Efficient adsorption of Crystal Violet Dye using Coir Pith-Derived Biochar: CCD-RSM modelling and Characterization

. Effective wastewater treatment techniques are needed to mitigate the effects of water contamination, which is a severe environmental issue. The high adsorption capacity and sustainability of adsorption techniques using biochar from agricultural waste, such as coir pith, have made them appear promising. This work intends to optimize the adsorption procedure utilizing biochar made from coir pith using Central Composite Design (RSM-CCD). The effects of pH, time, adsorbent dosage, and adsorbate concentration were examined on adsorption efficiency. In order to comprehend its structural and elemental characteristics, the synthesized biochar was characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). This study focuses on determining the optimal conditions for maximum adsorption efficiency and contributes to environmental engineering by utilizing agricultural waste for water remediation. The results showed that the biochar removed Crystal violet with a remarkable 72% efficiency, highlighting its potential as a powerful adsorbent for water purification.


Introduction
Textile effluents that get into natural streams and pollute the water have become a serious environmental problem that becomes a global problem.Contaminants in water are a major issue, and adsorption methods have been shown to be effective at resolving this problem.Biochar made from agricultural waste has gained attention because of its exceptional adsorption efficiency.[1].
Coir pith is a common waste product from agriculture that could be used to make biochar.Biochar with a porous structure can be made in two steps: drying in the air and pyrolysis at high temperatures.Its unique structure makes it better for adsorption, which makes it a good choice for water remediation.[2].
This study aims to optimize the dye adsorption process using coir pith biochar as the adsorbent.In order to accomplish this, a comprehensive understanding of biochar's properties is essential.Characterization techniques like Scanning Electron Microscopy (FE-SEM), Energy-Dispersive X-ray Spectroscopy (EDX), and X-ray Diffraction (XRD) were carried out.SEM gives information about the morphology of biochar, such as the size and distribution of pores [3].With EDX, the elemental composition and distribution of the biochar can be analyzed.XRD analysis provides the phase of the material [4].
The main goal of this study is to examine the effectiveness of synthesized biochar made from coir pith in adsorptive removal of Crystal Violet (CV) dye and other pollutants from wastewater.The study aims to find the optimal adsorption conditions by examining effects of factors like pH, time, adsorbent dosage, and adsorbate concentration.Response surface method -Central Composite Design (RSM-CCD) is employed to analysed the factorial influence on adsorption efficiency.
By optimizing coir pith biochar adsorption process, this study aims to make a contribution to the field of water treatment by by providing insights into the effectiveness of this sustainable adsorbent.The results have wider implications for dealing with water pollution problems caused by textile effluents, and they show the importance of characterization of biochar using SEM, EDX, and XRD methods to learn more about its properties and ability to absorb chemicals.

Preparation of Biochar
The coir pith was dried in an oven at 60°C for 24 hours to reduce moisture content.Subsequently, it was heated at 100°C for 12 hours to eliminate remaining moisture and volatile compounds.The dried coir pith was then calcinated at 550°C and ground into fine particles, resulting in a fine powder of biochar.The coir pith and synthesised biochar are shown in fig. 1.

Adsorption Treatment
The solvent for the adsorbate, Crystal Violet, was distilled water.Using the right buffering agents, the pH of the distilled water was maintained.An appropriate quantity of crystal violet dye was mixed with a specific amount of distilled water to make the dye concentration.The dye solution was mixed with a particular amount of biochar to start adsorption, ensuring a homogeneous dispersion.Using an orbital shaker, the mixture was stirred at room temperature for a certain amount of time.After the desired adsorption time, the samples were centrifuged to separate the biochar from the solution.Using a calorimeter, the remaining dye solution's absorbance was measured to determine the adsorption efficiency.

Experimental Design
The study employed the Central Composite Design (CCD) as the experimental design methodology.Four variables were investigated: pH, time, adsorbent dosage (biochar), and adsorbate concentration (Crystal Violet).The range of the variables was determined based on the specific requirements of the study.31 experiments were conducted according to the CCD, including factorial, center, and axial points.The factorial levels are shown in table 1.The obtained data were analyzed using MINITAB software to determine the significance of each variable and optimize the adsorption process for maximum efficiency.
Additionally, the synthesized biochar was characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) techniques to analyze its surface morphology, elemental composition, and crystalline phases.

Characterization Techniques
The synthesized biochar was characterized using Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDX), and X-Ray Diffraction (XRD) techniques to analyze its structural and elemental properties.
SEM analysis provided high-resolution images of the biochar's surface morphology, allowing for the examination of its pore structure and particle size distribution.EDX analysis was performed to determine the elemental composition of the biochar, providing information about the presence of different elements and their relative abundances.XRD analysis helped identify the crystalline phases present in the biochar and provided insights into its structural properties.

