1. Introduction
Hexavalent chromium (Cr(VI)) contamination in soil is a global environmental challenge because of its high toxicity, mobility, and carcinogenicity [1]. Industrial activities such as electroplating, tanning, and chromite smelting [2] have caused Cr(VI) concentrations in soil to exceed 600 mg/kg at some sites [3][4], far above the 150 mg/kg limit in the "Risk Control Standard for Soil Pollution in Agricultural Land" (GB 15618-2018). Cr(VI) occurs as an anion and readily migrates in groundwater [5], threatening ecosystems and human health. In contrast, Cr(III) forms stable precipitates, thereby reducing environmental risks. Therefore, reducing Cr(VI) to Cr(III) and achieving long-term immobilization is a key goal in soil pollution remediation [6][7].
Extensive research has focused on chemically modifying biochar (BC) for Cr(VI)-contaminated soil remediation [8]. Chemical modification is a key approach for improving biochar performance, including acid treatment, metal loading, and redox modification [9][10]. Acid treatments (e.g., H3PO4, H2SO4, HCl) enhance Cr(VI) adsorption by etching the surface, enlarging pore structure, and introducing oxygen-containing groups such as carboxyl and hydroxyl. H3PO4 activation greatly increases biochar specific surface area [11], but the resulting acidic waste stream poses secondary pollution risks if untreated, potentially acidifying water or altering soil pH.
Metal-loaded modifications, including nano-zero-valent iron (nZVI) and iron oxides, introduce reductive active sites [12] to convert Cr(VI) into less toxic Cr(III). nZVI offers strong reducing power, environmental compatibility, and practicality and is widely used in Cr(VI) remediation [13][14]. However, large-scale applications face high preparation costs, stringent inert-gas reaction requirements, particle agglomeration reducing surface area and reactivity [15], and oxidation by O2/H2O forming Fe(III) oxide layers that hinder electron transfer and long-term Cr(VI) reduction [16].
Biomass feedstock research has largely focused on traditional biomass such as walnut shells, peanut shells, and wood chips, which provide high cellulose content and stable pyrolysis yields [17][18]. However, geographic constraints limit feedstock availability, hindering large-scale remediation efforts, while reliance on inherent pore structure and surface chemistry restricts adsorption performance. In China, agricultural and forestry wastes like rice straw and corn stover are produced annually in large quantities and often discarded via burning or burial. Their high ash content and low calorific value render them low-value resources, with modification research still limited. Rice straw biochar typically exhibits a specific surface area below 80 m2/g and basic surface functional groups exhibiting weak affinity for anionic Cr(VI). Therefore, there is an urgent need to enhance its physicochemical properties via modification techniques.
Existing modification techniques mainly rely on single-acid treatment or metal loading [19][20], limiting the simultaneous optimization of pore structure, surface chemistry, and reduction capacity. While single-acid treatment can enhance specific surface area, it often compromises the carbon framework stability of biochar. Moreover, many acids pose secondary environmental risks due to strong corrosiveness, making the search for low-pollution, low-corrosivity acids essential for sustainable modification. Simply loading nZVI improves reduction capacity but does not overcome particle agglomeration or high costs. Commonly used FeSO4 has insufficient reduction potency for heavy metal ions, necessitating co-modification with Na2SO3 to achieve effective Cr(VI) removal.
Studies further show that single-modified biochar generally has low adsorption capacity for Cr(VI) and is highly susceptible to competitive adsorption from coexisting anions (e.g., SO42−, NO3−) in complex soil matrices, leading to much lower remediation efficiency in field conditions than in laboratory settings [21][22]. Therefore, developing novel modification strategies—such as acid activation combined with metal loading—to control both physical structure and chemical activity is essential to overcome current technological limitations.
Formic acid, occurring naturally in the secretions of bees, ants, and caterpillars (hence its alternative name “ant acid”), offers greater environmental safety than conventional acids[23]. Furthermore, as the simplest fatty acid, it is inexpensive to synthesise [24]. To date, no studies have examined formic acid-modified biochar loaded with iron salts for Cr(VI)-contaminated soil remediation.
