Coupling coagulation-flocculation-sedimentation with adsorption on biosorbent (Corncob) for the removal of textile dyes from aqueous solutions

doi:10.21203/rs.3.rs-4024263/v1

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Coupling coagulation-flocculation-sedimentation with adsorption on biosorbent (Corncob) for the removal of textile dyes from aqueous solutions

https://doi.org/10.21203/rs.3.rs-4024263/v1

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In this study, we assessed the efficacy of coupling coagulation-flocculation-sedimentation (CFS) with adsorption onto a biosorbent (corncob) for the removal of textile dyes from aqueous solutions. Two synthetic dyes, Bemacron Blue RS 01 (BB-RS01) and Bemacid Marine N-5R (BM-N5R), were selected for examination. Initially, aluminum sulphate (Al₂(SO₄)₃.18H₂O) served as the coagulant, followed by the addition of superfloc 8396 as a flocculation polymer. Corncob (CC) acted as the biosorbent material for adsorption. We optimized coagulation parameters, including coagulant and flocculant doses, and assessed pH influence. In the adsorption phase, we investigated adsorbent mass, particle size, pH, temperature, contact time, and initial dye concentration. Analytical techniques such as FTIR, SEM, TGA, BET, and pHpzc were employed to characterize corncob (CC). Both Langmuir and Freundlich isotherm models were employed to analyze equilibrium adsorption data, with the Langmuir isotherm yielding the highest correlation (0.94 < R² < 0.97). Our results demonstrated significant reduction in dye concentration through CFS, achieving elimination rates of 94% at pH 6 for BB-RS01 and 90.3% at pH 4 for BM-N5R. Corncob's adsorption for each dye was notably influenced by solution pH during the adsorption process, with removal rates of 26.19% observed at pH 6 for BB-RS01 and 7.69% for BM-N5R at pH 4. Maximum dye adsorption capacities were 99.01 mg/g for BB-RS01 and 46.08 mg/g for BM-N5R. This study underscores the effectiveness of coupling CFS with corncob adsorption for efficient and economical dye removal, especially with agricultural waste as an adsorbent material.

coagulation-flocculation

adsorption

spectrophotometry

synthetic dyes

corncob

adsorbent characterization

Isotherm

Effective dye removal: Combining coagulation-flocculation and adsorption processes achieved significant reduction rates;
Sustainable solution: Corncob proved to be an efficient adsorbent, highlighting the potential of agricultural waste in wastewater treatment;
Optimized treatment: Systematic parameter adjustments improved efficiency, offering a cost-effective approach for the treatment of synthetic-colored wastewater.

In today’s world, environmental pollution is a pressing issue, prompting extensive research into effective and sustainable wastewater treatment strategies. Synthetic dyes generated from industrial activities pose a notable challenge among the array of pollutants, primarily due to their enduring nature and detrimental impacts (Lin et al., 2023). These dyes find their way into wastewater from textile factories, which release undesirable effluents containing mixed contaminants, including dyes. Textile industries, in particular, have the potential to cause harm to both fauna and flora when they dispose of dyes in the environment. The textile industry uses dyes in its finishing processes and releases polluting effluents that harm biodiversity (Chanwala et al., 2019; Cripps et al., 1990). Synthetic dyes, which originate from various industrial activities, pose a significant challenge due to their persistent nature and relatively low biodegradability index (Faghihinezhad et al., 2022). It has been estimated that over 100.000 distinct dye varieties exist in the marketplace, with a yearly worldwide output surpassing 700.000 tons (Chittal et al., 2019). According to Zollinger H., the textile industry releases 50% of its dyes into the natural receiving environment (Zollinger, 2003). These effluents contain an extensive array of organic chemicals (e.g., dyes, solvents, pigments, and surfactants) as well as harmful substances including heavy metals (Verma, 2008; Lotito et al., 2012) that are harmful to flora and fauna (Stumm & Morgan, 1996) due to their low biodegradability (Bouras, 2003) and their chemical stability against oxidizing agents (e.g., UV rays, and chemical oxidants). Given the high organic load, intense and persistent colours, and elevated concentration of dissolved solids in these effluents, dyeing and finishing manufacturers are now mandated to treat their wastewater before releasing it into public treatment facilities (Arslan, 2001).

Researchers have investigated both actual textile effluents and simulated wastewater with properties resembling those of typical textile discharges. Through the analysis of components and treatment techniques, their goal is to develop effective strategies for addressing synthetic dye contamination in wastewater (Yaseen & Scholz, 2019). There is a pressing need for innovative and eco-friendly strategies to address this issue (Kishor et al., 2021). Numerous techniques have been developed to treat colour in textile wastewater. However, removing this type of pollution from water often proves challenging due to the inability of the standard treatment to degrade organic dyes (Guiza & Bagane, 2013). Therefore, chemical oxidation treatment (Jamal et al., 2015; Mokif, 2019); electrocoagulation (Manh, 2016; Perng & Bui, 2014); biodegradation (Bisht & Lal, 2019; Carlıell et al., 1995); adsorption (Boumediene et al., 2018; Yadav et al., 2020) and membrane processes (Allègre et al., 2006; Petrinić et al., 2007) can remove dyes from industrial wastewater. However, while many of them are prohibitively expensive, others are considerably more affordable.

Among the various treatment technologies available, combining coagulation and adsorption processes is a promising approach for sustainable wastewater treatment, highlighting the importance of environmental preservation and resource utilization. This method can have synergistic effects, resulting in improved dye removal efficiencies and enhanced treatment outcomes. This integrated approach leverages the strengths of processes, overcoming the limitations of individual techniques and offering a holistic solution for textile dye wastewater treatment.

