Red mud-based Fe/C nanostructured materials for multi-interface remediation of Cr(VI)-contaminated soil and stabilization

Abstract

The stabilization remediation performance of Cr(VI)-contaminated soil hinges on the remediation behaviors at soil–Cr(VI)–stabilizer multiple interfaces. Fe/C nanostructured materials featuring high chemical affinity, quick electron transfer and tunable active sites might tackle the problems of substance transport and structure evolution across multiple interfaces. Herein, we report that the co-pyrolysis of red mud and straw, two abundant solid wastes, can realize the scaled-up synthesis of biochar-supported nanoscale zero-valent iron (nZVI/BC). At an initial Cr(VI) concentration of 1000.00 mg kg⁻¹ and stabilizer dosage of 10%, the optimal nZVI/BC converted the Cr(VI)-contaminated soil into non-hazardous waste, with toxicity characteristic leaching procedure (TCLP) leaching concentrations of 3.13 mg L⁻¹ Cr(VI) and 11.26 mg L⁻¹ Cr(T). Experimental and theoretical results revealed that nZVI/BC altered the species evolution at the multiple interfaces of nZVI/BC–Cr(VI)–soil, where the acid-soluble Cr in soil shifted into stable residual Cr owing to the microscopically increased bidentate-binuclear inner-sphere coordination modes and the reduction process over the nZVI/BC surface. Meanwhile, the released iron species from nZVI/BC was immobilized on the soil surface, thereby regulating organic matter adsorption to recover soil agglomeration. Therefore, this study presents the feasibility of obtaining Fe/C nanostructured materials by one-step upgrading agricultural and industrial waste into eco-friendly stabilizers for remediating Cr(VI)-contaminated soils.

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Environmental significance

Stabilization technology has been documented to be an effective remediation strategy toward Cr(VI)-contaminated soil, while suffering from the lack of superior stabilizers and an unclear multi-interface remediation mechanism. Herein, we report that the co-pyrolysis of red mud and straw, two abundant solid wastes, can realize the scaled-up synthesis of biochar-supported nanoscale zero-valent iron (nZVI/BC). nZVI/BC enhanced Cr(VI) stabilization in soil by regulating bidentate-binuclear inner-sphere coordination and reduction, while the released iron species promoted soil agglomeration. Therefore, engineered Fe/C nanostructured materials synthesized from selected solid wastes can be promising amendments for heavily Cr(VI)-contaminated soils.

1. Introduction

Hexavalent chromium Cr(VI)-contaminated soil with high ecological toxicity and a dispersed structure poses mortal threats to human health and soil erosion resistance.^1–3 Stabilization remediation has been regarded as an ecologically compatible and quick-acting soil remediation technology to reduce the mobility and effectiveness of Cr(VI) and improve the soil structure. However, the stabilization remediation performance strongly hinges on advanced stabilizers, where the surface active sites determine the Cr(VI) affinity and the released substances may affect the surface agglomeration of soil particles.^4–7 Recently, iron-based materials with strong reducibility and Cr(VI) affinity, such as ferrous sulfate, have been often used to remediate Cr(VI)-contaminated soil yet they suffer from high cost or induce unstable Cr(VI) immobilization in soil.^8–11 Red mud, an iron oxide-rich solid waste produced from the aluminum industry, is a potentially superior iron-based stabilizer with an extreme cost advantage for treating Cr(VI)-contaminated soils but faces unstable Cr(VI) immobilization due to the covered iron sites and slow electron transfer.^12–16

Iron/carbon (Fe/C) nanostructured materials with easily harmonizing surface active sites can modulate the dispersibility and bonding strength of Fe species, as well as controllably bind or release surface iron sites.^17–20 Benefiting from the electron transfer from C to adjacent Fe through the Fe–O–C linkages, an electron-rich microcenter around Fe is thus constructed for the binding or reduction of Cr(VI).²⁰ Among various carbonaceous supports, straw, an abounding biomass waste in nature, can serve as a raw material to produce biochar with large specific surface areas, abundant oxygen-containing functional groups (OFGs) and soil structure improvement abilities.^21–25 More importantly, the evolved reductive gas components (H₂ and CO) of carbothermal reactions can convert iron oxides (e.g., Fe₂O₃) into low-valence iron species (e.g., Fe₃O₄, FeO, Fe⁰),^{23,24,26–28} which interact with the newborn carbon species to form Fe/C composites.^29–32 In this regard, Fe/C nanostructured materials prepared by the co-pyrolysis of straw and red mud might be endowed with abundant and tunable surface reductive sites, thereby becoming a potential low-cost stabilizer for remediating Cr(VI)-contaminated soil.

