Introduction

Considerable amounts of synthetic dyes are used in many process industries. Textile, pharmaceutical, leather, food processing, cosmetics, and paint industries discharge into the environment about 10–15% of over 0.8 million ton of several types of dyes produced annually worldwide. It is projected that the dye business will grow annually by about 2–3% due to the increased global production and consumption of dyes. In developing countries like India and China, the production rate of dyes is increasing steadily every year. Without proper treatment, these dyes harm the environment, marine life and pose serious health threats to humans particularly upon degradation as they become toxic, recalcitrant, mutagenic, and carcinogenic1,2,3.

Methylene blue (MB) (3,7-bis(Dimethylamino)-phenothiazin-5-iumchloride) is a thiazine cationic dye commonly used for biological staining as well as coloring paper, hair, cottons and wools4. In addition, MB injection is used in the treatment of methemoglobinemia and urinary tract infections. However, accumulation of MB in wastewater has adverse health effects such as difficulties in breathing, vomiting, eye burns, diarrhea and nausea5.

Several physicochemical and biological processes have been successfully carried out in order to treat dye effluents6, examples of which are membrane filtration7, photocatalytic degradation8, irradiation9, biological treatment10, conventional adsorption11 and ultrasonic-assisted adsorption12. Adsorption is a commonly employed strategy due to its relative facile operation, availability of its adsorbents, selectivity and potential for scale-up. Among employed adsorbents, activated carbon is commonly used due to its efficiency and high surface area13. However, it has substantial limitations due to its high cost of regeneration and its impact on the environment. Performance of the adsorption process could be improved via ultrasonication. The ultrasonication process produces waves that enhance and accelerate mass transfer by generating shear forces in the solution via micro-streaming, micro-turbulence, acoustic waves, and micro jets without significantly changing the equilibrium of the adsorption/desorption system. This, in turn, could speed up the adsorption14,15.

For the sake of environmental sustainability and for economic reasons, many researchers investigated biosorption as a low-cost, safe and potentially effective alternative to traditional sorption; specifically for the removal of heavy metals and dyes from wastewater16,17,18. Several studies reported MB removal using untreated biosorbents or those treated thermally and/or activated using acids or bases. For studies that utilized untreated biosorbents, some of the highest maximum adsorption capacities reported (18–333 mg/g) pertained to cellulose- and lignin-based adsorbents such as chitin19, rice straw lignin20, wood cherry tree14, wheat shells21, as well as fruit peels of banana, orange and pineapple22 in addition to biomass such as Aspergillus fumigatus23 and Bacillus subtilis24, and others like fly ash geopolymer25, white pine sawdust18, and Abelmoschus esculentus seeds26. As for pre-treated adsorbents, some of the highest maximum adsorption capacities (20–672 mg/g) were reported for walnut wood activated carbon treated with nitric acid27, spent tea modified by NaOH28, sulfuric-acid activated cotton stalk29, base-activated bamboo charcoal30, HCl-activated oil palm fiber31, and coconut husk activated carbon modified by KOH at 816 °C32, in addition to pre-treated peels such as oven-dried Artocarpus camansi peels33, banana peels activated with NaOH34, jackfruit peel modified with microwave induced NaOH activation35, and pomelo skin activated by NaOH using microwave heating36.

Fava beans (Vicia faba) are highly consumed in different parts of the world like the Mediterranean area in Europe and Africa, Latin America, China and India, thus massive amounts of the peels are disposed of as waste. Only a portion of this waste is used by farmers as animal feedstock. In a previous study, fava bean powder was used for the removal of heavy metal ions of Pb(II), Cd(II) and Zn(II)37 to achieve removal efficiencies of 100%, 92.86% and 36.86%, respectively. Broad bean peels, on the other hand, were utilized to remove MB dye with a maximum adsorption capacity of 192.7 mg g−1 at 30 °C38.