The coagulation-flocculation-sedimentation (CFS) process is a commonly utilized method in wastewater treatment due to its effectiveness, simplicity, and economic benefits (Anjaneyulu et al., 2005; Georgiou et al., 2003; Golob et al., 2005; Judkins Jr & Hornsby, 1978; Puchana-Rosero et al., 2018; Zonoozi et al., 2008). It is generally acknowledged as a cost-effective and efficient technology (Semerjian & Ayoub, 2003). This approach has been shown to be effective in reducing turbidity and eliminating suspended solids from wastewater (Jarvis et al., 2005). However, it is important to recognize that the CFS process may have limited effectiveness in removing certain dyes (Riera-Torres et al., 2010). Therefore, integrating complementary treatment methods may be necessary to improve dye removal efficiency from wastewater.

Among the array of methods available, the adsorption process stands out as a favourable technique for eliminating dye pollutants from aqueous solutions owing to its notable removal efficiency, simplicity, and widespread application (Faghihian et al., 2014; Nourmoradi et al., 2016). Adsorption is widely recognized as an effective and cost-efficient method for dye removal from water, particularly when dealing with dye concentrations below 100 mg/L(Bailey et al., 1999; Crini, 2006; Ho & McKay, 2004; Zollinger, 2003). It should be noted that for higher concentrations, pretreatment may be required. Additionally, recent research has explored the use of a combination of diverse separation methods to achieve superior separation quality(Chakraborty et al., 2005; Furlan et al., 2010; Raj et al., 2023).

This research assessed the effectiveness of combining coagulation-flocculation-sedimentation with adsorption processes for removing textile dyes from synthetic aqueous solutions. Activated carbon is traditionally favoured for its efficacy in the adsorption process; however, its application is limited by high costs and regeneration challenges, as documented in the literature (Allen & Koumanova, 2005; Bailey et al., 1999; Crini, 2006; Ho & McKay, 2004). In light of these limitations, there has been a burgeoning interest in the exploration of sustainable materials. A plethora of studies have delved into the potential of agricultural by-products as eco-friendly adsorbents, demonstrating their capacity to mitigate dye pollutants in aqueous environments (Ait Ahsaine et al., 2018; Akkari et al., 2022; Akperov & Akperov, 2019; Boumchita et al., 2016; Boumediene et al., 2015; Guiza et al., 2014; Hajialigol & Masoum, 2019; Munagapati et al., 2021; Senthil Kumar et al., 2011; Singh et al., 2018). These investigations underscore the viability of such materials in addressing the economic and environmental concerns associated with conventional adsorbents. To overcome these limitations, the integration of adsorption using corncob is proposed as an adjunctive treatment measure. Corncob, which is abundant, cost-effective, and environmentally friendly, presents a promising alternative as a biosorbent for dye removal. Its porous structure and extensive surface area offer numerous sites for the adsorption of dye molecules, resulting in decreased dye concentrations and enhanced treatment efficacy.

Throughout our research, we investigated the optimal operational parameters for both techniques. This study assessed the effectiveness of corncob in removing Bemacid Marine N-5R (BM-N5R) and Bemacron Blue RS 01 (BB-RS01) dyes. In the coagulation-flocculation-sedimentation (CFS) process, we adjusted several variables such as coagulant/flocculent dosage, contact time, and pH to determine the optimal conditions for maximum dye removal.

In some research, superfloc 8396 as a polymer was revealed to have promising performance in sludge conditioning, leading to enhancements in flocculation parameters through flock formation (Hocine et al., 2023; Kebaili et al., 2022). In our case study, we explored the efficiency of this product as a flocculent in textile wastewater treatment. During the adsorption process, the impacts of temperature, corncob mass, particle size, pH, and initial dye concentration on dye removal were investigated. These parameters significantly influence the adsorption process, subsequently affecting dye uptake.

2.1. Dyes, coagulants and flocculants

In our experiments, we used two synthetic dyes: Bemacron Blue RS 01 (BB-RS01), a disperse dye, and Bemacid Marine N-5R (BM-N5R), an acidic dye. These dyes were supplied by the SOITINE textile industry located in NEDROMA – TLEMCEN (West of Algeria). For the coagulation-flocculation process, we used aluminum sulphate (Al₂(SO₄)₃.18H₂O; Feralco, Germany) as a coagulant and Superfloc 8396 (Kemira, Finland) as a flocculent. We conducted experiments by varying certain parameters, including the coagulant and flocculent dosages, and the pH of the solution. These variations drew inspiration from the findings of previous studies (Lefebvre, 1990; Semmens & Field, 1980) that demonstrated the effectiveness of such parameter adjustments in the decolourization process through coagulation-flocculation. Microsoft Excel was used for data analysis and visualization.. All figures presented in the results section were plotted using Excel's charting tools.

2.2. Determination of the wavelength and absorbance of dyes

We conducted spectral analysis using a Perkin Elmer Lambda 25 UV-visible spectrophotometer that covers a wavelength range from 190 to 1100 nm. This instrument was used to determine the maximum wavelength and measure the absorbance of the solution within a carefully selected wavelength range. It is worth noting that all experiments were carried out using distilled water with a conductivity not exceeding 10µs/cm. The dye concentration in the solution was determined by absorbance measurements based on a calibration curve. To achieve this, we prepared concentrations of 10, 20, 30, 40, and 50 mg/L for both dyes, all of which were derived from a base solution with a dye concentration of 1 g/l.

