This investigation focused on the Congo red uptake, as the azo dye, from an aqueous environment. Herein, ash derived from sunflower seed pulp waste (SSPA) was used as an eco-friendly low-cost adsorbent. Based on the BET results, the specific active surface area of SSPA was approximately 102 m2/g. The effect of the initial analyte concentration (10–50 mg/L), the concentration of the adsorbent (1–5 g/L), and the processing time (10–240 min) on the rate of Congo red uptake was also evaluated and optimized. According to the results, the maximum dye removal from synthetic solution (91.89%) was achieved at a dye concentration of 50 mg/L, an adsorbent concentration of 3 g/L, and a processing time of 180 min. The maximum SSPA capacity for Congo red uptake from aquatic solution was 15.34 mg/g, achieving under optimized operational conditions. The adsorption process of SSPA also follows a pseudo-second-order kinetic model (qe = 15.85 mg/g; R2 > 0.99), considering the results of BET and FTIR, suggesting that the rate-controlling step in analyte removal is the chemical interaction between functional groups in SSPA and used dye. Finally, the SSPA washing with different solvents showed that the adsorbent treated with 1 M sodium hydroxide still performed well after his five reuses.

  • Sunflower seed pulp ash (SSPA) was prepared and applied as an eco-friendly low-cost adsorbent for Congo red uptake.

  • Experiments showed the rate-controlling step in dye adsorption is the chemical interaction of functional groups between SSPA and Congo red.

  • According to the washing process of the adsorbent used by different solvents, one molar solution had a high capacity to wash the SSPA.

There are limited resources of freshwater on earth, and the development of industry and urbanization is leading to the release of large amounts of dyes, pollutants, organic pollutants, and heavy metal ions into natural water resources (Drobyazko et al. 2021; Moghadam & Samimi 2022). Azo dyes are principal contaminants produced by industries, such as textile printing, cosmetics, textiles, and food dyes (Oliver Paul Nayagam & Prasanna 2023). Due to the toxicity and high level of non-biodegradability in the environment, dyes are dangerous to the lives of aquatic animals and humans, even in low concentrations (You et al. 2018; Samimi & Safari 2022). Polluted water released by industry contains highly carcinogenic and non-biodegradable dyes of Congo red. Wastewater containing Congo red dye is produced in the textile, printing, dyeing, paper, and plastic industries (Hamad & Saied 2021; Khan et al. 2022). To reuse these polluted waters, many new and effective adsorbents have been used to remove the contaminants (Mohammadi et al. 2021; Nuryadin & Imai 2021; Samimi 2024). Dye-contaminated water is decolorized by conventional methods such as coagulation (Zhou et al. 2017; Nimesha et al. 2022), photocatalysis (Magdalane et al. 2021; Mambwe et al. 2021), environmental bacteria (Manjarrez Paba et al. 2021), and adsorption (Samimi & Moeini 2020). Among these, the adsorption method is very effective and efficient because it has features, such as low cost, availability of various adsorbents, simplicity of the process, high efficiency, and ease of implementation (Samimi & Shahriari-Moghadam 2023). The adsorption method has been used in many studies to remove Congo red from wastewater. In some studies, different adsorbents such as composite layers made of melamine-formaldehyde and polyvinyl alcohol (Bhat et al. 2020), activated carbon nanocomposite made of Guar gum (Gupta et al. 2020), pineapple skin (Dai et al. 2020), synthetic FexCo3xO4 nanoparticles (Liu et al. 2019), synthesized zeolite and ZnO particles (Madan et al. 2019), chitosan/bentonite composite (Abukhadra et al. 2019), and NiO–SiO2 composite particles (Lei et al. 2016), were used for the removal of Congo red from wastewater.

The various biosorbents have attracted the attention of researchers due to their cost-effectiveness and acceptable performance in removing Congo red from aqueous solutions containing this type of dye (Siddiqui et al. 2023). Among these, it can mention the paper and algae waste collected from bio-related companies (Fawzy & Gomaa 2020), hydroxyapatite nanoparticles synthesized from eggshells (Vinayagam et al. 2023), hydroxyapatite nanorods adsorbents synthesized from phosphogypsum wastes (Bensalah et al. 2020), magnetically lignin-based bio-adsorbent (Zong et al. 2023), dried powdered cabbage waste (Wekoye et al. 2020), Jojoba seed wastes (Al-Zoubi et al. 2020), carbon from leaves and stem of water hyacinth (Extross et al. 2023), sugarcane bagasse prepared with tartaric acid (Said et al. 2020), activated carbon from ashitaba waste and walnut shells (Li et al. 2020), saffron stem shells (Dbik et al. 2020), eucalyptus leaf powder (Kumari et al. 2020), and natural and modified Clinoptilolite with a surfactant (Nodehi et al. 2020).