The objective of the present study is to develop a viable and efficient biosorption process for the removal of MB dye from aqueous solutions using fava bean peels (FBP) as low-cost adsorbents. For that purpose, biosorption was conducted using ultrasonic-assisted (US) shaking as an alternative to conventional (CV) magnetic stirring. Sorption performance, in terms of capacity and rate, was evaluated and compared to that of CV biosorption under various conditions including adsorbent dosage, initial dye concentration, pH and contact time. A statistical multiple linear regression model was developed using the backward method on R program to determine the influencing operating parameters.

Materials and methods

Materials

MB was purchased from Central Drug House (CDH, India), with λmax of 663–667 nm. The dye has a pKa of 3.8, hence is neutral at pH ≤ 3.8 and positively-charged above pH 3.8. FBP were obtained from a local market in Cairo, and were ground in a Braun blender and sieved to a particle size range of 0.25 to 2 mm, then washed with tap water to remove solid impurities and dust. The peels were then washed with distilled water to remove any adsorbed particles, left to dry out in a petri dish, then sealed and stored for future use. The composition of Egyptian FBP is 46.5% carbohydrates, 25.2% proteins, 10.3% dietary fibers and 1.5% lipids38.

Characterization

The chemical structure of FBP was analyzed using the KBr pellet method onto a Thermo Scientific Nicolet 380 Fourier Transform Infrared (FTIR) spectrometer, which houses an EverGlo lamp that emits infrared radiation in the spectral range from 7800 to 350 cm−1. Surface morphology of FBP before and after sorption was also studied using LEO Field Emission Scanning Electron Microscope (SEM) employed at 1 nm resolution and a magnification of 1000×. Dynamic light scattering was conducted using Malvern Nano-ZS90 to measure the particle size distribution before and after ultrasonication for the maximum operation time of 70 min. Brunauer-Emmett-Teller (BET) measurements were performed on a BETASP 2020 analyzer, and the nitrogen adsorption isotherm values were determined using an ASAP 2020-Micromeretics apparatus at 77 K. The time required to attain sorption or desorption equilibrium was between 15 and 20 minutes.

Kinetic studies

Batch kinetic experiments were performed using CV and US shaking. In each mode of operation, initial concentrations of MB (3.6, 7, 13, 25, 50, 75 and 100 mg/L) were prepared by simply weighing the desired amount and dissolving it in 100 mL of distilled water. The runs were conducted in 250-mL glass conical flasks in which the desired mass of adsorbent (0.1, 0.25, 0.5, 0.75 and 1g) was added to 100 mL of the MB solution. The experiments took place at room temperature (27 ± 2 °C), and at different initial pH values of 5.8, 7.2 and 9.2, adjusted using 0.1 N NaOH. CV experiments were conducted by shaking the conical flasks on a Burrell Wrist Action Shaker Model 75 shaker set to 416 rpm, whereas the US experiments were undertaken using an ultrasonicator (model VWR® Symphony™ Ultrasonic Cleaners 150HT) which operates at a frequency of 35 KHz. Each conical flask was left to shake for a specific time duration after which it was centrifuged at 7500 rpm for 10 minutes. CV biosorption durations were 2, 5, 10, 15, 20, 25, 30 and 40 minutes, while those of US biosorption were 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 minutes due to the faster kinetics achieved under sonication. Adsorption capacities were calculated according to the following equation:

$$q=({C}_{o}\,-\,C)V/W$$
(1)

where q is the amount adsorbed at time t, C is the concentration of the dye in solution at time t, Co is the initial concentration of the dye, V is the volume of the solution and W is the dry weight of the biosorbent.

The percentage removal was calculated using:

$$ \% R=(({C}_{o}\,-\,C)/{C}_{o})\ast 100$$
(2)

Kinetics of adsorption was examined using the pseudo-second order kinetic model, expressed by the following linearized form17,39

$$\frac{t}{q}=\frac{1}{k{q}_{e}^{2}}+\frac{t}{{q}_{e}}$$
(3)

where k is the rate constant and qe is the amount adsorbed at equilibrium.

