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Article

Application of Iron-Bimetal Biochar for As and Cd Reduction and Soil Organic Carbon Preservation Under Varying Moisture

by
Frank Stephano Mabagala
1,2,
Tingjuan Wang
1,
Qiufen Feng
3,
Xibai Zeng
1,
Chao He
1,
Cuixia Wu
1,
Nan Zhang
1 and
Shiming Su
1,*
1
Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Key Laboratory of Agricultural and Rural Eco-Environment, Ministry of Agriculture and Rural Affairs, Beijing 100081, China
2
Tanzania Agricultural Research Institution (TARI), TARI-Mlingano Centre, Tanga P.O. Box 5088, Tanzania
3
Hunan Cultivated Land and Agricultural Eco-Environment Institute, Key Laboratory of Agro-Environment in Midstream of Yangtze Plain, Ministry of Agriculture, Changsha 410125, China
*
Author to whom correspondence should be addressed.
Agriculture 2025, 15(11), 1114; https://doi.org/10.3390/agriculture15111114
Submission received: 27 March 2025 / Revised: 7 May 2025 / Accepted: 19 May 2025 / Published: 22 May 2025
(This article belongs to the Section Agricultural Soils)
Browse Figures
Versions Notes

Abstract

:
The contamination of paddy soils with arsenic (As) and cadmium (Cd), coupled with the depletion of soil organic carbon (SOC), poses significant threats to rice yields and quality. There is an urgent need to identify a suitable soil additive capable of achieving simultaneous heavy metal remediation and promotion of organic matter enrichment. The current study introduced two novel iron (Fe)/magnesium (Mg)-based bimetal-oxide-modified rice straw biochar (RSB), namely RSB-Fe/Mn and RSB-Fe/Mg. It evaluated their effectiveness in As/Cd immobilization and SOC preservation. An 8-week cultivation experiment was carried out in sequential drying–flooding moisture fluctuation conditions, with the soil pore water As/Cd (PWAs/Cd) and SOC fractions monitored. The mechanisms of As/Cd immobilization were investigated using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS) characterizations. Results revealed that PWAs and PWCd were reduced by up to 67.1% and 80.2% during the drying period and by 27.0% and 76.5% during the flooding period, respectively. Additionally, SOC content increased by 16.3% and 33.9% with RSB-Fe/Mn addition during the drying and flooding period, respectively, with an increase in the mineral-associated organic carbon (MAOC) fraction. The study proves that RSB-Fe/Mn and RSB-Fe/Mg are effective for soil As/Cd passivation and SOC stabilization, offering a promising solution to mitigate As and Cd pollution in paddy soils while maintaining soil quality.

1. Introduction

Arsenic (As) and cadmium (Cd), coming from mining, industrial processes, agricultural practices, and natural sources, contaminate paddy soils [1,2], which further threatens human health through their entry into the food chain [3,4]. Mitigating As and Cd in paddy soils is particularly challenging due to their contrasting behaviors and responses to water management practices and soil conditions [5]. The inconsistent transformation of Cd and As in paddy fields poses considerable challenges for their remediation. The bioavailability of Cd decreases with rising soil pH, primarily due to the formation of precipitates such as Cd(OH)2 [6]. However, as soil pH increases, more As is released from bulk soil. This occurs because hydroxide groups in the soil solution compete with As for binding sites, reducing its adsorption to the soil matrix. In paddy soils, an opposite trend is typically observed: the availability of Cd decreases, while that of As increases during the transition from the drying to the flooding stage. This shift occurs as the soil becomes anaerobic, and its pH rises.
In previous studies, the contradiction between Cd and As immobilization efficiency has been resolved by applying some materials that can simultaneously reduce the bioavailability of Cd and As [7]. Functional groups exist on materials targeting Cd and As immobilization, respectively, benefiting Cd and As passivation simultaneously. Biochar has been proven effective in Cd and As passivation because of its high surface area and many binding sites, such as carboxyl and phenolic hydroxyl groups. Biochar-metal oxide composites are more effective in reducing Cd and As bioavailability, as metal oxide provides more sites for Cd and As to connect to the modified biochar [8]. There have been numerous studies regarding biochar modified with metal oxide in the remediation of paddy soils contaminated with As and Cd. For example, enhanced iron and manganese plaque formation improved microbial community structure, and increased soil enzyme activities were found for biochar modified with iron (Fe) and manganese (Mn) applications in acidic and mining-influenced environments. Magnesium (Mg)-modified biochar, such as Mg ferrite biochar and Mg-aluminum-modified biochar, is effective in immobilizing heavy metals such as As, Cd, and lead in contaminated soils, thereby reducing their bioavailability [9,10]. However, these studies scarcely focused on comparing the influence of biochar-Fe/Mn and biochar-Fe/Mg on soil Cd/As immobilization. The effect of biochar-Fe/Mn and biochar-Fe/Mg materials on Cd/As decontamination in different water content states of paddy soils, i.e., drying and flooding periods, has not been thoroughly studied.
In many regions, paddy soils contaminated with As and Cd often face additional health issues, such as declining soil organic carbon (SOC) levels and soil acidification. These problems not only exacerbate soil degradation but also reduce crop productivity. Incorporating organic soil amendments, such as biochar and straw, into paddy soil has increased organic matter content, improved overall soil conditions, and enhanced rice yield, particularly in acidic soils [7,11,12,13]. Metal oxides such as Mg oxide, Fe oxide, and Mn oxide play a significant role in regulating the biogeochemical cycles and protecting organic carbon in soils, mainly through forming particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). It has been reported that iron-bound organic carbon contributes 37.8% of the total organic carbon [14]. With the help of iron hydroxide collaboration, 37.8% of the dissolved soil carbon accumulates [15], 21.5% carbon stabilized soil organic matter is formed [16,17], 6.2–31.2% carbon-protected plant-derived aliphatic compounds and polysaccharides are created [18], and mononuclear iron (III)-NOM complexes form through redox and complexation reactions [19]. Like iron, manganese and magnesium oxide (MgO) also have a high potential for stabilizing organic molecules through organo–mineral associations in the soil [20,21,22]. Different from Mg oxide, the chemical valence of Fe and Mn oxides changes when they interact with soil components and contaminants or with water content switching, thus influencing the redox potential (Eh) of paddy soils, affecting Cd/As passivation efficiency and the formation of MAOC and POC [16,20,23]. However, a significant gap remains in understanding the role of Fe/Mg oxide- or Fe/Mn oxide-modified biochar composite in regulating As and Cd bioavailability and SOC fractions. Also, few studies have focused on interpreting how the redox reaction of Fe and Mn elements in biochar-Fe/Mn functions in reducing As and Cd contamination and forming POC and MAOC.
This study aims to assess the effect of iron-bimetal modified rice straw biochar for the simultaneous reduction in As and Cd and preservation of SOC under varied moisture conditions in paddy soils. It targets determining the function of iron bimetal biochar in decreasing As and Cd’s mobility and increasing SOC retention under different wet conditions. To this end, two types of modified biochar have been prepared and applied: rice straw biochar doped with iron/manganese oxides (RSB-Fe/Mn) and iron/magnesium oxides (RSB-Fe/Mg). This investigation is directed towards improving the debilitation of As and Cd mobility, making it available during drying and flooding conditions of typical paddy field environments. We also investigate the impacts of these materials on soil pH and the stabilization of the SOC fractions. This specific research seeks to answer the question: In dynamic moisture conditions, can iron-bimetal-modified biochars effectively reduce the mobility of As and Cd while promoting SOC stabilization in paddy soils? With the findings, recommendations can easily be made on strategies that are likely effective for managing As and Cd from soils while enhancing soil health so that environmental protection and sustainable soil management goals can be met.
By this integrated approach, this study analyzed the essential research questions relating to the efficiency of each material regarding metal immobilization and the underlying role they play in SOC stabilization, as well as the differential pathways through which Fe/Mn- and Fe/Mg-modified biochars interact with contaminants and SOC. These findings will ultimately guide how biochar could be appropriately amended to enhance soil health against heavy metal risks in paddy agroecosystems. Therefore, this research aims to determine the function of iron bimetal biochar in decreasing the mobility of As and Cd and increasing the retention of SOC under various moisture conditions.

