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In this work, the potential of natural and pretreated palm tree trunk (PTT) as agents for adsorption of an organic dye, 2,6-dichlorophenolindophenol (2,6-DCPIP) from aqueous solutions was probed. Natural and acetic acid treated PTT were characterized by Fourier transform infrared (FT-IR) spectroscopy and by the point of zero charge (pzc). The biosorption of 2,6-DCPIP was investigated in batch mode using natural and treated PTT. This study was achieved by highlighting several parameters such as the contact time, biosorbents dosage, the initial concentration of 2,6-DCPIP, the pH of the solution, the ionic strength and the interfering ions. The results showed that 2,6-DCPIP was successfully adsorbed from aqueous solutions by natural and treated PTT. The equilibrium was attained after 40 minutes for treated PTT and 20 minutes for natural PTT. The maximum capacity of adsorption was obtained at pH = 2. The adsorption isotherms were investigated and it was found that the experimental data were best described by the Dubinin-Radushkevich isotherm for the natural PTT (R^{2} = 0.979) and by the Temkin isotherm for the treated PTT (R^{2} = 0.976). The maximum adsorption capacities determined by Langmuir isotherm were found as 108.932 and 157.233 μmol·g^{–1} for natural and treated PTT, respectively. The adsorption kinetics was analyzed and was best described by the pseudo-second order model (R^{2} ≥ 0.998). The diffusion mechanism was studied and the result showed that external mass transfer is the main rate controlling step. The desorption of 2,6-DCPIP is favorable in alkaline medium.

Dyes are synthetic aromatic water-soluble dispersible organic colorants, having potential applications in various industries. They are widely used in textile, paper, plastic, food and cosmetic industries in order to give the certain coloration to the desired product and also consume substantial volumes of water. It is reported that, over 100,000 dyes are commercially available and more than 700,000 ton/year are produced in the world [

All chemical reagents used in this experiment were of analytical grade, purchased and used without further purification. NaCl, CaCl_{2} and NaOH were purchased from Fisher, CH_{3}COOH and BaCl_{2} were purchased from BDH, HNO_{3} and 2,6-DCPIP were purchased from Riedel-de-Häen and HCl was purchased from Phillip Harris. The structure of 2,6-DCPIP is illustrated in

The palm tree trunk used in this work was collected from a local agricultural field of the Littoral region in Cameroon. The biomass was cut into small pieces, washed several times with tap water to remove dust and soil particles, and then dried in sun for 8 days. The dried biomass was ground into fine powder and sieved to obtain sizes ranging from 0 - 100 µm. The powder was washed several times with distilled water, air-dried for 2 days and then in an oven at 110˚C for 24 h before being kept in a bottle for further use.

In view of studying the effect of chemical pretreatment of biomass on 2,6-DCPIP uptake capacity, the biomass was submitted to pretreatment with acetic acid, according to the following procedure: 5 g of the natural biomass was put into contact with 100 mL of 2 mol∙L^{−1} acetic acid solution. The mixture was stirred in a mechanical platform shaker (EDMUND BÜHLER GmbH) for 2 h at a speed of 200 rpm. The resultant biomass was washed several times with distilled water in order to remove the excess of acetic acid until the pH (6.3) of washed water was stable. After washing thoroughly, the biosorbent was air-dried for 2 days and then in an oven at 110^{°}C for 24 h before being kept in a bottle for further use.

The determination of the pzc of natural PTT and treated PTT was performed according to the method previously described by [^{−1} NaCl solution whose initial pH ( pH i ) was measured and adjusted between 1 and 12 with NaOH or HCl solutions. The containers were sealed and placed on a mechanical platform shaker for 48 h at a speed of 150 rpm after which the final pH ( ) was measured. The pzc occurs when there is no change in pH after contact with the biosorbent. The pzc corresponds to the point where the curve of Δ pH = pH f − pH i = f ( pH i ) crosses the line pH i .

