Uptake of Methylene Blue from Aqueous Solution by Naturally Grown Daedalea africana and Phellinus adamantinus Fungi.

Herein, Daedalea africana and Phellinus adamantinus were evaluated for the uptake of the methylene blue (MB) dye. Various factors such as pH range, time of exposure, dye concentration, adsorbed quantity, etc. have been studied for the uptake. Adsorption isotherms investigated in this study include the Langmuir and Freundlich isotherms. The Langmuir isotherm has been long known to be the best fit in the process of adsorption. The maximum monolayer adsorption capacity for D. africana was reported to be 0.5210 mol/kg, and it is 1.8387 mol/kg for P. adamantinus at 298 K. The n values 0.8748 and 0.9524 obtained indicate that the dye is favorably adsorbed on both adsorbents. Kinetics data analysis has shown that methylene blue adsorbed on the fungus showed pseudo-second-order chemisorption and film as well as intra particle diffusion. These results reveal that the abovementioned fungi can be used as good sources for the uptake of the MB dye.

Introduction

Rapid worldwide industrialization to meet the demands of the growing populapan class="Chemical">tion has led to the utilization and release of chemicals to the environment.[1−3] The textile, plastic, and food sector industries use a variety of dyes during the industrial processes.[4,5] It is estimated that over 7 × 105 tons of dye is manufactured annually making available 100,000 commercial dyes, a greater percentage of which is discharged as waste into the water sources during the industrial processes.[4−6] Most of these dyes are toxic, carcinogenic, and have complex structures, which are highly resistant to degradation.[7,8] Their molecular structures are resistant to light, water oxidation, and biodegradation posing a threat to the environment. Due to the inhibition of sunlight, photosynthesis is also reduced in aquatic flora causing ecological imbalance in the aquatic system.[9]

Methylene blue, a capan class="Chemical">tionic thiazine dye, although widely used in coloring and coating paper, temporary hair colorant, and dyeing wool and cotton, in therapeutic doses is a safe drug, but acute or long-term exposure causes eye irritation, and ingestion may cause mouth dryness, rapid pulse, haziness, hemolytic anemia, and methemoglobinemia. Therefore, removal of methylene blue from industrial effluents by adopting safe and convenient methods is an environment and health concern.[6,10]

Diverse methods of treatment technologies have been adopted for wastewaters such as photocatalytic degradation, ozonation, ion exchange membranes, adsorption/precipitation processes, coagulation/flocculation, biological treatment, electrochemical techniques, and nanofiltration.[11−15] Simultaneous detection and photodegradation of industrial and sewage pollutants have recently been investigated on p-type semiconductors@metal organic frameworks (MOFs) with multifunctional platforms.[16] A number of parameters with regard to availability, cost, productivity, and accessibility in operation govern their adoption. Adsorption in which a gas or a liquid becomes attracted to and is attached to the surface of a solid is of recent examination as a significant method of wastewater remediation.[13] Recently, a number of attempts are in the way for use of cheaper, feasible, renewable, and efficient adsorbents. A number of cost-effective adsorbent materials from agricultural waste, industrial by-products,[17] and biomaterials are generally disposed of as garbage like garlic peel, prickly bark of cactus fruit, and hydrolyzed oak saw dust, and calcium alginate composite were used for the adsorption of methylene blue.[10,18,6] Activated carbon from different sources, i.e., cotton,[19] cocoon of the silk moth worm,[20] and tea seed shells,[21] has been evaluated as an adsorbent for wastewater treatment, but it is not economical.[22] A higher methylene blue adsorptive ability was exhibited by activated carbon derived from finger citron residues.[23] Novel MOFs with large pores and high surface areas and more advanced bimetallic MOFs overcoming the shortcomings of the former, with enhancement in uptake of dyes, have been studied.[24] Much attention has also been directed toward the use of microorganisms for bioremediation of organopollutants.[25] Biosorption is the uptake of pollutants by biological materials.[26] There are many publications of bacteria, fungi, and algae being used as biosorbents.[9] With fungi being more advantageous, researchers have already worked with the uptake of azo dyes by Candida tropicalis(27) and acid red B by Pichia sp. TCL.[28] Fungi produce extracellular lignolytic enzymes that are able to break down complicated organic compounds.[27] The uptake of textile waste dyes has also been evaluated on Daedalea flavida, Polyporus sanguineus, Irpex flavus,[29] and Inonotus dryadeus.[7]Fomes fomentarius and Phellinus igniarius were investigated for the adsorption of methylene blue depicting adsorption capacities of 232.73 and 202.38 mg/g, respectively.[30]

