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Acid Yellow 25 (AY25) is used in the textile industry for dyeing of natural and synthetic fibers, and is also used as a coloring agent in paints, inks, plastics, and leathers. Effluents from such industries are major sources of water pollution. Hence, it is important to find simple, efficient, and inexpensive ways to remove these dyes from wastewater. Here, we determined the suitability of chitin extracted from waste crab legs as an adsorbent for removing AY25 dye. The adsorption kinetics was modeled using pseudo-first order, pseudo-second order, and intraparticle diffusion equations to determine the rate controlling step. Results showed that the pseudo-second order adsorption mechanism is predominant, and the overall rate of the dye adsorption process is therefore controlled by an adsorption reaction. Adsorption isotherms were analyzed by utilizing the Langmuir, Freundlich, Dubinin-Radushkevich (D-R) and Temkin isotherm models at 23℃, with data collected by using various initial dye concentrations with different chitin dosages. Our results show the highest correlation with the Langmuir model, consistent with the fact that chitin contains both a monolayer and homogeneous adsorption sites. Based on the D-R model, the adsorption of AY25 dye onto chitin is via chemisorption. Furthermore, we have concluded that the rate constants of both pseudo-second order adsorption and film diffusion are correlated to the initial dye concentrations and chitin dosages. In conclusion, chitin from waste crab legs is a very suitable adsorbent material that is capable of rapidly removing up to 95% of the initial concentration of AY25 dye at a pH of 2 and room temperature.

Dyes are widely used by textile, food, paper, paint, and pharmaceutical industries [

Currently, there are many approaches for the treatment of dye wastes. Chemical methods like coagulation, precipitation, electrochemical destruction, electro-flocculation, irradiation, and oxidation, are used to treat wastewater dyes [

inexpensive and the breakdown products are completely mineralized. However, the microbiological approach needs a large land area due to the high volume of sludge generated, making this solution a challenge. Furthermore, some of the industrial dyes are resistant to aerobic digestion and thus may not be effectively removed due to their extremely slow oxidation rate [

The consumption rate of snow crabs in the United States is nearly 50,000 tons per year [

This crab-waste recycling method will benefit both the food industry and the textile industry by saving on their waste management expenses and their effluent treatment cost, respectively. The objective of this research is to study the suitability of using chitin prepared from waste snow crab legs to remove AY25 dye from aqueous solutions.

Acid Yellow Dye AY25 (CAS 6353-85-9) was purchased from Sigma-Aldrich (St. Louis, MO) without further purification. Waste snow crab (Chionoecetes opilio) legs were obtained from Dierbergs Market (Edwardsville, IL). All other reagents were purchased from Fisher Scientific.

The preparation of chitin was carried out as stated in previous reports. Briefly, loose tissues inside the crab legs were scraped out and the shells were washed thoroughly with tap water and finally with deionized water. The shells were dried at room temperature for 2 to 3 days and were ground to a fine powder, followed by demineralization as described previously [^{−1} and N-H stretching at 3265 and 3100 cm^{−1} using the Nicolet iS5 FT-IR spectrometer (Thermo Scientific), as well as X-Ray Power Diffractometry (XRD) [_{β} radiation (λ = 1.5406 Å). The relative intensities were recorded between 5^{o}-40° (2θ = 5˚ - 40˚) at room temperature. The X-ray diffractometer was operated with 1˚ diverging and receiving slits at 50 kV and 40 mA and a continuous scan was carried out with a step size of 0.015˚ and a step time of 0.2 seconds. Peaks were compared with the α-chitin peaks obtained from the MDI JADE data base. Five reflections at angles of 9.3˚, 19.3˚, 23˚, 26˚, and 36.6˚ were detected.

