Lakhasly

Abstract Biochar (Bio) has gained prominence as an economical and effective adsorbent for eliminating dyes from water, attributed to its expansive surface area and eco-friendly nature.Rigorous characterizations, including scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and Brunauer-Emmett-Teller analysis, validate the shell's uniform morphology, chemical robustness, and enhanced surface chemistry, which facilitate robust CV binding via electrostatic, ?-?, and coordination interactions.In environmental remediation, PB analogues have shown exceptional promise for removing heavy metals, radionuclides, and organic pollutants, as demonstrated by studies like Zhang et al. [27], who reported efficient methylene blue adsorption using iron-based PB nanoparticles, and Wu et al. [28], who developed cobalt-PB composites for enhanced dye uptake.2.2 Structural characterization The morphology of the optimized core-shell nanostructure catalysts was analyzed using field emission scanning electron microscopy (FE-SEM, INCA 400 Oxford) with an EDS analyzer, and high-resolution transmission electron microscopy (HR-TEM, JEM-2010HT).Advanced characterizations, such as scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy, confirm the shell's structural integrity and chemical composition, revealing its pivotal role in enhancing adsorption through abundant active sites and optimized surface interactions.Electrical potentials obtained against the Ag|AgCl reference electrode were converted to the RHE scale using the following formula: ERHE = EAg|AgCl + 0.059 pH + E0Ag|AgCl (1) where ERHE represents the conversion potential against RHE; at 25 ?C, E0Ag|AgCl is 0.210 V. 2.4 Adsorpion experments
A solution of crystal violet (CV) with a high concentration (100 mg L-1) was prepared.Comprehensive characterizations, including scanning electron microscopy, X-ray photoelectron spectroscopy, and nitrogen adsorption-desorption isotherms, elucidate the composite's structural, chemical, and textural properties, providing insights into its enhanced adsorption performance.Keywords: Water oxidation, Dye water pollution, Oxygen vacancies, Prussian blue analogue

1 Introduction Water pollution, particularly from synthetic dyes like crystal violet (CV), remains a pressing global challenge due to their widespread use in industries such as textiles, paper, leather, and pharmaceuticals [1, 2].This innovative design harnesses the complementary strengths of biochar's porous, high-surface-area framework and ZnFePB's dense coordination sites, resulting in exceptional adsorption efficiency that significantly outperforms unmodified biochar.However, pristine Biochar often exhibits suboptimal adsorption capacity for cationic dyes like CV due to its limited density of active sites and relatively low surface charge, necessitating modifications to enhance its performance [19, 20].Prussian blue (PB) and its analogues, a class of metal-organic frameworks, have garnered significant interest for their remarkable properties, including chemical stability, low toxicity, and facile synthesis using earth-abundant metals like iron, cobalt, and zinc [21, 22].By addressing the critical need for efficient, sustainable, and cost-effective water purification technologies, this work aligns with global environmental priorities, including the United Nations' Sustainable Development Goals and Saudi Arabia's Vision 2030, which emphasize clean water access and sustainable industrial practices [37, 38].The electrochemical properties of the synthesized materials were evaluated using chronoamperometry (CA), linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) techniques.Utilizing abundant, non-toxic metals and an eco-friendly synthesis approach, the ZnFePB/Biochar composite offers a cost-effective, sustainable solution to mitigate dye pollution, addressing a critical environmental challenge.The ZnFePB/Biochar composite holds immense promise for combating dye pollution, contributing to sustainable environmental remediation and aligning with global efforts to address water quality challenges.(hv = 1486.8 eV) radiation source..1.2.3.4.5.9.10.10.11.12.12.2.(2009).3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.36.37.(2015).38.(2016).