In this study, locally available fungi collected from natural sources, D. africana and P. adamantinus, were used as adsorbents in the uptake of the methylene blue dye. Daedalea spp. mushrooms grow on decaying oak trees and hard wood trunks. Daedalea spp. possesses natural antioxidant, anticancer, and antimicrobial properties.[31]Phellinus spp. is a wood-inhabiting fungus, which is cosmopolitan, inhibiting diverse angiosperms or gymnosperms.[32]P. adamantinus is reported to have anticancer activities and is used in traditional medicine for the treatment of cancer.[33] The constitution of fungi includes a multitude of functional groups, essential amino acids, carbohydrates, proteins, and phenols.[34]P. adamantinus contains phytochemicals having oxygen- and nitrogen-containing organic compounds, which act as ligands showing excellent binding to the cyclooxygenase enzyme inhibiting its activity.[33] The literature reveals that the phenolic compounds in the fungal extracts scavenge free radicals, also serve as hydrogen donors, support singlet oxygen quenching, and have a great reducing potential.[34] A greater population of the fungi is mesophilic, growing in the range of 10–40 °C, and can be grown at room temperature of 22–25 °C, with a pH range of 4–8.5 with broad optima of pH 5–7, suitable for the industrial wastewater discharge, which ranges from pH 6 to 9. Several fungi are also known to be acid-resistant. Fungi grow or can be commercially grown with appropriate and suitable sources of carbon, nitrogen, salts, and trace elements to support mycelial growth.[35]Pleurotus species were commercially cultivated in a lesser and cheap way using wider substrates and sporeless strains, tolerating wider temperatures and chemicals, procuring high yields in a shorter cycle of time.[36] The optimum conditions of culture media, carbon, nitrogen, vitamin, temperature, organic acid, and mineral salt sources for mycelia growth of Phellinus species were also examined.[37] Studies were also conducted to reuse agroindustrial waste, which is discarded, or a mixture of them as substrates for mushroom production,[36] thus serving a dual purpose in waste management. In this regard, the physical and chemical properties, accessible binding sites, favorable conditions for production and cultivation, nonhazardous nature, and environmentally friendly process frame these fungi for use as a better possible method for treating wastewater. The medicinal importance of these fungi serves as an added advantage for use in water pollution. In this study, the effects of variables like pH, dye concentration, adsorbent dose, contact time,[38] and agitation were examined for the uptake of the methylene blue dye. Adsorption isotherms of the sorption process were also studied. In addition to the efficiency of these adsorbents, they were also examined for the adsorption of industrially important anionic dyes Bromocresol green (BCG), Congo Red (CR), Eosin Yellow (EY) and are adjoined in supplementary studies.

Results and Discussion

Characterization of Adsorbents

Textural Characterization

In our manuscript, DA is D. africana, and PA is P. adamantinus. Field-emission scanning electron microscopy (FE-SEM) was employed for DA and PA, and images of DA are shown in Figure a–d from lower to higher magnification show rough surfaces with a porous nature and tubular shapes, while those of PA as seen in Figure a–d are a mass of clustered biconcave disc-like irregularly arranged pores of diameter nearly 4 μm stacked one over the other. The surface area and average pore radius of DA were found to be 51.886 m2/g and 2.99 Å, respectively (Figure e), while those of PA were 4.49 m2/g and 5.299 Å, respectively, from BET analysis (Figure e). The DA shows a higher surface area but a lower average pore radius and pore distribution; being not uniform possibly accounts for the low adsorption capacity as compared to PA with a lower surface area but a higher pore radius with high pore distribution and therefore more density of adsorption sites resulting in more interactions between the adsorbate and adsorbent. The images demonstrate a good adsorption for methylene blue (MB). The N2 adsorption isotherms of DA and PA are of type IV, corresponding to mainly mesoporous adsorbents (Supporting Information, Figures S1, S2, respectively). The hysteresis loops indicate that capillary condensation occurs in the mesopores, the initial loop indicates adsorption, and the second loop indicates desorption. In DA, there is a gradual increase in adsorption at increasing P/Po, while in PA, there is a slow increase in adsorption over a wide range of relative pressures, and a steep increase is observed at higher relative pressure. The pore size distributions (Barrett–Joyner–Halenda method) of DA and PA (Supporting Information, Figures S3, S4, respectively) show that the pore widths are mainly between 1 and 10 nm indicating a mainly mesoporous nature with low composition of micro- and macropores. Therefore, the surface of DA and PA gives knowledge of an arranged network ranging from micropores to macropores. The increase in adsorption at higher relative pressures for PA indicates more availability of macropores.[21]

Figure 1

FE-SEM images of DA at (a) 50 μm magnification, (b,c) 10 μm magnification, and (d) 4 μm magnification. (e) BET isotherm of DA. (f) ATR-IR spectra at room temperature of DA and MB adsorbed onto DA with the inset photograph of the fungi growing on the branch of the tree.

Figure 2

FE-SEM images of PA at (a) 50 μm magnification, (b,c) 10 μm magnification, and (d) 4 μm magnification. (e) BET isotherm of PA. (f) ATR-IR spectra at room temperature of PA and MB adsorbed onto PA with the inset photograph of the fungi growing on the branch of the tree.

FE-SEM images of DA at (a) 50 μm magnificapan class="Chemical">tion, (b,c) 10 μm magnification, and (d) 4 μm magnification. (e) BET isotherm of DA. (f) ATR-IR spectra at room temperature of DA and MB adsorbed onto DA with the inset photograph of the fungi growing on the branch of the tree.

FE-SEM images of pan class="Chemical">PA at (a) 50 μm magnification, (b,c) 10 μm magnification, and (d) 4 μm magnification. (e) BET isotherm of PA. (f) ATR-IR spectra at room temperature of PA and MB adsorbed onto PA with the inset photograph of the fungi growing on the branch of the tree.