The effects of chitin dosage, pH, and temperature were determined using batch experiments, in which the different amounts of chitin were placed in Erlenmeyer flasks containing different dye concentrations at varying pH and temperatures. A calibration curve of absorbance at 392 nm versus the known concentration of AY25 was obtained in order to determine the dye concentration that remained in the solutions for the characterization below. The total volume for each flask was 25 mL and the flasks were placed on a water-bath shaker (New Brunskwick Scientific, Edison NJ) at 70 rpm. Approximately 1 mL of dye solution from each flask was removed at different time intervals for 150 minutes. The samples were centrifuged at 13,000 rpm for 1 minute using a Brinkmann 5412 centrifuge. The absorbance values of the resulting supernatants after dilution were measured at 392 nm using a UV-1800 double-beam spectrometer (Shimadzu, Kyoto, Japan). The dye concentrations remaining in each solution (after correction for dilution factor if necessary) were determined. The dye removal efficiency (E%) was calculated as:

E % = C i − C f C i × 100 % or A i − A f A i × 100 % (1)

where C_{i} (or A_{i}) and C_{f} (or A_{f}) are the initial and final concentrations in mg/L (or absorbance at 392 nm) of AY25 in liquid, respectively. The adsorption capacity at time t (q_{t} in unit of mg/g) was determined by Equation (2).

q t = ( C o − C t ) V m (2)

where C_{o} and C_{t} are the initial dye concentration and the concentration at time t in the liquid phase (mg/L), respectively, m is the mass of chitin (g), and V is the volume of the aqueous phase (L). Adsorptive capacity at equilibrium (q_{e}) is a measurement of how well a certain adsorbent adsorbs the adsorbate in solution. This term is utilized for a better comparison and is important to judge the performance of an adsorbent. The value of q_{e} can be calculated by the equation below.

Adsorptive Capacity ( q e ) = Amount of dye adsorbed ( mg ) Amount of Chitin ( g ) (3)

The data presented in the results section are reproducible and are from at least four separated trials using at least two different batches of chitin.

We isolated chitin from waste crab legs by the demineralization and deproteinization processes as described previously [_{a}, as seen in P-toluenesulfonic acid with its pK_{a} = −2.1. Similarly, the solutions with a higher pH resulted in less protonation of the

amino group in chitin that weakens the ionic interaction as well. We also determined the effect of temperature on dye adsorption by chitin by utilizing 58 mg/L AY25 with 3 g/L chitin at pH 2 and 70 rpm. Our results revealed that higher temperatures have a lower efficiency of dye removal (

We then determined the dosage effect of chitin at 23˚C. At a low initial dye concentration (58 mg/L), the adsorption of dye onto chitin is very effective and reaches equilibrium quickly. For example, the dye removal reaches 90% within 15 min of contact time with the exception of the chitin dosage of 1 g/L, which reaches equilibrium in ~1 hr (

Adsorption kinetics provides information about the interactions between the adsorbent and adsorbate, which are important to understand the adsorption process conditions. Typically, dye adsorption kinetics is influenced by the adsorption and mass transfer steps, including external- and intra-particle diffusions that are related to the structure of the absorbent (e.g. particle size and

intrinsic property), the nature of adsorbate, and the process conditions such as temperature, pH, and mixing speed. Many models, including pseudo-first order, pseudo-second order, and intra-particle diffusion models, have been employed to describe the kinetic processes.

The pseudo-first order kinetic model is expressed as:

log ( q e − q t ) = log q e − k 1 2.303 t (4)

where q e is the amount of dye adsorbed at equilibrium (mg/g), q t is the amount of dye adsorbed at time t, and k_{1} is the equilibrium rate constant for pseudo-first order kinetics (min^{−1}). The pseudo-first order model is based on the rate of adsorption being related to the concentration difference of adsorbate between the solution and the surface of adsorbent. In such a process, the adsorption process is primarily controlled by the external mass transfer coefficient. For this model, a linear plot of log ( q e − q t ) versus t (Equation (4)) is anticipated and the resulting slope corresponds to the value of k_{1}. However, the pseudo-first order equation cannot be applied well for a slow adsorption process and, therefore, is generally limited for the adsorption with a short contact time [^{2} values (

