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In this work, low cost coconut biochar based activated carbon (CBAC) was used for adsorption of Butylparaben (BPB) from aqueous medium. The prepared CBAC was characterized using BET, Boehm analysis and the adsorption equilibrium, kinetics and thermodynamics studies of BPB adsorption were carried out. During batch adsorption runs, the effects of factors, such as contact time (0 - 300 min), CBAC dose (200 - 800 mg), pH (3 - 11) and solution temperatures (303 - 348 K) were investigated on BPB removal. Experimental results reveal that the BPB removal efficiency on CBAC is higher than 97% under acidic and neutral conditions. Equilibrium data were fitted by Langmuir, Freundlich and Temkin isotherm models with correlation coefficient more than 0.9. The pseudo-second order kinetic model was observed to fit well the adsorption data. Thermodynamic analysis shows positive values of standard Gibb’s free energy, suggesting the non-spontaneity of the process. The changes in enthalpy (0.2 J.mol
^{-1}) and entropy (19 J.mol
^{-1}) were found to be endothermic with an increase of randomness. The high adsorption efficiency of the synthesized coconut biochar materials with low cost indicates that it may be a promising adsorbent for removing organic compounds.

During these last 20 years, concerns about the consequences of human and wildlife to long term exposure to substances in the environment that can interact with the endocrine system, are increasingly criticized. These substances, leading to biological perturbations, are commonly referred to “endocrine disruptors”. According to the WHO definition, endocrine disruptors are natural or artificial chemical substances foreign to the body that can interfere with the operating systems and induce endocrine resulting in deleterious effects on the body or on his descendants [

Several studies have shown the potential and actual harmful effects of these compounds on wildlife and humans [^{−1} to μg・L^{−1} [

The objective of the present work is to prepare biochar based activated carbon from coconut shells of Côte d’Ivoire (West Africa), which are rejected by the farmers as waste materials [

Butyl parahydroxybenzoate (BPB) (purity > 99%) was obtained from an analytical grade reagent, Sigma Aldrich, US. The physico-chemical properties and molecular structure of BPB are summarized in _{2}SO_{4}. All the chemicals and reagents used in this study were of analytical grade and used as received. All the solutions and reagents were prepared with deionized water.

The adsorbent used in this study was prepared from coconut shell biochar. The experimental conditions and activated carbon characteristics were described by Atheba [

The Brunauer-Emmett-Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) pore size of the sample were measured by adsorption of nitrogen liquid at 77 K collected from a Micromeritics Tristar 3000 surface area and pore size analyzer. The surface functional groups were determined according to the method of Boehm [

Parameter | Value |
---|---|

Molecular Formula | C_{11}H_{14}O_{3} |

Molecular Weight (g・mol^{−1}) | 194.23^{ } |

Solubility in water (mg・L^{−1}) | 158 |

Absorption Maxima (nm) | 255 |

Activation parameters | ||||
---|---|---|---|---|

Time | Atmosphere | Flow rate | Temperature | Heating rate |

3 h | N_{2} + CO_{2} | 100 ml・min^{−1} | From 20˚C to 800˚C | 20˚C min^{−1} |

and phenol functions were dosed with increasing force bases (NaHCO_{3}, Na_{2}CO_{3}, NaOH) while the total basicity was determined with hydrochloric acid.

The ability of activated carbon to uptake BPB from aqueous solution was evaluated in batch mode adsorption. BPB stock solution of 100 mg・L^{−1} was prepared by dissolving the appropriate amount of BPB in 1000 ml of volumetric flask and completing the volume with deionized water up to mark. The experimental solutions of the desired concentration were obtained by successive dilutions.

The first set of tests consisted to determine equilibrium time of adsorption. For this purpose, 3 erlenmeyer flasks containing 200 ml of 10 mg・L^{−1} BPB and activated carbon (4 g・L^{−1}) were shaken at 150 rpm, rom temperature (25˚C ± 2˚C), during 300 min expected to reach equilibrium and the measured pH was 7.0 ± 0.5. At the regular interval time (0, 5, 15, 30, 60, 120, 180 and 300 min) samples were withdrawn and the amount of BPB adsorbed was determined by measuring the residual BPB concentration in the liquid phase using an OMEGA-UV-Vis (Thermoscientifique, France) spectrophotometer calibrated at the wavelength of 255 nm.

