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heavy-metal removal methods, adsorption techniques are energy efficient and eco- friendly for the selective removal of heavy metals. In particular, zeolites have been attractive candidate adsorbents for heavy metals since the first attempt using zeolites to purify water in the 19th century. Many types of zeolites have been tested for heavy-metal removal, such as zeolites X, Y, A, and P (GIS type) and clinoptilolite, etc. Among these, zeolites X and A are generally the best adsorbents for heavy- metal removal.73 Recent studies have focused on modifying zeolites with metals or metal oxides to improve their heavy-metal removal capability. For instance, iron-coated clinoptilolite displayed much improved adsorption capability for heavy metals, especially for Pt, in comparison with uncoated clinoptilolite.74 This was prob- ably because the iron could reduce PbII ions on the surface of clinoptilolite to Pb0. Taking advantage of the reductivity of iron, nanoscale-zero-valent-iron/zeolite com- posites could be made. With large surface areas of iron nanoparticles and the stabi- lization effect of zeolites toward iron nanoparticles, the uptake for PbII could be much improved.75 On the other hand, metal-oxide/zeolite composites could be made for heavy-metal removal. For instance, core-shell ZnO/Y particles with zeolite Y as the core and ZnO nanoflakes as the porous shell could be fabricated as a multifunctional water purifier;76 200 mg of ZnO/Y core-shell composites exhibited equilibrium PbII adsorption of 74.7 mg g1, which was much higher than that of pris- tine zeolite Y (47.93 mg g1). The enhanced PbII adsorption could be attributed to the formation of a ZnO nanoflake layer on the surface of zeolite Y, which attained negative charges, attracting the positively charged PbII ions. Meanwhile, with surface modification of ZnO nanoflakes, the ZnO/Y composite also exhibited high adsorption and photocatalytic activity toward organic pollutants (for instance, methylene blue dye) and antibacterial activity toward microorganisms (such as Escherichia coli, Staphylococcus aureus, and Pseudomonas). To realize practical large-scale heavy-metal removal by using zeolites, future efforts should be focused on metal recovery, zeolite regeneration, and competitive multi-component adsorption. Methods dealing with nuclear wastewater have attracted much attention, especially since the Chernobyl and Fukushima Daiichi disasters. Zeolites are radiologically stable to gamma, alpha, and beta irradiation. Furthermore, small-pore zeolites exhibit high affinity for radionuclides such as 90Sr and 137Cs at very low concentra- tions. A recent study attributed such high affinity to the preferential binding between radioactive cations and 8-ring windows in small-pore zeolites.77 These unique characteristics, as well as their easy availability, make zeolites currently the most widely used adsorbents for radioactive pollutants. For instance, natural zeolites, such as mordenite and clinoptilolite, were used to decontaminate wastewater discharged from the damaged nuclear power plants at Chernobyl.78 During the cleanup after the Fukushima disaster, zeolites were used to concentrate 137Cs in the radioactive waste.79 In addition, an 80-m-wide underground zeolite wall was installed at Fukushima Daiichi, aiming to block leaked 90Sr in the soil from migrating to the sea.80 Besides metal cations, zeolites are also capable of adsorbing other radioactive molecules, such as iodine released during the dissolution of nuclear fuel rods. Recently, Pham et al.81 investigated the capture of iodine by using various types of porous materials, including all-silica zeolites, aluminosilicate zeolites, active carbons, and metal-organic frameworks. All-silica zeolites exhibited superior capability over other porous materials for I2 removal from highly acidic dissolver so- lution. In particular, hydrophobicity-intensified silicalite-1 (which was prepared with fewer defects than conventional silicalite-1) and all-silica beta captured 0.26 and 0.21 g g1 of I2, respectively, from a 5 M HNO3 solution saturated with I2 within 1 hr. Both hydrophobic zeolites remained intact after immersion in 5 M HNO3 Chem 3, 928–949, December 14, 2017 945PDF Image | Applications of Zeolites in Sustainable Chemistry
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