Adsorptive removal of phenol from aqueous solution using chemically modified Saccharum officinarum biomass: modelling and process optimisation

ABSTRACT This article proposes a way to extract phenol from aqueous solution as synthetic wastewater through the biomass (saccharum officinarum)-derived ZnCl2-activated carbon named as SBAC. The surface area and pore radius of SBAC were found to be 415.960 m2/g and 0.725 nm, respectively. Batch adsorption studies were carried out to analyse the percentage removal of phenol under various conditions, such as initial concentration (50–500 mg/L), initial pH (2–12), adsorbent dosage (2–40 g/L), temperature (283–303 K) and contact time (30–360 min). The proposed study has achieved ~94% of maximum phenol extraction, with the adsorption capacity of 9.44 mg/g at an initial concentration of 50 mg/L, dose of 5 g/L, pH of 5.5, contact time of 60 min and temperature of 303 K. The kinetic and equilibrium studies confirmed pseudo-second-order reaction and Radke-Prausnitz isotherm as the best fit. In thermodynamic study, the negative values of ΔG0ads (−21.448 to −24.330 kJ/mol), ΔH0 (−1.059 kJ/mol) and positive value of ΔS0 (0.072 kJ/mol K) confirmed the spontaneity, exothermic nature and randomness of adsorption process, respectively. Regeneration and reusability of SBAC showed 86.45% removal of phenol till fifth consecutive cycles. Cost analysis revealed that the developed SBAC was 14 times less expensive (~718 INR/ 9 $) than commercial activated carbon. From the present study, the developed adsorbent found to be good for wastewater treatment under ambient reaction conditions to achieve clean water.


Introduction
Organic compounds such as dyes, phenol, petroleum, surfactants, pesticides, pharmaceuticals and their derivatives have been considered as toxic pollutants in wastewater [1].Mainly, phenolic compounds are those organic compounds that serve as precursor to the variety of products and valuable substances.Phenolic derivatives are used in the production of plastics, epoxy, nylon, bakelite, polycarbonates, detergents, matrixectomy, carbolic soap, phenoxy herbicides, aspirin, pharmaceutical drugs, etc.
The exposure of phenol in large quantity cause several health hazards such as irritation of nose, eye, skin and dermatitis.Phenol and its vapours enter the body through the air, water and food, causing harmful effects on the liver, kidney, heart and central nervous system as well [2].Even a small amount of phenol in water can be noxious to certain marine species and causes taste and odour problems in drinking water [3].The major physical properties of phenol are graphically depicted in Figure 1.
According to WHO guidelines, the permissible limit for phenol in drinking water is 2 mg/ L, which forces researchers to focus on wastewater detoxification [4].Permissible limit for phenol in wastewater as per the recommendations of the Environmental Protection Agency is less than 1 mg/L [5].Permissible limit for phenol in drinking water as per Bureau of Indian Standards is 0.002 mg/L [6].Phenol can be extracted from wastewater using several methods before it gets discharged into the water bodies, such as solvent extraction [7], chemical oxidation [8], adsorption [9], biological treatment [10][11][12][13][14], precipitation [15], electrochemical incineration [16], photocatalytic degradation [17] and biodegradation [18].Many researchers are concerned about phenol due to its slow degradation, high water solubility and the potential for mobility in the environment.One of the most robust strategies for the extraction of phenol is adsorption procedure among others due to its uncomplicated methodology, profoundly proficient and financially savvy approach.
Adsorption is a surface phenomena where impurities get concentrated on the surface of adsorbent with physical and chemical interactions.Phenol and other organic contaminants and lower amounts of inorganic compounds were effectively extracted with less sludge production in the adsorption study.If the accessibility of adsorbent is very high, then there is no need for regeneration except disposal.Activated carbon [19] remains the most broadly contemplated adsorbent and is generally prepared from coal, wood and coconut shell.The operation and maintenance of the adsorption mechanism are convenient and can be quickly taken care of since operating costs are virtually low.The adsorbent cost, and the costs incurred for the regeneration of the adsorbent, may also be a large proportion of the total cost.Coal is conventionally used to produce activated carbon.As per the 2006 report, the production of activated carbon costs almost $2500 per metric ton [20].The activated carbon is costly due to the rise in demand and limited resources; researchers are seeking cost-effective adsorbents for phenol removal.In general, for the extraction of phenol and its chemical compounds, cost-effective activated carbon prepared from agricultural waste materials such as zeolite [21], Beet pulp [22], bagasse fly ash [23], Olive pomace [24], Green macro alga [25], tea waste [26], pistachio nutshell [27], orange peel ash [28], babul sawdust [29], banana peel [30], peanut shells [31], rice husk [32] and guava tree bark [33] were used.Carbon nanotubes grafted with copolymer of acrylic acid and acrylamide, polyethylene glycol (PEG), poly (trimesoyl, m-phenylenediamine) [34][35][36], magnitized graphene oxide nanoparticles and scoria stone [37,38] have also been used for the removal of phenol and its compounds from wastewaters.
The overall production of saccharum officinarum (sugarcane) in India during the period 2017-2018 is estimated at a record of 376.9 million tons, second only to Brazil.Maharashtra is second-highest producer of sugarcane in India (about 98.92 million tons) i.e. approximately 26.22% of the national sugarcane production [39].Nagpur is considered the second capital of Maharashtra and has the largest market in which sugarcane is abundantly available around the year.
The purpose of this research was to evaluate the effectiveness and efficiency of biomass-derived adsorbent produced from saccharum officinarum bagasse (SBAC) in the adsorptive treatment of phenol synthetic wastewater to decrease the environmental pollution load.In this study, the prepared SBAC has been characterised by X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy, Brunauer, Emmett and Teller (BET) surface area and scanning electron microscopy (SEM) to investigate phase purity and crystallinity, functionality, textural properties and morphology of the adsorbent, respectively.The batch adsorption studies were conducted to get a better knowledge of the adsorption mechanism involved.The effect of initial pH (pH 0 ), initial concentration (C 0 ), contact time (t), adsorbent dosage (m), agitation speed (rpm) and temperature (T) on phenol adsorption was studied.The performance of SBAC was compared with commercial activated carbon of different makers, and the cost estimation was also performed.The COD test, scale-up design, cost analysis, regeneration and reusability of the adsorbent have been presented in the subsequent sections.