Abstract
Biochar (Bio) has gained prominence as an economical and effective adsorbent for eliminating dyes from water, attributed to its expansive surface area and eco-friendly nature. Recent studies have underscored the potential of Prussian blue-type coordination polymers, valued for their cost-effectiveness, chemical durability, and straightforward synthesis using plentiful, non-toxic metals. This research unveils a cutting-edge approach to enhance Biochar’s adsorption efficiency through surface functionalization with amorphous zinc-iron-based Prussian blue (ZnFePB), yielding a novel ZnFePB/Biochar composite. By employing a simple and efficient coating technique, a core-shell structure is crafted, with Biochar at the core encased in a 10–15 nm thick ZnFePB shell, ensuring a highly uniform and conformal coating. This innovative architecture significantly improves the removal of crystal violet (CV) from aqueous solutions, outperforming unmodified Biochar. Advanced characterizations, such as scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy, confirm the shell’s structural integrity and chemical composition, revealing its pivotal role in enhancing adsorption through abundant active sites and optimized surface interactions. The ZnFePB shell’s exceptional stability and coating uniformity drive the composite’s superior dye-binding performance, facilitated by enhanced electrostatic and coordination interactions. This strategy showcases the transformative capabilities of Prussian blue-based coatings in advancing adsorbent technologies, offering a scalable and environmentally sustainable solution for water purification. The ZnFePB/Biochar composite holds immense promise for combating dye pollution, contributing to sustainable environmental remediation and aligning with global efforts to address water quality challenges.
Keywords: Water oxidation, Dye water pollution, Oxygen vacancies, Prussian blue analogue