In this context, the present study prepared formic acid-iron salt modified rice straw biochar (FFBC) for the adsorption and immobilisation of Cr(VI) in soils. Orthogonal experiments were conducted to optimise preparation parameters, thereby enabling systematic investigation of the material’s immobilisation efficiency for Cr(VI). To elucidate the microstructural changes and reduction mechanisms of Cr(VI), Scanning Electron Microscopy (SEM), Brunauer–Emmett–Teller surface area analysis (BET), and X-ray Photoelectron Spectroscopy (XPS) characterisation techniques were employed. The novelty of FFBC is threefold: firstly, replacing conventional biomass with rice straw to facilitate the high-value utilisation of agricultural and forestry residues; secondly, employing formic acid as an environmentally benign activator to mitigate secondary pollution; and thirdly, introducing iron active sites to significantly enhance electron transfer efficiency and Cr(VI) reduction capacity. This study presents a low-cost and efficient technical approach for the remediation of Cr(VI)-contaminated soils. In addition, it offers theoretical insight and practical guidance for the green modification of biochar and the valorisation of waste biomass.
2. Materials and Methods
2.1. Materials
Rice straw samples were collected from paddy fields in Xiaogan City, Hubei Province. After harvest, they were rinsed with deionised water until all suspended matter and impurities were removed, then cut into 3 cm segments to prepare for subsequent experiments. The processed straw was dried in a Yiheng DHG-9055A oven (China) at 55°C for 12 h under constant temperature to ensure complete moisture removal while preventing organic degradation or structural alterations.
Soil samples were collected from Shenyang City, Liaoning Province. They were screened to remove coarse particles, weeds, and plant debris, then sieved through a 2 mm mesh. The processed soil was dried at 60°C for 12 h under constant temperature. The primary chemical composition was analysed using an X-ray fluorescence spectrometer (XRF), and the results are shown in Table 1.
The principal reagents in this study included deionised water, HCOOH, K2Cr2O7, FeCl3·6H2O, FeSO4·7H2O, and KOH. Deionised water was produced using a SYHL-VF ultra-pure water system (Shenyuan, China). HCOOH (99% concentrated) was supplied by Shandong Yinglang Chemical Co., Ltd. K2Cr2O7 was obtained from Sinopharm Chemical Reagent Co., Ltd. and used as the Cr(VI) source. FeCl3·6H2O and FeSO4·7H2O (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as iron sources for synthesizing iron-based compounds. KOH (analytical grade) was obtained from the same supplier and used to adjust solution pH, maintain alkalinity, and facilitate the activation reaction.
2.2. Methods
2.2.1. Preparation of modified biochar
2.2.1.1. Preparation of FFBC materials
Fig. 1 illustrates the schematic diagram for the preparation of modified FFBC material, with the specific procedure as follows: Three batches of rice straw, each weighing 400 g, were immersed in 80% HCOOH solution and soaked for 12 hours. After soaking, each batch of rice straw was thoroughly rinsed multiple times with deionised water to ensure complete removal of formic acid, continuing until the rinse water was clear and free of turbidity. The rinsed samples were then oven-dried to constant weight. The dried rice straw was placed in a vacuum tube muffle furnace pre-purged with nitrogen for pyrolysis. The pyrolysis programme commenced at 30°C, with a heating rate of 5°C/min to 600°C, which was maintained for 3 hours. Heating was then terminated, and the nitrogen atmosphere maintained until the samples cooled naturally to room temperature. The cooled samples were removed and set aside. The pyrolyzed samples were transferred to a mortar for grinding, and the particle size was controlled at approximately 0.112 mm (passing the 0.154 mm sieve but retained on the 0.1 mm sieve).After grinding, the samples were sealed and stored.
2.2.1.2. Preparation of contaminated soil
A standard Cr(VI) contaminated solution (concentration: 0.3 mg/mL) was prepared. Using a precision balance (Chinese standard), 84.75 mg of solid K2Cr2O7 was weighed and added to a beaker containing 100 mL deionised water.
After grinding, the samples were sealed and stored for subsequent modification. A total of 0.5 g of solid FeCl3·6H2O and FeSO4·7H2O in a 1:1 mass ratio was dissolved in 200 mL of deionised water under magnetic stirring until complete dissolution. Subsequently, 10 g of the ground biochar sample was added to the solution, and 5 mol/L KOH was slowly titrated dropwise until the pH stabilised at 10 ± 0.2. The mixture was then sealed and left to stand for 12 hours, followed by thorough rinsing with deionised water and drying to a constant weight. The obtained material was designated as FFBC1.Using the same procedure, FeCl3·6H2O and FeSO4·7H2O were mixed at mass ratios of 2:1 and 3:1 to prepare FFBC2 and FFBC3, respectively.