We then fit our data with a pseudo-second order model, which includes the same equation for both internal and external mass transfer mechanisms as expressed by the following formula:

t q t = 1 k 2 q e 2 + ( 1 q e ) t (5)

where k_{2} is the pseudo-second order rate constant (g・mg^{−1}・min^{−1}). An example showing a linear fit of t/q_{t} versus contact time (t) for 165 mg/L initial dye concentration with 3, 4, and 5 g/L of chitin is shown in ^{2} > 0.9993 yields the pseudo-second order rate constant k_{2} and the calculated q_{e} (q_{e}_{,cal}) agrees with the experimental ones (q_{e}_{,exp}) derived from the experiments

Initial Conc. (mg/L) | q_{e}_{,exp} (mg/g) | Pseudo-first order | Pseudo-second order | ||||
---|---|---|---|---|---|---|---|

q_{e,cal} (mg/g) | k_{1} (min^{−1}) | R^{2} | q_{e,cal} (mg/g) | k_{2} (g・mg^{−1}・min^{−1}) | R^{2} | ||

58^{a} | 17.73 | 0.097 | 0.002 | 0.0427 | 17.67 | 0.3027 | 0.9999 |

58^{b} | 13.54 | 0.124 | 0.008 | 0.7734 | 13.49 | 0.4210 | 0.9999 |

58^{c} | 10.85 | 0.163 | 0.016 | 0.7404 | 10.82 | 0.6668 | 0.9999 |

165^{a} | 53.88 | 1.16 | 0.031 | 0.8454 | 54.35 | 0.0385 | 1.0000 |

165^{b} | 35.87 | 0.434 | 0.014 | 0.5338 | 35.97 | 0.1137 | 1.0000 |

165^{c} | 28.82 | 0.113 | 0.008 | 0.6067 | 28.82 | 0.5016 | 1.0000 |

284^{a} | 92.07 | 17.29 | 0.040 | 0.9437 | 92.94 | 0.0070 | 0.9999 |

284^{b} | 69.32 | 2.354 | 0.027 | 0.8829 | 69.64 | 0.0277 | 1.0000 |

284^{c} | 55.58 | 0.719 | 0.013 | 0.5887 | 55.56 | 0.0562 | 0.9999 |

385^{a} | 127.17 | 65.90 | 0.045 | 0.9818 | 130.72 | 0.0021 | 0.9993 |

385^{b} | 98.08 | 25.55 | 0.047 | 0.9656 | 99.90 | 0.0049 | 0.9997 |

385^{c} | 78.64 | 4.61 | 0.032 | 0.8775 | 79.18 | 0.0137 | 0.9999 |

^{a,}^{b,c}The concentrations of chitin are 3, 4, 5 g/L, respectively.

conducted shown in _{2} is associated with a lower initial dye concentration for a fixed chitin dosage (see _{2} is a complex function of initial concentration C_{o} [

k 2 = C o ( A k C o + B k ) and

1 / k 2 = A k + B k ( 1 / C o ) (6)

where A_{k} (mg・min・g^{−1}) and B_{k} (mg^{2}・min・g^{−1}・L^{−1}) are fitting constants, and can be determined by the plot of 1/k_{2} versus 1/C_{o}. Our data shows the linearity for each plot at different chitin dosages (_{k} values of 3.346, 4.342, and 5.816 mg^{2}・min・g^{−1}・L^{−1} for 3, 4, and 5 g/L chitin, respectively. The values of B_{k} increase with the increase of dosage, indicating that k_{2} is also dependent on chitin dosage. Interestingly, our data can be fit well when considering the amount of adsorbent into Equation (6), as shown in the following.

