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Article

Nano Zero-Valent Iron—Rubber Seed Shell Biochar (nZVI-RSSB) Enhances Removal of Cadmium from Water

by
Guoyan Zhan
1,2 and
Zhenhua Zhang
2,*
1
Laboratory of Agricultural Products Processing Quality and Safety Risk Evaluation, Ministry of Agriculture and Rural Affairs (Zhanjiang)/Agricultural Product Processing Research Institute, Chinese Academy of Tropical Agricultural Sciences, Zhanjiang 524000, China
2
The School of Agriculture and Environment, The University of Western Australia, Crawley, WA 6009, Australia
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(17), 9807; https://doi.org/10.3390/app15179807
Submission received: 9 July 2025 / Revised: 20 August 2025 / Accepted: 4 September 2025 / Published: 7 September 2025
(This article belongs to the Special Issue Advanced Research in Activated Carbon Adsorption—2nd Edition)
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Abstract

Cadmium {Cd (II)} poses a high risk to ecological security and human health due to its high toxicity, easy migration and difficult degradation. Using waste rubber seed shell biochar (RSSB) as the carrier material of nZVI may inhibit the caking oxidation of zero-valent iron and improve the removal efficiency of Cd (II) from water. Through a series of batch experiments, the adsorption mechanism of modified biochar on Cd (II) clarified that the removal effect of nano-zero-valent iron-rubber seed shell biochar (nZVI-RSSB) on heavy metals in water was better than that of RSSB. The results showed that when the dosage of complex biochar was 80 mg, the initial concentration of Cd (II) was 50 mg/L, and the solution pH was 6, the maximum adsorption capacity of nZVI-RSSB for Cd (II) reached 30.42 mg/g, compared with the RSSB of 13.32 mg/g. The adsorption kinetics model showed that chemisorption and physical adsorption existed simultaneously. The results of the in-particle diffusion model show that the adsorption process may be divided into two stages. The Langmuir competitive adsorption model was followed. Electrostatic adsorption and precipitation/co-precipitation could be the main ways for the removal of Cd (II) by composite materials. Meanwhile, the synergistic adsorption of nZVI-RSSB composites with multiple metals in actual water showed its application potential in water pollution control. Hence, the nZVI-RSSB not only successfully inhibits the caking oxidation of zero-valent iron, but also effectively improves the removal efficiency of heavy metals from water.

1. Introduction

Cadmium {Cd (II)} is a typical heavy metal pollutant with the characteristics of high physiological toxicity, long half-life, bioaccumulation and difficulty in degradation [1,2]. Cd (II) pollution in water can accumulate in aquatic products and enter the human body through the food chain, eventually causing damage to the nervous system, kidneys and the liver [3,4]. Currently, several common methods for removing Cd (II) include plant extraction, adsorption, chemical precipitation, membrane separation and reverse osmosis. Among them, adsorption has become one of the most efficient and economical methods for removing Cd (II) due to its significant performance, low energy consumption, and simple operation [5,6,7]. In order to find efficient, cheap and environmentally friendly adsorbents to remove Cd (II) from water, researchers have been trying various materials, such as metal–organic frameworks, graphene oxide, bentonite, carbon nanofibers and biochar, etc. [8,9,10,11]. Biochar has become one of the materials that have received the most attention because of its wide source, low preparation cost, rich pore structure, high affinity for pollutants and good exchange capacity [12]. Under oxygen-limited conditions, Biochar is usually prepared by high-temperature pyrolysis of organic waste, such as sludge, animal feces, plant waste, etc. [13,14,15]. However, a single biochar has the disadvantages of insufficient number and types of surface oxygen-containing functional groups, few surface-active sites, and weak direct adsorption and fixation ability of pollutants, which is limited in practical application [16]. Hence, using surface-loaded functional materials is an effective way to make up for the inherent shortcomings of biochar.
Nanoscale zero-valent iron (nZVI) is an environmentally friendly adsorption material. It is not only a suitable non-toxic, cheap, magnetic and environmentally friendly material, but also has abundant reactive surface sites, high reactivity and a large specific surface area. For example, it can improve the reduction process of Cr (VI) by adsorbents through improving the electron transfer efficiency and repairing organic pollutants in water [17,18,19]. It has been studied as a means of adsorbing toxic pollutants such as heavy metals in water. However, due to its high reactivity and magnetic properties, it is easily oxidized and agglomerated, thus losing its reactivity, which limits its ability to adsorb heavy metal ions [20]. Based on these characteristics, it can be loaded on biochar to enhance its dispersibility and stability. At the same time, the rich reaction surface sites and the high reactivity of nZVI can be used to improve the adsorption effect of biochar [21,22,23,24,25]. The adsorption mechanism may include electrostatic adsorption and precipitation/co-precipitation, etc.
China is rich in biomass energy, among which the rubber tree is an important tropical woody plant. Rubber tree planting in our country produces more than 1 million tons of rubber seeds each year, and the available resources are very abundant [26]. Rubber tree seeds are mainly composed of rubber seed shells and rubber seed kernels, of which rubber seed shells account for about 30–50% of the total weight of rubber seeds. They are rich in lignin, cellulose, hemicellulose, etc., and are easy to pyrolyze and carbonize. For a long time, except for a small amount of rubber seeds produced by rubber tree planting for breeding and primary processing of seed oil, most of them have been abandoned because they have not been effectively utilized, which not only causes waste resources, but also pollutes the environment [27,28]. In recent years, some researchers have gradually realized the utilization value of rubber seed resources and have conducted relevant research on rubber seed shells, mainly transforming them into industrial activated carbon and producing organic matter reinforcing fillers [29]. For example, researchers used microwave irradiation and steam activation methods to prepare rubber seed shell activated carbon with excellent performance, with a yield of 23.5–40.5% [30,31]. However, there is no study on the preparation of biochar using rubber seed shells as raw material. Using biochar derived from rubber seed shells would offer low material costs, abundant agricultural waste reuse, and applications in wastewater purification.
Some studies have demonstrated the removal efficiency of different biochars and biochar composites loaded with nZVI on Cd (II) in solution, but there are few actual evaluations of their feasibility and efficiency in actual water bodies [32,33]. Because actual water bodies are often complex in composition and contain a variety of organic or inorganic pollutants, the presence of too many types of pollutants or an overly high pollutant content could greatly affect the adsorption effects of the selected materials. Therefore, the objectives of this study were to (1) evaluate the structural characteristics and adsorption performance for Cd (II) in water using RSSB and nZVI-RSSB at different temperatures; (2) compare the stability of Cd (II) adsorption between the RSSB at a temperature with better performance with nZVI-RSSB, and (3) verify the efficiency of metal removal by nZVI-RSSB from actual water. This study provides certain data references for the application of modified biochar in environmental governance.