Efficiency Calculation
The adsorption efficiency of the biochar was calculated based on the absorbance measurements obtained from the calorimetry analysis.The initial concentration of Crystal Violet and the concentration of Crystal Violet remaining in the solution after adsorption were used to determine the percentage of dye removed by the biochar.The adsorption efficiency was then calculated using the following formula:

Adsorption Efficiency (%) = [(Initial concentration -Final concentration) / Initial concentration] x 100 (1)
The obtained adsorption efficiency values were analyzed statistically to assess the significance of each variable and determine the optimal conditions for achieving maximum adsorption efficiency.

Results and Discussion
The test runs obtained from RSM-CCD using Minitab software were conducted for experimental studies.The test results were loaded for regression and statistical analysis, which provides factorial interaction in the adsorption process.The generated regression model was utilized for predictive analysis of adsorption efficiency based on the effect of selected factors.The factors considered as determining adsorption efficiency are pH, time, adsorbent dosage, and adsorbate concentration.The regression analysis revealed quadratic models for each variable, indicating the presence of optimal values that maximize the adsorption efficiency.
The statistical analysis showed that pH had the most substantial effect on the adsorption efficiency, followed by time, adsorbent dosage, and adsorbate concentration.Based on the statistical analysis, the optimal conditions for maximum adsorption efficiency were determined as follows: pH 7, time 30 minutes, adsorbent dosage 35 g/L, and adsorbate concentration 30 mg/L.Under these conditions, the biochar achieved an impressive 72% efficiency in removing Crystal Violet, highlighting its potential as an effective adsorbent for water treatment applications.

RSM -CCD
The Central Composite Design (CCD) experimental design was implemented to investigate the effects of pH, time, adsorbent dosage (biochar), and adsorbate concentration (crystal violet) on the adsorption efficiency.A total of 31 experiments were conducted, and the response variable, adsorption efficiency, was measured for each run the sets and experiments along with response are shown in table 2. Based on the run order and the corresponding absorbance values, the results indicate variations in the adsorption efficiency of CV by the biochar.It is essential to analyse the relationship between the experimental factors and the response variable to understand the overall adsorption behaviour [7].A regression equation has been developed to provide insights into the significance of each factor and their interactions on the adsorption process.By fitting a regression model to the data, it was able to determine the mathematical relationship between the factors and the response variable.

Response surface methodology plots
Response Surface Methodology (RSM) is a powerful statistical technique that can be used to optimize the critical factors in CV adsorption and determine the nature of the response surface.The aim of the RSM is to develop a predictive model for CV adsorption efficiency, which can be used to determine the optimal conditions for the process and improve its efficiency.
When Absorbate (Dye concentration) is Constant, with the increase in pH, Time and Absorbent, the efficiency also increases.A State of Equilibrium is attained after 60 minutes of concentration.From the RSM plots in fig 5 .It is can be seen that the reaction time has a linear relationship with adsorbate efficiency.

Conclusion
The utilization of biochar derived from coir pith as an adsorbent for water treatment was investigated in this study.The synthesized biochar exhibited a porous structure and showed promising adsorption properties for the removal of Crystal Violet from aqueous solutions.The CCD experimental design facilitated the optimization of the adsorption process by identifying the significant factors and their optimal levels.The statistical analysis revealed that pH, time, adsorbent dosage, and adsorbate concentration had a significant influence on the adsorption efficiency.
The characterization techniques of SEM, EDX, and XRD provided valuable insights into the structural and elemental properties of the biochar, confirming its suitability for adsorption applications.The achieved 72% efficiency in removing Crystal Violet demonstrates the potential of biochar derived from coir pith as an eco-friendly and efficient adsorbent for water treatment.
This research contributes to the field of environmental engineering by utilizing agricultural waste for sustainable water remediation.

Fig. 2 .
Fig. 2. SEM image of synthesised coir pith biocharThe morphology of the synthesised biochar is identified by employing SEM characterization, which reveals the porous structure with particle morphology of the biochar as shown in fig. 2. Chemical composition was confirmed by EDX analysis, presence of carbon, oxygen, and other elements in the biochar, indicating its carbon rich nature for adsorption.Results of EDX and XRD analysis are shown in fig 3 and 4. The high surface area and crystalline of the synthesised biochar were confirmed by XRD analysis [6].

Fig. 5 .
Fig. 5. Surface response plots of factors effecting the efficiency of adsorption

Table 1 .
Factorial levels for CCD model

Table 2 .
Set of experimental runs and efficiency response