Accepted Manuscript Title: Adsorptive potential of agricultural wastes for removal of dyes from aqueous solutions Author: Hemant Singh Garima Chauhan Arinjay K. Jain S.K. Sharma PII: DOI: Reference:
S2213-3437(16)30422-5 http://dx.doi.org/doi:10.1016/j.jece.2016.11.030 JECE 1343
To appear in: Received date: Revised date: Accepted date:
29-8-2016 20-11-2016 22-11-2016
Please cite this article as: Hemant Singh, Garima Chauhan, Arinjay K.Jain, S.K.Sharma, Adsorptive potential of agricultural wastes for removal of dyes from aqueous solutions, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2016.11.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ADSORPTIVE POTENTIAL OF AGRICULTURAL WASTES FOR REMOVAL OF DYES FROM AQUEOUS SOLUTIONS Hemant Singh, Garima Chauhan*, Arinjay K. Jain*, S. K. Sharma University School of Chemical Technology, Guru Gobind Singh Indraprastha University, New Delhi-110078 *Corresponding author: [email protected] [email protected]
Highlights : 1. Novel green adsorbents were explored for removal of dyes from waste water. 2. Proposed an economic incentive to industrial practice for waste management. 3. Possibility of mass transfer resistance and kinetic aspects was investigated. 4. More than 90% dye removal was obtained for all investigated dye-adsorbent systems. 5. Isotherms and kinetics were examined to look into adsorbate-adsorbent interaction. 1
Abstract Present work aims to investigate the adsorptive characteristics of agricultural wastes (Citrus Limetta Peel and Zea Mays Cob) for effective removal of dyes from aqueous solutions. Batch adsorption experiments were carried out in order to analyse sorption behaviour of dye-adsorbent systems at different adsorbent dosage and initial dye concentration. Possibility of mass transfer resistance was investigated to improve the diffusion rate, whereas kinetic aspects were examined to achieve thermodynamic equilibrium for the proposed adsorption process. Solution pH was observed to significantly affect the adsorption efficiency by regulating degree of ionization of the adsorbate’s functional groups. More than 90% removal of dyes was attained for all dye-adsorbent systems under optimum reaction conditions. Characterization studies were performed to examine the changes in morphology and functional groups of the adsorbents before and after adsorption process. Kinetic study suggested the pseudo-second kinetic model with normalized standard deviation Δqt (%) < 5% and regression coefficient > 0.999 as being able to better describe kinetic data than pseudo first order and elovich kinetic models. Adsorbateadsorbent interaction was investigated by looking into the applicability of Langmuir, Freundlich and Dubinin-Radushkevich isotherms for the proposed adsorption process. Maximum adsorption capacity Q° was observed to be highest for malachite green dye (8.733 mg/g and 16.72 mg/g) using CLP and ZMC respectively. Findings obtained from thermodynamic studies indicated endothermic and spontaneous nature of the proposed process. This work offers an economic incentive to the industrial practice for waste management and eofriendly approach for removal of toxic dyes from textile waste water. 2
Keywords Green adsorbents, equilibrium studies, kinetics, thermodynamics, characterization
1. Introduction Unprecedented growth of industries, in spite of playing an important role in mankind welfare, has left a significant negative impression on the ecosystem. Many industries such as textile, paint, leather etc. use dyes for colouring their products and large
volume of effluents containing several dye organics is heedlessly discharged into the surface water bodies without any prior treatment. Presence of dyes, even at small amounts (<1 mg/L) in the industrial effluent, is a sincere matter of concern for both toxicological and esthetical reasons . Dyes can affect the biological metabolism process by interfering the transmission of sunlight through water . Also dyes have tendency to sequester metal ions and thus may engender toxicity to aquatic life. Mutagenic and carcinogenic nature of dyes may impart hazardous health effects such as dysfunction of kidneys, central nervous system and reproductive system. Several biological, physicochemical and electrochemical treatment methods have been reported in literature [3-6] however high process cost, secondary pollution possibilities, use of toxic reagents and long process time confine the use of these conventional methods at industrial scale. Therefore, a strong need is felt to develop an effective eco-friendly process for the removal of dyes from industrial wastewater. Adsorption process has been found to be superior among the existing processes for water treatment due to high efficiency, flexibility, ease of operation, insensitivity towards toxic pollutants and economic feasibility [1,7,8]. Activated carbon (AC) has shown considerable good dye removal efficiency [9,10] though practical application of AC is limited due to problems associated with its regeneration or disposal, sludge production and economic feasibility [11,12]. Recently, agricultural and industrial wastes [13-17] have dictated the researcher’s interest owing to their wide availability, low cost, less commercial value and biodegradable properties. Babalola et al.  investigated the adsorption efficiency of a plant waste Cedrela odorata Seed Chaff (COSC) for the adsorption of industrial dyes from aqueous solutions. Microstructures of COSC indicated presence of various organic moieties which are responsible for efficient adsorption of toxic dyes. Unuabonah et al.  performed continuous 4
adsorption studies in a fixed bed reactor using a novel hybrid adsorbent (a combination of Carica papaya seeds and Kaolinite clay) and reported breakthrough adsorption capacity of 35.46 mg/g for the adsorption of methylene blue dye. Regeneration possibilities, significant adsorption capacity and economical feasibility make this proposed hybrid adsorbent as a promising alternative. Tree waste Pentaclethramacrophylla tree bark and Malacanthaalnifolia tree bark were employed as low-cost adsorbent for the removal of toxic dyes and heavy metals (Cd 2+, Pb2+) from aqueous solutions . Adsorption efficiency of the adsorbents was optimized by varying various process parameters such as pH, biomass dose, initial solute ion concentration, agitation time and temperature . Biosorption potential of abundantly available waste biomass Zea mays seed chaff was investigated for the removal of heavy metals (Cr3+,Cd2+ and Pb2+) from aqueous solutions. Authors suggested endothermic nature of the biosorption of these metals with large positive entropy values . Adsorption potential of lignocellulosic material (native and modified form) was investigated for the effective removal of Direct Yellow 50 dye from aqueous solution under batch and continuous mode . Modified biosorbents demonstrated the enhanced biosorption capacity than native and immobilized forms. Polyethylenimine treated peanut-husk biomass was also illustrated as an efficient biosorbent for the removal of Indosol Black NF and Indosol Orange RSN dyes from aqueous solutions and desorption study were conducted to look into the possibility of regeneration of bioadsorbent . Maximum dye removal (58.01 mg/g) was achieved at 200 mg/L initial dye concentration, pH 2.0, and 0.17 g peanut husk adsorbent dose by optimizing the process parameters using response surface methodology .