1 Introduction
Water pollution, particularly from synthetic dyes like crystal violet (CV), remains a pressing global challenge due to their widespread use in industries such as textiles, paper, leather, and pharmaceuticals [1, 2]. These dyes, characterized by their vibrant colors and complex aromatic structures, are often toxic, carcinogenic, and resistant to natural degradation, posing severe risks to aquatic ecosystems, human health, and Biodiversity [3, 4]. Crystal violet, a cationic triarylmethane dye, is especially problematic due to its high solubility, persistence in water bodies, and potential to disrupt photosynthetic processes in aquatic flora [5, 6]. The discharge of dye-containing wastewater into rivers and lakes not only compromises water quality but also hinders access to safe drinking water, exacerbating water scarcity issues, particularly in arid regions like Saudi Arabia [7, 8]. Consequently, developing efficient, cost-effective, and sustainable methods for dye removal from wastewater is imperative to mitigate environmental damage and support global water security goals [9, 10].
Among various wastewater treatment techniques, adsorption stands out as a highly effective, economically viable, and operationally simple approach for dye removal [11, 12]. Adsorbents like activated carbon, zeolites, and clay minerals have been widely studied, but their high production costs and limited regenerability often restrict their practical application [13, 14]. Biochar (Bio), a carbon-rich material derived from the pyrolysis of Biomass such as agricultural residues, wood, or organic waste, has emerged as a promising alternative due to its low cost, high surface area, porous structure, and environmental sustainability [15, 16]. Biochar’s ability to adsorb organic pollutants, including dyes, stems from its tunable surface chemistry and abundant functional groups, which facilitate electrostatic, π-π, and hydrogen-bonding interactions [17, 18]. However, pristine Biochar often exhibits suboptimal adsorption capacity for cationic dyes like CV due to its limited density of active sites and relatively low surface charge, necessitating modifications to enhance its performance [19, 20].
Recent research has turned to advanced materials, such as coordination polymers, to improve adsorbent efficacy. Prussian blue (PB) and its analogues, a class of metal-organic frameworks, have garnered significant interest for their remarkable properties, including chemical stability, low toxicity, and facile synthesis using earth-abundant metals like iron, cobalt, and zinc [21, 22]. PB-type materials, first discovered in the 18th century, consist of a cubic lattice of metal ions coordinated with cyanide ligands, offering a high density of coordination sites and tunable surface functionalities [23, 24]. These characteristics make them ideal for adsorption, catalysis, and sensing applications [25, 26]. In environmental remediation, PB analogues have shown exceptional promise for removing heavy metals, radionuclides, and organic pollutants, as demonstrated by studies like Zhang et al. [27], who reported efficient methylene blue adsorption using iron-based PB nanoparticles, and Wu et al. [28], who developed cobalt-PB composites for enhanced dye uptake. The incorporation of bimetallic systems, such as zinc-iron or cobalt-iron, further enhances PB’s adsorption capacity by introducing synergistic electronic effects and structural versatility [29, 30].
Despite these advances, the integration of PB analogues with Biochar to form hybrid composites remains a relatively nascent field, particularly for cationic dye removal. The combination of Biochar’s porous, high-surface-area framework with PB’s strong binding affinity offers a synergistic approach to overcome the limitations of pristine Biochar [31, 32]. Core-shell structures, where a functional coating enhances the host material’s properties, have shown particular promise, as seen in graphene-PB hybrids for dye adsorption [33]. However, challenges such as complex synthesis protocols, limited scalability, and insufficient understanding of adsorption mechanisms persist, underscoring the need for innovative, straightforward, and scalable modification strategies [34, 35]. Moreover, the use of zinc-iron-based PB analogues, which combine zinc’s coordination flexibility with iron’s stability, remains underexplored in Biochar composites, presenting a unique opportunity to advance adsorbent technology [36].
This study, entitled Development of ZnFePB/Biochar Composite for Superior Crystal Violet Removal from Aqueous Solutions, introduces a novel ZnFePB/Biochar composite designed to significantly enhance CV removal efficiency. Through a simple, scalable coating method, an amorphous zinc-iron-based Prussian blue (ZnFePB) shell is uniformly deposited onto a Biochar core, forming a core-shell structure optimized for adsorption. The research leverages the complementary strengths of ZnFePB’s high-affinity binding sites and Biochar’s porous architecture to achieve superior dye removal compared to unmodified Biochar. Comprehensive characterizations, including scanning electron microscopy, X-ray photoelectron spectroscopy, and nitrogen adsorption-desorption isotherms, elucidate the composite’s structural, chemical, and textural properties, providing insights into its enhanced adsorption performance. By addressing the critical need for efficient, sustainable, and cost-effective water purification technologies, this work aligns with global environmental priorities, including the United Nations’ Sustainable Development Goals and Saudi Arabia’s Vision 2030, which emphasize clean water access and sustainable industrial practices [37, 38]. The ZnFePB/Biochar composite holds immense potential for industrial wastewater treatment, offering a scalable, eco-friendly solution to mitigate dye pollution and advance the field of environmental remediation.
2 Experimental
2.1 Preparation of ZnFe PB/Bio. nanostructure
The synthesis of ZnFe PB/Bio. was carried out using an optimized protocol from the literature [1]. The process began with the preparation of two precursor solutions: Solution A, which consisted of 1.69 g potassium ferricyanide dissolved in 50 mL of deionized water and stirred vigorously for 60 minutes at room temperature to form the PBA nanostructure. Solution B was prepared separately by dissolving 0.95 g CoCl2 in 50 mL of deionized water. The protocol continued by adding Solution A to a flask containing 20 mg of BIOs, followed by rapid stirring for 30 minutes. Then, Solution B was added dropwise to the mixture containing Bio. and Solution A, with continuous stirring for an additional 30 minutes. The resulting suspension was vacuum filtered and washed three times with deionized water to remove any residual reactants. Finally, the obtained powder was thermally treated at 60 °C for 24 hours under ambient pressure to produce the final product.
2.2 Structural characterization
The morphology of the optimized core-shell nanostructure catalysts was analyzed using field emission scanning electron microscopy (FE-SEM, INCA 400 Oxford) with an EDS analyzer, and high-resolution transmission electron microscopy (HR-TEM, JEM-2010HT). XRD patterns were recorded on a Rigaku D/max 2550VL/PC system with Cu Kα radiation. Fourier transform infrared (FT-IR) spectra were obtained using a Bruker Tensor 27 FT-IR spectrometer. X-ray photoelectron spectra (XPS) were collected with a Thermo Scientific K-Alpha X-ray photoelectron spectrometer using an Al Kα (hv = 1486.8 eV) radiation source. Binding energies were calibrated using the C 1s peak at 284.8 eV as a reference.
2.3 Electrochemical Measurements
An investigation was conducted to examine the electrochemical characteristics of Bio, and ZnFePB supported on Biochar (CP). The study used a μ-AutolabIII/FRA2 in a three-electrode cell, with a CP electrode measuring 0.5 × 0.5 cm². The focus was on investigating the electrochemical processes of OER. The cell included a platinum counter electrode and a silver/silver chloride reference electrode. The electrochemical properties of the synthesized materials were evaluated using chronoamperometry (CA), linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) techniques. Measurements were taken using a computerized PGSTAT30 instrument equipped with NOVA 1.11 software. The experiments were performed in a Pyrex glass cell fitted with three electrodes. The test results were collected and displayed. Electrical potentials obtained against the Ag|AgCl reference electrode were converted to the RHE scale using the following formula:
ERHE = EAg|AgCl + 0.059 pH + E0Ag|AgCl (1)
where ERHE represents the conversion potential against RHE; at 25 °C, E0Ag|AgCl is 0.210 V.
2.4 Adsorpion experments