2. Materials and Methods

2.1. Materials

The raw materials used to prepare biochar in this study are derived from the seeds of the rubber tree. After collecting rubber seeds from the rubber tree forest in Mazhang District, Zhanjiang City, Guangdong Province, the seed kernels were broken and removed, and the remaining rubber seed shells were cleaned and dried. The dried rubber seed shell was crushed and passed through a 60-mesh sieve to prepare it for use.
The Cd (II) solution was prepared by dissolving Cd (II) (NO3)2·4H2O, and the pH value of the solution was adjusted by 0.1 M HCl or NaOH. HCl, NaOH, FSO4·6H2O, NABH4 and NaNO3 used in this work are all analytical grade (AR), purchased from Guangdong Guanghua Technology Co., Ltd., Shantou, China. The deionized water was used for solution preparation and washing.

2.2. Preparation of Biochar

The rubber seed shell biochar (RSSB) was prepared using the oxygen-limited temperature-controlled cracking method, and the material was placed in a tube furnace (DROIDE, SK2-4-12TPB3, Shanghai, China) [34]. When nitrogen gas was introduced, the temperature was increased at the rate of 15 °C/min and kept at 200 °C for 1 h. Subsequently, the temperature was raised further, to 300 °C and maintained at this value for 2 h. After cooling, biochar at the temperature of 300 °C was obtained. Similarly, the biochar was obtained, respectively, at temperatures of 450 °C, 600 °C, and 750 °C. The biochar at four different temperatures (300 °C, 450 °C, 600 °C, and 750 °C) was marked as RSSB300, RSSB450, RSSB600, and RSSB750.
Through screening tests, the optimal pyrolysis temperature for preparing the modified biochar was determined based on RSSB adsorption properties. The nZVI-RSSB was prepared by the method of “FeSO4 infiltration-NaBH4 liquid phase reduction” [35]. Briefly, 24.88 g FeSO4·6H2O and 10 g of biochar (RSSB450) were dissolved in 200 mL of ethanol-water (ethanol/water = 1:3, v/v). After ultrasonication of the mixture for 30 min, nitrogen gas was introduced into the above solution and was used to remove the dissolved oxygen at a constant temperature. The reaction was carried out in a shaker at 25 °C and 180 rpm for 24 h. Afterwards, 50 mL of NaBH4 solution (28.78 mg/L) was added dropwise to the mixed solution, which was completed after mixing for 60 min. The samples separated from centrifugation were washed three times with ethanol and water, then placed in a freeze-dryer to vacuum dry for 12 h to obtain composite material of nano zero-valent iron–rubber seed shell biochar, which was marked as nZVI-RSSB450 and stored in a vacuum dryer for later use.
Fe2+ + 2BH4− +6OH− → Fe0 + 2B(OH)3 + 4H2

2.3. Characterization of Biochar

The specific surface area and pore size distribution of biochar were tested by a pore size and specific surface area analyzer (Micromeritics, 3Flex 5.02, Norcross, GA, USA). The specific surface area was selected by the BET model, and the pore size distribution was selected by the Barrett-Joyner-Halenda (BJH) model. The surface morphology of biochar was observed by scanning electron microscopy (SEM; Thermo Fisher Scientific, Apreo 2, Waltham, MA, USA). The surface functional groups of different materials were analyzed by a Fourier microinfrared spectrometer (FT-IR; Thermo Fisher Scientific, NicoletiN10, USA). The surface crystal composition was studied using an X-ray diffractometer (XRD; Rigku, smartlab9, Izumisano, Japan), and the surface element (Fe, C, O) composition was analyzed using an X-ray photoelectron spectrometer (XPS; Thermo Fisher Scientific, NexsaG2, USA). The Cd (II) content in the solution before and after the adsorption of the biochar was determined using an atomic absorption spectrometer (HITACHI, ZA3000, Chiyoda City, Japan).