2. Materials and Methods

2.1. Materials and Reagents

All experimental data in this study were collected in 2024. The soil that was studied here was collected at 0–20 from a paddy field in Liuyang, Hunan, China, located at 28°0′33′′ N, 113°19′25′′ E. In the new World Reference Base (WRB) 2022 for soil resource classification system, based on the acidic nature, texture (silt), hydromorphic conditions typical of paddy fields, and soil disturbance and long-term management, the soil is classified as an Anthrosol soil. The basic soil properties and heavy metal content are presented in Table S1. After collecting the soil, we placed it in a dark place to expose it to the natural environment. After collection, the soil was cleaned of impurities and sieved through a 2 mm mesh. RSB was purchased from Henan Lize Environmental Protection Co., Ltd. and produced from rice straw through oxygen-limited pyrolysis at 700 °C for approximately 4.5 h. Reagents, including Fe(NO3)3·9H2O, Mn(NO3)2·4H2O, Mg(NO3)2·6H2O, urea, Cd(NO3)2·4H2O, and NaAsO2, were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). A rhizon sampler with a pore size of 0.15 μm was purchased from Rhizosphere Co., Ltd. (Wageningen, The Netherlands).

2.2. Synthesis of RSB Composites

RSB-Fe/Mn and RSB-Fe/Mg were synthesized using the hydrothermal method. Specifically, the process of synthesizing the materials involved four main steps. First, 15 g of RSB was added to 500 mL metal solutions containing 0.15 mol L−1 Fe(NO3)3·9H2O and 0.05 molL−1 Mn(NO3)2·4H2O to prepare RSB-Fe/Mn, and 0.15 molL−1 Fe(NO3)3·9H2O and 0.05 mol L−1 Mg(NO3)2·6H2O to prepare RSB-Fe/Mg. The mixture was homogenized with a magnetic stirrer to form a suspension. Five hundred milliliters of urea solution at a concentration of 1.65 mol L−1 was slowly added to the suspension under continuous stirring. After thoroughly mixing the suspension, the liquid was heated in an oil bath at 100 °C for 8 h with constant agitation. After the reaction, the solution was naturally cooled, and the formed gel was precipitated and aged for 12 h. The product was separated by centrifugation at 6000 rpm, then freeze-dried and ground to obtain the RSB-Fe/Mn and RSB-Fe/Mg materials.

2.3. As and Cd Adsorption Kinetics by RSB-Fe/Mn and RSB-Fe/Mg

The adsorption batch experiments were conducted on a constant temperature oscillator at a rate of 200 rpm under 298 ± 1 K. All adsorption experiments were carried out in triplicate. The stock solutions of Cd (II) and As (III) (1000 mg L−1) were prepared from the dissolution of Cd(NO3)2·4H2O and NaAsO2, respectively. The adsorption kinetics experiment of RSB-Fe/Mn, RSB-Fe/Mg, and RSB was examined by mixing 20 mg of adsorbent with 60 mL of water contaminated with 10 mg L−1 As or Cd, respectively. Samples of the adsorption solution were collected at various time points while shaking on a constant temperature oscillator: 0, 5, 10, 15, 20, 40, 60, 120, 180, 360, 720, 1440, 2880, and 4320 min. A 0.22 μm filtration membrane filtered the mixture, and As and Cd concentrations in the filtrate were measured. As concentration was determined by Atomic Fluorescence Spectrometry (HG-AFS, AFS-9120, Ji Tian Inc., Beijing, China), with a limit of detection (LOD) = 0.033 μg L−1, limits of quantitation (LOQ) = 0.109 μg L−1, and recovery rate = 87.2% for the As concentration test. Cd concentration was tested using Inductively Coupled Plasma Mass Spectrometry (ICP-MS, NEXION 2000 G, Perkin-Elmer Instruments Co., Ltd., Shelton, CT, USA), respectively, with LOD = 0.718 ng L−1, LOQ = 2.394 ng L−1, and recovery rate = 96.5% for Cd concentration measurement. The adsorption kinetic data for As and Cd on RSB-Fe/Mn, RSB-Fe/Mg, and RSB were analyzed using pseudo-first-order and pseudo-second-order models.
Pseudo-first-order model:
ln (Qe − Qt) = ln Qe − K1 * t
Pseudo-second-order model:
te/Qt =1/K2 * Q2e + t/Qe
where Qt (mg g−1) is the adsorption capacity of RSB-Fe/Mn, RSB-Fe/Mg, and RSB at time t (h); Qe (mg g−1) is the adsorption capacity at equilibrium; t = 0, which can be related to the other parameters as: h = k2·qe2; K1 is the rate constant for the pseudo-first-order; K2 is the rate constant for the pseudo-second-order model.

2.4. Soil Incubation Experiment

Fifty grams of dry soil was weighed and placed in 100 mL plastic containers. The experiment included four treatments: control treatment (CK), RSB, RSB-Fe/Mg, and RSB-Fe/Mn, each with four replicates and two incubation durations. For the drying stage, the incubation duration was set at 28 days. For the flooding stage, an additional 28-day incubation period was conducted following the initial 28-day drying stage. In a dry stage test, 1 g of RSB-Fe/Mg or RSB-Fe/Mn was thoroughly mixed with the dry soil. The CK treatment was set up in the same conditions as the RSB, RSB-Fe/Mg, and RSB-Fe/Mn treatments, except that no material was added to the dry soil during incubation. Distilled water was added to the adsorbent and dry soil mixture to maintain moisture at 70% of the field water holding capacity (FWHC). The FWHC was determined gravimetrically by calculating the soil’s water-holding capacity (WHC). The WHC for the soil was defined in terms:
WHC (%) = (W2 − W3)/W3 * 100
where
W2 is the weight of the wet soil after saturation and drainage, and
W3 is the oven-dry weight of the soil.
The moisture content was maintained by weighing the samples and adding water to the tube every five days. A rhizon sampler was inserted into the soil at the beginning of the incubation. After 28 days of incubation at 70% FWHC and 25 degrees, avoiding sunlight, 5 mL of soil solution was taken from the soil using a rhizon sample under the vacuum provided by a syringe. Soil’s Eh and pH were tested in situ using a microelectrode (Mettler Toledo GmbH, Im Langacher 44 8606 Greifensee, Switzerland). The soil was naturally dried for further analysis. For a flooding stage test, after 28-day incubation at 70% FWHC, distilled water was added to the incubation tube to maintain a water level of around 2 cm above the soil. The amount of distilled water added was the same for all trials, ensuring a consistent water content for all treatments. The incubation test lasted for another 28 days under flooding conditions. Then, a 5 mL soil solution was collected, and the soil’s Eh and pH were tested using the same method as previously mentioned. The soil was kept by natural drying, grinding, and storing for further analysis.