Chemical Surface Characterization

Attenuated total reflection infrared (ATR-IR) analysis was performed on both fungi. The ATR-IR peaks display a characterispan class="Chemical">tic fungus spectrum. The spectrum of DA showed peaks at 3310.53 (O–H stretching bonded hydroxyl groups), 2886.02 (C–H stretching), 1644.16 (C=O stretching), 1419.52 and 1371.51 (phenols), 1316.33 (C–O stretching), 1251.96 (C–N stretching), and 1038.44 cm–1 (C–OH stretching) as seen in Figure f. The spectrum corresponding to PA displayed peaks at 3280.32 (N–H stretching secondary amide), 3650 and 3466.52 (O–H stretching vibrations corresponding to alcohols, phenols, and carboxylic acids as in chitin and cellulose), 2905.47 and 2845.02 (CHO groups), 2800–2000 (C–H stretching vibrations), 1800–1500 (C=O and C=C stretching), 1628.97 (COO– stretching), 1371.30 (aromatic C=C stretching, polarized at one of the carbons due to the nearby bonded oxygen atom), 1303.88 (C–C stretching), and 1149.69 and 1036.46 cm–1 (C–O stretching) (Figure f). Therefore, ATR-IR, GCMS (Figures S5–S12), and HRMS (Figures S13 and S14) (Supporting Information) suggest the presence of mainly esters, acids, alcohols, phenols, and amides in DA and PA[6,7,39,40] with elemental composition of mainly carbon and oxygen. The most probable molecules present in DA and PA are shown in the Supporting Information (Schemes S1–S10 and Schemes S11–S15, respectively). When MB was adsorbed onto DA, the peaks mainly at 3310.53 cm–1 shifted to 3303.82 cm–1,1644.16 cm–1 to 1641.30 cm–1,and 1038.44 cm–1 to 1029.91 cm–1, and a new peak at 1369.36 cm–1 (N=O stretching) was observed (Figure f). Adsorption of MB onto PA showed peaks at 1642.87 cm–1 shifting to 1635.37 cm–1, 1379.74 cm–1 to 1376.66 cm–1, and 1036.46 cm–1 to 1030.49 cm–1 (Figure f) showing that −O–H, −C=O, −COO–, and −C–O are mainly involved during adsorption.

Adsorption Studies

Effect of Solution pH

The pH of the solution has a major contribution on the adsorption of MB onto the adsorbents. The surface charges of the dye and the fungal biomass are affected due to the increase in hydrogen or hydroxyl ions, also affecting the functional groups dissociation on the surface of the fungal biomass.[6] 1 × 10–5 m3 volume of an aqueous solution of MB of initial concentration 3.12 × 10–2 mol/m3 was studied for pH 2, 4, 6, 7, 8, 10, and 12 at 298 K. To each of the solutions, 0.00001 kg of the adsorbent was added. The pH was adjusted with 0.1 N HCl (Merck, India) and NH3 (Merck, India) solutions. As seen from Figure a, at low pH = 2, the adsorption was minimum on both the fungal biomass, and the adsorption of MB abruptly increased for a pH of 2 to 4 and thereafter remained constant. At lower pH, there exists on the surface of DA and PA a more positive charge, thereby causing repulsion between MB, a cationic dye, and the adsorbent surface. When the pH increased from 4 to 10, the adsorbent surface becomes more negatively charged enhancing the electrostatic force of attraction to the pollutant dyes, thereby increasing the adsorption capacity.[10] This is in accordance with the adsorption of MB on I. dryadeus(7) and Aspergillus niger.[41] The pH at zero point charge (pHzpc) where the graph intersects the plot that equals pHintial = pHfinal for the adsorbent DA was 5.85, and that for PA was 7.47 as seen in Figure b. The pHzpc of DA is lower than that of PA. At pH values below pHzpc, the electric charge on the adsorbent is positive favoring anion adsorption, and for pH values above the pHzpc, the surface charge is negative favoring adsorption of cationic species.[1] The optimum values of pH 8 and 6 were used to analyze the effect of other variables for adsorption on DA and PA, respectively. In our other experiments, the optimum pH values for the adsorption of CR, BCG, and EY onto DA were estimated to be 7, 2, and 4, respectively, and the adsorption of all these dyes onto PA depicted an optimum pH = 2.

Figure 3

(a) Effect of pH = 2 to 12 on the adsorption of MB on fungal biomass (adsorbent dose, DA, PA: 0.00001 kg, MB concentration: 3.12 × 10–2 mol/m3, volume of MB: 1 × 10–5 m3, T = 298 K). (b) pH value of point of zero charge for DA and PA. (c) Amount of MB adsorbed per unit mass at varying initial dye concentration (adsorbent dose DA, PA: 0.00001 kg, volume of MB: 1 × 10–5 m3, contact time 5 h, DA at pH = 8, PA at pH = 6, T = 298 K). (d) % dye removal of MB on fungal biomass (adsorbent DA, PA: 0.00001 kg, volume of MB: 1 × 10–5 m3, contact time 24 h, DA at pH = 8, PA at pH = 6, T = 298 K). (e) Effect of adsorbent dose on the adsorption of MB onto fungal biomass (initial dye concentration: 7.81 × 10–2 mol/m3, volume of MB: 1 × 10–5 m3, contact time 5 h, pH = 8 for DA, pH = 6 for PA, T = 298 K).

