Adsorption Characteristics of Allura Red AC onto Sawdust and Hexadecylpyridinium Bromide-Treated Sawdust in Aqueous Solution.

The Allura red AC (ARAC) dye adsorption onto natural sawdust (NSD) and hexadecylpyridinium bromide-treated sawdust (MSD) was investigated in aqueous solution as a function of contact time, solution pH, particle size, adsorbent dosage, dye concentration, temperature, and ionic strength. The adsorbents were characterized by Fourier transform infrared spectroscopy and X-ray diffraction crystallography. The dye adsorption onto both adsorbents was confirmed by field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy. The maximum dye adsorption was found within 120 min at pH 2.0 for NSD and pH 3.0 for MSD, respectively, with a particle size of 0-75 μm and an adsorbent dosage of 0.07 g/50 mL ARAC dye solution (50 μmol/L). The batch adsorption kinetic data were followed by the pseudo-second-order kinetic model rather than the pseudo-first-order and Elovich kinetic models. Equilibrium adsorption isotherms were explained by the Langmuir isotherm model, and the maximum extent of adsorption was found to be 52.14 μmol/g for NSD and 151.88 μmol/g for MSD at 55 °C. The values of activation energy (E a) and thermodynamic parameters (ΔG ⧧, ΔH ⧧, ΔS ⧧, ΔG°, ΔH° and ΔS°) proved that the ARAC dye adsorption onto both adsorbents NSD and MSD is a spontaneous-endothermic physisorption process. ARAC (98-99%) was released from dye-loaded adsorbents in aqueous solution (pH ≥ 12) within 120 min. The adsorbents NSD and MSD were reused for a second time without significant loss of their adsorption efficiency.

Introduction

Dyes are a kind of organic compounds with a complex aromatic molecular structure which makes them more stable and non-biodegradable.[1,2] Various industries often use large amount of dyes that are responsible for producing wastewater and discharged into water bodies. The removal of dyes from these industrial effluents is desired not only for aesthetic reasons but also this wastewater could have high levels of BOD, COD, color, toxicity, surfactants, fibers, and turbidity.[3,4] Moreover, the breakdown products of dyes are toxic to aquatic life and carcinogenic to humans. A number of methods such as photodegradation,[5] coagulation,[6] flotation,[7] ultrafiltration-exchange,[8] chemical oxidation,[9] ozonation,[10] membrane filtration,[11] and electrochemical[12] have been recommended to clean wastewater. However, these techniques are highly complicated and overpriced. Therefore, these techniques are not reasonable to clean effluents of textile and food industries in developing countries such as Bangladesh.

The adsorption technique has received great importance for the protection of environment by eliminating textile and food dyes from wastewater.[13,14] Literature survey shows that various low-cost adsorbents such as peanut shell,[15] sugarcane bagasse,[16] banana peel,[17] activated carbon prepared from rice hull,[18] jackfruit seed,[19] and chitosan[20−23] have been used to remove organic dyes from water. However, these adsorbents do not have good adsorption capacity. Therefore, new adsorbents are still under development to improve the adsorption performance.

Sawdust is an enormous and accessible byproduct of sawmills in our country, Bangladesh. Thus, the appropriate use of this biomaterial must bring conspicuous economic and social benefits. The raw and modified sawdust (MSD) were used to eliminate organic dyes such as methylene blue,[24−26] acid green and acid red 14,[27] Congo red,[28] torque blue,[29] and tartrazine[30] from aqueous solution. It is noted that the adsorption of Allura red AC (ARAC; Figure a) azo dye onto sawdust and surfactant-MSD in aqueous solution was not reported in literature yet. However, chitosan,[31] cross-linked chitosan,[32]Spirulina platensis,[33] vine-trimming waste,[34] and activated carbons[35−37] were used to remove ARAC from aqueous solution.

Figure 1

Structure of Allura red AC dye (ARAC; a) and hexadecylpyridinium bromide (CPB; b).

In this study, the natural sawdust (NSD) and cationic surfactant hexadecyl perydinium bromide (CPB; Figure b)-MSD were prepared and characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The adsorption characteristics of ARAC onto NSD and MSD were investigated in aqueous solution as a function of contact time, solution pH, particle size of the adsorbents, adsorbent dosages, dye concentration, ionic strength, and solution temperature, respectively. To understand the kinetics and mechanisms of ARAC dye adsorption onto NSD and MSD, the batch adsorption kinetic data were examined by using various kinetic models. The dye adsorption equilibrium was studied at different temperatures. The equilibrium dye adsorption data were examined by various isotherm models. The dye adsorption onto NSD and MSD was checked by field emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis. The release of ARAC from dye-loaded adsorbents and the reuse of dye-loaded NSD and MSD were investigated. The dye adsorption capacity of the present adsorbents was compared with other adsorbents reported in the literature. The thermodynamics of the dye adsorption process was also investigated.

Results and Discussion

Characterization of Adsorbents

The FTIR spectra of NSD and MSD are shown in Figure . The spectrum of NSD (Figure a) showed the characteristic bands at 3420 (broad) cm–1 for the N–H and O–H stretching, 2922 cm–1 for alkyl C–H stretching, 1647 cm–1 for amide I (C=O group), 1559 cm–1 for amide II (N–H bending), 1458 cm–1 for −CH2 bending, 1373 cm–1 for overlapping of C–H bending and symmetric CH3 deformation, 1317 cm–1 for C–N, 1113 cm–1 for overlapping of asymmetric bridge oxygen (C–O–C) and C–O stretching, 1057 and 1036 cm–1 for overlapping of C–O and C–N stretchings, and 669 cm–1 for O–H deformation (out of plane). In the spectrum of MSD (Figure b), the intensity of the characteristic vibration band at 2918 cm–1 for symmetric C–H stretching was increased while the shoulder peak at 2852 cm–1 was attributed to the asymmetric C–H stretching vibration of saturated hydrocarbon. This is because of the increase in the aliphatic carbon content (from CPB) in MSD.[38] These results indicate the existence of CPB on the surface of MSD.

Figure 2

FTIR spectra of natural sawdust (a: NSD) and modified sawdust (b: MSD). The spectra were recorded in KBr.

Crystalline nature of the material is determined through an efficient technique known as XRD. Crystalline materials show well-defined peaks while noncrystalline or amorphous materials show a hallow instead of a well-defined peak.[39] The XRD pattern of NSD showed no sharp peak (Figure a), which indicates that NSD was noncrystalline in nature. On the other hand, the XRD pattern of MSD showed two sharp peaks at 2θ: 26.43 and 39.44°, respectively (Figure b), which suggests that MSD is crystalline in nature.

Figure 3

XRD Spectra of NSD (a) and MSD (b).

The point of zero charge (pHpzc) describes the condition when the electrical charge density on a surface is zero. The pHzpc values of NSD and MSD were determined by using the pH drift method.[40] The values of pHzpc were estimated to be 5.03 for NSD (figure not shown) and 7.43 for MSD (figure not shown), respectively. These results indicate that the surfaces of NSD will act as the positive-charged surface at pH < 5.03 and as the negative charged surface at pH > 5.03. On the other hand, the surfaces of MSD will act as the positive-charged surface at pH < 7.43 and as the negative-charged surface at pH > 7.43. A similar result was also observed in cationic surfactant CPB-modified peanut husk.[38]

Effect of Contact Time on Adsorption Capacity

Contact time is one of the important parameters for the assessment of practical application of the adsorption process. In order to determine the equilibrium time, the adsorption of ARAC dye (50 μmol/L) onto both adsorbents was studied as a function of contact time at pH 2.0 for NSD and at pH 3.0 MSD, respectively. The typical changes in UV–visible spectra of ARAC with time during adsorption onto NSD and MSD in aqueous solution are shown in Figure . It is noticed that NSD adsorbed 60% of ARAC (50 μmol/L) within 120 min (Figure a), whereas MSD adsorbed 100% of ARAC (50 μmol/L) within 120 min (Figure b). The initial rate of dye adsorption onto MSD was faster than that of NSD in aqueous solution (Figure ).