2.4. Batch Adsorption Experiment

Batch adsorption experiments were performed to reveal the mechanism by which biochar removes Cd (II). Briefly, different biochar materials were added to a 250 mL plastic bottle containing 40.00 mL of a 50 mg/L Cd (II) (NO3)2·4H2O solution, and 0.01 mol/L of NaNO3 was used as the supporting electrolyte to oscillate on a 180 r/min multi-purpose oscillation box. Except for the temperature change experiment, the other experiments maintained the oscillation box temperature at 25 ± 0.5 °C. Approximately 0.6 mL of the sample was removed from the plastic bottle at certain time intervals (0–360 min) with a syringe (1 mL) and filtered through a 0.45 μm water filter membrane. Then, exactly 0.5 mL of the sample was taken with a pipette (1000 μL) and diluted by a factor of 25 (Cmax = 2 mg/L). An atomic absorption spectrometer (HITACHI, ZA3000, Japan) was used to detected the residual Cd (II) concentration (standard curve: 0~3 mg/L).
In order to explore the factors influencing Cd (II) adsorption of nZVI-RSSB450, the duration of the adsorption test was from 0 to 350 min; the amounts of biochar additions were selected as 10 mg, 20 mg, 50 mg, 80 mg, and 100 mg; the initial concentrations of Cd (II) were selected as 20 mg/L, 50 mg/L, 80 mg/L, and 100 mg/L; the pH in the initial solution varied within the range of 3~8; the temperature of the adsorption tests was set to 25, 30, 35, 40, 45, and 50 °C. All the adsorption experiments were repeated three times to ensure the reproducibility of the results. The amounts of Cd (II) adsorption by RSSB and nZVI-RSSB450 were calculated using the mass equilibrium equation [36]:
q t = c 0 − c t V m
where qt is the adsorption capacity of Cd (II) at time t (mg/g), C0 is the initial concentration of Cd (II) (mg/L), Ct is the equilibrium concentration of Cd (II) at time t (mg/L), V is the volume of Cd (II) solution (mL), and m is the mass of RSSB and nZVI-RSSB (mg).

2.4.1. Adsorption Kinetics

The adsorption kinetics process of RSSB and nZVI-RSSB were analyzed. Briefly, 80 mg of material was added to a 250 mL plastic bottle containing 40.00 mL of 50 mg/L Cd (II) solution, and 0.01 mol/L of NaNO3 was used as the support electrolyte. We adjusted the initial pH of the solution to 6.0 ± 0.2; then, we oscillated the solution at 180 r/min on the oscillation box and started timing. We took about 0.6 mL of the sample from the plastic bottle at certain time intervals (0–1440 min) with a syringe (1 mL) and filtered it through a 0.45 μm water filter membrane. Then, exactly 0.5 mL of the sample was taken with a pipette (1000 μL) and diluted by a factor of 25 (Cmax = 2 mg/L). The residual Cd (II) concentration was detected by an atomic absorption spectrometer (standard curve: 0~3 mg/L), and the equilibrium adsorption amount was calculated. The adsorption mechanism of Cd (II) was studied by using the pseudo-first-order kinetic equation, the pseudo-second-order kinetic equation, and the intra-particle diffusion equation. The calculation equations are as follows [37,38]:
q t = q e ( 1 − e − k 1 t )
q t = q e 2 k 2 t 1 + q e k 2 t
q t = k i p t 1 2 + C
where qt is the adsorption capacity of Cd (II) at time t (mg/g); qe represents the adsorption equilibrium of Cd (II) (mg/g); k1 is the pseudo-first-order kinetic constant (min−1), k2 is the pseudo-second-order kinetic constant (g/mg/min), both representing the adsorption rate constant; kip is the intraparticle diffusion kinetic constant, representing the diffusion rate constant of Cd (II) (mg·min−0.5/g); and C is a constant related to the thickness of the adsorption boundary layer.

2.4.2. Adsorption Isotherm

Adsorption isotherm is a theoretical method to calculate the surface adsorption amount of an adsorbent. Briefly, Cd (II) solutions with initial concentrations of 5 mg/L, 10 mg/L, 20 mg/L, 40 mg/L, 60 mg/L, 80 mg/L, 100 mg/L, 150 mg/L, and 200 mg/L were prepared. All the solutions used 0.01 mol/L NaNO3 as the support electrolyte, and the initial pH of the solution was adjusted to 6.0 ± 0.2. The exact amount of 40.00 mL was put into a numbered 250 mL plastic bottle, and 80 mg of biochar was added. The above solution was placed on the oscillation box, and the temperature of the oscillation box was set to 25 ± 0.5 °C, after which the box was oscillated at a speed of 180 r/min for 24 h. We took about 0.6 mL of the sample from the plastic bottle with a syringe (1 mL) and filtered it through a 0.45 μm water filter membrane. Then, exactly 0.5 mL of the sample was taken with a pipette (1000 μL). After dilution, we used an atomic absorption spectrometer to detect the residual Cd (II) concentration (standard curve: 0~3 mg/L) and we calculated the equilibrium adsorption amount. The adsorption isothermal lines of Cd (II) of RSSB and nZVI-RSSB and the thermodynamic parameters of the adsorption process were simulated using the Langmuir isothermal adsorption model and the Freundlich isothermal adsorption model. The parameter calculation equations are as follows [39]:
q t = k L q m c e 1 + k L c e
q t = K F c e 1 / n
where qe is the equilibrium adsorption amount of Cd (II) (mg/g); qm represents the maximum adsorption amount of Cd (II) (mg/g); KL is the Langmuir model parameter, representing the adsorption energy (L/mg); and KF and n are Freundlich model parameters, representing Cd (II) adsorption capacity [(mg/g) (mg/L)−1/n] and adsorption intensity, respectively.

2.4.3. Removal of Heavy Metals by Biochar in the Actual Water

The effect of RSSB450 and nZVI-RSSB450 on the removal of heavy metals in actual water was analyzed. The actual water was collected from a lake in Caojiang Town, Gaozhou City, Maoming, Guangdong Province. After the water sample was retrieved, it was filtered and its basic properties were immediately determined. The basic properties of the water were pH 6.04, COD 16 mg/L, oil 0.02 mg/L and ammonia nitrogen 0.185 mg/L. We used 40.00 mL water and 0.01 mol/L NaNO3 as the supporting electrolyte, and placed the container on a multi-purpose oscillation box, set the temperature to 25 ± 0.5 °C, and oscillated at a speed of 180 r/min. About 5 mL of the sample was taken from the plastic bottle with a syringe (5 mL) at certain time intervals (0–360 min) and filtered through a 0.45 μm water filter membrane, while the residual Cd (II) concentration was detected with an atomic absorption spectrometer (HITACHI, ZA3000, Japan). To ensure that the adsorption conditions of each sample were consistent, an oscillation experiment was performed for each time separately.

