Biosorption of acid yellow-99 using mango (Mangifera indica) leaf powder, an economic agricultural waste
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The competency of mango leaf powder (MLP), an eco-friendly and cost-effective adsorbent, has been extensively studied for the removal of a carcinogenic azo dye, acid yellow-99 (AY-99), from simulated wastewater on principal interest in combating water pollution and to comprehend the mechanisms of the phenomenon. Optimum dye adsorption occurs at pH 2.5, whereas the temperature has no significant effect. MLP can effectively adsorb 708.15 mg g−1 of dye as a function of the initial dye concentration with excellent fitting to the Langmuir isotherm model illustrating monolayer adsorption. The process is rather swift to be completed within ~ 160 min following film diffusion model and pseudo-second-order rate kinetics. The magnitude of hindrance exerted by NaCl in dye adsorption is found to be quite insignificant. The biomass is competent enough for anionic dye decolourization in the binary mixture (AY-99 + AR-88) determined by the first-order derivative spectrophotometric method. Thermodynamically the process is spontaneous, endothermic and entropy-driven. Zeta potential, XRD, SEM-EDXA and X-ray elemental mapping analyses have been used to assess the morphological changes and the mechanisms of dye interaction with MLP. FTIR spectroscopy and chemical modification of functional groups of biomass establish the major contribution of hydroxyl groups for effective dye decolourization through complexation and electrostatic interactions.
KeywordsDye adsorption Mango leaf Adsorption mechanism Simulated wastewater
Environmental pollution caused by anthropogenic activities and growing industrialization has attracted much attention to modern science worldwide. On increasing demand, various industries like textile, paper, printing, food, leather, cosmetic, pesticide, paint, etc. formulate their products with colour by dyes or pigments [41, 43]. Discharging their effluents into receiving streams not only affects environmental sustainability but also can impart their acute toxicity to aquatic organisms as well as human health. Most of these dyes are noxious and/or carcinogenic in nature [20, 26]. Therefore, an efficient treatment for controlling water pollution has become an imperative task . However, the fact is that these dyes are also very difficult to treat since they are the synthetic origin of complex molecular structure, highly resistant to aerobic digestion and stable when exposed to light, heat and oxidizing agents [26, 28]. Numerous efficient physical and chemical methods such as chemical coagulation, reverse osmosis, filtration and advanced oxidation processes are often encountered by several limitations like high operating cost, non-eco-friendly nature and generation of toxic secondary by-products [4, 14, 45]. In this regard, biosorption has become an eco-friendly viable substitute and has been found to be superior to conventional processes in recent years [34, 39]. This technology is primarily based on biomaterials to decontaminate colourants from an aqueous medium [52, 53]. Though various agricultural residues or its by-products [24, 38, 58] used for the removal of dye from its aqueous phase have been examined so far, increasing attention has been going on to explore cheap, renewable, locally available and effective alternative materials.
Mango (Mangifera indica) leaf is an abundant economical agricultural waste that is found in all seasons. This leaf comprises a large number of glucosides such as mangiferin, isomangiferin, neomangiferin and homomangiferin . These biopolymers possess a variety of functional groups like hydroxyl, carboxyl, amino, phosphate, etc. that can be the binding sites for dye molecules. To our best knowledge, there is to date no investigation available in the literature relating to the detailed characterization of MLP. Additionally, authenticated sorption phenomenon on the basis of mechanistic explication using of MLP in its natural form for the removal of AY-99 dye is unrevealed. Furthermore, the simulated effluent having ingredients like real effluent should be taken into account during the investigation of dye removal. Aside from highest concentration of dyes, traces of salts, surfactants, alarming quantity of a carbon source and caustic waste are the ubiquitous constituents of textile effluent [27, 42], which may affect on removing the target dye.
The present investigation reports the using of the raw mango leaf powder (MLP) for safe decolourization of a textile dye, AY-99, from both its aqueous solution and simulated textile wastewater for the first time. Since the dyeing wastewaters typically contain more than one type of dyes and sodium chloride, the adsorption process has also been studied in the presence of these substances. In addition, the present study aims at detailed characterization of MLP and executes comprehensive study of the dye adsorption with support from sophisticated instrumental analyses such as UV–visible spectrophotometer, surface area analyser, zeta potential analyser, scanning electron microscope equipped with energy-dispersive X-ray analyser, X-ray diffraction, Fourier transform infrared spectrophotometer and thermal gravimetric analysis and evaluates the usefulness of this adsorbent.