Regarding that the stabilization remediation of Cr(VI)-contaminated soil involves complex behaviors, such as Cr(VI) reduction, Cr(VI) redistribution, active site transformation, and soil particle agglomeration,^7,11,14,33 a deep understanding of these multi-interfacial reaction processes is crucial to promote the development of the stabilization remediation technology. Herein, we fabricated a red mud-based Fe/C nanostructured material for multi-interface remediation of Cr(VI)-contaminated soil, thereby efficiently immobilizing Cr(VI) and recovering soil agglomeration, aiming to explore its synthesis technology and reveal dynamic processes at the multiple interfaces of the stabilizer–Cr(VI)–soil. During the co-pyrolysis process of red mud and straw, in situ diffuse reflectance infrared Fourier transform spectroscopy combined with thermogravimetry and gas chromatography (DRIFTS-TG-GC) characterization was conducted to disclose the time-resolved pyrolysis and formation of chemical bonds, mass loss, and gas evolution. During the process of stabilization remediation, the in situ open circuit potential integrated with attenuated total reflectance Fourier transform infrared (OCP-ATR-FTIR) spectroscopy method we previously established,³⁴ combined with density functional theory (DFT) calculations, was employed to investigate the substance transport and structure evolution, as well as charge transfer behavior at the soil–Cr(VI) and Cr(VI)–stabilizer interfaces.

2. Materials and methods

2.1. Materials and preparation

Air-dried and 100 meshed soil, red mud and straw were obtained from Hubei, Shandong, and Jiangsu provinces in China, respectively. The properties and compositions of soil and red mud are listed in Table S1 (ESI†).

To simulate the working conditions of heavily Cr(VI)-contaminated industrial field soils, the Cr(VI) concentration in contaminated soil was set to 1000 mg kg⁻¹ in this study.^35–37 1000 mg kg⁻¹ simulated Cr(VI)-contaminated soil (CrSoil) was prepared according to the literature.³⁸

For the preparation of RM700, 100.00 g of red mud was pyrolyzed in a tube furnace under vacuum conditions over the temperature range of 25–700 °C at a rate of 5 °C min⁻¹, and kept at 700 °C for 2 h. The resultant powder was denoted as RM700. For preparing red mud-based Fe/C nanostructured materials, 10.00 g (10%, wt% of red mud) of straw was mixed with the initial red mud powder by ball milling at 150 rpm for 30 min, and the other steps were the same as those of RM700. Fe/C nanostructured materials with different straw dosages (0, 2%, 5%, 10%, and 15%) were synthesized by changing the mass of straw (0, 2.00, 5.00, 10.00, and 15.00 g) with the same procedure.

2.2. Batch experiments

During soil remediation, 5 g stabilizer was evenly mixed with 50 g CrSoil, which was aged indoors for 28 days. The temperature was kept at 25 °C, and the soil moisture content was 50% water holding capacity. All the treatments were conducted in triplicate. All sampled soils were air-dried, crushed, and passed through a 100 mesh sieve for further analysis.

The toxicity characteristic leaching procedure (TCLP) was conducted to appraise the soil stabilization remediation performance.^10,39 The environmental safety of soil TCLP leachate referred to the identification standards of hazardous waste (GB/T5085.3-2007), in which the leaching concentration limits of Cr(T) and Cr(VI) were stipulated to be 15 mg L⁻¹ and 5 mg L⁻¹, respectively.⁴⁰ Cr speciation distribution was tested by the BCR sequential extraction method.⁴¹ Extracts separated from solid residues were specified here as F1 to F4, representing acid-soluble, reducible, oxidisable, and residual, respectively. The Cr(VI) content was determined by the 1,5-diphenylcarbazide method using a UV-visible spectrophotometer (Shimadzu 2550, Japan). The element concentration was measured using an inductively coupled plasma optical emission spectrometer ICP-OES (Agilent 5110, USA). The size distribution of soil particles was characterized using a laser particle size analyzer equipped with a helium–neon laser with a wavelength of 633 nm (Malvern Mastersizer 2000, UK).⁴²