2.3. Coagulation-flocculation-sedimentation process

We conducted the coagulation-flocculation sedimentation process following the Jar-Test protocol using a VELP flocculator, as depicted in Fig. 1, as captured by a Xiaomi Redmi Note 8. Each 500 ml sample was introduced into a 1 L tall beaker according to this procedure. The process involved a 4-station flocculation, with each station having a unique rotation speed ranging from 0 to 300 rpm. These rotation speeds and the timing were preset and digitally displayed by the device.

In this phase, we introduced varying doses of the coagulant (Aluminium sulphate Al₂(SO₄)₃.18H₂O) into the prepared sample, which we agitated for 2 minutes at a high speed (200 rpm). We followed this with a 15-minute flocculation period, during which we added different doses of superfloc 8396 and stirred the mixture at a slower speed (40 rpm). After 30 minutes of sedimentation, the flocks were allowed to settle, and the supernatant was extracted for analysis. The percentage of BB-RS01 and BM-N5R dye removal was calculated using Eq. (1) as follows:

$$\% of dye removal=\frac{{C}_{0}-{C}_{t}}{{C}_{0}}\times 100$$

C₀ and C_t represent the initial and instantaneous concentrations of dye (mg/L) respectively.

The dye concentration in the initial solutions was set at 500 mg/L, which mirrors the typical concentrations found in industrial settings observed in textile wastewater emissions. For each experiment, precise doses of Aluminium sulphate (Al₂(SO₄)₃.18H₂O) and flocculent (superfloc 8396) were measured using an electronic balance 620 PRS model, with an accuracy of ± 2 mg. To measure the hydrogen potential (pH) of the solutions, we employed an electronic multiparameter CONSORT instrument with an accuracy error of ± 1%. Before using this instrument, we adjusted it with calibration solutions with pH values of 4 and 10, as recommended by the Standard Methods for the Examination of Water and Wastewater (AWWA Manual M32-Distribution network analysis for water utilities, 1989). The pH was adjusted using sodium hydroxide (NaOH: 0.1N) and hydrochloric acid (HCl: 0.1N). The aluminum sulphate salt Al₂(SO₄)₃.18H₂O facilitated the removal of dye at the determined optimal dose. Every experiment was performed twice, and the mean values were then utilized for subsequent analyses and calculations.

2.4. Adsorption processes

To accelerate dye removal and enhance the decolourization of our synthetic solution, we employed a coupled approach involving adsorption and coagulation-flocculation-sedimentation processes. Batch experiments were carried out using pretreated synthetic solutions for adsorption (obtained after the coagulation-flocculation-sedimentation method) and corncob as a biosorbent. During these experiments, we varied the contact time from 5 to 480 minutes to gather kinetic and equilibrium data. In each test, 1 liter of dye solution was combined with 1 gram of adsorbent (with a particle size ranging from 1.25 to 2 mm) in a 1-liter beaker. The adsorption process occurred within a thermostatic bath at 25 ± 2°C, with continuous agitation at 400 rpm using a heated magnetic stirrer from VELP. Various parameters, including contact time (0–8 hours), initial dye concentration (15–100 mg/L), adsorbent particle size (0.5–3.15 mm), adsorbent mass (0.5-3 g/L), temperature (25–45°C) and pH (2–11) were systematically investigated to understand their effects on the adsorption process. From the adsorption experiment outcomes, we formulated equilibrium isotherms and determined the quantity of solute adsorbed onto the adsorbent phase, as illustrated in Eq. (2).

$${q}_{e}=\frac{{(C}_{0}-{C}_{e})\times V}{W}$$

Where q_e represents the amount of dye adsorbed, measured in mg/g. C₀ and C_e represent the initial and equilibrium concentrations of the dyes, respectively (mg/L). V indicates the volume of the solution in liters (L), and W refers to the mass of the adsorbent in grams (g).

2.5. Adsorbent material

We selected corncob as the biosorbent material for this study due to its cost-effectiveness and abundant availability as an agricultural by-product in Algeria. Corncob (CC) was collected from Tlemcen (Western Algeria). We air-dried the material for 25 days at temperatures ranging from 303 to 308 °K to prepare the adsorbent. We then milled them finely using a Moulinex AR110830 and filtered them with an Automatic Sieve Shaker D403 (Controlab) to obtain particle sizes ranging from 1.25 to 2 mm. We thoroughly washed the produced material with distilled water to remove residual impurities and then dried it in an oven at 355 ± 5 °K for 24 hours. Figure 2 shows a photograph of the prepared corncob material.

Before their use in experiments, corncob adsorbents can be comprehensively characterized through various physical and chemical methods. These methods included Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Thermogravimetric / Differential thermal analysis (TGA/DTA), Brunauer-Emmett-Teller (BET) analysis, and determination of the pH at the point of zero charge (pHpzc).

2.5.1. Infrared spectroscopy analysis (FTIR)

Fourier Transform Infrared Spectroscopy (FTIR) analysis was employed to detect and showcase the functional groups and complexes in the corncob material. Roughly, 2 mg of the material was finely ground and mixed with 30 mg of high-purity potassium bromide (KBr, 99% purity, Sigma Aldrich). The resulting powder was then pelletized under a pressure of 7 bars. A Perkin Elmer Spectrum FTIR spectrophotometer (Version 10.4.00, USA) was utilized for spectrophotometric analysis within the wavelength range of 300 to 4000 cm^− 1.