The effect of activated carbon dose on BPB adsorption process was conducted at fixed pH (pH 7 ± 0.5) and fixed initial BPB concentration (10 mg・L^{−1}) in 200 ml of solution. Different activated carbon masses were used (1, 2, 3 and 4 g・L^{−1}). The flasks were shaken at 150 rpm for 180 min (according the equilibrium time determined above) and samples were collected for analysis.

The next set of tests was performed to determine the effects of pH on adsorption process. Adsorption was studied at pH 3, 5, 7, 9 and 11 at 25˚C ± 2˚C, in 10 mg・L^{−1} of initial BPB concentration, 200 ml of solution and 4 g・L^{−1} of activated carbon. The solution initial pH value solution was adjusted using 0.1 M H_{2}SO_{4 }and 0.1 M NaOH, solutions. The flasks were shaken at 150 rpm for 180 min, and samples were collected for analysis.

The adsorption isotherms experiments were carried out using different initial concentrations of BPB (5, 10, 15, 20, 25, 30, 40, 50 mg・L^{−1}) and the same dose of activated carbon (4 g・L^{−1}). The mass of activated carbon was fixed according to the result obtained during the effect of activated carbon dose on BPB adsorption process. The flasks were shaken at 150 rpm for 180 min and samples were taken for analysis. Room temperature and optimum pH were used for test.

The adsorption kinetic experiments were carried out using 200 ml of 10 mg L^{−1} of BPB solution and at a fixed concentration of activated carbon (4 g・L^{−1}). The flasks were shaken at 150 rpm, 25˚C ± 2˚C, and pH 7.0 ± 0.5. Samples were withdrawn at regular interval times and analyzed.

The last set of analysis was consisted to evaluate the thermodynamic parameters such as Gibbs free energy (ΔG˚), enthalpy (ΔH˚) and entropy (ΔS˚) changes, to understand the feasibility and nature of the adsorption process. For this, erlenmeyer flasks containing 200 ml of solution at 10 mg・L^{−1} of BPB and 4 g・L^{−1} of activated carbon were shacking at 150 rpm and different temperatures (10˚C, 22˚C, 30˚C, 40˚C) during 180 min.

The percentage of BPB removal is calculated using the following equation:

PercentageRemoval ( PR% ) = 100 × ( C i − C f ) / C i (1)

The amount adsorbed in (mg・g^{−1}) is calculated using Equation (2):

q e = ( C i − C f ) × V / m (2)

where C_{i} and C_{f} are the initial and final concentrations (mg・L^{−1}) of BPB in liquid phase, respectively; V (L) represents the volume of solution and m (g) the mass of activated carbon.

The most used materials in adsorption process are activated carbons. This is because of their high adsorption capacity, high surface area, micro porous structure, high degree of surface and high chemical and mechanical stability [_{2} adsorption isotherm was measured to determine the surface area and pore size of the adsorbent. The surface area was determined by a fitting analysis based on the BET equation. The quantitative assessment of the acido-basic functions of the surface was obtained by Boehm titration. According to the IUPAC classification of absorption isothermal curves, the N_{2}-sorption (^{2}・g^{−1} with a pore volume of ≈0.22 cm^{3}・g^{−1}.

Physical | S_{BET} (m^{2}・g^{−1}) | 443.134 | |
---|---|---|---|

Pore volume (cm^{3}・g^{−1}) | 0.220 | ||

Pore size (Ǻ) | 23.619 | ||

Chemical | Acid (meq・g^{−1}) | -OH | 0.172 |

-COO- | 1.038 | ||

-COOH | 2.780 | ||

Base (meq・g^{−1}) | 1.075 | ||

pH | 10.2 | ||

Structure | Grain size (mm) | 1 < ϕ < 2 | |

Appearance | Spangle |

structural and physico-chemical properties of the prepared adsorbent. It is interesting to note that the activated carbon is more acid, but has also some basic functions.

The impact of contact time on the adsorption capacity of activated carbon (AC) for BPB removal was evaluated. In

The effect of activated carbon dose on BPB (pH 7 and 10 mg・L^{−1}) removal was studied and the result obtained is depicted in ^{−1}. This may be due to an increase in the availability of surface active centers. The same observation is also reported in literature [

carbon of 4 g・L^{−1}, were thus considered for the following runs.

