Remediation of As(V) and Cd(II) contamination by a ZVI-biochar: experimental and DFT calculation

A novel zero-valent iron loaded biochar (ZVI-CSC) was synthesized in this study for the remediation of As((cid:0)) and Cd(II) contamination. The impact of ZVI-CSC on the adsorption performance of As((cid:0)) and Cd(II) in solution, as well as their migration properties in soil, were investigated through adsorption kinetics and soil column leaching experiments, respectively. The results showed that the adsorption capacity of As((cid:0)) by ZVI-CSC was signi�cantly improved to 14.42 g·kg -1 at pH=3, compared with unmodi�ed biochar. However, the adsorption capacity of Cd(II) was not improved. In the leaching experiments, the addition of ZVI-CSC to As((cid:0))-contaminated soil resulted in a signi�cant reduction of cumulative As((cid:0)) release rate from 32.26% to 3.11%, compared with CSC. Moreover, the role of ZVI in As(V)/Cd(II) remediation was analyzed. nZVI forms ≡ Fe-OH and ≡ Fe-OOH due to oxidation, which can form monodentate and bidentate complexes with As(V) via ligand exchange, thus promoting As immobilization. Furthermore, to further improve the adsorption performance of Cd(II), the electrostatic potential (ESP) of biochars with different surface functional group (C=O, C–O–C, –OH and –COOH) and their bond dissociation enthalpy (BDE) with As((cid:0)) and Cd(II) were calculated based on density functional theory (DFT). The results showed that –OH was the most effective for As(V) adsorption, and C–O–C was the most effective for Cd(II) adsorption. C=O and –COOH can be used to the simultaneous adsorption of As(V) and Cd(II). Therefore, the surface functional groups of ZVI-CSC can be selectively modi�ed to improve its adsorption performance of As((cid:0)


