Glow discharge plasma in water: A green approach to enhancing ability of chitosan for dye removal
Graphical abstract
Highlights
► The glow discharge plasma approach was applied to enhance the adsorption ability of chitosan for dye removal. ► Dye uptake on plasma modified chitosan was a faster process than adsorption on untreated chitosan. ► The plasma modification of chitosan could improve the adsorption of both acid and reactive dyes to varying degrees. ► This technically feasible, highly efficient and cost effective glow discharge plasma is a suitable pretreatment method for environmental adsorption materials.
Introduction
Chitosan, the deacetylated product of chitin, is a renewable biopolymer. Due to its biocompatibility, biodegradability, lack of toxicity, low cost and abundance in nature, chitosan has attracted considerable attention for its environmental applications, especially for the removal of dye pollutants, which are a serious source of environmental contamination in many countries [1], [2]. Because chitosan contains a large number of active hydroxyl (–OH) and amine (–NH2) groups, it exhibits good adsorption capacity for anionic dyes and heavy metal ions. Nevertheless, for the practical application of chitosan, there is a need to develop methods for enhancing the adsorption capacity of chitosan.
Recently, much attention has been paid to developing suitable methods for chitosan modification, including monocarboxymethylation, cross linking with epichlorohydrin, cross linking with polyvinyl alcohol blend beads and incorporation in magnetic resins [3], [4], [5], [6]. However, such modifications require inconvenient procedures and large quantities of organic solvents. Therefore, there still remains a need to explore new and green approaches to enhancing the ability of chitosan to remove contaminants from water.
Non-thermal plasmas, such as dielectric barrier discharge, corona discharge, and glow discharge, are a great alternative technology for degradation of aqueous organic pollutants [7], [8], [9], [10]. Among these plasma technologies, glow discharge plasma (GDP) is now considered as a promising advanced oxidation technology due to its stable operation, low discharge and much higher yields of oxidizing species, such as OH and H2O2, produced in the liquid-phase when compared with what is expected on the basis of Faraday's law [11], [12]. In addition, GDP can be run in water with higher conductivity compared with pulsed corona discharge plasma [13], [14], [15]. Nevertheless, most environmental research on glow discharge plasma has thus far been focused on investigating the degradation of contaminants. Today, gas plasma-based strategies for the surface modification of biodegradable polymers and biomaterials have demonstrated great potential [16], [17]. In this regard, gases are applied during the plasma treatment process. As a result, chemical functionalities are introduced onto the surfaces of the materials. The introduced functionalities can be subsequently used to bind polymers or other molecules to the surface in order to achieve desired surface properties [18]. However, application of glow discharge plasma in water to modify bio-macromolecules for the adsorption of hazardous compounds has not been reported.
Our recent work showed that chitosan could be efficiently pretreated with H2O2 to induce modifications that allow improved removal of C. I. Acid Red 73 from weak acid solutions [19]. The pretreated chitosan appeared to have a more ordered structure and a higher hydrophilic character than the untreated chitosan, but no significant changes in functional groups on the chitosan surface were detected. Capitalizing on the fact that glow discharge plasma in water can also generate active oxygen species, we hypothesized that it might be possible to explore a plasma approach to enhance the adsorption capacity of chitosan. Herein, the pretreatment of chitosan by glow discharge plasma in water is reported, and the effects of various pretreatment parameters on the modification of chitosan were investigated.
Section snippets
Chemicals
Chitosan was purchased from Zhejiang Golden-shell Biochemical Co., Ltd., Zhejiang, China (deacetylation degree = 91.04%, Fig. 1a). C. I. Acid Red 73 (AR 73, chemical structure is shown in Fig. 1b) and other dyes are all commercially available. Doubly distilled water was used throughout this study. Other chemicals were of laboratory reagent grade and used without further purification.
Modification of chitosan via glow discharge plasma
The GDP reactor contained a cylindrical vessel with an inner diameter of 40 mm. The DC high-voltage power unit
Plasma treatment of chitosan: morphology, structure and characterization analysis
Plasma is referred to as the fourth state of matter, which can be defined as a quasi-neutral particle system in the form of fluid-like mixtures of free electrons, ions and radicals, generally also containing neutral particles. In our experiments, plasma is generated by passing water through electrical fields during glow discharge. These plasmas show strong deviations from kinetic equilibrium and have electron temperatures that are much higher than the temperature of the ions and neutral species
Conclusions
In this study, glow discharge plasma in water is explored as a suitable and green approach to enhancing the adsorption properties of chitosan for dye removal. The results showed that the GDP treatment changed the morphology and crystallinity of chitosan particles, and that the number of –CH2 and –CH3 group in chitosan samples increased. Various pretreatment parameters, including discharge current and pretreatment time, played significant roles on the chitosan modification. C. I. Acid Red 73
Acknowledgments
This study was supported by the Qianjiang Talent Scheme, Zhejiang Province, China (No. 2011R10045), the National Basic Research Program of China (No. 2009CB421603) and the Fundamental Research Funds for the Central Universities.
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