2.3. In situ characterization

In situ DRIFTS-TG-GC characterization was used to monitor the fabrication of Fe/C nanostructured materials. A sealed tank equipped with two zinc selenide ATR crystals for transmitting incident light and diffusely scattered light was used for DRIFTS detection. There was a thermocouple and a mass sensor at the bottom of the tank, and the generated gas was conveyed to a gas chromatograph equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD) (TET Instrument GC-2030, China). First, 0.05 g of a red mud and straw mixture was added into the tank and then flattened. After evacuating the reaction vessel, the DRIFTS background noise was removed, and the baseline was recorded at the initial room temperature. Maintaining vacuum conditions, temperature-programmed heating was started. Meanwhile, the DRIFTS spectrogram, mass, and GC signals were recorded with time.

According to our previous study,³⁴ the in situ OCP-ATR-FTIR spectroscopy method was employed to reveal the multi-interface CrSoil remediation mechanism. The standard reference electrode and electrolyte were Ag/AgCl and 0.05 mol L⁻¹ Na₂SO₄, respectively. The conductivity of the working electrode relied on the transparent graphene film attached to the ATR prism. As for the CrSoil surface, 20 μL of 4 g L⁻¹ CrSoil ethanol dispersion was added onto the graphene film and dried to form the working electrode. As for the stabilizer surface, the powder in the initial CrSoil ethanol dispersion was changed into the Fe/C nanostructured materials. An electrochemical workstation (CHI760E, Chenhua, China) was employed to detect the OCP signals. An ATR-FTIR spectrometer (Nicolet IS50, Thermo, USA) equipped with a diamond ATR crystal and a liquid N₂-cooled mercury–cadmium–telluride detector (256 average scans, 4 cm⁻¹ resolution) was employed to detect the IR signals.

All of the theoretical calculations were performed using spin-polarized density functional theory (DFT) methods in the Vienna Ab initio Simulation Package (VASP 5.4). Other details of chemicals, sample characterization, and DFT calculations are provided in Text S1.†

3. Results and discussion

3.1. Characterization of Fe/C nanostructured materials

The crystalline nanoparticles and amorphous porous substrate of Fe/C nanostructured materials were identified by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) (Fig. 1a–f and S1†). The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and the elemental mappings showed that ∼100 nm bright iron nanoparticles were deposited on the ∼1500 nm gloomy carbon substrate along with oxygen (Fig. 1g). The thickness of the stepped Fe/C composites was around 700 nm, while that of the supported iron monomer was approximately 100 nm (Fig. 1h and i). Subsequently, XRD characterization confirmed the structure of nanoscale zero-valent iron (nZVI) (Fig. 1j). The distinct peaks at D and G bands in the Raman spectrum manifested the graphitic carbon of biochar in the Fe/C composites, and the high value of I_D/I_G = 0.93 pointed to abundant oxygenated functional groups (OFGs) (Fig. 1k).⁴³ OFGs were less distributed on the small particles at the edge of the sample, which may be due to the coverage of iron particles (Fig. S2†). Therefore, these OFGs might act as “anchors” to increase the roughness and promote the interlocking of biochar-supported nZVI (nZVI/BC) (Fig. 1l).

Fig. 1 Morphology and structure analysis of the red mud-based Fe/C nanostructured materials. (a) SEM image. (b) HRTEM image and (c–f) the magnified images. (g) HAADF-STEM image and the elemental mappings of C, O, and Fe, and their overlapped image. (h) AFM image and (i) the height statistics. (j) XRD pattern and (k) Raman spectrum. (l) Schematic illustration of the nZVI/BC structure.