Preparation of SBAC
Sugarcane bagasse was obtained from local market of Nagpur.The material was undergone thorough cleaning with tap water to eliminate the soil/residue, and afterwards it was subjected to sun drying for about 48 h.The dried sugarcane bagasse was grounded into fine particles and sieved for acquiring particles in the range of 850-300 µm.Sieved material was blended with 0.1 M zinc chloride solution (ZnCl 2 ) in 1:0.25 (g:mL) proportion.This mixture was placed in muffle furnace for carbonisation at a temperature of 673 K for 60 min.The obtained charred sample was washed many times by doubled distilled water (DDW) to make it free from the acid and dried at 378 K for about 3 h.The dried carbon passing through 600 µm sieve and retaining on 150 µm sieve (ASTM 11-70) was utilised for adsorption study.SBAC proximate analysis was conducted as per standard procedure IS 1350:1984.

Preparation of phenol stock solution
Phenol stock solution was prepared by diluting 1 g of phenol in 1 L of DDW to obtain a concentration of 1000 mg/L.By diluting the standard stock solution with DDW, batch adsorption experiments were performed on different initial concentrations.COD experiment was performed with the untreated and treated samples as per IS: 3025 (28) to measure the quantity of resulting phenol in the solution before and after adsorption.

Analytical measurement
A UV-vis spectrophotometer (UV-2450, Shimadzu, Japan) was used to analyse the absorbance of aqueous solution at maximum wavelength (λ max ) 269.5 nm referring to the phenol concentration.Standard solutions were prepared at a concentration of 1, 2, 3, 4, 5, 10, 20, 30, 40 and 50 mg/L by diluting the stock solution, and absorbances were noted down.A standard plotted graph of phenol concentration vs. absorbance was used to determine an unknown solution concentration.A linear variance of up to 50 mg/L concentration was seen in the standard chart.The effect of pH on adsorption was performed on µ controller-based pH system with electrode and temperature (Probe Type 361).

Batch adsorption study
Batch adsorption experiments were carried out to optimise several parameters including effect of dosage, initial pH, contact time, agitation speed, initial concentration, temperature, etc.The experiments were conducted by taking 50 mL of phenol containing aqueous solution with predetermined dosage of adsorbent in 250 mL Erlenmeyer flasks and were kept in the orbital shaking incubator [REMI, model: CIS 24-BL] at 303 K (except for temperature study) under 150 rpm.To determine optimum dose of adsorbent, a different dosage of SBAC (2 ≤ m ≤ 40 g/L) contacted with 50 mL of known phenol concentration was performed for adsorption until achieve its equilibrium.To analyse the impact of initial pH (2 ≤ pH 0 ≤ 12) on the adsorption, 0.1 N NaOH and 0.1 N HCl were used to alter the pH 0 of the solution.Adsorption kinetics was determined by studying the contact time (2-360 min) at different intervals.The temperature study was conducted in the range of 283 to 323 K and initial concentration (C 0 ) of phenol in the range 50 to 500 mg/L.The blank study was conducted by taking SBAC in 50 mL of DDW, and the remaining conditions were similar to the above experiments to investigate the colour leaching.The following equation gives the amount of phenol removed in percent and the adsorption capacity of SBAC at equilibrium conditions: Where, C 0 = Initial phenol concentration (mg/L) C e = Residual phenol concentration (mg/L) m = Quantity of SBAC (g) q e = Adsorption capacity of SBAC (mg/g) V = Phenol solution volume (L)