Figure 4

Typical changes in UV–visible spectra of ARAC with time during adsorption onto NSD (a) and MSD (b) in aqueous solution at 25 °C ([ARAC]0: 50 μmol/L; volume of dye solution: 50 mL; pH 2.0 for NSD and pH 3.0 for MSD; particle size of adsorbents: 0–75 μm; and adsorbent dosage: 0.07 g). The spectra were taken at 0, 1, 3, 5, 10, 15, 20, 30, 45, 60, 90, and 120 min.

The plots of q versus contact time, t, are shown in Figure . It is noted that the adsorption of dye onto both adsorbents occurred rapidly in the first 20 min, and thereafter, no noticeable dye adsorption was observed beyond a contact time of about 120 min. This may be because of strong attractive forces between the dye molecules and the adsorbents. The fast adsorption of dye molecules occurred on the external surface of the adsorbents followed by the pore diffusion into the adsorbents matrix to attain equilibrium. Further increase in contact time shows that there was no significant progress in the adsorption of ARAC dye onto both adsorbents (NSD and MSD) in aqueous solution. A similar result was found in textile dye adsorption onto activated carbons made from DVD and CD wastes.[41] Hence, all the adsorption experiments were conducted for 120 min.

Figure 5

Effect of contact time, t, on the amount of ARAC dye adsorption (q) onto NSD (circle open) and MSD (circle solid) in aqueous solution at 25 °C ([ARAC]0: 50 μmol/L; volume of dye solution: 50 mL; pH 2.0 for NSD and pH 3.0 for MSD; particle size of adsorbents: 0–75 μm; adsorbent dosage: 0.07 g). All lines are modelled pseudo-second-order adsorption kinetic traces produced by utilizing eq , and the qe(cal) and k2 values are recorded in Tables and 2, respectively.

Table 1

Comparison of Calculated and Experimental qe Values, and Kinetic Parameters for the Adsorption of ARAC onto NSD at Various Solution pHs, Initial Dye Concentration, Ionic Strengths, and Temperatures

		pseudo-first-order			pseudo-second-order				Elovich
parameter	qe(exp) (μmol/g)	qe(cal) (μmol/g)	k1 (min–1)	R2	qe(cal) (μmol/g)	k2 (g/μmol min)	h (μmol/g min)	R2	α (μmol/g min)	β (g/μmol)	R2
pH; [Dye]: 50 μmol/L, Tem.: 25 °C, Particle Size: 0–75 μm, Adsorbent: 0.07 g
2	22.08	4.79	0.049	0.931	22.27	0.036	18.02	0.999	7.70 × 103	0.569	0.928
3	16.91	3.99	0.034	0.963	17.04	0.032	9.35	0.999	7.56 × 104	0.937	0.994
4	5.75	3.02	0.035	0.997	5.92	0.032	1.11	0.997	1.67 × 101	1.392	0.970
5	3.84	2.49	0.032	0.996	4.01	0.030	0.49	0.995	0.35 × 101	1.788	0.945
6	2.96	1.62	0.040	0.928	3.06	0.071	0.67	0.999	0.35 × 101	2.189	0.970
[Dye] (μmol/L), pH 2.0, Temp.: 25 °C, Particle Size: 0–75 μm, Adsorbent: 0.07 g
50	22.08	4.79	0.049	0.931	22.27	0.036	18.02	0.999	7.70 × 103	0.569	0.928
70	27.04	9.28	0.063	0.991	27.47	0.021	15.95	0.999	1.19 × 103	0.382	0.943
100	34.97	9.30	0.042	0.977	35.46	0.012	15.48	0.999	6.28 × 103	0.351	0.985
150	42.82	11.04	0.043	0.987	43.67	0.008	15.13	0.999	1.72 × 104	0.306	0.991
200	43.20	11.15	0.049	0.981	44.05	0.008	14.56	0.999	2.03 × 103	0.267	0.992
Tem. (°C), pH 2.0, [Dye]: 50 μmol/L, Particle Size: 0–75 μm, Adsorbent: 0.07 g
25	22.08	4.79	0.049	0.931	22.27	0.036	18.02	0.999	7.70 × 103	0.569	0.928
35	23.71	5.40	0.061	0.913	23.92	0.040	22.62	0.999	1.40 × 104	0.553	0.917
45	24.66	4.74	0.062	0.928	24.88	0.042	26.25	0.999	2.68 × 104	0.558	0.927
55	25.61	4.42	0.069	0.957	25.84	0.047	31.15	0.999	2.40 × 105	0.630	0.927
Particle Size (μm); pH 2.0, [Dye]: 50 μmol/L, Tem.: 25 °C, Adsorbent: 0.07 g
0–75	22.08	4.79	0.049	0.931	22.27	0.036	18.02	0.999	7.70 × 103	0.569	0.928
75–150	20.99	4.72	0.046	0.953	21.19	0.034	15.38	0.999	7.4 × 103	0.602	0.926
150–300	18.98	5.90	0.052	0.985	19.23	0.029	10.56	0.998	1.38 × 103	0.579	0.971
300–600	17.48	5.34	0.048	0.961	17.73	0.029	9.24	0.998	1.25 × 103	0.630	0.968
Ionic Strength (mol/L); pH 2.0, [Dye]: 50 μmol/L, Tem.: 25 °C, Particle Size: 0–75 μm, Adsorbent: 0.07 g
0.05	19.56	4.48	0.039	0.946	19.72	0.034	13.16	0.999	7.33 × 103	0.654	0.952
0.10	17.99	4.71	0.034	0.979	18.15	0.028	9.07	0.999	1.11 × 104	0.759	0.996
0.20	16.09	4.03	0.033	0.985	16.21	0.031	8.14	0.999	2.85 × 104	0.926	0.995
0.30	14.93	4.21	0.033	0.952	15.06	0.030	6.78	0.999	1.79 × 103	0.793	0.991
0.40	13.88	4.73	0.032	0.975	14.03	0.024	4.79	0.998	4.43 × 102	0.756	0.995

Table 2

Comparison of Calculated and Experimental qe Values, Kinetic Parameters for the Adsorption of ARAC onto MSD at Various Solution pHs, Initial Dye Concentration, Ionic Strengths, and Temperatures