2.5.2. Scanning Electronic Microscopy (SEM)

We employed Scanning Electron Microscopy (SEM) to observe and analyze the shape and structure of microstructural particles. The high-resolution SEM used for this purpose was of the TESCAN VEGA type.

2.5.3. Thermogravimetric/Differential thermal analysis (TGA/DTA)

Thermogravimetric analysis (TGA) and differential thermal analysis were performed using a TGA instrument (PerkinElmer STA 8000). A 15.201 mg sample of corncob (CC) was placed into an alumina crucible under a nitrogen atmosphere with a flow rate of 18 ml/min. The sample was then heated from 50 to 700°C at a rate of 10°C/min.

2.5.4. Brunauer-Emmett-Teller (BET) analysis

The specific surface area, average pore size, and distribution of the samples from N₂ adsorption at 77,3 K were evaluated using a Micromeritics Gemini 2380.

2.5.5. Determination of the point of zero charge pH (pHpzc)

The determination of the corncob pH at the zero point charge (pHpzc) involved adding 0.15 g of the material to 50 mL of 0.01 M NaCl solution. We altered the pH of each mixture to a range between 2 and 12 using 0.1 M NaOH or HCl solutions. We continuously stirred the mixtures for 48 hours at of 25°C. Subsequently, we measured the final pH of each mixture. A graph was constructed by plotting ΔpH as a function of pHi and identifying the pHpzc at the point where the curve intersects the zero axis.

3.1. Corncob characterization

The characterization of corncob as an adsorbent offers a detailed analysis of its physicochemical properties and adsorption potential. This comprehensive evaluation is essential for understanding how the corncob adsorbent interacts with different contaminants.

3.1.1. Infrared spectroscopy analysis

The results of the infrared spectroscopy analysis conducted on the dried corncob within the spectral range of 4000 − 300 cm^− 1 are presented in Fig. 3.

Figure 3 shows that this material contains different absorption peaks:

The broad and intense band around 3352 cm^− 1 is ascribed to the stretching vibrations of the -OH and -NH groups. (Mahadevan et al., 2022);
The asymmetric C–H and symmetric C–H bands correspond to bands at 2914 cm^− 1 and 2868 cm^− 1, respectively (Daoud et al., 2019);
The ester group in hemicellulose represents a band at approximately 1639 cm^− 1 with a C = O stretching vibration. (Xiao et al., 2016);
The peaks at 1238 and 1153 cm^− 1 may be assigned to amino C‒N stretching and the C‒O vibration of carboxylic acid (Akkari et al., 2023; Yu & Luo, 2014);
The prominent band at approximately 1028 cm⁻¹ is indicative of the C-O-H or C-O-R linkages, found in alcohols or esters (Bouchelkia et al., 2023; Boumediene et al., 2015).

Following the adsorption of both dyes, subtle shifts were detected in nearly all spectral peaks, particularly in the regions corresponding to hydroxyl (-OH) and amino (-NH) groups, showing significant changes at 3317 cm^− 1 for BM-N5R and 3356 cm^− 1 for BB-RS01. Furthermore, alterations were observed in the carbonyl (C = O) peaks at 1712 cm^− 1 for BM-N5R and 1624 cm^− 1 for BB-RS01, as well as in the peaks associated with ether (C‒O) groups at 1026 cm^− 1 for BM-N5R and 1031 cm^− 1 for BB-RS01. These observations indicate that hydroxyl, amino, and carboxyl groups serve as the predominant functional groups on the surface of corncob (CC), likely playing pivotal roles in the adsorption mechanisms of BB-RS01 and BM-N5R. Our findings align well with other results reported in the literature regarding FTIR analysis of lignocellulosic materials (Boumediene et al., 2015; Derrick et al., 2000; Yuen et al., 2005).

3.1.2. TGA Thermogravimetric / Differential thermal analysis (TGA/DTA)

Thermogravimetric analysis (TGA) under a nitrogen atmosphere was used to evaluate the thermal stability of the samples in the temperature range of 50 to 700°C, as shown in Fig. 4. All the samples exhibited multistage degradation. A slight mass loss, attributed to the elimination of adsorbed water, was observed in all samples at approximately 50 to 150°C. In the thermal decomposition study of corncob (CC), three distinct phases of mass reduction were observed. The initial phase, transpiring between 60 and 150°C, was characterized by the evaporation of bound water, accounting for approximately (6%) of the mass. Subsequently, the second phase, spanning 180 to 450°C, was marked by the thermal degradation of organic constituents, contributing to a substantial mass loss of (73%) approximately. The final phase, extending from 475 to 675°C, corresponded to the carbonization process of the residual material, resulting in an additional (17%) mass decrease (Arica et al., 2022).

3.1.3. BET analysis

As depicted in Fig. 5, the nitrogen adsorption-desorption isotherms for corncob are showcased. According to the Brunauer-Emmett-Teller (BET) classification, physisorption isotherms are typically divided into six distinct categories. The isotherms of the CC predominantly align with those of types II and IV, as evidenced by a hysteresis loop, which suggests the existence of mesopores (type IV) along with a fraction of macropores (type II). The distribution of pore sizes, inferred from the isotherm's adsorption trajectory, is also illustrated in Fig. 5 (refer to the inset). The CC sample demonstrates a wide range of pore sizes, extending from mesopores (with diameters between (17Å < and < 3000 Å) to macropores (with diameters exceeding > 500Å). Additionally, the BET analysis revealed that the sample possesses a specific surface area of 0,4127 m².g^− 1 and an average pore diameter of 106,879 Å.