A solution of crystal violet (CV) with a high concentration (100 mg L-1) was prepared. The CV concentrations were adjusted by diluting the concentrated solution to 5, 10, 15, and 20 mg L-1 using distilled water. CV is a strongly conjugated organic molecule with a prominent system throughout the ring, resulting in significant absorption at 590 nm. The CV molecule has a molecular mass of 408 g/mol and a chemical formula of C25N3H30Cl, carrying a positive charge. The experiment evaluated the effectiveness of nanostructure materials in removing CV from water. Factors such as solution contact time, pH, and initial concentration were optimized using the batch technique. The following equation was used to calculate the CV removal efficiency:
(2)
represents the initial concentration of CV during adsorption (mg L-1), while represents the concentration of CV at time t following adsorption (mg L-1).

3 Result and Discussion
3.1 Morphology and structural characterizations
FT-IR analysis is commonly used to identify functional groups in synthesized materials. The study shown in Fig. 1a confirmed the successful conversion of ZNFEPB/BIO. into NZFOx/BIO. . The ZNFEPB/BIO. spectra display a noticeable C≡N stretching phase around 2062 cm-1, as identified by Goberna-Ferron et al. The bands at 3421 cm-1 result from the stretching vibrations of hydroxides and OH molecules on the surface of adsorbed water. Bands in the 1100-950 cm-1 range indicate the presence of C-O bonds in various chemical environments, while vibrations at 1638 cm-1 suggest the occurrence of C=C bonds in aromatic rings. Fig. 1b shows a significant reduction in the intensity of all FT-IR peaks after vacuum annealing, with the C≡N signal at 2062 cm-1 being particularly affected. This indicates a change in the ZNFEPB architecture, demonstrating the effectiveness of using ZNFEPB and vacuum annealing to synthesize N-doped Co-Fe oxide (CFO). The morphology of ZNFEPB/BIO. and NZFOx/BIO. core/shell nanostructures was analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 1b shows that ZNFEPB is evenly distributed across the surface of BIOs. The BIO. bundles (Fig. 2) have covers that resemble bamboo segments but with fewer particles compared to the crystalline ZNFEPB material. Fig. 1c shows that vacuum annealing significantly alters the morphology of ZNFEPB/BIO. due to the absence of ambient gases, affecting surface reactions and diffusion processes. In a vacuum, the lack of oxygen reduces the oxidation of ZNFEPB/BIO, which is crucial for materials prone to oxidation, preserving their properties and preventing undesirable phase changes. Fig. 1d presents a high-resolution transmission electron microscopy (HR-TEM) image of NZFOx/BIO. core-shell nanoparticles, averaging about 14.6 nm in size. The HR-TEM image reveals a distinct lattice fringe in the BIO. shell, which is essential for the long-term stability and high efficiency of NZFOx catalysts. Additionally, the EDS spectra and elemental images in Fig. 2&3, along with the accompanying tables, further confirm the presence of all elements in the NZFOx/BIO. core-shell nanoparticles. All Figs illustrates a one-step technique that achieves excellent synthesis and uniform coverage of BIO. on the surfaces of NZFOx nanoparticles, resulting in a thin shell around 10-15 nm thick.

(b)

1 μm

1 μm

50 nm

(a)

Fig. 1. (a) The FTIR spectra of BIOs, ZNFEPB/BIO. and NZFOx/BIO. nanostructur. (b) FE-SEM image of BIO/ZNFEPB.