Present study explores the adsorption potential of Citrus Limetta Peel (CLP) and Zea Mays Cobs (ZMC) for the removal of three different textile dyes (Malachite Green (MG), Methylene Blue (MB), Congo Red (CR)) from synthetic samples. Citrus Limetta (Sweet Lemon) and Zea Mays (Corn) are considered important fruit and cereal crops respectively across the globe and are produced in more than 100 countries. Nearly 115 million ton production of citrus across the globe was reported in year 2012 which is predominantly contributed by China, Brazil, USA and India . Zea Mays is currently the second most abundant crop globally [26,27] and is predicted to surpass both wheat and rice to become the number one grain at global scale by 2020 . Nearly 968 million tons of corn production is reported globally in year 2015-16 in which USA contributes to more than 35% of total production . Consequently, large amount of sweet lemon peels and corn-cobs are produced every year from these crops. These agricultural by-products are structurally composed of cellulose, hemicellulose, lignin and protein content, therefore their direct disposal in ecosystem may increase the biological oxygen demand of water and putrefaction of biomass . To the best of our knowledge, removal of MB, MG and CR textile dyes has not been examined yet using CLP and ZMC as green adsorbents. The focus of the study was to investigate mass transfer limitations, equilibrium and kinetic aspects in order to optimize the dye adsorption efficiency of the proposed novel low-cost adsorbents. Various characterization techniques were employed to investigate the changes in physico-chemical properties of fresh and dye-loaded adsorbents.
2. Material And Methods 2.1.
Methylene blue (MB), also known as tetramethylthionine chloride is a cationic thiazine dye which consists of dark green crystals or crystalline powder and has a bronze-like luster. It is one of the most commonly used dyes in textile industry for dying cotton, wool and silk. Exposure to MB has been reported to cause increased heart rate, cyanosis, nausea, jaundice, quadriplegia, Heinz body formation and tissue necrosis in humans. Iris epithelium, corneal and conjunctival injury , neurotoxic effects on central nervous system , serotonin toxicity  and teratogenic effects  have also been reported widely due to acute exposure to MB dye. Table 1 describes the main characteristics of each dye employed in present study. Malachite Green (MG), a cationic N-methylated diaminotriphenylmethane dye, exists as a mixture of chromatic malachite green cation and its carbinol base in solution . It is extensively used in dyeing of textile material and in distilleries. Applicability of MG as a therapeutic agent in aquaculture has also been reported widely. However, excessive exposure of MG may cause teratogenicity, respiratory toxicity carcinogenesis, mutagenesis and chromosomal fractures [36,37]. Congo Red (1- Naphthalenesulfonic acid, 3, 3'-(4, 4' biphenylene bis (azo) bis 4amino) di sodium salt) is known to metabolize to a human carcinogen benzedene. Acute exposure of this dye may cause an allergic reaction and anaphylactic shock. Therefore, the treatment of effluent containing toxic dyes is of sincere concern these days.
Preparation And Characterization of Low-Cost Adsorbents
CLP and ZMC samples were collected from local vendor, New Delhi (INDIA) and washed thoroughly using distilled water in order to remove the impurities present on the surface. The samples were dried in presence of sunlight for 5 - 7 days. 7
Completely dried material was grinded to powder and then sieved using 40 mesh screen in order to obtain the desired particle size. The powdered sample was finally dried in a hot air oven at 100°C for 2 h and was stored in an airtight vessel. It was used directly for batch adsorption experiments without any further treatment. Figure 1 demonstrates raw material and the grinded form (150 µm particle size) of the prepared adsorbents. Figure 1 : Preparation of Low-cost adsorbents (A) CLP (B) ZMC Proximate analysis was performed in order to estimate the moisture, ash, volatile matter and carbon content in fresh samples of CLP and ZMC adsorbents. Fourier Transform
determination of functional groups on the adsorbent surface. Samples for FTIR analysis were prepared using KBr powder as illustrated in literature . Pellets were prepared by mixing 0.5 g of adsorbent in sufficient amount of potassium bromide (KBr). FTIR spectra of the raw material and residue obtained after adsorption experiments were recorded using FTIR spectrophotometer (Nicolet IS50) within the range 500 - 4000 cm-1. Surface morphology of raw and dye loaded adsorbents was investigated with the aid of Scanning Electron Microscopic Analysis (SEM EVO 50) operated at 20 kV accelerated voltage and magnification of 3KX and of 20KX. Gold coating was provided on non-conducting adsorbents with the aid of vacuum evaporation to get uniform thickness of specimen during analysis . 2.3.
100 mg/L stock solutions were prepared by dissolving accurately weighed amounts of MB, CR and MG dyes in 1 L of distilled water. Working solutions were prepared by diluting the stock solutions into various working concentrations ranging from 5 mg/L 8
to 25 mg/L. Amount of adsorbent was varied for a wide range (0.25 g – 2.5 g) in order to determine the optimum adsorbent dose. pH of the aqueous solutions was adjusted by adding 0.1 M HCl or NaOH solutions. Adsorption experiments were performed at room temperature and constant stirring speed of 150 rpm for a certain agitation time in order to achieve equilibrium. Once equilibrium is attained, the supernatant liquids were filtered and residual dye concentration was determined using UV/vis spectrophotometer (Hitachi UV-2900). Influence of reaction pH (3.0 12.0), initial dye concentration (5 - 25 mg/L), reaction time (10 - 60 min) and reaction temperature (303 - 333 K) were investigated. The amount of dye adsorbed was calculated using following expression given in equation (1): 𝐶0 −𝐶𝑒
𝑞𝑒 = (
where, qe = Amount of dye adsorbed per unit mass of adsorbent (mg/g) at equilibrium, C0 = initial dye concentration (mg/L), Ce = dye concentration in the solution at equilibrium (mg/L), V = Volume of the synthetic solution (L), w = weight of the adsorbent (g). 2.4.