In some previous studies, synthetic adsorbents, which have a high price and are harmful to the environment, or cheap adsorbents, whose surface modifications require using chemical materials, which are dangerous to nature and microorganisms, have been used. Many studies have been carried out on using sunflower derivatives for adsorption, such as sunflower head and stem as adsorbents to remove heavy metals from aqueous media (Mohmmadkhani et al. 2016) and sunflower stalks to remove lead and cadmium from aqueous solution (Jalali & Aboulghazi 2013). This work aimed to use an inexpensive and eco-friendly adsorbent produced during a simple process to remove colored wastewater. Sunflower seed pulp ash (SSPA) was used as an adsorbent to remove Congo red dye from aqueous solutions in a continuous fluidized bed reactor. The effects of operational variables on Congo red uptake, such as initial dye concentration, process time, and SSPA amount, were also studied.

Materials

Using Congo red dye (Merck, high purity), synthetic solutions were prepared and used in this study as feed. The sunflower seed pulp was obtained from local shops producing sunflower oil (Kermanshah, Iran). Ethanol (Merck, 99.8%) and sodium hydroxide (NaOH) (Merck, ACS reagent, ≥97.0%, pellets) were used in the adsorbent washing step. All experiments used double-distilled water.

Preparation of adsorbent

The ash powder used in the present study was prepared from sunflower seed pulp as agricultural waste. First, the prepared sunflower seed pulp was put into the furnace and the temperature was increased to 600 °C at a rate of 5 °C/min, held at this temperature for 1 h, and then was cooled to ambient temperature. The resulting material, SSPA, was crushed and grinded, and the particles smaller than 45 μm were separated for testing. Finally, the prepared powder particles were stored in an insulated container. Before the experiments, no chemical operations or physical treatment were conducted on ash obtained from sunflower seed pulp.

Methods

The scheme of the setup prepared for all experiments is shown in Figure 1. In this study, Congo red dye removal was investigated using the adsorption process in a fluidized bed reactor (the volume of the reactor is 100 mL). The fluidized bed reactor was specifically customized to investigate the adsorption of various pollutants. It has not only the same characteristics as a fluidized bed but also being able to establish proper mixing to increase the contact between the adsorbent surface and pollutant molecules. The feed containing the dye solution made with a desired concentration and specific amounts of ash made of sunflower seed pulp was tested at different contact times in the mentioned fluidized bed reactor. The feed solution in a container was pumped using a peristaltic pump (BT100-1F model) into the reactor from the bottom, leading to the adsorbent fluidization.
Figure 1

Schematic of the setup used in this study to remove Congo red dye by SSPA.

Figure 1

Schematic of the setup used in this study to remove Congo red dye by SSPA.

Close modal
To increase the efficiency of mixing and adsorption, the laboratory setup used an electrical motor with a speed of 600 rpm to rotate two mechanical stirrers. After a specified period, after centrifugation (to separate the adsorbent from the treated wastewater), the sample was removed from the setup and discarded as the final treated sample. Congo red dye concentration was measured using a UV-Vis spectrophotometer (PG Instruments, UK, Model: T80 + +) at 497 nm with a calibration curve. Device calibration curves were generated individually by preparing solutions with different dye concentrations. The variables studied included initial dye concentration, process time, and amount of adsorbent. The SSPA capacity in dye uptake and Congo red removal percentage were determined as follows (Samimi & Nouri 2023):
formula
(1)
formula
(2)
where is the concentration of initial dye in mg/L, is the concentration of dye at residence time t in mg/L, (mg/g) is the SSPA capacity in Congo red removal at residence time t, and M (g/L) is the SSPA amount in dye solution.

Characterization of SSPA

FTIR spectroscopy was applied to identify the functional groups of the SSPA adsorbent. The surface morphology and specific surface area, as well as the surface particle size, were evaluated through SEM and BET analysis (N2 gas adsorption at 77 K), respectively.