3.1.4. pHpzc corncob determination

The pH of the point of zero charge (pHpzc) for corncob reveals the nature of its surface charge, indicating whether it has a positive or negative charge. Figure 6 presents the results obtained for this parameter.

The point of zero charge (pHpzc) for the corncob-based adsorbent (CC) was determined to be 7.4. This empirical value is in close agreement with the pHpzc of 6.5 reported by Suteu et al. for corncob material (Suteu et al., 2011). It is inferred that at pH values surpassing the pHpzc, the adsorbent surface acquires a negative charge, thereby facilitating the adsorption of cationic dyes. In contrast, at pH values below pHpzc, the surface is likely to become positively charged, influencing the adsorption behavior differently. This characteristic is pivotal for the strategic application of CC in the targeted removal of specific pollutants from aqueous solutions.

3.2. Determination of the wavelength for each dye

Figure 7 illustrates absorbance curves as a function of various wavelengths for two studied dyes, BB-RS01 (a) and BM-N5R (b), across five different concentrations between 10 and 50 mg/L.

The maximum wavelengths for the five concentrations were determined to be 596 nm for BB-RS01 and 565 nm for BM-N5R, as shown in Fig. 7.

These five solutions were utilized for daily spectrophotometric calibration. Figure 8 displays the calibration curve for the five concentrations, plotting dye absorbance at the previously determined maximum wavelength for each dye.

We used calibration curves, which were prepared daily, to determine dye concentrations from the spectrophotometric absorbance for our experiments.

3.3. Coagulation-flocculation sedimentation study

In our experiments, we generated solutions with a concentration of 500 mg/L for both BB-RS01 and BM-N5R. We maintained a consistent flocculent concentration of 50 mg/L throughout the study to ensure the reliability and efficacy of the coagulation-flocculation process. These experiments were conducted at a natural pH of 6 to 8. Throughout the experiments, we carefully maintained the temperature at approximately 25 ± 2°C. Given the high concentrations of the colour solutions involved, we strategically implemented a dilution factor of F = 5.

3.3.1. Effect of coagulant and flocculent doses

The efficiency of the coagulation-flocculation process relies on precise dosages of coagulant and flocculent, considering the initial contaminant concentrations. Aluminum sulphate (Al₂(SO₄)₃.18H₂O) demonstrates optimal performance within the 100 to 600 mg/L range, ensuring effective particle destabilization without excessive or insufficient chemical usage. Likewise, the selected flocculent (superfloc 8396) exhibited peak efficiency between 30 and 130 mg/L when dosed. These ranges encourage large flock formation and enhance settling, ensuring successful water purification across various initial contaminant levels. Figure 9 shows the results obtained for the coagulant dose variation and the dye removal rate in solution.

The optimal coagulant dose was 300 mg/L, resulting in a dye concentration of 48.42 mg/L for BB-RS01, and 400 mg/L of coagulant, corresponding to a dye concentration of 70 mg/L for BM-N5R. The percentage of dye removed increased proportionally with higher coagulant dose, reaching a peak at the optimal coagulant dose. Conversely, exceeding this optimal coagulant amount resulted in decreased dye removal efficiency.

Figure 10 illustrates the impact of varying flocculent doses, varying from 30 mg/L to 125 mg/L, on dye removal. It should be highlighted that we maintained the coagulant doses at the previously optimized levels of 300 mg/L for BB-RS01 and 560 mg/L for BM-N5R, as determined in Fig. 9, keeping them constant throughout this experiment.

We observed that the optimal flocculent dose for BB-RS01 dye was 50 mg/L, corresponding to a treated dye concentration of 48.42 mg/L. The optimum flocculent dose for the BM-N5R dye was identified as 75 mg/L, associated with a treated dye concentration of 61.2 mg/L. As we increased the flocculent dose, the percentage of dye removal (%) also increased proportionally. Specifically, we achieved remarkable 90.6% removal for BB-RS01 and 88% removal for BM-N5R. Chemical injection and the quick dispersion of the coagulant induce the agglomeration of dye particles and facilitate their decantation (Degremont, 2005).

3.3.2. Effect of pH

During the coagulation process, the pH of the solution holds a crucial role (Beltrán-Heredia et al., 2009). In this study, the effect of solution pH (from 2 to 11) on dye removal was studied at the optimal coagulant and flocculent doses. The optimal concentrations of coagulant and flocculent obtained from the previous experiments were kept constant during the pH change. The results are given in Fig. 11.

Our results indicate that there was a notable increase in dye removal efficiency within the pH range of 2 to 12. The removal efficiency of BB-RS01 started at pH 6 (94%) and stabilized at pH 8, achieving 95.1% dye elimination, at a final dye concentration 25 mg/L. In contrast, for BM-N5R, the maximum removal efficiency occured at pH 4 with a removal rate of 90.3% and the final dye concentration was 50 mg/L. These results can be ascribed to the nature of the dyes and their corresponding pH values. The variation in pH can be explained by the introduction of aluminum salts into the water, which leads to the release of H⁺ ions through hydrolysis reactions:

Al³⁺ + nH₂O Al(OH)n + nH⁺ (3)

3.4. Adsorption study

Dye adsorption experiments were conducted focusing on two aspects: kinetic and equilibrium adsorption.