Adsorption Kinetic Studies
Kinetics of the adsorption process was evaluated in the present study to determine the rate of adsorption. Pseudo first order and second order models have been used widely in literature to investigate the kinetics associated with the adsorption of metals and dyes from aqueous solutions [13,17,40]. Applicability of these kinetic models was investigated in the present study to estimate the rate of adsorption of MB, MG and CR dyes on CLP and ZMC adsorbents. Elovich equation was also chosen to model the kinetic data obtained from this study.
Lagergren pseudo-first order kinetic model is employed to investigate adsorption of liquid-solid system on the basis of adsorbent capacity  in which one adsorbate species reacts with one active site on surface. The linearized form of the pseudo first order kinetic model is given in equation (2)  : ln(𝑞𝑒 − 𝑞𝑡 ) = ln(𝑞𝑒 ) − 𝑘1 𝑡
where qe is the amount of adsorbate adsorbed at equilibrium (mg/g), qt is the amount of solute adsorbed per unit weight of adsorbent at time t (mg/g), k 1 is the rate constant of pseudo first order adsorption (min-1). Plot of ln (qe-qt) vs time was drawn for all the three employed dyes and values of k1 and qe,cal were calculated from the slope and intercept respectively. The linearized form of pseudo-second-order equation is given here in equation (3)  : 𝑡 𝑞𝑡
Linear plot of t/qt versus t was drawn and values of qe and k2 were calculated from the slope and intercept respectively. Initial sorption rate (h = k2qe2) at t = 0 was estimated using the value of pseudo second order rate constant k2. Linear form of the elovich kinetic model is given in equation (4)  : 𝑞𝑡 =
ln(𝛼𝛽) + 𝛽 ln(𝑡) 𝛽
where α is the initial desorption rate (mg/(g min)) and β is the desorption constant (g/mg). Available adsorption sites (1/β) was calculated from the slope of linear plot of qt versus ln(t). Intercept (1/β)ln(αβ) indicates the adsorption quantity when ln(t) = 0 2.5.
Error Analysis 10
The applicability of the employed kinetic models was analysed on the basis of linear regression coefficient (R2), normalized standard deviation (Δqt (%)) and average relative error (ARE). Δqt(%) and (%)ARE were calculated using equation (5) and equation (6)  respectively.
∆𝑞𝑡 (%) = 𝐴𝑅𝐸(%) =
(𝑞𝑒𝑥𝑝 −𝑞𝑐𝑎𝑙 ) ⁄𝑞 ) 𝑒𝑥𝑝
𝑁−1 100 𝑁
∗ ∑𝑁 𝑖=1 |
where N is the number of data points, qexp and qcal (mg/g) are the experimental and calculated adsorption capacity values. 2.6.
Adsorption Equilibrium Studies
In order to understand the nature of interaction between adsorbate and adsorbent, different isotherm models namely Langmuir, Freundlich and Dubinin–Radushkevich (D–R) were employed in present study. 2.6.1. Langmuir Isotherm The Langmuir isotherm is based upon an assumption of monolayer adsorption onto adsorbent surface with a finite number of adsorption sites of uniform energies . Linearized form of langmuir equation is given as mentioned in equation (7)  : 𝐶𝑒 𝑞𝑒
1 𝑄0 𝑏
where, Ce is the equilibrium concentration of adsorbate (mg/L), qe is the amount of adsorbate adsorbed per unit mass of adsorbent (mg/g), Q 0 is the monolayer adsorption capacity (mg/g) and b is the Langmuir constant related to the free energy of adsorption (L/mg). Slope and intercept of the linear plot of C e/qe versus Ce give values of b and Q0 respectively. Separation factor (RL) is also an essential 11
characteristic of langmuir isotherm to determine the feasibility of adsorption process. It is represented as shown in equation (8) : 𝑅𝐿 =
where Co and b denote the dye concentration and langmuir constant respectively. If RL = 0, adsorption is irreversible; 0 < RL < 1, adsorption is favourable and RL > 1, adsorption is unfavourable.
2.6.2. Freundlich Isotherm The Freundlich isotherm assumes that heterogeneity of the adsorbent surface and the capacity of adsorption are related to the dye concentration at equilibrium. The freundlich equation  is expressed in linear form as shown in equation (9) : 𝑙𝑛𝑞𝑒 = 𝑙𝑛𝐾𝑓 +
where Kf is the freundlich constant related to bonding energy and n represents surface heterogeneity factor that indicates the adsorption intensity. The values of K f and n were calculated from the intercept and slope of linear plot ln(q e) vs ln(Ce). The value of ‘n’ is considered a measure to estimate the favourability of the sorption process. Values of n > 1 represent favourable adsorption condition [48,49]. 2.6.3. Dubinin-Rahushkevich (D-R) isotherm The equilibrium data were also applied to the Dubinin-Radushkevich (D–R) isotherm model  in order to determine the type of sorption (physical or chemical) process. The linearized form of D–R equation is given in equation (10) : ln(𝑞𝑒 ) = ln(𝑞𝑚 ) − 𝛽𝜀 2
where qe (mg/g) is the amount of dye adsorbed onto per unit dosage of adsorbent (CLP or ZMC), qm is the theoretical monolayer sorption capacity (mg/g), β (mol 2/J2) is related to the average energy of sorption per mole of the adsorbate as it is transferred to the surface of the solid from infinite, ε is Polanyi potential which is calculated using equation (11)  : 1
𝜀 2 = 𝑅𝑇𝑙𝑛 (1 + 𝐶 )
where, R is universal gas constant (8.314 J/mol.K), T is the absolute temperature (K). Slope and intercept of linear plot of ln(qe) vs ε2 give the value of β and qm respectively. The mean adsorption energy (E) was calculated from constant β using the relation given in equation (12) : 𝐸=
If the magnitude of E is between 8 and 16 kJ/mol, then adsorption process proceeds via chemisorption, while for values of E < 8 kJ/mol, the adsorption process is considered to be of physical nature [30,45]. 2.7.