(a) Effect of pH = 2 to 12 on the adsorppan class="Chemical">tion of MB on fungal biomass (adsorbent dose, DA, PA: 0.00001 kg, MB concentration: 3.12 × 10–2 mol/m3, volume of MB: 1 × 10–5 m3, T = 298 K). (b) pH value of point of zero charge for DA and PA. (c) Amount of MB adsorbed per unit mass at varying initial dye concentration (adsorbent dose DA, PA: 0.00001 kg, volume of MB: 1 × 10–5 m3, contact time 5 h, DA at pH = 8, PA at pH = 6, T = 298 K). (d) % dye removal of MB on fungal biomass (adsorbent DA, PA: 0.00001 kg, volume of MB: 1 × 10–5 m3, contact time 24 h, DA at pH = 8, PA at pH = 6, T = 298 K). (e) Effect of adsorbent dose on the adsorption of MB onto fungal biomass (initial dye concentration: 7.81 × 10–2 mol/m3, volume of MB: 1 × 10–5 m3, contact time 5 h, pH = 8 for DA, pH = 6 for PA, T = 298 K).

Influence of Starting Concentration of the Dye

Adsorption is influenced by the pan class="Chemical">adsorbate concentration, which acts as a pushing force to counter all other forces operating between the adsorbent and the aqueous solution. To study the adsorption capacity of the fungal biomass, 0.00001 kg of each of the fungal biomass was added separately to 1 × 10–5 m3 volume of MB solution of different initial concentrations 1.56 × 10–2, 3.12 × 10–2, 4.68 × 10–2, 6.25 × 10–2, and 7.81 × 10–2 mol/m3 (5–25 ppm) at predetermined pH. The experimental results for MB adsorption at different initial dye concentrations are shown in Figure c–d. Observation reveals a proportional increase in adsorption with an increase in initial concentration for the concentrations studied. For the increase of the initial MB concentration from 1.56 × 10–2 to 7.81 × 10–2 mol/m3, the amount adsorbed increased from 1.47 × 10–2 to 7.29 × 10–2 mol/m3 for DA and 1.34 × 10–2 to 6.64 × 10–2 mol/m3 for PA (Figure c). The average percent dye removal is shown in Figure d. Results reveal efficient adsorption by both the fungal biomass for MB for the concentrations performed. For much higher initial concentrations, there was not much appreciable difference in the increase in the adsorption capacity indicating adsorbent saturation.

Effect of the Adsorbent Dose

The extent of adsorption is strongly affected by the dosage of the adsorbent.[26] To study the quantity of the uptake of each adsorbent, varying amounts of adsorbent in the range of 0.00001–0.00005 kg were added separately to 1 × 10–5 m3 of 7.81 × 10–2 mol/m3 MB solutions at the effective pH at 298 K for 5 h. Figure e shows the removal of MB by the fungal biomass at different adsorbent doses. With an increase in the amount of adsorbent, the adsorption was found to increase. A higher amount of adsorbent facilitates more surface area for adsorption making possible more active sites.[2] The adsorption capacity was higher at low dosages of adsorbent, and thereafter, the adsorption capacity decreased with increasing doses. Figure e shows that, with the increase of the adsorbent dose from 0.00001 to 0.00005 kg, decreases in the adsorption capacity from 0.01758 to 0.01506 mol/kg for DA and 0.05498 to 0.01284 mol/kg for PA for an initial concentration of 7.81 × 10–2 mol/m3 MB solution were observed. This may be due to the insufficiency of the available dye particles to completely cover the adsorbent sites available at higher doses.[26] Similar results were also observed with adsorption of MB onto the biopolymer oak saw dust composite.[6] The percent dye removal values were 96.52% by DA and 82.30% by PA for a dosage of 0.00005 kg of adsorbent. For the same experimental conditions, the anionic dyes BCG, CR, and EY at effective pH = 2, 7, and 4, respectively, showed removal percent values of 71.9, 52.5, and 57.3%, respectively, by DA (Supporting Information, Figure S15) and 68.7, 47.9, and 43.7%, respectively, by PA at pH = 2 (Supporting Information, Figure S16). Therefore, the adsorbents showed a greater removal percentage of MB in comparison with the other dyes studied.

Effect of Contact Time in Stationary and Agitation Conditions

Contact time studies were conducted separately to identify the rate of adsorption using 7.81 × 10–2 mol/m3 initial concentration of 5 × 10–5 m3 MB solution at the desired pH adding 0.00001 kg of each adsorbent at 298 K. The supernatant was withdrawn at specified time intervals and tested for dye adsorption. For the initial period of time, the adsorption rate rapidly increases, and thereafter, a gradual increase with not much significant change in equilibrium concentration is observed.[42] This is due to the immediate occupancy of the available vacant active sites by the concentrated dye, decreasing the surface area of the adsorbent for adsorption subsequently.[10] Agitation is also an important phenomenon in adsorption. On agitation, the solute particles become distributed in the bulk solution increasing the contact between the adsorbate and adsorbent.[6] 5 × 10–5 m3 volume of 7.81 × 10–2 mol/m3 MB solutions made at the effective pH was agitated using a magnetic stirrer at 400 rpm to which 0.00001 kg of each adsorbent was added. The solution was removed at different time intervals and centrifuged at 8000 rpm, and the supernatant was tested to study dye adsorption. Figure a shows that the equilibrium time was reached between 150 and 180 min for DA and PA. For PA, the percent removal increases from 32 to 53% at the end of 30 min on agitation, while for DA, an increase from 52 to 59% was observed for the same duration of time on agitation. It was also observed that, beyond 80 min of agitation, the removal percent does not change significantly for DA (Figure b). This may be due to the adsorption–desorption tendency of the dye molecules or due to the adsorbent particles and dye molecules having the same speed, subsequently causing not much change in the adsorption after formation of a layer enveloping the adsorbent,[6] or the electrostatic forces of attraction between DA and MB dye are weak as compared to that between PA and MB dye with also an observation of the pore size difference between DA and PA. Agitation is effective in the adsorption of MB on PA attaining 90% removal at the end of 180 min as seen in Figure b, while it is only 72% removal on DA. Figure c shows the percent removal increase of MB on DA and PA after 24 h in stationary and agitation conditions.