To study sorption equilibrium and to determine the maximum sorption capacity (qm), sorption experiments were conducted in a manner similar to that described for kinetic studies. However, sorption was carried out for 24 h at 27 ± 2 °C and pH of 5.8 using different initial concentrations. Sorption isotherms were constructed as qe versus Ce and were then fitted to Langmuir and Freundlich models whose equations can be expressed by the following linear forms (Eqs. 4 and 5, respectively)40

$$\log \,{q}_{e}=\,\log \,{K}_{f}+\frac{1}{n}log{C}_{e}$$
(4)
$$\frac{{C}_{e}}{{q}_{e}}=\frac{1}{{q}_{m}b}+\frac{{C}_{e}}{{q}_{m}}$$
(5)

Kf and n are parameters specific for each adsorbent, while qm is the maximum sorption capacity which can be calculated from the slope of Eq. 5, and b is Langmuir constant.

Statistical analysis

Multiple linear regression was used to analyze the experimental results employing the program R 3.5.1. The most significant factors affecting the amount adsorbed at equilibrium (qe) were selected via the backward method.

In general, the dependent variable (qe) is expressed as follows

$${q}_{e}={\beta }_{0}+{\beta }_{1}{X}_{1}+{\beta }_{2}{X}_{2}+\ldots +{\beta }_{n}{X}_{n}+{\epsilon }$$
(6)

where \({\beta }_{1},{\beta }_{2},\ldots ,{\beta }_{n}\) are the regression coefficients, \({X}_{1},{X}_{2},\ldots ,{X}_{n}\) are the factors affecting qe, \({\epsilon }\) are the random errors due to other factors not included in the study.

Results and discussion

Biosorption indicators

In this section, the effect of different operating parameters (time, initial concentration, adsorbent dose and pH) on two sorption indicators (equilibrium sorption capacity qe and % removal) is investigated. To do so, the sorption time profiles for the uptake of MB (q) onto FBP using CV and US shaking were first constructed under different operating conditions (Figs. 1 and S1–S3, supplementary material). To investigate the effect of adsorbent dose, the uptake profiles of 50 mg/L MB onto different doses of FBP at pH 5.8 were plotted in Fig. 1. In each of these profiles (Fig. 1), the effect of time on the adsorption capacity is manifested. Initially, the capacity increases rapidly with time then it flattens out as equilibrium is approached. Equilibrium time varies between 20 and 40 min depending on the adsorbent dose and mode of operation. Similar profiles were constructed to examine the effect of initial concentration at pH 5.8 using 5 g/L FBP at low (3.6–13 mg/L) and high (25–100 mg/L) MB concentration ranges (Figs. S1 and S2, respectively). Similarly, the effect of pH was studied for 50 mg/L MB onto 5 g/L FBP at different pH values (Fig. S3). For all profiles, q increased with time until it flattened out when qe was approached.

Figure 1
figure 1

Uptake profiles for the CV versus the US biosorption of MB onto FBP. Sorption conditions are 50 mg/L initial concentration and pH 5.8 at different adsorbent doses.

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By comparing the CV and the US uptake profiles at the same operating conditions, it can be observed that they have similar shapes as well as comparable qe values (p > 0.05). The US profiles however, have higher initial rates as observed in their steeper initial slopes allowing them to reach equilibrium at a shorter time.

To study the effect of operating parameters on the sorption, values of qe and % removal were obtained from each sorption profile and were plotted versus each varying parameter. The effect of initial dye concentration is depicted in Fig. 2, which shows that qe increases linearly with increasing the initial concentration under both CV and US conditions. This behavior has been previously reported for the sorption of various heavy metals and dyes onto biosorbents, and it can be ascribed to an increase in the driving force which overcomes the mass transfer resistance17,41. As for the % removal, it increases with concentration in a hyperbolic manner until it flattens out and remains almost constant at high concentrations. Similar behavior was reported for the biosorption of lead (5–100 mg/L) onto peach/apricot stones42; and was attributed to the saturation of sorption active sites. Comparing the qe versus C0 profiles for CV as opposed to US biosorption, it can be inferred that ultrasonication did not significantly affect the value of qe. A similar conclusion can be drawn for the % R versus C0 profiles. The linearized forms of the hyperbolic relations for the CV and US profiles are shown in the inset of Fig. 2, and the corresponding hyperbolic equation for both profiles can be expressed as follows:

$$ \% R=\frac{a\times {C}_{0}}{(b+{C}_{0})}$$
(7)

where a is the maximum % removal and b is the C0 corresponding to 50% removal. Values of a and b for CV biosorption are not significantly different from those of US biosorption.

Figure 2
figure 2

Maximum percentage removal efficiency and equilibrium uptake capacity of MB onto FBP as a function of initial concentration. Removal was achieved using CV and US biosorption at pH 5.8 with an adsorbent dose of 5 g/L. Inset shows the linear forms of the % R-C0 hyperbolic relations.

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Figure 3 illustrates the effect of adsorbent dose on biosorption indicators, where qe for both CV and US biosorption is shown to decrease logarithmically with increasing adsorbent dose. The logarithmic relation is evident from the figure inset. Similar behavior was observed for biosorption of Cr (III) onto orange peel wastes and for that of Pb (II), Cd (II) and Cu (II) onto olive pomace wastes. This could be owed to the reduction in surface area caused by the aggregation of particles and overlapping of sorption active sites at the high adsorbent dose17. Percent removal, on the other hand, increases with increasing adsorbent dose until it reaches a maximum at about 5 g/L dose, after which it declines. This behavior was previously reported for biosorption of Cu (II) onto antibiotic waste as well as sorption of Pb (II) onto solid waste produced from olive-oil industry17,43 and the decline in % removal was due to the saturation of active sites. Corresponding plots for pH are not shown since no statistical difference (p > 0.05) according to t-test was obtained in the values of the equilibrium uptake capacity or % removal with change in the pH of the solution.

Figure 3
figure 3

Maximum percentage removal efficiency and equilibrium uptake capacity of MB onto FBP as a function of adsorbent dose. Removal was achieved using CV and US biosorption at pH 5.8 at an initial concentration of 50 mg/L. Inset shows the linear forms of the qe- adsorbent dose relations.

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Kinetic parameters for biosorption

To quantitatively assess the effect of operating conditions on sorption rates, the kinetic profiles alluded to earlier were fitted to the pseudo-second order rate model and values of the relevant kinetic rate constants were determined and presented in Table 1. The predicted profiles were found to be in good agreement with their experimental counterparts as evident from the high correlation coefficient (R2) values compiled in Table 1.

Table 1 Pseudo second-order (k) kinetic rate constants for conventional and ultrasonic biosorption of MB onto FBP. Sorption conditions for section A: 5 g/L adsorbent dose, pH 5.8 and different initial concentrations, section B: 50 mg/L initial concentration, pH 5.8 and different adsorbent doses. Correlation factors are also included in the table.
Full size table

Section A of Table 1 presents the variation in k with initial concentration. This section of the table along with Fig. 4 (top panel) show that k varies inverse proportionally with C0 in a logarithmic manner. The same trend is observed for both CV and US biosorption. Reduction in k and hence in rate could be a result of the increased diffusional resistance across the boundary layer. Percentage increase in k as a result of ultrasonication is also shown in section A of Table 1 and Fig. 4 (bottom panel) which display that the percentage increase drops with increasing C0 in a linear manner. This indicates that the effect of ultrasonication in enhancing the rate is mitigated at the higher concentrations, where the mass transfer resistance is reduced by virtue of the high concentration gradient driving force regardless of ultrasonication and hence the influence of ultrasonication is not highly pronounced. The highest % increase in k recorded at the low concentration range (3.6–25 mg/L) corresponds to % removal of 70–80%. The advantage that ultrasonication provides at this low-concentration range is particularly important from the practical point of view since removing dyes in this concentration range is challenging, although various dye effluents exist in this range44.