3.4.1. Adsorption kinetics

In the coupling study, the adsorption process followed coagulation flocculation sedimentation, and our kinetic dye adsorption experiments were performed under the following conditions; these results were obtained previously for the CFS process:

The initial pH for the BM-N5R dye was set to 4 and the initial dye concentration (C₀) was 50 mg/L;
For BB-RS01 dye, we set the initial pH (pH_i) at 8 and used an initial dye concentration (C₀) of 25 mg/L;
In both cases, the m/V ratio was maintained at 1 g/L (CC had a mass of 1 g and the volume of solution was 1 L), and CC with a uniform particle diameter (dp) of 1.25/2 mm was used to ensure consistency. The solution was kept at a constant temperature of 25 ± 2°C and the agitation speed (W) was held at 400 tr/min to facilitate contact between the adsorbate and the adsorbent.

- Influence of initial dye concentration and contact time

We explored the initial dye concentration and the influence of time on the adsorption kinetics of the waste material CC using various initial dye concentrations: 15, 50, 75, and 100 mg/L. Figure 11 shows the sorption kinetics results for the various initial dye concentrations (C₀) studied.

Figure 12 shows a distinct trend in the kinetic curves, marked by a swift increase in the quantity of dye adsorbed during the initial contact minutes, leading to an equilibrium phase. Equilibrium was achieved in 30 minutes for BB-RS01 and 60 minutes for BM-N5R. At this stage, the removal percentage of BB-RS01 by corncob ranged from 18.1% at an initial dye concentration (C₀) of 15 mg/L to 30.5% at C₀ = 100 mg/L. In contrast, for BM-N5R, the percentage of adsorped corncob increased from 6.29% at C₀ = 25 mg/L to 12.09% at C₀ = 100 mg/L.

- Effect of biosorbent mass

To assess the impact of adsorbent mass on the kinetics of dye adsorption, we employed various masses (0.5, 1, 1.5, and 3g) of the adsorbent. The objective was to optimize the quantity of adsorbent required to attain maximum dye adsorption. Figure 13 shows the results.

Figure 13 shows that as the mass of the corncob increased, the equilibrium adsorption of both BB-RS01 and BM-N5R also increased. Both dyes typically reach equilibrium in approximately 60 minutes. As we increase the sorbent mass, the percentage of the dye sorbed at equilibrium also rises: from 18.23–30.86% for BB-RS01 and from 3.25–16.71% for BM-N5R over a mass range of 0.5 to 3.0 g. The observed augmentation in dye sequestration efficiency is attributable to the mass increment of the adsorbent, which correlates with the proliferation of available sorptive loci, culminating in an escalated assimilation of the dye constituents.

- Influence of adsorbent particle size

The particle size of the material can affect the adsorption kinetics of BB-RS01 and BM-N5R. We investigated the impact of different particle sizes: 0.5/0.63 mm, 1.25/2 mm, 2/2.5 mm, and 2.5/3.15 mm on the adsorption of these two dyes by the waste material. The results are presented in Fig. 14.

Figure 14 demonstrates that reducing the particle size of CC results in an increased percentage of dye removal at equilibrium. This is attributable to the fact that a smaller particle size offers a greater surface area for adsorption. The time required to reach equilibrium remained constant at 60 minutes for both the BB-RS01 and BM-N5R dyes. However, as the particle size of the corncob expands from 0.5/0.63 mm to 2.5/3.15 mm, the equilibrium dye sorption percentage rises from 16.27–24.23% for BB-RS01 and from 4.11–8.68% for BM-N5R.

- Effect of pH

We analysed the CC's adsorption of the dye at various pH values, and Fig. 15 illustrates the results of this analysis based on the kinetics of adsorption at 25 ± 2°C. We conducted this experiment to determine the favourable pH for dye fixation.

As illustrated in Fig. 15, the effectiveness of dye removal varies and fluctuates across the pH range of 2 to 11. Specifically, for BB-RS01, a dye removal efficiency of 26.19% is observed at pH 6, indicating that the non-ionic nature of the dye plays a role (Zhang et al., 2014). A basic medium is not favourable for dye uptake. The molecular properties of CC affect its interaction with dye molecules under neutral or basic pH conditions. On the other hand, for BM-N5R, the effectiveness ranged from 9.93% at pH 2 to 7.59% at pH 4 and 3.97% at pH 11. This fluctuation can be elucidated by taking into consideration the pHpzc value of CC (pHpzc = 7.4), wherein a solution pH lower than the pHpzc promotes the adsorption of anionic dyes due to the negatively charged surface. The point of zero charge (pHpzc) offers valuable insights into the active sites of the biomass and its capacity to adsorb substances.

- Effect of temperature

Temperature is a critical factor in adsorption processes, significantly influencing their efficiency and outcomes. Three adsorption experiments were performed at different solution temperatures (25, 35 and 45°C) to examine how temperature affects dye adsorption by corncob. The results are presented in Fig. 16.

According to the results (Fig. 16), the highest adsorption efficiency increased from 23.93–37.23% as the temperature increased from 25 to 45°C for the BB-RS01 dye. In contrast, the highest adsorption efficiency increased from 7.68–13.11% as the temperature climbed from 25 to 45°C for the BM-N5R dye, suggesting that this process is endothermic. The time necessary to reach equilibrium was 60 minutes for both dyes.

3.4.2. Equilibrium adsorption

We carried out equilibrium adsorption, commonly known as adsorption isotherms, of both dyes by CC under the optimal operating conditions that we previously determined in kinetics studies and are presented in Table 1.

