Powdered grape seeds (PGS) as an alternative biosorbent to remove pharmaceutical dyes from aqueous solutions

Dyes are commonly employed in pharmaceutical formulations to enhance the aesthetic appearance and reduce errors in medication (Pérez-Ibarbia et al. 2016). During the pharmaceutical manufacturing, some of these dyes are lost and, consequently, discarded in effluents (Fernández et al. 2010). The incorrect disposal of dye-containing effluents is a well known problem, which can cause several damages to the environment and human health (Gupta & Suhas 2009). In this way, some countries created severe restrictions for the discharge of colored effluents (Hessel et al. 2007) and, consequently, many studies have been focused to search for treatments able to remove dyes from aqueous effluents (Crini & Badot 2008; Álvarez et al. 2013; Yagub et al. 2014; Dotto et al. 2015a; Khandare & Govindwar 2015).

Several treatments are used to remove dyes from industrial effluents, including chemical precipitation, ion flotation, ion exchange, membrane filtration, AOPs, adsorption, biosorption and electrochemical methods (Yagub et al. 2014; Ahmed et al. 2017). Among these, biosorption, which is defined as ‘the removal of contaminants from aqueous media by inactive or non-living biomass’, has gained attention due to advantages such as low cost, ease of operation, fast kinetics and use of diverse types of biomasses as biosorbents (Dotto et al. 2015a). In this sense, several biosorbents have been used to remove dyes from aqueous media, for example, papaya seeds (Weber et al. 2014), lotus seedpod (He et al. 2016), spent bleaching earth (Belhaine et al. 2016) and oricuri fiber (Meili et al. 2017). The use of grape wastes has been studied in recent years. Torab-Mostaedi et al. (2013) verified the potential of grape peels to remove cadmium and nickel from aqueous media. Al Bsoul et al. (2014) studied the potential of grape seeds as biosorbent for cooper ions. In spite of these efforts, the use of grape seeds as biosorbent for dye removal is scarce.

Currently, there is a great interest in the exploitation of the residues generated by the wine industry (Spiridon et al. 2016; Al-Hamamre et al. 2017). During wine production, it is estimated that approximately 25% of the grape weight results in by-product/waste (termed ‘pomace’ which is comprised of skins and seeds) (Dwyer et al. 2014). Grape seeds contain oil, lignin, cellulose and hemicellulose (Yedro et al. 2015). These compounds, in turn, contain several functional groups that can act as biosorption sites. Based on this information, we believe that grape seeds can be an available, low cost and efficient material, that can be used as biosorbent to remove dyes from aqueous media.

In this research, an alternative low-cost and efficient biosorbent named powdered grape seeds (PGS) was prepared from wastes of a wine industry, and used to treat synthetic solutions containing the pharmaceutical dyes brilliant blue (BB) and amaranth red (AR). The biosorbent was characterized according to the point of zero charge (pH_zpc), Boehm titration, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and energy X-ray dispersive spectroscopy (EDS). The effects of initial pH of the solution and biosorbent dosage on the biosorption were studied. The biosorption kinetic data were evaluated by the pseudo-first-order (PFO), pseudo-second-order (PSO) and Elovich models. Langmuir, Freundlich and Sips models were used to fit the biosorption equilibrium data. Thermodynamic parameters such as standard Gibbs free energy change (ΔG⁰), standard enthalpy change (ΔH⁰) and standard entropy change (ΔS⁰) were also estimated.

PGS biosorbent was prepared from wastes of a wine industry located in Mendoza, Argentina. Vitis vinifera L. grapes were collected from vineyards located in Mendoza province, Argentina (33°04′S, 68°19′W). The grapevine bunches (red cultivar of the Bonarda variety) were subjected to the wine processing. During the wine processing, a pomace containing husks, stems and seeds was generated. This pomace was refrigerated and carried to the laboratory. The grape seeds were manually separated from the pomace, washed with drinking water and then rinsed with Milli-Q water. The seeds were lyophilized for 48 h (Virtis freeze mobile, model 6, USA), pulverized with a mill (Ultracomb, MO-8100A, Argentina) and sieved until the discrete particle size ranging from 80 to 110 μm. The resulting material was named PGS biosorbent.

