Biosorption of textile dye reactive blue 221 by capia pepper (Capsicum annuum L.) seeds

Industrialization grows very quickly, and the use of dyes in industries all over the world increases with the need of people for colors (Geetha et al. 2015). Contamination of the aqueous environment by several dyes resulting from various industries such as textile, leather, paper, printing, iron-steel, petroleum, solvent, pharmaceutics and cosmetics has become a crucial environmental problem at the present time (Daneshvar et al. 2012a, 2012b; El Haddad et al. 2014; Oguntimein 2015; Deniz & Kepekci 2016). Approximately 60% of the total dyes produced for coloring various fabrics are consumed by textile mills. A percentage (10–15%) of these dyes originating from the textile industry are dispersed to wastewaters (Oguntimein 2015). The effluents of this industry are intensely polluting to the aquatic environment. The color resulting from dyes reduces the sunlight and oxygen penetrating to the aqueous environment and creates a negative impact on photosynthetic aquatic life. Furthermore, some dyes may contain toxic or carcinogenic groups (Argun 2012; El Haddad et al. 2013; Guerrero-Coronilla et al. 2015; Yavari et al. 2015).

Reactive dyes, among the synthetic dyes used in industries, are very crucial in the contamination of aqueous media. Cotton is the most commonly-used textile fiber to be colored by this kind of dye, therefore the production of these dyes continues incrementally. Reactive dyes are highly soluble in water, thus 20–50% of unfixed and hydrolyzed dye can be discarded from textile mills as an effluent (Wangpradit & Chitprasert 2014). These dyes have low biodegradability and a high salt content, and contribute extensively to the organic load of waters. They contain triarylmethane, azo and anthraquinone chromophoric groups. The concentrations of reactive dyes in textile mill wastewaters are in the range of 60 to 250 mg/L (Argun 2013).

There are numerous dye removal techniques in the literature. Oxidative processes, adsorption, ion exchange, membrane filtration, electro coagulation, biological treatment with living organisms and biosorption can be used to decolorize wastewaters (Robinson et al. 2001; Aksu & Karabayir 2008). Biosorption is a challenging technology that uses several biosorbents such as bacteria, algae, yeast, fungi, and agricultural and industrial by-products to remove various ionic contaminants and dyes from wastewaters (Kim et al. 2015). This technology is a cheap and effective way to remove color from aqueous solutions (Ozcan et al. 2005; Srinivasan & Viraraghavan 2010). Cheap and alternative types of biomass have been studied by researchers worldwide, and new biosorbent materials are also being investigated today (Deniz 2013).

Some of the types of biomass used in various researches include malt bagasse (Fontana et al. 2016), S. platensis (Dotto et al. 2012), Cladosporium sp. (Fan et al. 2012), Spirogyra sp. (Khataee et al. 2013), olive stone biomass (Albadarin & Mangwandi 2015), Aspergillus versicolor (Huang et al. 2016), marine brown macroalgae (Daneshvar et al. 2012a, 2012b), Cucumis sativus peel (Lee et al. 2015), Cupressus sempervirens cone chips (Fernandez et al. 2012), and rice husk (Safa & Bhatti 2011).

In this study, capia pepper seeds were used as a biosorbent material to remove a textile dye, reactive blue 221, from aqueous solutions. There are have only been a few studies conducted using this biomass to remove ionic substances and dye from aqueous solutions. In one of these studies, Ozcan et al. (2005) investigated the biosorption performance of Capsicum annuum seeds on copper removal from aqueous solutions. Seeds were used without any pre-treatment in this study, and the copper adsorption equilibrium was obtained in 60 minutes with this biomass (Ozcan et al. 2005). Also, another study on the pepper seeds was reported by Akar et al. (2011). In this work, the seeds were pre-treated with acetone and successfully removed reactive blue 49 from dye solutions (Akar et al. 2011).

So, there are no more studies known in the literature conducted with various pepper seeds. Also, a few adsorption studies treating reactive blue 221 dye with sepiolite (Alkan et al. 2005, 2007) and kaolinite (Karaoğlu et al. 2010) are found in the literature.