Introduction
The risk of soil contamination and accumulation of heavy metals (HMs) in crops has increased due to rapid agricultural and industrial development (Li et al. 2019, Roberts 2014, Song et al. 2022, Zhao et al. 2009).The contamination of metals and metalloids, such as arsenic (As) and cadmium (Cd) in paddy soils, has become a critical issue threatening food safety and human health (Li et al. 2011, Panaullah et al. 2009, Yu et al. 2016).Therefore, it is imperative to reduce the bioavailability of As and Cd in paddy soils co-contaminated with these two elements.Due to the opposite geochemical behavior of As and Cd in paddy soils (Chen et  Biochars are carbonaceous materials produced by pyrolysis of biomass and can be used as organic amendments and carbon sequestration in soil systems.Due to its high surface area, large pore volume, and abundant availability of functional groups, biochar has become a promising material for the remediation of As and Cd in both solution and soil (Chen et al. 2022, Qin et al. 2018, Rathnayake et al. 2021).Yang et al. (Yang et al. 2021) found that the bioavailable DTPA fractions of Cd in soils adding straw biochar were 48.09%, which was lower than that in soils without biochar.Raw biochar still has limitations in repairing arsenic-contaminated soils, which generally increases the mobility or bioavailability of As (Kim et al. 2018) For example, the application of rice husk biochar increases the bioavailability of As in soil by 28-47% (Mensah et al. 2022).The application of sawdust biochar increased the bioavailability of arsenic in soil from 28.5-33.2%(Fan et al. 2020).However, typical biochar produced from plant biomass tend to have poor treatment e ciency for inorganic pollutants due to the scarcity of oxygen-containing functional groups (Yoon et al. 2023).For this reason, several recent studies have attempted to produce modi ed biochar with speci c functions suitable for the treatment of inorganic pollutants.Compared with the original biochar, the modi ed biochar showed better results in repairing As contaminated soil.Metal ion modi cation can improve the pore structure and surface chemistry of biochar, especially iron or iron with other metals, and increase the number of oxygen-containing functional groups of biochar (Wan et al. 2021, Wu &Feng 2017).Iron modi ed biochar has been widely used in remediation of soil contaminated by As or Cd.
Zero-valent iron (ZVI) has been successfully applied in water treatment and soil remediation contaminated by heavy metals and chlorinated organic compounds (Li et Ullah et al. 2020c).Houben and Sonnet discovered that the bioavailability of Cd and Zn in soils was signi cantly reduced when using ZVI(Houben &Sonnet 2020).Similarly, Zhu et al. (Zhu et al. 2022) reported that the mobility of As and Cr in groundwater was effectively decreased after adding ZVI.ZVI exhibited high e ciency in removing HMs through various interaction mechanisms, including adsorption, precipitation, and co-precipitation.HMs can also be reduced from the oxidized to the reduced state due to the generated Fe 2+ , H 2 from Fe 0 dissolution, and tertiary corrosion products (e.g., green rust and magnetite) (Liang et al. 2017).A cirtical issue with ZVI is its corrosion passivation (Jiao et (Lu et al. 2012, Tong et al. 2011, Vithanage et al. 2015).The types of functional groups appearing on the surface of biochar will directly affect its adsorption performance (Tan et al. 2021).Lin et al. found that the process of iron impregnation modi ed biochar changed the functional groups on its surface, thus improving its adsorption capacity for As( ) (Lin et al. 2017).Moradi et al. and Ni et al. found that iron-modi ed biochar has unique physical and chemical properties that facilitate ion exchange, complexation, electrostatic attraction and precipitation between its surface and Cd( ), thus, Cd( ) can be converted into a stable component (Moradi &Karimi 2021, Ni et al. 2019).The rich functional groups of biochar are the main advantages of its adsorption of As and Cd.To further explore its adsorption properties, it is necessary to analyze the in uence of different functional groups on the adsorption properties of As and Cd.However, current research on this aspect has not delved into the underlying mechanisms.
Furthermore, although the potential adsorption mechanisms of biochar on heavy metals have been investigated by various adsorption experiments (e.g.kinetics and isotherms), there are still some microscopic mechanisms based on molecular and electronic scales that have not been well addressed.Based on rst principles, density functional theory (DFT) calculations are well suited to study adsorption behavior from the atomic and electronic scales (Geerlings et al. 2020, Yan et al. 2017).DFT calculations are widely used as an effective method to evaluate the interactions between metal cations and surface oxygen-containing groups on carbon materials.Recently, DFT calculations have been gradually used to explore the adsorption properties and mechanisms between different organic compounds/metal ions and adsorbents (Cao et  ) was purchased from Aladdin.The natural hematite mineral samples were con rmed by Xray diffraction (XRD) analysis to be almost pure, with only a small amount of quartz.The minerals were ground and passed through a 100-mesh sieve for subsequent experiments.The corn straw was purchased from Nanjing, Jiangsu Province.The corn straw was air-dried, sheared and sieved to obtain particles smaller than 0.5 mm in size, and then dried at 105°C to constant weight and store in seal until used.

Preparation of biochar
Firstly, ZVI-CSC was prepared by suspending dried hematite (5 g) and straw (25 g) in 50 ml of deionized water.The suspension was sonicated for 30 min and then the mixture was dried in an oven at 60°C for 24 hours.Subsequently, ZVI-CSC were prepared in a xed bed under nitrogen atmosphere.The pyrolysis temperature was set to be 800°C and the heating rate was set to be 10°C/min.The prepared ZVI-CSC was washed several times with deionized water and then dried under vacuum at 60°C for 24 h.The prepared CSC and ZVI-CSC was stored in a sealed desiccator for subsequent use.