Figure 4

(a) Effect of contact time on the adsorption of MB without agitation (initial dye concentration: 7.81 × 10–2 mol/m3, volume of MB: 5 × 10–5 m3, adsorbent dose DA, PA: 0.00001 kg, pH = 8 for DA, pH = 6 for PA, T = 298 K). (b) Effect of agitation at 400 rpm on the adsorption of MB (initial dye concentration: 7.81 × 10–2 mol/m3, volume of MB: 5 × 10–5 m3, pH = 8 for DA, pH = 6 for PA, adsorbent amount DA, PA: 0.00001 kg, T = 298 K). (c) Effect of agitation on the removal percent of MB on DA and PA after 24 h.

(a) Effect of contact pan class="Chemical">time on the adsorption of MB without agitation (initial dye concentration: 7.81 × 10–2 mol/m3, volume of MB: 5 × 10–5 m3, adsorbent dose DA, PA: 0.00001 kg, pH = 8 for DA, pH = 6 for PA, T = 298 K). (b) Effect of agitation at 400 rpm on the adsorption of MB (initial dye concentration: 7.81 × 10–2 mol/m3, volume of MB: 5 × 10–5 m3, pH = 8 for DA, pH = 6 for PA, adsorbent amount DA, PA: 0.00001 kg, T = 298 K). (c) Effect of agitation on the removal percent of MB on DA and PA after 24 h.

Adsorption Isotherms

Studies on the adsorption isotherm give an idea on the pan class="Chemical">MB interaction with the adsorbents and help to optimize their applicability. A number of isotherm models have been formulated. The experimental data was plotted for the Langmuir and Freundlich isotherm models. The Langmuir isotherm refers to adsorption occurring on a monolayer at a fixed number of adsorption sites having similar energy.[41] The mathematical Langmuir isotherm can be represented as eq .where Ce is the concentration at equilibrium of the MB (mol/m3), qe is the quantity of MB adsorbed per gram of fungal biomass under equilibrium (mol/kg), qmax is the maximal monolayer adsorption capacity (mol/kg), and b is the energy of adsorption (m3/kg).

The experimental data were plotted using the Langmuir isotherm in linear form (eq ).

The values of b, qmax, and the corresponding coefficient of correlation R2 given in Table have been deduced using the slope as well as the intercept of the graph of 1/qe vs 1/Ce shown in Figure a.[43] With the dimensionless constant RL denopan class="Chemical">ting the adsorption behavior called as the separation factor, the equilibrium parameter can be calculated by using eq .[6]

Table 1

Isotherm Constants for the Adsorption of MB at 298 K onto Fungal Biomass DA (0.00001 kg) and PA (0.00001 kg)a

model	adsorbent	estimated isotherm parameters
Langmuir		qmax (mol/kg)	b (m3/kg)	R2
	DA	0.5210	48.6	0.9939
	PA	1.8387	60.1	0.9975
Freundlich		n	Kf ((mg/g)(L/mg)1/n)	R2
	DA	0.8748	8.1940	0.9918
	PA	0.9524	37.7656	0.9966

Maximum adsorption capacity qmax (mol/kg), energy of adsorption b (m3/kg), adsorption capacity Kf ((mg/g)(L/mg)1/), adsorption intensity n, coefficient of correlation R2.

Figure 5

(a) Langmuir isotherm showing the variation of 1/qe with respect to 1/Ce for the adsorption of MB on fungal biomass. (b) Freundlich isotherm showing the variation of ln qe with respect to ln Ce for the adsorption of MB on fungal biomass (MB: 1.5 × 10–2-4.68 × 10–2 mol/ m3, 5 × 10–5 m3, DA: pH = 8, 0.00001 kg, PA: pH = 6, 0.00001 kg, T = 298 K).

(a) Langmuir isotherm showing the variation of 1/qe with respect to 1/Ce for the adsorppan class="Chemical">tion of MB on fungal biomass. (b) Freundlich isotherm showing the variation of ln qe with respect to ln Ce for the adsorption of MB on fungal biomass (MB: 1.5 × 10–2-4.68 × 10–2 mol/ m3, 5 × 10–5 m3, DA: pH = 8, 0.00001 kg, PA: pH = 6, 0.00001 kg, T = 298 K).

Maximum adsorption capacity qmax (mol/kg), energy of adsorppan class="Chemical">tion b (m3/kg), adsorption capacity Kf ((mg/g)(L/mg)1/), adsorption intensity n, coefficient of correlation R2.