The natural and treated PTT were also characterized by using Fourier transform infrared spectroscopy (FT-IR) which allowed to identify different chemical

functional groups present in natural and treated PTT powder. FT-IR spectra were obtained by means of the ATR technique with a Bruker α-P FT-IR spectrophotometer within a range of 4000 - 400 cm^{−1} with a resolution of 4 cm^{−1}; 200 scans were collected for each spectrum.

The stock solution of 2,6-DCPIP was prepared at 10^{−3} mol∙L^{−1} by dissolving 0.327 g of the hydrated sodium salt of 2,6-dichlorophenolindophenol in 1 L of distilled water. Solutions of different concentrations (2 × 10^{−5} - 10^{−4} mol∙L^{−1}) were prepared by dilution of the stock solution with distilled water. The pH of each solution of 2,6-DCPIP was adjusted to the required value using HCl or NaOH solutions.

The biosorption studies were achieved in aqueous solution, in a shake flask, at room temperature. In this study, the batch biosorption experiments were carried out by mixing pre-weighted amounts (5 - 60 mg) of biosorbent with 10 mL of 2,6-DCPIP of various initial concentrations (2 × 10^{−5} - 10^{−4} mol∙L^{−1}) into a flask. The mixture was stirred at constant agitation speed of 150 rpm for an interval time between 5 - 70 min on a mechanical platform shaker. After agitation, the suspensions were filtered using whatman filter paper. The filtrates were analyzed by measuring the absorbance using UV-Vis Spectrophotometer (JENWAY) at a maximum adsorption wavelength of 600 nm [_{e} by using the equation of calibration curve. The amount of 2,6-DCPIP adsorbed at equilibrium q_{e} (mol∙g^{−1}) (Equation (1)), at time t q_{t} (mol∙g^{−1}) (Equation (2)) and the percentage of adsorption ( % ads ) (Equation (3)) were calculated as follows [

q e = C i − C e m V s (1)

q t = C i − C t m V s (2)

% ads = C i − C e C i 100 (3)

where C i , C e and C t (mol∙L^{−1}) are the initial concentration, the final concentration at equilibrium and the final concentration at time t of 2,6-DCPIP, respectively. V s (L) is the volume of 2,6-DCPIP solution and m (g) is the weight of the biosorbent.

The desorption studies were achieved by mixing 300 mg of biosorbents with 100 mL of 2,6-DCPIP of initial concentrations 10^{−4} mol∙L^{−1} into a flask. The mixtures were stirred at constant agitation speed of 150 rpm for 40 min. After agitation, the suspensions were filtered using whatman filter paper. The filtrates were analyzed by measuring the absorbance using UV-Vis Spectrophotometer. The 2,6-DCPIP loaded biosorbents were recovered and dried in an oven. After drying, 30 mg of 2,6-DCPIP loaded biosorbents were mixed with 10 mL of the desorption solutions of H_{2}O, 10^{−2} mol∙L^{−1} of NaOH or HNO_{3}. The mixtures were stirred at constant agitation speed of 150 rpm for 40 min on a mechanical platform shaker. After agitation, the suspensions were filtered. The filtrates were also analyzed by measuring the absorbance using UV-Vis Spectrophotometer. The desorption percentages ( % des ) were calculated as follows (Equation (4)) [

% des = C f − C r C f 100 (4)

where; C f and C r (mol∙L^{−1}) are the initial concentration and the final concentration of 2,6-DCPIP loaded biosorbents, respectively.

The FT-IR spectra of PTT were recorded in order to explore the surface functional groups as shown in ^{−1}, this peak shows the presence of alcohol, phenol or carboxylic acids [^{−1} was attributed to asymmetric and symmetric C - H stretching of aliphatic methyl and methylene. The peaks localized at 1725.38 cm^{−1} and 1628.20 cm^{−1} are characteristic of carbonyl (C = O) of carboxylic acids and carboxylate, respectively. The peak at 1510.65 cm^{−1} is a constant value for all the lignin esters. The IR peak at 1422.53 cm^{−1} may be due to the symmetrical bending vibration of alkane bonds (-CH_{2}). The absorption peak at 1238.72 cm^{−1} could be due to C - O, C - H or C - C stretching vibrations of carboxyl groups (-COOH). The band localized at 1031.92 cm^{−1} is attributed to C - O stretching vibrations of lignin [^{−1} is the fingerprint zone and the absorption cannot clearly be assigned to any particular vibration because they correspond to complex interacting vibration systems [