Characterisation methods
The X-ray diffraction (XRD) spectra of sample was recorded on XRD (X'Pert pro, PAN analytical), and the profiles were obtained by using X'celerator detector employing Cu-Kα radiation (λ = 1.54060Å) at 45 kV and 40 mA in the range of 10-100° with scan rate of 10 s per step with step size of 0.0170 at 40 mA and at 45 kV.The texture and surface morphology of SBAC was studied with the help of scanning electron microscopy (SEM) (JSM-7610 F, Japan).The pore size distribution was estimated with the help of Image J software.The Fourier transform infrared (FTIR) spectroscopy of SBAC was recorded on FTIR spectrometer (IR Affinity −1 , Miracle 10, Shimadzu, Japan) in the range of 500-2000 cm −1 .The surface area, pore size and pore volume of SBAC was studied by Brunauer-Emmet-Teller (BET) method using Quantachrome Nova touch 1.1 analyser by nitrogen adsorption at 77 K, and 0.35 g dry powder sample was degassed about 2 h at 200°C.

Adsorbent characterisation
The proximate analysis of SBAC revealed that the moisture, volatile matter, ash and fixed carbon content of the adsorbent were 5.65, 8.13, 20.88 and 65.33%, respectively.SBAC had high fixed carbon, which improves the absorption potential of the adsorbent.The above values of the adsorbent can be suggestive in the management of adsorbent after utilisation for the adsorption process.For example, less amount of moisture and volatile matter content gives more calorific value, the amount of ash content of adsorbent can be utilised for the decision on management of the ash after utilisation and the high content of fixed carbon can give a better calorific value of the material, which can be disposed-off by incineration, subject to the less emissions of some toxic substances in the air.
The XRD profile of SBAC is shown in the Figure 2, and the pattern provides evidence that it was amorphous in nature, as indicated by the two major diffraction peaks at 2θ of around 25.14° attributed to hemicellulosic hydroxyl, carboxylic, aldehyde and ketone groups and 44.94° attributed to lignin components, which were commonly found for XRD spectrum of biomass-derived activated carbon [40].
The SEM study revealed the heterogeneous morphology with numerous pores present on the surface having tendency to enhance the adsorption process (Figure 3(a,b)).To investigate further, the sizes of the pores were measured, and the size distribution of the same is provided in Figure 3(c).Interestingly, the pore structure referring to the high surface area disappeared after phenol adsorption (Figure 3(d)).Subsequently, the smoother and even surface was observed for the same [41][42][43][44][45].ZnCl 2 chemical activation has resulted in the formation of new micropores and expansion of existing micropores, resulting in an increase in volume and surface area of the micropores, and the pores were generated during carbonisation as a result of ZnCl 2 evaporation.ZnCl 2 activation developed enormous quantities of micropores, which helped in the removal of pollutants from the solution [46].
The N 2 adsorption-desorption experiments were performed to evaluate the surface area using BET-BJH method.According to IUPAC, the adsorption-desorption plot approaches type-II isotherm, indicating multilayer adsorption phenomenon (Figure 4).The obtained surface area of SBAC was 415.96 m 2 /g, average pore radius was 0.7252 nm and average pore volume was 0.1285 cm 3 /g.FTIR spectrum of SBAC before and after adsorption is shown in Figure 5.There was a dip in the spectral bands at 3856 and 3714 cm −1 ascribed to -OH functional groups phenol, alcohol and carboxylic acid [47], at 2356 cm −1 due to O-C=O [48], at 1681 cm −1 due to C=O vibrations [49], at 1542 cm −1 due to aromatic C=C [50], at 1175 cm −1 due to ν C-O phenols, ethers [51], band at 874 cm −1 represents isolated hydrogens; a band at 808 cm −1 , two adjacent hydrogens; a band at 746 cm −1 , four adjacent hydrogens; and a band at 679 cm −1 , five adjacent hydrogens [52].
Owing to the observed surface area and microporous structure of the SBAC, there obtained an efficient adsorption of phenol.