2 Materials and methods
Acid yellow-99 (C.I. 13900) and acid red-88 (C.I. 15620) used in this study were obtained from Sigma-Aldrich, USA. All other chemicals were of analytical grade and procured from E-Merck, Germany.
2.2 Preparation of the stock dye solution
The stock solution of dye, AY-99 (1000 mg L−1) was prepared by dissolving the weighted amount of dye into double-distilled water in a volumetric flask. The working solutions were obtained by diluting the stock solution with desired double-distilled water.
2.3 Biomass preparation and constituent analysis
Mango (Mangifera indica) leaves were collected from the local area. The leaves were thoroughly washed with running tap water to remove dust or any unwanted adhering particles and then with double-distilled water to remove the remaining dirt materials and dried in an oven at 80 ± 5 °C for 8 h. The dried leaves were then crushed, grounded by the mechanical grinder (Bravo, Bajaj) and sieved in various particle sizes (177–841 µm) using different meshes. The powdered biomass was stored in desiccators at room temperature until further use.
2.3.1 Moisture content
2.3.2 Ash content
2.3.3 Extractives analysis
2.3.4 Hemicellulose analysis
2.3.5 Lignin analysis
2.3.6 Cellulose analysis
2.4 Batch biosorption experiments
The effect of pH on the dye adsorption experiments was estimated over a range of solution pH from 1.0 to 7.0. The MLP biomass was conditioned in a buffer of the desired pH for 2 h under shaking (120 rpm), collected by filtration and washed with double-distilled water for used in this study. To study the effect of temperature on the adsorption process, experiments were performed by varying the temperature (20 °C–40 °C) at the optimum pH. Equilibrium isotherm studies were conducted with a range of different initial dye concentrations (50–4500 mg L−1) at the optimum temperature and pH. The adsorption rate of dye adsorption by MLP was observed at different time intervals ranging from 0 to 24 h, keeping other experimental conditions same. The effect of MLP biomass dosage (0.1–0.5 g) and particle size (177–841 µm) on AY-99 dye sorption was also investigated. The influence of NaCl salt concentration (0.05 to 0.5 M) on the sorption process was investigated. In addition, the adsorption of AY-99 onto MLP in the presence of other dye (AR-88) in the medium (100 mg L−1, pH 2.5) was evaluated. Each of the data was obtained from the individual flask, and therefore, no correction was necessary for withdrawal of sampling volume.
2.5 Instrumental characterization of biosorbent
2.5.1 Surface area analyses
The surface area of MLP biomass was determined by the Brunauer–Emmett–Teller (BET) using nitrogen adsorption and desorption isotherms using a Quantachrome Instruments (NOVA 1200e, USA). The pore size distribution was obtained by the Barrett–Joyner–Halenda (BJH) method.
2.5.2 SEM-EDXA and elemental analyses
The change in surface morphologies of MLP biomass before and after AY-99 dye adsorption and the X-ray elemental mapping was observed with a scanning electron microscope (SEM, JEOL JSM-6360, Japan) equipped with energy-dispersive X-ray (EDX). The samples were coated with platinum by a vacuum electric sputter coater to the finest thickness.
2.5.3 X-ray diffraction (XRD) study
In order to determine the biomass phase types and possible dye removal mechanisms, both pristine and dye-treated MLPs were further analysed using X-ray diffractometer (RIGAKU, model: ULTIMA-III, Japan) equipped with Cu Kα (λ = 0.15406 nm) radiation. The data were recorded with a scanning speed of 5° min−1 operated at the voltage of 40 kV.
2.5.4 Fourier transform infrared (FTIR) spectroscopy analysis
FTIR spectra of pristine and dye-absorbed MLP biomass were recorded in transmittance mode in the region of 4000–400 cm−1 (over 500 scans) with a resolution of 4 cm−1 by a Shimadzu FTIR spectrophotometer (IR Prestige-21, Japan) equipped with high-sensitivity pyroelectric detector (DLATGS) to evaluate the occurrence of surface functional groups of biomass and their possible involvement in dye adsorption. Pressed pellets were prepared by grinding the samples with KBr (spectroscopic grade) where a sample/KBr ratio ~ 1/100.