A detailed study was conducted to observe the dye removal performance of capia pepper seeds, without any pre-treatment, as a natural biosorbent. Some optimization tests were also carried out. The effects of solution pH, adsorbent dosage, contact time and initial dye concentration were studied. Also, kinetic and isotherm studies were made to define the biosorption process.

Capia peppers were bought from a traditional market place in Dikili, Izmir, Turkey. The seeds in the peppers were separated from the body and washed thoroughly with tap, distilled and deionized water, respectively. The biomass was dried in an oven at 70 °C for 2 days. After the drying process, the seeds were ground and sieved to a particle size of 250–500 μm. The sieved biosorbents were kept in an oven at 55 °C for 2 days and then stored in a container without contact with air for further tests.

Commercial grade CI reactive blue 221 (Molecular weight: 1082.83 g/mol, chemical formula: C₃₃H₂₄ClCuN₉Na₃O₁₅S₄, CAS number: 93051-41-3) (Argun 2012) textile dye was chosen as the model dye in this work and supplied by Alfa Kimya. This dye was dissolved in deionized water to prepare the desired dye solutions. 0.1–4.0 N HNO₃ or NaOH solutions were used to adjust the pH of the wastewater to be treated. HNO₃ and NaOH were obtained from Merck Co. All reagents were of analytical grade and used without any further purification.

In the above equation, the highest initial dye concentration used in the study is shown as C₀. The change in suitability of the adsorbent to the adsorbate with the R_L value is given in Table 1 (Deniz & Karaman 2011).

Linear regression analyses were conducted by plotting against t.

It can be clearly seen from Figure 1 that the maximum removal efficiency and uptake capacity using a natural biosorbent were obtained at a pH of 2. The dye removal efficiencies at pH 1 and 2 were nearly the same, while the uptake capacities were found to be higher at pH 2. The removal efficiency of the pH 1 test was high because of the dilution effect resulting from the pH adjustment. Therefore, in the other tests conducted to examine the biomass performance, the pH of dye solutions was adjusted to 2. At pH values above 3, the removal efficiency and uptake capacity decreased. Therefore, the studies were not conducted at higher pH values. The same pH trend for several dyes was observed in the studies conducted in the literature by several researchers (Çolak et al. 2009; Kiran et al. 2009; Asgher & Bhatti 2010; Akar et al. 2011). Despite the high removal efficiency of the biosorption process and the low cost of the biomass, it has some disadvantages. In the textile sector, the pH of wastewater may be so variable that it needs to be neutralized before discharge to the environment. Before treatment by biosorbent, the pH value should be adjusted to 2 for effective removal of dye. After treatment process neutralization is unavoidable.

The pH value of the treated media affects the biosorption capacity of the biomass, the solubility properties of the dyes and also the hue of the aforementioned media. Reactive dyes such as Reactive Blue 221 represent anionic characteristics. At lower pH values, the biosorbent surface is positively charged, thus attracting the negatively-charged dye ions. So, the anionic groups of the dye solution may be bound by the protonated surface of the biosorbent material, and the color of the dye has been successfully removed from the contaminated aqueous media probably by this electrostatic attraction (Akkaya & Özer 2005). At higher pH values, the binding areas deprotonate and a negative charge occurs. Dye removal efficiency decreases due to the repulsive forces between the biosorbent surface and the dye (Akar et al. 2011). These cases prove the results obtained in this study.

The performance of the biomass regarding removal efficiency increased with increasing biosorbent dosage, and reached a maximum 97% with 2.8 g capia pepper seeds per liter. It can be concluded that a higher biomass dosage affects the treatment efficiency and hue removal performance in a positive way. However, the increment of the biomass amount in the solution reduced the biosorption capacity. Doses of 1.6 g L⁻¹ and 2.2 g L⁻¹ were found to be optimum in terms of both removal efficiency and uptake amounts.