Adsorbent washing with solvents

To wash ash derived from sunflower seed pulp, ethanol, one molar of NaOH, and double-distilled water at 65 °C were used and the results were compared. Accordingly, the sunflower seed pulp collected from the experiment was stirred with 100 mL of each solvent (ethanol, NaOH solution, and double-distilled water) for 180 min and after filtration using a filter paper and drying at ambient temperature for 12 h, it was used again.

Characterization of SSPA adsorbent

The BET results of SSPA showed a total pore volume of 0.0990 cm3/g, a specific surface area of 102.68 m2/g, and a mean pore diameter of 3.859 nm. The specific surface area results of SSPA with other similar adsorbents are presented in Table 1. The SEM image of SSPA is also shown in Figure 2.
Table 1

Comparison of BET results of the used adsorbent in this work with other studies

AdsorbentSpecific surface area (m2/g)Reference
Natural clinoptilolite 11.93 Nodehi et al. (2020)  
Activated carbon prepared from sunflower seed hull 21.06 Thinakaran et al. (2008)  
Sunflower waste – manganese iron oxide composite 47.14 Uygunoz et al. (2022)  
Fly ash from a local power plant 7.53 Harja et al. (2022)  
SSPA 102.68 This study 
AdsorbentSpecific surface area (m2/g)Reference
Natural clinoptilolite 11.93 Nodehi et al. (2020)  
Activated carbon prepared from sunflower seed hull 21.06 Thinakaran et al. (2008)  
Sunflower waste – manganese iron oxide composite 47.14 Uygunoz et al. (2022)  
Fly ash from a local power plant 7.53 Harja et al. (2022)  
SSPA 102.68 This study 
Figure 2

SEM image of SSPA.

Figure 2

SEM image of SSPA.

Close modal
FTIR spectroscopy is an essential technique to characterize functional groups and the variation study of these groups in the adsorbent (Ehzari et al. 2022). The FTIR spectra of the ash adsorbent before and after Congo red adsorption are shown in Figure 3(a) and 3(b), respectively. The bonds of 3,422.58 and 3,502.9 cm−1 are attributed to the tensile frequency of the OH at the SSPA surface. The adsorption peak of 2,856.99 and 2,923.47 cm−1 is related to the symmetric tensile frequency of the –CH3 group. The peak of 1,627.66 cm−1 shows the tensile frequency of C = O bond in carboxylic acid, which is bonded with intramolecular hydrogen (Han et al. 2010). The peak of 1,387.86 cm−1 is caused by the symmetric curvature of –CH3. The presence of an adsorption peak at 1,084.54 cm−1 may be related to the frequency change at OH and the tensile frequency at C–O–C in the cellulosic structure of the adsorbent. The FTIR results showed that the surface of the adsorbent is abundant with different oxygen functionalities (O–H, C = O, C–O–C). These functionalities will act as active binding sites for the effective uptake of anionic dyes from the water phase (Al-Zoubi et al. 2020). As shown in Figure 3(b), after the adsorption of Congo red, the peak of tensile frequencies of OH as well as the tensile frequency of C = O in carboxylic acid with bounded intramolecular hydrogen changed, and frequencies of 3,502.90 and 1,627.66 cm−1, respectively, reduced to 3,426.20 and 1,625.33 cm−1.
Figure 3

FTIR spectrum of the SSPA; (a) before adsorption and (b) after adsorption.

Figure 3

FTIR spectrum of the SSPA; (a) before adsorption and (b) after adsorption.

Close modal

Effect of operational variables on the elimination of Congo red

In this study, the effects of different variables such as initial concentration of Congo red, adsorbent concentration, and process time on Congo red dye removal were evaluated. The values of each of these variables along with their dimensions are presented in Table 2. As shown in Table 2, the initial concentration of the dye in the range of 10–50 mg/L, the concentration of adsorbent in the range of 1–5 g/L, and the processing time in the range of 10–240 min were investigated.