2.5.5 Thermal analysis
Thermal gravimetric analysis (TGA) of dye-laden and pristine MLP biomass was examined to resolve thermal degradation of different constituents using a TG/DTA instrument (PerkinElmer, Singapore) from 25 to 500 °C in airflow at a heating rate of 10 °C min−1.
2.6 Functional groups modification of MLP
The functional groups of MLP were modified chemically to assess their quantitative involvement in dye decolourization as described below. The modified biomass was thoroughly washed with deionized water, dried and used for biosorption studies.
2.6.1 Methylation of amine group
1 g of MLP biomass was refluxed with 15 mL of 10% formaldehyde and 30 mL of 50% formic acid for 4 h .
2.6.2 Esterification of carboxyl group
1 g of biomass was stirred for 6 h at room temperature with 70 mL of anhydrous CH3OH and 0.6 mL of concentrated HCl .
2.6.3 Acetylation of hydroxyl group
0.55 g of biomass was refluxed with acetic anhydride at 80o C for 10 h .
2.7 Desorption study
After sorption experiment, dye-loaded biomass was rinsed with double-distilled water and dipped into 50 mL of eluant medium (0.1 M NaOH or NaCl) in 250-mL Erlenmeyer flasks separately. The flasks were kept under shaking (120 rpm) for 2 h at 30 °C and centrifuged, and the concentration of the dye in the supernatant was determined by measuring the absorbance at 450 nm. The dye-desorbed MLP was then reused for repetitive adsorption–desorption cycles.
2.8 Simulated textile wastewater treatment
The simulated wastewater (feed solution) was prepared by dissolving dye with other auxiliary contaminants in tap water at room temperature as described by . The ingredients of feed solution were: 50 mg L−1 potassium di-hydrogen phosphate, 58 mg L−1 calcium chloride, 500 mg L−1 ferrous sulphate, 20 mg L−1 nickel sulphate, 220 mg L−1 magnesium sulphate, 1300 mg L−1 ammonium chloride, 17 mg L−1 ferric chloride, 4 mg L−1 zinc chloride, 7 mg L−1 manganese chloride, 4 mg L−1 cobalt chloride, 5000 mg L−1 sodium bicarbonate, 1 mg L−1 sodium metaborate, 300 mg L−1 EDTA, 1000 mg L−1 glucose and 50–1000 mg L−1 AY-99. The final pH of the feed solution was adjusted to pH 2.5.
2.9 Statistical analysis
All results reported here are the means of three replicate experiments and statistically different at P < 0.05. The mean standard deviation was determined from experimental results and represented as error bars in the figures. To check the suitability of the fit, correlation coefficient (R2) and Chi-square (χ2) test were used using Origin 8.0 software.
3 Result and discussion
3.1 Characterization of biosorbent
3.1.1 Analysis of MLP
The quantitative analysis of MLP biomass constituents as represented in Table S1 (Supplementary data) revealing hemicellulose is the primary constituent of MLP, while secondary and tertiary components are found to be cellulose and lignin, respectively. Alongside, the specific surface area (16.72 m2 g−1) and total pore volume (6.84 × 10−3 cm3 g−1) of the MLP biomass have been estimated by the standard Brunauer–Emmett–Teller (BET) method. The biomass is mostly mesoporous with an average pore diameter of ~ 42 Å determined from the BJH method, and the pore size distribution is depicted in Fig. S1 (Supplementary data).