1 / k 2 = A k + B ′ k ( 1 / C o D ) (7)

where D is the amount of chitin in g/L and B ′ k (mg^{2}・min・L^{−2}) is the new fitting parameter. The values of A_{k} and B ′ k obtained by the fitting with Equation (7) are 0 mg・min・g^{−1} and 0.222 mg^{2}・min・L^{−2}, respectively, with R^{2} = 0.9824 (the insert in

Our observations suggest an intra-particle diffusion that may be involved in this sorption phase. The possibility can be explored by plotting q_{t} versus t^{1/2} (i.e. W-M plot) based on the Weber and Morris model [

q t = k t 1 / 2 + C (8)

where k is the intra particle diffusion rate constant (mg・g^{−1}・min^{1/2}). While k provides information for the diffusion process, the value of C carries profound information, such as the thickness of the boundary layer that controls the process. For C = 0 (i.e. the fitting line passes through the origin), the intraparticle diffusion is the rate controlling step. Multilinearity of W-M plots has been reported. Typically for adsorption, the first linear sorption process involves the fastest dye migration to external surfaces of chitin via either external surface adsorption or boundary layer diffusion. The second linear adsorption phase is attributed to a slower intra-particle diffusion followed by a third linear final equilibrium stage, in which the diffusion is very slow due to very low dye concentration left in the solution. We only observed two linear portions of the adsorption in our case. The adsorption process is very rapid such that the adsorption kinetics cannot be determined precisely during the onset of the adsorption. _{t} versus t^{1/2} for 385 mg/mL dye and 3, 4, and 5 g/L chitin, in which the first

linear region corresponds to the boundary layer diffusion with the k_{1} values of 9.3235, 3.5211, and 1.4478 min^{−1}. The second linear portions can be explained by the intraparticle diffusion, a slow process giving k_{2} values of 0.3457, 0.1582, and 0.0972 mg・g^{−1}・min^{1/2} (_{1} (the uptake process) is more significantly affected by the initial concentration (C_{o}), but not by the mass of chitosan. They obtained straight lines from the plot of lnk_{1} versus lnC_{o} and demonstrated that the first phase of adsorption can be fitted by:

k 1 = A n C o B n (9)

where A_{n} and B_{n} are the fitting parameters. However, our study showed that the first phase of the adsorption of AY25 onto chitin is also affected by the amount of adsorbent (D). The plot of ln(k_{1}) versus ln([dye]/[chitin]) in our study suggests

Initial Conc. (mg/L) (dye/chitin: mg/g) ___ | Film and Intra-particle diffusion models | ||||
---|---|---|---|---|---|

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

58 (19.33)^{a} | 2.2595 | 0.9798 | 0.0253 | 0.8761 | |

58 (14.50)^{b} | 1.0496 | 0.5875 | 0.001 | 0.5875 | |

58 (11.60)^{c} | 0.9553 | 0.8717 | 0.011 | 0.7828 | |

165 (55.00)^{a} | 2.6921 | 0.7421 | 0.1855 | 0.9827 | |

165 (41.25)^{b} | 3.5274 | 0.9617 | 0.0461 | 0.7548 | |

165 (33.00)^{c} | 1.352 | 0.7226 | 0.017 | 0.9663 | |

284 (94.67)^{a} | 4.1732 | 0.8276 | 0.1719 | 0.9397 | |

284 (71.00)^{b} | 2.7384 | 0.9189 | 0.0991 | 0.9799 | |

284 (56.80)^{c} | 2.5635 | 0.9614 | 0.0704 | 0.8624 | |

385 (128.33)^{a} | 9.3235 | 0.9749 | 0.3457 | 0.9628 | |

385 (96.25)^{b} | 3.5211 | 0.9795 | 0.1583 | 0.9669 | |

385 (77.00)^{c} | 1.4478 | 0.8432 | 0.0972 | 0.8584 |

^{a,}^{b,c}The concentrations of chitin are 3, 4, 5 g/L, respectively.

a correlation among the film diffusion rate, the dye and chitin concentrations. Thus, Equation (9) can be modified as:

k 1 = A n ( C 0 D ) B n (10)

Note that the poor fitting (R^{2} = 0.7945) is mainly due to the experimental errors associated with the measurements for the rapid adsorption of this phase. Though the basis of the correlation is not known, our results indicate that the mass transfer process of AY25 adsorption onto the chitin surface from the onset of the process is very quick, followed by a slower intraparticle diffusion, and both processes are controlled by the “availability” of dye to the surface of chitin.