Three pre-weighed soil samples (5 g each) were prepared. The standard Cr(VI) solution was added to each soil sample as follows: for the first group, 1 mL of the standard Cr(VI) solution was added, achieving a Cr(VI) content of 0.3 mg and a final concentration of 60 mg/kg in the soil; for the second group, 2.5 mL of the solution was added (0.75 mg Cr(VI)), giving a concentration of 150 mg/kg; for the third group, 5 mL was added (1.5 mg Cr(VI)), resulting in a concentration of 300 mg/kg. Consequently, the chromium contamination levels for the three soil groups were 60 mg/kg, 150 mg/kg, and 300 mg/kg, corresponding to 2, 5, and 10 times the risk control value of 30 mg/kg for Class I land use, as stipulated in the Chinese national standard Soil Environmental Quality Standard for Soil Pollution Risk Control of Construction Land (Trial) (GB 18588-2018).
Throughout the treatment process, care was taken to ensure that Cr(VI) was fully penetrated and uniformly distributed within the soil, thus avoiding uneven contaminant distribution that could compromise subsequent experiments. Each group of contaminated soil samples was then sealed and stored for subsequent use.
A standard Cr(VI) solution with a concentration of 0.3 mg/mL was prepared by dissolving 84.75 mg of K2Cr2O7 in 100 mL of deionized water using an analytical balance (Yingheng, China). Three 5 g soil samples were prepared in advance, and the standard Cr(VI) solution was added to each group as follows: 1 mL for the first group, 2.5 mL for the second, and 5 mL for the third. This resulted in Cr(VI) concentrations of 60, 150, and 300 mg/kg, corresponding to 2, 5, and 10 times the risk control value (30 mg/kg) for Class I land use specified in the Chinese national standard Soil Environmental Quality Standard for Soil Pollution Risk Control of Construction Land (Trial) (GB 18588-2018).During preparation, after adding the Cr(VI) solution, the soil samples were thoroughly mixed using a glass rod to ensure uniform Cr(VI) distribution and prevent heterogeneity that could affect subsequent experiments. The treated soil samples were then sealed for later use.
2.2.2. Orthogonal experimental design
Building on extensive preliminary testing, a three-factor, three-level orthogonal experiment was designed to evaluate the adsorption performance of FFBC on Cr(VI)-contaminated soil under different conditions. Three main factors were considered: FFBC dosage, Cr(VI) contamination concentration, and Fe(III)/Fe(II) ratio. Preliminary studies identified 16.6% soil moisture as optimal for adsorption; therefore, this parameter was held constant throughout the orthogonal experiments. The experimental design for FFBC materials is summarised in Table 2.
In this study, adsorption efficiency was a key indicator of biochar performance, reflecting its capacity to capture target substances during adsorption. Biochar with high adsorption efficiency can efficiently remove harmful substances or pollutants from the environment within a shorter timeframe. This capability is critical for improving the material's practical applicability. Adsorption efficiency was calculated from leaching toxicity concentrations using the following formula:
Eq. (1):η — adsorption efficiency; M — mass of Cr(VI) added; C — leaching toxicity concentration; V — volume of water.
2.2.3. Toxicity leaching test
This study employed toxicity leaching tests to evaluate the efficacy of soil remediation, focusing on the immobilisation capacity of the test materials for heavy metals and organic pollutants. The experiments followed the “Method for Toxic Leaching of Solid Waste—Sulfuric Acid and Nitric Acid Method (HJ/T299-2007),” a standard issued by the Ministry of Ecology and Environment of the People's Republic of China. By conducting leaching tests under acidic conditions, the study analysed the leaching behaviour of heavy metals and organic pollutants in soil, thereby providing data support for contaminated soil remediation technologies.
This study used toxicity leaching tests to evaluate soil remediation efficacy, focusing on the immobilisation capacity of adsorbent materials for heavy metals and organic pollutants. The tests followed the “Solid Waste Toxicity Leaching Methods – Sulphuric Acid and Nitric Acid Method (HJ/T299-2007)” issued by the Ministry of Ecology and Environment of the People's Republic of China. Leaching tests under acidic conditions allowed analysis of the leaching behaviour of heavy metals and organic pollutants in soil, supporting the development of remediation technologies.