The PGS biosorbent was characterized according to several important aspects regarding biosorption. The point of zero charge (pH_zpc) was determined using the 11 points experiment (Park & Regalbuto 1995) in order to assess the surface charge of the biosorbent as a function of the pH. The total acidity and basicity of the biosorbent was verified by the Boehm titration method (Goertzen et al. 2010). FT-IR (Shimadzu, Prestige 21, Japan) was used to identify the main functional groups of the biosorbent (Silverstein et al. 2007). The textural characteristics of the biosorbent and the main elements on the surface were visualized by SEM coupled to EDS (Jeol, JSM-6610LV, Japan) (Goldstein et al. 1992). FT-IR and SEM were also performed after the biosorption process (optimum condition) in order to verify possible modifications on the biosorbent surface.

The biosorption experiments were realized in batch mode at 200 rpm using a thermostated agitator (Marconi, MA 093, Brazil) in order to verify the effects of initial pH, PGS dosage and also to obtain the kinetic and isotherm curves. Four experimental steps were performed:

• (I) The effect of initial pH was studied (from 1.0 to 8.0) (adjusted with HNO₃ and NaOH) under the following conditions: initial dye concentration of 50 mg L⁻¹, contact time of 1 h, volume of solution of 25 mL, PGS dosage of 2.00 g L⁻¹ and temperature of 25 °C (PGS was put directly in contact with the solutions).

• (II) The effect of PGS dosage (from 0.25 to 5.00 g L⁻¹) was investigated under the same conditions, using the optimum pH defined elsewhere.

• (III) Kinetic experiments were performed using the optimum pH and PGS dosage defined above. The initial dye concentration was 50 mg L⁻¹. The experiments were performed at 25 °C with contact time varying from 0 to 120 min and the volume of solution was 25 mL.

• (IV) Isotherm curves were constructed at 25, 35, 45 and 55 °C, using the optimum pH and PGS dosage defined above. The initial dye concentration ranged from 25 to 300 mg L⁻¹ and the volume of solution was 25 mL, with the aliquots stirred until the equilibrium (maximum 6 h).

For R_L = 1, the isotherm is linear, 0 < R_L < 1 indicates a favorable process and, R_L = 0 indicates an irreversible process (Hamdaoui & Naffrechoux 2007).

PGS biosorbent was characterized according to the point of zero charge (pH_zpc), total acidity and basicity, FT-IR, SEM and EDS. The pH_zpc of the biosorbent was 6.85 (see supplementary material, available with the online version of this paper). This shows that at pH values lower than 6.85 the biosorbent is positively charged, while at pH values higher than 6.85, PGS is negatively charged. The values of carboxylic, lactonic and phenolic groups were, respectively, 0.05, 0.28 and 2.98 meq g⁻¹. Consequently, the total acidity on the PGS surface was 3.31 meq g⁻¹. The total basicity was 0.01 meq g⁻¹.

The PGS spectrum (before biosorption) in Figure 2 shows the main intense bands around 3,400, 2,900, 2,800, 1,750, 1,650, 1,510, 1,400 and from 1,300 to 1,000 cm⁻¹. The broad band centered at 3,400 cm⁻¹ is the -OH stretching. The stretchings of C-H and CH₂ can be visualized at 2,900 and 2,800 cm⁻¹, respectively. The band at 1,750 cm⁻¹ could be assigned to the carbonyl groups (C = O). The C = O link of the acetyl groups can be seen at 1,650 cm⁻¹. The C = C link of aromatic ring is visualized at 1,510 cm⁻¹. The CH deformation can be seen at 1,400 cm⁻¹. The vibrational bands in the region 1,300–1,000 cm⁻¹ can be assigned to -CO, C-O-C and carboxylic acids. These bands reveal that the PGS biosorbent is composed by several functional groups that are able to bind with the pharmaceutical dyes AR and BB. In the spectra after biosorption (PGS loaded BB and PGS loaded AR) (Figure 2), no significant changes were observed. This shows that no links were formed or broken during the biosorption process, indicating that a physical biosorption occurred.

