Single and binary adsorption of perfluorooctanoic acid and perfluorooctane sulfonic acid on chitosan-carbon nanotubes hydrogel beads: Adsorption kinetics, isotherms, and thermodynamic parameters

Siphesihle Mangena Khumalo, Babatunde Femi Bakare, Sudesh Rathilal
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Abstract

The occurrence of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) in bodies of water that receive contaminants has presented substantial health hazards for humans and aquatic organisms. Herein, the adsorption of PFOA (1.208E-05 mol/L to 1.208E-04 mol/L) and PFOS (9.997E-06 mol/L to 9.997E-05 mol/L) on chitosan-carbon nanotube (CCNT) hydrogel beads from aqueous solutions was studied. Findings on the adsorption kinetics studies suggest that the nonlinear pseudo-first-order kinetic model fits experimental data well at a contact duration of 48 h and an adsorbent load of 1.5 g/L for both PFOA and PFOS. Single adsorption isotherm data were accurately modeled by the nonlinear Freundlich model, with R2 =0.991 for PFOA and R2 =0.997 for PFOS, suggesting that the adsorption of PFOA and PFOS on the adsorbent was not restricted to the monolayer adsorption process. Binary adsorption isotherm data were well fitted by the extended-Langmuir isotherm model (R2=0.996 for PFOA and R2=0.995 for PFOS) and extended-Sips isotherm model (R2=0.996 for PFOA and R2=0.997 for PFOS). As such, it was inferred that the uptake of one adsorbate in the presence of the other resulted in antagonistic effects, which reduced the overall efficacy of the adsorbent. Thermodynamic studies explicitly indicated that the adsorption of PFOA and PFOS on CCNT was an endothermic process that can be characterized as a physicochemical adsorption process. The presence of sodium chloride as a competing ion synergized the uptake of adsorbates from an aqueous solution. It was concluded that hydrophobic interaction and electrostatic attraction were the predominant mechanisms in the sorption of PFOA and PFOS on CCNT hydrogel beads.
壳聚糖-碳纳米管水凝胶珠对全氟辛酸和全氟辛烷磺酸的单一和二元吸附:吸附动力学、等温线和热力学参数
在接受污染物的水体中出现全氟辛酸(PFOA)和全氟辛烷磺酸(PFOS)对人类和水生生物的健康构成了重大危害。研究了壳聚糖-碳纳米管(CCNT)水凝胶珠对水溶液中PFOA (1.208E-05 ~ 1.208E-04 mol/L)和PFOS (9.997E-06 ~ 9.997E-05 mol/L)的吸附性能。吸附动力学研究结果表明,在接触时间为48 h、吸附剂负荷为1.5 g/L时,PFOA和PFOS的非线性拟一阶动力学模型与实验数据吻合良好。单次吸附等温线数据采用非线性Freundlich模型精确建模,PFOA的R2 =0.991, PFOS的R2 =0.997,表明吸附剂对PFOA和PFOS的吸附并不局限于单层吸附过程。采用扩展langmuir等温线模型(PFOA R2=0.996, PFOS R2=0.995)和扩展sips等温线模型(PFOA R2=0.996, PFOS R2=0.997)对二元吸附等温线数据进行了较好的拟合。因此,可以推断,在另一种吸附剂存在的情况下,一种吸附剂的吸收会产生拮抗作用,从而降低了吸附剂的总体功效。热力学研究明确表明,全氟辛烷磺酸和全氟辛烷磺酸在CCNT上的吸附是一个吸热过程,可表征为物理化学吸附过程。氯化钠作为竞争离子的存在协同了水溶液中吸附剂的吸收。结果表明,疏水相互作用和静电吸引是CCNT水凝胶珠吸附PFOA和PFOS的主要机理。
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