Effect of SBAC dosage
Figure 6(a) refers to the effect of SBAC dosage for the removal of phenol at ambient reaction conditions (C 0 = 50 mg/L, V = 50 mL, pH 0 = 5.5, T = 303 K, t = 60 min, and rpm = 150).Phenol removal increases quickly with the rise in SBAC dosage up to 5 g/L, and there was a marginal difference for 5 ≤ m ≤ 40 g/L of SBAC.The rise in phenol removal with SBAC dose is due to the increment in surface active sites, so the more active surface sites, more adsorption takes  place.Adsorbent capacity q e , on the other hand, declines with the rise in SBAC dosage at constant phenol concentration, and constant volume causes saturation in the adsorption procedure [29,53].Since there was a nominal increase in the percentage removal after 5 g/L dosage, 5 g/L was regarded as the optimum dose, and the later experiments were performed using 5 g/L as SBAC dose.removal from 90.26 to 91.96%, then it drops to 90.05% when the pH rises to 12 but maximum phenol removal occurred at about 93% at pH 0 5.5 of the solution, then further experiments were conducted at natural pH.The pH at which adsorbent surface posses zero charge is known as point of zero charge (PZC).PZC of SBAC takes place at pH 6 as shown in Figure 7(a).Decreasing the pH < 6 causes positive charge and increasing the pH > 6 causes adsorbent surface negative on the adsorbent surface.Phenol was a very weak acid with a pK a value of 9.89, which was dissociated if the pH value exceeds pK a , and at low pH, it was mainly in molecular state as shown in log C-pH graph Figure 7(b).With higher pH values, the removal of phenol decreases because of phenol ionisation and repugnance between phenolate anions and negative SBAC sites [54].At lesser pH (<PZC), phenol molecules get easily attached onto the negative SBAC surface, and moreover, the pH of the phenol is less than the PZC i.e. 5.5 < 6.As a result, at pH 5.5, the maximum removal of phenol takes place [30].

Effect of contact time
Figure 8(a) illustrates the effect of contact time on phenol removal by SBAC at concentration C 0 ranging from 50 to 200 mg/L and m = 5 mg/L at 303 K to determine the equilibrium contact time.Phenol adsorption increases rapidly with the increasing contact time due to active surface sites available on the SBAC to reach the equilibrium state.Once it attains the equilibrium, there was no significant removal as a result of the existence of repulsive force between the adsorbent surface and phenol cause some surface sites of adsorbent remain unoccupied.The test was conducted for 360 min, and it was observed that adsorption of phenol was fast in the first 60 min, later there was a marginal difference in the removal of phenol [55].For C 0 = 50 mg/L, about 94% of phenol adsorbed in 60 min, for C 0 = 100 mg/L, about 88% of phenol adsorbed in 60 min and for C 0 = 200 mg/L, about 84% of phenol adsorbed in 60 min of contact; afterwards the phenol adsorption is very slow.However, with increasing concentration of phenol, the adsorption of phenol reduces at any time t, and adsorptive capacity q t (mg/g) rises [54].Based on the results, further tests were performed at time t = 60 min.

Kinetic study
The kinetic study measures the rate at which phenol was adsorbed on the surface of SBAC.Tests were performed for initial concentration C 0 ranging from 50 to 200 mg/L with m = 5 mg/L at 303 K under continuous mixing at various time interval.

Pseudo first order and pseudo second order models
The pseudo first order equation [56]: Where, q t is the amount of phenol adsorbed at time t (mg/g) q e is the amount of phenol adsorbed on SBAC at equilibrium (mg/g) k f is pseudo first-order rate constant (min −1 ) t is time (min) q t values are found from the graph log (q e -q t ) vs. t shown in Figure 8(b), and the k f values for C 0 = 50, 100 and 200 mg/L were 0.011, 0.003 and 0.012 min −1 , respectively, shown in the Table 1.
The pseudo second-order equation [56]: Where k s = pseudo second-order rate constant (g/mg min).q e value calculated from the graph of t/q t vs. t is shown in Figure 8(c) at C 0 = 50-200 mg/L.h and q e values were measured from the slope, k s was calculated from the h value and is shown in Table 1.The values of q e, exp, and q e, cal of the pseudo secondorder kinetic model is almost equal, and the R 2 value is unity compared to the pseudo first-order kinetic model.The unit value of R 2 and the similarity between experimental and calculated adsorption capacities (q e ) agrees the validity of model for the adsorption of phenol onto SBAC.The adsorption of phenol on SBAC is most likely the monolayer with chemical process controlling the adsorption rate [34,35].