Adsorption isotherms can be used to explain the adsorption in equilibrium at a specific temperature, which provides a better understanding of the surface properties of the adsorbent and the adsorption behavior. Thus we analyzed our data using several established adsorption isotherm models, including Langmuir, Freundlich, and Dubinin-Radushkevich (D-R). To do so, we studied the effect of chitin dosage on equilibrium dye concentration (C_{eq}) by using an initial dye concentration of 385 mg/L and chitin dosages from 1 to 5 g/L, shown in

is shown in the following.

q e q max = b C e 1 + b C e (11)

where q_{e} and q_{max} are the equilibrium concentration of adsorbate adsorbed per adsorbent (mg/g) and saturation capacity of adsorbent (mg/g) forming the monolayer, respectively; C_{e} is equilibrium concentration of the adsorbate (mg/L) and b is the Langmuir adsorption constant (L/mg) that is related to the heat of adsorption. The essential characteristic of the Langmuir isotherm can be expressed by the dimensionless constant called the equilibrium parameter (separation factor), R_{L}, as defined below.

R L = 1 1 + b C o

where C_{o} is the initial dye concentration (mg/L).

Our results reveal that the dye adsorption process fits the Langmuir adsorption model well (_{L} values indicate the type of isotherm to be irreversible (R_{L} = 0), favorable (0 < R_{L} < 1), linearly adsorptive (R_{L} = 1), or unfavorable (R_{L} > 1) [_{L} value for the adsorption of AY25 dye onto chitin is 0.045 (

In contrast to the Langmuir isotherm, the empirical Freundlich isotherm is used to describe the adsorption onto a heterogeneous surface or surfaces containing binding sites with different binding affinities. In this model, the strong binding site is occupied first followed by a weaker binding. The Freundlich isotherm is given by Equation (12) [

q e = K F C e q 1 / n

log q e = 1 n log C e q + log K F (12)

where K_{F} and n are the Freundlich constant and heterogeneity factor (or adsorption intensity), and C_{eq} is the concentration of adsorbate remaining in solution. The 1/n values indicate the type of isotherm to be irreversible (1/n = 0), favorable (0 < 1/n < 1), or unfavorable (1/n > 1). If n = 1 then the partition between the two phases are independent of the concentration. The calculated R^{2} value (R = 0.9884) indicates that the equilibrium data in this study fairly agrees with the Freundlich model, presenting heterogeneous adsorption sites on the surface of chitin. This is probably because multiple interactions are present in this system where dye could interact with different sites of the adsorbent. Our results also show that the 1/n value (0.4195) for AY25 dye adsorption is between 1 and 0,

Isotherm Model | Parameters | |||
---|---|---|---|---|

Langmuir | q_{max} (mg/g) | b (L/mg) | R_{L} | R^{2} |

303 | 0.045 | 0.055 | 0.9951 | |

Freundlich | K_{F} (L/g) | 1/n | R^{2} | |

37.35 | 0.4195 | 0.9884 | ||

D-R | q_{D} (mol/g) | β_{D} (mol^{2}/J^{2}) | E_{DR} (kJ/mol)_{ } | R^{2} |

0.00202 | 3.36 × 10^{−9} | 12.20 | 0.9934 | |

Temkin | A_{T} (L/mg) | B (mol^{2}/kJ^{2}) | b_{T} (J/mol)_{ } | R^{2} |

0.888 | 46.14 | 53.34 | 0.9467 |

and thus the adsorption process is favorable under the studied conditions.

