Neolamarckia cadamba (cadamba) waste pulp as a natural and techno-economic scavenger for methylene blue from aqueous solutions

Abstract In this work, Neolamarckia cadamba (cadamba), also known as bur flower tree has been exercised to demonstrate as an excellent methylene blue scavenger from simulated as well as industrial wastewater. The particle morphology and structural insights were gained from FESEM, BET surface area, FTIR, and pHZPC. The adsorption behavior was mapped by different physico-chemical parameters such as contact time, pH, input concentration, and temperature. Experimental data reveal rapid adsorption, and >90% uptake was successful within the first 15 min and reaches equilibrium by 45 min (removal efficiency = 94.15%) at neutral pH. The maximum adsorption capacity was found to be 115.60 mg/g. The uptake process follows pseudo-second-order kinetics (R2 = 0.99), confirming a chemisorption process while the Langmuir model (R2 = 0.99) satisfactorily addresses the adsorption path. Thermodynamic parameters suggest a spontaneous, feasible, and exothermic process with increased entropy. Spent adsorbent could easily be regenerated in up to 74% using 1:1 MeOH/H2O with a potential of three-cycle use. Real-time efficacy has been established with an MB containing industrial effluent and up to 44.70% adsorption, which confirms the material’s practical applicability. Statistical reliability was confirmed by the relative standard deviation. Altogether, the present material offers clean and green removal of methylene blue dye from versatile wastewater. NOVELTY STATEMENT The search for cleaner and greener protocols for water treatment is on the rise. With this line, we have chosen non-edible fruit pulps of Neolamarckia cadamba for extraordinary methylene blue uptake from diverse contaminated water bodies. Compared to contemporary materials, the excellent adsorption capacity (115.60 mg/g) with methylene blue dye offers an edge. The material could be regenerated easily and reused for three cycles. The method doesn’t involve any chemical treatment, is greener, and could be applied on a large scale. Due to huge availability, excellent adsorption capacity, reusability, and simple preparation provide advantages to the material for sustainable water treatment. Graphical Abstract


Introduction
Water pollution has been the biggest problem in the world.Water pollution happens due to the high population growth, industrialization of rural areas, agricultural waste, radioactive waste, marine dumping, and many others (Bayomie et al. 2020).One of the major causes of groundwater and surface water pollution is organic dyes escaping from the textile, pharmaceutical, cosmetic, paper, and food industries (Ahmad et al. 2020;Mahato et al. 2023).Dyes are organic and colored compounds and classified according to their origin (natural, synthetic), water solubility (water-soluble: reactive, acidic, basic; water insoluble: vat, sulfur, pigment dye), chemical structure, presence of chromophore (azo, nitro, xanthene dyes) and industrial application (Berradi et al. 2019).Dyes can also be categorized as cationic or acidic, anionic or basic, and nonionic dye.(Sillanp€ a€ a et al. 2023).
The wastewater affects not only humans but also the flora and fauna (Alp Arici 2022).The textile-contaminated water contains dyes having a complex molecular structure with functional groups like aldehyde (CHO), carbonyl (C¼O), amine (NH 2 ), carboxylic (COOH), and azo (N¼N) groups (Temesgen et al. 2018;Rigueto et al. 2020).Such dyes are non-biodegradable, carcinogenic, and mutagenic in nature.Moreover, dyes lower photosynthetic activity, resist light penetration, and lower oxygen demand in water bodies (Sharafzad et al. 2021;Rezaei et al. 2023).In most developing countries, textile wastewater pollutes groundwater and affects crop production.Exposure to textile dyes leads to irritation in the eyes, skin disease, lungs damage, digestive & respiratory tract irritation, tumors, etc (Sillanp€ a€ a et al. 2023).The workers who are engaged in handling these reactive dyes can be easily prone to health hazards like rhinitis, dermatitis, occupational asthma allergic conjunctivitis, and many more (Mehra et al. 2021).
Therefore, it is important to remove dye toxicity from water bodies.Different physical and chemical methods such as coagulation, ion exchange, flocculation, electrochemical treatment, membrane separation, photo-catalysis, oxidation, filtration, Fenton's oxidation, reverse osmosis (Al-Hawary et al. 2023), and precipitation are implicated in reducing the bad effect of chemicals (Hashem et al. 2020;Rout et al. 2023).These conventional techniques have so many disadvantages including low biodegradability of side products, formation of toxic byproducts, a large amount of chemical consumption, high sludge treatment cost, and longer biosorbent regeneration time (Bhattacharjee et al. 2020).These methods have restraints regarding the cost, efficiency, reliability, difficulty in operation, handling the chemicals, and impression on biodiversity.The adsorption process is an efficient alternative technique for pollutant removal due to the low-cost investment, flexibility, and easy operation (Kumari et al. 2023).The best advantage of the adsorption process is it generates less hazardous byproducts (Aguayo-Villarreal et al. 2020;Cusioli et al. 2020).
Methylene Blue (MB) is mostly used in pharmaceutical and chemical industries.It is a cationic thiazine dye.The illegal dumping of dyes and their additional chemicals into water bodies generate a dreadful effect on the marine system.MB may also lead to other health hazards like vomiting, diarrhea, nausea, and eye irritation.So, methylene blue removal from waste streams has become a vital concern (Duhan and Kaur 2021;Chandarana et al. 2021).During the adsorption process, the Columbia force and stacking interaction between methylene blue, and adsorbent administer excellent adsorption of MB dye (Eltaweil et al. 2020).
The present study focuses on preparing adsorbent from Neolamarckia cadamba fruit pulp for efficient removal of MB from the water solution.The adsorbent was characterized by FTIR, FESEM, and BET surface area analysis.The batch experiments, kinetic, isotherm models, thermodynamic studies, tests with different interfering ions, and spiked effluents were done under optimized conditions for a better understanding of adsorption.The regeneration capacity of spent material was also checked by using different eluents to know the reusability.The material was tested with real industrial wastewater and different spike wastewater to determine the practical utility.Due to high adsorption capacity, huge availability, non-toxicity, and superiority over other phytosorbent, the present material can be used as a potential scavenger for Methylene blue.