		pseudo-first-order			pseudo-second-order				Elovich kinetic model
parameter	qe(exp) (μmol/g)	qe(cal)(μmol/g)	k1 (min–1)	R2	qe(cal) (μmol/g)	k2 (g/(μmol min))	h (μmol/g min)	R2	α (μmol/g min)	β (g/μmol)	R2
pH; [Dye]: 50 μmol/L, Tem.: 25 °C, Particle Size: 0–75 μm, Adsorbent: 0.07 g
2	31.84	4.85	0.051	0.928	32.05	0.041	41.67	1.000	2.08 × 105	0.489	0.859
3	35.07	3.87	0.040	0.742	35.21	0.044	54.68	1.000	1.05 × 105	0.417	0.768
4	34.59	4.97	0.055	0.883	34.84	0.040	49.05	1.000	4.34 × 104	0.396	0.823
5	34.01	5.82	0.049	0.906	34.25	0.032	37.06	0.999	5.16 × 104	0.412	0.844
6	26.97	8.96	0.056	0.969	27.40	0.020	15.09	0.999	7.39 × 102	0.363	0.938
7	18.64	7.38	0.032	0.909	18.94	0.015	5.31	0.999	7.61 × 101	0.431	0.982
8	16.73	8.54	0.035	0.976	17.12	0.012	3.55	0.998	3.29 × 101	0.438	0.996
9	13.64	10.33	0.054	0.913	14.20	0.012	2.40	0.998	1.26 × 101	0.467	0.985
10	11.84	7.31	0.037	0.968	12.42	0.012	1.77	0.999	0.59 × 101	0.468	0.988
[Dye] (μmol/L), pH 3.0, Temp.: 25 °C, Particle Size: 0–75 μm, Adsorbent: 0.07 g
50	35.07	3.87	0.040	0.742	35.21	0.044	54.68	1.000	1.05 × 105	0.417	0.768
70	48.81	8.25	0.054	0.927	49.26	0.020	48.78	1.000	6.34 × 104	0.277	0.887
100	68.44	18.97	0.051	0.936	69.44	0.009	40.82	0.999	7.15 × 103	0.166	0.956
150	82.35	26.69	0.034	0.903	83.33	0.004	30.49	0.999	2.49 × 103	0.126	0.938
200	101.33	36.45	0.030	0.929	102.04	0.003	30.30	0.998	3.12 × 103	0.105	0.981
Tem. (°C), pH 3.0, [Dye]: 50 μmol/L, Particle Size: 0–75 μm, Adsorbent: 0.07 g
25	35.07	3.90	0.040	0.742	35.21	0.044	54.05	0.999	1.05 × 105	0.417	0.776
35	35.17	4.60	0.082	0.928	35.34	0.067	83.33	0.999	2.50 × 107	0.580	0.799
45	35.34	2.45	0.055	0.859	35.46	0.088	111.11	1.000	2.24 × 108	0.677	0.744
55	35.58	1.78	0.051	0.868	35.59	0.122	153.85	0.999	6.82 × 1013	1.005	0.735
Particle Size (μm); pH 3.0, [Dye]: 50 μmol/L, tem.: 25 °C, Adsorbent: 0.07 g
0–75	35.07	3.87	0.040	0.742	35.21	0.044	54.68	1.000	1.05 × 105	0.417	0.768
75–150	34.35	10.62	0.054	0.983	34.84	0.017	20.00	0.999	2.72 × 104	0.312	0.963
150–300	32.76	8.77	0.033	0.911	33.00	0.015	16.31	0.999	2.15 × 103	0.336	0.967
300–600	29.97	10.05	0.045	0.918	30.49	0.014	12.95	0.998	2.85 × 102	0.291	0.945
Ionic Strength (mol/L); pH 3.0, [Dye]: 50 μmol/L, Tem.: 25 °C, Particle Size: 0–75 μm, Adsorbent: 0.07 g
0.05	34.90	4.35	0.044	0.772	35.09	0.040	49.51	0.999	2.76 × 104	0.378	0.782
0.10	34.66	5.00	0.046	0.931	34.84	0.036	43.48	0.999	1.63 × 104	0.365	0.808
0.20	34.15	5.91	0.050	0.887	34.36	0.032	37.31	0.999	1.01 × 104	0.357	0.842
0.30	33.78	6.35	0.048	0.898	34.01	0.028	32.79	0.999	6.76 × 103	0.349	0.854
0.40	33.13	7.13	0.047	0.916	33.44	0.024	26.88	0.998	3.99 × 103	0.341	0.882

Effect of pH

The functional groups of the adsorbent and adsorbate can be protonated or deprotonated in aqueous solution depending on its pH. This causes electrostatic attraction or repulsion between charged adsorbates and adsorbents. The kinetics of dye adsorption onto NSD and MSD at various solution pHs is depicted in Figure . The adsorption of ARAC dye onto NSD was examined in aqueous solution at pH ranging from 2 to 6. The time of equilibrium dye adsorption gradually increased with increasing solution pH (Figure a). It is noticed that the initial dye adsorption rate and the extent of dye adsorption capacity of NSD was significantly decreased with an increase in solution pH (Figure a). The maximum amount of ARAC dye adsorbed onto NSD was obtained to be 22.08 μmol/g at solution pH 2 (Figure ). The ARAC dye behaves as an anionic dye molecule, and two sulfonate (−SO3–) groups (Figure a) may be electrostatically attracted by protonated amino (−NH2+) groups of NSD in acidic solution (pH 2). This electrostatic attraction between dye molecules and adsorption sites may become the reason to observe highest dye adsorption onto NSD at solution pH 2 (Figure ).

Figure 6

Effects of solution pHs on the adsorption of ARAC dye onto NSD (a) and MSD (b) in aqueous solution at 25 °C. Experimental conditions [ARAC]0: 50 μmol/L; volume of dye solution: 50 mL; particle size of adsorbents: 0–75 μm; and adsorbent dosage: 0.07 g. Solution pHs: circle open: pH 2; circle solid: pH 3; triangle up open: pH 4; triangle up solid: pH 5; box: pH6; box solid: pH 7; diamond open: pH 8; diamond solid: pH 9; multiplication: pH 10. All lines are modelled pseudo-second-order adsorption kinetic traces produced by utilizing eq , and the qe(cal) and k2 values are recorded in Tables and 2, respectively.

Figure 7

Plot for qe vs solution pH for the adsorption of ARAC dye onto NSD (circle solid) and MSD (circle open) in aqueous solution at 25 °C. Data were taken from Figure .

The adsorption of ARAC dye onto MSD was examined in solution at pH ranging from 2 to 10. The results are depicted in Figure b. The MSD was unstable in very acidic solution (pH 2). It is observed that the initial rate of dye adsorption, h (μmol/g min), amplified with rising solution pH up to pH 3 (Table ), and thereafter, it significantly reduced with expanding solution pH from 4 to 10 (Table ). After 120 min, the amount of equilibrium dye adsorption (qe) onto MSD versus solution pH is shown in Figure . The dye binding capacity of MSD at equilibrium increased from 32.05 to 35.21 μmol/g with increasing pH 2 to 3, slightly decreased in the region of 5 < pH < 6 and then significantly decreased at pH > 6. The maximum amount of ARAC adsorbed onto MSD was estimated to be 35.21 μmol/g at solution pH 3. A similar result was also observed in the adsorption of light green anionic dye onto CPB-modified wheat straw in aqueous solution.[42] Therefore, all further kinetic studies were implemented in solution at pH 2.0 for NSD and at pH 3.0 for MSD, respectively.

Table 3

Diffusion Rate Constants (kfd and kid) for the Adsorption of ARAC on NSD at Different Solution pHs, ARAC Dye Concentrations, Ionic Strengths, Particle Size, and Temperatures

	film diffusion model		intraparticle diffusion model
parameter	kfd (min–1)	R2	kid1 (μmol/g min0.5)	R2	kid2 (μmol/g min0.5)	R2	kid3 (μmol/g min0.5)	R2
pH; [Dye]: 50 μmol/L, Tem.: 25 °C, Particle Size: 0–75 μm, Adsorbent: 0.07 g
2	0.049	0.930	3.117	0.980	0.628	0.878	0.076	0.973
3	0.037	0.982	1.649	0.999	0.545	0.979	0.222	0.939
4	0.035	0.997	0.500	0.993	0.438	0.959	0.151	0.922
5	0.032	0.996	0.333	0.993	0.381	0.996	0.151	0.922
6	0.040	0.928	0.541	0.973	0.356	0.988	0.037	0.960
[Dye] (μmol/L), pH 2.0, Temp.: 25 °C, Particle Size: 0–75 μm, Adsorbent: 0.07 g
50	0.049	0.930	3.117	0.980	0.628	0.878	0.076	0.973
70	0.063	0.991	5.263	0.993	1.066	0.968	0.110	0.890
100	0.030	0.947	5.031	0.999	1.192	0.986	0.524	0.992
150	0.034	0.967	5.029	0.998	1.504	0.968	0.550	0.977
200	0.031	0.977	5.129	0.997	1.826	0.969	0.609	0.987
Tem. (°C), pH 2.0, [Dye]: 50 μmol/L, Particle Size: 0–75 μm, Adsorbent: 0.07 g
25	0.049	0.930	3.117	0.980	0.628	0.878	0.076	0.973
35	0.046	0.899	3.183	0.999	0.865	0.894	0.154	0.84
45	0.050	0.904	3.610	0.999	0.863	0.952	0.067	0.951
55	0.050	0.967	3.418	0.982	0.557	0.966	0.105	0.872
Particle Size (μm); pH 2.0, [Dye]: 50 μmol/L, Tem.: 25 °C, Adsorbent: 0.07 g
0–75	0.049	0.930	3.117	0.980	0.628	0.878	0.076	0.973
75–150	0.046	0.953	3.906	0.984	0.654	0.942	0.157	0.791
150–300	0.052	0.985	2.809	1.000	0.983	0.975	0.140	0.825
300–600	0.048	0.961	2.852	0.985	0.859	0.971	0.145	0.721
Ionic Strength (mol/L); pH 2.0, [Dye]: 50 μmol/L, tem.: 25 °C, Particle Size: 0–75 μm, Adsorbent: 0.07 g
0.05	0.039	0.946	3.043	0.991	0.590	0.984	0.162	0.959
0.10	0.034	0.979	1.848	0.999	0.672	0.968	0.230	0.956
0.20	0.033	0.985	1.281	0.991	0.572	0.989	0.215	0.96
0.30	0.033	0.952	1.756	0.986	0.796	0.982	0.219	0.985
0.40	0.032	0.975	1.959	0.989	0.674	0.991	0.258	0.983

Effect of Particle Size

The effects of adsorbent particle size on the adsorption kinetics of ARAC onto NSD and MSD in aqueous solution are depicted in Figure . It is noted that the dye adsorption capacity of both adsorbents (NSD; Figure a and MSD; Figure b) was decreased with increase in their particle size. This is because of the decrease in the surface area of the adsorbents with increase in their particle size. Similar results were also noticed for the removal of Congo red and methylene blue from aqueous solution by using marine alga Porphyra yezoensis Ueda[43] and activated carbons prepared from waste biomass,[44] respectively.