The procedure included preparing ultrapure water, concentrated sulphuric acid, and concentrated nitric acid. These acids were mixed in a 2:1 ratio, with concentrated sulphuric acid added slowly to prevent splashing. The mixture was added to 1 L of water and adjusted to pH 3.20 ± 0.05. Sample moisture content was determined, and samples were dried at 105°C to constant weight; if liquid components were present, pressure filtration was performed, and the moisture content of the filter residue was measured. Coarse soil particles were crumbled to pass through a 9.5 mm sieve. Extraction solvent was added at a liquid-to-solid ratio of 10:1, and the mixture was shaken for 18 h. After shaking, the mixture was vacuum filtered, and the filtrate was stored at 4°C for leaching toxicity analysis using an inductively coupled plasma optical emission spectrometer (ICP-OES, PerkinElmer ICP 2100, USA).
2.2.4. Microscopic characterisation tests
FFBC materials were analysed using a nitrogen adsorption–desorption isotherm analyser (AUTOSORB-1-C, Quantachrome, USA) to determine specific surface area and pore size distribution. Pore size distribution was obtained using the Barrett–Joyner–Halenda (BJH) model. Based on capillary condensation theory (Kelvin's equation), the BJH model analyses the adsorption and desorption branches of the isotherm to extract mesopore and partial macropore distribution information within the 2–50 nm range. This approach was appropriate for evaluating the pore structure of the modified biochar.
The surface morphology and microstructure of FFBC materials were examined by scanning electron microscopy (SEM) using a Gemini SEM 300 (Carl Zeiss, Germany) and a Tescan 120-0283 (Tescan BMO, Czech Republic). Before analysis, all specimens were dried at 105°C, and all samples were gold-sputtered.
To investigate the molecular structure, chemical composition, and vibrational characteristics of FFBC materials and post-reaction samples, a Thermo Scientific Nicolet 10 Fourier Transform Infrared Spectrometer (Thermo Fisher Scientific, USA) was used for sample characterisation. Spectra were collected in the range of 4000–400 cm−1 with 32 scans at a resolution of 2 cm−1.
To investigate the crystal structure and phase composition of FFBC materials, X-ray diffraction (XRD) analysis was performed using a Rigaku Smart Lab SE diffractometer (Rigaku Corporation, Japan). A copper target was used as the X-ray source at 40 kV and 40 mA. The X-ray wavelength was 0.15406 nm, with a scanning speed of 5°/min over a 2θ range of 10–80°.
To characterise the surface elemental composition, chemical state, and valence state of the samples, X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo ESCALAB 250XI spectrometer (Thermo Fisher Scientific, USA).
To assess the leaching toxicity of chromium-contaminated soil, this study used a PerkinElmer ICP 2100 Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, PerkinElmer Inc., USA) for analysis.
3. Results
3.1. Orthogonal Experiment Results Analysis
The results show that sample adsorption performance is primarily governed by the Fe(III)/Fe(II) ratio. Leaching concentrations and adsorption efficiencies are shown in Fig. 2. Compared with the unmodified control, all modified samples exhibited significantly reduced leaching concentrations, indicating that biochar modification markedly enhances Cr(VI) adsorption.
The lowest leaching concentration occurred in FFBCCS1 (10.66 mg/L), while the unmodified control at the same contamination level (60 mg/kg) reached 36.9 mg/L. The highest leaching concentration occurred in FFBCCS3 (118.65 mg/L), while the control at the same contamination level (300 mg/kg) reached 180.9 mg/L. These findings confirm that adding modified biochar substantially reduces Cr(VI) leaching.
FFBCCS8 achieved the highest adsorption efficiency (90.44%), whereas FFBCCS3 had the lowest (60.45%). FFBCCS7, sharing the same Fe(III)/Fe(II) ratio as FFBCCS3 but with a higher biochar dosage, exhibited an adsorption efficiency 12.23% lower than that of FFBCCS8. Despite a fivefold difference in modification content, FFBCCS1 and FFBCCS6 showed only a 3.32% variation in adsorption efficiency, suggesting that modification content alone is not the decisive factor.
Overall, the analysis shows that FFBC1 consistently outperforms FFBC2 and FFBC3 across all modification levels and contamination concentrations. This confirms that the Fe(III)/Fe(II) ratio of the modified biochar is the key determinant, with FFBC1 exhibiting stronger and more stable adsorption properties and indicating strong potential for practical application.