Chemicals and instrumentation
Methylene Blue(C 16 H 18 ClN 3 S) was procured from Merck, India (99% pure).NaOH (97% pure, Himedia, India) and HCl (98% pure, Himedia, India) have been procured from Avra Synthesis Pvt. Ltd.India.Redline Binder Oven was used to dry the sample.For shaking purposes, OS 100 orbital shaking incubator was used.Model R-8M Remi Benchtop Centrifuge was handled for centrifugation.Hitachi Double Beam Spectrophotometer (model U-2900) operated at k max ¼ 664 nm to take down concentration data of the Sample.Vanira LI 613 pH Meter was employed to measure Solution pH.The surface Morphology of biomaterial was studied by using Field Emission Scanning Electron Microscopy (Zeiss Gemini SEM 300).The functional group characterization of adsorbent was examined with Fourier Transform Infrared Spectroscopy (FTIR-Spectrum (Perkin Elmer)).Braunaur-Emmett-Teller (BET) surface area was evaluated in Nova Touch 2LX.

Material preparation (Neolamarckia cadamba fruit pulp -NCP)
Neolamarckia cadamba (Bur-Flower) tree is an evergreen and fast-growing tree bearing spherical flowers which blossom in monsoon.During the end of the monsoon, flowers are converted into fruit that has a pulpy center.Approx 50-60 cadamba fruits were collected from Talcher Town (20.9528N, 85.2334 E), Angul, Odisha.The outer green peel along with the seed was separated and a bulk amount of pulp was obtained.Then it was washed with tap water to remove left-out things attached to its surface.For extraction of yellowish-orange pulp, it was refluxed with distilled water (D.W) for about 8-10h.the pulp was collected separately and oven dried at 67 C (by using Redline Binder Oven) for 48h.The dried material was then ground with a commercially available grinder.It was sieved to get uniform particles.The material prepared from this process was named as NCP.The scheme of NCP material preparation was given below (Scheme 1).