Characterization
The speci c surface area and pore structure of CSC and ZVI-CSC was examined by the Brunel-Emmett-Taylor (BET) method using an N 2 adsorption/desorption analyzer (Autosorb-iQ, Quantachrome, USA).
Scanning electron microscopy (SEM) (JSM-5610LV, Nippon Electronics Co., Japan) and energy dispersing X-ray spectroscopy (EDS) were used to identify the micromorphology and microcomponent element species and contents of CSC and ZVI-CSC.Energy dispersive X-ray diffraction (XRD) analysis was also performed to determine the crystal structure of the major minerals in the biochar using a computercontrolled X-ray diffractometer equipped with a Cu Kα radiation source, a stepper motor, and a graphite crystal monochromator (D/max2500/PC, Science Corporation, Japan).Biochar surface functional groups were measured using (Fourier Transform Infrared Spectrometer) (FTIR) (NEXUS670, Nekoli, USA).A nonmonochromatic Al Kα anode target (ESCALAB 250xi) was used for X-ray photoelectron spectroscopy (XPS) analysis.The high energy resolution scanning of Fe 2p, O 1s, C 1s and As 3d peaks was obtained, respectively.Raman spectra were measured using a Raman spectrometer (HORIBA LABRAM HR Evolution).
2.4 Adsorption experiments for As( ) and Cd( ) 40 mg/L As( ) solution and 40 mg/L Cd( ) solution were pre-prepared.Transfer 10 mL of As( ) and Cd( ) solutions to a 25-mL centrifuge tube, respectively.100 mg of ZVI-CSC was weighed and put into the centrifuge tubes.The initial pH of the suspensions was adjusted to 3.0 and 6.0 by using 0.1 mol/L NaOH or 0.1 mol/L HCl solution.The suspensions were shaken in a mechanical vibrator (205 rpm, 20 ± 0.5°C) for 240 min.The suspensions were sampled at 5 min, 10 min, 30 min, 1 h, 2 h, 3 h, 4 h, 8 h, 12 h and 24 h, respectively.The sampled suspension was then immediately ltered through a 0.45 µm aqueous lter membrane.The residues of As( ) and Cd( ) in the ltrate were measured by inductively coupled plasma atomic emission spectrometry (ICP).

Soil leaching experiment
Soil samples were collected from top soil (0-20 cm) in Nanjing, pretreated by As( ) and Cd( ) solution spraying and then aged for 14 days.The As( ) and Cd( ) concentrations of the pretreated soil were 47 and 3 mg•kg − 1 , respectively.The eld capacity was maintained at 40%.Soil leaching experiment was conducted to explore the effect of biochars on the As( ) and Cd( ) remediation in actual soil and under rain conditions.The experimental system of soil leaching is shown in Fig. S1.The system consists of water supply device, pump, soil leaching column and collection device.As( ) contaminated soil and As( )/Cd( ) co-contaminated soil were placed in soil leaching columns with the addition of CSC or ZVI-CSC.Deionized water was used as the leaching solution and its total amount was calculated by referring to the annual precipitation data of Nanjing, Jiangsu Province for the past 50 years.The soil leaching was carried out in 6 batches.The amount of leaching solution in each batch was one sixth of the total amount, i.e., 350 ml, which is equivalent to the daily natural precipitation in Nanjing in summer.Leaching solution was rst added to the soil leaching column to maintain its maximum water holding capacity for 24 hours.The leaching experiment was then carried out with the ow rate of leaching solution maintained at 0.4 ml/min.The experiment was repeated six times, with each interval of 24 hours.The ltrate was then collected for measurement.The single and cumulative release amount, and the single and cumulative release rate of heavy metals in soil leaching experiment are shown in Text S2.
2.6 DFT calculation 2.6.1.Model construction and geometry optimization.These model compounds were constructed for the following DFT calculations to investigate the adsorption mechanism of oxygen-containing functional groups on the surface of biochar with Cd and As through ESP analysis and BDE analysis.The geometry optimization were carried out by Gaussian 09 program (Frisch et al. 2016).The B3LYP method and 6-31G* basis set were used for geometric optimization.

Electrostatic potential (ESP) analysis
The electrostatic potential describes the interaction energy of a unit positive charge located at a point r with the current system: 1 In this study, the quantitative electrostatic potential analysis of the optimized molecular structure of biochae on van der Waals surfaces was performed using the Multiwfn 3.5 program(Lu &Chen 2012).The wave functions were obtained based on the B3LYP method and the 6-31G* basis set.The les obtained from the calculations were imported into GaussView software for visualization and analysis.The van der Waals surface refers to an equivalent surface with electron density equal to 0.001e/bohr3.