3. Results and Discussions

3.1. Characterization of Biochar

In order to comprehensively explore the structural properties of RSSB and nZVI-RSSB450, several different types of characterization and analysis were performed on them. Figure 1 is a SEM image of RSSB and nZVI-RSSB450. RSSB gradually becomes fragmented with high-temperature carbonization, and the surface is smooth. And as the temperature continues to increase, RSSB carbonizes more completely and the fragmentation is more obvious. When the temperature reached 750 °C, RSSB began to deform and partially collapsed, indicating that overly high temperature would cause some damage to the structure of biochar [40]. When nZVI was loaded on the biochar surface, fine particulate matter appeared on the biochar surface and porous structure, indicating that nZVI was successfully loaded on the biochar. Due to nZVI’s inherent magnetic properties and high surface energy, it easily agglomerates into large particles. After being combined with biochar, the biochar carrier can prevent it from forming a large area of stacking structure, which promotes its distribution more uniformly, thereby improving the solution to the nZVI aggregation problem [25,41].
The specific surface area, pore volume and pore size of RSSB and nZVI-RSSB450 were analyzed, and the results are shown in Table 1. There was a certain difference in the specific surface area of biochar obtained at different temperatures, which was manifested as the specific surface area first increasing, and then decreasing, reaching the maximum value at 450 °C. The pore volume showed a change pattern similar to the specific surface area, while the change in the pore size was opposite to the change pattern of the specific surface area. Compared with RSSB450450, after the modified load of nZVI, the specific surface area, pore volume and pore size of nZVI-RSSB450 had been reduced. This was mainly because the surface and pores of RSSB450 were occupied by nZVI, thus reducing its specific surface area [42,43]. The pore structures of RSSB and nZVI-RSSB450 were further analyzed by N2 adsorption–desorption isotherms (Figure 2). It can be seen that the isotherms of these biochars conformed to the IV curve and the H3 hysteresis ring, indicating that all materials had mesoporous structures [44]. From the pore size distribution diagram, it could also be seen that the pore sizes of different biochar materials were mainly distributed between 2 and 13 nm, indicating that the pore structure of RSSB and nZVI-RSSB450 was mainly mesoporous [4].
The changes in the functional groups of RSSB and nZVI-RSSB450 were studied by FT-IR characterization. As shown in Figure 3, the characteristic bands at 3745 and 3405 cm−1 correspond to -OH stretching vibration caused by iron oxide or cellulose. The C-H stretching vibration of the alkane was observed at 2935 cm−1. The characteristic band near 1608 cm−1 was attributed to the C=O stretching vibration. The band at about 1103 cm−1 was the stretching vibration of the C-O bond [45,46]. RSSB300 was rich in functional groups, such as -OH, C-H, and C=O. As the pyrolysis temperature increased, these functional groups gradually decreased or disappeared [47]. The characteristic bands of nZVI mainly appeared at about 1300 cm−1 and 600 cm−1. Compared with RSSB450, nZVI-RSSB450 showed a Fe-O stretching vibration band at 574 cm−1, proving that nZVI has been loaded to the biochar surface [48]. The dispersed distribution of nZVI in biochar was observed in the SEM image, which also indicated that it successfully reduced the aggregation phenomenon. The abundant pore structure of nZVI-RSSB450 contains a large number of organic components and silicon-rich minerals, which provides additional reaction sites for nZVI. The weak band at 574 cm−1 indicated that nZVI-RSSB450 had low oxidation [33]. After loading nZVI, -OH and C=O both undergo certain deviations, indicating that due to the adhesion of iron, vibrations such as -OH were affected. At the same time, compared with unmodified biochar, the characteristic band intensity of nZVI-RSSB450 near 1608 cm−1 decreased, indicating that there could be a reaction between nZVI and the carbonyl group in biochar, forming a compound, which is not a simple attachment [49]. Compared with the other biochar, nZVI-RSSB450 had rich surface oxygen-containing functional groups, which would facilitate the removal of Cd (II). The surface oxygen-containing functional groups could generate strong coordination compounds through the donation of lone pairs of electrons to the vacant orbit of the metal ions [50].
The crystallinity and material structure of RSSB and nZVI-RSSB can be further understood through XRD spectra (Figure 4). The diffraction peaks appearing at 15–30 are mainly carbon-containing organic compounds, and with the increase in preparation temperature, the diffraction peaks of biochar materials are more obvious, indicating that the content of organic components in the materials increases. The XRD pattern showed that the main peaks of nZVI-RSSB450 were not only the peaks of organic components, but also the characteristic peaks of Fe0 and the weak oxidation peaks of iron oxide, which proved the formation of zero-valent iron. And the appearance of weak oxidation peaks could be caused by the oxidation of nZVI-RSSB450 during preparation, drying and storage [47]. The rapid oxidation of nZVI was inevitable and detrimental to heavy metal removal, but this defect could be minimized by aging nZVI under controlled oxygen exposure to form a stable shell [33].
XPS spectra were used to further examine the structure of the different biochars (Figure 5a,b). Broad scan spectra showed that Fe 2p, O 1s, C 1s photoelectron peaks were observed, and with the increase in preparation temperature, the C 1s peak gradually increased and the O 1s peak gradually decreased. The results were similar to those of the XRD analysis. Further analysis from the Fe 2p peak showed that peaks at 725.7 eV and 711.8 eV were assigned to Fe2O3 and Fe3O4, respectively, and the weak peak at 706.8 eV was assigned to Fe0 [51]. The presence of Fe2O3 and Fe3O4 peaks also verified the aging phenomenon during the preparation of nZVI-RSSB450, indicating that an iron oxide shell coating was formed, thus delaying the aging of Fe0, so the peak of Fe0 was not detected obviously, which was consistent with the results of XRD analysis [33].
The presence of Fe2O3 and Fe3O4 peaks also verified the aging phenomenon during the preparation of nZVI-RSSB450, indicating that an iron oxide shell coating was formed, thus delaying the aging of Fe0 and resulting in the peak of Fe0 not being easily detected, which is consistent with the results of the XRD analysis [33].