3.2. Fe–O–C bonds induced nZVI/BC fabrication

In situ DRIFTS-TG-GC characterization was thus conducted to monitor the structure evolution and interfacial mass transport during nZVI/BC synthesis (Fig. 2a–d and S3 and S4†). The red mud mixed with 10% straw was heated to 700 °C with a rate of 5 °C min⁻¹ under vacuum conditions (stage 1), and then kept at 700 °C for 2 h (stage 2). At the beginning of stage 1, the dehydration–condensation reaction produced carbonyl –COO– groups (Fig. 2a) and resulted in a mass loss (Fig. 2c).^43,44 Starting from 400 °C, the peak of C=O groups shifted to higher wavenumbers with the appearance of the peaks of C–OFe and CO–Fe bonds.⁴⁵ As these Fe–O–C bonds increased, the amide CO–NH and –COO– groups suddenly decreased, accompanied by the production of H₂ and CO (Fig. 2d and S2†).^44,46 When reaching 600 °C, the accumulated gas concentrations exceeded 5% and Fe–O–C bonds began to decrease. Subsequently, the significant mass loss starting from 609 °C indicated the reduction of iron oxide (Fig. 2c), which proved to be beneficial to Cr(VI) adsorption (Fig. S5a†). At stage 2, iron oxide continued to be reduced, accompanied by the decrease of the Fe–O–C peaks (Fig. 2b). However, –CH₂ groups began to form and covered the other sites at the end of stage 2.⁴⁷ As a result, the subsequently extended co-pyrolysis hindered Cr(VI) removal (Fig. S5b†).

Fig. 2 Characterization of nZVI/BC fabrication. (a–d) In situ DRIFTS-TG-GC signals during nZVI/BC synthesis. DRIFTS spectra of (a) the heating process (stage 1) and (b) the subsequent temperature-keeping process (stage 2). (c) TG and (d) GC signals. (e) Raman spectra of pyrolysis products under different straw dosages. (f) XRD patterns. XPS spectra of (g) Fe 2p and (h) C 1s. (i–k) Pyrolysis product characterization under different raw material mixing states. (j) GC signals. (k) XRD patterns.

Furthermore, we explored the impact of raw materials on nZVI/BC preparation. As expected, the straw was a necessary raw material for the formation of the biochar structure (Fig. 2e). The straw dosages (except 0%) had little effect on OFGs of biochar, but significantly affected Cr(VI) removal by nZVI/BC, indicating the surface adsorption site of nZVI rather than biochar (Fig. 2e and S5c†). As a component of red mud, chlorite was reduced to nZVI by forming Fe–O–C bonds (Fig. 2f–h and S6 and Table S2†). In order to explore the preparation conditions, the pyrolysis products of raw materials in different mixing states were also detected (Fig. 2i–k). Neither the reducing gas nor the nZVI structure was found after the pyrolysis of red mud (the solid products named RM700). When red mud and straw were pyrolyzed in separate double tanks, the produced gas was similar to that in the single tank, but no obvious nZVI structure was found in the products (DouRM700), proving the importance of the in situ generated gases for Fe₂O₃ reduction.^29–32

3.3. Stabilization remediation performances of Cr(VI)-contaminated soil

The widely used TCLP method¹⁰ was employed to evaluate the Cr(VI) immobilization performance in soil (Fig. 3a). First, 50% water proportion and 10% stabilizer dosage proved to be the best parameters and were therefore chosen for subsequent soil remediation (Fig. S7†). With the concentration of 1000.00 mg kg⁻¹ Cr(VI) in the soil, 54.02 mg L⁻¹ Cr(VI) and 55.15 mg L⁻¹ Cr(T) were leached from the initial Cr(VI)-contaminated soil (CrSoil) by TCLP extraction. After 28 days of amendment, in the case of RM700, the leaching Cr(VI) and Cr(T) from the amended soil were 18.50 and 19.99 mg L⁻¹, respectively, exceeding the recognized regulation standard of 5 mg L⁻¹ Cr(VI) and 15 mg L⁻¹ Cr(T).⁴⁰ As for nZVI/BC, the final detectable TCLP leaching concentrations of Cr(VI) and Cr(T) were only 3.13 mg L⁻¹ Cr(VI) and 11.26 mg L⁻¹ Cr(T), respectively. Meanwhile, the leaching concentrations of all possible toxic compositions met the GB/T5085.3-2007 standard (Table S3†). Thus, this heavily polluted CrSoil remediated by nZVI/BC can be considered non-hazardous waste. Impressively, nZVI/BC could halve the acid-soluble (F1) Cr and convert it into more stable forms (mainly F4 – residual) (Fig. 3b). These results demonstrated that the nZVI/BC amendment could promote the transformation of soil Cr from unstable to stable forms, decreasing Cr leachability. Meanwhile, the particle size distribution of soil aggregates recovered to a larger range after remediation (Fig. 3c).