BDE analysis
The bond dissociation enthalpy (BDE) represents the reaction enthalpy change during the cleavage of chemical bonds, and it can re ect the reaction reactivity.The binding energy ΔE of biochar molecules with As and Cd was calculated through BDE analysis and its calculation method was introduced in our previous work (Chen et al. 2020a).Moreover, the bond lengths were obtained based on the optimal structure by the calculation of B3LYP/6-31G* level.
3 Results and Discussion

Characterization
As shown in Fig. 1, the microscopic morphology of CSC, ZVI-CSC before and after adsorption was analyzed by scanning electron microscope (SEM).Irregularly shaped biomass is transformed into laminar structures with porous surfaces and hematite is transformed into aggregated mineral particles.Clusters of ne iron particles can be observed ranging in size from 40 to 80 nm on the surface of ZVI-CSC (Zhou et al. 2014), indicating that the nZVI particles were uniformly loaded to the surface of the biochar.However, these particles were not found on the surface of CSC.Moreover, based on SEM mapping, iron can be oberved on the surface of biochars, further demonstrating the successful loading of nZVI particles.
Speci c surface area and pore structure of CS, CSC, and ZVI-CSC were listed in Table 1.Compared to CSC and ZVI-CSC, the speci c surface area of ZVI-CSC was signi cantly increased to 44.24 m 2 •g − 1 , which may be due to the enhanced dispersion of ZVI particles on the porous and rough surface of biochar(Devi &Saroha 2014).More available adsorption sites were thus formed onto ZVI-CSC, resulting in higher adsorption capacity of heavy metals (Zou et al. 2022).The pore size distribution curves in Fig. S2 also con rmed that ZVI-CSC have signi cantly higher surface area and pore volume compared to CSC.The improved pore structures directly affected the diffusion rate of heavy metals from the surface of biochar to its internal pore structure (Ahmad et al. 2014), enhancing the reaction between As(V) and Cd(II) and ZVI-CSC.indicating that the biochar has a distinct graphene-like structure (Cheng et al. 2020).The intensity ratio of peak D and G (ID/IG ratio) indicates the degree of graphitization (Sevilla et al. 2017).The ID/IG value of ZVI-CSC was higher than that of CSC, indicating that the graphitization of ZVI-CSC was higher than that of CSC.This may be due to the loading of ZVI increased the degree of defects in biochar.The ID/IG ratio of composites prepared by co-pyrolysis of magnetite preloading or Fe 3+ impregnated biomass showed a similar trend(El-Azazy et al. 2021).
The XPS spectra of CSC and ZVI-CSC were illustrated in Fig. 2(d).Based on Fig. 2(d), peaks of C 1s (~ 285 eV), O 1s (~ 532 eV), and Fe 2p (712 eV) can be detected in ZVI-CSC, indicating Fe was successfully loaded onto biochar.Moreover, As 3d can also be observed in CSC and ZVI-CSC after adsorption.The spectra of C 1s, O 1s and As 3d were given in Fig. S3.For O 1s spectra, it can be found that compared with CSC, new peaks at 530.3 eV and 531.3 eV can be observed for ZVI-CSC, which belong to inorganic oxygen Fe-O and adsorbed oxygen, respectively.This also indicated that the Fe was successfully loaded onto ZVI-CSC.For As 3d spectra, main spectral peaks were found to belong to As(V), As(III) and As(0), respectively.For CSC, As(III) was the main form of As.However, for ZVI-CSC, As(V) was the main form of As.