3.1.2 Thermal gravimetric analysis (TGA)
Thermogravimetry is considered as a superior analytical tool in adsorption process since this methodology can monitor the thermal degradation of different components of MLP and the physicochemical changes in biomass during the heating process. The weight loss profile of the pristine and dye-loaded biomass during the heating process is depicted in Fig. S2 (Supplementary data) where pristine biomass has two distinct weight loss regions. The first one (weight loss around 10.06%) to the temperature rise of 190 °C may be exemplified for the evaporation of the physically absorbed water and volatile matters of the biomass. In the second region of 190 to 330 °C, decaying of complex organic matters like primary constituents such as hemicelluloses occurred with a maximum decline in the sample mass and residual weight of the biomass after this phase was ~ 57%. In the third phase, secondary constituents such as cellulose in the biomass was degraded at a temperature range of 330–480 °C after which merely 33% of sample residue left. Afterwards, lignin degradation continued beyond 480 °C [8, 13], supporting the findings for mentioned component analysis of MLP (Table S1, Supplementary data). The TGA curve of the dye-laden MLP biomass differs from that of the pristine biomass. The selected weight loss is altered with a slight increase in thermal stability. This difference of weight loss in the thermogram may be related to higher stability of material formed through the incorporation of dye onto biomass [3, 48].
3.1.3 SEM-EDXA and elemental analyses
3.1.4 X-ray diffraction study
3.1.5 FTIR analyses
3.2 Chemical modification of Functional groups of MLP
A decrease in adsorption by 72–78% due to the modification of hydroxyl groups is noted. Thus, the hydroxyl groups abundant in hemicellulose and cellulose structure play a crucial role in dye adsorption.
Stirring the biomass with methanol and HCl causes the esterification of carboxyl group where a strong alkylating agent replaces hydrogen atom from the carboxyl group. The esterification does not significantly alter the dye adsorption capacity. On the other hand, formaldehyde and formic acid methylate the secondary amines present on the biomass surface. Since no such change in adsorption of dye is observed due to the blocking of amine groups of MLP biomass, it is likely that this group is not involved in the adsorption process. The results of the functional group modifications are in good agreement with the FTIR findings.
3.3 UV–visible spectroscopic study for dye adsorption
3.4 Effect of influencing process variables on AY-99 removal by MLP
3.4.1 Effect of pH, temperature and thermodynamic properties
Temperature is another important parameter affecting the biosorption process, as dyeing process is carried out at a relatively higher temperature. Thus, the dye sorption experiments were carried out at three different temperatures and the results are presented in Fig. S3(a) (Supplementary data). The adsorption of AY-99 increases from 83.24% (20.81 mg g−1) to 91.48% (22.87 mg g−1) with rising the temperature from 20 to 30 °C. Further increment of temperature at about 40 °C offers on significant effect in dye removal (93.80%) . This phenomenon can be clarified on the basis of kinetic energy. Rising temperature leads to an increase in the mobility of the dye molecules for interaction with MLP, thereby enhancing the dye adsorption .
The calculated thermodynamic parameters are summarized in Table S2 (supplementary data). The negative values of ΔG° designate that the dye decolourization process is spontaneous and favourable at all the studied temperatures. The process is endothermic as the obtained ΔH° value becomes positive, indicating the adsorption of heat energy during dye–biomass interaction. Alongside, the dye attraction mostly follows the chemisorption phenomenon by considering the magnitude of ΔH° (> 40 kJ mol−1) . The positive magnitude of ΔS° corresponds to the increase in randomness at the solid–liquid interface during the sorption of AY-99 onto MPL biomass [26, 44].
3.4.2 Effect of biomass particle size and biomass dosage
The particle size of biomass plays an important role in dye removal process since it determines the rate as well as effective sorption yield of biomass. Figure S4a (Supplementary data) represents that the smaller particles adsorb more dyes over larger particles. The sorption capacity of MLP for AY-99 decreases to 16.72 mg g−1 from 23.22 mg g−1 for 841 µm to 177 µm particle sizes, respectively. This phenomenon may be exemplified by the fact that the small particle having their higher surface areas and surface-active sites enhances dye removal from an aqueous solution. In addition, for smaller particles, the diffusion path becomes shorter and dye molecules can easily penetrate into the mesopores and micropores of the biomass [11, 26].
Sorption studies were also conducted by varying the biomass dosage from 0.1 to 0.5 g and presented in Fig. S4(b) (Supplementary data). The percentage removal of dye is increased with increasing biomass quantity up to 0.2 g, which may be due to the exposure of the enhanced number of vacant sites on MLP for occupying the readily available dye molecules. Further increase in biomass dosage has no appreciable changes in dye removal. In addition, maximum dosage (0.5 g) leads to a slight decrease in dye removal pattern. This differential adsorption behaviour may be accounted for the agglomeration of active sites, leading to a small increase in percentage adsorption, followed by the decline in dye removal (%). Alongside, higher dosage also renders the adverse effect in dye uptake capacity (mg g−1) of biomass as illustrated by .