The influence of the initial aqueous solution pH value is an important parameter that affects the adsorption process [^{−1} BPB solution. This pH range (

^{+} ion in solution. The electrostatic attraction between BPB molecule and the adsorbent surface increase the amount of BPB adsorbed. At the high pH range, the surface of activated carbon might become negatively charged due to excess of OH^{−} ions concentration, which will compete with BPB for the available positively charged sites on the adsorbent

surface, thus decreasing the removal efficiency of BPB.

Some authors [

The equilibrium adsorption isotherms are important in the development of adsorption process. Adsorption characteristics of an adsorbent are used to estimate the adsorbent/adsorbate interaction. In this study, four isotherm models were used to investigate and elucidate the adsorption behavior of BPB onto activated carbon. Data obtained from the experiments were fitted by Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models.

1) Langmuir isotherm

The Langmuir isotherm model was chosen for the estimation of maximum adsorption capacity corresponding to complete monolayer coverage on the biomass surface [

1 / Q e = 1 / Q m K L C R + 1 / Q m (3)

where, Q_{e} is the amount of BPB adsorbed per mass of adsorbent (mg・g^{−1}), C_{r} is the residual concentration of BPB, Q_{m} is the monolayer adsorption capacity (mg・g^{−1}) and K_{L} is the Langmuir constant. The isotherm parameters, Q_{m} K_{L} and the coefficient of correlation are presented in _{m}, which is a measure of the maximum sorption capacity corresponding to the complete monolayer coverage showed that the activated carbon adsorption capacity for BPB was 7.519 mg・g^{−1}. The adsorption coefficient, K_{L}, related to the apparent energy of sorption is 0.093 L・mg^{−1} and the correlation coefficient 0.979. This observation showed that the energy of adsorption is not very favorable to BPB, probably due to its weak interaction between BPB molecules and the activated carbon surface properties. The same capacity order was reported [

The essential factor K_{L} is used to evaluate the feasibility of the adsorption process as favorably and nature of the isotherm.

R L = 1 / ( 1 + K L C i ) (4)

where K_{L} is the Langmuir constant and is the initial concentration of BPB.

The R_{L} value for BPB adsorption on activated carbon is 0.64 (0 < R_{L} < 1), indicating that the adsorption process is favorable [

2) Freundlich isotherm

The Freundlich model was chosen to estimate the adsorption intensity of the adsorbate on the adsorbent surface. The linearized form of the Freundlich isotherm is given by:

ln Q e = ln ln K F + ln C e × 1 / n (5)

where K_{F} is the Freundlich isotherm constant related to the adsorption capacity, whereas describes the adsorption intensity [

Isotherm | |||||
---|---|---|---|---|---|

Langmuir | Freundlich | ||||

Q (mg・g^{−1}) | K_{L} (L・mg^{−1}) | R^{2} | K_{F} (mg・g^{−1}) | n | R^{2} |

7.52 | 0.093 | 0.979 | 0.773 | 1.553 | 0.973 |

Temkin | Dubinin-Radushkevich | ||||

b_{T} (kJ・mol^{−1}) | K_{T} (L・mg^{−1}) | R^{2} | Q_{m} (mg・g^{−1}) | K_{D}_{−}_{R} (mol・kJ^{−1})^{2} | R^{2} |

24.49 | 0.874 | 0.909 | 0.390 | 2.681 | 0.785 |

adsorbent [

3) Temkin isotherm

The Temkin isotherm [

Q e = R T ln K T × 1 / b T + R T ln C e × 1 / b T (6)

where, b_{T} is the Temkin constant related to the heat of adsorption (J mol^{−1}), K_{T} is the equilibrium binding constant (L g^{−1}), R the gas constant (8.314 J mol^{−1}・K^{−1}), T is the absolute temperature (K), Q_{e} is the amount of BPB adsorbed per mass of adsorbent (mg・g^{−1}) and C_{e} is the residual concentration of BPB at the equilibrium. The values of the parameters are given in ^{2} = 0.909) indicates that Temkin’s model fits the BPB adsorption on the activated carbon. The constant of Temkin K_{T} (0.317 mg・g^{−1}) is less than unity, suggesting a low affinity between the adsorbate/adsorbent, and the positive value of b_{T} (24.49 kJ mol^{−1}) indicates that the heat of adsorption due to interactions with adsorbate decreases linearly with the recovery rate [