Feasibility of experimental data obtained from the adsorption studies were analysed through thermodynamic investigation. Thermodynamic parameters such as standard free energy change (ΔG°, kJ/mol), enthalpy change (ΔH°, kJ/mol) and entropy change (ΔS°, J/mol/K) were determined using Van't Hoff equation as expressed in equation (13) and equation (14) : ln(𝐾𝑑 ) =
∆𝑆 0 𝑅
∆𝐺 0 = ∆𝐻 0 − 𝑇∆𝑆 0
Kd refers to the standard thermodynamic equilibrium defined by C ad/Ce, where Cad (mg/L on solid adsorbent) and Ce (mg/L in the solution). R is the universal gas constant (8.314 J/mol.K), and T is the absolute temperature (K).
3. Results and Discussions 3.1.
Dye Concentration in each aqueous solution was determined with the aid of Hitachi U-2900 UV spectrophotometer fitted with 3 mm quartz cells. Standard solutions of varying concentration of individual dye (MB, MG, CR) were prepared by dissolving suitable amount of the dye in 100 mL of water. The absorbance was measured at wavelength range from 800 nm to 200 nm against the reagent blank. Maximum absorbance for MB, MG and CR dye were detected at 666 nm, 615 nm and 499 nm respectively. Calibration plot was prepared for each dye by plotting concentration of dye (mg/L) versus absorbance. The unknown concentration of dye in the residual supernatant was calculated using equations (15) – (17) obtained from calibration curve. Methylene Blue :
(R2 = 0.97)…...……………………...(15)
Y = 0.143x + 0.0558
Malachite Green : Y = 0.0236x - 0.0029
(R2 = 0.99)...………………………...(16)
Congo Red :
(R2 = 0.99)..…………………………(17)
Y = 0.1639x - 0.0218
where Y is the absorbance of spectrophotometer and X is the amount of dye (mg/L). 3.2.
Characterization of Adsorbents
3.2.1. Proximate Analysis Proximate analysis was performed in order to estimate the moisture, ash, volatile matter and carbon content in fresh samples of CLP and ZMC adsorbents. 2 g of raw
material was weighed and kept in hot air oven at 105°C (90 min.), 650°C (30 min.), 910°C (7 min.) for the estimation of moisture, ash and volatile matter content respectively using following equations (18) – (20). 𝑆−𝐷
𝑀 = 100 ∗ ( 𝑆−𝐹 )
𝐴 = 100 ∗ ( 𝑆−𝐹 )
𝑉 = 100 ∗
Here, F = Mass of empty crucible; S = mass of crucible + sample; D = mass of crucible plus sample after drying. Percentage of carbon content was determined by summing up the percentage content of the above described parameters and then subtracting it from 100 as shown in equation (21) : % Carbon Content = 100 – (% M + % V + % A)
Table 2 lists the moisture, ash, volatile matter and carbon content obtained from proximate analysis for both the low-cost adsorbents. It can be observed from Table 2 that the raw materials are rich in moisture and volatile matter content, whereas ash content was found to be non-significant in the adsorbents. Carbon content was more than 10% in both the samples which substantiate the acceptability of these raw materials as green adsorbents for dye removal from waste water. Surface Chemistry FTIR spectra of raw material and dye loaded adsorbents are shown here in Figure 2(A) and Figure 2(B) for CLP and ZMC respectively. FTIR spectra clearly revealed reduction, disappearance or broadening of the peaks after adsorption of MB, MG or CR dyes on the CLP and ZMC adsorbents. Figure 2(A) reveals the characteristic 15
broad peak at 3424 cm-1 due to O-H stretching vibration of hydroxyl functional groups. The absorption band at 2925 cm−1 corresponds to the symmetric C–H stretching of alkane group in cellulose and hemicellulose . Other major peaks at bandwidths of 1752 cm-1, 1611 cm-1, 1521 cm-1, 1383 cm-1 attributed to C=C stretching of alkynes, vibration of C=O stretching of lactones, ketones, and carboxylic anhydrides, C=C of aromatic ring and C–H stretching in alkanes or alkyl group respectively. The C-O stretching vibration in cellulose, hemicellulose and lignin can be corroborated for the absorption peaks at 1233 cm −1 and 1100 cm−1 [52,53]. Dye loaded adsorbents exhibited disappearance of peaks at band position 1521cm-1, 1438 cm-1 and 1233 cm-1 whereas appearance of some new peaks suggested the possibility of adsorbate-adsorbent interactions . The peak representing O–H stretching vibration gets shifted from 3424 cm−1 to 3484 cm−1 for MB while CR and MG loaded adsorbent did not show any shift in the bandwidth. Figure 2(A) : FTIR spectra of raw and dye loaded CLP adsorbent. Figure 2(B) demonstrates the FTIR spectra of fresh and dye loaded ZMC. A broad band around 3447 cm-1 indicates the stretching vibration of O–H bond in hydroxyl groups, though the peaks were shifted from 3447 cm -1 to 3416, 3425 and 3429 cm-1 for CR, MG and MB respectively. The peak at 1654 cm -1 in raw material corresponds to bending vibration of O–H groups and was observed to shift at 1634 cm -1 for MG and MB dye loaded ZMC. It may correspond to the involvement of hydrogen of O-H group in the formation of surface complex with intramolecular hydrogen bonding which facilitate the sorption of dye molecule onto the adsorbent surface . The peaks observed at 2923 and 1383 cm -1 were assigned to stretching and bending vibration of C–H bond in methyl groups respectively whereas peak located at 1735
cm-1 attributed to carbonyl group stretching . The strong C–O band at 1053 cm-1 corroborates the lignin structure of the ZMC . The peak at 1246 cm-1 may represent the stretching vibration of C–O in phenols. The slight changes in vibration frequencies as a result of the adsorption of dyes onto ZMC and CLP corresponds to complexation, chelation, precipitation and ion-exchange reactions which probably took place in the functional moieties on the adsorbent surface . Figure 2(B) : FTIR spectra of raw and dye loaded Zea Mays Cobs Adsorbent.