3.2. Extreme Range Analysis
To verify whether FFBC1 exhibits stronger and more stable adsorption properties, this study systematically analysed the orthogonal experimental results of FFBC materials using range analysis to comprehensively evaluate the influence of each factor on adsorption efficiency. In Fig. 3, k1, k2, and k3 denote the average adsorption efficiencies at levels 1, 2, and 3 for each factor, respectively. R denotes the range, calculated as the difference between the maximum and minimum average adsorption efficiencies for each factor across all levels, and reflects its relative influence on experimental outcomes.
Analysis shows that the Fe(III)/Fe(II) ratio exerts the strongest influence on FFBC adsorption efficiency, with an R value of 17.017, indicating substantial variation in adsorption efficiency across different Fe(III)/Fe(II) ratios. This highlights the critical role of modification in regulating adsorption performance. This enhancement likely arises from the introduction of hydrophilic groups and surface charge modification, improving interactions between the material and Cr(VI) and thus increasing adsorption capacity.
The FFBC dosage also significantly affected adsorption efficiency (R = 10.393), with higher dosage effectively improving performance by providing more active sites for Cr(VI) adsorption and increasing overall capacity. In contrast, the effect of initial Cr(VI) contamination concentration was relatively minor (R = 5.967), suggesting that contamination level has a limited impact on adsorption efficiency.
3.3. Analysis of the Structure and Surface Characteristics of Modified Biochar
Fourier Transform Infrared Spectroscopy (FTIR) was used to characterise three samples with distinct Fe(III)/Fe(II) ratios (FFBC1, FFBC2, FFBC3) to investigate functional group alterations during modification. The results are shown in Fig. 5(b). Broad peaks at 3519 and 3445 cm−1 correspond to −OH stretching vibrations [25], indicating substantial retention of hydroxyl groups. The peak intensity is highest in FFBC1 and progressively decreases with increasing FeCl3 dosage (FFBC2 and FFBC3). The strong absorption peak at 1581 cm−1 corresponds to C=O stretching vibrations [26], suggesting the formation of carboxylate (COO−) groups capable of coordinating with metal ions to form stable complexes [27]. The weak peak at 1416 cm−1 represents C=C bending of the aromatic skeleton, indicating that the biochar framework remains largely intact during modification. A C–O stretching vibration [28], characteristic of alcohol or ester groups, appears at 1090 cm−1. The strong absorption peak at 798–882 cm−1 is attributed to Fe–O vibrations [29], confirming effective iron loading. Overall, the Fe(III)/Fe(II) ratio affects the type and abundance of oxygen-containing functional groups on the biochar surface, with higher Fe(III) contents reducing −OH and −COOH groups and potentially influencing Cr(VI) remediation efficiency.
SEM analysis was conducted on FFBC1, FFBC2, and FFBC3, with results shown in Fig. 4(a–c). FFBC1 exhibits a relatively intact porous structure with smooth pore walls and uniform pore distribution. Minor particle deposits, identified by FTIR as iron oxides or iron salt crystals, were distributed uniformly without blocking the pores, facilitating Cr(VI) adsorption and reduction. FFBC2 shows wrinkling and partial deformation of pore walls with denser iron oxide and salt crystal deposition, partially obstructing pore connectivity. At higher FeCl3 dosages, FFBC3 exhibits severe pore collapse and extensive surface deposition of iron oxides and salts, reducing specific surface area and pore volume. These observations indicate that excessive Fe(III) loading induces structural degradation and pore blockage.
Nitrogen adsorption–desorption isotherm analyses were performed on FFBC1, FFBC2, and FFBC3, with pore size distributions calculated using the BJH model (Fig. 4(d–f)). FFBC1 exhibits a mesoporous structure with pore sizes mainly between 2–10 nm, a gradually increasing cumulative pore volume, and multiple narrow peaks in the differential curve, indicating structural diversity. FFBC2 shows a slightly larger cumulative pore volume and extended pore sizes up to 50 nm, although dominant mesoporosity persists in the 2–5 nm range. FFBC3 displays a narrower pore size distribution (2–8 nm) with reduced differential pore volume, reflecting pore blockage and collapse at high Fe(III) dosages.
Collectively, moderate iron salt modification introduces iron-based active components and optimises mesoporous architecture, whereas excessive FeCl3 causes pore collapse and structural degradation, reducing surface area and adsorption performance.