The pzc of a material in a solution is the pH value at which the net surface charge of the material is equal to zero [

The pzc value of the pretreated PTT with acetic acid (3.8) is lower than the pzc value of PTT (

The biosorbent was treated with acetic acid. The treatment affected the functional groups contained at the surface of the material. ^{−1} for natural PTT and 26.007 µmol∙g^{−1} for treated PTT. As can be seen, the amount of 2,6-DCPIP adsorbed at equilibrium is higher with treated PTT than natural PTT. This result can be explained by the fact that the treatment with acetic acid like all the acid treatments, lead to the protonation of the surface functional groups of the material [

The effect of biosorbents dosage on the removal of 2,6-DCPIP was studied and the results of this study are shown in

The effect of contact time on biosorption of 2,6-DCPIP is presented in

The effect of initial concentration on the biosorption of 2,6-DCPIP was investigated and the results are shown in ^{−1} and from 4.644 to 23.451 µmol∙g^{−1} for natural and treated PTT, respectively. As can be seen, the amount adsorbed at equilibrium for both biosorbents increases as the 2,6-DCPIP concentration increases. This result can be explained by the fact that increasing the initial 2,6-DCPIP concentration would increase the mass transfer driving force, and hence, the rate at which 2,6-DCPIP molecules pass from solution to the particle surface [

The pH is an important factor that affects biosorption processes. It is used in

industry to increase the adsorption of dyes. The effect of initial pH on biosorption of 2,6-DCPIP on natural and treated PTT is shown in ^{−1} and from 36.993 to 3.529 µmol∙g^{−1} for natural and treated PTT, respectively. This can be explained by the fact that, the PZC of natural PTT is 4.8 and treated PTT is 3.8. Thus, at low pH (pH < PZC) more protons will be available for the protonation of the biosorbent surface which increases the electrostatic attraction between the positively charged biosorbent sites and the negatively charged 2,6-DCPIP. There is nearly no electrostatic repulsion between the biosorbent and the 2,6-DCPIP at pH = 2 and hence, the amount adsorbed is at its maximum. Increasing the pH (pH > PZC) leads to an increase in 2,6-DCPIP anions in the solution as well as the number of negatively charged sites on the adsorbent due to the increase in hydroxyl ions. This results in electrostatic repulsion between the 2,6-DCPIP and the adsorbent, which is the reason for the decrease in the amount of adsorption. A similar trend was observed for the adsorption of azo dyes by glutaraldehyde-crosslinked chitosans [

The ionic strength of the solution is one of the factors that control both electrostatic and non-electrostatic interactions between the adsorbate and the adsorbent surface [

The effect of interfering ions on biosorption of 2,6-DCPIP was carried out using different salts; BaCl_{2}, CaCl_{2} and NaCl. The results show that the capacity of adsorption increases with the presence of salts in the order NaCl < CaCl_{2} < BaCl_{2} (

2,6-DCPIP and the biosorbents. This can also be explained by the fact that the divalent ions favour more aggregation of dye molecules and decreases the solubility than the monovalent ions. The higher capacity of adsorption of BaCl_{2} than CaCl_{2} can be explained by the fact that, the higher molecular weight of BaCl_{2} favours more aggregation of dye molecules and decreases the solubility than the lower molecular weight of CaCl_{2}.

In order to understand the mechanism of biosorption, it is important to perform the adsorption isotherms. In this study, four adsorption isotherms were used to describe the obtained equilibrium data: Langmuir, Freundlich, Dubinin-Radushkevich and Temkin isotherms.