Table 1

Optimal conditions for dye removal by adsorption on corncob
Parameters	Optimal conditions
Parameters	BB-RS01	BM-N5R
Agitation (rpm)	400	400
pH of solution	6	2
Temperature (°C)	45	45
Adsorbant size (mm)	0,5/0,63	0,5/0,63
Adsorbant mass (g)	3	3
Agitation time (h)	8	8

Figure 17 shows the CC adsorption isotherms (qe vs. Ce) for BB-RS01 and BM-N5R. The classification by Giles et al (Giles et al., 1960) for liquid-solid adsorption indicates that these isotherms are of the L type.

The data presented in Fig. 17 provide information on the capacity of CC to remove dyes per unit of mass under system conditions. The plot of these adsorption isotherms shows that the maximum adsorption capacities for both dyes by corncob are approximately 67.98 mg/g for BB-RS01 and 30.87 mg/g for BM-N5R. These results indicate that BB-RS01 has a greater affinity for CC than BM-N5R.

3.4.3. Scanning electronic microscopy (SEM)

Figure 18 illustrates the particle morphologies of the corncobs both before (Fig. 18a) and after dye adsorption (Fig. 18(b) and Fig. 18(c)). Figure 18(a) provides a clearer view of the surface shape of the initial corncob material. However, Fig. 18(b) and Fig. 18(c) reveal that the surface of the particles becomes irregular and exhibits rough heterogeneity after the adsorption process. The SEM image presented in Fig. 18 elucidates a pronounced metamorphosis of the initial nonporous precursor, designated as CC, into an advanced adsorbent exhibiting a restructured surface architecture. This alteration is indicative of the material’s enhanced adsorptive interaction potential.

- Modelling of the adsorption isotherms

This study examined the configuration of a sorption system aimed at reducing pollution from effluents by evaluating two isotherm equations (Freundlich, 1906; Langmuir, 1918).

Their linearized models are as follows:

Langmuir model: $\frac{{C}_{e}}{{q}_{e}}=\frac{{C}_{e}}{{q}_{m}}+\frac{1}{{K}_{L}\times {q}_{m}}$ (4)

Freundlich model: ${ ln}{q}_{e}={ln}{K}_{F}+n.{ln}{C}_{e }$(5)

Where q_e represents the amount of dye adsorbed at equilibrium per gram of adsorbent (mg/g), and q_m denotes the maximum dye adsorption capacity of the adsorbent material (mg/g). C_e is the equilibrium dye concentration in the solution (mg/L). K_L is the equilibrium constant (L/mg) that depends on the conditions applied. The values of K_F and n are positive constants that are determined by both the solute-adsorbent interaction characteristics and the ambient temperature. The intercept and slope values of Ce/q_e versus Ce linear plots were used to determine the constants q_m and K_L. However, the values of the constants n and K_F can be derived from the slope and the intercept at the origin of the Ln qe vs. Ln Ce plots, respectively. Figure 19 and Fig. 20 display the equilibrium data modelling curves for both BB-RS01 and BM-N5R.

The parameter results of each model are given in Table 2.

Table 2

Langmuir and Freundlich isotherm parameters for dye adsorption
Isotherms	Parameters	BB-RS01	BM-N5R
Langmuir	q_max(mg/g)	99.01	46.08
	K_L(L/mg)	0.0044	0.0038
	R²	0.9401	0.9710
Freundlich	K_F	0.8415	2.2791
	n	1.453	1.53
	R²	0.9101	0.9509

From the information depicted in Fig. 19 and Fig. 20, and in consideration of the model parameters listed in Table 2, the Langmuir model seems to be more appropriate for representing the experimental results across the tested dye concentration range. It has stronger regression coefficients (R² range: 0.94 < R² < 0.97) than does the Freundlich model (R² range: 0.91 < R² < 0.95). Under our experimental conditions, the maximum dye adsorption capacities (q_m) by CC were approximately 99 mg/g for BB-RS01 and 46 mg/g for BM-N5R.

Compared to the literature, the dye adsorption capacities of CC tested surpassed those of other agricultural wastes, as detailed in Table 3.

Table 3

Comparison of the maximum adsorption capacities of dyes onto various adsorbents: a literature review
Adsorbent	Dye	q_max(mg/g)	References
Almond shell	Methylene Blue (MB)	833,33	(Ait Ahsaine et al., 2018)
Raphia Hookerie	Rhodamine-B	666,67	(Inyinbor et al., 2016)
Almond shell	Crystal Violet (CV)	625	(Ait Ahsaine et al., 2018)
Acid-Activated Carbon Orange Peels	Disperse Blue 183	149.344	(Taifi et al., 2022)
Orange peels	Methyleneblue	218	(Boumediene et al., 2015)
Banana Peel	Congo Red	164,6	(Munagapati et al., 2021)
Corncob	Bemacron Blue-RS01 (BB-RS01)	99,01	In this study
Cactus fruit peel	Basic Red 46	82,58	(Akkari et al., 2022)
Palm tree date	Congo red	73,53	(Guiza et al., 2014)
Wood sawdust	Neutral Red C.I.50040	64,06	(Belaid & Kacha, 2011)
Banana Peel	Reactive Black 5	49,2	(Munagapati et al., 2021)
Corncob	Bemacid Marine-N5R (BM-N5R)	46,08	In this study
Cotton stalks	Basic Green 5	42,37	(Akperov & Akperov, 2019)
Potato peel	Methyleneblue	32,70	(Boumchita et al., 2016)
Corncob	Reactive dye orange 16	26,11	(Suteu et al., 2011)
Walnut shell	Malachite Green	11,76	(Hajialigol & Masoum, 2019)
Corncob	Methyl Orange	7,50	(Salih et al., 2022)
Cashew nutshell	methylene blue	5,311	(Senthil Kumar et al., 2011)
Banana peel powder	Rhodamine-B	3,88	(Singh et al., 2018)

Despite the advancements presented in this study, the adsorption capacities delineated herein fall short of those documented for alternative adsorbents, as delineated in Table 3. The discrepancies in dye adsorption efficacy can be attributed to the intrinsic characteristics of the adsorbent materials. These characteristics encompass the molecular architecture, specific surface area, and presence of active functional groups. Furthermore, the dye class and the particular experimental conditions used in each investigation play a crucial role in the adsorption process. This underscores the necessity for a comprehensive understanding of the adsorption mechanisms to optimize the material design for superior dye removal efficiency.