FTIR
Figure 1(a) shows the FTIR spectra of NCP biomaterial before and after the adsorption of MB dye.A sharp peak was seen which represents the O-H stretching of polyphenolic moieties.Such a peak was found to shift from 3435 cm À1 to 3430 cm À1 which signifies a possible H-bonding interaction prevails between NCP and the dye molecule.A similar interaction was seen in Ipomoea carnea biosorbent for MB removal (Mathivanan et al. 2021) The peak at 3006 cm À1 signifies the aromatic C-H stretching.Peaks from 2857 cm À1 to 2851 cm À1 , and from 2927 cm À1 to 2921 cm À1 show the aliphatic C-H bond stretching.The sword-like intense peak at 1713 cm À1 indicates a C¼O stretching arising from the presence of the carbonyl group.It was slightly shifted to 1711 cm À1 after MB adsorption.It suggests the presence of either ester, ketonic, or aldehydic groups.The peak at 1627 cm À1 represents -OH bending.The peak from 1439 cm À1 and 1432 cm À1 were due to C-C stretching and shifting of these peaks to 531 cm À1 and 523 cm À1 to give information about MB dye loading.
BET surface area analysis BET surface area was analyzed by adsorption-desorption of N 2 gas onto adsorbent material.Figure 1(b) indicates the type (II) adsorption isotherm curve.Such type of curve is obtained for microporous adsorbent on which unlimited monolayer and multilayer adsorption takes place (Abebe et al. 2018).The flattened portion in the middle part of the curve shows the formation of the monolayer.The surface area NCP was 1.84Â10 À1 m 2 /g, and the pore volume was 8.11Â10 À4 cc/g.This is consistent with other phytosorbents reported (Qaiyum et al. 2022(Qaiyum et al. , 2023)).

FESEM
Figure 1(c-d) represents the FESEM image of NCP before and after stacking of MB dye.From Figure 1(c) it was observed that the NCP surface is rough, irregular, and heterogenous in structure.Micro and macro pores are also visible.After MB adsorption, the covered dye-loaded structure on the NCP surface was found as evident from a smooth and compact surface (Figure 1(d)).This confirms that the pores are responsible for MB uptake.

Batch adsorption studies
Adsorption study was carried out in Batches in 100 mL Erlemeyer Flask.0.1 g of NCP powder was added to MB solution (30 mL, 20 mg/L) and the solution was shaken in temperature controlled orbital shaker at 110 ± 5 rpm for 45 min.As the pH of the dye solution was 7.02, all experiments were done with this pH.After the experiment, samples were centrifuged at 3000 rpm for 3 min and the remaining dye concentrations have been evaluated from the standard calibration curve of MB.
The adsorption capacity of NCP and %Adsorption of MB are calculated based on Equations ( 1) and ( 2) respectively.
Where q e (mg/g) represents adsorption capacity.V is the MB solution volume (L) and m denotes NCP mass (g).Efficiency was calculated as follows: Efficiency ¼ Dye uptake in second run Dye uptake in first run Â 100 (3)

Effect of contact time
The MB adsorption capacity of NCP was tested for (0-60 min) at room temperature (Figure 2  The percentage adsorption of MB on the NCP surface decreases from 92.60% (10 mg/L) to 86% with increasing dye concentration (60 mg/L).Such changes are because the concentration of solution increased, and the number of dye molecules increased; there were no sufficient vacant sites available for adsorption for which competition may occur between the dye molecules on the NCP surface to interact.A similar finding was seen earlier (Ahmad et al. 2020).The maximum adsorption capacity was determined by taking 500 mg/L of MB dye.To this 0.1 g, NCP was added and stirred for 12h, and the capacity was calculated to be 115.60 mg/g.

Effect of pH
The pH effect (Figure 2(c)) was examined by varying the pH from 3 to 11; 0.1 M NaOH and HCl were used for pH adjustment.At lower pH 3, adsorption was less (84.80%) but it increases (97.20%) upon increasing the pH 11.The dye adsorption was well explained by zero point of charge (pH zpc ) in Figure 2(d).pH zpc is the pH value at which there are equal positive and negative charges distributed on the surface of the adsorbent.When the pH < pH zpc, the NCP surface is positive charge and it favors anionic dye to be adsorbed.If the pH > pH zpc , the NCP surface is negatively charged and it prefers positively charged (cationic dye) adsorbate for adsorption (Cemin et al. 2021).The pH zpc of NCP was calculated to be 3.56.The lowest MB adsorption (84.80%) was observed at a pH less than 3.56 because of the repulsion between the positively charged NCP surface and MB.Moreover, less adsorption of MB in an acidic medium may be due to competition of excess H þ ions and cationic dye for the active sites.MB dye removal was favorable in an alkaline medium at pH 11, the optimum MB adsorption value was found to be 97.20%.This can be understood by the reduction of H þ ions on the active sites and the predominant negatively charged surface of NCP, augmenting the electrostatic interaction between negatively charged bio-sorbent and the cationic MB dye (Carvalho et al. 2018;Alghamdi and El Mannoubi 2021).Since, drinking water's pH lies between 6.5-7.5, neutral pH was maintained in all the experiments.