An RL value greater than 1 indicates that the adsorption behavior is not favorable, RL = 1 indicates linearity, 0 < RL < 1 is favorable, and RL = 0 is irreversible.[6] The values of correlapan class="Chemical">tion coefficient R2 = 0.9939 (DA) and R2 = 0.9975 (PA) obtained from the Langmuir plot indicate that the Langmuir expression provided good linearity. For the experimental data, the RL values range between 0.578 and 0.804 for DA and 0.525 and 0.768 for PA indicating that the adsorption was favorable. The maximum monolayer adsorption capacities were 0.5210 (DA) and 1.8387 mol/kg (PA). The results are more appreciable than our previous report of 0.428 mol/kg.[7] A similar adsorption capacity of 0.593 mol/kg was reported for the adsorption of the dye Remazol Black B on Rhizopus arrhizus.[44] A nearly similar adsorption uptake of 581.40 mg/g MB was also reported by the activated carbon derived from the plant finger citron residues.[23]Table lists the adsorption of MB on other adsorbents with varying surface areas.[7,10,21,23,45−47] Though the surface areas of DA and PA are relatively small, they exhibit a high adsorption for a planar molecule like MB. The adsorption capacities of DA and PA in the present study are appreciable as they are naturally occurring and directly utilized without any chemical action. In our supplementary studies, the adsorption isotherms of DA and PA for adsorption of BCG, CR, and EY revealed adsorption capacities of 112.36, 105.26, and 54.35 mg/g, respectively, by DA (Supporting Information, Figure S17 and Table S1) and adsorption capacities of 147.06, 71.43, and 90.09 mg/g, respectively, by PA (Supporting Information, Figure S18 and Table S1). The data has been fitted in the equation of the Freundlich isotherm represented as follows (eq ).

Table 2

Maximum Monolayer Adsorption Capacities qmax (mol/kg) of MB on Different Adsorbents

adsorbent	BET surface area (m2/g)	adsorption capacity qmax (mol/kg)	ref
tea seed shell	1530.67	1.0712	(21)
oil palm trunk fiber	64.44	0.4669	(45)
garlic peel	0.561	0.2584	(10)
Caulerpa lentillifera	4.946	1.3037	(46)
I. dryadeus	4.332	0.4283	(7)
finger citron residue activated carbon	2887	1.8177	(23)
carbonized cotton linen fiber cloth-BiFeO3	442.55	0.3068	(47)
D. africana	51.886	0.5210	present work
P. adamantinus	4.49	1.8387	present work

Kf ((mg/g)(L/mg)1/) and n are the Freundlich constants, which describe the adsorption capacity as well as adsorption intensity, respectively, whose values were determined from the plot representing ln qe vs ln Ce as shown in Figure b.[43]Table tabulates the calculated Kf, n, and the coefficient of correlation R2 values. For a value of n = 1, the adsorption indicates linearity and involves a chemical process when n < 1 and a physical process when n > 1. The n values ranging from 1 to 10 describe good adsorption.[42] The coefficients of correlation R2 = 0.9918 and 0.9966 are in line with the experimental data of MB on DA and PA, respectively. The n values 0.8748 (DA) and 0.9524 (PA) obtained indicate that the dye is favorably adsorbed on both the adsorbents, which is in agreement with the observation of the RL values. A higher value of Kf indicates higher capacity of adsorbents.[48] Linear graphs were obtained for both the isotherms indicating applicability for adsorption. Almost the same results were seen in the literature for Langmuir and Freundlich isotherms.[42] The Langmuir model is the best applied to the adsorption examining the high R2 values indicating that the adsorption of MB onto DA and PA is monolayer with no interaction of the dye particles with each other. PA with a surface area of 4.495 m2/g exhibited a higher adsorption capacity than DA possessing a surface area of 51.886 m2/g. This difference in the uptake capacity of MB can be credited to the larger size of MB sufficiently accommodating into the compatible average pore radius of PA (5.299 Å) in contrast to a smaller average pore radius of DA (2.99 Å) resulting in an enhanced interaction. As seen from the FE-SEM images (Figure a–d), PA exhibits a higher porosity in comparison to DA (Figure a–d) facilitating more adsorption.[49] BET analysis also reveals PA with more macropores than DA facilitating easy removal of a relatively larger-sized MB molecule. The Freundlich isotherms of DA and PA adsorption of BCG, CR, and EY in our simultaneous studies showed n values ranging from 0.9 to 1.0 depicting good adsorption. The adsorption isotherm and isotherm parameters are given in the Supporting Information (Figures S19–S20 and Table S2). The adsorption isotherm plots for the Langmuir and Freundlich isotherms for the dyes studied show high R2 values on the straight line curves indicating good applicability of these adsorbents (Supporting Information, Figures S17–S20 and Tables S1–S2).

Adsorption Kinetics

The kinetic data were applied to pseudo-first-order, pseudo-second-order, and diffusion models.

Pseudo-first-order Kinetics

The rate constant with which the pseudo-first-order reaction takes place can be calculated using Lagergrens[10] (eq ).qe and qt (mol/kg) in the equapan class="Chemical">tion are the quantities of the MB adsorbed when the reaction is in equilibrium and at time t (min), respectively, and k1 (min–1) is the rate constant of the first-order reaction. The values of k1 and qe obtained using the slope along with the intercept from the line log(qe – q) vs t for DA and PA at various concentrations are listed in Tables and 4, respectively.