Figure 8

Effects of adsorbent particle size on the adsorption of ARAC onto NSD (a) and MSD (b) in aqueous solution at 25 °C. Experimental conditions: [ARAC]0: 50 μmol/L; volume of dye solution: 50 mL; adsorbent dosage: 0.07 g; and pH 2.0 for NSD and pH 3.0 for MSD. Particle size: circle open: 0–75 μm; circle solid: 75–150 μm; triangle up open: 150–300 μm; triangle up solid: 300–600 μm. All lines are modelled pseudo-second-order adsorption kinetic traces produced by utilizing eq , and the qe(cal) and k2 values are recorded in Tables and 2, respectively.

Effect of Adsorbent Dosage

The effects of adsorbent dosages on the dye removal percentage (%) and the extent of equilibrium dye adsorption (qe) onto the both adsorbents are exhibited in Figure , where the dye concentration was 50 μmol/L (50 mL) fixed. It is noticed that the dye removal percentage increased with increasing masses of both adsorbents (Figure a,b). This is because of the increase in surface areas of adsorbents and expanding the number of adsorption sites available for adsorption. On the other hand, the extent of dye uptake per gram of adsorbent (qe) was remarkably decreased with increasing the masses of both adsorbents (Figure a,b). These results can be mathematically explained by combining eqs and 22. As observed in eq , the amount of dye uptake (qe) and the mass of adsorbent (m) are inversely proportional. For a fixed dye percentage removal, the increase in adsorbent mass leads to a decrease in qe values because the volume (V) and initial dye concentrations (C0) are always constant. Similar results were also observed in the removal of Congo red and brilliant green dye from aqueous solutions by using the marine alga P. yezoensis Ueda[43] and home-made activated carbons,[45] respectively.

Figure 9

Effects of adsorbent dosages on the adsorption of ARAC onto NSD (a) and MSD (b) in aqueous solution at 25 °C. Experimental conditions: [ARAC]0: 50 μmol/L, volume of dye solution: 50 mL, particle size: 0–75 μm; equilibrium time: 120 min; and pH 2.0 for NSD and pH 3.0 for MSD. Symbols: circle open: qe (μmol/g) and red circle solid: dye removal (%).

Effect of Initial Dye Concentration

It is known that the initial concentration delivers the key driving force to overwhelm all mass transfer resistances of all molecules between the aqueous and solid phases. An increase in the initial adsorbate concentration leads to an increase in the adsorption capacity of the adsorbate onto the adsorbent. The impacts of initial concentration of dye on the kinetic of ARAC dye adsorption on both adsorbents (NSD and MSD) in aqueous solution are depicted in Figure . The rate of dye adsorption was significantly rapid for the initial 5 min, and afterward, it progressed at a slow rate and eventually attained equilibrium. The dye adsorption attained equilibrium within 10 min for 50 μmol/L and 60 min for 200 μmol/L ARAC dye solutions, respectively. However, the data were received for 120 min to confirm perfect equilibrium. The rate of initial ARAC dye adsorption, h (μmol/g min), decreased with rising concentration of dye (Tables and 2), showing that the adsorption of ARAC dye onto NSD and MSD is really dye concentration-dependent. One of the possible reasons for this phenomenon is at lower dye concentration, solute concentrations to the adsorbent site ratio is high, which cause an increase in the color removal rate, and at higher dye concentrations, solute concentrations to the adsorbent site ratio is low because of the saturation of adsorption sites, which results a slower rate of color removal. The values of equilibrium dye uptake (qe) were increased from 22.08 to 43.20 μmol/g for NSD (Figure a) and 35.07 to 101.33 μmol/g for MSD (Figure b) with increasing initial dye concentrations from 50 to 200 μmol/L, respectively. An analogous phenomenon was also found in the adsorption of reactive black 5 (RB5), reactive yellow 145 (RY145), and Remazol Brilliant Violet on chitosan in liquid solution.[21−23]

Figure 10

Plots of q vs contact time (t) for ARAC dye adsorption onto NSD (a) and MSD (b) at different initial dye concentrations (pH 2.0 for NSD and pH 3.0 for MSD; temperature: 25 °C; dye solution volume: 50 mL; particle size of adsorbents: 0–75 μm; adsorbent dosage: 0.07 g; [ARAC]0: circle open: 50 μmol/L; circle solid: 70 μmol/L; triangle up open: 100 μmol/L; and triangle up solid: 150 μmol/L; box 200 μmol/L). All lines are modelled pseudo-second-order adsorption kinetic traces produced by utilizing eq , and the qe(cal) and k2 values are recorded in Tables and 2, respectively.

Effect of Ionic Strength

The effects of ionic strength on the adsorption of ARAC dye onto both adsorbents (NSD and MSD) were investigated in aqueous solution. The dye solution ionic strength was controlled by using 1 mol/L Na2SO4 solution keeping other parameters constant. It is noticed that the initial rate and the extent of equilibrium dye adsorption onto both adsorbents declined with rising the dye solution ionic strengths (Figure a for NSD and Figure b for MSD). It can be stated that the addition of salts allows the neutralization of the negative sites of dye molecules by an extra Na+ ion, and hence, the electrostatic repulsion barrier is hindered, and non-electrostatic interactions between the adsorbent and neutral site can occur. Again, the negative SO42– ions compete with dye anions for adsorption. Thus, the adsorption capacity decreases with increasing ionic strength of the aqueous solution. A similar result was also noted in Remazol Brilliant Violet dye adsorption on chitosan in aqueous solution.[23]

Figure 11

Plots of q vs contact time (t) for ARAC dye adsorption onto NSD (a) and MSD (b) at different solution ionic strength solutions ([ARAC]0: 50 μmol/L; volume of dye solution: 50 mL; particle size of adsorbents: 0–75 μm; adsorbent dosage: 0.07 g; temperature: 25 °C; and pH 2.0 for NSD and pH 3.0 for MSD). Solution ionic strengths: circle open: 0.05 mol/L; circle solid: 0.1 mol/L; triangle up open: 0.2 mol/L; triangle up solid: 0.3 mol/L; and box: 0.4 mol/L. All lines are modelled pseudo-second-order adsorption kinetic traces produced by utilizing eq , and the qe(cal) and k2 values are recorded in Tables and 2, respectively.

Effect of Temperature

Temperature is a significant issue which can change the rates and equilibrium sorption processes. The temperature has two major effects on the adsorption process.

Increasing the temperature is known to increase the rate of diffusion of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particle, owing to the decrease in the viscosity of the solution.

And changing the temperature will change the equilibrium capacity (qe) of the adsorbent for a particular adsorbate.

Figure shows the effect of temperature on the adsorption kinetics and equilibrium adsorption of ARAC onto both adsorbents NSD and MSD in aqueous solution. It is noticed that both the initial rate (h) and the extent of equilibrium dye adsorption (qe) onto both adsorbents NSD (Figure a) and MSD (Figure b) increased with increasing solution temperatures. The values of qe were estimated to be 22.08 μmol/g at 25 °C and 25.61 μmol/g at 55 °C for NSD and 35.07 μmol/g at 25 °C and 35.58 μmol/g at 55 °C for MSD, respectively.