Figure 4
figure 4

CV and US kinetic rate constants (top panel), and % increase in the rate constants with ultrasonication (bottom panel) as a function of initial concentration.

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The effect of adsorbent dose on the kinetic rate constants is shown in section B of Table 1, where it can be deduced that k increases logarithmically with increasing the dose (i.e. the plot of log k-log dose is linear). This could be due to the increase in the number of vacant sites encountered as the amount of adsorbent increases. The rate, being proportional to the number of vacant sites, consequently increases. The percentage increase in the value of k relative to that obtained through CV was determined at each adsorbent dose in order to investigate the effect of ultrasonication (Table 1, section B). At low adsorbent doses, values of k for CV and US biosorption are not statistically different. However, with higher doses, k is significantly enhanced with ultrasonication and this enhancement becomes more pronounced as the dose increases. One plausible explanation could be that localized high temperatures caused by ultrasonic waves produce cavities with more exposed functional groups (vacant active sites) in the large particles.

The data presented in Table 1, section C along with Fig. 5 (top panel) also show that for both CV and US biosorption, k increased with the first three pH values (2.8, 3.8 and 5.1) at which measurements were obtained. In both cases, k significantly decreased at about pH 6 then increased significantly to reach their respective maximum values at about pH 7.2 where further increases in pH up to a value of 9.2 did not show significant changes in k.

Figure 5
figure 5

Rate constants for sorption of 50 mg/L MB onto 5 g/L FBP at different pH values (top panel) and determination of PZC (bottom panel).

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The point of zero net charge (PZC) which corresponded to pH 6.2 was determined by plotting the final versus the initial pH values and determining the point at which they equate as shown in Fig. 5 (bottom panel). It is interesting to note that the pH value corresponding to the PZC is close to that at which the lowest observable k value was obtained in both biosorption modes. This is probably due to the minimal electrostatic attraction between the zero-charged fava peels and the positively-charged MB (PZC > pKa of dye). At pH values below PZC and above pKa of the dye, relatively higher values of k were obtained in both cases despite the electrostatic repulsion between the positively charged adsorbent and positively charged dye. This implies that the adsorbent-dye interaction is not purely electrostatic. Above PZC, k increases with pH due to the electrostatic attraction between the positively-charged dye and the negatively-charged adsorbent. This behavior again indicates that the interaction involves both electrostatic and non-electrostatic binding. The latter could be van der Waal’s, dipole interactions or chemical bonding. Furthermore, the highest %increase in k due to ultrasonication is observed near PZC. Despite the net zero charge on the adsorbent at PZC, ultrasonic waves could have induced partial charge on the surface of the produced cavities and thus enhanced sorption.

Biosorption under equilibrium conditions

The equilibrium isotherm for the CV sorption of MB onto FBP was fitted to each of the Langmuir and Freundlich models (Fig. S4), where it was shown to be better described using the Langmuir model as indicated by the higher value of the correlation factor (R2 = 0.9636) relative to that of Freundlich (R2 = 0.9097). It is to be noted here that the qm obtained under equilibrium conditions (140 mg/g) was not attained in the kinetic studies presented earlier. This could be due to the much longer operational time of the equilibrium study (24 h) that has allowed for further adsorption to take place through pore diffusion mechanism. Values of qm reported in previous literature for other treated and untreated fruit peels are presented in Table 2. By comparing these values to that obtained in this study, it can be inferred that FBP are superior to most of the other untreated peels. In all of these studies, the capacities of untreated peels ranged from 18 to 133 mg/g except for one study which reported a capacity of 333 mg/g for melon peels. However, FBP have lower performance relative to some of the treated ones whose capacities ranged from 19.7 to 409 mg/g and the activated carbons derived from fruit peels which ranged from 400–1193 mg/g. Nevertheless, working with untreated peels could provide a more cost effective and greener option than working with treated peels. This is yet to be investigated through looking into various technical and economic parameters like, for instance, performance of the treated versus the untreated peels, cost of materials, equipment and energy involved in the treatment process, recovery of the treatment solvents, in addition to the environmental impact of the treatment process.