It can be seen clearly in Figure 5 that the dye removal percentage increased with the pH decrease from 8.0 to 1.0. For both dyes, no biosorption occurred from pH 8.0 to 6.0, while the dye removal percentage was higher than 80% at pH 1.0. This shows that the biosorption of BB and AR dyes by PGS is strongly pH dependent. This behavior can be explained on the basis in the characteristics of the PGS and dye molecules. BB and AR are anionic dyes (Figure 1) and its sulphonated groups are negatively charged independent of the pH (since the pK_a of these groups are negative). In parallel, the PGS biosorbent is positively charged at pH values lower than 6.85 (pH_zpc of the biosorbent is 6.85). In this way, at pH of 1.0, the negatively charged dye molecules are attracted by the positively charged surface of the PGS biosorbent, leading to high values of removal percentage. Similar trends were found using other materials to remove BB, like chitosan (Dotto & Pinto 2011), flower wastes (Echavarria-Alvarez & Hormaza-Anaguano 2014) and Hydrilla verticillata (Rajeshkannan et al. 2011). Also, for the removal of AR using chitosan films (Cadaval et al. 2015), water hyacinth leaves (Guerrero-Coronilla et al. 2015) and tamarind pod shells (Ahalya et al. 2012) were used. Based on these results, the subsequent biosorption tests were performed at pH of 1.0.

For BB dye (Figure 6(a)), the increase in biosorbent dosage from 0.25 g L⁻¹ to 1.00 g L⁻¹ caused an increase from 56 to 87% in the dye removal percentage (R). However, the biosorption capacity (q) decreased from 120 to 40 mg g⁻¹. A new increase from 1.00 g L⁻¹ to 5.00 g L⁻¹ caused no significant effect on the dye removal percentage but, the biosorption capacity continued to decrease. In the case of AR dye (Figure 6(b)), the increase in biosorbent dosage from 0.25 g L⁻¹ to 1.50 g L⁻¹ caused an increase from 32 to 85% in the dye removal percentage (R). However, the biosorption capacity (q) decreased from 58 to 18 mg g⁻¹. A new increase from 1.50 g L⁻¹ to 5.00 g L⁻¹ caused no significant effect on the dye removal percentage but the biosorption capacity continued to decrease. This behavior is common, since the increase in biosorbent dosage provides more biosorption sites and, consequently, more dye is removed from the solution. On the other hand, the biosorption capacity decreases, since these additional sites can block one each other. Aiming to obtain suitable values of R and q for both dyes, 0.50 g L⁻¹ was selected as the optimum biosorbent dosage to be used in further studies.

A typical kinetic behavior was observed for both dyes, where the biosorption capacity presented a strong increase in the first 5 min, followed by a gradual increase until 90 min. After, the biosorption rate decreased strongly, and the equilibrium was attained within 6 h. Also, Figure 6 shows that the biosorption capacity was higher for BB than for AR dye. This can occur because the BB molecule is higher than the AR molecule (see Figure 1), increasing the probability of this molecule being docked in an active site. This behavior was proved in other work, using physical statistics approaches (Dotto et al. 2015b).

PFO, PSO and Elovich models were fitted with the experimental data, in order to find an adequate and mathematically easy model to represent the experimental kinetic data. The results are presented in Table 1. The high R² and low ARE values presented in Table 1 show that the PSO and Elovich models were adequate to explain the experimental kinetic data. The q₂ value for BB was higher than the q₂ value for AR, confirming its higher biosorption capacity. This behavior is corroborated by the b parameter of the Elovich model, which was lower for BB. The k₂ values were similar for both dyes, indicating that the biosorption rate was also similar, during the entire biosorption period. However, the h₀ value was higher for BB, indicating that, at the initial stages, the BB biosorption was faster. Finally, Table 2 shows that for both dyes, q₂ closely very well with the experimental value q_e (exp) confirming that the PSO model can be used to predict the experimental values of biosorption capacity.

