. 2022; Yang et al. 2019; Song et al. 2022). As reported, Tl adsorption capacity of MnFe2O4-modified biochar in aqueous solution could reach 170.55 mg g−1 (Liu et al. 2021). Modified biochar composites can also successfully reduce As mobility and V mobility in soils, as indicated by previous studies (Beiyuan et al. 2023; Lin et al. 2017). Even though pertinent research has shown that modified biochar exhibits superior effectiveness in the remediation or removal of Tl and other metal(loid)s compared to pristine biochar, there still remains a notable gap in our understanding regarding the remediation of complex Tl-As-V contamination in agricultural soils.

The specific objectives of this work are delineated as follows: (1) to examine the effect of BC and FMBC on the passivation efficiency in terms of adsorption capacity to Tl/As/V at different application dosages, and (2) to investigate the mechanism of passivation effects in contaminated soils through comprehensive characterization means including Fourier Transform Infrared Spectrometer (FTIR), Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM–EDS), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), and Transmission Electron Microscopy-Energy Dispersive Spectroscopy (TEM-EDS).

2 Materials and methods

2.1 Soil properties

Soil samples were collected from the contaminated farmland near the Tl mineralization region in southwestern Guizhou, China in this study. The mining area is characterized by sedimentary rocks dating back to the Permian and Triassic epochs, which are overlaid by Quaternary alluvium. Consequently, it contains elevated concentrations of As, Tl and V. After sampling, soil samples were grinded through a 10-mesh sieve and were subsequently kept in polyethylene-sealed bags at 4 °C after air drying.

2.2 Preparation of the Fe–Mn modified biochar composites (FMBC)

The FMBC was prepared according to a co-precipitation method, which was modified according to Liu et al. (2021). In brief, 2.0 g of BC was firstly dispersed in 100 ml of 0.4 M FeSO4·7H2O solution. The mixture was then added into 100 ml of 0.2 M KMnO4 solution. The pH of the mixture was adjusted to 10.0 by NaOH and the mixture was stirred continuously at 25 ℃ for 3 h. The precipitate was collected and washed with ethanol and deionized water. Lastly, the solid product was dried in a vacuum oven at 60 °C for 24 h.

2.3 Soil amendment experiment

Four dosages of 5, 10, 15 and 20 g kg−1 of BC and FMBC were added into the soil and the treated groups, respectively, and labeled as BC-5, BC-10, BC-15, BC-20, FMBC-5, FMBC-10, FMBC-15 and FMBC-20, respectively. 100 g of each soil sample was sieved through a 200-mesh sieve and subsequently combined with the passivation material in a clean plastic jar. Soil samples from each treated group were maintained at 60% of their field capacity for water retention by adding deionized water as necessary. Appropriate soil samples were dried for subsequent analysis at intervals of 15, 30, 60 and 90 days.

2.4 Extraction of heavy metals in bioavailable states

0.5 g of soil was placed in a centrifuge tube, with 10 ml of 0.5 mol L−1 HCl added. After shaking at 250 r min−1 for 1 h and centrifuging at 4500 rpm for 30 min, the supernatant was collected. The process, involving the addition of 10 ml of deionized water, shaking, and centrifuging, was repeated to combine the supernatants, resulting in a total of 40 ml of the extract. This was then concentrated by heating at 80 °C until it was completely dry, redissolved in a 1:1 HNO3 solution adjusted with 2% HNO3 to a total volume of 10 ml, filtered through a 0.45 μm membrane, and analyzed with ICP-OES (Optima 8000, PerkinElmer, USA).

Passivation efficiency was calculated as follows:

$$Passivation\, ratio = \frac{{C_{{0}} { - }C_{{\text{n}}} }}{{C_{{0}} }} \,$$
(1)

wherein C0 represents the available content of heavy metals in the soil of the control group (mg kg−1); Cn represents the available content of heavy metals in the soil of the test group (mg kg−1).