Table 2 Summary of the recent studies performed on MB removal using fruit peels and their activated forms.
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Characterization of FBP pre- and post biosorption

The FTIR spectra of the FBP before and after CV and US sorption (Fig. S5) showed a peak at 3500 cm−1 that could be ascribed to the OH stretching vibration, in addition to a peak at 1700 cm−1 that could be attributed to the amide carbonyl stretch18,33,34. Comparing the IR spectra pre-adsorption to those post-adsorption under CV or US agitation, it can be concluded that both peaks were shifted to the left after adsorption. This indicates the formation of higher energy bonds which consequently implies binding as a result of adsorption. Possible electrostatic interaction between the positively charged nitrogen or sulfur on MB and the negatively charged lone pair on the carbonyl oxygen of FBP could have occurred, or alternatively a redox reaction. Additionally, hydrogen bonds may have been formed between the amine groups of the dye and hydroxyl groups of FBP. Furthermore, the position of the peaks pertaining to CV and US biosorption remained unchanged, indicating that ultrasonication did not decompose or chemically alter the structure of the peels and therefore ultrasonication could be used as an alternative to conventional agitation.

Surface morphology of FBP before and after biosorption is also depicted in Fig. S6 where it can be seen that the surface of the peels became rougher after adsorption of MB. In addition, no change in the morphology can be observed with ultrasonication of FBP. Furthermore, the hydrodynamic diameter of FBP before and after ultrasonication was measured to be 2520 ± 764 and 2406 ± 753 nm, respectively implying that ultrasonication had no significant effect on particle size. This data along with the FTIR measurements suggest that the structure of FBP was not damaged by the process performed.

The BET adsorption isotherm for FBP is depicted in Fig. 6. According to IUPAC, the isotherm is Type II which describes strong interaction pertaining to macroporous adsorbents. A wide pore size distribution ranging from 1.7 to 264 nm pore diameter was obtained as shown in Fig. 7, which indicates that some mesopores also exist along with macropores. In addition, the total surface area was found to be 0.2108 ± 0.0035 m²/g, while the total pore volume was 0.00067 cm3/g.

Figure 6
figure 6

BET type II isotherm for FBP describing strong interaction to macroporous adsorbents.

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Figure 7
figure 7

BET isotherm for pore size distribution with magnified inset for clarity.

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Predicting the sorption capacity via statistical modeling

In order to run the regression analysis, a correlation study was performed to examine if the linear regression analysis can be applied. The variables or factors included in the study were the initial concentration, C0, adsorbent dose, time, t, pH and the stirring method, X. X was defined as a binary variable which takes the value 0, if CV or the value 1 if US. The correlation analysis revealed that qe is significantly correlated with C0, t, and dose and hence regression analysis was carried out. All variables were included at the first stage and then the backward method was performed to select the best set of factors affecting qe. The pH factor was dropped because it was not significant in presence of the other factors in the equation, thus the final model has the following form

$$predicted\,{q}_{e}=20.41+0.16{C}_{0}+0.15t-13\,ln(dose)+2.5\,X$$
(8)

The model developed after dropping the pH is highly significant with a p-value of 1.074e-15. The adjusted R2 is 91% which means that the chosen factors explain 91% of the variation in qe. This strong relation could be clearly shown (Fig. S7) by comparing the predicted qe values to their corresponding experimental values.

Conclusion

CV and US biosorption were successfully utilized for the removal of MB by FBP under different conditions. A multiple linear regression statistical model revealed that initial concentration, adsorbent dose and time were influencing factors. The US process is recommended since it provides faster removal than CV while achieving the same maximum sorption capacity and maintaining the chemical structure of the adsorbent. Future work will consider optimizing the regeneration process for the exhausted FBP, and validating the cost effectiveness and potential of scale-up of the US process. This proof-of-concept study could also be extended to other contaminants of emerging concern.