2.5. Fractionation of SOC

A simplified physical size fractionation using the method described by Morfidou et al. [24] and Han et al. [25] was conducted. Briefly, 10 g of air-dried soil was subjected to reciprocal shaking in 40 mL of 5 g L−1 sodium hexametaphosphate (Na6O18P16) solution in a 50 mL centrifuge bottle at 0.00817 mol L−1 concentration at 180 rpm overnight (16 h) in a 1:8 w/v ratio to disperse aggregates. The mixture was shaken at 300 r min−1 for 16 h and centrifuged at 5000× g for 20 min. The materials that remained on a 0.53 mm sieve’s upper surface (referred to as the POC fraction) and those that passed through it (referred to as the MAOC fraction) were transferred into glass beakers that had been pre-weighed. The soil fractions were dried in a forced-air oven at 105 °C for 48 h. The POC and MAOC fractions and representative soil samples were subjected to milling to provide a fine and homogenous powder material before C analysis.

2.6. Determination of Iron Oxide Fractions

The iron oxide fractions were determined by modified sequential extraction techniques of Patzner, M.S. et al. [26] and Coward, E.K. et al. [27]. Free Fe oxides (Fef) were extracted via the dithionite–citrate–bicarbonate (DCB) technique. Organic-associated Fe oxides (Feo) were extracted using sodium pyrophosphate, and poorly crystallized Fe oxides (Fed) were determined with acid ammonium oxalate in dark conditions.

2.7. Characterization of the Materials

The FTIR spectra of RSB-Fe/Mn, RSB-Fe/Mg, and RSB before and after adsorption were collected on a Nicolet 6700 FTIR spectrometer (Thermo Fisher Co., Waltham, MA, USA) in the 400–4000 cm−1 range at a 0.09 cm−1 measurement resolution. The mineral compositions of materials before and after adsorption were detected by XRD using a Rigaku Ultima IV instrument (Akishima-shi, Japan) with Cu Kα radiation (λ = 0.1541 nm) at 40 KV and 40 mA current. The 2θ range scanned was 10 to 80°, with a step size of 0.01°. The Jade 6.0 software was used to determine the potential mineral phases of the samples. The different states of Fe, Mn, Mg, O, C, Cd, and As elements in RSB-Fe/Mn and RSB-Fe/Mg materials before and after adsorption were determined by XPS with monochromatic Al Kα radiation (1486.8 eV) using an Amicus XPS instrument (Shimadzu Co., Kyoto, Japan) in full scan mode and fine spectra scan for respective elements. The morphology of the RSB-Fe/Mn and RSB-Fe/Mg materials was investigated by Scanning Electron Microscopy (SEM, ZEISS GeminiSEM 300, Oberkochen, Germany).

2.8. Statistical Analysis

Data analysis was made with R software, version 4.5.0. The results are expressed as the mean values with standard deviations (SD). The Pearson correlation test was carried out using SPSS 22 (SPSS Inc., Chicago, IL, USA), with a significance level of p < 0.05. Characterization data analysis was performed with Jade 6 (Materials Data, Inc., Livermore, CA, USA), XPSPEAK (Raymund W.M. Kwok, Department of Chemistry, The Chinese University of Hong Kong, Shatin, Hong Kong), and ImageJ version 1.54p (Wayne Rasband). All the figures were created using Origin 2022 Pro (OriginLab Corp., Northampton, MA, USA).

3. Results

3.1. Cd/As Adsorption Kinetics and Isotherm on RSB-Fe/Mn and RSB-Fe/Mg

We used pseudo-first-order and pseudo-second-order kinetic models to study the adsorption kinetics of As and Cd on RSB-Fe/Mn, RSB-Fe/Mg, and RSB. As and Cd underwent fast adsorption in the first 200 min, as shown in Figure 1, and eventually reached equilibrium in 4320 min. The higher correlation coefficients (R2 values greater than 0.99) shown in Table 1 support the fitting curves’ suggestion that the pseudo-second-order model more accurately described the adsorption behavior than the pseudo-first-order model. This suggests that chemisorption mechanisms mainly control the adsorption process for both metals on the modified and unmodified biochars. Under both kinetic models, RSB-Fe/Mn showed a higher capacity for adsorption of both As and Cd than RSB-Fe/Mg and raw RSB. The pseudo-second-order model exhibited a higher adsorption capacity than the pseudo-first-order kinetics for all tested materials. The initial speedy adsorption regime, followed by moderate saturation equilibrium, corroborates the kinetic constants from the pseudo-second-order model. The prime conclusion here is that the presence of metal ions, such as Fe/Mn or Fe/Mg, significantly increases the adsorption rate and loading capacity of RSB, confirming the beneficial effects of loading metal ions for surface reactivity and available sites.
The fitting curves in Figure 1 and the parameters listed in Table 1 offer compelling proof that surface modification successfully enhanced the adsorption performance. In Table 1, kinetic parameters, such as equilibrium adsorption capacities and rate constants, are broken down in detail. These results highlight the significance of material modification strategies in maximizing adsorbent performance for removing As and Cd from aqueous environments.

3.2. Influence of RSB-Fe/Mn and RSB-Fe/Mg Addition on Soil Cd/As Content, pH, and Eh

The findings of the screening test for soil incubation are shown in Figure 2. Before cultivation, porewater had comparatively high As and Cd concentrations. Significant decreases in both contaminants were seen in all treatments following four weeks of cultivation during the drying period, with the most noticeable reductions occurring under the addition of RSB-Fe/Mn. During both the drying and flooding phases, it was noteworthy that Cd removal was always more successful than As removal. Nevertheless, under flooding conditions, the As decontamination rate decreased, especially in treatments with RSB and RSB-Fe/Mg, suggesting that As immobilization may be limited by water saturation. However, the Cd concentrations under RSB-Fe/Mn and RSB-Fe/Mg amendments stayed noticeably lower during both periods (p < 0.05). The pH and Eh of the soil varied with treatments and moisture regimes, as shown in Figure 2c,d. All of the amendments raised the pH of the soil in comparison to the control, and the flooding period generally produced higher pH values than the drying period. In both periods, adding materials based on RSBs raised the soil’s Eh, with RSB-Fe/Mn consistently attaining the highest Eh values. These modifications improved soil oxidation conditions and pH buffering, which may have helped explain the observed decreases in metal mobility.

3.3. Evaluation of Kinetic Model Performance Using Error Metrics for Cd and As Adsorption

Kinetic models for Cd and As adsorption were evaluated for their predictive performance using the coefficient of determination (R2), Mean Absolute Error (MAE), and Root Mean Square Error (RMSE). The results from Table 2 showed that the pseudo-second-order (PSO) model consistently outperformed the pseudo-first-order (PFO) model for Cd adsorption across all adsorbents, indicating lower MAE and RMSE values and demonstrating greater fitting and predictive accuracy. In contrast, the PFO model provided a better fit for As adsorption, as evidenced by consistently lower error metrics compared to the PSO model. The As adsorption was better described by the PFO model, as indicated by significantly lower MAE and RMSE values. These results highlight distinct kinetic behaviors for Cd and As, underscoring the importance of using multiple error metrics alongside R² to assess model performance. Table 2 supports the preferential application of the PSO model for Cd and the PFO model for As systems. These results validate the suitability of PSO for Cd and PFO for As, highlighting the necessity of contextual interpretation of MAE and RMSE in addition to a mechanistic understanding and enhancing the dependability of kinetic model selection.