Fig. 3 Remediation performance of Cr(VI)-contaminated soil. (a) TCLP leaching concentrations of Cr(VI) and Cr(T). (b) Cr speciation distribution (F1: acid-soluble; F2: reducible; F3: oxidisable; F4: residual). (c) Size distribution of soil particles.

3.4. Multi-interface mechanisms of Cr(VI)-contaminated soil remediation by nZVI/BC

In situ OCP-ATR-FTIR characterization was employed to investigate the interfacial remediation behaviors of CrSoil by nZVI/BC, consisting of CrSoil–solution and nZVI/BC–solution interfaces (Fig. 2). The OCP and ATR-FTIR methods can respectively reveal interfacial charge transfer and surface coordination, and the integration of both can simultaneously reveal changes in charge and chemical bonds at multiple interfaces.^34,38 When soil was contaminated by Cr(VI), the peaks of desorbed organic matter (OM) were different from those of OM solution, while the peaks of the adsorbed Cr(VI) were similar to those of Cr(VI) solution (Fig. S9†). In short, Cr(VI) desorbed chemically adsorbed OM, and mostly existed in the weak outer-sphere coordination. This OM desorption weakened the adhesion between soil particles, explaining the decrease in soil particle size.⁴⁸ During CrSoil remediation by nZVI/BC, the electric double layer on the CrSoil surface and the nZVI/BC surface consisted of a Helmholtz layer (HL) and a diffuse layer (DL), and the potential change followed the formulas: ΔE_HL = ΔE_OCP − ΔE_Zeta, ΔE_DL = ΔE_Zeta. The zeta potentials of nZVI/BC and CrSoil were −23.3 mV and −15.2 mV, respectively. Based on the detected OCP (Fig. 2c and d) and zeta potentials, the calculated potential changes in HL and DL are summarized in Table 1. Opposite variations of ΔE_DL were observed, which were −2.7 mV for CrSoil and +5.4 mV for nZVI/BC, indicating the desorption and the electrostatic adsorption of Cr(VI), respectively. Intriguingly, only 39% of electrons flowed from nZVI/BC (ΔE_HL = +174.0 mV) to CrSoil (ΔE_HL = −67.4 mV), which originated from the redistribution of chemically bonded Cr(VI), and the remaining 61% electrons were consumed by the Cr(VI) reduction.

Table 1 Changes of different potentials

Surface	ΔEOCP (mV)	ΔEZeta (mV)	ΔEDL (mV)	ΔEHL (mV)
CrSoil	+179.4	+5.4	−2.7	−67.4
nZVI/BC	−70.1	−2.7	+5.4	+174.0

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The addition of nZVI/BC onto the CrSoil surface caused some new IR peaks to appear in the range of 1080–900 cm⁻¹, indicative of the adsorption of OM and Cr(VI) (Fig. 4e). Among them, the peaks at 1025 and 997 cm⁻¹ differed from those of the dissolved OM (Fig. S9d†) and thus could be attributed to the inner-sphere adsorbed OM. The change of OM content proved the ability of nZVI/BC to restore OM in soil (Fig. S10†). As is known, OM could act as a “cementing agent” to promote soil aggregation, explaining the increase in soil particle size (Fig. 3c).⁴⁸ The IR peaks in the range of 980–900 cm⁻¹ belonged to Cr(VI) adsorption, where the small peak at 942 cm⁻¹ was attributed to the outer-sphere coordination, and the significant peaks at 975 and 923 cm⁻¹ belonged to bidentate binuclear (BB) inner-sphere coordination, which always exhibited the strongest binding ability compared to the other Cr(VI) binding structures over iron-based materials.^34,49 According to the calculation of the IR peak area, the adsorption amount of BB inner-sphere coordinated Cr(VI) was 5 times that of the outer-sphere coordinated Cr(VI). The increase in adsorbed OM and Cr(VI) on the CrSoil surface both originated from those initially free or electrostatically adsorbed ones, and might be induced by the adsorbed iron species released from nZVI/BC.⁴⁸ As expected, the addition of CrSoil onto the nZVI/BC surface also resulted in significant IR signals (Fig. 4f). Because of the strong electron-donating ability of nZVI/BC, the IR peaks of the inner-sphere coordinated OM moved to higher wavenumbers of 1097, 1027 and 1001 cm⁻¹. Interestingly, only the BB inner-sphere coordination, with peaks at 975 and 923 cm⁻¹, appeared within the Cr(VI) adsorption peak range. By comparing the peak areas, the adsorption capacity of nZVI/BC was 2.5 times that of CrSoil.