Table 3

Kinetic Parameters in Adsorption Studies Using MB 0.0156–0.0781 mol/m3, DA 0.00001 kg, pH = 8, T = 298 K, Adsorption Capacity at Equilibrium qe (mol/kg), Pseudo-first-order Rate Constant k1 (min–1), Pseudo-second-order Rate Constant k2 (kg/mol min), and Coefficient of Correlation R2

	pseudo first order			pseudo second order
C0 (mol/m3)	qe (mol/kg)	k1 (min–1)	R2	qe (mol/kg)	k2 (kg/mol min)	R2
0.0156	0.0211	0.015	0.9810	0.0535	1.1279	0.9974
0.0312	0.0413	0.016	0.9796	0.1120	0.6014	0.9978
0.0468	0.0630	0.015	0.9862	0.1671	0.3608	0.9971
0.0625	0.0725	0.016	0.9824	0.2350	0.3692	0.9987
0.0781	0.0782	0.016	0.9846	0.3095	0.3631	0.9992

Table 4

Kinetic Parameters in Adsorption Studies Using MB (Concentration 0.0156–0.0781 mol/m3) at pH = 7, PA (0.000010 kg) at 298 K, Adsorption Capacity at Equilibrium qe (mol/kg), First-Order Rate Constant k1 (min–1), Second-Order Rate Constant k2 (kg/mol min), and Coefficient of Correlation R2

	pseudo first order			pseudo second order
Co (mol/m3)	qe (mol/kg)	k1 (min–1)	R2	qe (mol/kg)	k2 (kg/mol min)	R2
0.0156	0.0815	0.0190	0.9512	0.0962	0.1403	0.9546
0.0312	0.1643	0.0214	0.9651	0.1828	0.0943	0.9852
0.0468	0.2585	0.0218	0.9665	0.2791	0.0576	0.9860
0.0625	0.3029	0.0219	0.9488	0.3473	0.0675	0.9892
0.0781	0.3217	0.0230	0.9663	0.4168	0.0826	0.9954

Pseudo-second-order Rate Kinetics

The following pseudo-second order reaction (eq ) was used for studying the kinepan class="Chemical">tic analysis.

The slope as well as intercept present in the line t/q vs t gives the adsorption capacity under equilibrium qe (mol/kg) and the rate constant k2 (kg/mol min) of the pseudo-second-order reacpan class="Chemical">tion (Figure a,b). Tables and 4 tabulate the values of qe as well as k2. The correlation coefficient values for the pseudo-second order are higher than those of the pseudo-first-order rate kinetics for both DA and PA (Tables and 4). This implies that pseudo-second-order kinetics with high R2 best explains the process of adsorption indicating adsorption involving some levels where chemical interactions between MB and the adsorbents are predominantly responsible for dye removal.

Figure 6

Pseudo-second-order kinetics for the adsorption of MB onto (a) DA and (b) PA. Intraparticle diffusion model for the adsorption of MB onto (c) DA and (d) PA (DA: pH = 8, 0.00001 kg, PA: pH = 6, 0.00001 kg, MB: 1.5 × 10–2 to 7.81 × 10–2 mol/m3, 5 × 10–5 m3, T = 298 K).

Pseudo-second-order kinetics for the adsorppan class="Chemical">tion of MB onto (a) DA and (b) PA. Intraparticle diffusion model for the adsorption of MB onto (c) DA and (d) PA (DA: pH = 8, 0.00001 kg, PA: pH = 6, 0.00001 kg, MB: 1.5 × 10–2 to 7.81 × 10–2 mol/m3, 5 × 10–5 m3, T = 298 K).

Intraparticle Diffusion

The intraparticle diffusion model is given by eq .

The plotted graph showing q vs t0.5 (Figure c,d) gives the intraparticle rate constant k (mol/kg min0.5) in addipan class="Chemical">tion to C, a constant describing the importance of the boundary layer. As seen from Figure c,d, for all initial concentrations, the plots for both DA and PA did not pass the origin, which indicates the influence of other factors and not only intraparticle diffusion for determination of the rate of reaction, which suggests that, initially, there is immediate occupation of the adsorbent sites on the surface film diffusion and then a slow intraparticle diffusion into the micropores.[50]

From the pHzpc and ATR-IR data, the possible mechanism would suggest that the aromatic esters on the fungal surface dissociate to give carboxylate groups with a negative surface above pHzpc (pH > 5.85) in DA attracting MB cationic species enhancing adsorption by electrostatic attraction (Figure ). In PA, below pHzpc (pH < 7.47), the surface is positive where the carbonyl group of ester polarizes, and the slightly positive charge carbon attracts the lone pair of electrons on the nitrogen of MB favoring more adsorption (Figure ) at pH lower than pHzpc where optimum adsorption was seen at pH = 6. The BET and FE-SEM data reveal that the adsorption process involves physical interactions between the adsorbate and adsorbent. The ATR-IR, pHzpc, n values, and kinetics study suggest also the contribution of chemisorption in adsorption,[7] with −O–H, −C=O, −COO–, and −C–O groups mainly involved in adsorption. Therefore, adsorption of MB onto DA and PA involves a combination of both physisorption and chemisorption.

Figure 7

Plausible mechanism of chemical reaction between adsorbents (DA and PA) with MB.

Plausible mechanism of chemical reaction between pan class="Chemical">adsorbents (DA and PA) with MB.