Table 2

Isotherm parameters for BB biosorption onto PGS

Models .	Temperature (K) .
	298 .	308 .	318 .	328 .
Langmuir
qm (mg g−1)	324.4	381.6	537.3	599.5
KL (L mg−1)	0.020	0.021	0.022	0.038
RL (C0= 300 mg L−1)	0.131	0.142	0.136	0.080
R2	0.9790	0.9979	0.9939	0.9985
	0.9736	0.9967	0.9918	0.9888
ARE (%)	5.91	4.18	5.76	5.54
Freundlich
KF ((mg g−1)(mg L−1)−1/nF)	28.1	26.2	30.1	50.9
1/nF	2.26	2.03	1.83	1.94
R2	0.9424	0.9717	0.9757	0.9868
	0.9303	0.9656	0.9708	0.9834
ARE (%)	16.15	16.15	16.69	13.77
Sips
qs (mg g−1)	311.8	353.1	515.11	830.1
Ks (L mg−1)	0.025	0.024	0.023	0.017
M	1.066	1.111	1.044	0.776
R2	0.9790	0.9970	0.9930	0.9925
	0.9676	0.9950	0.9890	0.9885
ARE (%)	7.42	5.11	5.40	6.96

Models .	Temperature (K) .
	298 .	308 .	318 .	328 .
Langmuir
qm (mg g−1)	324.4	381.6	537.3	599.5
KL (L mg−1)	0.020	0.021	0.022	0.038
RL (C0= 300 mg L−1)	0.131	0.142	0.136	0.080
R2	0.9790	0.9979	0.9939	0.9985
	0.9736	0.9967	0.9918	0.9888
ARE (%)	5.91	4.18	5.76	5.54
Freundlich
KF ((mg g−1)(mg L−1)−1/nF)	28.1	26.2	30.1	50.9
1/nF	2.26	2.03	1.83	1.94
R2	0.9424	0.9717	0.9757	0.9868
	0.9303	0.9656	0.9708	0.9834
ARE (%)	16.15	16.15	16.69	13.77
Sips
qs (mg g−1)	311.8	353.1	515.11	830.1
Ks (L mg−1)	0.025	0.024	0.023	0.017
M	1.066	1.111	1.044	0.776
R2	0.9790	0.9970	0.9930	0.9925
	0.9676	0.9950	0.9890	0.9885
ARE (%)	7.42	5.11	5.40	6.96

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In order to find an adequate representation for the equilibrium experimental data, the models named Langmuir, Freundlich and Sips were used. The fitting results are presented in Table 2 (for BB) and Table 3 (for AR). The higher values of R² and and the lower values of ARE (Table 2) indicate that the Langmuir model was the best to represent the biosorption equilibrium for the BB dye. However, for the AR dye, the Sips model was the more adequate.

Table 3

Isotherm parameters for AR biosorption onto PGS

Models .	Temperature (K) .
	298 .	308 .	318 .	328 .
Langmuir
qm (mg g−1)	59.8	65.6	85.6	94.4
KL (L mg−1)	0.041	0.040	0.031	0.045
RL (C0= 300 mg L−1)	0.075	0.076	0.097	0.068
R2	0.8687	0.9218	0.9681	0.9981
	0.8415	0.9053	0.9613	0.9972
ARE (%)	22.77	16.04	9.29	1.16
Freundlich
KF ((mg g−1)(mg L−1)−1/nF)	14.3	14.7	13.4	20.2
1/nF	4.03	3.84	3.07	3.62
R2	0.8100	0.8611	0.9493	0.9686
	0.7720	0.8332	0.9384	0.9613
ARE (%)	28.59	22.44	12.89	8.32
Sips
qs (mg g−1)	50.9	55.4	83.9	94.2
Ks (L mg−1)	0.045	0.047	0.033	0.048
m	7.994	4.686	1.047	1.006
R2	0.9999	0.9957	0.9682	0.9989
	0.9988	0.9922	0.9522	0.9975
ARE (%)	0.69	1.68	9.24	1.12

Models .	Temperature (K) .
	298 .	308 .	318 .	328 .
Langmuir
qm (mg g−1)	59.8	65.6	85.6	94.4
KL (L mg−1)	0.041	0.040	0.031	0.045
RL (C0= 300 mg L−1)	0.075	0.076	0.097	0.068
R2	0.8687	0.9218	0.9681	0.9981
	0.8415	0.9053	0.9613	0.9972
ARE (%)	22.77	16.04	9.29	1.16
Freundlich
KF ((mg g−1)(mg L−1)−1/nF)	14.3	14.7	13.4	20.2
1/nF	4.03	3.84	3.07	3.62
R2	0.8100	0.8611	0.9493	0.9686
	0.7720	0.8332	0.9384	0.9613
ARE (%)	28.59	22.44	12.89	8.32
Sips
qs (mg g−1)	50.9	55.4	83.9	94.2
Ks (L mg−1)	0.045	0.047	0.033	0.048
m	7.994	4.686	1.047	1.006
R2	0.9999	0.9957	0.9682	0.9989
	0.9988	0.9922	0.9522	0.9975
ARE (%)	0.69	1.68	9.24	1.12