3.4. Influence of RSB-Fe/Mn and RSB-Fe/Mg on Fe-Oxides and SOC Fractions

The RSB, RSB-Fe/Mn, and RSB-Fe/Mg amendments affect variations in soil Fe-oxides and SOC fractions, as seen in Figure 3 and Figure 4. Treatments showed some variation, especially in drying, when a significant increase in Fef, POC, and MAOC was found in amendment-treated soils compared to the control. The treatments showed that the RSB-Fe/Mn amendment consistently increased these fractions, providing strong evidence for its greater ability to modify stable Fe and SOC pools instead. On the other hand, decreased concentrations of the more labile forms, such as Feo, Fed, and DOC, indicate a transformation from active into more passive forms of iron and carbon. The RSB-Fe/Mn again showed the most significant increases in Feo, Fed, DOC, and MAOC during the flooding period (Figure 4), indicating its efficacy in anaerobic environments. Fef and POC have been shown to decrease across treatments, which suggests that SOC may be redistributing or changing into more stable forms, especially MAOC. Figure 5 summarizes the normalized distributions of SOC fractions. In drying conditions, POC dominated the SOC pool across treatments; however, there was a noteworthy increase in MAOC fractions with RSB-Fe/Mn and RSB-Fe/Mg amendments. During the flooding, a decrease in POC and a simultaneous increase in MAO and DOC fractions were observed, which indicates that the labile carbon was converted to a more stable form under modified conditions. These findings highlight the role of Fe(Mn)-modified biochar in promoting SOC stabilization by increasing MAOC formation and controlling DOC dynamics.

3.5. Spectral Characterization Results

The SEM images in Figure S1 show significant morphological changes in RSB-Fe/Mn and RSB-Fe/Mg compared to the unmodified raw RSB. In Figure S1a,b, the unmodified biochar samples exhibited a relatively smooth surface, where after modification with Fe/Mn (c,d) and Fe/Mg (e,f) oxides, nanoparticles with a size of 10–40 nm or thin flasks with a thickness of ~10 nm were formed on the RSB surface, making a densely covered rough surface, indicating successful impregnation of the Fe, Mn or Mg oxides. Notably, more flasks were formed on RSB-Fe/Mg, while more nanoparticles were found for RSB-Fe/Mn. The FTIR spectra in Figure 6a showed the absorption patterns of different material samples and indicated the presence of various functional groups and chemical interactions. The peak at 1036 cm−1 with strong intensity was attributed to the =C-H and =CH2 bending vibration of the carbon skeleton of RSB. The 910 and 792 cm−1 peaks corresponded to aromatic CH out-of-plane vibrations. The bands between 1600 and 1670 cm−1 belonged to C=O stretching vibration; after As and Cd adsorption, it can be seen that a new peak appeared at 1659 cm−1 beside the original peak at 1616 cm−1, indicating that carboxyl or ester groups were involved in the As and Cd adsorption. The C-O-H bending vibration at 1384 cm−1 increased after As and Cd adsorption on RSB-Fe/Mn and RSB-Fe/Mg. Furthermore, the O-H stretching vibration was observed at 3340 cm−1, increasing after Cd and As adsorption.
The XRD patterns obtained for RSB, RSB-Fe/Mn, RSB-Fe/Mg, and their respective oxide materials are presented in Figure 4b. In all three samples, RSB, RSB-Fe/Mg, and RSB-Fe/Mn, distinct peaks of SiO2 were observed at 26.58°, 50.1°, and 59.82° for the two metal adsorbents and metals. Additional peaks at 21.00°, 36.56°, and 59.05° could be attributed to the formation of Fe/Mn and Fe/Mg hydrotalcites during the latter stages of the process. Characteristic peaks of lepidocrocite appeared at 14.00° and 47.04° for RSB-Fe/Mg, while they were absent in RSB-Fe/Mn before adsorption. Nevertheless, lepidocrocite formation was evident with both materials after As and Cd adsorptions, based on the formation of newly developed peaks. MnO2 formation for the RSB-Fe/Mn was confirmed by the presence of peaks at 31.34°, 37.49°, 41.35°, and 51.57°. Moreover, the peaks located at 28.03°, 42.57°, and 81.55° that appeared after adsorption were due to CdCO3 and FeAsO5 formation and suggested successful chemical interaction between the materials and target contaminants.
After adsorption, Cd and As were detected on RSB-Fe/Mg and RSB-Fe/Mn materials by XPS analysis. The Cd3d spectra of RSB-Fe/Mg and RSB-Fe/Mn, as illustrated in Figure 7 and Figure 8, showed peaks at 406.14–407.24 eV and 405.76–406.92 eV, respectively (Table 3). Cd-OM was the predominant species for RSB-Fe/Mg (36.10%), while Cd-CO3 and Cd-OH had the highest content for RSB-Fe/Mn (30.09%). Similarly, two subpeaks were identified in the As3d spectra of RSB-Fe/Mg and RSB-Fe/Mn at 49.38 eV, 50.08 eV, and 47.93 eV and 48.76 eV, respectively. The ratios changed to 35.12% and 64.88%, respectively, on RSB-Fe/Mn, whereas the As (III) and As (V) species accounted for 57.30% and 42.70% on RSB-Fe/Mg (Table 3). The RSB-Fe/Mg O1s spectra shifted following adsorption, with peaks shifting from 529.5–531.74 eV to a lower range (529.31–531.59 eV). However, RSB-Fe/Mn did not exhibit any discernible change. For RSB-Fe/Mg, the M–O ratio rose from 11.72% to 16.35%; RSB-Fe/Mn went from 7.30% to 18.16%. While the M–OH ratio rose for RSB-Fe/Mn (from 44.95% to 51.72%), it fell for RSB-Fe/Mg (38.24% to 34.92%). RSB-Fe/Mn had a significant drop in H2O content (47.75% to 30.12%), while RSB-Fe/Mg had a less noticeable drop (50.04% to 48.73%). The Mn2p spectra (Figure 7) showed a change in oxidation states; Mn2+ decreased from 33.27% to 17.28%, while Mn3+ and Mn4+ increased from 38.58% and 28.15% to 44.86% and 37.86%, respectively. In the Mg1s spectra (Figure 8), the increase in the Mg-O-M ratio was 35.65% to 40.86%, whereas that of Mg-OH decreased from 36.56% to 30.14%. For Fe2p spectra (Figure 7 and Figure 8), the Fe2+ increased from 45.70% to 49.38% after adsorption, whereas Fe3+ decreased from 54.30% to 50.62%.