Conclusions

The present investigation revealed the use of DA and PA, naturally available fungi, in the adsorption studies of MB present in solution form. The process of adsorption is explained with the help of the Langmuir isotherm model suggesting monolayer adsorption. The maximum adsorption capacities were 0.5210 and 1.8387 mol/kg for DA and PA, respectively. A small amount of 0.00001 kg of DA and PA demonstrated 72.3 and 90.4% removal, respectively, for 7.81 × 10–2 mol/m3 MB. PA exhibited 3.5 times more adsorption than DA under the same experimental conditions. The process of adsorption has been observed to follow pseudo-second-order reaction kinetics suggesting chemisorption over a heterogeneous surface. The BET and FE-SEM images suggest physical interactions also involved in the adsorption process. PA with a low surface area in comparison to DA shows more affinity for MB indicating here that pore size and distribution play a major role in physisorption. A high adsorption capacity by PA, also possessing inherent medicinal use, is an advantage for water treatment processes. The varied use of these adsorbents as supported by our other supplementary studies on BCG, CR, and EY is appreciable as they are applied without any chemical processing, are ecofriendly producing no harm on the ecosystem during their application, can be grown and produced with the best interacting combination of nutrients, temperature, and other factors, and serve as a continuous supply for use of their application. Therefore, the applications of these adsorbents can be further reinforced in the study of removal of a combination of dyes.

Experimental Section

Adsorbate

The adsorbate under study pan class="Chemical">methylene blue (MB), [7-(dimethyl-amino)phenothiazin-3-ylidene]-dimethylazanium chloride, a cationic dye, was supplied by Merck, India (molecular weight (g/mol) 319.90). MB stock solution (3.126 × 10–1 mol/m3) was made by dissolving 0.0001 kg of dye in 1 × 10–3 m3 of Milli Q water. Further dilutions were made to obtain the required experimental concentrations. To study the dye removal, the unadsorbed concentration of the dye was estimated in a UV spectrophotometer (UV-1800 Shimadzu). Absorbance values were recorded at λmax = 658 nm. The source and purity of the chemicals used are tabulated in Table . The dyes Bromocresol green and Congo Red were purchased from HiMedia, India, and Eosin Yellow was purchased from Merck, India. Absorbance values were recorded at λmax = 442 nm for BCG, 498 nm for CR, and 517 nm for EY.

Table 5

Source and Purity of Chemicals

chemicals	CAS Reg. No.	source and supplier	purity (% wt)
methylene blue	61-73-4	Merck, India. Rathod Chemicals	0.98
hydrochloric acid	7647-01-0	Merck, India. Rathod Chemicals	0.37
ammonia solution	7664-41-7	Merck, India. Rathod Chemicals	0.25

Preparation and Characterization of Adsorbents

The fungi used as adsorbents were collected from natural sources. pan class="Species">D. africana (DA) was collected from the branch of the Terminalia catappa tree, and P. adamantinus (PA) was collected from the branch of the Delonix regia tree in Pune, India. The fungi were extensively washed to remove unwanted particulate matter and then kept in an oven for drying at 323 K for 6 h. The dried fungal biomass was ground and sieved to make available more surface area for adsorption. The prepared biomass was packed in air-tight containers and used for adsorption studies.

Textural features of the fungi were determined by pan class="Chemical">N2 adsorption at 77 K by the Brunauer–Emmet–Teller (BET) method using a Quantachrome Autosorb Q2 surface area analyzer. Surface morphology and the characteristics of the porosity of the fungi were determined by a field-emission scanning electron microscope (FE-SEM) (FEI Quanta −450) at 5 kV. Identification of the surface functional groups of the fungal biomass was performed by an attenuated total reflection infrared (ATR-IR) spectrophotometer (FTIR Tensor-37 Bruker Platinum ATR) in the range of 4000–400 cm–1. Gas chromatography mass spectroscopy (GCMS, Shimadzu, GC 2010 Plus) (Supporting Information, Figures S5–S12) and high-resolution mass spectroscopy (HRMS) (Supporting Information, Figures S13, S14) were employed for chemical characterization.

Adsorption Equilibrium and Kinetic Studies

Adsorption equilibrium analysis was performed using the pan class="Chemical">MB dye in 2.5 × 10–4 m3 conical flasks, which contain solutions of various concentrations 1.5 × 10–2 to 4.68 × 10–2 mol/ m3 (5–15ppm) with a fixed amount of 0.00001 kg of adsorbent at predetermined pH = 8 (DA) and pH = 6 (PA) at 298 K to establish the adsorption isotherms onto the fungal biomass. The flasks were sealed and agitated at a constant speed of 120 rpm for 5 h ensuring equilibrium. The flasks were then removed and centrifuged at 4000 rpm for 15 min. The supernatant was estimated for the remaining dye concentration at λmax 658 nm using a UV spectrophotometer (UV-1800 Shimadzu). The quantity of dye adsorbed at equilibrium qe (mol/kg) was determined using eq .

Here, C0 (mol/m3) and Ce (mol/m3) represent initial and equilibrium concentrapan class="Chemical">tions of the dye in the solution state, respectively, V (m3) represents the solution volume under study, and W (kg) is the mass of the adsorbent used. The influence of different conditions on the uptake of methylene blue upon the fungal biomass was studied. The percent dye removal (% dye removal) was determined from eq where Co (mol/m3) is the initial MB concentration and C (mol/m3) is the concentration at any instant t. To understand the mechanism, kinetic experiments were also performed using 0.00001 kg of the adsorbent in diluted concentrations of 1.5 × 10–2 to 7.8 × 10–2 mol/ m3 (5–25 ppm) using 5 × 10–5 m3 MB at 298 K from initial time to 5 h at pH = 8 (DA) and pH = 6 (PA). The amount of dye adsorbed q (mol/kg) at various time intervals was made known from eq .