3.2. Factors Influencing Cd (II) Adsorption of Biochar

3.2.1. Effect of Time on Cd (II) Adsorption of RSSB and nZVI-RSSB

RSSB300, RSSB450, RSSB600 and RSSB750 were tested for Cd (II) adsorption under the same parameters, and the adsorption capacities were compared in order to screen the optimal pyrolysis temperature. As shown in Figure 6, the adsorption capacities of Cd (II) by the four materials were 4.90 mg/g, 12.12 mg/g, 9.50 mg/g, and 7.55 mg/g, respectively, among which RSSB450 had the highest adsorption capacity, which was more than double that of RSSB300. This is because RSSB450 has a good carbonization degree, contains more hydroxyl functional groups, has a more porous structure, has a larger contact area with Cd (II), and has better adsorption capacity than those prepared at other temperatures [52]. RSSB450 was selected to be modified to load nano zero-valent iron to obtain nZVI-RSSB450. Under the same parameters as the above materials, the adsorption capacity of Cd (II) was obtained as shown in Figure 6. The adsorption capacity of Cd (II) on nZVI-RSSB450 reached 25.17 mg/g, which was more than double that of RSSB450. nZVI has abundant active surface sites and high reactivity, but due to its small particle size, it is easy to agglomerate and oxidize, and nZVI-RSSB450 can effectively solve these shortcomings of nZVI [20]. The results show that the biochar carrier decomposed at medium temperature is beneficial to the removal of Cd (II), and that the modified treatment significantly improves the adsorption capacity of RSSB450. The adsorption capacity of Cd (II) by nZVI RSSB450 is much higher than that of RSSB450, and it can efficiently adsorb Cd (II) from water.

3.2.2. Effect of Dosages on Cd (II) Adsorption of nZVI-RSSB450

As shown in Figure 7, the adsorption capacity of Cd (II) increased with the increase in adsorbent dosage. After 360 min of reaction, when the dosage of nZVI-RSSB450 was 10, 20, 50, 80, and 100 mg, the adsorption capacity of Cd (II) was 16.98 mg/g, 18.98 mg/g, 22.86 mg/g, 25.17 mg/g, and 25.26 mg/g, respectively. This was because Fe0 dissolved Fe2+ and active sites increased with the increase in nZVI-RSSB450 dosage, which consequently increased the Cd (II) removal capacity [53]. When the dosage of nZVI-RSSB450 was 100 mg, the removal capacity of Cd (II) did not increase, compared with the 80 mg dosage. Considering economy, the dosage of nZVI- RSSB450 is 80 mg for the Cd (II) adsorption test.

3.2.3. Effect of Initial Cd (II) Concentration on Cd (II) Adsorption of nZVI-RSSB

The initial concentration of the Cd (II) solution also has great influence on its adsorption removal. The results showed that the adsorption capacity of Cd (II) increased gradually with the increase in Cd (II) concentrations, in the range of 20~100 mg/L (Figure 8). When the initial concentration was 20, 50, 80, and 100 mg/L, the adsorption capacities of Cd (II) were 9.86 mg/g, 25.17 mg/g, 22.08 mg/g, and 31.48 mg/g, respectively, after 360 min. When the initial concentration was 20 mg/L, the removal rate of Cd (II) in the aqueous solution reached more than 90% within 30 min, while when the initial concentration is 100 mg/L, 40% of Cd (II) remained in the aqueous solution after 360 min. Admittedly, when other conditions remain unchanged, the active sites of biochar are insufficient. When the Cd (II) concentration reached a certain level, the adsorption capacity of nZVI-RSSB450 reached saturation and remained stable, and excessive Cd (II) would not be adsorbed, thus affecting its adsorption removal [54]. Among the initial Cd (II) concentrations investigated, 50 mg/L Cd (II) was efficient and rational for the nZVI-RSSB450 adsorption test.