4. Discussion

4.1. Impact of RSB-Fe/Mn(Mg) on Soil As/Cd Content, pH, and Eh During Flooding and Drying Stages

Heavy metal dynamics and soil chemical properties are greatly impacted by periodic soil pH and Eh variations during alternating flooding and drying phases in paddy soils treated with biochar modified with metal oxides [28]. It is commonly known that adding modified biochar to soil raises its pH, affecting the mobility and bioavailability of heavy metals like As and Cd [28,29]. In line with earlier research, our investigation found that flooding led to higher soil pH levels and levels of dissolved organic carbon and amorphous iron oxides, which help stabilize metals like Pb and Cd by changing them into less bioavailable forms [28]. Figure 2 clearly shows that soil pH increased and Eh decreased significantly during the transition from drying to flooding, consistent with the processes described by Wang et al. [30]. Specifically, the rise in soil pH under flooded conditions can be attributed to the oxidized compounds’ depletion and bicarbonate formation as soils become anaerobic [31]. The decline in Eh is likely driven by the reduction in electron acceptors such as iron and manganese oxides, consistent with the mechanisms outlined in Wang et al. [30]. However, compared to Wang et al., who reported near-complete depletion of Eh under prolonged flooding, our results showed that Eh, while significantly reduced, remained positive and did not approach zero (Figure 2d). Reducible iron and manganese oxides, providing alternative electron acceptors and increasing the biochar matrix’s electron buffering capacity [32], can explain this difference.
Furthermore, as well as slowing the decline in Eh in our study compared with untreated controls and past studies with unmodified biochar, the roles of biochar in moderating electron exchange processes [33], enhancing microbial electron transfer [34], and contributing to oxygen retention [34,35] are more likely to be there. These findings suggest that incorporating RSB-Fe/Mn and RSB-Fe/Mg oxides onto RSB can significantly influence soil redox behavior, leading to more stable redox conditions even under saturated environments. This adds new insight into the role of oxide-modified biochar in improving nutrient retention and heavy metal immobilization under fluctuating moisture regimes. This area has not been fully addressed in earlier studies [36].
Numerous studies have found that modified biochar materials have good potential to reduce the mobility and limit the bioavailability of As and Cd in paddy soils, especially when combined with different metals [37]. Compared with unmodified biochar, modified biochar significantly reduces As and Cd availability in soil and plant uptake [38,39]. Research shows an average reduction of 65.01% in the availability of Cd in soil and 70.72% in the Cd content of plants, with specific modifications achieving a reduction of up to 96.34% in the availability of Cd in soil [40]. Results from this investigation indicated that RSB-Fe/Mn addition could alleviate As and Cd content in porewater, varying between dried and flooded conditions, such as 77.50% and 67.06% lower during dried periods and 76.51% and 27.02% lower during flooding events for As and Cd, respectively. RSB-Fe/Mg also recorded a good performance but was less efficient when compared to RSB-Fe/Mn. These results cohere with Shaheen et al. [41] and Yang et al. [42], who reported that iron-modified biochars intensify As and Cd’s immobilization under flooded paddy soils due to increased surface area and chemical reactivity. Different from some studies stressing that iron is the most significant modifying agent [41,42], our results denoted that introducing manganese enhances further efficiency of materials, especially in variable redox conditions, as also shown in [23,43] where dual-modified (Fe/Mn) biochars present diversified reactive sites for stabilizing heavy metals. Unlike those recorded by Li et al. [44], where continuous reductions in As and Cd immobilization were observed under anaerobic conditions, a slight decrease in As immobilization and improvement in Cd fixation were observed under flooded conditions. The differences in soil type, pH buffering capacity, and the particular modified biochar synthesis method can explain this discrepancy. In cases where we observed an increase in pH during flooding, the increase is thought to have inhibited As adsorption through competition with hydroxyl ions while promoting precipitation of Cd, a mechanism also reported in Lee et al. [45]. The superior performance of RSB-Fe/Mn over RSB-Fe/Mg may be due to Mn’s redox-active nature and its capability to form Mn oxides, which contribute extra binding sites for metal ions [34]. These findings highlight how crucial it is to modify biochar to accommodate paddy soils’ varying moisture regimes. The dual function of RSB-Fe/Mn under both oxic and anoxic conditions makes it seem a unique and dynamic option for long-term bio-remediation of As and Cd. Our studies rather drive the unison of Fe and Mn in stabilizing heavy metals as an improved approach to soil health and food safety well-being for contaminated rice-growing areas from previous studies on iron or magnesium modifications [41,42,43,44].
A kinetic study was carried out to examine the adsorption kinetics of As and Cd on RSB-Fe/Mn, RSB-Fe/Mg, and RSB in aqueous solution. The adsorption data were fitted using pseudo-first-order and pseudo-second-order kinetic models (Figure 1). The observed experimental adsorption rates were consistent with the pseudo-first-order rate constants (K1), which had the order RSB-Fe/Mn > RSB-Fe/Mg > RSB. Additionally, for both As and Cd, the regression coefficients (R2) for the pseudo-second-order model were consistently higher than those of the pseudo-first-order model, as indicated in Table 1. This suggested a chemisorption process involving electron sharing or exchange between the adsorbates and functional groups on the adsorbent surface better describes the adsorption mechanism [43]. The ranking of constant rates for the pseudo-second-order model (RSB-Fe/Mn > RSB-Fe/Mg > RSB) verified that the presence of Fe/Mn modification remarkably corresponded to the increase in adsorption rate over Fe/Mg and unmodified RSB. These results agree with the findings of [46,47], who stated that Fe/Mn-modified biochars exhibit higher efficiency in heavy metal adsorption due to increased surface reactivity and redox-active sites. Nevertheless, in contrast with some older reports showing similar adsorption capacities between Fe/Mn and Fe/Mg modifications [48,49], our findings seemed to suggest a definite favor for Mn incorporation, possibly due to Mn’s synergistic effects across its multiple oxidation states, enabling efficient complexation of both As and Cd. Such discrepancy could be attributed to variations in synthesis modes, biochar feedstock, or solution pH conditions used in adsorption experiments [50]. Accordingly, our results strengthen previous conclusions and render new perspectives on the relative effectiveness of Mn and Mg co-modification for multimetal adsorption systems.

4.2. Impact of RSB-Fe/Mn and RSB-Fe/Mg on the Protection of SOC

Organic compounds can interact directly with mineral surfaces through various sorption reactions or stabilize through co-precipitation and aggregate formation [51]. The interaction of minerals with organic matter is an important mechanism that stabilizes SOC and shields it from microbial metabolism. In particular, anion exchange, ligand exchange/surface complexation, hydrophobic interactions, entropic effects, hydrogen bonding, and cation bridging are some of the ways that iron and manganese oxides bind organic material [20,52]. Thus, these oxides have gained recognition for their significant potential to stop the deterioration of organic matter. Our study demonstrated the superior SOC stabilization capacity of RSB-Fe/Mn by showing the highest POC and MAOC levels under drying conditions when compared to RSB-Fe/Mg, raw RSB, and control treatments. A change from POC to MAOC was noted upon entering a flooding regime, with the RSB-Fe/Mn treatment showing the most noticeable increase. An increase in the fraction of organically associated Feo coincided with this rise in MAOC content, indicating that Fe oxides regulate the SOC stabilization process. The effects of iron, manganese, and magnesium oxides on SOC stabilization have been documented in the past [53], with ligand exchange, chelation, cation bridging, flocculation, and precipitation mechanisms serving as the primary mechanisms for protection [54]. Our results agree with Zhuang Y et al. [55], who showed how manganese oxides serve as essential micronutrients that improve microbial processes that support the persistence of organic matter and aid SOC stabilization through organo–mineral interactions. We also found that Mn-rich conditions promote the formation of MAOCs, which is supported by the affinity of N-rich organic compounds for Mn-oxides [56]. Previous studies have demonstrated that the interaction of biochar with iron and manganese minerals is also essential in promoting the formation of MAOC [57].
Furthermore, incorporating carbon into macroaggregates is one way biochar components can promote physical protection mechanisms and improve SOC stability [58]. This aligns with our discovery that SOC storage under variable moisture conditions was considerably enhanced by RSB-Fe/Mn and RSB-Fe/Mg treatments (Figure 5). Therefore, using RSB-Fe/Mn and RSB-Fe/Mg materials could be a good way to keep SOC levels stable and encourage long-term carbon sequestration. These advantages should significantly improve soil health and support the development of sustainable farming methods, especially in the face of changing moisture regimes and climate variability [59].