Weber-Morris intra-particle diffusion equation
The intraparticle diffusion model equation [57]: Where k id = intraparticle diffusion rate constant (mg/g min 1/2 ) C = constant (mg/g) The Weber-Morris plot of q t versus t 0.5 satisfies the linear relationship with the experimental results; then only intra-particle diffusion regulates the sorption mechanism.Figure 8(d) illustrated that phenol removal onto SBAC was not only controlled by intraparticle diffusion, since the obtained plot was not passing through the origin, and two distinct slopes were formed.The first linear segment describing mesopore diffusion and the second reflecting micropore diffusion and comparable results were reported [58].Parameters were measured from the plotted graph and reported in Table 2.

Effect of agitation speed
Agitation speed effect on the process for C 0 = 50 mg/L at varying speed of 50, 100, 150 and 200 rpm at 303 K at varying contact time is represented in Figure 9(a).The adsorption of phenol improves from 50 to 150 rpm on account of increased turbulence, and as an outcome, the lessening thickness of the boundary layer over the adsorbent particle shoots up the mixing.Higher adsorption capacity occurred at the speed of 150 rpm and after that there was no critical increment in adsorption due to the disassembling of activated carbon.The investigation of phenol removal proceeded for 360 min at 50 rpm and 100 rpm, where the equilibrium was achieved at 180 min.At 150 rpm, maximum removal occurred at 60 min, and at 200 rpm, maximum removal occurred at 120 min.However, there was no noteworthy increment seen in the phenol adsorption beyond contact time where maximum removal occurred w.r.t agitation speed.Thus, equilibrium speed and contact time for further analysis was optimised under 150 rpm and 60 min, respectively.

Effect of initial concentration and temperature
Temperature (283 K ≤ T ≤ 323 K) and C 0 (50 ≤ C 0 ≤ 500 mg/L) effect on the adsorption and equilibrium capacity of phenol by SBAC at m = 5 mg/L and t = 60 min studied, and outcomes are illustrated in Figure 9(b).The results revealed that there was decreasing phenol removal with increasing C 0 because at a fixed quantity of adsorbent  adsorbs same or fixed amount of phenol, and SBAC adsorptive capacity q e (mg/g) also increases with the increment of C 0 because of less resistance to phenol mass transfer from the solution [59]. Figure 9(b) additionally showed with an increment of temperature from 303 to 323 K, the extraction of phenol from the solution decreased because phenol molecules have a large kinetic energy at high temperatures, and binding to the SBAC surface was difficult.

Adsorption isotherms and error analysis
Adsorption equilibrium analysis has been conducted at an equilibrium temperature and time to find the relation among amount of phenol adsorbed on SBAC surface and remaining in solution.By creating a systemic link between equilibrium curves, the adsorption mechanism for phenol removal can be optimised.Isotherm models like Langmuir, Freundlich, Temkin, Redlich-Peterson, Toth and Radke and Prausnitz isotherm have been studied to discuss equilibrium characteristics of the adsorption process.

Langmuir isotherm model
The Langmuir adsorption isotherm determines the relationship between the number of active sites on the surface of adsorbent and concentration of adsorbate species in the solution at equilibrium [60].
Where C e is the equilibrium phenol concentration (mg/L), K L is the adsorption energy (L/ g), q e is the amount of phenol adsorbed onto SBAC at equilibrium (mg/g) and q m is the monolayer maximum adsorptive capacity (mg/g).The C e vs. C e /q e graph was plotted (Figure S1) for calculating above constants and are shown in the Table 3.The q m values increased from 63.291 to 81.967 mg/g and then decreased to 75.188 mg/g with increase in temperature of the solution from 283 to 323 K, which described that the adsorption of phenol onto SBAC was an exothermic process.The Langmuir adsorption coefficient, K L , was found to be 0.030, 0.032, 0.034, 0.032 and 0.030 L/mg at 283, 293, 303, 313 and 323 K. Langmuir constant, b, reflects the affinity between adsorbent and the adsorbate, whose value illustrated strong binding of phenol ions onto SBAC.The favourability can be expressed in terms of a dimensionless constant called separation factor, R L , which is defined by the following relationship R L value describes the feasibility of adsorption process.If R L > 1, the adsorption process would be unfavourable; R L = 0, the adsorption would be irreversible, whereas 0 < R L < 1 indicates the adsorption process as energetically favourable.The separation parameters, R L , for phenol adsorption were 0.889, 0.912, 0.913, 0.907 and 0.903 at 283, 293, 303, 313 and 323 K, were less than unity, indicating that SBAC was the appropriate adsorbent for phenol ions [61].