3.2.2. Surface Morphology The morphological analysis of the adsorbent surface was conducted through SEM analysis. Figure 3(A) and Figure 3(B) demonstrate the textural and porous features of CLP and ZMC adsorbents respectively. Well-developed cavernous porous structures were clearly visible in micrographs which may assist in accommodating large amount of dye molecules from synthetic aqueous solutions to the adsorbent surface. After the adsorption of MB, MG and CR dyes on CLP and ZMC surface, significant change is observed in surface texture of the dye loaded adsorbent. Surface was observed to be partially covered by dye compounds. SEM images of dye loaded adsorbents are shown in Figure S-1 of the supporting information. Agglomeration, cluster formation and a thin layer of dye on adsorbents were clearly seen in the SEM images which articulate efficient adsorption of dyes. Figure 3 : SEM Images of (A) Fresh CLP (B) Fresh ZMC Adsorbents 3.3.
Effect of Reaction pH on Dye Adsorption
Solution pH influences the surface properties of adsorbents by regulating degree of ionization of the dye’s functional groups and thus plays an important role in the dye
removal process from wastewater . The effect of pH on the adsorption of dyes using CLP and ZMC was investigated by varying the solution pH over a range of 2.0 to 12.0 and results are shown in Figure 4(A) & Figure 4(B) respectively. It can be clearly depicted from Figure 4 that MG and MB dye removal efficiency increased with an increase in solution pH whereas decrease in CR adsorption efficiency was observed at alkaline pH. Similar results for the adsorption of MB, MG and CR were reported in literature [30,58]. Figure 4 (A) : Effect of solution pH on adsorption capacity (mg/g) (red lines) and (%) adsorption efficiency of CLP adsorbent (black lines) (Adsorbent dose : MB (1.5 g), MG (0.5 g), CR (0.5 g); Initial dye conc. = 10 mg/L; agitation time = 30 min.; settling time = 1 h; stirring speed = 150 rpm) Figure 4 (B) : Effect of solution pH on adsorption capacity (mg/g) (red lines) and (%) adsorption efficiency of ZMC adsorbent (black lines) (Adsorbent dose : (MB (1.5 g), MG (0.25 g), CR (1.25 g); Initial dye conc. = 10 mg/L ; agitation time = 30 min.; settling time = 1 h; stirring speed = 150 rpm) The pH dependent adsorption performance of these dyes can be explained on the basis of pKa value of adsorbate and the pHzpc of the adsorbent. pKa values of MG and MB were reported as 6.9 and 5.8 respectively [30,59] which can be related with the possibility of existence of these dyes in cationic form at pH > pKa and in anionic form at pH < pKa. Number of hydroxyl group ions increases at the alkaline solution pH which results into an increase in number of negatively charged sites and consequently, enhances the attraction between positively charged dye and adsorbent surface. The adsorbent surface becomes deprotonated due to availability of large number of hydroxyl ions (pH > pHzpc), thus adsorbate cations move towards 18
negatively charged sites on CLP/ZMC surface through electrostatic attraction. Similar results were reported in literature for the adsorption of MB/ MG dyes. Kushwaha et al.  suggested that at pH 7.0, surface of Dacus Carota adsorbent was negatively charged to its maximum extent, therefore further increase in pH did not increase surface charge intensity as well as adsorption capability. Amode et al.  illustrated a steady decrease in the amount of MB being adsorbed with increase in pH from 4.0 – 12.0. Maximum adsorption of dye between pH values of 6.0 – 10.0 can be attributed to electrostatic attraction between the negative charges of the adsorbent surface and the positive charge of the MB cation, since the amount of dye being removed was high (>140 mg/g) at ambient pH (MB = 4.6)). On the other hand, Babalola et al.  reported that the removal of MB onto plant waste biosorbents was highest at pH 12.0 due to the low proton densities on the surfaces of these biosorbents at alkaline pH. In contrast to MB and MG dye, CR is an anionic dye and therefore, relatively less amount of dye (< 60%) was removed at alkaline pH. This observation was attributed to electrostatic repulsion between adsorbate and adsorbent in excess of hydroxyl ions which hindered the adsorption of CR anions on adsorbate. At acidic pH, the positive charges (H+) at the solution interphase increased and the adsorbent surface became more positively charged, thus electrostatic attraction of CR dye anions led to high removal efficiency. More than 90 % dye removal was attained at pH = 7.0, pH = 10.0 and pH = 1.0 - 2.0 for MB, MG and CR dye respectively using CLP and ZMC adsorbents.