4.3. The Cd/As Interaction Mechanisms with RSB-Fe/Mn and RSB-Fe/Mg

The SEM images in Figure S1 show significant morphologic changes in the RSB following Fe Mn and Fe Mg modification compared to the unchanged raw RSB. Modified RSB-Fe, RSB-Fe, Mn, and RSB-Fe, Mg showed more rough surfaces, indicating increased surface area and generation of more reactive As and Cd binding sites. These morphological changes align with previous findings that the modification of manganese induces a flaky structure and promotes agglomeration on the surface of biochar, thus increasing the material’s stability [60]. The material specimens’ FTIR spectra (Figure 6a) showed different absorption patterns, revealing modifications to the functional groups and chemical bonds before and following As and Cd adsorption. Precipitation, complexation, ion exchange, and electrostatic attraction are some processes that lead to the immobilization of As and Cd [61]. Oxygen-containing functional groups like C=O, -OH, and C-O were actively involved in forming inner-sphere and outer-sphere complexes with As and Cd, according to an analysis of the FTIR spectra. Significantly, the number of active sites that could form M-O-As and M-O-Cd complexes (where M = Fe, Mn, or Mg) increased with the introduction of Fe/Mg and Fe/Mn oxides.
Furthermore, FTIR data showed that the pure biochar was abundant in oxygenated groups like esters, carboxyls, and phenols, as well as C=C (from alkenes and aromatic structures). These groups led to a high binding ability between As/Cd and RSB-Fe/Mn or RSB-Fe/Mg, comparable to the results of Xiang et al. [62] and Yang et al. [23], who found that the superior performance of the RSB-Fe/Mn and RSB-Fe/Mg materials in adsorbing As and Cd was due to the fact that Mn and Fe improved the surface properties and metal binding abilities of the RSB. In another study, Tang et al. [63] reported that iron and manganese facilitate the formation of hydroxyl and oxide functional groups on the RSB surface, thereby increasing the number of active sites available for metal adsorption. XRD patterns confirmed that precipitation and co-precipitation processes occurred during As and Cd adsorption [64,65]. Specifically, the formation of crystalline precipitates such as CdCO3 and FeAsO5 indicated direct reactions between As/Cd and the modified biochars. Complementary XPS analysis revealed that heavy metal adsorption onto RSB-Fe/Mn was accompanied by Mn (II) generation, suggesting that Mn (IV) was reduced during As immobilization. Simultaneously, partial oxidation of As (III) to As (V) was detected, consistent with earlier studies reporting that Mn oxides can oxidize As(III), thereby enhancing its sorption onto Fe oxides [66].
The XPS analysis indicated that Fe (III) was partially reduced to Fe (II) (Table 3), suggesting the formation of goethite-Fe (II)-As (III) intermediates [67], which are known to accelerate As adsorption processes. These redox reactions, the oxidation of As (III) and the reduction of Mn (IV) and Fe (III), likely explain the superior As adsorption capacity of RSB-Fe/Mn compared to other treatments [68,69]. However, cannibalizing the RSB-Fe/Mg adsorbent resulted in lesser oxidation of As (III) into As (V) and a lesser extent of Fe (III) reduction, which denotes that none of the goethite-Fe (II)-As (III) intermediate formation during As adsorption was present. Yet, XPS indicated that Mg–OH species were converted to Mg–OM complexes after adsorption, implying that the hydroxyl groups on the surfaces of MgO became substituted by either As or Cd. This concurs with reports from studies investigating MgO-modified biochars, where Cd adsorption has been attributed mainly to mineral precipitation and ion exchange processes. In general, the amalgamation of SEM, FTIR, XRD, and XPS evidence strongly confirms the successful immobilization of As and Cd on RSB-Fe/Mn and RSB-Fe/Mg materials. These findings further highlight the tremendous potential of iron and manganese modifications in enhancing biochar’s reactivity and environmental remediation capacity for As and Cd-contaminated soils.

4.4. Correlation Analysis of As/Cd Contents, SOC Fractions, and Soil Physicochemical Properties with Material Addition

The correlation analysis between soil properties and the Mantel test results for DOC, MAOC, and POC in the drying and flooding phases is shown in Figure 9. In the drying period, significant positive correlations were found between Free Fe and PWAs/PWCd (Spearman’s r > 0.5, p < 0.01). Free Fe was negatively correlated with soil pH due to the introduction of free Fe ions with RSB-Fe/Mg and RSB-Fe/Mn addition. SOC adversely correlated with PWAs, meaning improving SOC benefits the soil as passivation. For SOC fractions, it can be seen that MAOC was correlated with PWCd, and pH/Eh was an important factor influencing POC, MAOC, and DOC. MAOC and DOC showed a higher correlation with Fed and Feo in the flooding period. MAOC also exhibited higher correlations with PWCd; however, no SOC fractions showed significance with PWAs. pH still claims to be the main reason for POC, MAOC, and DOC change. pH also showed a higher negative correlation with Fed and Feo, consistent with the Fe oxide transformation into Fed and Feo during the flooding stage.
From the analysis, a hypothesis can be deduced that the relationship between iron species and carbon content under different moisture conditions greatly influences soil As and Cd stabilization. The oxidation of iron (II) during drying can lead to the formation of reactive iron (III) minerals, which facilitate the stabilization of MAOM and POM through co-precipitation and adsorption processes [70]. Conversely, iron (III) can be reduced to iron (II) during flooding, releasing the previously stabilized SOC and making it accessible for microbial degradation. By acting as an electron shuttle, DOC can increase iron reduction and CO2 emissions [71]. Strong positive correlations between PWAs and free Fe were observed in the drying phase, suggesting that oxidative conditions promote As co-precipitation with Fe2O3, thereby improving As retention in soil. SOC showed significant correlations with MAOC fractions, indicating that drying conditions enhance the stabilization of organic matter in soil aggregates and mineral phases. Analysis of the Mantel test showed that pH is a key factor influencing soil As/Cd availability, Free Fe content, and SOC fractions. This indicates that RSB-Fe/Mn or RSB-Fe/Mg affect Cd/As passivation and soil carbon retention mainly through a pH-regulating mechanism.
Conversely, flooding conditions altered soil chemistry, with notable negative correlations between soil pH and Fed/Feo, suggesting the reductive dissolution of Fe2O3 under flooding possibly releases As and affects the stability of carbon fractions. However, the positive relationship between SOC and pH and between MAOC/DOC and Feo/Fed implies that RSB-Fe/Mn and RSB-Fe/Mg addition facilitated the stabilization and preservation of SOC by switching Fe oxides fractions into Feo and Fed. Our results align with Patel et al. [72], who have shown that short-term changes in preceding moisture conditions significantly influence soil carbon dynamics. These results indicate that soil redox dynamics, iron speciation, and pH are crucial factors affecting the stabilization and mobility of carbon and associated elements during the drying and flooding phases, with transparent chemical processes between oxidative and reductive soil environments.

4.5. Limitations of the Study

This study was conducted under laboratory-controlled conditions that may not capture all the complexities of real field environments. These findings, drawn from short-term effects, also point toward required long-term field experiments to assess the prolonged impact of iron-based bimetal-oxide-modified biochar in suppressing heavy metals and preserving SOC. Further, applying rice-straw biochar on paddy soils may limit the generalization of these results to other soil types and environmental conditions. All other considerations related to large-scale biochar application concerning feasibility and economy fell outside the purpose of this study and require further investigation.