Freundlich isotherm model
Freundlich isotherm can be applied to an adsorption system when the numbers of adsorption sites exceeds the number of contaminant molecules/ions.The isotherm thus describes multilayer and physical adsorption over the heterogeneous surface [62].
Where K F is the capacity of adsorbent (L/g) and 1/n is the iIntensity of adsorption.The graph of ln C e vs. ln q e was plotted (Figure S2) for calculating above constants and are shown in the Table 3. K F is a coefficient that represents the quantity of adsorbed phenol from a solution having unit concentration and indicates the relative adsorption capacity of an adsorbent.Freundlich constants, n, is exponent, which is the measure of adsorption intensity or surface heterogeneity, whose value lies in the range 1-10 for favourable adsorption phenomenon.K F values for phenol come out to be 5.  [63].The regression coefficients for straight line plots were around 0.99 at five test temperatures, which suggested better fitting of the adsorption data to Freundlich model as compared to Langmuir isotherm.Moreover, Freundlich model describes adsorption on solid heterogeneous surface, and better fitting of adsorption data suggested SBAC surface to be heterogeneous.The better fitting of adsorption data to Freundlich isotherm indicated that the adsorption of phenol preferably followed multilayer and heterogeneous adsorption process.

Temkin isotherm model
The derivation of Temkin adsorption isotherm assumes that the fall in the heat of adsorption is linear and not logarithmic as implied in Freundlich isotherm discussed above.This sorption isotherm contains a factor, which explicitly takes into account the interaction between adsorbate and adsorbent.Due to interactions between adsorbent and adsorbate, the heat of sorption of adsorbing ions in the layer decreases linearly with the coverage of adsorbent surface [64].Where B is the Temkin energy constant (kJ/mol) and K T is the constant describing the interaction between phenol molecules and SBAC surface.Graph ln C e vs. q e was plotted (Figure S3) for calculating above constants and are shown in the Table 3. Temkin constant B values were 12.133, 13.080, 15.630, 14.800 and 14.39 kJ/mol at 283, 293, 303, 313 and 323 K, respectively.It has been reported that the typical range of bonding energy for ionexchange mechanism is 8.0-16 kJ/mol, and low values in this study indicates a weak interaction between phenol ion and SBAC, supporting an ion-exchange mechanism for the present study.These results complemented Langmuir and Freundlich isotherm results.However, the regression coefficient values ranges between 0.973 and 0.989, indicating poor fitting of adsorption data to Temkin isotherm.Non-fitting of data may be due to energetic homogeneity of adsorption sites on the surface of SBAC [65].

Toth isotherm model
Toth isotherm model is given by the equation [66]: Where q Th is the adsorption capacity parameter in Toth isotherm (mg/g), K Th is theIsotherm constant ((mg/L) Th ) and Th is the dimensionless constant.Graph C e Th vs.
(C e /q e ) Th was plotted (Figure S4) for calculating above constants and is shown in the Table 3. Th values ranges from 0.261 to 0.461, K Th values ranges from 0.160 to 0.278 and q Th ranges from 105.948 to 299.655 mg/g at different temperature varied from 283 to 323 K.

is3.8.5. Redlich-Peterson isotherm model
Redlich-Peterson (R-P) isotherm model is given by the equation [67]: Where K R (L/g) and a R (L/g) were R-P isotherm constants and β constant ranges from 0 to 1. Graph ln C e vs. ln (K R *C e /q e -1) was plotted (Figure S5) for calculating above constants and is shown in the Table 3. K R values were 2.3, 6.737, 6.9, 6 and 5.2 L/g at 283, 293, 303, 313 and 323 K. β values 0.549, 0.674, 0.527, 0.655 and 0.662 at different temperature lies between 0 and 1, which showed better adsorption of phenol.

Radke-Prausnitz isotherm model
The isotherm model of the Radke and Prausnitz was given by the equation [68]: Where K RP is the constant L/g, k rp is the constant [(mg/g)/(mg/L) 1/P ] and P is the dimensionless constant.The Radke and Prausnitz constants were calculated from the graph C e P vs. C e /q e (Figure S6) and reported in the Table 3. P values were 0.792, 0.672, 0.710, 0.729 and 0.727 and K RP values were 0.158, 0.560, 0.294, 0.250 and 0.240 L/g at different temperature ranges from 283 to 323 K. R 2 values at all temperatures showed 0.999, indicating that SBAC was the appropriate adsorbent for phenol removal from aqueous solution [69].
Values of all isotherm model constants, error functions and coefficients (R 2 ) to fit the data at several temperatures are presented in Table 3.The regression coefficient (R 2 ) was more than 0.97, so consideration of error function values will be crucial.From the table it was found that adsorption equilibrium data fits best in the sequence of Radke Prausnitz isotherm-Redlich Peterson-Freundlich-Toth-Langmuir and Temkin.Figure 10 shows the comparison of isotherm model for phenol adsorption onto SBAC.