Fig. 4 In situ OCP-ATR-FTIR characterization during CrSoil remediation by nZVI/BC. Schematic diagram of (a) the device and (b) the position of multiple interfaces. Detection of (c and e) the CrSoil surface and (d and f) the nZVI/BC surface. (c and d) OCP signal change. (e and f) Infrared spectral change and peak fitting of the final infrared spectrum (green).

DFT calculations were then employed to unveil the CrSoil remediation by nZVI/BC. Among the surface coordination structures of Cr(VI) on nZVI/BC (Fig. 5a and b), the chemisorption energy (−8.63 eV) of the BB coordination was more negative than that (−4.84 eV) of the monodentate mononuclear (MM) coordination, reflecting the stronger binding ability. Structurally, the optimized configuration presented the breakage of Fe–OC bonds, and the redistribution of these iron ions could help regulate subsequent coordination behaviors over the soil surface. Meanwhile, the dual channel built on nZVI/BC was found to favor electron transfer to adsorbed Cr(VI) through the inner-sphere coordinated Fe–O–Cr bonds (Fig. 5c), pointing to the strong electron-donating ability of nZVI/BC.

Fig. 5 DFT and multi-interface mechanisms of CrSoil remediation by nZVI/BC. Theoretical configuration of (a) MM and (b) BB adsorbed Cr(VI) on nZVI/BC. (c) Charge density difference. The yellow isosurfaces and the blue isosurfaces represent accumulation and depletion of negative charge, respectively. Schematic diagram of (d) the soil agglomeration and (e) the substance transfer process.

In addition, the concentration change of iron ions in solution (Fig. S11†) confirmed the transfer of iron ions from the stabilizer to CrSoil. During CrSoil remediation, the iron ions released from nZVI/BC and RM700 samples were captured by soil and were responsible for the surface binding of Cr(VI) and OM. With the help of OM adhesion, soil particles became large aggregates (Fig. 5d). Cr 2p XPS of remediated soil revealed the existence of 79% Cr(III) and 21% Cr(VI) after nZVI/BC remediation and 46% Cr(III) and 54% Cr(VI) after RM700 remediation (Fig. S12†). Combined with our previous analysis of OCP-ATR-FTIR, the presence of chromium in nZVI/BC-remediated soil is summarized in Table S4.† Only 1% of initially free or electrostatically attracted Cr(VI) was adsorbed on the CrSoil surface in the form of a weak outer-sphere coordination, while 5% and 15% were respectively adsorbed on the CrSoil surface and nZVI/BC surface by forming the stable BB inner-sphere coordination, and the other 79% was reduced to Cr(III) and stably immobilized on the nZVI/BC surface (Fig. 5e).