The general equation of Langmuir isotherm is described as follows (Equation (5)) [

C e q e = 1 K L ⋅ q max + C e q max (5)

where; q e and q max ( µmol ⋅ g − 1 ) are the equilibrium and maximum biosorption capacities of biosorbent, respectively. C e ( µmol ⋅ L − 1 ) is the equilibrium concentration of solution. K L ( L ⋅ μ mol − 1 ) is the Langmuir biosorption constant. The constants q max and K L were calculated from the slopes and intercepts of

linear plots of C e q e versus C e . The essential characteristic of Langmuir isotherm

can be expressed in terms of a dimensionless constant called separation factor ( R L ), which is defined as follows (Equation (6)) [

R L = 1 1 + K L C i (6)

The value of R L indicates whether the type of adsorption isotherm will be favorable ( R L < 1 ) , unfavorable ( R L > 1 ) , linear ( R L = 1 ) or irreversible ( R L = 0 ) .

The general equation of Freundlich isotherm is described as follows (Equation (7)) [

log q e = log K f + 1 n log C e (7)

where K f ( L ⋅ g − 1 ) is the Freundlich constant related to the biosorption capacity and 1 n is an empirical parameter related to the biosorption intensity of the adsorbent. The Freundlich isotherm constants 1 n and K f were calculated from the slopes and intercepts of linear plots of log q e versus log C e .

The general formula of Dubinin-Radushkevich isotherm is given by the following Equation (8) [

ln q e = ln q max − β ε 2 (8)

where; β (mol^{2}∙kJ^{−2}) is the activity coefficient related to the mean free energy (E (kJ∙mol^{−1})) obtained from Equation (10) and ε is polanyi potential which is determined from Equation (9):

ε = R T ln ( 1 + 1 C e ) (9)

where, R is the universal gas constant (8.314 × 10^{−3} kJ∙mol^{−1}∙K^{−1}) and T is the absolute temperature in kelvin in our case (298 K). The constants β and q max were calculated from the slopes and the intercepts of linear plot of ln q e versus ε 2 .

E = 1 − 2 β (10)

The general formula of Temkin isotherm is given by the following Equation (11) [

q e = q max R T Δ Q ln K T + q max R T Δ Q ln C e (11)

where; Δ Q (kJ∙mol^{−1}) is the heat of adsorption, K T (L.µmol^{−1}) is the Temkin isotherm constant. The constants Δ Q and K T were calculated from the slopes and the intercepts of linear plot of q e versus ln C e .

The isotherms obtained for 2,6-DCPIP adsorption onto natural and treated PTT are shown in

The values of correlation coefficients of Dubinin-Radushkevich and Temkin isotherms are closest to unity, implying that Dubinin-Radushkevich and Temkin isotherms are most appropriate to describe the biosorption of 2,6-DCPIP on natural and treated PTT, respectively. The values of 1/n determined by the Freundlich isotherm (

Biosorbents | Langmuir | Freundlich | ||||
---|---|---|---|---|---|---|

K_{L} (L∙µmol^{−1}) | q_{max} (μmol∙g^{−1}) | R^{2} | K_{f} (L∙g^{−1}) | 1⁄n | R^{2} | |

PTT-CH_{3}COOH | 0.006 | 157.233 | 0.147 | 0.454 | 0.938 | 0.953 |

PTT | 0.004 | 108.932 | 0.302 | 0.191 | 0.922 | 0.975 |

Biosorbents | Dubinin-Radushkevich | Temkin | ||||

E (kJ∙mol^{−1}) | q_{max} (μmol∙g^{−1}) | R^{2} | ΔQ (kJ∙mol^{−1}) | K_{T} (L∙µmol^{−1}) | R^{2} | |

PTT-CH_{3}COOH | 8.562 | 2409.033 | 0.958 | 33.969 | 0.248 | 0.976 |

PTT | 8.476 | 1339.244 | 0.979 | 31.202 | 0.159 | 0.958 |

The adsorption energies obtained from Dubinin-Radushkevich isotherm (^{−1}, implying that chemisorption (anionic exchange) is the mechanism which controls the biosorption process [

The values of the separation factor determined by the Langmuir isotherm (

The controlling mechanisms of adsorption process such as chemical reaction, diffusion control or mass transfer coefficient are used to determine kinetic models. Thus, the kinetics of dye onto various adsorbent materials was analyzed using different kinetic models which are presented below.