2.5 Characterizations

For the characterization of FMBC, Fourier Transform Infrared Spectrometer (FTIR, Bruker ALPHA) was used for analyzing surface functional groups. The specific surface area and pore size distribution of the materials were determined by Brunauer–Emmett–Teller (BET, ASAP 2460). Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM–EDS) was used for morphological analysis by TESCAN-MIRA-LMS instrument (Czech Republic) equipped with Oxford X-ray EDS. The surface composition and valence state of the composites were evaluated by X-ray Photoelectron Spectroscopy (XPS, Thermo Scientific K-Alpha, USA) and then analyzed by Avantage software. For the characterization of the studied soil samples, X-ray Diffraction (XRD, Ultima IV, Japanese Science) was applied to investigate the crystal structure, and FEI Talos-F200S Transmission Electron Microscopy-Energy Dispersive Spectroscopy (TEM-EDS) performed at 200 kV voltage was employed to investigate the mineral compositions and element distribution in the fine particles.

3 Results and discussion

3.1 Characterization of passivation materials

The raw BC had a relatively low specific surface area, only 4.0 m2 g−1 (Fig. S1). In comparison, FMBC had a higher surface area, 202.0 m2 g−1, which is 50.5 times larger than that of pristine BC. The increase of specific surface area could be ascribed to the formation of Mn/Fe oxides and activation of the modified FMBC surface. Compared with BC, FMBC displayed richer pore structure and larger specific surface area, which may produce a better adsorption performance during soil remediation.

As displayed in Fig. S2, many irregular small particles were found to be attached to the porous structure of the as-prepared materials. Fe, Mn, C and O were the major components confirmed by the EDS-mapping. TEM-EDS results indicated that Fe and Mn were distributed along the shape surface of the material (Fig. 1). Further analysis combined with FFT and HRTEM displayed that the fine particles were composed of magnetite (Fe3O4), maghemite (Fe2O3), braunite (Mn2O3) and hausmannite (Mn3O4).

Fig. 1
figure 1

The STEM-EDS mappings of FMBC material: a STEM image, b–e HRTEM image and FFT results (inset images) of the corresponding labeled areas in the STEM image of FMBC, (f1-f5) EDS mapping of Fe, Mn, C, O and Ca on FMBC

Full size image

Four peaks were identified for FMBC at 850.89, 1522.46, 1628.04, 3412.65 cm−1 by FTIR (Fig. S3), coinciding with the vibrations of C–H, –COOH, C=C and O–H (Yin et al. 2020; Zhang et al. 2020). Moreover, the characteristic peak of 518.33 cm−1 was attributed to metal–oxygen stretching (Fe–O and Mn–O) (Ahmed et al. 2016; Liu et al. 2021), which indicated a successful loading of Fe/Mn oxides on the biochar surface. Notably, a weak band was exhibited at 1063.41 cm−1, which may be associated with bending vibration of metal oxide hydroxyl (such as Fe-OH) (Zhang et al. 2005).

Three peaks were identified in deconvoluted Mn 2p spectra and each displayed a binding energy of 640.42 eV, 641.81 eV and 643.74 eV, corresponding to Mn(II), Mn(III) and Mn(IV), respectively (Fig. 2). The proportions of Mn(II), Mn(III) and Mn(IV) were 3.20%, 89.08% and 7.73%, respectively, and Mn(III) was the chief Mn species in FMBC. For the spectra of Fe 2p, two discernible peaks were observed, with binding energies at 710.24 eV and 713.28 eV, indicating the presence of Fe(II) and Fe(III), respectively. The main species of Fe in FMBC was Fe(II) in the proportion of 66.48%. In summary, FMBC composites contained high-valent Mn oxides with strong oxidizability, which can oxidize Tl(I) to Tl(III) and decrease Tl mobility, and Fe oxides loaded on surface possessed higher adsorption capacity (Chu et al. 2022).

Fig. 2
figure 2

The XPS spectra of FMBC composites: a Fe 2p; b Mn 2p

Full size image

3.2 Effect of passivated materials on soil Tl morphology

Compared with the original soils, the bioavailable Tl (Tlbio) contents increased significantly after incubation (in a range of 0.70 to 1.01 mg kg−1). This increase was primarily attributed to the water addition, which led to the dissolution of some minerals in the soil and consequently lowered the soil pH (Hsu et al. 2022). Under acidic conditions, Tl(III) is more prone to spontaneous conversion to Tl(I), which is very soluble (Yao et al. 2018).