Figure 12

Plots of q vs contact time (t) for ARAC dye adsorption onto NSD (a) and MSD (b) in aqueous media at different temperatures ([ARAC]0: 50 μmol/L; volume of dye solution: 50 mL; particle size of adsorbents: 0–75 μm; adsorbent dosage: 0.07 g; and pH 2.0 for NSD and pH 3.0 for MSD). Solution temperatures: circle open: 25 °C; circle solid: 35 °C; triangle up open: 45 °C; and triangle up solid: 55 °C. All lines are modelled pseudo-second-order adsorption kinetic traces produced by utilizing eq , and the qe(cal) and k2 values are recorded in Table and 2, respectively.

This may be attributed to increase penetration of dye molecules inside microspores at higher temperatures or the creation of new active sites. The formation of more than one molecular layer on the surface of adsorbent appears to be achieved in the case of ARAC dye. This suggests that the equilibrium time became shorter with increase in the solution temperature (Figure ). Analogous results were also noticed in the adsorption of RB5 and Remazol Brilliant Violet on chitosan in aqueous solution.[21,23]

Adsorption Kinetics Modeling

The adsorption kinetic data obtained from different batch adsorption trails were analyzed by applying the pseudo-first-order,[46] pseudo-second-order,[47] and Elovich[48] kinetic models. The pseudo-first-order adsorption kinetics can be written as eq where q (μmol/g) and qe (μmol/g) indicate the extent of adsorption at time t and at equilibrium time, respectively. The pseudo-first-order adsorption rate constant, k1 (min–1), can be estimated from the slope of the plot log(qe – q) versus t.

The pseudo-second-order kinetic model can be written by the following equations

Nonlinear form

Linear form

The pseudo-second-order rate constant, k2 (g/μmol min), can be computed from the slope of the plot of t/q versus t. The initial adsorption rate, h (μmol/g min), can be estimated by eq

The Elovich model is written aswhere α (μmol/g min) is the rate of initial adsorption and β (g/μmol) is associated to the degree of surface exposure and the activation energy for chemisorption.

The various kinetic plots of the batch adsorption experiments are not shown in this paper. However, the values of correlation coefficients (R2) and kinetic parameters obtained from various utilized kinetic models are presented in Table for NSD and Table for MSD, respectively. It is noted that the values of R2 obtained from the pseudo-first-order model (≤0.913) for NSD and (≤0.742) for MSD, and from the Elovich model (≤0.917) for NSD and (≤0.768) for MSD were low. On the other hand, the value of R2 obtained from the pseudo-second-order kinetic model was found to be ≥0.998 for both NSD and MSD adsorbents. Moreover, the calculated qe(cal) value obtained from pseudo-second-order kinetic model was very similar to the experimental qe(exp) values for both adsorbents NSD and MSD (Tables and 2). These results indicate that the adsorption of ARAC onto NSD and MSD followed pseudo-second-order adsorption kinetics.[21−23]

Adsorption Mechanism

It well known that the adsorption mechanism is regulated by the film diffusion or intraparticle diffusion, that is, one of the methods should be the rate-limiting step. The film diffusion model suggested by the McKay[22] and intraparticle diffusion model proposed by Weber and Morris[49] equations is expressed as

Film diffusion model

Intraparticle diffusion modelwhere kid (μmol/g min0.5) is the intraparticle diffusion rate constant, kfd (min–1) is the film diffusion rate constant, F = q/q∞ is the fractional attainment of the equilibrium, and q (μmol/g) and q∞ (μmol/g) are amounts of ARAC adsorbed after t time and infinite time, respectively. Typical plots of ln(1 – F) versus t and q versus t0.5 for ARAC dye adsorption onto NSD and MSD in aqueous solution are shown in Figures and 14. The values of diffusion kinetic parameters (kfd and kid) and R2 found in employed models are shown in Tables and 4, respectively. From Figures and 14, it is noticed that none of the linear plots at any concentration of ARAC solution passed through the origin, which suggests that both film diffusion and intraparticle diffusion are the rate-limiting steps for the ARAC dye adsorption onto NSD and MSD. As expected, the order of diffusion rate constant (kid1) in the first stage was higher than that in the second (kid2) and third (kid3) stages. At the beginning, the ARAC dye adsorbed on the external surface of the both adsorbents NSD and MSD; hence, the initial dye adsorption rate was very high. The external surface of the adsorbents reached saturation, and the adsorption rate became slower. Then, the dye molecule entered into the internal pores within the particles and finally was adsorbed by the internal surface of the adsorbents. When the dye molecules diffused through the internal pores or along the surface wall of the pores within the particles, the diffusion resistance increased, which caused the slow diffusion rate.[23]

Figure 13

Plots of ln(1 – F) vs contact time (t) for ARAC dye adsorption onto NSD (a) and MSD (b) at different initial dye concentrations (pH 2.0 for NSD and pH 3.0 for MSD; temperature: 25 °C; dye solution volume: 50 mL; particle size of adsorbents: 0–75 μm; adsorbent dosage: 0.07 g; [ARAC]0: circle open: 50 μmol/L; circle solid: 70 μmol/L; triangle up open: 100 μmol/L; triangle up solid: 150 μmol/L; and box: 200 μmol/L).

Figure 14

Plots of q vs square root of contact time (t0.5) for ARAC dye adsorption onto NSD (a) and MSD (b) at different initial dye concentrations (pH 2.0 for NSD and pH 3.0 for MSD; temperature: 25 °C; dye solution volume: 50 mL; particle size of adsorbents: 0–75 μm; adsorbent dosage: 0.07 g; [ARAC]0: circle open: 50 μmol/L; circle solid: 70 μmol/L; triangle up open: 100 μmol/L; triangle up solid: 150 μmol/L; and box: 200 μmol/L).

Table 4

Diffusion Rate Parameters (kfd and kid) for the Adsorption of ARAC on MSD at Different Solution pHs, ARAC Concentrations, Ionic Strengths, Particle Size, and Temperatures

	film diffusion model		intraparticle diffusion model
parameter	kfd (min–1)	R2	kid1 (μmol/g min0.5)	R2	kid2 (μmol/g min0.5)	R2	kid3 (μmol/g min0.5)	R2
pH; [Dye]: 50 μmol/L, Tem.: 25 °C, Particle Size: 0–75 μm, Adsorbent: 0.07 g
2	0.051	0.928	5.605	0.999	0.582	0.927	0.095	0.949
3	0.040	0.742	8.449	0.992	0.605	0.959	0.107	0.968
4	0.055	0.883	7.377	0.999	0.750	0.895	0.079	0.829
5	0.049	0.906	6.852	0.999	0.337	0.977	0.174	0.723
6	0.056	0.969	5.223	0.947	1.045	0.977	0.215	0.681
7	0.032	0.909	3.748	0.954	1.193	0.977	0.239	0.926
8	0.035	0.976	3.221	0.974	1.015	0.928	0.494	0.938
9	0.054	0.913	1.835	0.998	1.233	0.971	0.396	0.909
10	0.037	0.968	2.509	0.986	1.367	0.981	0.281	0.984
[Dye] (μmol/L), pH 3.0, Temp.: 25 °C, Particle Size: 0–75 μm, Adsorbent: 0.07 g
50	0.040	0.742	8.449	0.992	0.605	0.959	0.107	0.968
70	0.050	0.889	7.913	0.937	1.037	0.931	0.104	0.983
100	0.051	0.936	9.859	0.944	3.174	0.812	0.318	0.822
150	0.034	0.903	4.713	0.999	6.137	0.969	0.938	0.903
200	0.030	0.929	10.098	0.959	5.483	0.969	2.392	0.762
Tem. (°C), pH 3.0, [Dye]: 50 μmol/L, Particle Size: 0–75 μm, Adsorbent: 0.07 g
25	0.040	0.742	8.449	0.998	0.605	0.959	0.107	0.968
35	0.142	0.871	5.591	0.999	0.833	0.917	0.042	0.653
45	0.055	0.859	5.509	0.999	0.318	0.884	0.036	0.858
55	0.051	0.868	3.903	0.986	0.212	0.989	0.030	0.926
Particle Size (μm); pH 3.0, [Dye]: 50 μmol/L, Tem.: 25 °C, Adsorbent: 0.07 g
0–75	0.040	0.742	8.449	0.992	0.605	0.959	0.107	0.968
75–150	0.054	0.983	5.151	0.996	1.849	0.971	0.202	0.852
150–300	0.033	0.911	4.867	0.996	1.592	0.911	0.426	0.975
300–600	0.045	0.918	3.498	0.981	2.513	0.939	0.264	0.932
Ionic Strength (mol/L); pH 3.0, [Dye]: 50 μmol/L, Tem.: 25 °C, Particle Size: 0–75 μm, Adsorbent: 0.07 g
0.05	0.044	0.772	8.964	0.994	0.672	0.912	0.079	0.911
0.10	0.046	0.831	8.268	0.981	0.614	0.791	0.112	0.995
0.20	0.050	0.887	7.880	0.979	0.932	0.834	0.118	0.962
0.30	0.048	0.898	7.888	0.967	0.907	0.864	0.112	0.995
0.40	0.047	0.916	7.833	0.977	1.172	0.920	0.139	0.976