Error analysis
this study, different functions for error analysis were employed to verify the best-fit isotherm.For the closeness of the experimental and calculated values, q e, exp and q e, cal were compared for all of the other error functions [70].The following detailed descriptions for error analysis functions used throughout the study were 'sum of the squares of errors (SSE), average relative errors (ARE), sum of the absolute errors (SAE), Marquardt's percent standard deviation (MPSD) error function, Chi-square error function (χ 2 ), hybrid fractional error function (HYBRID), which have been explained earlier' [30].The hybrid fractional error function [71], MPSD error function [72], and Chi-square error function (χ 2 ) [73] explained earlier researchers.

Thermodynamic analysis
The free energy change of Gibbs is given by the equation [74]: ads , ΔH 0 and ΔS 0 were measured from the Van't Hoff graph ln K ads vs. (1/T) (Figure S7) and represented in the Table 4, using the equation: The physical adsorption is indicated by the values of ΔG 0 ads in the 0 to −20 kJ/mol range and chemisorption is indicated by the values in the −80 to −400 kJ/mol range [75].The ΔG 0 ads values and driving force increases with the rise in temperature, i.e. higher negative values of ΔG 0 ads , higher driving force.All the values of ΔG 0 ads were obtained in the range of −21.448 to −24.330 kJ/mol in the current analysis, suggesting that the process approaches physisorption followed by chemisorption.The negative ΔH 0 (−1.059kJ/mol) values suggest the exothermic existence of SBAC's phenol adsorption mechanism [76].The positive ΔS 0 (0.0720 kJ/mol K) values confirm the higher stability of the phase of adsorption.

Mechanistic study
The feasible mechanism is based on a thorough understanding of all conceivable physical and chemical interactions between the substrate (phenol) and adsorbent (SBAC), as evidenced by characterisation data and experimental results.Due to van der Waals forces, chemical affinity, electrostatic attraction and a number of different functional groups, such as hydroxyl, carbonyl, lactone, ketone, aldehyde, etc., are present on the surface of activated carbon and contribute to the formation of physical and chemical bonds with phenol during the adsorption process [63].The emulous behaviour of ZnCl 2 -treated activated carbon could be attributed to additional chemical interactions via Lewis acid catalysing reactions (Figure 11) between nucleophilic groups and aromatic hydrocarbon, ketone or aldehyde groups [77].

Design of batch sorption system from isotherm data
A batch adsorption process was designed by using isotherm study [69].It gives the greater understanding of the amount of adsorbent needed in industrial applications to achieve the optimal efficiency when handling phenol wastewater.The schematic diagram for the single-stage batch sorption model for phenol onto SBAC is shown in Figure S8, where M a is the SBAC mass (g), V is the phenol volume (L), C 0 and C 1 are the initial and final concentration of phenol (mg/L), q 0 and q 1 are adsorbent capacity SBAC (mg/g) for blank and phenol loaded adsorbent.
The mass balance equation is: Equation ( 11) can be updated and rearranged as follows for the equilibrium conditions C 1 = C e , q 1 = q e , and q 0 = 0 for fresh SBAC: Since the Radke-Prausnitz isotherm was the ideal isotherm for phenol adsorption on SBAC, so Radke-Prausnitz isotherm equation was given by: Substitute equation ( 13) in the equation (12) and rearrange the equation: The value of M a can be determined using equation (14) for the different volumes of phenol solution.Table 5 shows the mass of SBAC required in (g) for a batch sorption process with varying volumes of phenol solution at 303 K and removal efficiencies of 50, 60, 70, 80, 90 and 95% respectively, for an initial concentration of 50 mg/L.

Desorption studies
Desorption, as well as the viability of the adsorption and desorption behaviour of the SBAC, was studied by using HCl, NaOH, H 2 SO 4 , hot water and NH 3 solutions at 303 K.
Desorption study was carried out by taking 0.25 g of phenol-loaded SBAC mixed with 50 mL of eluents, and it was agitated in shaker at 303 K about 60 min.It was found that NaOH solution gave better results compared to other eluents shown in the Figure 12(a), which describes that when phenol interact with NaOH solution, it forms the soluble salt C 6 H 5 O − Na + , which aids phenol desorption from the SBAC [78].The mechanism that causes higher desorption for NaOH solution comparative to other solutions were explained somewhere else [79].The desorption was carried out up to 180 minutes, and it was rapid up to 90 minutes after that there was a minor shift in the desorption.It was observed that desorption of phenol was fast at 0.2 N NaOH, later there was a marginal difference as show in the Figure 12(b).To test the adsorbent regeneration potential, five consecutive adsorption-desorption cycles were performed with recycled activated carbon as the regenerated adsorbent and 0.2 N NaOH as the effective agent as shown in Figure 12(c,d).