The increment of dye removal efficiency with higher dosages of biosorbents and the contrast between efficiency and biosorption capacity was also observed in several dye removal studies in the literature (Akar et al. 2009a, 2009b). It can be told that increasing the biosorbent dosage raises the biosorbent surface area and also the quantity of prospective binding sites. The decrease in the uptake capacity of the biomass occurs because of probable biosorbent aggregation. Hence, the effective surface area of the biomass diminishes (Barka et al. 2011).

Dye removal efficiency decreased with increasing concentrations of dye solution. On the other hand, the biosorption capacity of the biomass increased directly proportionally to the initial dye solution concentration. A maximum removal efficiency above 96% was obtained with an initial dye concentration of 217 mg L⁻¹. Biosorption uptake capacity for this concentration was determined as 95.35 mg g⁻¹.

Removal efficiency was also reported to decrease at higher dye concentrations in various studies. With the increase in the dye concentration, the available active adsorption sites on the biomass surface diminish, so there are no more places for the adsorption of dye onto pepper seeds (Khataee et al. 2013; Geetha et al. 2015).

Uptake by the biomass increased with an increase in initial dye concentrations in the other studies in the literature (Safa & Bhatti 2011; Oguntimein 2015). This may be due to the increasing driving force (Aksu & Tezer 2005). The mass transfer resistance between the aqueous and solid phases was overcome by this driving force. Also with higher initial dye concentrations, interaction between the dye molecules and biosorbent can increase by affecting the biosorption in a positive way (Aksu & Tezer 2005).

The dye biosorption performance with raw biomass enhanced with time. Tests were conducted for 48 hours. In 21 hours, dye removal started to decelerate and approximately reached equilibrium. In the first 2.5 hours of the treatment a biosorption capacity of 52.44 mg g⁻¹ was obtained, which is nearly half of the 48 hour uptake amount (99.83 mg g⁻¹). The biosorption process was very rapid at the beginning of the process, but the removal rate decreased with advancing contact time. This case can be explained as follows: at the beginning of the study, there are too many available adsorption sites for rapid dye adsorption onto the biomass. In advancing periods of biosorption, the dye molecules fill all the free sites of the biomass, so the removal slows down (Oguntimein 2015).

This biomass retained remarkable amounts of dye. This process can be accelerated by pretreating this biomass with several chemicals. It should be kept in mind that the cost of this biosorption process will increase with this pretreatment.

To design an adsorption system, the rate of the process is a very important factor. Also, the kinetic parameters are important for the modeling of biosorption systems (Srivastava et al. 2015). In order to investigate the kinetics of RB221 dye biosorption onto pepper seeds, two kinetic models, pseudo-first order and pseudo-second order models, were used in this study.

These kinetic studies are important, because they provide knowledge regarding the probable biosorption mechanism and also make it possible to improve suitable mathematical models for describing the interactions. The parameters and rates obtained from kinetic studies can be used to improve biosorbents for industrial applications, and also the complicated dynamics of the biosorption can be understood by these data (Sen Gupta & Bhattacharyya 2011).

The R² value for this model was found to be 0.98. The other parameters, q_e and k₁, were 57.80 mg g⁻¹ and 0.071 h⁻¹, respectively. The calculated and experimental equilibrium values are 57.80 and 99.83 mg g⁻¹, respectively. The calculated and experimental uptake capacities do not match each other. So, this model cannot describe the sorption kinetics.

According to the model, q_e and k₂ were found to be 97.09 mg g⁻¹ and 0.005 g mg⁻¹ h⁻¹ with a correlation coefficient of 0.99. This R² value is higher than that of the pseudo-first order model. Also, the calculated and experimental values for q_e were 97.09 mg g⁻¹ and 99.83 mg g⁻¹, respectively. These values are closer to each other, showing that the more appropriate model for RB221 biosorption is the pseudo-second order kinetic model. The kinetic results are also summarized in Table 2.