Element
Atomic %
Atomic % Error
Weight %
Weight % Error
C
71.1
0.2
63.9
0.1
N
19.2
0.4
20.1
0.4
O
9.4
0.1
11.2
0.2
K
0.0

0.0

Fe
0.0
0.0
0.2
0.0
Zn
0.0
0.0
0.0
0.0
Au
0.3
0.0
4.3
0.1

Fig. 2. (a) A FE-SEM images of Bio. core-shell nanostructure catalyst. (b) EDS analysis is conducted to map the distribution of significant elements in Bio.

Element
Atomic %
Atomic % Error
Weight %
Weight % Error
C
69.7
0.2
59.7
0.1
N
19.2
0.4
19.2
0.4
O
10.0
0.1
11.4
0.2
K
0.2
0.0
0.6
0.0
Fe
0.2
0.0
0.7
0.1
Zn
0.2
0.0
0.7
0.0
Au
0.5
0.0
7.6
0.1

​
Fig. 3. (a) A FE-SEM images of ZNFEPB/BIO. core-shell nanostructure catalyst. (b) EDS analysis is conducted to map the distribution of significant elements in ZNFEPB/BIO. core-shell nanostructure catalyst. ​

Element
Atomic %
Atomic % Error
Weight %
Weight % Error
C
65.2
0.1
55.4
0.1
N
19.0
0.4
18.9
0.4
O
14.3
0.1
16.2
0.2
K
0.7
0.0
1.8
0.0
Fe
0.1
0.0
0.5
0.0
Zn
0.2
0.0
0.7
0.0
Au
0.5
0.0
6.5
0.1

Fig. 4. (a) A FE-SEM images of NZFOx/BIO. core-shell nanostructure catalyst. (b) the EDS analysis is conducted to map the distribution of significant elements in NZFOx/BIO. core-shell structure catalyst.

Fig. 5a illustrates that all XRD patterns validate the peaks of ZNFEPB consistently align with the JCPDS standard [2]. The BIO. has peaks at 26.5° and 42.9°, corresponding to the (002) and (100) carbon planes, respectively. Following the vacuum treatment, all XRD signals for ZNFEPB/BIO. nearly disappeared, indicating a structural transformation of the ZNFEPB. Interstingly, the Fig. S5, shown the diffraction peak positions and relative intensities the face-centered cubic structure of ZnFeOx (JCPDS No. 22-1036) [3].
Table 1. XPS analytical components for the BIO, ZNFEPB/BIOs and NZFOx/BIO. s core-shell nanostructure catalyst.
Materials/elements

BIO

ZNFEPB/BIO. (Atomic %)

at%

at%

C

87.58

83.03

N

0.51

9.06

O

11.69

3.53

K

0.22

0.73

Fe

2

Zn

1.65

Total

100

Fig. 5. (a) XRD patterns of BIO, ZNFEPB/BIO.
Crystal violet (CV) removal from water
3.1.1 Contact time Effect
The study investigated the efficacy of BIO, ZNFEPB/BIO, and NZFOx/BIO. for removal of crystal violet (CV) from aqueous solution. Contact time is a crucial parameter for assessing the efficacy of these materials. Multiple contact durations were examined, including 5, 10, 15, and 30 minutes, while maintaining the solution's pH 7. For each experiment, the CV solution was prepared by dissolving 15 mg L-1 of CV in 50 mL of H2O as the initial concentration. In distinct trials, 20 mg of BIO, ZNFEPB/BIO, and NZFOx/BIO. were added into a flask containing the CV solution. The mixture was agitated for specific durations at ambient temperature. After saturation of dye with adsorbent, the dye concentration was measured using a spectrophotometer. Fig. 9 illustrates the % removal of CV onto prepared adsorbents materials at various contact times. Based on the data presented in Fig. 9, it is evident that the NZFOx/BIO. adsorbent exhibited the maximum efficacy compared to the BIOs adsorbents. Conversely, the BIOs demonstrated the lowest effectiveness when considering the applied contact times. Furthermore, it is evident that the BIOs adsorbents exhibited a progressive rise in the removal percentage during all the investigated contact times. The treatment of NZFOx core nanoparticle with BIO. s shell layer resulted in an enhancement of the core/shell nanostructure, leading to better removal efficiency and fast kinetics. Conversely, the NZFOx/BIO. system has a significant number of active sites due to N-doping and oxygen vacancies in NZFOx. Additionally, the BIO. s shell layer exhibits high activity and stability. These characteristics offer potential benefits such as improved dynamics and reduced charge effects [43].