Kinetic analysis
Pseudo-rst-order model (PFO) and pseudo-second-order model (PSO) were used in this study for kinetic analysis of As  S1.Based on Fig. 3 and Table S1, the adsorption of As( ) and Cd( ) was more consistent with the pseudo-second-order model.Thus, it can be inferred that the control of adsorption rate of As( ) and Cd( ) mainly depends on chemisorption.For As( ) adsorption, when pH = 3, the maximum adsorption of As( ) by ZVI-CSC was 14.42 g•kg − 1 , which was 2.1 times that of CSC.When pH = 6, the maximum adsorption of As( ) by ZVI-CSC was 4.1 g•kg − 1 .Compared to CSC, the As( ) adsorption capacity of ZVI-CSC was signi cantly improved, especially at lower pH.For Cd( ) adsorption, the adsorption capacity of CSC and ZVI-CSC was similar, in the range of 2 to 4 g•kg − 1 .The Cd( ) adsorption capacity of ZVI-CSC was better when pH = 3, however it was better for CSC when pH = 6.
To better understand the adsorption process of As( ) and Cd( ) from solution to ZVI-CSC, the adsorption data were further plotted using an intraparticle diffusion model as shown in Fig. 3.The curves of As( ) and Cd( ) can be divided into three phase stages, namely the external diffusion stage (I), the intra-particle diffusion stage (II) and the adsorption equilibrium stage (III) (Gao et al. 2020).The results show that the adsorption of As( ) and Cd( ) at stage (I) was the most rapid, i.e., the diffusion process in the surface boundary layer of ZVI-CSC was the most rapid.At this stage, there were many adsorption sites on the surface of ZVI-CSC and the concentration of As( ) and Cd( ) in the solution was relatively high.A greater mass driving force causing rapid adsorption was thus generated(Sewu et al. 2017), so As( ) and Cd( ) can be quickly diffused and adsorbed to the surface of ZVI-CSC.As the adsorption proceeds, the internal diffusion process within the pores of ZVI-CSC dominated the retention of As( ) and Cd( ), which was considered as the rate limiting phase with a low rate constant at stage (II).The low rate may be related to the formation of inner layer complex or kinetic inhibition of heavy metals through narrow pore channels (Wang et al. 2015).Moreover, at stage (III), the curve gradually leveled off, indicating that the adsorption sites within ZVI-CSC were already occupied by As( )/Cd( ) and the remaining ions were not available for adsorption.That is, the removal of As( ) and Cd( ) by ZVI-CSC was controlled by both surface adsorption and intraparticle diffusion, and mainly by intraparticle diffusion (Liu et al. 2022).

Soil leaching
Based on the kinetic analysis, ZVI-CSC showed excellent arsenic adsorption performance.Thus, soil leaching experiments were also carried out to investigate the effect of ZVI-CSC on arsenic migration in soils.Single and cumulative release amount and release rate of As in As( ) contaminated soil and As( ) & Cd(II) co-contaminated soil after adding CSC and ZVI-CSC were given in Fig. 4. Based on Fig. 4(a), compared to adding CSC, the single release amount of As in both As( ) contaminated soil and cocontaminated soil after adding ZVI-CSC was signi cantly lower, almost zero even for the rst leaching.However, the single release amount of As in soils adding CSC was relatively high for the rst leaching and it gradually decreased with the increasing leaching times.
Based on Fig. 4(b), the cumulative release amount and release rate of As in soils adding CSC was 15.16 mg•kg − 1 and 32.261%, respectively.However, after adding ZVI-CSC, the cumulative release amount and release rate of As decreased to 1.4639 mg•kg − 1 and 3.1146%, respectively, about one-tenth of the As released when adding CSC.Based on the single and cumulative release amount of As, it can be concluded that the migration of As in soil decreased signi cantly after adding ZVI-CSC.This is because the immobilization of As was achieved through the complexation between ZVI-CSC and As.Moreover, compared to As( ) contaminated soil, the cumulative release amount and release rate of As in As( ) and Cd(II) co-contaminated soil were relatively higher, which may due to the competitive adsorption of As( ) and Cd(II).

Role of ZVI in As(V)/Cd(II) remediation
Based on the SEM analysis, nZVI particles were proved to be loaded to the surface of the biochar.Due to its small particle size and high speci c surface area, nZVI is easily oxidized by oxygen in the environment, forming a passivation layer on its surface.The main components of the passivation layer were Fe 3 O 4 , Fe 2 O 3 , and FeOOH, formed by the passivation of highly reactive Fe 0 core (Zhang et al. 2011).The presence of the "outer" passivation layer effectively prevents further oxidation of the nZVI inside the passivation layer, but leads to a reduction in the reduction capacity of the nZVI.Thus, nZVI particles have the dual nature of both components, which leads to unconventional chemical behavior, such as the simultaneous oxidation, reduction and adsorption of As(V) as shown in the XPS spectrum of As 3d in Fig. S3.
The high As(V) removal capacity of ZVI-CSC was mainly due to its surface complexation with As.Speci c functional groups with a nity for As(V), in particular ≡ Fe-OH and ≡ Fe-OOH, were formed due to the oxidation of nZVI, which can form monodentate and bidentate complexes with As through ligand exchange (Zhang et al. 2010).Moreover, HAsO 4 2− was the main form of As in aqueous solutions, with a partial negative charge on the oxygen atom (-0.895 for HAsO 4 2− ) and a partial positive charge on As (+ 1.125) (Kish &Viola 1999, Suazo-Hernández et al. 2019), which favors As adsorption through electrostatic attraction.Furthermore, As(V) can diffuse through the oxide layer or be adsorbed directly on the surface (Sun et al. 2011).Moreover, due to the electrons released by the oxidation of nZVI, analyzed in combination with the XRD and XPS results, As(V) may be reduced to As(III) and subsequently adsorbed or even reduced to As(0) and precipitated (Xu et  reduce Cd(II) to zero-valent Cd even without the presence of a passivated layer of ZVI.Therefore, for heavy metals such as Cd(II) with its standard electrode potential close to Fe 0 , the removal effect by nZVI is mainly through the adsorption of the outer passivation layer.Thus, compared to As(V) remediation, the remediation effect of Cd(II) by ZVI was not so good.