Activation Parameters

Activation energy because of the ARAC dye adsorption onto NSD and MSD in aqueous solution was determined by utilizing the values of k2 at various temperatures (Tables and 2). Presuming that the correlation among the pseudo-second-order rate constant (k2), temperature (T), and activation energy (Ea) obeys the Arrhenius equation[50] expressed by the eq where R (8.314 J/mol K) is the ideal gas constant. The slope of the plot of ln k2 versus 1/T (R2 = 0.995 for NSD and 0.994 for MSD) was employed to assess Ea, which was estimated to be 6.67 kJ/mol for NSD and 11.70 kJ/mol for MSD in the temperature range 25–55 °C (Table ). The low activation energy range (4–40 kJ/mol) indicates physisorption, while higher activation energies (40–400 kJ/mol) suggest chemisorption. The values of Ea indicate that the ARAC dye adsorption was a physiosorption process.

Table 5

Values of the Activation Parameters for the Adsorption of ARAC Onto NSD and MSD in Aqueous Solution

	activation energy		thermodynamic activation parameters
temperature (K)	Ea (kJ/mol)	R2	ΔG⧧ (kJ/mol)	ΔH⧧ (kJ/mol)	ΔS⧧ (J/mol.K)	R2
Dye Adsorption onto NSD
298	6.67	0.995	53.95	4.07	–167.40	0.987
308			55.62
318			57.29
328			58.97
Dye Adsorption onto MSD
298	11.70	0.994	53.50	9.10	–148.98	0.987
308			54.99
318			56.48
328			57.97

To determine the thermodynamic activation parameters such as changes in enthalpy of activation (ΔH⧧), entropy of activation (ΔS⧧), and Gibb’s free energy of activation (ΔG⧧), the following equations were employed[22]where k2 (g/mol min), R, and T remain the same as earlier, kB: 1.381 × 10–23 J/K is the Boltzmann constant and hP: 6.626 × 10–34 Js is the Planck constant. The slope and y-intercept of the plot ln(k2/T) versus 1/T (R2 = 0.987 for NSD and MSD) were utilized to determine ΔH⧧ and ΔS⧧, respectively (Table ). The values of ΔH⧧ were estimated to be 4.07 kJ/mol for NSD and 9.10 kJ/mol for MSD, which is consistent with endothermic nature of the diffusion process. The values of ΔS⧧ were found to be −167.40 J/mol K for NSD and −148.98 for MSD, which indicates that no significant alteration developed in the internal structure of the adsorbent material during adsorption. The values of ΔG⧧ were found to be 53.62, 55.62, 57.29, and 58.97 kJ/mol for NSD and 53.50, 54.99, 56.48, and 57.97 kJ/mol for MSD at 25, 35, 45, and 55 °C, respectively (Table ). The positive ΔG⧧ values indicate the presence of an energy barrier in the adsorption process.[22]

Adsorption Isotherm

An adsorption isotherm is a graphical picture for stating the relation between the extent of dye adsorbed per unit mass of adsorbent and equilibrium dye concentration in the liquid phase at constant temperature. It shows how dye can be distributed between the liquid and solid phases at various equilibrium concentrations. There are a number of factors that determine the shape of isotherm. The major factors are the number of compounds in the solution, their relative adsorbing abilities, initial concentration of the adsorbate in the solution, and the degree of competition among solutes for adsorption sites. The relationship between the amount of equilibrium dye adsorption onto both adsorbents (NSD and MSD) and the equilibrium dye concentrations in aqueous solution at different temperatures is given in Figure . It is noticed that the extent of equilibrium dye adsorption onto NSD was increased with increasing solution temperature (Figure a); however, this effect was very insignificant in the case of MSD (Figure b).

Figure 15

Equilibrium adsorption isotherm of ARAC dye onto NSD (a) and MSD (b) in aqueous solution at different temperatures (particle size 0–75 μm; pH 2.0 for NSD and pH 3.0 for MSD); circle open: 25 °C; circle solid: 35 °C; triangle up open: 45 °C; and triangle up solid: 55 °C. All lines are modeled Langmuir adsorption isotherm using eq , and the values of KL and aL are recorded in Table .

Table 6

Langmuir, Freundlich, and Temkin Isotherm Constants at Different Temperatures for the Adsorption of ARAC Onto NSD (pH 2.0) and onto MSD (pH 3)

		temperature (°C)
isotherm	parameters	25	35	45	55
Dye Adsorption onto NSD
Langmuir model	KL (L/g)	2.07	2.48	2.71	3.17
	aL (L/μmol)	0.041	0.048	0.052	0.061
	qm (μmol/g)	50.98	51.25	52.04	52.14
	R2	0.999	0.999	0.999	0.999
Freundlich model	KF ((μmol/g) (μmol/L)−1/n)	6.33	7.72	8.29	9.25
	n	2.41	2.64	2.59	2.82
	R2	0.948	0.961	0.952	0.929
Temkin model	KT (μmol/L)	0.37	0.51	0.57	0.69
	b (J/mol)	217.45	228.16	227.07	231.18
	R2	0.989	0.994	0.994	0.986
Dye Adsorption onto MSD
Langmuir model	KL (L/g)	9.50	10.27	10.79	11.15
	aL (L/μmol)	0.063	0.068	0.071	0.074
	qm (μmol/g)	151.46	151.65	151.72	151.88
	R2	0.999	0.999	0.999	0.999
Freundlich model	KF [(μmol/g)(μmol/L)−1/n]	41.59	43.18	44.83	46.83
	N	5.05	5.17	5.33	5.55
	R2	0.955	0.956	0.959	0.965
Temkin model	KT (μmol/L)	7.83	9.85	13.03	18.56
	b (J/mol)	147.62	150.83	155.69	162.18
	R2	0.982	0.979	0.977	0.975

The equilibrium adsorption data were evaluated by using various isotherm models such as Ferundlich,[51] Temkin,[52] and Langmuir.[53] Nonlinear and linear forms of all the utilized models are described as follows:

Freundlich model

Nonlinear form

Linear form

Temkin model

Nonlinear form

Linear form

Langmuir model

Nonlinear form

Linear formwhere Ce (μmol/L) is the equilibrium ARAC concentration in solution, qe (μmol/g) is the extent of dye adsorption onto the adsorbent at equilibrium time, KF [(μmol/g) (μmol/L)−1/], and n are Freundlich isotherm constants signifying the ability and strength of the adsorbent, respectively. KT (μmol/L) is a Temkin isotherm constant, b (J/mol) is a constant associated to the heat of adsorption, R (8.314 J/mol K) and T (K) remain the same as earlier. KL (L/g) and aL (L/μmol) are the Langmuir isotherm constants, and the ratio of KL/aL denotes the maximum dye adsorption capacity, qm (μmol/g), of NSD and MSD, respectively.