5. Conclusions

In the present study, we investigated the synergistic effects of applying RSB-Fe/Mn and RSB-Fe/Mg amendments in As and Cd co-contaminated paddy soils, which proved that the materials could increase organic carbon retention while reducing As and Cd concentration at the same time. RSB-Fe/Mn materials reduced As and Cd in dry and flood periods, achieving up to 77.50% PWCd removal and 67.06% PWAs removal in the dry period. RSB-Fe/Mg also demonstrated high efficiency in decreasing up to 80.18% PWCd and 41.74% PWAs, respectively. RSB-Fe/Mn and RSB-Fe/Mg led to a pH increase in the drying period and an Eh improvement in the flooding period. Furthermore, RSB-Fe/Mn and RSB-Fe/Mg contributed to the preservation of SOC by a 16.3% and 6.7% increase in the dry period and a 27.2% and 33.9% increase in the flooding period. The change in SOC content is also reflected in its composition change, which consists of 63.0% of POC and 35.3% of MAOM in the dry regime, switching to 53.4% of POC and 42.1% of MAOM in the flooding regime. The increased SOC content and more stable SOC fraction were correlated with the higher Fed and Feo contents. The primary mechanism of RSB-Fe/Mn and RSB-Fe/Mg for Cd and As immobilization involved complexation and precipitation. At the same time, As (III) oxidation-Mn (IV)/Fe (III) reduction accounted for increased Cd/As passivation ability. This study highlighted the potential of RSB-Fe/Mn and RSB-Fe/Mg as efficient passivation for Cd and As co-immobilization in paddy soils, also as a sustainable carbon sequestration material, which could improve SOC stability and promote sustainable agricultural practices at the same time. The RSB-Fe/Mn amendment simultaneously reduces As and Cd concentrations, enhances soil pH and redox potential, and stabilizes SOC in paddy soils. These multifunctional benefits make RSB-Fe/Mn a practical and sustainable remediation technique for contaminated paddy soils, while also promoting soil health and supporting climate-resilient agricultural practices.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agriculture15111114/s1, Figure S1: Scanning Electron Microscopy (SEM) images of RSB (a,b), RSB-Fe/Mn (c,d) and RSB-Fe/Mg (e,f). The magnification times of SEM images were 10K and 40K; Table S1: Basic properties of the studied soils.

