Water is essential for living systems. However, water pollution has become a serious problem with the development of industry. The main pollutants in wastewater are emerging organic pollutants (EOCs), which may be caused by a wide variety of pharmaceuticals, pesticides, and industrial chemicals. Therefore, water remediation is necessary in order to achieve water reuse.
Water remediation is the process of cleaning up and repairing contaminated water to a certain extent. There are various remediation methods, among which techniques based on cyclodextrin (CD) have attracted much attention. CD can be used in water remediation for the reduction of pollutant toxicity, the solubilization of weakly polar organic pollutants, and the absorption of pollutants in water remediation, as described below [1].
Reduction of Pollutant Toxicity
Adding CD to industrial wastewater can make it easier for pollutants to enter the cavity and be temporarily wrapped. Since the surface of CD molecule has certain hydrophilic properties, it can effectively avoid the affinity and damage of the hydrophobic groups of toxic substances to the microbial cell wall.
As early as 1988, a study was conducted to evaluate the effect of β-CD complexation on the toxicity of certain pollutants in activated sludge systems. It was studied that the addition of β-CD significantly improves the detoxification ability of the activated sludge system. As the concentration of β-CD increases, the toxicity of certain pollutants in wastewater gradually decreases and can even be counteracted. β-CD complexation also improved the adaptability of sludge to captan (a kind of pesticide) and pharmaceutical wastes, which may have practical implications in the treatment of industrial wastewater containing toxic substances [2].
Solubilization of Weakly Polar Organic Pollutants
Nonionic weakly polar chemicals in water are difficult to remove using conventional methods due to their low solubility. The inclusion complexation of CDs can enhance the solubility of non-ionic weakly polar organic compounds in water, thereby realizing the removal of pollutants.
A research team studied the solubilization effect of hydroxypropyl-β-CD (HPCD) on five weakly polar organic pollutants: trichlorethylene, chlorobenzene, naphthalene, anthracene and dichlorodiphenyltrichloroethane (DDT). The solubilization data of HPCD, miscible solvents and surfactants showed that the solubilization ability of HPCD for weakly polar organic pollutants is between miscible solvents and typical surfactants. But for the dissolution of anthracene and DDT, the dosage of HPCD is smaller, so it has obvious advantages. Moreover, HPCD can be completely dissolved in water and cannot cause secondary pollution [3].
Absorption of Pollutants
CDs can also be used to absorb pollutants in water, such as metal ions and dyes. The adsorption mechanism mainly includes electrostatic interactions between adsorbent and adsorbate, hydrogen bonding, hydrophobic interactions (pollutant-polymer and pollutant-pollutant interactions), and complexation. Table 1 shows instances of CD absorption of metal ions and dyes.
- Absorbing metal ions: There are a large number of hydroxyl groups in CD molecules that can chelate metal ions. With the introduction of new functional groups by modification, CD derivatives have certain advantages in removing heavy metals from polluted water.
- Absorbing dyes: On the other hand, the discharge of printing and dyeing wastewater has brought serious pollution to the environment. CD derivatives can absorb various dye pollutants from water through inclusion complexation of CD cavities, and it was found that β-CD-based materials exhibited high adsorption capacities toward dyes [4].
Table 1. Instances of CD absorption of metal ions and dyes.
| Intances | Summary |
| Absorbing metal ions | Cathum et al. modified α-CD to prepare a CD-based solid material, which has a removal rate of 70% for total heavy metal ions (such as calcium, lead, chromium, nickel, cobalt, and mercury). Besides, the CD-based material could also selectively remove the heavy metals of concern without being consumed by alkaline and alkaline-earth metals [5]. |
| Absorbing dyes | Ozmen et al. synthesized a CD-based polymer by adding 4,4'-methylene-bis-phenyldiisocyanate (MDI) as a crosslinking agent and studied its sorption capacity for dyes with a model system of aqueous Congo red (1-naphthalenesulfonic acid, 3,3-(4,4-biphenylenebis(azo)) bis (4-amino-) disodium salt) solutions. The results showed that the CD-based polymer has a relatively high sorption capacity for Congo red [6]. |
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References
- Bhattacharya, S.; et al. Introduction to water remediation: Importance and methods. [Energy, Environment, and Sustainability] Water Remediation. 2018, 3–8.
- Ola´h, J.; et al. β-Cyclodextrin enhanced biological detoxification of industrial wastewaters. Water Res. 1988, 22 (11): 1345-1351.
- Wang, X.; Brusseau, M. L. Solubilization of some low-polarity organic compounds by hydroxypropyl-β-cyclodextrin. Environ. Sci. Technol. 1993, 27(13): 2821–2825.
- Crini, G; Peindy, H. N. Adsorption of C.I. Basic Blue 9 on cyclodextrinbased material containing carboxylic groups. Dyes Pigments 2006, 70: 204–211.
- Cathum, S. J.; et al. Treatment of mixed contamination in water using cyclodextrin-based materials. Remediation. 2006, 16(4): 43-56.
- Ozmen, R.Y.; et al. Use of β-cyclodextrin and starch based polymers for sorption of Congo red from aqueous solutions. 2007, Hazard Mater. 148(1-2): 303-310.
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