As displayed in Fig. 3, Tlbio content was significantly reduced after adding passivation materials. The content of Tlbio in the soil passivated by BC material exhibited no obvious regularity in time gradient, while Tlbio content after passivation with different dosing levels of FMBC demonstrated a significant pattern in the time gradient, which was positively correlated with the dosing level. The lowest Tlbio content (0.123 mg kg−1) was achieved after 60 days of passivation with FMBC-20, with passivating rate of 83.9%. It suggests that FMBC can reduce the Tlbio content more efficiently than the pristine BC. The better passivation ability of FMBC can be explained by the fact that the different mineral components of the soil started to stabilize Tl after the addition of FMBC, which may facilitate the transformation of Tl geochemical fractions. For example, Tl in exchangeable fraction can be transformed into other more stable fractions (i.e., oxidizable, reducible and residual fractions).

Fig. 3
figure 3

Bioavailable Tl contents and passivation rate of Tl in the soil sample after passivation for: a 15 days; b 30 days; c 60 days; and d 90 days

Full size image

3.3 Effect of passivated materials on soil pH

Soil pH controls metal morphology, mineral surface metal transformation, and solubility, thus constituting one of the most pivotal factors influencing the bioavailability of the toxic metal(loid)s (Lan et al. 2021; Wei et al. 2023). The pH values increased in all groups with increasing passivation time, which was related to the alkalinity of the biochar (e.g., the dissolution of carbonates and hydroxides of the biochar, and the release of cations) (Virk et al. 2021). Almost all pH values of soil samples treated with FMBC composites were higher than those of BC-treated soils (Fig. 4). This may be induced by the depletion of acidic functional groups and the increase in alkaline functional groups of the modified biochar containing Fe and Mn (Lehmann et al. 2006). Geng et al. (2022) found that biochar incorporation into acidic soils elevated pH levels by 8.48−79.25%, attributed to the solubilization of alkaline metals in biochar such as calcium (Ca) and magnesium (Mg), which in turn increased soil pH. Both biochar and biochar-derived substances can contribute to the alkalization of soil, effectively reducing its acidity (Zhang et al. 2022). The elevation of soil pH led to more exchangeable sites on soil surface for metal adsorption, thereby facilitating metal fixation (Cheng et al. 2020). The pH value also played a significant role in the immobilization of Tl, as Tl(III) had a propensity to precipitate in alkaline environments and form stable complexes with halogen ions.

Fig. 4
figure 4

Variations of soil pH value after passivation for 15, 30, 60, and 90 days

Full size image

3.4 Effect of passivated materials on other toxic metal(loid)s in soils

Compared with the control group and BC passivated soil samples, the FMBC passivated soil samples exhibited an increment of Fe and Mn content, which increased with the dosage of FMBC (Fig. 5). Both passivation materials exerted elevating passivation effects on V and As when applied at increasing rates. It is worth noting that the FMBC composite exhibited greater passivation efficiency compared to pristine BC. FMBC-20 was identified as the most effective passivator, achieving the highest passivation rates of 71.92% for V and 71.09% for As.

Fig. 5
figure 5

Bioavailable contents of other studied elements: a As, b V, c Fe, and d Mn in soil samples after passivation

Full size image

As displayed in Fig. S4, a high negative correlation was observed between Tl, Fe and Mn. The application of FMBC increased the passivation of Tl, with the best performance by using FMBC-20. Positive correlations were also found between the dosage of FMBC and Tl, V and As, respectively. It has been reported that the Fe/Mn-modified biochar can immobilize As in soil matrixes (Lin et al. 2019). This observation can be elucidated by the similarity in the passivation mechanisms governing Tl and As, where Fe in FMBC mainly provided adsorption sites for the metals while Mn oxidized Tl(I) and As(III) to Tl(III) and As(V), respectively. On the other hand, the adsorption, complexation and coprecipitation of As by amorphous Fe(III) oxides and FeO(OH) resulted in the formation of Fe–As–O complexes, which passivated As (Zhang et al. 2023). The presence of Fe(II) in the material could reduce VO2+ to VO2+ and VO+, thereby weakening the mobility of V. In addition, VO43− can conjugate with Fe3+ to precipitate as (xFe2O3⋅yV2O5⋅zH2O), thereby increasing the passivation rate of V (Li et al. 2022).