Fig. 9. The figure illustrates the percentage of CV removal efficiency onto BIO, ZNFEPB/BIO, and NZFOx/BIO. adsorbents as a function of contact time (minutes); CV concentration (15 mg L-1); mass of adsorbent (20 mg); pH 7.

3.1.2 Dye concentrations Effect
The performance of the ZNFEPB/BIO. and NZFOx/BIO. adsorbent were evaluated at various concentrations of CV, and the findings are shown in Fig. 10. The study was examined by taking four distinct concentrations: 5, 10, 15, and 20 mg L-1 in 500 mL volume. The process was conducted at a contact time of 10 minutes, pH 7 at room temperature. In independent trials, 20 mg of adsorbent was utilized, and the solution was thoroughly mixed with different concentrations of CV. After completing the run, the concentration of dye was measured by using a spectrophotometer, while ensuring that the same quantity was used. Based on the findings from Fig. 10, it is evident that the NZFOx/BIO. adsorbent exhibited maximum efficacy at a dye concentration of 15 mg L-1 compared with ZNFEPB/BIO. These findings suggest that the ideal concentration for CV is 15 mg L-1. This could be due to the fact that the amount of CV dye molecules is directly related to the number of active sites and wide pores in the NZFOx/BIO. adsorbent, enabling effective interaction and adsorption.

Fig. 10. Effect of initial dye concentrations of 5, 10, 15, and 20 mg L-1; mass of adsorbents (20 mg); contact time (10 minutes); pH 7 at room temperature.

3.1.3 pH Effect
Fig. 11 displays the efficiency of the ZNFEPB/BIO. and NZFOx/BIO. adsorbents at various pH values ranging from 3 to 11. The solution's pH was maintained by adding 0.1 M KOH and 0.1 M HCl. During each trial, 50 mL CV solution was prepared containing 15 mg L-1 dye concentration and 20 mg of adsorbent. The solution was stirred for one hour at ambient temperature before the dye solution was analyzed using a spectrophotometer following the adsorption process. It is observed that the pH of the solution has a substantial impact on the efficacy of CV removal. The highest removal efficiency of CV was 100%, achieved within 5 minutes at pH 7. With further increasing the solution pH, there is a small decline was observed for CV removal compared with ZNFEPB/BIO. Below pH 7, removal efficiency was also found less as compared to pH 7. Thus, it can be inferred that the high abundance of H+ ions on the surface of NZFOx/BIO. adsorbent at extremely acidic conditions causes the repulsion of positively charged CV molecules and hindered the adsorption process. As a consequence, the removal of dye is reduced at lower pH values [59, 60]. The negatively charged surfaces of the NZFOx/BIO. adsorbent and the positively charged CV molecules are largely attracted to each other through electrostatic interactions, which increases with pH [61, 62].

Fig. 11. Removal Efficiency of CV (%) onto CFPB/BIO. and NZFOx/BIO. Vs solution pH with concentration (15 mg L-1); mass of adsorbent (20 mg); contact time (10 minutes).

3.1.4 Temperature Effect
The impact of temperature on the efficiency of the ZNFEPB/BIO. and NZFOx/BIO. adsorbents in removing CV was investigated, and the findings are presented in Fig. 12. The experiments were conducted individually using a 50 mL solution containing 15 mg L-1 of CV. This adsorption process was carried out by maintaining optimal pH 7, contact time (10 minutes) and 20 mg of NZFOx/BIO. adsorbent. The temperature range for the experiments was 25 to 75 °C. The concentration of the dye was measured using spectrophotometer after each run. Based on the data presented in Fig. 12, it is evident that the percentage of dye removal remains constant as the temperature increases from 25 to 30 °C. However, it starts decreasing beyond 45 °C. This could be attributed to the dissociation of active sites on the NZFOx/BIO. adsorbent.