3.2.4. Effect of pH on Cd (II) Adsorption of nZVI-RSSB

The initial pH of the solution is an important factor affecting the adsorption of nZVI-RSSB450. The pH has a significant effect on the morphological distribution, ionization degree and surface charge of the adsorbent [55]. Figure 9 shows the change in Cd (II) adsorption capacity within 360 min under a series of pH values (3~8). At pH = 3.0~6.0, the adsorption removal of Cd (II) increased with the increase in initial pH, and the adsorption capacity reached the maximum at pH = 6. Subsequently, the adsorption capacity of Cd (II) decreased in the range of pH 6.0~8.0, but there was no sharp decline, indicating that nZVI-RSSB450 had pH adaptability and buffering capacity, and could maintain good adsorption capacity at higher pH values. From the results, it appears that nZVI-RSSB450 in weak acid to neutral solution for Cd (II) removal effect is good. When the pH value is low, the proton occupying the active site suppresses the electrostatic interaction through protonation of functional groups [56]. In addition, protons attack the adsorbed heavy metals, causing them to dissociate from the solid phase [57]. In addition, most Cd (II) in the solution at low pH exists as Cd2+ ions, which are not easily adsorbed [58]. Meanwhile, high concentrations of H+ at low pH promote the dissolution of nZVI, greatly reducing the adsorption sites. Conversely, as pH increases, protonated functional groups undergo deprotonation and active sites are released [59]. In addition, the increase in the pH value is conducive to the formation of Fe hydroxide and inhibits the dissolution of Fe oxide. Due to the rich hydroxyl ions on the surface of nZVI-RSSB450, the increase in pH value is also conducive to the formation of CdxFe(1−x)(OH)2 precipitate [60]. The removal of Cd (II) by nZVI-RSSB450 increased with the increase in pH value, indicating that precipitation and coprecipitation play a crucial role in Cd (II) fixation [61]. In conclusion, the increase in the initial pH value is an important factor affecting Cd (II) removal, a higher pH value is conducive to Cd (II) adsorption, and Cd (II) adsorption is optimal when the initial solution pH is 6. Electrostatic adsorption and precipitation/coprecipitation may be involved in Cd (II) removal by nZVI-RSSB450.

3.2.5. Effects of Temperature on Cd (II) Adsorption of nZVI-RSSB450

Adsorption experiments were performed on nZVI-RSSB450 at different temperatures prior to isothermal adsorption (Figure 10). The results showed that temperature had little effect on the amount of Cd (II) adsorbed by nZVI-RSSB450 in the range of 25–50 °C. Therefore, isothermal adsorption experiments were performed on nZVI-RSSB450 at a constant room temperature of 25 °C by varying the initial Cd (II) concentration, and the adsorption isotherm of RSSB450 was analyzed as a comparison.

3.3. Cd (II) Adsorption Kinetics of RSSB450 and nZVI-RSSB450

Based on the adsorption kinetics model, the adsorption kinetics of Cd (II) on nZVI-RSSB450 at pH 6 were studied, and the adsorption kinetics of Cd (II) on RSSB450 were analyzed as a comparison (Figure 11a). The adsorption capacity of RSSB450 and nZVI-RSSB450 for Cd (II) increased sharply from the beginning, and the adsorption of Cd (II) on nZVI-RSSB450 reached equilibrium faster. This was because there were abundant adsorption sites on the surface of the adsorbent, and a high initial Cd (II) concentration quickly occupied a large number of adsorption sites [62]. As the adsorption time increased, the adsorption process became slow and eventually entered a stable state. This could be due to the decrease in available adsorption sites [63].
On the basis of statistical analysis, the adsorption kinetics of Cd (II) by RSSB450 and nZVI-RSSB450 was further analyzed. The quasi-first-order, quasi-second-order and intra-particle diffusion models were used for a fitting analysis (Table 2). The fitting results showed that the fitting coefficient (R2) of the pseudo-second-order kinetic model was greater than that of the pseudo-first-order kinetic model, indicating that the adsorption rate was controlled by the number of sites on RSSB450 and nZVI-RSSB450, and chemical adsorption played a major role [64]. Compared with RSSB450, nZVI-RSSB450 show a higher adsorption capacity and a smaller specific surface area, which was due to the large number of functional groups and active sites on nZVI-RSSB450, indicating that the nZVI modification improved the application potentials in wastewater purification. However, the pseudo-first-order kinetic fitting results of Cd (II) adsorption on RSSB450 and nZVI-RSSB450 show that R2 values were higher than 0.9, which indicated physical adsorption also took place [4].
The intraparticle diffusion model can be used to describe the behavior of the material during the adsorption process. As shown in Figure 11b and Table 3, the adsorption of Cd (II) by RSSB450 and nZVI-RSSB450 diffused from the aqueous phase to the outer surface of the adsorbent under the action of electrostatic attraction. This process was faster and the adsorption rate was higher. Then, intraparticle diffusion adsorption was carried out, and Cd (II) was transferred from the outer surface of the adsorbent to the active sites in the pores. The adsorption rate of this process gradually decreased until the adsorption process reached equilibrium. The second stage curve fitting results also indicated that the active sites in the adsorbent played an important role in the adsorption process [65]. All the curves fitted by the intra-particle diffusion model were linear, and the intercept did not pass through the zero point, indicating that, in addition to intra-particle diffusion, the boundary layer could also have a greater impact on the adsorption process [66].

3.4. Cd (II) Adsorption Isotherm of RSSB450 and nZVI-RSSB450

Adsorption data were fitted and analyzed by the Freundlich model and the Langmuir model. The fitting results were shown in Figure 12 and Table 4. The results showed that the adsorption capacity of RSSB450 and nZVI-RSSB450 increased with the increase in the initial concentration, and the fitting results showed that the results obtained by the Langmuir model were better than the results obtained by the Freundlich model, indicating that monolayer chemical adsorption dominated the adsorption process in RSSB450 and nZVI-RSSB450, which was consistent with the kinetic results [67]. The adsorption equilibrium constants (KL) of Cd (II) on RSSB450 and nZVI-RSSB450 were greater than zero, which was positively correlated with adsorption affinity and adsorption capacity, indicating that adsorption occurred spontaneously [4]. In addition, the maximum adsorption capacity of RSSB450 and nZVI-RSSB450 for Cd (II), as calculated by the Langmuir model, was 13.32 and 30.42 mg/g, respectively. These results, higher than those of other biochar materials, indicated that the modification of nZVI iron increased the active groups on the surface of RSSB450, improved the probability of chemical reaction with Cd (II), and thus improved the adsorption capacity [68,69].