3.5. Interaction between Cr(VI) and stabilizers

To validate and elucidate the interaction between Cr(VI) and stabilizers, batch adsorption experiments were first employed to compare the Cr(VI) removal by RM700 and nZVI/BC. Within 6 h, nZVI/BC removed 76% of Cr(VI), much higher than that (40%) of RM700 (Fig. 6a). As shown in Fig. 6b, both removal curves obeyed the pseudo-first-order kinetics equation, and the rate constant (k = 0.38 h⁻¹) of nZVI/BC was 2.5 times that (k = 0.15 h⁻¹) of RM700. Subsequently, we systematically evaluated the discrete contributions of different adsorption components. As expected, the filtrates (possible dissolved components) from RM700 and nZVI/BC could not effectively remove Cr(VI) (≤8%), indicating the significant contribution of surface sites (Fig. 6a). Owing to the strong electron-donating and adsorption abilities, surface Fe(II) sites (≡Fe(II)) always combined more Cr(VI) than other surface iron sites, which could be subtly complexed by 1,10-phenanthroline.⁵⁰ Limited by the amount of ≡Fe(II), the contribution of ≡Fe(II) to RM700 was just around 40% (Fig. 6c and d). In contrast, during Cr(VI) removal by nZVI/BC, ≡Fe(II) made the main contribution, ranging from 85% to 76% throughout the removal process (Fig. 6e), confirming the important role of oxide shells on the nZVI surface. In addition, SEM and elemental mappings were employed to visualize the morphology and substance distribution of the Cr(VI)-adsorbed materials (Fig. 6f and g). As expected, Cr was distributed along Fe, indicative of the ≡Fe binding sites for Cr(VI). As indicated in Fig. 6g, nZVI was detached from the porous biochar surface, demonstrating the possibility of iron transport from nZVI/BC to the soil surface.

Fig. 6 (a–e) Cr(VI) removal by RM700 and nZVI/BC. (a) Removal curves of materials and filtrates from their solution. (b) Plots of −ln(C/C₀) versus time. (c) The effect of 1,10-phenanthroline (phen). Time-dependent contributions of different adsorption sites in (d) RM700 and (e) nZVI/BC. The initial Cr(VI) concentration and material dosage were 4 mg L⁻¹ and 0.4 g L⁻¹, respectively. SEM images of (f) Cr(VI)-adsorbed RM700 and (g) Cr(VI)-adsorbed nZVI/BC.

4. Conclusions

Cr(VI)-contaminated soil has extremely high ecotoxicity and a dispersed agglomeration structure, causing serious environmental and engineering problems. Stabilization technology has been documented to be an effective remediation strategy toward Cr(VI)-contaminated soil, while suffering from the lack of superior stabilizers and an unclear multi-interface remediation mechanism. In this study, an all-solid waste stabilizer of nZVI/BC material synthesized from the co-pyrolysis of red mud and straw was used to realize Cr(VI) stabilization and soil agglomeration, further revealing the multi-interface remediation process of nZVI/BC–Cr(VI)–soil. 10% nZVI/BC (wt% of soil) could convert 1000.00 mg kg⁻¹ Cr(VI)-contaminated soil into non-hazardous waste (GB/T5085.3-2007 standard), with TCLP leaching concentrations of 3.13 mg L⁻¹ Cr(VI) and 11.26 mg L⁻¹ Cr(T). Theoretical and in situ experimental results manifested that 79% of the initially free or electrostatically attracted Cr(VI) was reduced into Cr(III) and then stably bonded onto the nZVI/BC surface, approximately 5% and 15% of the unreduced Cr(VI) immobilized onto the soil and nZVI/BC surface, respectively, with a stable BB inner-sphere coordination, and only 1% of the unreduced Cr(VI) still resided on the soil surface via a weak outer-sphere coordination. Meanwhile, the released iron species from the nZVI/BC surface promoted soil agglomeration by anchoring organic matter at the soil surface. This multi-interface remediation mechanism of Cr(VI)-contaminated soil provides theoretical guidance on rationally designing high-performance red mud-based Fe/C nanostructured materials for Cr(VI) stabilization and soil aggregation.

Data availability

The authors declare that data will be made available on request

Author contributions

Shiyu Cao: data curation, formal analysis, funding acquisition, investigation, visualization, writing – original draft. Jiangshan Li: conceptualization, funding acquisition, supervision, writing – review & editing. Jing Nie, Yanbiao Shi, and Jiaqi Dong: formal analysis. Lizhi Zhang: writing – review & editing. Qiang Xue: resources, supervision.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (52400195, 42177163 and 22306119), the National Key Research and Development Program of China (2023YFC3707801), and the Fundamental Research Funds for the Central Universities (YJ202460).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4en01087f

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