X-ray diffraction (XRD) analysis was performed on three FFBC materials, as shown in Fig. 5(a). The diffuse peak at 2θ = 22.6° corresponds to an amorphous carbon structure. The absence of distinct crystalline diffraction peaks confirms that the carbon framework lacks long-range atomic order. The formation of amorphous carbon is likely associated with the modification treatments applied during FFBC synthesis, which suppress carbon crystallinity. Moreover, SEM imaging reveals numerous surface defects and irregular morphologies, corroborating the amorphous character of the carbon structure. This amorphous configuration exhibits high surface reactivity, which strongly affects the adsorption capacity of FFBC materials.
3.4. Adsorption and Reduction Mechanism
Fourier Transform Infrared Spectroscopy (FTIR) was performed on FFBCCS8 (3% FFBC1 loading, 150 mg/kg Cr(VI) concentration) and FFBCCS3 (0.2% FFBC3 loading, 300 mg/kg Cr(VI) concentration), with spectra shown in Fig. 6(d). Broad and intense absorption peaks at 3444 cm−1 (FFBCCS8) and 3439 cm−1 (FFBCCS3) correspond to the stretching vibration of hydroxyl groups (−OH) [30], indicating abundant polar oxygen-containing functional groups on both materials. FFBCCS8 shows stronger absorption intensity due to its higher modifier dosage (3%), introducing more hydrophilic groups and enhancing Cr(VI) affinity. At 1635 cm−1, prominent peaks correspond to C=O/O–H bending vibrations of carboxyl (−COOH) or phenolic hydroxyl (−OH) groups [31], confirming that modification enriches surface oxygen-containing functional groups. The peak at 1024 cm−1 corresponds to C–O–C symmetric stretching or Si–O absorption [32], indicating possible ester or silicate groups. Low-wavenumber peaks (600–500 cm−1) arise from Fe–O vibrations [33], confirming successful iron loading.
FTIR results indicate that the Fe(III)/Fe(II) ratio and loading level influence the distribution of oxygen-containing functional groups. FFBC1 at low Fe(III) proportion and high dosage (FFBCCS8) introduced more hydrophilic −OH and −COOH groups, enhancing Cr(VI) adsorption, whereas FFBC3 at high Fe(III) proportion (FFBCCS3) exhibited fewer oxygen-containing groups despite clearer Fe–O signals, leading to reduced adsorption capacity.
To elucidate Cr(VI) reduction mechanisms, X-ray photoelectron spectroscopy (XPS) was used to analyse FFBCCS8, FFBCCS3, and a control group (FFBCCS8–none Cr(VI)) prepared with 3% FFBC1 but no Cr(VI). In the C 1s spectrum (Fig. 6(b)), C–C (284.4 eV), C–O (285.9 eV), and C=O (288.8 eV) peaks were identified [34], with C=O more abundant (9.65%) in FFBCCS8 than FFBCCS3, indicating greater availability of polar functional groups for Cr(VI) adsorption. The Cr 2p spectrum (Fig. 6(a)) shows Cr(VI) peaks at 580.0 eV and 589.1 eV [35] and Cr(III) peaks at 576.8 eV and 583.6 eV [36]. Cr(III) accounted for 60.07% in FFBCCS8, exceeding the 55.11% in FFBCCS3, demonstrating that high-dosage FFBC1 enhances Cr(VI) reduction to Cr(III). In the Fe 2p spectrum (Fig. 6(c)), Fe(III) and Fe(II) peaks appear at 711.1 eV and 713.6 eV [37], respectively. The control group showed no significant change in the Fe(III)/Fe(II) ratio, while in FFBCCS8, Fe(II) decreased as Fe(III) increased, suggesting Fe(II) reduced Cr(VI) to Cr(III) before being oxidised. The overall mechanism (Fig. 6(e)) involves: (1) electrostatic adsorption of Cr(VI) by surface −OH and −COOH groups and (2) Fe(II)-mediated reduction of Cr(VI) to Cr(III).
4. Discussion
Compared with previous studies using strong acids [38] to modify biochar, this work employed formic acid as a mild carbon source in combination with a FeCl3–FeSO4 composite iron source. The resulting material achieved Cr(VI) adsorption efficiencies exceeding 70% under three different FeCl3/FeSO4 ratios, significantly outperforming biochars modified with ZnCl2 or H3PO4 reported in earlier studies [39], while avoiding the secondary environmental pollution risks commonly associated with strong acid treatments. Moreover, unlike materials modified with single iron salts [40] or zero-valent iron [41], the FFBC material utilised a Fe(III)/Fe(II) composite system to enable a synergistic adsorption–reduction mechanism. This combined capacity for adsorption and electron transfer mitigates the limitations of nanoparticle zero-valent iron, including agglomeration and surface oxidation leading to reduced long-term reactivity. Orthogonal experiments identified the optimal FeCl3/FeSO4 ratio, enhancing the practical feasibility and offering valuable guidance for real-world engineering applications.