The complexity of removing dyes from textile wastewater stems from their diverse hazardous and toxic components. In our study, we aimed to address this challenge by coupling the coagulation-flocculation method with adsorption on corncob for effective dyestuff removal from textile wastewater. Synthetic solutions representing disperse and acid dye classes were utilized in our tests. The efficacy of coagulation-flocculation is primarily influenced by pH and reagent concentration, allowing for robust solid-liquid separation. The optimal dosages of coagulant (Al₂(SO₄)₃.18H₂O) were determined to be 300 mg/L for Bemacron Blue (BB-RS01) and 560 mg/L for Bemacid Marine (BM-N5R), while the flocculent concentrations (superfloc 8396) were set at 25 mg/L for BB-RS01 and 75 mg/L for BM-N5R. The pH solution significantly enhanced the colour removal effectiveness with a decolourization rate of 94% achieved at pH 6 for BB-RS01 and 90.3% at pH 4 for BM-N5R. During the adsorption process, corncob's adsorption for each dye was notably influenced by the solution pH, with removal rates (26.19%) observed at pH 6 for BB-RS01 and 7.69% for BM-N5R at pH 4. Increasing the temperature from 25°C to 45°C, enhanced the dyes adsorption indicating an endothermic process. Corncob characterization revealed the presence of mesoporous and irregular particle morphologies. The analysis of the equilibrium adsorption data of dyes using the Langmuir and Freundlich isotherm models demonstrated the best fit of the Langmuir model, with maximum dye adsorption capacities (q_m) of approximately 99 mg/g for BB-RS01 and 46 mg/g for BM-N5R. The results show that the adsorption process using CC enhances dye uptake and improves the quality of treatment. Corncorb (CC) can be considered an effective biosorbent for BB-RS01 and BM-N5R removal from solutions with an advantage for BB-RS01. Therefore, the synergistic coupling of coagulation-flocculation-sedimentation with adsorption on corncob proved to be effective for treating synthetic textile effluents contaminated with disperse or acidic dyes, offering an economical solution for dye removal from aqueous solutions and enhancing the safeguarding of the environment. These findings provide valuable insights into the complementary nature of the two methods and offer practical guidance for industries adopting sustainable wastewater treatment practices.

Acknowledgments The authors would like to express their gratitude to the Director of the Natural and Bioactive Substances Laboratory at the University of Tlemcen (Algeria) and the Director of the Applied Mineralogy and Applied Geochemistry Laboratory in the Department of Geosciences at the University of Aveiro (Portugal) for their help in material analysis for our research.

Authors Contributions Hadj Boumedien Rahmoun, Maamar Boumediene, and Abderahmane Nekkache Ghenim: conceptualization, formal analysis, interpretation of results, writing, original draft preparation and editing. Eduardo Ferreira Da Silva, João Labrincha: formal analysis, interpretation of results, review and editing.

Funding This work was financially supported by the Erasmus + Program (Project: 2020-1-PT01-KA107-077895) and the Algerian Higher Ministry of Education and Scientific Research (PRFU Project, Code: 20001UN01N17A).

Data availability Not applicable.

Ethical approval Not applicable.

Consent to participate Not applicable.

Consent to Publish All the authors have approved the final version of the manuscript.

Competing interests The authors declare no competing interests.

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No competing interests reported.

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Coupling coagulation-flocculation-sedimentation with adsorption on biosorbent (Corncob) for the removal of textile dyes from aqueous solutions

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Abstract

Figures

Highlights

1. Introduction

2. Materials and methods

2.1. Dyes, coagulants and flocculants

2.2. Determination of the wavelength and absorbance of dyes

2.3. Coagulation-flocculation-sedimentation process

2.4. Adsorption processes

2.5. Adsorbent material

2.5.1. Infrared spectroscopy analysis (FTIR)

2.5.2. Scanning Electronic Microscopy (SEM)

2.5.3. Thermogravimetric/Differential thermal analysis (TGA/DTA)

2.5.4. Brunauer-Emmett-Teller (BET) analysis

2.5.5. Determination of the point of zero charge pH (pHpzc)

3. Results and Discussion

3.1. Corncob characterization

3.1.1. Infrared spectroscopy analysis

3.1.2. TGA Thermogravimetric / Differential thermal analysis (TGA/DTA)

3.1.3. BET analysis

3.1.4. pHpzc corncob determination

3.2. Determination of the wavelength for each dye

3.3. Coagulation-flocculation sedimentation study

3.3.1. Effect of coagulant and flocculent doses

3.3.2. Effect of pH

3.4. Adsorption study

3.4.1. Adsorption kinetics

3.4.2. Equilibrium adsorption

3.4.3. Scanning electronic microscopy (SEM)

4. Conclusion

Declarations

References

Additional Declarations

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