The pseudo-first order equation of Lagergren is generally expressed as follows (Equation (12)) [

log ( q e − q t ) = log q e − K 1 ads 2.303 t (12)

where; q e and q t (µmol∙g^{−1}) are the amounts of 2,6-DCPIP adsorbed at

equilibrium and at time t, respectively. K 1 ads (min^{−1}) is the rate constant of pseudo-first order. The values of K 1 ads and q e were calculated from the slopes and intercepts of the linear plots of log ( q e − q t ) versus t.

The pseudo-second order equation is generally expressed as follows (Equation (13)) [

t q t = 1 K 2 ads ⋅ q e 2 + 1 q e ⋅ t (13)

where K 2 ads (g.µmol^{−1}.min^{−1}) is the rate constant of pseudo-second order. The values of q e and K 2 ads were calculated from the slopes and intercepts of the

linear plots of t q t versus t. This model allows determining the initial rate of

reaction h (µmol∙g^{−1}∙min^{−1}) (Equation (14)) and the half time of the reaction t_{1}_{⁄}_{2} (min) (Equation (15)).

h = K 2 ads ⋅ q e 2 (14)

t 1 / 2 = 1 q e ⋅ K 2 ads (15)

The Elovich model is represented by the following Equation (16) [

q t = 1 β ln ( α β ) + 1 β ln t (16)

where; α (mmol∙g^{−1}∙min^{−1}) is the initial adsorption rate and β (g∙µmol^{−1}) is related to the extent of surface coverage and activation energy for chemisorption. The values of β and α were calculated from the slopes and intercepts of the linear plots of q t versus ln t .

Weber and Morris demonstrated that in intraparticle diffusion studies, rate processes are usually expressed in terms of square root of time [

q t = K i p t 1 / 2 + C (17)

where; K i p (µmol∙g^{−1}.min^{−1/2}) is the intraparticle diffusion rate constant and C (µmol∙g^{−1}) is the thickness of boundary layer. The values of K i p and C were calculated from the slopes and intercepts of the linear plots of q t versus t 1 / 2 .

This model assumes that the surface solute concentration C s , on the sorbent is negligible at t = 0, and that intraparticle diffusion is also negligible; it is used to calculate the initial rate of solute sorption. The initial rate of sorption can be determined using the classical mass transfer equation, which describes the evolution of solute concentration C t in solution (Equation (18)) [

d C t d t = − β L S ( C t − C s ) (18)

where; β L is the external mass transfer coefficient, C t is the liquid phase solute concentration at time t, C s is the liquid phase solute concentration at the particle surface and S is the specific surface area for mass transfer. This equation can be simplified by substituting the following boundary conditions: C t → C 0 and C S → 0 when t → 0 ; C_{0} = initial solute concentration, to Equation (19) [

d ( C t / C 0 ) d t = − β L S (19)

So the external mass transfer rate β L S , was approximated by the initial slope of the C t / C 0 versus time graph.

In order to interpret the rate-controlling step during the adsorption process, the experimental data were further analyzed by the model given by Boyd (Equation (20)) [

F = 1 − 6 π 2 exp ( − B t ) (20)

Since F = q t q e , Bt could be represented as follows (Equation (21)):

B t = − 0.4977 − ln ( 1 − F ) (21)

where; F is the fraction of solute sorbed at different times t and Bt is a mathematical function of F. The calculated B values are used to calculate the effective diffusion coefficient, D i ( cm 2 / s ) using Equation (22) [

B = π 2 D i r 2 (22)

where, r represents the mean radius of the particle calculated by sieve analysis and by assuming them as spherical particles.

