ReviewRole of chitosan-based hydrogels in pollutants adsorption and freshwater harvesting: A critical review
Introduction
Freshwater is one of the resources on which all life depends, and people cannot live or produce without the supply of water. The amount of freshwater available for human use on earth is only 2.8% of the total water content of the planet. However, with the development of modern industry and the improvement of people's life, the water pollution caused by various human activities (e.g., family, industry, commerce, agriculture, etc.) has become increasing serious and has attracted considerable attention [1]. The consequent problem is the shortage of freshwater. The production of agriculture and industry produces a large amount of wastewater containing pollutants that pollute the environment and are harmful to human health, such as heavy metal ion, pesticide residues, various organic dyes, microplastics, and other organic and inorganic compounds [2], [3]. These wastewaters usually need to undergo certain treatment and meet the discharge standards before they are allowed to be discharged [4]. The ministry of ecology and environment spends a lot of financial and human resources to deal with water pollution every year. In addition, individual families incur an additional medical financial expense due to health problems caused by unsanitary drinking water [1]. Therefore, it is urgent to adopt effective and economical water treatment technologies to effectively improve these problems. This is not only protecting freshwater resources but also saving financial expenses.
There are three strategies have been summarized to protect freshwater resources, including 1) conserving water and reducing unnecessary waste of water resources, 2) wastewater purification and recycling to improve the utilization rate of water resources, and 3) obtaining freshwater resources from other sources, such as desalination of seawater and atmospheric condensation. The emphasis of water conservation is on the individual. Undeniably, this is very important. However, the subsequent water treatment and water harvesting are more important methods to protect freshwater resources. Several technologies have been developed for water treatment, including precipitation, adsorption [5], oxidation [6], membrane filtration, electrochemical methods [7], and biological treatment [8]. But many of these technologies are complex to implement, have high input costs, high energy requirements, and even low efficiency to meet specific needs. Among them, adsorption treatment is widely concerned because of its simple operation, low cost, and good effect. Many adsorption methods can be used in different environments to achieve the desired purpose. In addition, the use of suitable adsorbents or adsorption methods can also achieve the effect of resource recovery such as heavy metal ions in the process of wastewater decontamination [9]. This reversible adsorption-desorption property has greatly expanded the use of adsorption in water treatment. The commonly used adsorbents in the adsorption treatment can be inorganic activated carbon, zeolite, clay ore, alumina, etc., which have strong adsorption capacity for various pollutants [10]. Meanwhile, considering the economic benefits, some adsorbents prepared from agricultural by-products (e.g., rice husk, walnut shell, corn cob, and fruit shell) or biological wastes (e.g., shells of marine crustaceans) have gained wide attention due to their cost-effectiveness and environmental friendliness [11]. In addition, some other interesting strategies are used as adsorbents for water treatment such as nanomaterials, hydrogels, etc.
Hydrogels are widely used in delivery systems and tissue engineering due to their good water-holding, swelling, embedding and delivery capabilities [12], [13]. Moreover, due to its high adsorption, reversible swelling, antibacterial properties, biocompatibility and biodegradability, its use in water treatment has received increasing attention in recent years [14]. Chitin, is a polysaccharide that is abundant in nature (second only to cellulose), consisted of repeated β-(1-4)-N-acetyl-D-GlcN unit, which is mainly derived from the crustacean shells, insect bone, and fungal mycelia. Usually, those marine crustacean shells are disposed of as garbage, which creates many environmental problems and unnecessary waste. There are considerable studies that have been reported chitin has a lot of application in the treatment of industrial pollutants, enzymes immobilization, and textile industry [15]. Moreover study has shown that chitin can adsorb dyes and suspended matter from wastewater and is expected to be used as a natural source of adsorbent for wastewater treatment [16]. However, the solubility of chitin in common solvents is poor. Chitosan (CH), as a deacetylation product of chitin, composes of repeating 2-amino-2-deoxy-β-1-4-D-Glu unit [13]. It can be dissolved in dilute acid due to the dissociation of NH2 [17]. It has better prospects for use in various fields including biomedicine, environment and agriculture fields due to its favorable bioactivity such as antioxidant, antibacterial, adsorption, and film formation [18]. In addition, due to the large number of amino groups and hydroxyl groups in the main chain of CH, it can adsorb dyes and heavy metal ions through electrostatic and hydrogen bonding, which has been widely used in various wastewater treatments [19]. It has the advantages of lower cost and regeneration than the currently used adsorbents such as activated carbon [20]. However, studies have shown that the adsorption of metal ions by CH can only be done to the amorphous region, which greatly limits its application [21]. To enhance the adsorption efficiency, it is an effective strategy to prepare CH-based hydrogels for wastewater treatment by combining the advantages of CH and hydrogels. The porous network structure of hydrogels provides more sites for CH to bind to dyes or heavy metal ions [21]. Currently, the application of CH in wastewater treatment has been reviewed, but there is still a lack of systematically correlate reviews on hydrogels prepared based on CH and its derivatives for water treatment, especially in pollutants adsorption, oil-water separation, and water harvesting. Herein, the purpose of this review is to provide a comprehensive and comparative review of CH-based hydrogels for the adsorption of pollutants including heavy metal ions, industrial dyes, microplastics, and organics, as well as for freshwater harvesting such as seawater desalination and atmospheric condensation. It is hoped that this review will provide some usable ideas for researcher in solving the problem of freshwater shortage.
Section snippets
Pollutants adsorption
Freshwater is the indispensable substance for human living, but the freshwater resource is limited. With the rapid development of industries, causing more and more wastewater is discharged, leading a large amount of water pollution, and reducing the storage content of freshwater on the earth. Hence, it is necessary for freshwater collection or cyclic utilization of industrial wastewater after treatment. Moreover, there are many heavy metal (e.g., Cu2+, Zn2+, Hg2+, Pb2+, Cd2+, Fe3+, As5+, and Cr
Desalination of seawater
In addition to the above-mentioned treatment of wastewater, collecting freshwater from other sources is also a common way to solve the earth's shortage of freshwater resources. Seawater is one of the most abundant resources on earth, but since human production cannot use seawater directly, desalination is an important way to meet the growing demand for freshwater [73]. However, desalination process requires a series of complex and expensive equipment, resulting in high costs and low returns.
Future prospect and challenge
The shortage of freshwater has been a common concern and an urgent livelihood issue. The large amount of untreated domestic, industrial, and agricultural wastewater discharged not only aggravates the problem of freshwater shortage but also causes many environmental problems and damages the ecological balance, especially for the marine and lake ecosystems. Many kinds of technologies have been developed for water treatment, each with its own advantages and disadvantages. In this paper, we focus
CRediT authorship contribution statement
Jun Yang: Writing-Original Draft, Visualization, Data Curation
Xianxiang Chen: Writing-Review& Editing
Jiahui Zhang: Writing-Review& Editing
Yuanxing Wang: Writing-Review& Editing
Huiliang Wen: Writing-Review& Editing
Jianhua Xie: Validation, Resources, Writing-Review& Editing, Supervision
Declaration of competing interest
The authors declare that they have no conflict of interest.
Acknowledgments
This work was financially supported by the National Ten Thousand Talent Program Young Top-Notch Talent Program of China.
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