Adsorption of Cr(Ⅵ) from Aqueous Solutions on Organic Modified Laponite†

doi:10.7503/cjcu20160462

Abstract

Abstract:

Laponite was synthesized by hydrothermal method and modified by cetyltrimethyl ammonium bromide(CTMAB). The adsorption of Cr(Ⅵ) on modified laponite(CTMABL) as a function of pH, CTMABL dosage, ionic strength, contact time, and temperature was studied via batch technique. The results showed that the specific surface area and pore volume of modified samples were decreased, while the average pore size was increased. The d₍₀₀₁₎ value of CTMABL raised from 1.23 nm to 1.79 nm after the modification, which indicated that the CTMAB had entered into the interlayer of laponite. The removal of Cr(Ⅵ) from solution increased with the increase of solution pH value. At pH<8.5, the surface Zeta potential of CTMABL particles is positive, and thus the adsorption efficiency can be promoted by the electrostatic attraction between CTMABL particles and Cr(Ⅵ) anions. The adsorption of Cr(Ⅵ) on CTMABL increased with the increase of adsorbent dosage, maintaining a removal efficiency of >99% at the dosage of 4 g/L. The effect of ionic strength on the adsorption of Cr(Ⅵ) was not obvious. The adsorption kinetic data of Cr(Ⅵ) on CTMABL were well described by the pseudo-second-order kinetics model, and the adsorption rate was influenced by the membrane diffusion and intra-particle diffusion. The adsorption isotherms of Cr(Ⅵ) on CTMABL can be described well by the Langmuir model. The thermodynamic parameters of Cr(Ⅵ) adsorption on CTMABL at different temperatures indicated that the adsorption was endothermic and spontaneous. The adsorption of Cr(Ⅵ) was dominated by surface complexation, whereas electrostatic attraction could also promote the Cr(Ⅵ) adsorption on CTMABL.

Key words: Laponite, Hexavalent chromium, Organic modification, Cetyltrimethyl ammonium bromide, Surface complexation

CLC Number:

O611.4
O647

TrendMD:

CAO Xiaoqiang, YAN Bingqi, WANG Qian, WANG Yaping, QIU Jun, HUANG Yongqing, LI Lin, ZHANG Yan, HU Shugang, KANG Ling, LÜ Xianjun. Adsorption of Cr(Ⅵ) from Aqueous Solutions on Organic Modified Laponite^†[J]. Chem. J. Chinese Universities, 2017, 38(2): 173.

Figures/Tables 12

Fig.1 XRD patterns of laponite(a) and CTMABL(b) samples

Fig.2 FTIR spectra of laponite(a) and CTMABL(b) samples

Fig.3 Nitrogen adsorption-desorption isotherms of laponite(a) and CTMABL(b) samples

Fig.4 Effect of initial pH value of solution on Cr(Ⅵ) adsorption

Fig.5 Zeta potential of CTMABL at different pH values

Fig.6 Species distribution of Cr(Ⅵ) at different pH values

Fig.7 Effect of adsorbent dosage on Cr(Ⅵ) adsorption

Fig.8 Effect of ionic strength on Cr(Ⅵ) adsorption Ionic strength/(mol·L-1): a. 0; b. 0.1; c. 1.0; d. 3.0.

Fig.9 Kinetic curve of Cr(Ⅵ) adsorption on CTMABL a. 293 K; b. 308 K; c. 323 K.

Table 1 Kinetic parameters for Cr(Ⅵ) adsorption on CTMABL

T/K	Pseudo-first-order kinetics		Pseudo-second-order kinetics		Intra-particle diffusion
T/K	k₁	R²	k₂	R²	k_i	C	R²
293	0.000982	0.941	0.140	0.995	0.00252	0.0251	0.917
308	0.002260	0.983	0.169	0.995	0.00356	0.0611	0.984
323	0.006490	0.986	0.197	0.999	0.00515	0.0742	0.956

Table 2 Isotherm parameters of Cr(Ⅵ) adsorption on CTMABL

T/K	Langmuir model			Freundlich model			D-R model
T/K	Q	K_L	R²	K_F	1/n	R²	β	q_m	R²
293	10.186	0.338	0.998	4.296	0.203	0.872	1.757×10^-8	0.206	0.976
308	10.709	0.439	0.995	4.357	0.253	0.867	1.597×10^-8	0.219	0.963
323	11.126	0.321	0.993	4.166	0.267	0.906	1.263×10^-8	0.232	0.971

Table 3 Thermodynamic parameters of Cr(Ⅵ) adsorption on CTMABL

T/K	ΔH/(kJ·mol^-1)	ΔS/(kJ·mol^-1·K^-1)	ΔG/(kJ·mol^-1)
293		-26.103
308	120.390	0.466	-33.093
323		-40.122

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