Chemical oxygen demand (COD) test
In this study, COD test was conducted to measure the amount of phenol, or its other forms, present in the aqueous solution.The percentage removal COD of original and treated phenol sample was ~94% that means amount of organic matter was removed ~94% and the phenol removal through adsorption was about 94%.Study revealed that SBAC works efficiently to remove the phenol from the aqueous solution, and it showed that there were no formations of secondary products of phenol.

Comparative study of SBAC with other biomass
All the experiments were carried out in triplicate to determine the standard deviation and reproducibility of data in order to compare the efficiency of the produced adsorbent, and the better results were compared with the previously recorded adsorbents in Table 6.The table shows that SBAC's phenol adsorption efficiency was substantially greater than that of the other adsorbents.

Cost analysis of SBAC
The current study attempted to assess the cost of adsorbent such as chemically treated sugarcane bagasse.Table S1 shows the costs of each activity including physical and chemical activation processes, as well as the total cost for adsorbent preparation.For SBAC, the total cost of producing 1 kg of adsorbent was calculated to be INR 718.525.The cost of activated carbon was obtained from a commercial suppliers listed below in the Table S2.

Conclusions
The current research focused on the synthesis and characterisation of saccharum officinarum activated carbon (SBAC) utilisation for phenol adsorption from aqueous solution as synthetic wastewater.The grain size used for the study of SBAC ranged between 150 and 600 µm.The proximate analysis revealed that SBAC contains ~65% of fixed carbon, which ameliorates the adsorption capacity of SBAC.The developed SBAC was further characterised by powder X-ray diffraction for phase purity and crystalline, FTIR analysis to validate functional groups, N 2 adsorption-desorption isotherm to study its textural properties and SEM analysis for morphological study.The obtained surface area of SBAC was 415.96 m 2 /g with heterogeneous surface structure and numerous microsize pores indicating effective results towards adsorption.The optimum conditions found through batch adsorption study for the maximum removal of phenol, i.e.50 mg/L phenol content, 60 min contact time, 5 g/L dosage, 303 K temperature, 150 rpm agitation speed, at the pH 5.5.The maximum phenol removal was found to be 94.4% with the adsorption capacity of 9.44 mg/g under optimised conditions.The equilibrium data was fitted well to the Radke-Prausnitz isotherm model.The pseudo second-order kinetic model showed the best fit to the kinetic data resulting in the similar experimental and calculated q e values.The obtained thermodynamic results exhibited that the process was random, spontaneous and exothermic in nature with negative values of ΔH 0 (−1.059(kJ/mol) and ΔG 0 ads (−21.448 to −24.330 kJ/mol).The COD test results revealed that the reaction was carried out effectively without forming any secondary products of phenol.Desorption studies confirmed that the SBAC was reusable even after five consecutive cycles and the removal efficiency maintained more than 86%.Cost analysis revealed that the developed activated carbon was 14 times less expensive than commercial activated carbon.Hence, the chemically modified saccharum officinarum bagasse was the best low-cost adsorbent for the removal of phenol from synthetic wastewater.The SBAC can be the best solution for the removal of pollution from wastewaters.

Figure 3 .
Figure 3. (a and b) SEM micrographs of unloaded SBAC, (c) Pore size distribution graph, and (d) SEM micrograph of phenol loaded SBAC.

Figure 6 (
Figure 6(b) refers to the effect of initial pH on the phenol adsorption onto SBAC at a temperature of 303 K for m = 5 g/L, C 0 = 50 mg/L and t = 60 min studied at 2 ≤ pH 0 ≤ 12. Increasing the pH 0 of the solution from 2 to 6 results in a rise in phenol

Figure 5 .
Figure 5. FTIR spectra of SBAC before and after adsorption of phenol.

Figure 9 .
Figure 9. (a) Agitation speed effect on phenol adsorption at T = 303 K, C 0 = 50 mg/L, V = 50 mL, pH 0 = 5.5, m = 5 g/L and the maximum removal takes place at 150 rpm, and (b) Temperature and C 0 effect on the removal phenol at pH 0 = 5.5, t = 60 min, rpm = 150, m = 5 mg/L and the maximum removal takes place at C 0 = 50 mg/L and at 303 K.

Table 5 .
Mass of SBAC (g) for various amounts of phenol (L).