Kinetics of The 2,6-DCPIP adsorption onto natural and treated PTT are shown in

From

Biosorbents | Pseudo-first order model | Pseudo-second order model | ||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|

q_{exp} | q_{e} | k 1 ads | R^{2} | q_{e} | k 2 ads | h | t_{1⁄2} | R^{2} | ||||||||||||||

PTT-CH_{3}COOH | 23.966 | 8.485 | 0.066 | 0.988 | 24.900 | 0.015 | 9.523 | 2.615 | 0.999 | |||||||||||||

PTT | 16.754 | 38.295 | 0.322 | 0.994 | 18.242 | 0.019 | 6.342 | 2.876 | 0.998 | |||||||||||||

Biosorbents | Elovich model | External mass transfer | Boyd model | |||||||||||||||||||

α | β | R^{2} | β L S × 10 3 | R^{2} | B | D i × 10 9 | R^{2} | |||||||||||||||

PTT-CH_{3}COOH | 0.972 | 0.419 | 0.958 | 2.450 | 0.768 | 0.066 | 2.787 | 0.988 | ||||||||||||||

PTT | 0.067 | 0.411 | 0.733 | 2.170 | 0.442 | 0.322 | 13.592 | 0.994 | ||||||||||||||

Biosorbents | Intraparticle diffusion model | |||||
---|---|---|---|---|---|---|

k i p 1 | k i p 2 | C_{1} | C_{2} | R 1 2 | R 2 2 | |

PTT-CH_{3}COOH | 1.485 | 0.071 | 14.858 | 23.301 | 0.991 | 0.043 |

PTT | 4.263 | 0.320 | 0.398 | 14.876 | 0.944 | 0.779 |

adsorption. Also, the intraparticle diffusion model curve does not pass through the origin, which is an indication that 2,6-DCPIP diffusion in the bulk of natural and treated PTT is not the only process that governs the biosorption [

^{−13} to 10^{−5} cm^{2}⁄s, indicating that chemisorption occurs during the biosorption process [

The repeated availability of the adsorbents after adsorption-desorption cycles is crucial to illustrate the stability and potential recovery of the adsorbents. In this study, NaOH, HNO_{3} and H_{2}O were used as desorbing agents to regenerate the biosorbents. The results show that the maximum desorption percentage, 67.371% for natural PTT and 54.260% for treated PTT is obtained in NaOH medium (^{−}) of NaOH solution and 2,6-DCPIP loaded biosorbent. However, the low percentage of desorption obtained with treated PTT compared to natural PTT is due to the strong bond formed between the 2,6-DCPIP and the treated PTT [

From this study, the capacity of using natural and treated PTT for the removal of

2,6-DCPIP from aqueous solution has been proven. Both materials are efficient biosorbents, but the treated PTT showed better performance than natural PTT. The adsorption was highly dependent on various operating parameters such as; treatment of biosorbent, contact time, pH of solution, biosorbents dosage, initial concentration of 2,6-DCPIP, ionic strength and interfering ions. The adsorption isotherms indicate that the equilibrium data are the best described by the Dubinin-Radushkevich and Temkin isotherms for natural and treated PTT, respectively. Results of adsorption kinetics demonstrated that the adsorption processes were controlled by pseudo-second order kinetics. The mechanism of diffusion was studied and the results showed that external mass transfer was the main rate controlling step. Desorption using NaOH as desorbing agent recovers a maximum quantity of 2,6-DCPIP. From the results obtained, the utilization of PTT for the removal of 2,6-DCPIP from aqueous solution is promising.

The authors acknowledge the support of the International Foundation for Science (Grant n˚ W/5859-1 awarded to Evangeline NJANJA). Financial support from The World Academy of Sciences for the Advancement of Science in Developing Countries (TWAS grant no. 12-117 RG/CHE/AF/AC-G) is gratefully acknowledged.

The authors declare no conflicts of interest regarding the publication of this paper.

Ngaha, M.C.D., Djemmoe, L.G., Njanja, E. and Kenfack, I.T. (2018) Biosorption Isotherms and Kinetics Studies for the Removal of 2,6-Dichlorophenolindophenol Using Palm Tree Trunk (Elaeis guineensis). Journal of Encapsulation and Adsorption Sciences, 8, 156-177. https://doi.org/10.4236/jeas.2018.83008