Table 2

Operational variables in Congo dye adsorption process and their levels

FactorSymbolUnitLevels
Congo red initial concentration  mg/L 10, 30, 50 
Adsorbent concentration  g/L 1, 3, 5 
Time  min 10, 20, 30, 60, 90, 120, 150, 180, 240 
FactorSymbolUnitLevels
Congo red initial concentration  mg/L 10, 30, 50 
Adsorbent concentration  g/L 1, 3, 5 
Time  min 10, 20, 30, 60, 90, 120, 150, 180, 240 

Figure 4 presents the curves of Congo red dye removal percentage as a function of time at different concentrations of the SSPA. As shown in Figure 4, after 10 min, more than 57% of the dye in the aqueous solution was removed. However, with a further passage of time beyond 10 min, the Congo red uptake decreased over time, while the color removal percent still increased. At 120 min, the color removal percent had become almost constant over time, but still, the dye removal rate slightly increased until 180 min. Thus, the Congo red adsorption process on the adsorbent by changing the residence time can include two stages: rapid initial adsorption and slow adsorption at the process end. It is clear that the initial rapid adsorption occurs due to the high affinity of Congo red and the adsorbent. Furthermore, the high adsorption sites and mass transfer gradient between the adsorbent and Congo red can be found among the explanations for this rapid adsorption. On the other hand, because the adsorption sites are saturated, which leads to a decrease in the vacancy numbers, the amount of adsorbent decreases at the last time (Nodehi et al. 2020).
Figure 4

Congo red dye removal diagrams over time at different concentrations of SSPA: (a) initial dye concentration of 10 mg/L, (b) initial dye concentration of 30 mg/L, and (c) initial dye concentration of 50 mg/L.

Figure 4

Congo red dye removal diagrams over time at different concentrations of SSPA: (a) initial dye concentration of 10 mg/L, (b) initial dye concentration of 30 mg/L, and (c) initial dye concentration of 50 mg/L.

Close modal

As the diagrams in Figure 4 indicate, the adsorbent concentration plays an important role in the percentage of Congo red dye removal. With an increase in the adsorbent concentration from 1 to 3 g/L, a significant increase in Congo red dye removal percentage was observed. When increasing the adsorbent concentration from 3 to 5 g/L, the Congo red uptake decreased slightly and remained unchanged in some cases.

The diagrams in Figure 5 present the amount of Congo red adsorbed in grams of the adsorbent () over time. As shown in these figures, at low adsorbent concentrations ( = 1 g/L), the highest amount of adsorbed dye per gram of the adsorbent was observed, and with increasing the adsorbent concentration to 3 and then 5 g/L, the amount of adsorbed dye in grams reduced. In other words, the more adsorbents there are, the lower the ratio of adsorbed dye per gram of the adsorbent. Considering the diagrams in Figure 5, it is also worth noting that with increasing the adsorbent concentration, the time required to reach the final amount of adsorbed dye decreased due to more availability of adsorbent and the lower time of the adsorption process.
Figure 5

Diagrams of the amount of Congo red dye adsorbed (adsorption capacity) over time at different concentrations of SSPA; (a) initial dye concentration of 10 mg/L, (b) initial dye concentration of 30 mg/L, and (c) initial dye concentration of 50 mg/L.

Figure 5

Diagrams of the amount of Congo red dye adsorbed (adsorption capacity) over time at different concentrations of SSPA; (a) initial dye concentration of 10 mg/L, (b) initial dye concentration of 30 mg/L, and (c) initial dye concentration of 50 mg/L.

Close modal

Considering the results presented in Figures 4 and 5, the dye concentration of 50 mg/L, the adsorbent concentration of 3 g/L, and the time of 180 min can be introduced as the optimal operating conditions. Under these conditions, the percentage of dye removal was 91.89% and the amount of dye adsorbed per gram of the adsorbent was 15.32 mg/g.

Kinetic modeling of Congo red adsorption on SSPA

In this work, two adsorption kinetic models were used, pseudo-first-order and pseudo-second-order to determine the boundary phase velocity and adsorption behavior (Samimi & Shahriari-Moghadam 2021). Primarily, a pseudo-first-order model is used to explore the adsorption processes (Piri & Sepehr 2022). The pseudo-first-order kinetic equation, also known as the Lagergren equation, assumes that the absorption rate of the adsorption sites is proportional to the number of vacancies (Azimi et al. 2019). This term is represented as follows:
formula
(3)
where and are the Congo red amount adsorbed at the residence time of t and equilibrium, respectively, and is the pseudo-first-order equation per min.

When the adsorption fits the first-order model, versus t must represent a linear relationship, and the values of and can be determined using the slope and the intercept (Samimi et al. 2023). Table 3 represents the values for , , , and for the pseudo-first-order model. As shown in this table, despite the high correlation coefficient ( > 0.90), the calculated values deviate from the experimental . Therefore, the adsorption process of Congo red cannot be described by a pseudo-first-order model.