4) Dubinin-Raushkevich isotherm

Dubinin and Radushkevich (D-R) proposed another isotherm used for analysis [

ln Q e = ln ln X m − K D − R ϵ 2 (7)

where, ϵ = R T ln ( 1 + 1 / C e ) , Q_{e} is the amount adsorbed per unit mass of adsorbent (mg・g^{−1}), X_{m} is the adsorption capacity (mg・g^{−1}), C_{e} is the equilibrium concentration of BPB in solution (mg・L^{−1}), R the gas constant (8.314 J mol^{−1}・K^{−1}), K_{D}_{−}_{R} is the constant related to the adsorption energy (mol^{2}・kJ^{-2}) and T is the absolute temperature (K).

The plot of lnQ_{e} versus ϵ^{2} gives a straight line. The values of X_{m} (0.309 mg・g^{−1}) and K_{D}_{−}_{R} (2.681 mol^{2}・kJ^{-2}) are calculated from the intercept and slope (_{D}_{−}_{R}) for activated carbon toward BPB was found to be more than unity, indicating that sorption of BPB on activated carbon may not be significant. That result confirms the weak adsorption capacity calculated by the Langmuir isotherm (_{D}_{−}_{R} value, the mean energy of adsorption E (kJ・mol^{−1}) is calculated using the following equation:

E = − 1 / 2 K D − R (8)

The adsorption mechanism estimated by the magnitude of E (0.43 kJ・mol^{−1}) is less than 16 kJ・mol^{−1} for activated carbon, which indicates that adsorption seems to be given by particle diffusion.

Since there are no other published results on BPB removal by adsorption at our knowledge, the comparison with other adsorbents is done based on the adsorption capacity of activated prepared from coconut shell (

The kinetics of the adsorption phenomenon are determined by the mass transfer

Pollutant | Adsorption Capacity (mg・g^{−1}) | Reference |
---|---|---|

Reactive Blue 19 | 2.78 | [ |

Ammonium ions | 2.32 | [ |

Zn (II) | 4.28 | [ |

BPB | 7.52 | This study |

to the liquid-solid interface. Adsorption kinetics modelling provides best information about the reaction pathways. The results are fitted according to different kinetics models such as the pseudo-first order equation [

There are three steps in an adsorption process [

1) Pseudo-first order model

The adsorption of pseudo-first order equation is established by Lagergren in 1898 [

ln ln ( Q e − Q ) = ln Q e − k 1 t (9)

where k_{1} (min^{−1}) is the rate constante of the adsorption, Q and Q_{e} are the amount adsorbed at any time and at equilibrium respectively. The plot of ln(Q_{e}−Q) versus t give a straight line with correlation coefficient of 0.988, indicating the applicability of this equation. The values of k_{ad} and Q_{e} are calculated from the slope and intercept. The calculated value Q_{theo} (0.31 mg・g^{−1}) found from pseudo-first order equation (_{exp} (0.39 mg・g^{−1}).

2) Pseudo-second order model

The experimental data are tested for pseudo-second order, according to Ho and McKay’s [

t / Q = 1 / K 2 Q e 2 + t / Q e (10)

where k_{2} is the rate constant of sorption (g mg^{−1}・min^{−1}); Q_{e} is the amount of adsorbate at equilibrium (mg・g^{−1}); Q is the amount of adsorbate on the surface of the adsorbent at any time (mg・g^{−1}). The plot of t/Q versus t gives a straight line with correlation coefficient 0.995. The calculated adsorption capacity from the pseudo-second order equation (_{exp} (0.39 mg・g^{−1}).

3) Normalized standard deviation equation (ΔQ(%))

According to the correlation coefficient, the two models are close to the unity which accurately shows that the adsorption of BPB on this activated carbon follows pseudo second order. A normalized standard deviation, ΔQ(%) is

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

q_{e,exp} (mg g^{−1}) | q_{e,cal} (mg・g^{−1}) | k_{1} (min^{−1}) | ∆q (%) | R^{2} | q_{e,cal} (mg・g^{−1}) | K_{2} (g mg^{−1}・min^{−1}) | ∆q (%) | R^{2} |

0.39 | 0.31 | 0.022 | 20.4 | 0.988 | 0.43 | 0.109 | 10.2 | 0.995 |

calculated to compare the efficiency of adsorption isotherms.

