Adsorption Kinetics/Thermodynamic Behavior and Adsorption/Desorption Mechanism of Crystal Violet by Semi-interpenetrating Sodium Alginate/Polyacrylamide Hydrogel†

doi:10.7503/cjcu20170101

Abstract

Abstract:

The sodium alginate(SA), a kind of polyanion electrolyte including many carboxylic acid(—COO^-), was introduced into polyacrylamide hydrogel network to prepare the SA/PAM hydrogels with semi-interpenetrating network structure via free-radical solution polymerization. The pore morphology and chemical composition of SA/PAM hydrogels before and after adsorption of crystal violet(CV) were comparative analyzed by scanning electron microscope(SEM), X-ray photoelectron spectroscopy(XPS) and Fourier transform infrared spectroscopy(FTIR). All kinds of models were used to study adsorption kinetics and thermodynamics behavior for the adsorption of crystal violet(CV) on the SA/PAM hydrogel. Meanwhile, the desorption efficiency was discussed, and the mechanism of adsorption and desorption was proposed. The results showed that, with the introduction of SA, the pore size reduced, but pore numbers increased. The SA/PAM-10 hydrogel shows the maximum adsorption amount, about 13.5838 mg/g. Four kinds of SA/PAM hydrogels agree with pseudo first-order kinetic model, and the adsorption rate depends on both film diffusion and particle diffusion. The isothermal adsorption process is consistent with the Temkin and the D-R model, which belongs to the microporous multi-layer adsorption. Thermodynamic analysis results show that the adsorption process is driven by entropy rather than chemical factor. The desorption was achieved by adding HCl, the maximum desorption rate reached up to 94.18%, and reversible adsorption of SA/PAM hydrogel could be found by adding NaOH. The high adsorption capacity can be ascribed to the electrostatic interaction between COO^- of SA molecular chain and —C=N⁺— of CV molecules. Due to protonation of —COO^- and —NH₂ under low pH value, leading to increased electrostatic repulsion with CV molecules. Accordingly, the desorption rate was naturally increased.

Key words: Polyacrylamide, Sodium alginate, Crystal violet, Hydrogel, Adsorption kinetics

CLC Number:

O631
TQ319

TrendMD:

LI Zhigang, ZHANG Yixuan, ZHANG Qingsong, MA Youwei, HU Tao, BAI Haihui, LIU Pengfei, WANG Ke, ZHANG Xiaoyong. Adsorption Kinetics/Thermodynamic Behavior and Adsorption/Desorption Mechanism of Crystal Violet by Semi-interpenetrating Sodium Alginate/Polyacrylamide Hydrogel^†[J]. Chem. J. Chinese Universities, 2017, 38(11): 2118.

Figures/Tables 12

Fig.1 Appearance of SA/PAM hydrogels before(A) and after(B—D) adsorption of CV molecules(A) Surface appearances before adsorption of SA/PAM hydrogels; (B) surface appearances after adsorption of SA/PAM hydrogels; (C) cross-section appearances after adsorption of SA/PAM hydrogels; (D) appearances of CV solution after adsorption of SA/PAM hydrogels.

Fig.2 Surface morphology and pore size of SA/PAM hydrogels before(A—D) and after(A'—D') adsorption of CV molecules (A, A') SA/PAM-0; (B, B') SA/PAM-5; (C, C') SA/PAM-10; (D, D') SA/PAM-15.

Fig.3 Cross-section morphology of SA/PAM hydrogels before(A—D) and after(A'—D') adsorption of CV molecules(A, A') SA/PAM-0; (B, B') SA/PAM-5; (C, C') SA/PAM-10; (D, D') SA/PAM-15.

Fig.4 Content of oxygen curve of SA/PAM-x hydrogels

Fig.5 C1s(A1, A2), O1s(B1, B2) and XPS(C1, C2) spectra of the SA/PAM-10 hydrogel before(A1—C1) and after(A2—C3) CV adsorption

Fig.6 Fitting curves of adsorption kinetics on crystal violet by SA/PAM hydrogelsa. SA/AM-0; b. SA/PAM-5; c. SA/PAM10; d. SA/PAM15.