3.4.3 Effect of the initial dye concentration and equilibrium isotherm study
Langmuir and Freundlich isotherm model constants for adsorption of AY-99 onto MLP biomass
8.9 × 10−4
The dimensionless Langmuir parameter RL is found to have small values in the range 0.24–0.91, thereby indicating the favourable sorption for MLP towards AY-99. Hence, the chemisorption and monolayer coverage are mainly involved for decolourization of dye from an aqueous solution.
3.4.4 Effect of contact time and kinetic study
The dye sorption rate, as well as the mechanism, can be explained on the basis of the kinetic study. The adsorption process on a solid surface is considering as the rapid attachment of sorbate on sorbent surface, followed by liquid film diffusion where boundary plays an important role and finally slower intraparticle diffusion . This fact was well reflected in the experiment where the process was very fast initially, but in due course of time it became slower in a gradual manner and finally attained the equilibrium state plateau within 160 min (Fig. 6b).
Calculated kinetic parameters for removal of AY-99 onto MLP
Dye (mg L−1)
8.7 × 10−4
1.5 × 10−2
2.4 × 10−4
1.4 × 10−2
The experimental data for the intraparticle diffusion model are depicted in Table 2. The results presented in Fig. S5(d) (Supplementary data) show a triphasic nature of the curve. Accordingly, the nonlinear nature of the curve and the plot do not intercept the origin indicating intraparticle diffusion is involved but not the sole rate-controlling step for the entire dye removal process . On the other hand, the fitted straight-line plot of ln(1 − F) versus t for film diffusion-controlled transport mechanism (Fig. S5(c), Supplementary data) shows a high degree of correlation coefficient (R2 = 0.995). In addition, a small magnitude of intercept is found to be 7.03 × 10−5 and 2.74 × 10−3 for 100 mg L−1 and 200 mg L−1, respectively. This indicates that the film diffusion stage may be considered as the rate-limiting step deeming the entire process.
3.5 Mechanism of AY-99 removal by MLP biomass
Since smaller pore volume and specific surface area of MLP, the chemical interaction seems to be substantial in the present dye removal process . This phenomenon is proved thoroughly by the results obtained from isotherm as well as kinetic study. Again low pH rendering the electrostatic force of attraction of dye anions towards the positive surface of MLP supports the above assertions. The polysaccharides like hemicellulose and cellulose in MLP structure possess plenty of hydroxyl groups bonded with secondary forces which prove to be efficient docking sites for such chemical interactions with dye anions at low pH evident from FTIR findings. The outcomes of functional group modification study further outline a similar trend. These polysaccharides are the ingredients of MLP revealed by quantitative measurement of MLP constituents, TGA and XRD analyses. Not only the process follows the chemical interactions with unique binding energy of active sites, but also film mass transfer plays a key role in dye decolourization. Alongside, intraparticle diffusion through mesopores and micropores occurs in part of the process but may not to be conferred as the rate-limiting step. It is noteworthy that dye structure is not degraded or transformed during its removal by MLP biomass as evident from UV–Vis spectroscopic study and elemental mapping showing the spatial allocation of dye-containing metals on MLP. Therefore, this can be assumed that the decolourization of AY-99 on MLP is predominantly followed by the process of chemisorption with the concomitant involvement of the physical force of attraction.
3.6 Effect of NaCl
The effluent of textile industries contains a large amount of sodium chloride (NaCl). Hence, the dye sorption experiments were conducted in the presence of NaCl in the medium. The results presented in Fig. S6 (supplementary data) shows that the dye sorption capacity of MLP biomass decreases from 23.15 mg g−1 to 19.34 mg g−1 with increase in NaCl concentration up to 0.1 M. Initially, the decrease in dye adsorption may occur due to the negatively charged chloride ions and their competition with dye anions for binding to positively charged biomass surface. A slightly decrease in dye adsorption is noted for further increase in NaCl concentration, possibly due to the saturation of Cl− ion concentration. The results further confirm that the electrostatic force of attraction plays a crucial role in the binding of AY-99 to MLP. The findings are in good agreement with .