3.5. Removal of Metals by Biochar in the Actual Water

The adsorption effect of RSSB450 and nZVI-RSSB450 on Cd (II) in actual water was further tested to evaluate its practical application performance. As shown in Figure 13, RSSB450 could adsorb most Cd (II) in the water sample within 60 min, and finally reach 100% adsorption rate, while nZVI-RSSB450 could adsorb most Cd (II) within 30 min, and finally reach 100% adsorption rate. The pH and concentrations of heavy metals were measured before and after adsorption (Table 5). The pH value of the water sample, close to 6, provides a good pH environment for RSSB450 and nZVI-RSSB450 adsorption. At the same time, it also produces certain adsorption effects on other heavy metal ions in the water sample. Especially nZVI-RSSB450 could very efficiently also adsorb Pb, Cr (VI), As, Hg, etc., showing its good adsorption performance [70]. The synergistic adsorption effect of biochar on the adsorption process of various heavy metals in water promoted the efficient removal of various heavy metal ions in the water sample by nZVI-RS450, including electrostatic interaction and co-precipitation [71,72]. Therefore, nZVI-RSSB450 could be used as an adsorbent for heavy metals in water. As such, it provides additional utility as a resource, demonstrating it can remediate the pollution of heavy metals in water.
The results indicated that the combination of nZVI and RSSB improved the adsorption performance of the material and improved the removal efficiency of metals. The abundant oxygen-containing functional groups and minerals on the surface of RSSB450 provide a good supporting environment for nZVI, which improves its stability and reactivity. The active site in the composite plays an important role in the adsorption process, which is influenced by the concentration of Cd (II) and the pH of the solution. In addition, the abundant pore structure and the oxygen-containing functional group of RSSB450 itself could also contribute to the adsorption of Cd (II). Electrostatic adsorption and precipitation/co-precipitation may be the main mechanisms of Cd (II) removal by composite materials, and the synergistic adsorption of nZVI-RSSB450 of multiple metal ions in actual water bodies indicates its potential for application in water pollution control.
Considering the limited adsorption performance of biochar alone and the difficulty of using nZVI, we chose to combine them to form a composite material, thereby further improving the adsorption performance of the material. In the simulated adsorption experiment, we found that the adsorption capacity of nZVI-RSSB450 was more than double that of RSSB450, showing excellent adsorption capacity. Since the concentration of heavy metals such as Cd (II) in actual water was very low, after the addition of biochar RSSB450 and nZVI-RSSB450, the heavy metals in the water body were quickly almost completely adsorbed, so the superiority of the composite material is not obvious in this test.
Although the prepared nZVI-RSSB450 showed good heavy metal adsorption capacity and demonstrated its application effect in actual water bodies, this study does not systematically evaluate the competitive effect of coexisting pollutants (such as natural organic matter, other metal ions) on Cd (II) adsorption. Organic matter, such as humic acid in real water, may occupy active sites, while cations such as Ca2+/Mg2+ may inhibit Cd (II) adsorption through electrostatic shielding. This limits the applicability prediction of materials in complex water conditions [70]. At the same time, the study also failed to investigate the cyclic regeneration performance of materials (such as the retention rate of the adsorption capacity after pickling/thermal regeneration). In practical applications, it is necessary to solve the difficulty of material recovery and the risk of iron dissolution in repeated use. These engineering bottlenecks will restrict the implementation of the technology [17].

4. Conclusions

In this study, RSSB at different temperatures was first prepared, and the optimum temperature of RSSB450 was determined through adsorption screening experiments, and nZVI-RSSB450 was prepared by the liquid phase reduction method as the supporting material. The results showed that nZVI was successfully attached to the surface of RSSB450, and RSSB450 could effectively improve the stability and reactivity of nZVI and increase its surface active sites. The adsorption properties of nZVI-RSSB450 were affected by the dosage of the material, the initial concentration of Cd (II) and the pH of the solution. The adsorption mechanism mainly includes electrostatic adsorption and precipitation/co-precipitation (Figure 14). Through the adsorption experiment in actual water, it was found that nZVI-RSSB450 had a synergistic adsorption effect on various metals, which further verified the application potential of nZVI-RSSB450 as an adsorbent for the removal of heavy metals from water. Therefore, nZVI-RSSB could be an environmentally friendly and economical material for the treatment of wastewater containing heavy metals.

Author Contributions

Conceptualization, G.Z.; methodology, G.Z.; writing—original draft preparation, G.Z.; writing—review and editing, G.Z. and Z.Z.; project administration, G.Z. and Z.Z.; supervision, Z.Z.; funding acquisition, G.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Program for Quality and Safety Risk Assessment of Agricultural Products of China (GJFP20240501) and Hainan Provincial Natural Science Foundation of China (No. 224QN322).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The datasets used and analyzed during the current study are available from the corresponding author upon reasonable request.