3.4 Effect of adsorbent dose on dye adsorption Adsorbent dose play a significant role in determining the adsorption capacity at a given initial concentration of dye molecules in aqueous solution. Effect of adsorbent dosage on removal of MB, MG and CR dyes was investigated for a wide range of adsorbent amount (0.25 g to 2.5 g) in 100 mL solution of 10 mg/L dye concentration. Results are shown here in Figure 5(A) and Figure 5(B) for CLP and ZMC adsorbent respectively. Significant increase in percentage adsorption of dyes was observed with increase in adsorbent dosage from 0.25 g to 2.5 g which can be attributed to the increase in availability of adsorption sites on adsorbent surface with increase in dose of the adsorbent. Maximum 99.6 % and 94 % of MB dye removal was obtained at 1.5 g dose of CLP and ZMC respectively. Significant removal of MG and CR was observed at relatively lower adsorbent dose. Nearly 96 % MG dye was removed 0.5 g and 0.25 g of CLP and ZMC respectively. CR dye could not be removed more than 90 % using both adsorbents at optimum dose of the adsorbents i.e. 0.5 g and 1.25 g of CLP and ZMC respectively. Further increase in adsorbent dosage did not show significant increase in percentage removal of dye. Though, percentage removal of all dyes increased with increase in adsorbent dosage, the equilibrium adsorption capacity was observed to decrease with increase in the amount of adsorbent as shown in Figure 5. This observation may be attributed to the overlapping or aggregation of the adsorption sites [61,62] which decreases the total surface area of adsorbent and thus, limits the availability of active sites during adsorption process [17, 63]. Figure 5: Effect of adsorbent dose on adsorption capacity (mg/g) (red plots) and (%) adsorption efficiency (black plots) of (A) CLP adsorbent (B) ZMC
Adsorbent (Initial conc. = 10 mg/L; agitation time = 30 min.; settling time = 1 h; stirring speed = 150 rpm; pH = 7.0). 3.5 Possible Mass Transfer Limitations in Dye Adsorption Process Possible internal and external mass transfer limitations for the proposed dyeadsorbent systems were investigated on the basis of particle size distribution and stirring speed respectively. Internal mass transfer resistance was assumed to be negligible in present study due to very fine particle size distribution (<150 µm). Verification of external mass transfer resistance was accomplished by varying the stirring speed from 0 - 150 rpm at room temperature, agitation time 30 min. and initial dye concentration 10 mg/L. Results are shown in Figure 6(A) and 6(B) for dye (MB, MG, CR) removal using CLP and ZMC adsorbent respectively. Figure 6: Effect of stirring speed on dye adsorption efficiency using (A) CLP Adsorbent (B) ZMC Adsorbent (Initial conc.= 10 mg/L, contact time = 30 min, settling time = 1 h, pH = 7.0). More than 70 % dye adsorption efficiency was observed using both adsorbents even when experiments were performed without stirring, though increase in dye removal efficiency was attained with an increase in stirring speed. No any significant change in adsorption efficiency was observed beyond a stirring speed of 50 rpm for CLP and 100 rpm for ZMC. Therefore, 50 rpm and 100 rpm were considered as the optimum stirring speed for dye-CLP and dye-ZMC adsorbent systems respectively in order to minimize external mass transfer resistance. Only 1 - 2 % improvement in adsorption efficiency was observed with increase in stirring speed beyond the optimum stirring speed as shown in Figure 6.
Adsorption Kinetic Studies
3.6.1. Effect of Agitation Time Effect of agitation time on adsorption of MB, MG and CR dyes was studied at room temperature (303 K) using CLP and ZMC green adsorbents and results are shown in Figure 7(A) and Figure 7(B) respectively. A rapid removal was observed in first 5 - 10 min., followed by gradual adsorption process at slower rate which may be due to the fact that initially all active sites were vacant and readily available for the adsorption of dye ions from aqueous solution. Since the external surface is occupied by dye molecules at earlier stages, slower adsorption rate at later stages can be explained on the basis of diffusion of dye molecules into the interior area of the adsorbent. Equilibrium stage was attained within 25 - 30 min. at initial dye concentration of 10 mg/L. No further increase in adsorption efficiency was observed, once the equilibrium is attained. This observation can be attributed to the fact that remaining vacant sites are difficult to occupy probably due to predominance of repulsive forces between the molecules present on the adsorbents and in the bulk phase . Results were found to be in concordance with the literature [61,62]. CLP adsorbent removed 99 % MB dye in 20 min. whereas maximum 97 % MG removal was observed in 30 min. using ZMC adsorbent. Nearly 89 % and 83 % CR removal efficiency was observed using CLP and ZMC respectively in 30 min. agitation time. Therefore, further batch experiments with all employed adsorbate-adsorbent systems were conducted for 30 min. agitation time. Figure 7: Effect of agitation time on dye adsorption efficiency using (A) CLP Adsorbent (B) ZMC Adsorbent (Initial conc.=10 mg/L, settling time = 1 h, pH = 7.0, stirring speed = 150 rpm).
3.6.2. Batch Kinetic Studies Different kinetic models (pseudo first order, pseudo second order and elovich) were employed in present study for the analysis of kinetic sorption data using linear regression method. Linear plots for each kinetic model were drawn by employing equation (2) – equation (4). Plots obtained for pseudo first order, pseudo second order and elovich kinetic model are shown in Figure S-2 of supporting information. Data points for pseudo first order model do not fit to a straight line whereas straight lines and high values of correlation coefficient (R2) were obtained from data points of pseudo second order kinetic model. Plots obtained for the data points of elovich kinetic models also do not follow the linear regression analysis. Table 3 lists the kinetic parameters obtained using the linear regression method for different kinetic models. It can be inferred from Table 3 that correlation coefficient (R 2) approaches unity for pseudo second order kinetic model while performing dye adsorption experiments using CLP and ZMC adsorbents. The calculated qe,cal values obtained from pseudo second order kinetic model was found to be in good agreement with experimental values qexp and thus, indicated the applicability of pseudo-second order kinetic model for CLP-MB, CLP-MG, CLP-CR, ZMP-MB, ZMP-MG and ZMP-CR adsorption systems. Normalized standard deviation and (%) ARE have been compared in order to find out the best fitted kinetic model. Results for error analysis are also listed in Table 3. Least values of the normalized standard deviation Δq t (%) and (%) ARE substantiate the pertinence of pseudo second order kinetic model for all the employed adsorption systems.