Δ Q ( % ) = 100 × ( ∑ [ ( Q t exp − Q t c a l ) / Q t exp ] 2 ) 1 / 2 / ( n − 1 ) (11)

where, Q t exp is the experimental amount adsorbed at different time t, Q t c a l is the calculated amount adsorbed at different times and n is the number of observations. The ΔQ(%) value of pseudo second order (ΔQ(%) = 10.2) is smaller than that of the pseudo first order (ΔQ(%) = 20.4), confirming that the adsorption of BPB on activated carbon follows the pseudo second order equation (_{e} for the activated carbon (

4) Intra-particle diffusion model

The intra-particle diffusion model (

q t = K p t 1 / 2 + C (12)

where, q_{t} is the amount of BPB adsorbed at time t, C is the resistance to the mass transfer in the film and K_{p} is the intra-particle diffusion rate constant (mg g^{−1}・min^{1/2}).

The value of (K_{p}), C and the correlation coefficient are given in ^{2} = 0.943) indicates that the intra-particle transport is not the only rate of BPB adsorption onto activated carbon. This could be due to the adsorption of the solvent on the activated carbon area. Which would prevent the molecules of BPB to penetrate into the pores of activated carbon.

The effect of temperature on the adsorption of BPB by AC is shown in

Thermodynamic adsorption parameters are determined from the following relationships:

Δ G ˚ = − R T ln K L (13)

Δ G ˚ = Δ H ˚ − T Δ S ˚ (14)

where, K_{L} is Langmuir constant determined from the Langmuir isotherm.

Intra-particle diffusion | ||
---|---|---|

k_{p} (mg g^{−1} min^{−1/2}) | C (mg g^{−1}) | R^{2} |

0.029 | 0.058 | 0.943 |

The results are summarized in ^{−1}). The positive value of ΔS˚ (0.019 kJ mol^{−1}・K^{−1}) explains that the degree of randomness at the solid-liquid interface increased during BPB adsorption onto the activated carbon. Similar behavior was also reported for adsorption of Reactive Blue-19 on coconut shell based activated carbon [

Temperatures (K) | ΔG˚ (kJ mol^{−1}) | ΔH˚ (kJ mol^{−1}) | ΔS˚ (J mol^{−1 }K^{−1}) |
---|---|---|---|

303 | 5.97 | ||

323 | 6.37 | 0.2 | 19 |

335 | 6.61 | ||

348 | 6.86 |

A cost-effective activated carbon was used as adsorbents for the removal of BPB from aqueous solution by adsorption process. The effects of some parameters (contact time, temperature, pH, and ionic strength) on adsorption were evaluated. The adsorption increased with increasing adsobent dose, while it was decreasing with increasing pH, temperature and ionic strength. The experimental adsorption capacities of the AC for the adsorption of BPB determined as 98% in acidic media. The Langmuir adsorption capacity of the BPB was found to be 7.52 mg・g^{−1}. The adsorption was in consistent with the Langmuir, Freundlich and Temkin isotherm models and with the pseudo-second order kinetic model. The adsorption was of non-spontaneous and endothermic nature. The CBAC could be also used for the process design and it could be used as a potential adsorbent for the removal of various compounds from wastewaters.

The coconut shells used in this present work are considered as waste and abundantly available with promises affordable adsorbent for the removal of BPB from aqueous solution that is suitable for the wastewater treatment.

The authors declare no competing financial interest.

This study was financially supported by The National Sciences and Engineering Research Council of Canada. Authors thank Dr. Nicolas Keller at Institut de Chimie et Procedés Pour l’Energie, l’Environnement et la Santé (ICPEES), CNRS University of Strasbourg, France, for his support. Author N.B.A. acknowledges CSIR-India and TWAS-Italy for award of the CSIR-TWAS fellowship (FR number: 3240280453) for postgraduate studies at CSIR-NEIST, Jorhat, India.

Atheba, P., Allou, N.B., Drogui, P. and Trokourey, A. (2018) Adsorption Kinetics and Thermodynamics Study of Butylparaben on Activated Carbon Coconut Based. Journal of Encapsulation and Adsorption Sciences, 8, 39-57. https://doi.org/10.4236/jeas.2018.82003