3.7 Dye removal from a binary mixture
Wastewater from dye industries may contain more than one type of dyes in the effluent; hence, it is very important to study the quantitative removal of target dye molecules from mixed dye solutions. In this study, the first-order derivative spectrophotometric method was used to determine the concentration of AY-99 and AR-88 concentration in binary mixture simultaneously . Figure S7(a) and S7(b) (Supplementary data) reveals the zero-order and first-order derivative spectra of both single- and two-dye systems. In practice, calibration graphs were constructed for standard solutions in the two-dye systems (AA) at corresponding wavelengths (Fig. S7(c), Supplementary data). Based on experimental observations where both dyes are adsorbed by biomass showing in the decay plot (Fig. S7(d), Supplementary data), the colour of the solution turned from deep red to light red. There is a slight difference in dye removal performance for each dye in the mixture of AY-99 and AR-88 solution, which can be considered as counteraction effect in terms of competition between like-charged species for the available active sites of the concerned biomass. Thus, the significant potential of MLP can be exemplified by the enhanced accessibility to active sites for both anionic dyes in a binary mixture.
3.8 Desorption and recycling of biomass
3.9 Removal of AY-99 from simulated textile wastewater
In order to explore the potentiality of low-cost MLP biomass for treating simulated textile wastewater containing AY-99, adsorption characteristics were studied with varying dye concentrations (50–1000 mg L−1) at optimum pH. In addition, comparison of the dye removal efficiency of the biomass from an aqueous dye solution as well as from simulated textile wastewater was monitored. Dye sorption capacity by MLP from simulated wastewater exhibits merely 92%, at the 50 mg L−1 dye concentration. A gradual decrease in dye adsorption is found to be noted from 86.02 to ~ 78% with increasing initial dye concentrations in wastewater from 100 to 400 mg L−1, respectively, which is followed by reaching just about equilibrium (Fig. 7b). The result is quite similar to a single-dye solution in which the dye uptake efficiency of MLP for AY-99 displays ~ 90% removal at 100 mg L−1 initial dye concentration (Fig. 6a). The superior adsorption phenomenon from simulated textile wastewater may be accounted for more pH stability and buffering effect of the solution and also the ionization of functional groups of biomass surface . It is pertinent to mention here that the anionic dye (AY-99) removal efficiency of MLP from simulated textile wastewater was found to be very much effective even at higher dye concentration (1000 mg L−1). Accordingly, the discharge from the said dye extraction process becomes safe to aquatic life and water bodies, leading to ecological balance as per Environmental Regulation Act, 1986, Government of India.
The MLP biomass is found to be the most efficient for adsorption of AY-99 from its aqueous solution. Biomass is composed mostly by hemicelluloses, cellulose and lignin evident from TGA and chemical constituents’ analysis. The adsorption process verified by UV–visible spectroscopic study depends strongly on the solution pH being optimum at 2.5. Equilibrium adsorption data are well fitted to the Langmuir isotherm model, pseudo-second-order rate model and film diffusion model. Thermodynamic parameters revealed that the process is spontaneous and endothermic nature. Scanning electron microscopic studies depict the consequent structural alteration of the surface topography on the sorption of dye, while X-ray elemental analysis of the dye-laden biomass confirms the binding of the dye molecules onto the biomass surface. XRD study indicates both amorphous and crystalline characteristics of the biomass even after binding of dye. The major involvement of hydroxyl functional group has been observed during the adsorption process, supported by FTIR study and functional groups modifications of biomass. MLP displays the excellent capability of decontaminating both dyes in binary mixture by lowering their concentrations. The presence of NaCl in the medium exhibits no such significant influence on AY-99 adsorption. Desorption was successful, and the biomass can be used for five consecutive adsorption–desorption cycles. Overall, the study reveals the greater decolourization efficiency of MLP, which makes sure its potential for practical application.
The authors gratefully acknowledge Dr. A. R. Das (Polymer Science Unit, IACS, Kolkata) and Dr. Rajib Majumder (Adamas University, Kolkata) for their valuable suggestions and help during this study.
This study was funded by the University Grants Commission (Grant No.: RGNF-2011-12-13460), New Delhi, India.
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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