3.5 Effect of passivated materials on mineral composition of soils

The mineral composition and structure of soil affect its physicochemical properties (Matichenkov et al. 2001). As shown in Table S1, cancrinite was found in the soil sample treated with FMBC-20 after 60 days. Previous studies have shown that cancrinite, a feldspar-like mineral with a high surface area and porous aluminum silicate framework, is often associated with metal minerals (e.g., Fe oxides and sulfides) (Reyes et al. 2013; Sadeghalvad et al. 2021). The formation of cancrinite may be related to Fe oxide-rich FMBC and calcite in the original soils. Due to the replacement of Si4+ by Al3+, it has a negatively charged framework that requires positively charged cations to pass through porous channels, which was found useful in adsorption of Cu and Zn (Selim et al. 2019). High cation exchange capacity of cancrinite facilitates the adsorption of metal cations, such as Tl(I), into the mineral framework during soil passivation, thereby reducing the migration ability of Tl (Wernert et al. 2020). The existence of avicennite (Tl2O3) was found in areas where Tl and Mn were dense in our study (Fig. 6). It has been reported that avicennite usually forms only under strongly oxidizing and alkaline conditions (Migaszewski et al. 2021). By the addition of Mn(IV) oxides, Tl(III) was stabilized in soil or as Tl2O3 in soils contaminated with Tl-containing metal sulfides (Rinklebe et al. 2020). Therefore, Tl(III) was strongly associated with the strong oxidizing properties of Mn, oxidizing highly bioavailable Tl(I) to Tl(III) and achieving a passivating effect. Raguinite (TlFeS2) was also found in Fe and Tl enriched areas, which may result from the displacement of Tl(I) into the mineral skeleton by adsorption of Fe-minerals, thus reducing Tl mobility (Lin et al. 2020).

Fig. 6
figure 6

The STEM-EDS mappings of soil samples passivated by FMBC-20 after 60 days: a STEM image, b, c HRTEM image and FFT results (inset images) of the corresponding labeled areas in the STEM image of soil samples, d1–d4 EDS mapping of Tl, Fe, As, and Mn on soil samples

Full size image

Moreover, significant amounts of calcium silicate hydrate were generated at the end of passivation. Calcium silicate hydrate is a widely used material for cement immobilization, which can immobilize heavy metals by adsorption, complexation and isomorphic replacement (Chen et al. 2020). It is noteworthy that gibbsite was found in the soil amended by FMBC-20 after 15, 60 and 90 days of passivation by adding mineral amendments. It is well known that gibbsite is an Fe/Al-bearing mineral, which originated from primary carbonate minerals and underwent a series of weathering hydrolysis to gibbsite (Mamedov et al. 2022). Gibbsite can adsorb As to form internal complexes to reduce the effective As content in the soils, while gibbsite may also exhibit some adsorption capacity for Tl (Chen et al. 2010). Therefore, the bioavailability of both Tl and As were reduced compared to the control group and the original soil.

3.6 Effect of passivated materials on valence state of Tl

The XPS analysis of soil samples treated with FMBC-20 revealed the characteristic peaks for Tl(III) (117.64 eV and 118.51 eV) and Tl(I) (119.02 eV and 118.92 eV), respectively (Fig. 7a and b). The proportion of Tl(III) at 0 and 60 days was 21.42% and 50.53%, respectively. The addition of FMBC significantly increased redox potential of soil samples (Lin et al. 2019), promoting the oxidation of Tl(I) to Tl(III), thereby enhancing the immobilization of Tl. Further analysis revealed that Fe–Mn(OH) oxide particles like pyrolusite (MnO2) and goethite (FeO(OH)) occurred in the soils after the addition of FMBC (Fig. 8). The negatively charged surface of Mn could possibly be responsible for the adsorption of Tl(I) (Cruz-Hernández et al. 2019; Liu et al. 2019). Various Mn (hydr)oxides can readily oxidize the adsorbed Tl(I) to Tl(III) through catalytic processes (Eqs. (2) and (3)) (Jiang et al. 2022). The presence of avicennite (Tl2O3) served as additional evidence confirming the oxidation of Tl(I) by Mn (hydr)oxides during the soil passivation process. Moreover, under neutral or alkaline soil pH conditions, Tl(III) tends to precipitate spontaneously as Tl(OH)3, thus reducing the Tl transport in soil (Voegelin et al. 2015). Nevertheless, with the elevation of the pH in soil, the charge of soil tends to be more negative. This implies a potential shift in the adsorption process, changing from adsorption/oxidation to the adsorption of Tl(I) ions (Eq. (4)) (Antić-Mladenović et al. 2017). This might be the underlying reason for the decrease of Tl(III) in the later stages of passivation. Additionally, during passivation processes, the newly formed Fe (hydr)oxides, specifically goethite (FeO(OH)), demonstrate notable efficacy in adsorbing Tl(III) within the soil matrix. The Fe(III) oxides present may be transformed into crystalline structures, such as goethite, through electron transfer or atomic substitution mechanisms (Eq. (5)) (Antić-Mladenović et al. 2017). These transformations facilitate immobilization of Tl via forming outer-sphere and/or inner-sphere complexes (Gomez-Gonzalez et al. 2015). This supposition was further substantiated by XPS analysis (Fig. 7c and d), which revealed a substantial presence of Fe(III) in the soil matrices.