Kinetic study
The adsorption mechanism can be well evaluated through different kinetic studies such as pseudo-first-order, pseudosecond-order, and intra-particle diffusion kinetic models.These are represented in Figure 3.The calculated rate constant values, maximum adsorption capacity, and correlation coefficient R 2 are noted in Table 1.Equation ( 4) represents the pseudo-first-order (Figure 3(a) rate equation: Here, q e and q t represent the amount of dye adsorbed at equilibrium and at time t.k 1 is the pseudo-first-order rate constant (min À1 ).
Pseudo-second-order kinetic model (Figure 3(b)) shows whether chemical sorption or physical sorption is taking place on the adsorbent surface.This model assumes that the rate-limiting step is chemisorption.The Equation ( 5) for the pseudo-second-order rate is given below.
K 2 (g/mg.min) is the rate constant of pseudo-second-order.
Weber & Morris proposed an intra-particle diffusion model to study adsorption kinetics.According to this model (Figure 3(c)), mass transfer happens through the pore diffusion process.The equation is given by, In Equation ( 6), k 3 (mg/g.min 1/2 ) refers to the intraparticle diffusion rate constant, and C refers to the boundary layer thickness which was drawn from the intercept and the slope of the linear plot respectively (Ullah et al. 2022).
The best-fitted kinetic model was selected according to the closeness of the correlation coefficient (R 2 ) value.It was seen that the pseudo-second-order kinetic model fitted best for MB adsorption onto NCP where R 2 is 0.99 suggesting chemisorption.However, intra-particle diffusion (Figure 3(c), Figure S1) shows three-step MB adsorption.Rapid bulk adsorption occurs in the first step followed by pore diffusion in the second step.In the third step, the equilibrium point is achieved.A similar study was also found in Citrus reticulata peel for the removal of safranin orange dye (Janu ario et al. 2022).Adsorption isotherm study Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherm models were used to put the view on the adsorption mechanism and focus on the interaction of dye molecules with the surface of the biosorbent at equilibrium (Figure 4, and Table 2).Langmuir isotherm model (Figure 4(a)) assumes monolayer adsorption on homogeneous surfaces (Foo and Hameed 2010).Adsorption occurs on a fixed number of active sites.Equation ( 7) is given below.
Where, C e , q e , and q max have usual meanings and b is Langmuir constant (L/g).The separation factor R L can be drawn from Equation (8), Freundlich Adsorption isotherm model (Equation 9) is not restricted to the formation of the monolayer.It is applied to multilayer adsorption on the heterogeneous surface.Figure 4(b) represents the Freundlich isotherm model.
Where k f is the Freundlich constant.n f is the heterogeneity factor that denotes whether bio-sorption is linear (n f ¼ 1), or favors the chemical process (n f < 1) or physical process (n f > 1).If the values of 1/n f < 1 then it will show Langmuir isotherm and if 1/n f >1 then it will be cooperative adsorption (Vargas et al. 2011).From Table 2, the value of n f > 1 indicates favorable adsorption and strong interaction between MB and NCP.
The Temkin model (Figure 4c) compliments the heat of adsorption and the interaction of adsorbate and adsorbent (Kini et al. 2014).Temkin model is based upon two assumptions.The first one is the heat of adsorption of molecules in the layers, which decreases linearly with the coverage; because of the interaction among adsorbate-adsorbent.The second one is the uniform distribution of binding energies (Koyuncu and Kul 2020).
Here, K T and B T are Temkin constants.K T value indicates equilibrium binding constant concerning maximum binding energy and K T values are interlinked with the heat of adsorption.
Dubinin-Radushkevich (D-R) Isotherm model (Equation 11; Figure 4(d)) describes whether the MB adsorption occurs physically or chemically.ln q e ¼ ln q max À K ad e 2 (11) In the above equation, q e and q max have their usual meaning.K ad is the D-R isotherm constant that denotes the  Pseudo-first-order (min À1 ) 1.81 0.10 0.98 Pseudo-second-order (g.mg À1 .minÀ1 ) 5.80 0.12 0.99 Intra-particle-diffusion (mg.g À1 .min 1/2 ) 5.74 0.18 0.93 free energy of adsorption per mole of the dye molecule.e is Polanyi potential (Balarak et al. 2021) that can be noted as Here, R is the universal gas constant (8.314JK À1 mol À1 ).This model is very useful to estimate the nature of adsorption with its free energy.Energy value < 8 kJ/mol signifies physisorption and 8-16 kJ/mol signifies chemisorption.(Balarak et al. 2021).In the present study, the E value is calculated by using Equation 13.
The adsorption of MB onto the NCP surface follows the Langmuir isotherm model (R 2 ¼ 0.99) having an adsorption capacity of 23.11 mg/g at 298K.R L values lie between 0.18 to 0.21, suggesting favorable adsorption onto NCP.Moreover, the MB uptake capacity was decreased from 23.11 mg/g to 18.62 mg/g with a rise in temperature from 298K to 328K suggesting the exothermic nature of MB adsorption.Different interactions between MB and NCP may cause a decrease in adsorption capacity (Samal et al. 2022, Aldawsari et al. 2021).From the Freundlich isotherm, "n f " values are between 1.47 to 1.90 confirming the favorable MB adsorption which is following favorable R L values.In the Temkin model, B T values lie between 3.68 to 4.94 kJ/mol, suggesting the physical nature of adsorption (Dey et al. 2022).From the D-R model, it was found that the energy value varies from 2.21 to 2.48 kJ/mol which implies the physisorption of MB onto the material surface.This is consistent with the calculated E value (2.25 kJ/mol) for Ciprofloxacin adsorption onto AFAC (Azolla filiculoides activated carbon) (Balarak et al. 2021).Another contemporary report suggests an E value of 2.75 to 9.65 kJ/mol, thereby confirming the validity of the model.