It is clearly seen from Table 2 that the data obtained from the kinetic tests fitted the pseudo-second order model better than the first-order model. This means that the rate limiting step in the biosorption of RB221 dye by this biomass may be chemisorption. This conformity has been interpreted as surface sorption in the literature (Saha et al. 2012). As a result of chemical interactions, the electrons are probably exchanged or shared between the dye adsorbent and adsorbate (Ho & McKay 2000; Yeddou-Mezenner 2010).

There are many studies in the literature showing that the pseudo-second order model dominates many dye biosorption processes. Some of them are: dye removal by marine brown macroalgae (Daneshvar et al. 2012a, 2012b), Cladosporium sp. (Fan et al. 2012), Cucumis sativus peel (Lee et al. 2015), sepiolite (Alkan et al. 2007).

It can be clearly seen from Figure 7 that the data fit very well to the Langmuir model with a correlation coefficient value of 0.997. Maximum biosorption uptake capacity was found to be 142.86 mg/g. The constant b was calculated as 0.127 L/mg for biosorption of RB221 dye by pepper seed biomass. Also, the separation factor was calculated using initial dye concentrations and constant ‘b’ obtained from the Langmuir model.

It can be seen from Figure 8 that the dimensionless separation factor is reduced with an increase in the initial RB221 dye concentration. This means that the biosorption of RB221 onto pepper seed biomass improves with higher concentrations of dye solution. Also, the R_L value is in the range of 0.016 to 0.035, and shows that the biosorption process is favorable at all concentrations studied in this work. The biosorbent pepper seed is appropriate for RB221 dye solution in the light of these data. This case was observed in the other studies in the literature (Flores-Garnica et al. 2013; Guerrero-Coronilla et al. 2015).

In Figure 9, the correlation coefficient was estimated to be 0.991 according to the Freundlich isotherm. The k_f and n parameters of the Freundlich isotherm were 75.28 (mg g⁻¹) (L mg⁻¹)^1/n and 8.576, respectively.

The results of the two isotherm studies are summarized in Table 3. The correlation coefficients are very high for both Langmuir and Freundlich isotherms. But with a 0.997 R² value, the Langmuir model seems more appropriate for the experimental data.

So, these results show that the monolayer coverage of the biosorbent surface is the main situation in the biosorption of RB221 dye by pepper seed biosorbent. In some studies of dye removal conducted in the literature, the same trend was observed using bivalve shell and treated Zea mays L. (maize) husk leaf (Jalil et al. 2012), sea shell powder (Chowdhury & Saha 2010), treated Capsicum annuum seeds (Akar et al. 2011), malt bagasse (Fontana et al. 2016). For the comparison of some biosorbents used for the removal of dye from wastewaters, some uptake capacities from various works are given in Table 4.

Capia pepper seeds, as a natural waste material, were used to remove RB221 from contaminated solutions. The effects of pH, initial dye concentration, biosorbent dosage and contact time have been evaluated to optimize the dye biosorption process. Also, kinetic analyses and isotherm studies were conducted to define the biosorption of RB221 dye onto Capia pepper seeds. Langmuir and Freundlich isotherm models were used to assess the experimental data. Experimental results were very well suited to the Langmuir isotherm model, with a correlation coefficient of 0.997. Maximum biosorption uptake capacity of the capia pepper seeds was found to be 142.86 mg g⁻¹ for RB221 dye removal. This uptake capacity seems very high compared with the other biosorbents used for dye removal in the literature. In the kinetic studies, pseudo-first order and pseudo-second order kinetic models were used, and it was found that the pseudo-second order kinetic model was appropriate for the experimental data with a correlation coefficient of 0.99. This work shows that a natural waste material, capia pepper seeds, has a large potential for removal of color originating from dyes used mostly in textile industries. As a result, it is possible to evaluate this waste material as a source for treatment. The optimum pH region for this biosorbent in dye removal studies was found to be 2. To decrease the amount of pH adjustment reagents, a pH value up to 3 can be used in future biosorption studies. After the treatment process, further studies may be carried out to determine the regeneration potential of the spent biomass. If regeneration is not possible, it should be disposed of as a hazardous material.