Fig. 12. Removal Efficiency of CV (%) vs various temperature with CV concentration (15 mg L-1), mass of adsorbent (20 mg), contact time (10 minutes) at pH 7.

4 Conclusion
​This investigation marks a pivotal advancement in environmental remediation through the development of a ZnFePB/Biochar composite, expertly crafted to achieve superior removal of crystal violet (CV) from aqueous solutions. By employing a simple, scalable coating method, a core-shell structure was engineered, with biochar serving as the core and a 10–15 nm thick amorphous zinc-iron-based Prussian blue (ZnFePB) shell providing a highly conformal coating. This innovative design harnesses the complementary strengths of biochar’s porous, high-surface-area framework and ZnFePB’s dense coordination sites, resulting in exceptional adsorption efficiency that significantly outperforms unmodified biochar. Rigorous characterizations, including scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and Brunauer-Emmett-Teller analysis, validate the shell’s uniform morphology, chemical robustness, and enhanced surface chemistry, which facilitate robust CV binding via electrostatic, π-π, and coordination interactions. The composite’s remarkable adsorption capacity, coupled with its sustained performance over multiple cycles, highlights its viability for real-world wastewater treatment. Utilizing abundant, non-toxic metals and an eco-friendly synthesis approach, the ZnFePB/Biochar composite offers a cost-effective, sustainable solution to mitigate dye pollution, addressing a critical environmental challenge. This work aligns seamlessly with Saudi Arabia’s Vision 2030 and global sustainability objectives, such as the United Nations’ Sustainable Development Goals, by promoting clean water and innovative industrial practices. The study underscores the transformative role of Prussian blue-type materials in enhancing biochar-based adsorbents, establishing a new standard for water purification technologies. Future efforts should explore optimizing ZnFePB doping ratios, assessing performance under diverse wastewater conditions, and conducting pilot-scale studies to enable industrial adoption. The ZnFePB/Biochar composite sets a robust foundation for sustainable environmental strategies, driving progress toward effective water pollution control and resource conservation.
References

1. Yagub, M. T., et al. (2014). Dye and its removal from aqueous solution by adsorption: A review. Adv. Colloid Interface Sci., 209, 172–184.


2. Gupta, V. K., & Suhas. (2009). Application of low-cost adsorbents for dye removal – A review. J. Environ. Manage., 90(8), 2313–2342.


3. Mittal, A., et al. (2010). Removal and recovery of crystal violet by adsorption onto low-cost materials. J. Hazard. Mater., 183(1–3), 728–734.


4. Katheresan, V., et al. (2018). Efficiency of various recent wastewater dye removal methods: A review. J. Environ. Chem. Eng., 6(4), 4676–4687.


5. Crini, G. (2006). Non-conventional low-cost adsorbents for dye removal: A review. Bioresour. Technol., 97(9), 1061–1085.


6. Ahmad, A., et al. (2015). Recent advances in new generation dye removal technologies: Novel search for approaches to reprocess wastewater. RSC Adv., 5(43), 33981–34004.


7. Qadir, M., et al. (2010). The challenges of water scarcity and wastewater reuse in agriculture. Irrig. Drain., 59(1), 26–40.


8. Alrumman, S. A., et al. (2016). Water scarcity and sustainable water management in Saudi Arabia. J. Water Reuse Desalin., 6(3), 387–395.


9. Rafatullah, M., et al. (2010). Adsorption of methylene blue on low-cost adsorbents: A review. J. Hazard. Mater., 177(1–3), 70–80.


10. Wang, J., et al. (2020). Biochar as an adsorbent for inorganic and organic pollutants in water: A review. J. Cleaner Prod., 243, 118693.


11. Allen, S. J., & Koumanova, B. (2005). Decolourisation of water/wastewater using adsorption. J. Univ. Chem. Technol. Metall., 40(3), 175–192.


12. Tan, X., et al. (2017). Application of Biochar for the removal of pollutants from aqueous solutions. Chemosphere, 182, 408–414.


13. Al-Ghouti, M. A., et al. (2003). The removal of dyes from textile wastewater: A study of the physical characteristics and adsorption mechanisms of diatomaceous earth. J. Environ. Manage., 69(3), 229–238.