The values of isotherm parameters and correlation coefficients are given in Table . The Langmuir isotherm showed the highest correlation coefficients (R2), which indicates significantly good fit compared to Freundlich and Temkin adsorption isotherms. The highest extent of dye adsorption was observed to be 52.14 μmol/g for NSD and 151.88 μmol/g for MSD at 55 °C, respectively. Thus, the equilibrium constants (aL) obtained from Langmuir isotherms were used to calculate the thermodynamic parameters for the adsorption process.

An essential dimensionless term RL is the separation factor (Table ) which can be used to explain the characteristics of Langmuir isotherm.Here, C0 is highest initial concentration of dye taken in the adsorption isotherm processes.

Table 7

Relation between RL Values and the Types of Adsorption

values of RL	types of adsorption
RL > 1.0	unfavorable
RL = 1.0	linear
0 >>RL < 1.0	favorable
RL = 0	irreversible

The calculated values of RL were found to be 0.110, 0.094, 0.088, and 0.076 for NSD and 0.011, 0.010, 0.009, and 0.009 for MSD at 25, 35, 45, and 55 °C, respectively, which suggests that the present adsorption was proposing at all temperatures.

The maximum amount of ARAC dye adsorption onto various adsorbents are shown in Table . The values of qmax (μmol/g) imparted in Table were obtained from an appropriate experimental conditions of each investigation. It is noted that the ARAC dye adsorption capacity of MSD was significantly high compared to NSD and activated carbons.[35,36] Therefore, MSD can be used as an efficient biosorbent to eliminate ARAC dye from wastewater.

Table 8

Comparison of ARAC Dye Adsorption Capacities (qmax) of Different Adsorbents

adsorbents	pH	temperature (°C)	qmax (μmol/g)	references
chitosan	5.7	25	604.33	(31)
Spirulina platensis	4.0	25	944.16	(33)
vine-trimming waste	6.0	25	272.23	(34)
activated carbon	3.5	25	8.86	(35)
activated carbon	2.0		146.75	(36)
activated carbon from biological sludge (ACBS)	2.0	55	578.34	(37)
natural sawdust (NSD)	2.0	55	52.14	this study
CPB-treated sawdust (MSD)	3.0	55	151.88	this study

Confirmation of ARAC Dye Adsorption onto Both Adsorbents (NSD and MSD) by SEM and EDX

FE-SEM has been used to determine the particle shape, porosity, morphology, and nature of the surface of the adsorbent.[25] It is a useful tool for the analysis of the surface morphology of an adsorbent. Similarly, EDX is also a very useful technique for elemental analysis.[26] The FE-SEM images and EDX data of NSD before and after ARAC dye adsorption are shown in Figure . It is noticed that the irregular surface structure of NSD was clearly observed in the FE-SEM image, as shown in Figure a, and corresponding EDX data are shown in Figure b. The FE-SEM image of the surface of NSD became brighter after ARAC dye adsorption (Figure c), and the presence of precipitates also suggest that the layer of ARAC dye was present on the surface of NSD. Moreover, the mass percentage of nitrogen, oxygen, and sulfur were increased in NSD after adsorption of ARAC (Figure d). Hence, it is confirmed that the ARAC dye adsorption occurred on the surface of NSD.

Figure 16

SEM photograph and EDX data of NSD before (a,b) and after (c,d) adsorption of ARAC.

The surface morphology and EDX data of MSD before and after ARAC dye adsorption are shown in Figure . The figure shows surface texture, porosity, and elemental composition of the materials analyzed. It is noticed that the FE-SEM micrograph of MSD showed rough and microporous surfaces before adsorption of ARAC (Figure a). The corresponding elemental compositions of MSD are shown in Figure b. The elements of carbon, nitrogen, oxygen, and bromine were present in the surface of MSD before dye adsorption; however, the sulfur peak was not detected in Figure b. The brightness of MSD surfaces was increased after dye adsorption (Figure c). It is noted that the mass percentage of carbon and nitrogen increased on the surface of MSD after the adsorption of ARAC dye, and the presence of sulfur element was also detected (Figure d). These results confirm that the ARAC dye adsorbed onto the surfaces of MSD.

Figure 17

SEM photograph and EDX data of MSD before (a,b) after (c,d) adsorption of ARAC.

Thermodynamics

Thermodynamic factors associated with the adsorption phenomena, that is, changes in Gibb’s free energy (ΔG, kJ/mol), enthalpy (ΔH, kJ/mol), and entropy (ΔS, J/mol K) were computed by utilizing the Langmuir isotherm constant (aL, L/mol) and the subsequent equations[21]where R (8.314 J/mol K) and T (K) remain the same as before. The ΔH and ΔS were estimated from the slope and y-intercept of the plot of ln aL versus 1/T (R2 = 0.982 for NSD and R2 = 0.981 for MSD). The values of thermodynamic parameters are shown in Table . The values of ΔH suggest that the dye adsorption pursued endothermic processes. The positive ΔS values prove the presence of randomness at the solid–liquid edge throughout ARAC dye adsorption onto NSD and MSD, which develops from the translational entropy produced by the exchanged water molecules as related to that lost as a cause of dye uptake.[54] Negative ΔG values suggest that the adsorption of ARAC dye onto NSD and MSD adsorbents was natural adsorption and more promising at high temperature. The growth of adsorption capacities of NSD and MSD at higher temperatures might be ascribed to the enriched movement and permeation of ARAC dye molecules in the porous adsorbents by overwhelming the barrier of activation energy and boosting the intraparticle diffusion rate.[54]

Table 9

Thermodynamics of ARAC Dye Adsorption onto NSD and MSD in Aqueous Media

	dye adsorption onto NSD				dye adsorption onto MSD
temperature (°C)	ΔG (kJ/mol)	ΔS (J/mol. K)	ΔH (kJ/mol)	R2	ΔG (kJ/mol)	ΔS (J/mol. K)	ΔH (kJ/mol)	R2
25	–26.29	123.48	10.49	0.982	–27.37	106.21	4.26	0.981
35	–27.62				–28.48
45	–28.72				–29.54
55	–30.03				–30.55

Desorption and Reuse of NSD and MSD

The typical adsorption–desorption-adsorption phenomena of ARAC dye onto NSD and MSD in aqueous are shown in Figure . In adsorption steps, the concentration of dye solution was 50 μmol/L fixed with a working solution pH 2 for NSD (Figure a) and pH 3 for MSD (Figure b) at a temperature of 25 °C, while the discharge of ARAC from dye-included adsorbents was observed in 0.01 mol/L NaOH solution (pH 12) for NSD (Figure a) and in 0.1 mol/L NaOH solution (pH 13) for MSD (Figure b) at a temperature of 25 °C. In the first adsorption step, the intensity of equilibrium ARAC dye adsorption was estimated to be 22.08 μmol/g for NSD and 35.07 μmol/g for MSD (Figure a,b). It is observed that the initial dye desorption rate was very quick, and 90% of ARAC dye was released within 10 min. The 98–99% of ARAC dye was liberated from dye-included adsorbents within 120 min. This is due to the fact that in a strong basic solution, the electrostatic interaction between the adsorbents and the dye molecules was much weaker. Behind the dye-releasing step, the subsequent dye adsorption occurred and found similar adsorption phenomena as observed in the first adsorption step. The extent of dye adsorption was similar as observed in the first adsorption step. These results suggest that the present adsorbents NSD and MSD can be reused for further dye adsorption.

Figure 18

Adsorption and desorption kinetics of ARAC on NSD (a) and MSD (b) in aqueous solution at 25 °C and 50 μmol/L dye with three steps: adsorption step 1 at initial pH 2 for NSD and pH 3 for MSD, desorption step at initial pH 12 for dye NSD and at pH 13 for dye MSD, and adsorption step 2 at initial pH 2 for NSD and pH 3 for MSD.