$${\text{Mn}}^{{{2} + }} + {\text{2H}}_{{2}} {\text{O}} \rightleftharpoons {\text{MnO}}_{{2}} \left( {\text{s}} \right) + {\text{4H}}^{ + } + {\text{2e}}^{ - }$$
(2)
$${\text{MnO}}_{{2}} \left( {\text{s}} \right) + {\text{4H}}^{ + } + {\text{Tl}}^{ + } \rightleftharpoons {\text{Mn}}^{{{2} + }} + {\text{Tl}}^{{{3} + }} + {\text{2H}}_{{2}} {\text{O}}$$
(3)
$${\text{Tl}}^{{{3} + }} + {\text{2e}}^{ - } \rightleftharpoons {\text{Tl}}^{ + }$$
(4)
$$\equiv {\text{FeOH}} + {\text{Tl}}^{ + } + {\text{H}}_{{2}} {\text{O}} \rightleftharpoons \equiv {\text{FeOTlOH}}^{ - } + {\text{2H}}^{ + }$$
(5)
Fig. 7
figure 7

The XPS spectra of soil samples passivated by FMBC-20: a Tl 4f spectra (after 0 day), b Tl 4f spectra (after 60 days), c Fe 2p spectra (after 0 day), and d Fe 2p spectra (after 60 days)

Full size image
Fig. 8
figure 8

The STEM-EDS mappings of soil samples passivated by FMBC-20 after 15 days: a STEM image, b–d HRTEM image and FFT results (inset images) of the corresponding labeled areas in the STEM image of soil samples, e1–e4 EDS mapping of Tl, Fe, As, and Mn on soil samples

Full size image

In summary, these Fe–Mn(OH) oxides provide additional adsorption sites for metal ions and facilitate the oxidation of Tl(I) to Tl(III), leading to the generation of avicennite (Tl2O3) with reduced reactivity. Previous research also found that Tl(I) in the soil was firstly adsorbed by Fe(III) colloids, which was subsequently oxidized to Tl(III) complexes by Mn(IV) oxides in the soil, leading to Tl stabilization in the mineral lattice of soil with decreased mobility (Rinklebe et al. 2020; Voegelin et al. 2015).

4 Conclusion

This study investigated the applications of FMBC to passivate co-contamination of Tl, As and V in soils. Compared with the raw BC, the bioavailable Tl contents were reduced by 83.9% after 60 days of passivation by using FMBC-20. Meanwhile, the use of FMBC led to a notable enhancement in the soil pH values and decreased the exchangeable sites on the soil surface, thus promoting the immobilization of toxic metal(loid)s. Varied characterizations demonstrated that a considerable number of high-valent Mn oxides with strong oxidizing properties on FMBC composite surface improved the redox potential of the soil, oxidizing Tl(I) to Tl(III) and subsequently decreasing Tl mobility. Moreover, FMBC can also passivate As and V with a maximum passivation rate of over 70%. All these findings provide a new management solution for co-immobilization of Tl, As and V under the complicated contaminated soil circumstance. It can be used as a candidate method for minimizing the bioavailability of key toxic elements to the crop.