Layered titanates (Na2Ti3O7·nH2O) with exchangeable sodium cations located in the interlayer have been synthesized by simple hydrothermal treatment of Ti precursor in concentrated NaOH solutions. By proper control of the synthesis conditions, different morphologies of nanofibers and nanosheets are obtained. The metastable layered structure of the titanates collapses during the ion exchange, resulting in irreversible ion exchange. Target cations (eg., Ag+, Cu2+, Pb2+ and Eu3+) are completely concentrated from water and then tightly immobilized in the interlayer which is of great significance for the removal and subsequent safe disposal of hazardous metal cations. The ion exchange of the nanosheets is much more efficient than that of the nanofibers and other inorganic ion exchangers due to the larger surface area, less stable layered structure and larger amount of interlayer water of the nanosheets. The ion exchange of the titanates is also very selective. Valence, hardness, and radius of cations are main factors affecting the selectivity. Cations trapped in the interlayer are released by an acid‐induced phase transformation of the titanate nanosheets to rutile. Then the rutile can be used as a new Ti precursor to synthesize the titanate nanostructures, resulting in a full cycle of material use. The nanosheets may find applications in the decontamination and safe disposal of radioactive and heavy metal cations and also in the collection of valuable cations from water.

This work demonstrates that Fe 0 nanowires modified with chitosan embedded in porous anodic alumina are good adsorbents for the removal of Cr 6+ ions from water. Results show that the adsorption capacity can reach 113.2 mg g À1 , moreover, most of the toxic Cr 6+ ions transform into nontoxic Cr 3+ ions after adsorption.Surface water and groundwater are often contaminated by chromium ions from industrial and military activities. 1 In aqueous solution, it is mainly in Cr 3+ and Cr 6+ valence states. Cr 6+ ions show higher mobility than Cr 3+ and hence are severe contaminants to the environment due to their carcinogenic, mutagenic and teratogenic features in biological systems. 2 Cr 6+ is more toxic than Cr 3+ . 3 Chemical precipitation, chemical reduction, ion exchange and electrochemical treatment are some of the conventional techniques for the removal of heavy metal ions from aqueous solutions and industrial effluents. 4-8 In recent years, zero-valent iron (Fe 0 ) has been recognized as an effective material for the reduction of metal contaminants in surface water and in industrial effluents. 5,7 However, the conventionally employed Fe 0 is mainly in the form of particles that are covered with a surface oxide layer. 9-11 The oxide layer greatly impedes the Fe 0 reduction activity. Moreover, the particles tend to agglomerate in water through direct interparticle interactions such as van der Waals forces and magnetic interactions leading to poor particle size suitable for any reactions. 12 A stabilizer can reduce agglomeration of Fe 0 nanoparticles through electrostatic repulsion and steric hindrance. 12-14 Some researchers observed that carboxymethyl cellulose could be used as an efficient stabilizer to prepare Fe 0 nanoparticles. The efficiency of stabilized nanoparticles shows that stabilized nanoparticles degraded trichloroethene at least 17 times faster than the non-stabilized counterparts. 12 Thus, a proper stabilizer should disperse the metals well and stimulate the reaction on its surface. Then, it must be cheap, nontoxic, and widely available. Chitosan is obtained by deacetylation of chitin, which is extracted from layers of crustaceans and cell walls of many fungi. Chitosan is not only inexpensive and abundant in nature, but also a good and nontoxic adsorbent for heavy metals owing to the amine and hydroxyl functional groups in its structure. Previous investigations have demonstrated that chitosan-gold, platinum, silver and palladium composite nanoparticles are stable in aqueous solutions. 15,16 So far, the electrodeposition-combined-withchitosan method has never been used to prepare Fe 0 nanowires modified with chitosan embedded in porous anodic alumina (PAA) as an adsorption system to remove heavy metal ions. PAA with positive charges was used in this study to protect iron from oxidation, limit the size and supply electrostatic attraction. The iron nanowires embedded in PAA can be prepared by an alternating current electrodeposition method. The protonated amino groups of the chitosan and PAA were used a...

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