3.6.3. Adsorption Mechanism The proposed solid-liquid adsorption process is mainly carried out in three steps i.e. external mass transfer, pore diffusion and reaction controlled process. External mass transfer was neutralized in present study by maintaining optimum stirring speed during adsorption experiments. Reaction controlled process is relatively rapid process, therefore, it cannot be considered as the rate determining step. SEM studies clearly indicated CLP and ZMC as porous materials as mentioned in section 3.2.3; therefore, it is important to investigate the role of pore diffusion in order to ascertain it as a rate limiting step. The experimental kinetic data was fitted to intraparticle diffusion model which is given as equation (22)  : 𝑞𝑡 = 𝑘𝑑 𝑡1/2 + 𝐶
kd (mg/g.min1/2) is the rate constant for intraparticle diffusion and C is the intercept for the linear plot of qt versus t1/2 which reflects the boundary layer effect. Linear plot of qt versus t1/2 is shown in Figure S-3 of supporting information for the three employed dyes using CLP and ZMC adsorbent respectively and the kinetic parameters are listed in Table 3. It was depicted from experimental observations (Figure S-3) that intraparticle diffusion plots should be divided into two different linear regimes: the linear segment during initial phase (first 5 min.) corresponds to adsorption of dyes on the surface of adsorbent which can be referred as film diffusion. Linear plot in the second regime for later time duration represented gradual diffusion of dyes to adsorption site i.e. pore diffusion . Thus, it was inferred that removal of MB, MG and CR dyes using CLP and ZMC green adsorbents was governed by film diffusion and pore diffusion mechanism. Values of correlation
coefficient (R2) indicated film diffusion as the more dominating mechanism than pore diffusion in the present study. Results were found to be in good agreement with literature for the removal of MG and MB using different low-cost adsorbents [30,54,56]. 3.7. Batch Equilibrium Studies 3.7.1. Effect of initial dye concentration Equilibrium studies were conducted at optimum reaction conditions by varying the concentration of dye from 5 mg/L to 25 mg/L in aqueous solution. Experiments were performed at room temperature, stirring speed 150 rpm, pH 7.0 and agitation time 30 min. Results are shown in Figure 8(A) and Figure 8(B) for CLP and ZMC respectively. It was observed that the removal efficiency of CLP and ZMC decreases with an increase in initial concentration of dyes in synthetic solution, though the actual amount of dye adsorbed per unit mass of adsorbent increased. Figure 8: Effect of initial dye concentration on dye adsorption efficiency using (A) CLP Adsorbent (B) ZMC Adsorbent. The uptake of dye was increased from 0.06 to 1.62 mg/g, 0.17 to 4.70 mg/g, 0.17 to 3.77 mg/g using CLP with an increase in concentration of MB, MG, CR dyes respectively. Increase in dye uptake may be attributed to an increase in the concentration gradient with increase in initial dye concentration which provides a driving force to minimize mass transfer resistances of dyes between the aqueous and solid phase . Percentage removal of dye decreased nearly 6 – 10 % for MB, MG and CR dyes with increase in dye concentration of aqueous solution from 5 mg/L to 25 mg/L for ZMC adsorbent.
3.7.2. Equilibrium Isotherms Sorption isotherms represent the specific relation between the equilibrium concentration of adsorbate in the bulk and the adsorbed amount at the surface . Plots of langmuir, freundlich and D-R isotherm have been shown in Figure S-4 of supporting information for all adsorption systems and calculated parameters have been listed in Table 4. The linear plot of specific sorption (Ce/qe) against the equilibrium concentration (Ce) demonstrate that sorption of MB on CLP and ZMC follow the langmuir isotherm with the highest value of correlation coefficient R 2, though the maximum adsorption capacity Q 0 was noted highest for MG dye (8.733 mg/g and 16.72 mg/g) using CLP and ZMC respectively. Separation Factor (RL) was also calculated for langmuir isotherm using equation (9) and is plotted in Figure 9. The RL values were found to be in the suggested range for favourable adsorption i.e. 0 < RL < 1 at 303 K for all the employed adsorption systems. It was also depicted from Figure 9 that RL value approaches zero with increase in initial dye concentration (C0) which indicated less favourable adsorption at high initial dye concentration. Figure 9 : Variation of separation factor (RL) as a function of initial dye concentration. Experimental data obtained for MG and CR dye fitted well to the freundlich isotherm with R2 > 0.97 whereas removal of MB dye using CLP adsorbent did not correspond to freundlich isotherm. ZMC adsorbent was found to be suitable for the adsorption of all three employed dyes (MB, MG and CR) with high value of R 2 ( > 0.995) and high dye uptake capacity (Kf). Value of freundlich constant ‘n’ was found to be greater than 1 for all adsorption systems and clearly indicated a favourable adsorption
process. The dye uptake capacity Kf was found to be the highest for MG-ZMC system which inferred efficient adsorption of MG dye on Zea Mays cobs. D-R isotherm was observed to be a well fitted model with high value of correlation coefficient (R2 > 0.95) for all employed adsorption systems. Adsorption capacity (q m) was again found to be maximum for MG using ZMC adsorbent. Value of mean adsorption energy (E) lies between 8 - 16 kJ/mol for MB-CLP, MG-CLP, CR-CLP, MB-ZMP, MG-ZMP and CR-ZMP adsorption system which suggested chemisorption process for the present study. 3.8. Thermodynamics Studies 3.8.1. Effect of solution temperature of dye adsorption systems Effect of solution temperature on the adsorption of MB, MG and CR dyes using CLP and ZMC adsorbents was investigated by varying the solution temperature from 303 K to 333 K. Figure 10 demonstrates the increase in percentage adsorption of dyes on CLP and ZMC with an increase in solution temperature under optimum reaction conditions. This observation led to the conclusion that the proposed dye adsorption may be considered an endothermic process. The increasing rate of dye removal may be attributed to increase in rate of diffusion of the dye molecules across the external boundary layer and into the internal pores of the adsorbent particles . Results were found to be in concordance with the literature data where adsorption of dyes from aqueous solution was investigated using sepiolite , durian seed activated carbon  and oat hull  as efficient adsorbents. Figure 10 : Effect of solution temperature of dye adsorption systems. 3.8.2. Thermodynamic parameters
Van’t Hoff plot of ln(Kd) vs 1/T is shown in Figure 11 for all the adsorption system employed in present study. Thermodynamic parameters (ΔH° and ΔS°) were calculated from the slope and intercept of Van’t Hoff plot respectively using equation (14). Table 5 summarizes the thermodynamic parameters determined at different temperatures ranging from 303 K to 333 K. The positive value of ΔH° substantiated the endothermic nature of the dye sorption process. The positive ΔS° values suggest the increased randomness at solid-liquid interface and reflect favourable condition for the removal of MB, MG and CR dyes from synthetic solutions. Figure 11 : Van’t Hoff plot of ln (Kd) vs 1/T. Gibbs free energy ΔG° was calculated using equation (15) and values are reported in Table 5 at different solution temperatures. Negative values ΔG° indicated that adsorption of dyes on CLP and ZMC adsorbents is feasible and spontaneous in nature. Increase in values of ΔG° with increase in solution temperature depicts higher temperature to be favourable for the dye adsorption. The thermodynamic findings are in good agreement with the literature information for the adsorption of dyes on different types of adsorbents.