Conclusions

The adsorption characteristic of ARAC onto the both adsorbents (NSD and MSD) was investigated in aqueous solution. Both adsorbents (NSD and MSD) were prepared and characterized by FTIR and XRD methods. The values of pHzpc were estimated to be 5.03 for NSD and 7.43 for MSD, respectively. The dye adsorption onto both adsorbents was confirmed by FE-SEM and EDX analysis. The batch adsorption kinetic experiments were carried as a function of contact time, solution pHs, particle sizes, adsorbent dosages, dye concentrations, ionic strengths, and temperatures, respectively. The equilibrium dye adsorption onto both adsorbents were studied in aqueous solution at various temperatures. The batch adsorption kinetic data were examined by pseudo-first-order, pseudo-second-order, Elovich, intraparticle, and film diffusion models. The equilibrium dye adsorption isotherms were fitted with Temkin, Freundlich, and Langmuir adsorption isotherm models. The acidic solution pH was favorable for the ARAC dye adsorption onto both adsorbents. The maximum dye adsorption was found to be at pH 2.0 for NSD and at pH 3.0 for MSD, respectively. The amount of equilibrium dye adsorption onto both adsorbents was enhanced with rising concentration of dye solutions. The initial rate and amount of equilibrium dye adsorption onto both adsorbents were increased with increasing solution temperatures. However, the initial rate and the extent of dye adsorption reduced with rising dye solution ionic strengths. Batch adsorption kinetics of data was followed by the pseudo-second-order kinetic model rather than pseudo-first-order, Elovich, film-diffusion, and intraparticle diffusion models. Moreover, the observed values of qe(exp) in all cases were very similar to the calculated values of qe(cal) obtained from the pseudo-second-order kinetic model. The equilibrium dye adsorption isotherm was fitted well by the Langmuir adsorption isotherm rather than Temkin and Freundlich isotherm models. The activation and thermodynamic parameters suggest that the present dye adsorption onto both adsorbents (NSD and MSD) is an endothermic spontaneous physisorption process. The ARAC was released from dye-loaded adsorbents in very basic solution. Moreover, dye-loaded NSD and MSD were reused in the adsorption of ARAC in aqueous solution without loss of their adsorption efficiency. Hence, the NSD and MSD could be used to clean dye-containing industrial wastewater.

Experimental Section

Chemicals

The chemicals used in the present research were of pure analytical grade and used without further purification. The sources and purity along with the CAS registration numbers of the chemicals used have been summarized in Table . Deionized water was prepared by passing distilled water through a deionizing column (Barnstead, Suyboron Corporation, Boston, USA). Petroleum ether was prepared by distillation of petrol purchased from local gasoline station.

Table 10

CAS Registry Number, Purity and Source of the Chemicals Used

chemicals	CAS registry number	source	purity
sodium hydroxide	1310-73-2	Merck (Germany)	≥98.0%
hydrochloric acid 37%	7647-01-0	Sigma-Aldrich (Germany)	ACS reagent 37%
potassium chloride	7447-40-7	Merck (Germany)	≥99.0%
allura red AC (ARAC)	25956-47-6	Sigma-Aldrich (Germany)	dye content 80%
hexadecylpryridinium bromide (CPB)	140-72-7	Sigma-Aldrich (Germany)	≥97.0%

Preparation and Characterization of NSD and MSD

Sawdust used in present research was collected from the local sawmill of Savar, Dhaka, Bangladesh. The raw sawdust was frequently rinsed with distilled water to eliminate the dust-like impurities and dried in open air. The dried sample was pulverized into powder. To make it free from organic materials and other impurities, the powder of sawdust was treated with petroleum ether for two days. The organic material free sawdust was washed with warm distilled water (60 °C) and dried in the oven at 120 °C for 24 h. The dried sawdust was sieved using different meshes to separate different particle size sawdust and to preserve in the desiccator for further use.

The MSD was prepared according to the method used by Zhao et al.[38] with a little modification. Sawdust (10.0 g) was taken into a 1000 mL beaker containing 500 mL of 0.5% (w/v) CPB solution. The solution was stirred with a magnetic stirrer hot plate at 120 rpm and at room temperature (30 °C) for 24 h. Then, crude MSD was separated from the solution by filtration and repeatedly washed with distilled water to remove loosely bound CPB with sawdust. Finally, MSD was dried at 80 °C for overnight and stored in an airtight glass bottle for further use.

The FTIR spectra of NSD and MSD were taken in KBr at the frequencies 400–4000 cm–1 by utilizing FTIR spectrometer (IRPrestige-21 FTIR Spectrophotometer, Shimadzu, Japan). The X-ray powder diffraction outlines of NSD and MSD were observed by using an X-ray diffractometer maintaining a voltage of 30 kV and a current of 20 mA, using Cu Kα radiation (λ = 1.54 Å) and a frequency range of 10 μHz to 32 MHz. The representative data were collected in a 2θ range (5–70°) with a step size of 0.01° 2θ with an X-ray diffractometer (GNR-EXPLORER, GNR Analytical Instruments Group, Italy). The surface morphology and elements of NSD and MSD were examined before and after dye adsorption by using FE-SEM and EDX analysis (JSM-7610F Schottky field emission scanning electron microscope, JEOL Ltd., Japan). The pHzpc values of NSD and MSD were determined by using the pH drift method.[40]

Batch Adsorption Experiments

To examine the ARAC dye adsorption features on NSD and MSD, batch adsorption experiments were conducted in 125 mL stoppered bottles having 0.07 g of the adsorbent and 50 mL of dye solution (50 μmol/L). The solution pH was fixed by adding small amounts of 1 mol/L NaOH or HCl solution. The dye solution pH was determined by the pH meter (Adwa 101 pH meter). The sample bottles were agitated in a thermostated shaker at room temperature (25 ± 0.2 °C), with a velocity of 120 rpm, until achieving equilibrium. Each sample bottle was covered to prevent vaporization at elevated temperatures. Sample bottles were removed at preferred time periods for measuring dye concentration in the liquid phase. The samples were centrifuged at a speed of 4000 rpm for 5 min. The dye concentration in the supernatant was analyzed by the spectrophotometric method, using a Shimadzu UV-1800 spectrophotometer (Shimadzu, Japan), at a λmax value of 504 nm (pH 2–10). The molar absorptivity of ARAC was determined to be 21.0 × 103 L/mol cm at 504 nm. The extent of ARAC adsorbed per unit mass of NSD or MSD at time t, q (μmol/g) was determined by[22]where C0 (μmol/L) and C (μmol/L) are the concentration of ARAC dye in the liquid phase at initial and any time t, respectively; V (L) is the volume of ARAC dye solution, and m (g) is the amount of dry NSD or MSD used.

The kinetics of dye adsorption onto both adsorbents was also studied changing particle size of adsorbents (0–600 μm), dosages of adsorbents (0.02–0.20 g), dye concentrations (30–500 μmol/L), ionic strengths (0.05–0.40 mol/L), and temperatures (25, 35, 45, and 55 °C), respectively. Na2SO4 solution (1 mol/L) was used to adjust ionic strength of dye solutions. The equilibrium dye adsorption was conducted in aqueous solution at four different temperatures (25, 35, 45, and 55 °C), and at pH 2 for NSD and pH 3 for MSD in the absence of Na2SO4.

The extent of ARAC dye adsorption onto NSD or MSD at equilibrium time, qe (μmol/g), and % removal of ARAC dye were determined by[22,29]where Ce (μmol/L) is ARAC dye concentration in the liquid phase at equilibrium time; C0, V, and m remain the same as discussed earlier.

In the desorption study, the released amount of ARAC dye was measured in 50 mL of 0.01 mol/L NaOH solution (pH 12) for dye-loaded NSD and in 50 mL of 0.1 mol/L NaOH solution (pH 13) for dye-loaded MSD, respectively. At first, the adsorbent with dye solution (50 mL of 50 μmol/L) was agitated for 120 min, filtered, and the dye inserted-adsorbent was desiccated at room temperature (25 °C) for overnight. The dye inserted-adsorbent was kept into 50 mL desorbing solution, and the mixture was shaken in a thermostated shaker at room temperature (25 ± 0.2 °C), with a speed of 120 rpm, until complete desorption. The extent of dye adsorption was measured in similar way as explained earlier. All data given in this study are the average of double measurements.