Regarding feedstock selection, rice straw is widely available in agricultural countries such as China, providing both economic advantages and ease of large-scale implementation. The proposed strategy therefore outperforms the use of more specialised or geographically restricted materials such as cycad leaves [42], algae[43], or panda faeces [44], highlighting superior resource recycling potential and cost-effectiveness.
Interestingly, high-Fe(III) samples (e.g., FFBC3) showed reduced adsorption capacity despite increased iron loading. This reduction resulted from structural agglomeration and pore wall collapse, which lowered the specific surface area and prevented proportional increases in adsorption efficiency. Furthermore, all samples demonstrated favourable pH adaptability, contrasting with many materials reported to be highly efficient only under strongly acidic conditions [45]. These findings indicate that multifunctional synergistic mechanisms on the FFBC surface contribute to its broad application potential.
Several results align with existing literature: iron salt-modified biochars generally enhance Cr(VI) adsorption and reduction capacity, with oxygen-containing functional groups and Fe(II) species mediating adsorption and reduction processes, respectively. FTIR and XPS analyses confirmed the partial reduction of Cr(VI) and the possible involvement of surface complexation mechanisms, consistent with previous studies on the synergistic roles of oxygenated functional groups and iron species [40]. However, owing to insufficient XRD evidence, this study could not directly verify the presence of mineral precipitation or specific coordination structures [46]. Consequently, the immobilisation mechanism remains hypothetical and warrants further validation. Moreover, the present work was limited to static batch adsorption experiments; flow-through column studies, field-scale trials, and long-term stability assessments under complex contamination scenarios remain necessary. Future research should prioritise elucidating remediation mechanisms, evaluating performance in multi-component systems (e.g., Cd, Pb), and exploring integration with phytoremediation strategies to expand practical applications.
The FeCl3:FeSO4 mass ratios investigated in this study—1:1, 2:1, and 3:1—were selected based on preliminary results, which showed no significant improvement in adsorption performance at ratios below 1:1. Nonetheless, systematic evaluation of lower FeCl3:FeSO4 ratios remains a promising direction for future research.
5. Conclusion
This study synthesised iron-loaded modified biochar (FFBC) from straw biochar via composite modification using formic acid with FeCl3·6H2O and FeSO3·7H2O at varying Fe(III)/Fe(II) ratios. The adsorption performance under different Cr(VI) contamination levels was systematically investigated, process parameters were optimised via orthogonal experimental design, and the adsorption–reduction mechanism was elucidated using FTIR, XPS, SEM, and BET analyses. The main findings are as follows:
Enhanced Adsorption and Reduction Performance: Composite iron salt modification markedly enhanced the adsorption and reduction capacity of biochar towards Cr(VI). The FFBC1 material, prepared with a FeCl3:FeSO4 ratio of 1:1, exhibited the highest adsorption efficiency (90.44%), a well-preserved pore architecture, and abundant surface oxygen-containing functional groups, thereby facilitating synergistic adsorption and reduction processes.
Adsorption–Reduction Mechanism: Cr(VI) was initially adsorbed via electrostatic interactions with surface hydroxyl and carboxyl groups and subsequently reduced to Cr(III) through redox reactions mediated by Fe(II), effectively decreasing its toxicity and mobility in soil.
Factor Influence Hierarchy: Orthogonal experimental analysis demonstrated that the FeCl3/FeSO4 ratio exerted the greatest influence on adsorption efficiency, followed by biochar loading, whereas the initial Cr(VI) contamination concentration had a comparatively minor effect.
Structural and Functional Optimisation: Characterisation results confirmed that moderate Fe(III) loading preserved pore integrity and enhanced functional group density, while excessive Fe(III) induced pore collapse and reduced active site availability, leading to diminished adsorption performance.
Environmental and Engineering Advantages: The modification strategy employed in this study is environmentally sustainable, operationally controllable, and resource-efficient. It avoids strong acid treatments and costly precursors, highlighting its strong potential for large-scale engineering applications in contaminated soil remediation.
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