4. Conclusion Present study is a successful demonstration of the agricultural waste management and application of novel green low-cost adsorbents for industrial waste water treatment. Adsorption potential of green adsorbents CLP and ZMC was pioneered for the removal of dyes (MB, MG, CR) from aqueous solutions. The process was found to be highly dependent on solution pH, initial dye concentration, adsorbent dose, agitation time and solution temperature. Characterization studies substantiated the efficient adsorption of dyes on both adsorbents. The calculated qe,cal values 28
computed from pseudo second order equation showed good agreement with experimental values qexp, and indicated the applicability of pseudo-second order kinetic model for all the employed adsorption systems. The dye uptake process was found to be controlled by intraparticle diffusion in two stages i.e. film diffusion at earlier stages followed by pore diffusion after 5 min. agitation time. ZMC was observed to be the most suitable adsorbent for adsorption of MB, MG and CR dyes with highest dye uptake capacity. Thermodynamic studies indicated dye-CLP and dye-ZMC adsorption systems as feasible, spontaneous and endothermic in nature. Proposed low-cost adsorbents can be considered an environmentally benign substitute of chemically modified adsorbents for the effective removal of toxic components from industrial waste water.
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List of Tables : Table 1 :
Characteristics and Structure of the dyes MB, MG and CR
Table 2 :
Proximate Analysis for Citrus Limetta peel and Zea Mays Cob
Table 3 :
Kinetic Parameters and Error Analysis for Pseudo First Order, Pseudo Second Order and Elovich Kinetic Models
Table 4 :
Parameters for Langmuir, Freundlich and D-R isotherms
Thermodynamic Parameters for dye adsorption on CLP and ZMP adsorbent
List of Figures : Figure 1 :
Preparation of Low-cost adsorbents (A) CLP (B) ZMC.
Figure 2 :
FTIR spectra of raw and dye loaded (A) CLP (B) ZMC adsorbent.
Figure 3 :
SEM Images of (A) Fresh CLP (B) Fresh ZMC adsorbent. 40
Figure 4(A): Effect of solution pH on adsorption capacity (mg/g) (red lines) and (%) adsorption efficiency of CLP adsorbent (black lines) (Adsorbent dose : MB (1.5 g), MG (0.5 g), CR (0.5 g); Initial dye conc. = 10 mg/L; agitation time = 30 min.; settling time = 1 h; stirring speed = 150 rpm).
Figure 4(B): Effect of solution pH on adsorption capacity (mg/g) (red lines) and (%) adsorption efficiency of ZMC adsorbent (black lines) (Adsorbent dose : (MB (1.5 g), MG (0.25 g), CR (1.25 g); Initial dye conc. = 10 mg/L ; agitation time = 30 min.; settling time = 1 h; stirring speed = 150 rpm).
Effect of adsorbent dose on adsorption capacity (mg/g) (red plots) and (%) adsorption efficiency (black plots) of (A) CLP adsorbent (B) ZMC Adsorbent (Initial conc. = 10 mg/L; agitation time = 30 min.; settling time = 1 h; stirring speed = 150 rpm; pH = 7.0).
Effect of stirring speed on dye adsorption efficiency using (A) CLP Adsorbent (B) ZMC Adsorbent (Initial conc.= 10 mg/L, agitation time = 30 min, settling time = 1 h, pH = 7.0).
Effect of agitation time on dye adsorption efficiency using (A) CLP Adsorbent (B) ZMC Adsorbent (Initial conc.=10 mg/L, settling time = 1 h, pH = 7.0, stirring speed = 150 rpm).
Effect of initial dye concentration on dye adsorption efficiency using (A) CLP Adsorbent (B) ZMC Adsorbent (agitation time = 30 min, settling time = 1 h, pH = 7.0, stirring speed = 150 rpm)
Figure 9 :
Variation of separation factor (RL) as a function of initial dye concentration.
Figure 10 : Effect of solution temperature of dye adsorption systems.
Figure 11 : Van’t Hoff plot of ln (Kd) vs 1/T.
Table 1 : Characteristics and Structure of the dyes MB, MG and CR Characteristics
Pvt. Ltd. Class
methane dye Molecular Formula
Table 2 : Proximate Analysis for Citrus Limetta peel and Zea Mays Cob
Volatile Matter (%)
Table 3 : Kinetic Parameters and Error Analysis for Pseudo First Order, Pseudo Second Order and Elovich Kinetic Models Kinetic Model
s/ Error Analysis
Table 4 : Parameters for Langmuir, Freundlich and D-R isotherms Kinetic Isotherm
rs/ Error Analysis
Thermodynamic Parameters for dye adsorption on CLP and ZMP
adsorbent Adsorbents Dye
ΔG° (kJ/mol) 303K