DFT calculation
The role of ZVI in As(V)/Cd(II) remediation has been analyzed.However, the role of biochar itself in As(V)/Cd(II) remediation has not been studied, especially for the electrostatic attraction and surface complexation between As(V)/Cd(II) and the oxygen-containing functional group on biochar.To analyze the mechanism of these two aspects, surface electrostatic potential (ESP) and bond dissociation enthalpy (BDE) analysis of biochar and As(V)/Cd(II) were studied in this paper through DFT calculation.
To simplify calculations, the surface model of biochar was simulated using a seven-ring pure graphene structure, which has been effectively applied in previous studies and yielded good ndings (Gao et al. 2018, Yang et al. 2022).The surface electrostatic potential (ESP) is the key to study and predict intermolecular interactions and helps to understand the interactions between biochar and heavy metals.To gain more insight into the structure-property relationship between oxygen-containing functional groups on biochar and As(V)/Cd(II) adsorption characteristics, BDE analysis was used to calculate the binding energy of oxygen-containing functional group in biochar and As(V)/Cd(II)(Chen et al. 2018) as presented in Fig. 6.Based on the binding energy of biochar and As(V)/Cd(II), it can be inferred whether the reaction with oxygen-containing functional groups is likely to occur, and in the form of chemisorption or physisorption (Liu et al. 2022).Generally, if the binding energy is negative, a stable interaction can be formed.If the binding energy is positive, it is unstable.Moreover, the larger the absolute value of binding energy, the stronger the interaction.When the absolute value of binding energy of the reaction is greater than 10 kcal/mol, the reaction is chemisorption.
Based on Fig. 6, for AsO

Conclusions
In this study, ZVI-CSC was successfully prepared by the one-step method.The kinetic analysis showed that the maximum adsorption capacity (pH = 3) of ZVI-CSC for As(V) and Cd(II) in aqueous solution were 14.35 and 6.95 g•kg − 1 , respectively.In As( ) contaminated soil and As( )/Cd( ) co-contaminated soil, the cumulative As release rate was only 3.11% and 3.38%, respectively, after adding ZVI-CSC, which was obviously lower than that of unmodi ed biochars (32.26% and 41.97%).The adsorption and immobilization characteristics of As(V) in solution and soils were demonstrated to be superior after adding ZVI-CSC compared to umodi ed biochar, as evidenced by kinetic studies and soil leaching experiments.It is noteworthy that the passivation layer on the surface of nZVI gives the nZVI particles the dual property of two components to oxidize, reduce and adsorb As simultaneously.However, in comparison to the remediation of As(V), ZVI exhibits limited e cacy in treating Cd(II). To al. 2019, Zhang et al. 2022), i.e. the nonlinear and slow kinetics of Fe0 corrosion and the subsequent low e ciency in removing target contaminants.Another disadvantage of ZVI is their high surface energy and magnetic properties, which resulted in a strong tendency to agglomerate(Shen et al. 2023).Their low reactivity with certain hard-to-degrade pollutants and loss of permeability may further hinder their large-scale application in environmental remediation(Ullah et al. 2020a).The incorporation of ZVI into biochar matrix can effectively mitigate the agglomeration of ZVI and enhance its long-term stability(Gil et al. 2018) (Desalegn et al. 2019, Li et al. 2021, Wang et al. 2021).Zhang et al.(Zhang et al. 2021) synthesized several-layerred graphene-encapsulated iron nanoparticles, which effectively improving the absorption capacity of As(III) and Pb(II) with the enhanced electron transfer to contaminants.Moreover, possible mechanisms for heavy metal immobilization by biochar in contaminated soils can be attributed to (i) ion exchange(Chen et al. 2015a, Zhou et al. 2017), (ii) chemical or physical adsorption and surface precipitation(Chen et al. 2015b, Liu et al. 2010), and (iii) ligand complexation.Functional groups on biochar, such as carboxyl, alcohol hydroxyl or phenolic hydroxyl groups, play a pivotal role in the immobilization of heavy metals through electrostatic attraction and surface complexation reactions According to the basic structure proposed by Sun et al.(Sun et al. 2023), biochar consists of a large number of benzene rings as well as surface functional groups, thus a simple multi-phenylene ring construct with one oxygen-containing functional group attached to the surface was chosen for the construction of the molecular model.C = O, C-O-C, -COOH, and -OH were chosen as representatives of oxygen-containing functional groups.AsO 4 3− and Cd 2+ were chosen to represent heavy metal cations.