Thermodynamic application
The adsorption process depends upon different parameters such as Gibbs free energy change (DG), enthalpy change  (DH), and entropy change (DS).These parameters are calculated by using the equations given below.
DH and DS values are calculated from the slope and intercept of the Van't Hoff plot (Figure S2) respectively, and the DG is derived from Equation 16.From Table 3, it was observed that the DG value is negative (À3.27 kJ/mol to À2.27 kJ/mol) which clarifies that the adsorption process is spontaneous and thermodynamically feasible (Koyuncu and Kul 2020).The negative value of DH (À2.67 kJ/mol to À1.54 kJ/mol) suggests an exothermic process (Mohanta et al. 2020a(Mohanta et al. , 2020b;;Dey et al. 2022).Positive DS (1.80 kJ/K.mol to 2.45 kJ/K.mol)defines that the interfacial uniformness decreased during MB adsorption.A similar observation was found in alkali-assisted coconut fiber for the removal of methylene blue dye (Mohanta et al. 2021).

Statistical analysis
In any experimental tests, under the same conditions, different results may be obtained.The reproducibility of the adsorption of MB by NCP is one of the most important factors for establishing workability and checking validity.For that, all the experiments are conducted in triplet sets with ten replicates, and average data was used in this study.According to all results, the standard deviation (SD) varies from 0.010 to 0.011 and the percentage of relative standard deviation(%RSD) varies from 5.99% to 6.53%.The said statistical reliability check was performed with 30 mL, 20 mg/L dye solution with 0.1 g adsorbent material.The obtained result satisfies the workability and stability of the method (Piri et al. 2019).

Mechanism
MB dye adsorbed onto NCP through a combination of electrostatic interaction, p-p stacking, and hydrogen bonding as shown in Figure 5.At pH > pH zpc, electrostatic interaction occurs between the negatively charged NCP surface and N þ of MB dye.Moreover, the -OH group on the NCP surface is responsible for H-bonding, and p-p stacking occurs between polyphenolic moieties of NCP and phenyl rings of MB.Moreover, from the FESEM image, it was found that pores on the NCP surface were responsible for MB uptake.