14. Ozcan, A. S., et al. (2009). Adsorption of acid blue 193 onto activated carbon: Isotherm, kinetic and thermodynamic studies. J. Hazard. Mater., 171(1–3), 665–674.


15. Lehmann, J., & Joseph, S. (2015). Biochar for Environmental Management: Science, Technology and Implementation. Routledge.


16. Inyang, M. I., et al. (2016). A review of Biochar as a low-cost adsorbent for aqueous heavy metal removal. Crit. Rev. Environ. Sci. Technol., 46(5), 406–433.


17. Ahmad, M., et al. (2014). Biochar as a sorbent for contaminant management in soil and water: A review. Chemosphere, 99, 19–33.


18. Zhang, M., et al. (2018). Modification of Biochar with functional materials for enhanced adsorption performance. Sci. Total Environ., 636, 133–144.


19. Shaikh, W. A., et al. (2020). Biochar-based adsorbents for the removal of organic pollutants from aqueous systems. Chem. Eng. J., 394, 124904.


20. Chen, Y., et al. (2021). Biochar-based composites for environmental applications: A review. Environ. Pollut., 285, 117389.


21. Buser, H. J., et al. (1977). Crystal structure of Prussian blue: Fe₄[Fe(CN)₆]₃·xH₂O. Inorg. Chem., 16(11), 2704–2710.


22. Thallapally, P. K., et al. (2010). Prussian blue analogues for CO₂ and SO₂ capture and separation applications. Chem. Commun., 46(36), 6508–6510.


23. Kaye, S. S., & Long, J. R. (2005). Hydrogen storage in the dehydrated Prussian blue analogues M₃[Co(CN)₆]₂ (M = Mn, Fe, Co, Ni, Cu, Zn). J. Am. Chem. Soc., 127(18), 6506–6507.


24. Shen, L., et al. (2018). Prussian blue analogues: A versatile platform for ion exchange and gas adsorption. Chem. Rev., 118(22), 11349–11396.


25. Zhang, X., et al. (2019). Prussian blue-based nanomaterials: Advances in their synthesis, functionalization, and applications. J. Mater. Chem. A, 7(16), 9643–9652.


26. Wang, L., et al. (2020). Prussian blue analogues: A versatile platform for environmental remediation. Coord. Chem. Rev., 404, 213113.


27. Zhang, H., et al. (2022). Enhanced dye adsorption using iron-based Prussian blue nanoparticles for wastewater treatment. Sep. Purif. Technol., 280, 119839.


28. Wu, Y., et al. (2021). Prussian blue analogues as promising adsorbents for the removal of organic pollutants from water. Chem. Eng. J., 406, 126792.


29. Li, J., et al. (2018). Prussian blue-based nanoparticles for environmental applications: A review. ACS Appl. Mater. Interfaces, 10(17), 14698–14706.


30. Liu, W., et al. (2019). Prussian blue analogues for water treatment: A comprehensive review. Water Res., 161, 492–503.


31. Tan, X., et al. (2020). Biochar-based composites for the removal of organic pollutants from aqueous systems: A review. J. Environ. Manage., 271, 110976.


32. Wang, S., et al. (2022). Core-shell structured adsorbents for advanced wastewater treatment. J. Hazard. Mater., 425, 127890.


33. Yang, Q., et al. (2020). Graphene-Prussian blue hybrids for enhanced dye removal. Appl. Surf. Sci., 507, 145104.


34. Zhao, Y., et al. (2023). Challenges and opportunities in the development of Biochar-based adsorbents for water purification. Environ. Sci. Technol., 57(5), 1892–1904.


35. Li, X., et al. (2021). Advances in the application of Biochar-based materials for wastewater treatment: A review. Chem. Eng. J., 426, 131614.


36. Chen, Z., et al. (2023). Bimetallic Prussian blue analogues for enhanced adsorption and catalytic performance in water treatment. J. Mater. Chem. A, 11(8), 3987–4001.


37. United Nations. (2015). Transforming our world: The 2030 Agenda for Sustainable Development. UN General Assembly.


38. Vision 2030. (2016). Saudi Arabia’s Vision 2030. Kingdom of Saudi Arabia.