FTIR
Fig.2(c).Defective (D)-bands (~ 1347 cm -1 ) and graphitic (G)-bands (~ 1597 cm -1 ) were observed, ( ) and Cd( ) adsorption by CSC and ZVI-CSC(Chen et al. 2022, Feng et al. 2021, Suazo-Hernández et al. 2019).Intraparticle diffusion model (IPD)(Cao et al. 2023a, Kypritidou et al. 2022) was also used for mechanism analysis.The speci c formulas were given in Text S1.The results of kinetics and intraparticle diffusion model were shown in Fig. 3.The tting parameters for the kinetic models were illustrated in Table Generally, the ability to accept and donate electrons of molecular can be obtained based on ESP, and it is easier to donate electrons at lower ESP(Liu et al. 2022, Zhu et al. 2020).The ESP of biochars with different oxygen-containing functional group (C = O, C-O-C, -OH and -COOH) and its area percentage were illustrated in Fig.5, where red indicates negative ESP and blue indicates positive ESP.The aromatic ring region in the middle of biochar appears red and the edge appears blue due to the superposition of π-electron clouds on the surface of biochar.Thus, the outer surface of biochar was positively charged.However, the rich π-bond makes it can also be used as an electron donor(Sun et al. 2023, Zhu et al. 2020).Based on Fig. 5, for biochar with C = O, C-O-C, and -COOH, the lowest ESP appears near the oxygen atom in functional group, indicating that the O atoms have strong electronegativity and changed the ESP of the biochar edge, where electrostatic attraction or complexation with Cd 2+ can be formed(Lin et al. 2022).For biochar with C = O, -OH and -COOH, the highest ESP appears near the oxygen-containing functional group, where electrostatic attraction or complexation with AsO 4 3− can be formed.Based on the area percentage analysis, when -OH and -COOH were modi ed on biochars, the ESP of the whole molecule decreased.Therefore, electrostatic attraction or complexation was more easier to be formed at the site with high ESP near the functional group.However, for biochar with C-O-C, the ESP of biochar increased, which may weaken the interaction with AsO 4 3− (Zhu et al. 2020).