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The R_L values of the Langmuir model (Table 2) ranged between 0 and 1, indicating that the BB biosorption was a favorable process. The K_L (Table 2) increased with the temperature, suggesting that the biosorbent–BB affinity was higher at 328 K. The same trend was found for K_s (Table 3). The q_m (BB dye in Table 2) and q_s(AR dye in Table 3) parameters increased with the temperature, corroborating that the biosorption capacity is favored at 328 K.

The maximum biosorption capacities were found at 328 K (55 °C) and were 599.5 and 94.2 mg g⁻¹ for BB and AR, respectively. A comparison between the maximum biosorption capacities (q_max) of several materials used to remove BB and AR dyes from aqueous solutions is presented in Table 4. Based on this table, it can be stated that PGS is an excellent biosorbent to remove BB, since presented a very high biosorption capacity. Also, PGS is a suitable biosorbent for AR, since its biosorption capacity was comparable with the other materials. Furthermore, PGS has low cost, since is obtained from wastes using a simple processing.

The biosorption thermodynamic of BB and AR was evaluated according to the standard values of Gibbs free energy change (ΔG⁰, kJ mol⁻¹), enthalpy change (ΔH⁰, kJ mol⁻¹) and entropy change (ΔS⁰, kJ mol⁻¹ K⁻¹). The results are presented in Table 5.

For both dyes, negative ΔG⁰ values were obtained, demonstrating that the BB and AR biosorption onto PGS was a spontaneous and favorable process. The temperature increase provided more negative ΔG⁰ values, corroborating that the biosorption was favored at 328 K. The positive ΔH⁰ values indicated that the biosorption was endothermic in nature. The endothermic nature was also observed in the malachite green adsorption by Moroccan clay (Elmoubarki et al. 2015) (ΔH⁰ = 14.7 kJ mol⁻¹) and reactive blue 19 adsorption on coconut shell activated carbon (Isah et al. 2015) (ΔH⁰ = 7.771 kJ mol⁻¹). For both dyes, the magnitude of ΔH⁰ values suggests that physical electrostatic interactions occurred. The positive ΔS⁰ values indicated some rearrangements on the biosorbent surface during the biosorption.

Based on the FT-IR, pH_zpc, Boehm titration, pH studies, thermodynamic parameters and dye properties, a possible main biosorption mechanism was proposed: it is known that at pH lower than 6.85, the PGS surface is positively charged. At pH 1.0, the majority of the PGS groups are protonated (basic groups, carboxylic and hydroxyl groups). In parallel, the sulphonated groups of the dye molecules are negatively charged (negative pK_a). Then a physical electrostatic interaction occurs between the dyes and the PGS biosorbent. This mechanism is corroborated by the FT-IR spectra, which were not modified after the biosorption and also by the magnitude of ΔH⁰ values.

In this work, PGS biosorbent was developed using wastes of a wine industry, and then, applied as an alternative, low cost and efficient material to remove BB and AR dyes from aqueous solutions. The material characterization revealed that PGS has potential features for biosorption, like functional groups on the surface, cavities and protuberances. The biosorption of BB and AR was favored at pH of 1.0 and biosorbent dosage of 0.50 g L⁻¹, where the dye removal percentage was higher than 80%. PSO and Elovich models were adequate to represent the biosorption kinetic. The biosorption equilibrium of BB on PGS was well represented by the Langmuir model, while for AR, the Sips model was the most adequate. The maximum biosorption capacities were 599.5 and 94.2 mg g⁻¹ for BB and AR, respectively, attained at 328 K. The biosorption was a spontaneous, favorable and endothermic process. The biosorption occurred by physical electrostatic interaction between the dyes and the PGS biosorbent. These findings show that PGS is a potential candidate for biosorption purposes, since has low cost, availability, high biosorption capacity and high efficiency.