We then used the Dubinin-Radushkevich (D-R) isotherm model to test the nature of the chemical and physical means of adsorption [

q e = q D exp ( − β D ε 2 )

ln q e = ln q D − β D ε 2 (13)

where q_{D} is Dubinin-Radushkevich monolayer saturation capacity (mol/g); β_{D} is the constant related to the mean free energy of adsorption per mole of the adsorbate (mol^{2}/J^{2}); and ε is the Polyani potential (J/mol) and defined as ε = R T ln ( 1 + 1 / C e q ) ) (R is the gas constant and T is absolute temperature). By plotting lnq_{e} versus ε^{2}, q_{D} and β_{D} can be obtained. The constant β_{D} gives an idea about the mean free energy, where the mean free energy can be calculated using

E D R = 1 2 β D . All initial dye concentrations of AY25 dye tested show a good

agreement to the D-R isotherm model. The correlation for this isotherm is higher when compared to the Freundlich isotherm.The fitted results indicate the E_{DR} value is 12.20 kJ/mol (

The Temkin adsorption isotherm is used to determine the interaction between adsorbate and adsorbent [

q e = R T b T ln ( A T C e )

q e = B ln A T + B ln C e (14)

where A_{T} is the Temkin isotherm equilibrium binding constant (L/mg); b_{T} is the Temkin isotherm constant related to the heat of adsorption (J/mol), and B = RT/b_{T} (R is 8.314 J mol^{−1} K^{−1} and T is absolute temperature in Kelvin). A plot of q_{e} versus lnC_{e} enables the determination of the isotherm constants B and A_{T} from the slope and the intercept, respectively. The calculated coefficient of determination (R^{2}) shows that the experimental data does not have a very good fit (R^{2} = 0.9471) with the Temkin isotherm (

Here we have shown that the AY25 adsorption onto chitin (particle size of 297 to 177 μm) at pH 2 and room temperature with a shaking rate of 70 rpm can be described best by a pseudo-second order model. The intraparticle study indicates that the adsorption is mainly controlled by a film adsorption process. By varying the amounts of AY25 and chitin, we have determined that the rates of pseudo-second order adsorption and boundary layer diffusion that are a complex function of both initial concenentration of AY25 (C_{o}) and the dosage of chitin (D) as defined in Equations (7) and (10).

The adsorption isotherms of AY25 onto chitin were also analyzed using different models, including the Langmuir, Freundlich, D-R, and Temkin isotherms. Our adsorption data showed a good correlation for all isothermal models except for the Temkin model. Among them, the Langmuir isotherm showed the highest correlation, suggesting that the surface of chitin contains a monolayer with heterogeneous adsorption sites. Additionally, the separation factor (R_{L}) from the Freundlich isotherm indicated that such an adsorption is favorable. The mean free energy of adsorption in D-R was 12.2 kJ/mol, which accounts for the adsorption of AY25 dye onto chitin being chemisorption, primarily through ion-exchange interactions. The maximum AY25 adsorption capacities of chitin (q_{max}) obtained from the Langmuir and Freundlich isotherm models were ~300 and 320 mg/g, which are comparable to those of chitin with other acidic dyes, such as Acid Blue 25 and 158 [

In summary, our study demonstrates that chitin can be effectively used for the removal of AY25 dye as shown by its high adsorption capacity and rapid adsorption via chemisorption and film mass transfer processes. Our study on the effects of pH and temperature also indicates that wide ranges of pH (2 - 7) and temperature (20˚C - 70˚C) are adequate for removal of the majority of the dye from solution, making chitin as an ideal adsorbent for AY25 in textile water.

We thank Marie Gipson for assisting with the preparation of chitin. We thank the internal financial support from Southern Illinois University Edwardsville: Seed Grants for Transitional and Exploratory Projects (STEP) to CW and the Undergraduate Research and Creative Activities (URCA) Associate and Assistant Awards to EF, KS, and NS. This work was supported in part by the National Science Foundation grant NSF-CHE #1608484 to CW.

Wei, C.-C., Pathiraja, I.K., Fabry, E., Schafer, K., Schimp, N., Hu, T.-P. and Norcio, L.P. (2018) Removal of Acid Yellow 25 from Aqueous Solution by Chitin Prepared from Waste Snow Crab Legs. Journal of Encapsulation and Adsorption Sciences, 8, 139-155. https://doi.org/10.4236/jeas.2018.83007