4 3 −
adsorption, except for the weak interaction between C-O-C and AsO 4 3− , C = O, -OH and -COOH in biochar were all chemically adsorbed with AsO 4 3− , which can form complexs.The reaction with AsO 4 3− can be listed in order from easy to di culty: -OH (-140.98kcal/mol)> C = O (-111.46kcal/mol) > -COOH (-60.64kcal/mol).For Cd 2+ adsorption, -OH and Cd 2+ could not form stable interaction.C = O, C-O-C and -COOH in biochar were all chemically adsorbed with Cd 2+ .The reaction can be listed in order from easy to di culty: C-O-C (-111.46kcal/mol) > C = O (-101.33kcal/mol) > -COOH (-48.88kcal/mol).Thus, it can be concluded that C = O and -COOH can be used to the simultaneous adsorption of As(V) and Cd(II), and the interaction between C = O and As(V)/Cd(II) was stronger.However, -OH and C-O-C showed completely opposite properties on the adsorption of As(V) and Cd(II).-OH is only suitable for As(V) adsorption and C-O-C is only suitable for Cd(II) adsorption.
improve the simultaneous remediation of As( ) and Cd( ) by ZVI-CSC, the ESP of biochar with different functional groups (C = O, C-O-C, -OH and -COOH) was analyzed based on DFT calculation.According to the results of ESP calculation, the O atom changes the electrostatic potential of biochar edge, promoting the electrostatic attraction and the complexation of AsO 4 3− and Cd 2+ .Moreover, the interaction between As( )/Cd( ) and these functional groups on biochar were also analyzed based on DFT calculation.The results showed that C = O and -COOH can be used for simultaneous adsorption of As(V) and Cd(II),and -OH showed the best adsorption effect for As( ) (ΔE=-147.91 kcal/mol, bond length 2.805Å).C-O-C showed the best adsorption effect for Cd( ) (ΔE=-111.46kcal/mol, bond length 3.105Å).

Figure 4 Single
Figure 4

Figure 5 Surface
Figure 5 al. 2020b, Tian et al. 2021, Yao et al. 2019, Yao et al. 2022), the challenge in remediation of As and Cd lies primarily in selecting appropriate immobilization reagents.

Table 1
Speci c surface area and pore structure parameters of CS, CSC, and ZVI-CSC (Qian et al. 2019, Su et al. 2016) ZVI-CSC was performed, and the results are shown in Fig.2(a).The diffraction peak at 26.6° indicated the presence of SiO 2 in hematite and biochar(Huang et al. 2020), which can act as a carrier for ZVI and promote the oxidation of Fe 0 to provide electrons(Cai et al. 2021).The diffraction peaks at 33.16°, 33.6°, 40.8°, 49.56°, 62.54°, 63.96° correspond to Fe 2 O 3 for hematite.The diffraction peaks at 44.64° and 64.94° for ZVI-CSC correspond to Fe 0 peaks, indicating the successful loading of ZVI onto biochar.Moreover, it is noteworthy that the diffraction peak observed at 44.64°e xhibits a higher, narrower and sharper peak compared to the others, indicating that the Fe 0 crystallinity was increased after loaded onto biochar.According to previous studies, nano SiO 2 was found to increase Fe 0 crystallinity(Qian et al. 2019, Su et al. 2016), Therefore, the Fe 0 crystallinity caused by biochar is mainly attributed to the generation of SiO 2 from biochar.
al. 2021, Yan et al. 2012).For Cd remediation, mineral precipitation, ion exchange, complexation with oxygen-containing functional groups, electrostatic attraction and interaction with π-electrons are considered to be the main adsorption mechanisms of Cd by biochar(Lee et al. 2022, Tian et al. 2023, Wang et al. 2018).For ZVI, the surface charge adsorption or chemotactic adsorption between FeOOH and Cd(II) depends mainly on the presence state of Cd(II) in water.Cd exists mainly as Cd 2+ under acidic and neutral conditions.It is di cult to Declarations Author Contributions Statement Authors contributions Bangwei Liu, Conceptualization, Methodology, Formal analysis, Investigation, Resources, Data Curation, Visualization, Writing -Original Draft.Dandan Chen, Conceptualization, Methodology, Validation, Supervision, Writing -Review & Editing, Funding acquisition.Yan Zhou, Experiment, Software.Yiwei Zhang, Experiment, Software.Wenhui Liu, Experiment, Validation.Tian Xia, Software, Validation.Xiaoyu Su, Experiment, Validation.Ping Lu, Supervision, Validation, Project administration.Bangwei Liu, Yan Zhou, Yiwei Zhang, Dandan Chen, Wenhui Liu, Ping Lu, Xiaoyu Su, Tian Xia Corresponding author Correspondence to Dandan Chen Ethical approval This study does not involve human and/or animal subjects.Consent to participate and consent for publication All authors agreed with the content, and all gave explicit consent to submit, and they obtained consent from the responsible authorities at the institute/organization where the work has been carried out, before the work is submitted.Declaration of Interest Statement The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.