Author Contributions

Conceptualization, S.S.; Methodology, T.W.; Software, F.S.M.; Validation, S.S. and X.Z.; Formal Analysis, F.S.M.; Investigation, F.S.M.; Resources, Q.F.; Data Curation, N.Z.; Writing—Original Draft Preparation, F.S.M.; Writing—Review and Editing, N.Z.; Visualization, C.H.; Supervision, S.S.; Project Administration, C.W.; Funding Acquisition, S.S. and N.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Scientific and Technological Innovation Project of the Chinese Academy of Agricultural Sciences (CAASZDRW202408 and CAAS-ASTIP-2021-IEDA), the National Natural Science Foundation of China (No. 42207369), and the Central Public-interest Scientific Institution Basal Research Fund (No. BSRF202211), and XinJiang Production and Construction Corps Key Technology Research Program (2022AB009).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author due to reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Kinetics and isotherms: pseudo-first-order and pseudo-second-order kinetic model fitting curves for the adsorption of As and Cd on RSB-Fe/Mn, RSB-Fe/Mg, and RSB. (a) Pseudo-first-order model for As adsorption. (b) Pseudo-second-order model for As adsorption. (c) Pseudo-first-order model for Cd adsorption. (d) Pseudo-second-order model for Cd adsorption.
Figure 1. Kinetics and isotherms: pseudo-first-order and pseudo-second-order kinetic model fitting curves for the adsorption of As and Cd on RSB-Fe/Mn, RSB-Fe/Mg, and RSB. (a) Pseudo-first-order model for As adsorption. (b) Pseudo-second-order model for As adsorption. (c) Pseudo-first-order model for Cd adsorption. (d) Pseudo-second-order model for Cd adsorption.
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Figure 2. Effect of synthesized materials on (a) Cd and (b) As content in soil pore water and the change in soil (c) pH and (d) Eh during the drying and flooding periods of the incubation experiment. Bars with a different lowercase letter(s) indicate significant differences at p < 0.05 among the treatments.
Figure 2. Effect of synthesized materials on (a) Cd and (b) As content in soil pore water and the change in soil (c) pH and (d) Eh during the drying and flooding periods of the incubation experiment. Bars with a different lowercase letter(s) indicate significant differences at p < 0.05 among the treatments.
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Figure 3. Change in the (a) free Fe oxides (Fef), (b) organic-associated Fe oxides (Feo), and (c) poorly crystallized Fe oxides (Fed) content in soil under RSB-Fe/Mn, RSB-Fe/Mg, and RSB addition during the drying period. Change in different SOC fractions: (d) particulate organic carbon (POC), (e) mineral-associated organic carbon (MAOC), and (f) dissolved organic carbon (DOC) content under RSB-Fe/Mn and RSB-Fe/Mg addition during the drying period. Bars with a different lowercase letter(s) indicate significant differences at p < 0.05 among the treatments.
Figure 3. Change in the (a) free Fe oxides (Fef), (b) organic-associated Fe oxides (Feo), and (c) poorly crystallized Fe oxides (Fed) content in soil under RSB-Fe/Mn, RSB-Fe/Mg, and RSB addition during the drying period. Change in different SOC fractions: (d) particulate organic carbon (POC), (e) mineral-associated organic carbon (MAOC), and (f) dissolved organic carbon (DOC) content under RSB-Fe/Mn and RSB-Fe/Mg addition during the drying period. Bars with a different lowercase letter(s) indicate significant differences at p < 0.05 among the treatments.
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Figure 4. Change in the (a) free Fe oxides (Fef), (b) organic-associated Fe oxides (Feo), and (c) poorly crystallized Fe oxides (Fed) content in soil under RSB-Fe/Mn, RSB-Fe/Mg, and RSB addition during the flooding period. Change in different SOC fractions: (d) particulate organic carbon (POC), (e) mineral-associated organic carbon (MAOC), and (f) dissolved organic carbon (DOC) content under RSB-Fe/Mn and RSB-Fe/Mg addition during the flooding period. Bars with a different lowercase letter(s) indicate significant differences at p < 0.05 among the treatments.
Figure 4. Change in the (a) free Fe oxides (Fef), (b) organic-associated Fe oxides (Feo), and (c) poorly crystallized Fe oxides (Fed) content in soil under RSB-Fe/Mn, RSB-Fe/Mg, and RSB addition during the flooding period. Change in different SOC fractions: (d) particulate organic carbon (POC), (e) mineral-associated organic carbon (MAOC), and (f) dissolved organic carbon (DOC) content under RSB-Fe/Mn and RSB-Fe/Mg addition during the flooding period. Bars with a different lowercase letter(s) indicate significant differences at p < 0.05 among the treatments.
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Figure 5. The tendency of the SOC percentage change normalized to 100% during the drying and flooding period. Bars with a different lowercase letter(s) indicate significant differences at p < 0.05 among the treatments.
Figure 5. The tendency of the SOC percentage change normalized to 100% during the drying and flooding period. Bars with a different lowercase letter(s) indicate significant differences at p < 0.05 among the treatments.
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Figure 6. (a) Fourier Transform Infrared (FTIR) spectra and (b) X-ray Diffraction (XRD) patterns of material samples of RSB, RSB-Fe/Mg, and RSB-Fe/Mn before and after Cd/As adsorption, respectively.
Figure 6. (a) Fourier Transform Infrared (FTIR) spectra and (b) X-ray Diffraction (XRD) patterns of material samples of RSB, RSB-Fe/Mg, and RSB-Fe/Mn before and after Cd/As adsorption, respectively.
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Figure 7. XPS spectra analysis of RSB-Fe/Mn material before and after Cd and As adsorption: (a) Fe2p, (b) Mn2p, (c) O1s, and (e) Cd3d, (d) As3d spectra for RSB-Fe/Mn after Cd and As adsorption. For (a–e): The black curve is the measured curve; The red line is the fitted curve; The blue, green, orange, and purple lines are the subtitles of the respective peaks.
Figure 7. XPS spectra analysis of RSB-Fe/Mn material before and after Cd and As adsorption: (a) Fe2p, (b) Mn2p, (c) O1s, and (e) Cd3d, (d) As3d spectra for RSB-Fe/Mn after Cd and As adsorption. For (a–e): The black curve is the measured curve; The red line is the fitted curve; The blue, green, orange, and purple lines are the subtitles of the respective peaks.
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Figure 8. XPS spectra analysis of RSB-Fe/Mg material before and after Cd and As adsorption: (a) Fe2p, (b) Mg1s, (c) O1s, (d) As3d, and (e) Cd3d, spectra for RSB-Fe/Mn after Cd and As adsorption. For (a–e): The black curve is the measured curve; The red line is the fitted curve; The blue, green, orange, and purple lines are the subtitles of the respective peaks.
Figure 8. XPS spectra analysis of RSB-Fe/Mg material before and after Cd and As adsorption: (a) Fe2p, (b) Mg1s, (c) O1s, (d) As3d, and (e) Cd3d, spectra for RSB-Fe/Mn after Cd and As adsorption. For (a–e): The black curve is the measured curve; The red line is the fitted curve; The blue, green, orange, and purple lines are the subtitles of the respective peaks.
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Figure 9. Correlation analysis of soil properties and the Mantel test analysis of POC, MAOC, and DOC, and the variables, including redox potential (Eh), soil pH, pore water Cad (PWAs), and pore water cadmium (PWCd), under (a) drying, (b) and flooding situations. The line connection denotes the relationship between variables, line color thickness indicates p-values, and line thickness represents R values.
Figure 9. Correlation analysis of soil properties and the Mantel test analysis of POC, MAOC, and DOC, and the variables, including redox potential (Eh), soil pH, pore water Cad (PWAs), and pore water cadmium (PWCd), under (a) drying, (b) and flooding situations. The line connection denotes the relationship between variables, line color thickness indicates p-values, and line thickness represents R values.
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Table 1. Fitting parameters of As and Cd adsorption kinetics on different adsorbents.
Table 1. Fitting parameters of As and Cd adsorption kinetics on different adsorbents.
Pseudo-First Order qe (mg·g−1)K1 (h−1)R2Pseudo-Second Order qe (mg·g−1)K2 (g·mg−1·h−1)h (mg·g−1·h−1)R2
AsRSB-Fe/Mn7.5040.07740.8469.4460.011160.3200.995
RSB-Fe/Mg5.4730.02470.8636.8690.021190.1360.987
RSB4.7640.03810.7476.4930.023720.1160.978
CdRSB-Fe/Mn55.2680.09760.97754.1710.000346.3470.999
RSB-Fe/Mg50.8790.04470.98551.1250.0003813.9880.999
RSB30.9170.04270.96532.7980.000932.8700.999
Table 2. Model performance for Cd and As adsorption.
Table 2. Model performance for Cd and As adsorption.
Heavy MetalAdsorbentModel Type(Mean Absolute Error) MAERoot Mean Square Error (RMSE)R²
CdMnPSO0.440.440.66
CdMnPFO1.796.092.46
CdMgPSO0.410.400.63
CdMgPFO1.804.652.15
CdRSBPSO0.661.041.02
CdRSBPFO1.713.831.95
AsMnPSO2.609.283.04
AsMnPFO0.720.830.91
AsMgPSO5.1843.286.57
AsMgPFO0.570.460.68
AsRSBPSO7.4980.568.97
AsRSBPFO0.600.690.83
Table 3. Elemental attribution of RSB-Fe/Mn and RSB-Fe/Mg XPS spectra before and after adsorption and the relative ratio of respective species’ content based on peak area.
Table 3. Elemental attribution of RSB-Fe/Mn and RSB-Fe/Mg XPS spectra before and after adsorption and the relative ratio of respective species’ content based on peak area.
SpectraMaterialAttributionRSB-Fe/MgRSB-Fe/Mn
B.E. (eV)AreaRatio %B.E. (eV)AreaRatio %
Fe2pBefore adsorption2p3/2, Fe2+710.2743,520.2546.13 710.2255,730.6945.70
2p1/2, Fe2+723.625,427.42 723.8240,918.89
2p3/2, Fe3+712.0158,345.5953.87 712.0675,086.8954.30
2p1/2, Fe3+725.6822,181.62 725.8339,745.8
After adsorption2p3/2, Fe2+710.2357,029.445.76 710.2749,853.249.38
2p1/2, Fe2+723.743,518.42 723.831,340.03
2p3/2, Fe3+711.9381,603.4354.24 712.1354,753.8850.62
2p1/2, Fe3+725.8237,599.7 725.8228,484.61
Mg1sBefore adsorptionMg-O-M1302.7912,785.6635.65
Mg-OH1303.3813,11136.56
Mg2+1304.059965.4927.79
After adsorptionMg-O-M1302.9113,855.3240.86
Mg-OH1303.5810,220.4530.14
Mg2+1304.39837.0829.01
Mn2pBefore adsorptionMn2+ 640.343193.8417.28
Mn3+ 641.338293.1344.86
Mn4+ 642.596997.7437.86
After adsorptionMn2+ 640.265759.7233.27
Mn3+ 641.266679.0138.58
Mn4+ 642.354874.3828.15
O1sBefore adsorptionM-O529.541,920.4811.72 529.3427,138.227.30
M-OH530.88136,710.838.24 530.8167,010.444.95
H2O531.74178,922.250.04 531.64177,43547.75
After adsorptionM-O529.3162,861.8516.35 529.3364,052.8918.16
M-OH530.75134,31634.92 530.8182,437.851.72
H2O531.59187,406.948.73 531.67106,232.730.12
Cd3dAfter adsorptionCd-OM406.14385.76136.10 405.76398.6624.81
Cd-OH406.51360.44133.73 406.18483.4930.09
Cd-CO3406.95183.72117.19 406.52483.5130.09
Cd2+407.24138.55312.97 406.92241.2415.01
As3dAfter adsorptionAs(III)49.38707.84757.30 47.931115.9735.12
As(V)50.08527.43542.70 48.762061.4864.88
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Mabagala, F.S.; Wang, T.; Feng, Q.; Zeng, X.; He, C.; Wu, C.; Zhang, N.; Su, S. Application of Iron-Bimetal Biochar for As and Cd Reduction and Soil Organic Carbon Preservation Under Varying Moisture. Agriculture 2025, 15, 1114. https://doi.org/10.3390/agriculture15111114

AMA Style

Mabagala FS, Wang T, Feng Q, Zeng X, He C, Wu C, Zhang N, Su S. Application of Iron-Bimetal Biochar for As and Cd Reduction and Soil Organic Carbon Preservation Under Varying Moisture. Agriculture. 2025; 15(11):1114. https://doi.org/10.3390/agriculture15111114

Chicago/Turabian Style

Mabagala, Frank Stephano, Tingjuan Wang, Qiufen Feng, Xibai Zeng, Chao He, Cuixia Wu, Nan Zhang, and Shiming Su. 2025. "Application of Iron-Bimetal Biochar for As and Cd Reduction and Soil Organic Carbon Preservation Under Varying Moisture" Agriculture 15, no. 11: 1114. https://doi.org/10.3390/agriculture15111114

APA Style

Mabagala, F. S., Wang, T., Feng, Q., Zeng, X., He, C., Wu, C., Zhang, N., & Su, S. (2025). Application of Iron-Bimetal Biochar for As and Cd Reduction and Soil Organic Carbon Preservation Under Varying Moisture. Agriculture, 15(11), 1114. https://doi.org/10.3390/agriculture15111114

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