Table 1 Parameters of pseudo-first-order and pseudo-second-order model for the adsorption of CV by SA/PAM hydrogels

Hydrogel	Pseudo-first-order		Pseudo-second-order
Hydrogel	k₁/min^-1	R²	q_e/(mg·g^-1)	K₂(h·g/mg)	R²	q_e/(mg·g^-1)
SA/PAM-0	0.005377	0.9936	1.9516	0.003320	0.8230	2.2049
SA/PAM-5	0.002498	0.9985	8.8436	0.01495	0.7479	10.7631
SA/PAM-10	0.002323	0.9983	13.5838	0.006666	0.6634	18.2515
SA/PAM-15	0.004227	0.9981	8.2453	0.02581	0.9071	10.3616

Fig.7 Stripping rates of CV on SA/PAM-x hydrogels(A), SA/PAM-10 hydrogel with different pH value(B) and cyclic adsorption dynamics on CV by SA/PAM-15 hydrogels(C) at 20 ℃Inset of (B): appearances of CV solution after alesorption in different pH; inset of (C): surface appearances of SA/PAM-15 hydrogel through three cyclic adsorption and desorption.

Table 2 Rate parameters for the adsorption of CV by SA/PAM hydrogels

Hydrogel	Intra-particle diffusion model(q_t=k_idt^0.5+C)			Film diffusion[In(1-F)=-k_ft]
Hydrogel	k_id/(mg·g^-1·min^-1/2)	C	R²	k_f/min^-1	R²
SA/PAM-0	0.09546	-0.1076	0.9724	0.005377	0.9936
SA/PAM-5	0.3226	-0.9807	0.9757	0.002499	0.9985
SA/PAM-10	0.4789	-1.515	0.9731	0.002323	0.9983
SA/PAM-15	0.3437	-0.3554	0.9800	0.004227	0.9980

Table 3 Equilibrium isotherms parameters for the adsorption of crystal violet by SA/PAM-0 and SA/PAM-10 at 35 ℃

Equilibrium isotherms model	Parameter of model			Slope		Intercept			R²
SA/PAM-x	x	0	10	0	10	0	10	0	10
Langmuir	Q_m/(mg·g^-1)	36.75	11.03	-0.02721	-0.09066	4.960	3.687	0.2023	0.5216
	b/(L·mg^-1)	-5.486×10^-3	-24.789×10^-3
Freundlic	n	0.8715	0.5808	1.147	1.721	-1.888	-2.584	0.9733	0.9542
	1/n	1.147	1.721
	K_f/(mg·g^-1)· (L·mg^-1 $) 1 / n$	0.1513	0.07547
Temkin	B/(L·mg^-1)	5.028	11.00	5.028	11.00	-8.929	-27.28	0.9967	0.9948
	A	0.1693	0.08377
D-R	Q_m/(mg·g^-1)	61.45	1938	-1.272×10^-6	-1.618×10^-6	11.03	14.48	0.9976	0.9999
	B₁/(mol²·kJ^-2)	-1.272×10^-6	-1.618×10^-6

Table 3 Equilibrium isotherms parameters for the adsorption of crystal violet by SA/PAM-0 and SA/PAM-10 at 35 ℃

Equilibrium isotherms model	Parameter of model			Slope		Intercept			R²
SA/PAM-x	x	0	10	0	10	0	10	0	10
Langmuir	Q_m/(mg·g^-1)	36.75	11.03	-0.02721	-0.09066	4.960	3.687	0.2023	0.5216
	b/(L·mg^-1)	-5.486×10^-3	-24.789×10^-3
Freundlic	n	0.8715	0.5808	1.147	1.721	-1.888	-2.584	0.9733	0.9542
	1/n	1.147	1.721
	K_f/(mg·g^-1)· (L·mg^-1 $) 1 / n$	0.1513	0.07547
Temkin	B/(L·mg^-1)	5.028	11.00	5.028	11.00	-8.929	-27.28	0.9967	0.9948
	A	0.1693	0.08377
D-R	Q_m/(mg·g^-1)	61.45	1938	-1.272×10^-6	-1.618×10^-6	11.03	14.48	0.9976	0.9999
	B₁/(mol²·kJ^-2)	-1.272×10^-6	-1.618×10^-6

Table 4 Adsorption thermodynamic parameters for the adsorption of CV by SA/PAM-x hydrogels

Hydrogel	ΔS/(kJ·mol^-1·K^-1)	ΔH/(kJ·mol^-1)	ΔG/(kJ·mol^-1)
SA/PAM-0		11.86
SA/PAM-5	-0.003645	-1.388	<0
SA/PAM-10	-0.001904	-0.8510	<0
SA/PAM-15	-0.003249	-1.655	<0

Fig.8 Adsorption and desorption mechanism of SA/PAM hydrogels on CV molecules

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