Acknowledgments

We thank all the participants involved in this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. SEM images of RSSB and nZVI-RSSB450.
Figure 1. SEM images of RSSB and nZVI-RSSB450.
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Figure 2. N2 adsorption–desorption isotherms of RSSB and nZVI-RSSB450.
Figure 2. N2 adsorption–desorption isotherms of RSSB and nZVI-RSSB450.
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Figure 3. FT-IR spectra of RSSB and nZVI-RSSB450.
Figure 3. FT-IR spectra of RSSB and nZVI-RSSB450.
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Figure 4. XRD pattern of RSSB and nZVI-RSSB450.
Figure 4. XRD pattern of RSSB and nZVI-RSSB450.
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Figure 5. XPS spectra of RSSB and nZVI-RSSB450: wide scan (a), Fe 2p (b).
Figure 5. XPS spectra of RSSB and nZVI-RSSB450: wide scan (a), Fe 2p (b).
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Figure 6. Effects of time on Cd (II) adsorption of RSSB and nZVI-RSSB450.
Figure 6. Effects of time on Cd (II) adsorption of RSSB and nZVI-RSSB450.
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Figure 7. Effects of dosages on Cd (II) adsorption of nZVI-RSSB450.
Figure 7. Effects of dosages on Cd (II) adsorption of nZVI-RSSB450.
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Figure 8. Effects of initial Cd (II) concentration on Cd (II) adsorption of nZVI-RSSB450.
Figure 8. Effects of initial Cd (II) concentration on Cd (II) adsorption of nZVI-RSSB450.
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Figure 9. Effects of pH on Cd (II) adsorption of nZVI-RSSB450.
Figure 9. Effects of pH on Cd (II) adsorption of nZVI-RSSB450.
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Figure 10. Effects of temperature on Cd (II) adsorption of nZVI-RSSB450.
Figure 10. Effects of temperature on Cd (II) adsorption of nZVI-RSSB450.
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Figure 11. Cd (II) adsorption kinetics of RSSB450 and nZVI-RSSB450 (a), and intra-particle diffusion model of RSSB450 and nZVI-RSSB450 (b).
Figure 11. Cd (II) adsorption kinetics of RSSB450 and nZVI-RSSB450 (a), and intra-particle diffusion model of RSSB450 and nZVI-RSSB450 (b).
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Figure 12. Cd (II) adsorption isotherm of RSSB450 and nZVI-RSSB450.
Figure 12. Cd (II) adsorption isotherm of RSSB450 and nZVI-RSSB450.
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Figure 13. Adsorption effect in actual water.
Figure 13. Adsorption effect in actual water.
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Figure 14. Schematic diagram of the adsorption mechanism.
Figure 14. Schematic diagram of the adsorption mechanism.
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Table 1. The specific surface area, pore volume and pore size in RSSB and nZVI-RSSB450.
Table 1. The specific surface area, pore volume and pore size in RSSB and nZVI-RSSB450.
MaterialBET Surface Area (m2/g)Total Pore Volume (m3/g)Pore Size (nm)
RSSB3001.2490.003089.844
RSSB45057.2980.03033.944
RSSB60013.0270.01485.076
RSSB75020.9220.02394.824
nZVI-RSSB45039.4160.02406.466
Table 2. Pseudo-first- and pseudo-second-order kinetic parameters of the biochar on Cd (II) adsorption.
Table 2. Pseudo-first- and pseudo-second-order kinetic parameters of the biochar on Cd (II) adsorption.
MaterialPseudo-First-Order Kinetic ModelPseudo-Second-Order Kinetic Model
qe/(mg/g)k1/min−1R2qe/(mg/g)K2/g/mg·min−1R2
RSSB45011.6530.02660.965212.3940.00340.9850
nZVI-RSSB45024.5010.09400.980625.5360.00570.9979
Table 3. Intra-particle diffusion model parameters of the biochar on Cd (II) adsorption.
Table 3. Intra-particle diffusion model parameters of the biochar on Cd (II) adsorption.
MaterialIntraparticle Diffusion Model
C1kip1/mg·min−0.5/gR2C2kip2/mg·min−0.5/gR2
RSSB4501.17620.89320.986411.3000.02070.4844
nZVI-RSSB4509.72101.55090.873924.5680.01780.6098
Table 4. Langmuir and Freundlich model parameters of RSSB450 and nZVI-RSSB450 on Cd (II) adsorption.
Table 4. Langmuir and Freundlich model parameters of RSSB450 and nZVI-RSSB450 on Cd (II) adsorption.
MaterialLangmuir ModelFreundlich Model
qm/(mg/g)KL/(L/mg)R2KF/[(mg/g)(L/mg)1/n]nR2
RSSB45013.9770.11840.98623.62860.27570.9157
nZVI-RSSB45029.2510.37540.987510.5490.23370.9360
Table 5. Changes in pH and concentrations of heavy metal in actual water before and after the addition of RSSB450 and nZVI-RSSB450.
Table 5. Changes in pH and concentrations of heavy metal in actual water before and after the addition of RSSB450 and nZVI-RSSB450.
ParameterpHCd (II) (μg/L)Pb (μg/L)Cr(VI)(μg/L)As (μg/L)Hg (μg/L)
Initial concentration6.04 ± 0.014.73 ± 0.1238 ± 1.532 ± 2.217 ± 1.20.8 ± 0.04
Final concentration (RSSB450)6.72 ± 0.02ND2.2 ± 0.116.1 ± 0.244.9 ± 0.3ND
Final concentration (nZVI-RSSB450)7.52 ± 0.02ND0.8 + 0.02NDNDND
Note: the data is mean ± SD (n = 3).
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Zhan, G.; Zhang, Z. Nano Zero-Valent Iron—Rubber Seed Shell Biochar (nZVI-RSSB) Enhances Removal of Cadmium from Water. Appl. Sci. 2025, 15, 9807. https://doi.org/10.3390/app15179807

AMA Style

Zhan G, Zhang Z. Nano Zero-Valent Iron—Rubber Seed Shell Biochar (nZVI-RSSB) Enhances Removal of Cadmium from Water. Applied Sciences. 2025; 15(17):9807. https://doi.org/10.3390/app15179807

Chicago/Turabian Style

Zhan, Guoyan, and Zhenhua Zhang. 2025. "Nano Zero-Valent Iron—Rubber Seed Shell Biochar (nZVI-RSSB) Enhances Removal of Cadmium from Water" Applied Sciences 15, no. 17: 9807. https://doi.org/10.3390/app15179807

APA Style

Zhan, G., & Zhang, Z. (2025). Nano Zero-Valent Iron—Rubber Seed Shell Biochar (nZVI-RSSB) Enhances Removal of Cadmium from Water. Applied Sciences, 15(17), 9807. https://doi.org/10.3390/app15179807

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