Table 3

Results and parameters of pseudo-first-order and pseudo-second-order adsorption equations

Pseudo-first-order
Pseudo-second-order
(min−1) (g·mg−1·min−1)
10 3.06 1.76 0.0353 0.9616 3.14 0.0603 0.9999 
30 9.67 5.48 0.0339 0.9821 9.92 0.0187 0.9999 
50 15.34 11.48 0.0334 0.9570 15.85 0.0084 0.9995 
Pseudo-first-order
Pseudo-second-order
(min−1) (g·mg−1·min−1)
10 3.06 1.76 0.0353 0.9616 3.14 0.0603 0.9999 
30 9.67 5.48 0.0339 0.9821 9.92 0.0187 0.9999 
50 15.34 11.48 0.0334 0.9570 15.85 0.0084 0.9995 

Therefore, a pseudo-second-order model was used to determine the adsorption process. The second-order kinetics model is based on the assumption that the filling rates of the adsorbed sites are proportional to the square of the unoccupied sites (Samimi & Mansouri 2023). Furthermore, the number of occupied sites is directly proportional to the percentage of dye adsorbed, which is widely used in uptake processes (Özcan & Özcan 2005). This model is represented by the following equation:
formula
(4)
where and are the Congo red amount adsorbed at the residence times of t and equilibrium, respectively, and is the constant adsorption rate of the pseudo-second-order equation (g·mg−1 min−1).
According to Equation (4), when the diagram per t has a linear relationship, the pseudo-second-order kinetic model is suitable for adsorbing the Congo red dye on the SSPA. Figure 6 shows that has a significant linear relationship with t, which is proved by a high correlation coefficient (≫ 0.99). In addition, as seen in Table 3, the calculated values correspond to the experimental ones. These results indicate a higher correlation of the pseudo-second-order equation than the first-order equation. Similar results on the Congo red adsorption have been observed in other works (Kumari et al. 2020; Li et al. 2020; Said et al. 2020; Extross et al. 2023; Vinayagam et al. 2023).
Figure 6

Linear graphs of pseudo-second-order equations ( per ) at different concentrations of the SSPA; (a) initial dye concentration of 10 mg/L, (b) initial dye concentration of 30 mg/L, and (c) initial dye concentration of 50 mg/L.

Figure 6

Linear graphs of pseudo-second-order equations ( per ) at different concentrations of the SSPA; (a) initial dye concentration of 10 mg/L, (b) initial dye concentration of 30 mg/L, and (c) initial dye concentration of 50 mg/L.

Close modal

Adsorption reusability

The reusing process of the adsorbent was carried out according to the stated method under optimal conditions (initial dye concentration of 50 mg/L, residence time of 180 min and adsorbent concentration of 3 g/L). Different solvents were used to wash the adsorbent, and 1 M NaOH solution, 65% double-distilled water, and ethanol, respectively, had an acceptable performance in the washing process. Figure 7 summarizes the results of the washing process of SSPA. Based on the results presented in Figure 7, the use of one molar NaOH solution made it possible to wash the adsorbent for five rounds of reuse while retaining the Congo red dye removal capacity. Double-distilled water at 65 °C was able to wash the adsorbent well for four times. Finally, using ethanol, the adsorbent washing process was performed for three rounds while maintaining the Congo red dye removal capacity.
Figure 7

Results of the washing process of the adsorbent used in this study by different solvents.

Figure 7

Results of the washing process of the adsorbent used in this study by different solvents.

Close modal

In general, most synthetic adsorbents, which can be used for dye removal from aquatic solutions are expensive and harmful to the environment. In this study, SSPA, a cheap and eco-friendly adsorbent, was prepared and used as an unusable waste to adsorb Congo red dye. The effect of initial dye concentration, SSPA concentration and process time on Congo red removal was evaluated. The results showed that the SSPA had a good adsorption capacity of Congo red dye. Moreover, in the present research, both pseudo-first-order and pseudo-second-order kinetic models were used for modeling, resulting in higher accuracy of the pseudo-second-order model (≫0.99). Finally, the washing process of the adsorbent used by different solvents was investigated, confirming the solution with one molar concentration had a high capacity to wash the adsorbent used in this work.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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