Comparison of MB adsorption capacity with other phytosorbent materials
Wide varieties of phytosorbents were tested in the world for dye removal from water systems.In Table 4, NCP was compared with diverse biomaterials for efficient removal of MB.It was very clear from the consequence results of the  comparative table that NCP enjoys an edge with a high adsorption capacity for the removal of MB.

Effect of co-existing ions
Since wastewater contains many additive salts, it is important to identify the influence of co-existing ions on the selective adsorption of dye.The adsorption of MB in the presence of cations and anions such as Pb 2þ (1 mg/L), Hg 2þ (1 mg/L), AsO 4 3À (0.5 mg/L), and PO 4 3À (1 mg/L) was carried out.The influence of co-existing ions on the uptake of MB was shown in Figure S3(a).
The presence of the above-mentioned ions had a smallscale effect on the adsorption of methylene blue.The removal of MB in the presence of ions was 93% À 94.05% which was close to the removal percentage in the absence of ions.

Regeneration
Regeneration is important for Industrial use of the adsorbent material.A regeneration study can evaluate the potent ability and cost-effectiveness of the material (Janu ario et al. 2022).The desorption performance of MB from the NCP surface in Figure S3(b) was examined with 0.1M HCl, NaOH, NaCl, and (1:1) MeOH and it was seen that maximum regeneration happened in (1:1) MeOH solution with an efficiency of 73.48% due to better movement and higher solubility of the dye molecules within the pores (Samal et al. 2022).NCP also reused up to 3 cycles.After the third cycle, NCP shows only 37% efficiency.Thus, the material can be used in tech-economic prospectives.

Industrial sample analysis
To check the viability of NCP, it was used in industrial sewage collected from the nearby textile industries.7 mL of industrial effluent was taken and it was diluted for 5times.The pH of the effluent was maintained at 7.2.Then, 30 mL of it was taken for the experiment; to this 0.1 g, NCP was added and put in a shaker for 45 min.The concentration of effluent after adding NCP was compared with the concentration of effluent before the addition of material.44.70% adsorption was found which implies the practical use of NCP.Moreover, the efficiency of NCP was checked by using four spike wastewaters as distillation setup wastewater, RO wastewater, tube well water, and tap water, and their initial pHs were 6.72, 4.88, 9.44, and 7.25 respectively.20 mg/L dye was prepared with this spike water, and % of dye removal has been measured.The highest adsorption % was obtained in tube well water (90.83%) and the lowest in RO waste (81.86%) as shown in Figure S3 (c).

Conclusion
The non-edible Neolamarckia cadamba (Cadamba) fruit pulp was demonstrated for MB removal from wastewater.The FTIR data confirms the presence of phenolic and carbonyl functional groups which were responsible for hydrogen bonding between MB, and NCP material.The dye uptake via micro and macro pores of NCP was seen in the FESEM images.The material displays a maximum MB adsorption competency of 115.60 mg/g under ambient conditions.The adsorption process was fitted best in the pseudo-second-order kinetic model and Langmuir Isotherm model with R 2 ¼ 0.99.The material was successfully revived in a 1:1 methanol solution and could be reused for up to 3 cycles.The practical utility was determined by using real industrial effluent and spike wastewater.The adsorption mechanism is proposed as a combination of H-bonding, p-p stacking, and electrostatic interaction.Statistical reliability was confirmed from the relative standard deviation (5.99 to 6.53%) Its practical applicability, high adsorption capacity, and availability highlight it as a potential adsorbent for dye decontamination from wastewater.

Disclosure statement
The authors report there are no competing interests to declare.Data availability statement Data will be made available upon reasonable request.
Scheme 1. Material preparation and dye loading process.
Figure 2(b) portrayed the relationship between initial MB concentration (10-60) mg/L and adsorption percentage.

Figure 1 .
Figure 1.(a) FTIR spectra of NCP before (black line) and after (red line) MB adsorption, (b) Nitrogen adsorption-desorption BET curve of NCP, (c) FESEM image of NCP before and (d) after loading of MB dye.

Figure 5 .
Figure 5. Mechanism of MB adsorption onto NCP surface.

Table 4 .
Maximum adsorption capacity of various biomaterials for MB removal.