Adsorption for phosphate by crosslinked/non-crosslinked-chitosan-Fe(III) complex sorbents: Characteristic and mechanism
Graphical abstract
This figure shown that the adsorption mechanism of the two adsorbents and the pathway of phosphate transition under different pHs. It can be speculated that the electrostatic interaction and ligand exchange processes might be the dominant mechanism for phosphate adsorption on CTS-Fe and CTS-Fe-CL under acidic conditions. The mechanism of phosphate adsorption on CTS-Fe and CTS-Fe-CL at a high value of pH were mainly depended on ligand exchange process. This figure allows the reader to more intuitively understand the mechanism of phosphate adsorption at different pH conditions.
Introduction
Phosphorus is an essential element for the metabolism of all living organisms, which occurs naturally in a variety of chemical forms. The enrichment of phosphate in surface and groundwater is due to the extensive use of synthetic fertilizers and non-standard treatment of industrial, livestock and domestic wastewater [1]. Phosphorus plays a crucial role in ecosystems, the excess release of phosphate has become one of the leading factors for eutrophication in water resources, which promotes algae growth and leads to deterioration of water quality [2]. Although eutrophication has gained significant attention around the world, this problem has not been solved thus far.
To solve the problem of increasing concentrations of phosphate in aquatic ecosystems, several methods, such as biological, physical and chemical treatment techniques have been developed for phosphate removal, including aerobic granules sludge technique [3], ion exchange [4], electrodialysis, reverse osmosis [5], chemical precipitation [6] and adsorption [7]. Nevertheless, reverse osmosis, ion exchange, and electrodialysis require high cost [8]. Although the biological method is valid, it needs strict control of operating conditions, high energy consumption and large area [9]. Notably, adsorption is considered to be the most attractive method due to its ease of operation, the flexibility of design and cost efficiency [10]. So far, a lot of natural minerals and organic polymers have been evaluated to be adsorbents for removing phosphate from water, including activated carbon [11], zeolite [12], iron-based compounds [13], PVA [14], Chitosan [15]. Among them, chitosan has been widely concerned in removing pollutants from wastewater because of its excellent properties such as rich source, nontoxic, environmental protection and biocompatibility. Chitosan, a product of chitin N-deacetylation, is atypical nitrogen-containing basic polysaccharide in nature, which has usually been used to remove cation and anion species [16]. However, chitosan dissolves in wastewater at a low pH and loses its absorptive capacity because of its relatively weak base (pKa ∼ 6.2) [17]. To overcome this shortcoming, many studies have been focused on the modification process to improve the stability of chitosan such as surface protonation [18], mixed metal oxides [10] and polymer combinations [19]. Due to the abundant amino and hydroxyl functional groups, chitosan can be modified extensively by the cross-linking process to form variable morphology materials. The surface area and pore structure of chitosan were changed due to the insertion of new functional groups on chitosan after cross-linking process, which exhibited a higher performance on adsorption capacity [20].
In addition, the previous studies pointed out that metal ions were facilely combining with phosphate because a metal ion behaved as Lewis acid when losing their electrons present in the out layer [17]. Meanwhile, chitosan had the ability to coordinate with many transition metal ions through a chelation mechanism due to the liberal modifiable functional groups (OH and NH2) on the surface of chitosan [2]. Consequently, removing phosphate by single metal or multi-metal oxides loading chitosan adsorbents have been increasingly reported, such as chitosan-stabilized nano Zero-valent Iron (CS-nZVI) [21], chitosan/Al2O3/Fe3O4 composite nanofibrous [22], Zr4+-CSBent, Fe3+-CSBent and Ca2+-CSBent biocomposite [17]. Among these synthetic adsorbents, because of the strong buffering ability of trivalent iron, Fe loading was the most widely used method for modification. It has been previously demonstrated that the adsorption of phosphate on chitosan-Fe(III)-crosslinked in an experiment of solid-phase extraction was high efficiently [2], and phosphate adsorption capacity of iron(III) chitosan complex was much higher than iron(II) chitosan complex [23]. Recently, several studies have shown that chitosan-Fe(III) complex was a mesoporous material with excellent ability to remove anions [16]. However, the information on the mechanism of phosphate adsorption by Fe modified chitosan composite was limited.
In the present study, the physico-chemical characterization on chitosan-Fe(III)-crosslinked/non-crosslinked composites were explored during the adsorption process. Specifically, this study aims to investigate the changes in functional groups on the adsorbent surface and the differences between the structure of raw and crosslinked CTS-Fe composites. Moreover, the mechanism of phosphate transportation at different pH conditions and the role of Fe in the process of phosphate adsorption were revealed in this study.
Section snippets
Materials
Chitosan with a degree of deacetylation of 80.0% to 95.0% was purchased from Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. FeCl3·6H2O was obtained from Tianjin Fuchen Chemical Regents factory, Tianjin, China. All chemical reagents used in this study were of analytical grade, which were purchased from Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. Moreover, all stock solutions were prepared with distilled water. A stock solution of phosphate (50 mg L−1) was prepared by adding KH2PO
Characterization
The SEM images of chitosan-Fe(III)-crosslinked/non-crosslinked complex before and after phosphate adsorption were illustrated in Fig. 1. It was obvious that the non-crosslinked complex had tightly inhomogeneous folds in surface and tight core with extremely few pores. By contrast, the structure of crosslinked chitosan-Fe particles can be divided into three layers: brain striatum surface, mesoporous transitional layer, and the honeycomb inner core. The pleated surfaces on both adsorbents could
Conclusions
The CTS-Fe and CTS-Fe-CL were successfully synthesized and conducted to remove phosphate from aqueous solutions. The adsorption mechanism of the two adsorbents and the pathway of phosphate transition were also revealed. Based on the results of SEM and BET analysis, the CTS-Fe had a compact structure with tightly inhomogeneous folds and extremely few pores (average pore width 6.6 nm), while the CTS-Fe-CL exhibited a three layer of structure: brain striatum surface, mesoporous transitional layer
Acknowledgment
The authors acknowledge financial support from the National Natural Science Foundation of China (NSFC) (No. 51578519).
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