Acid mine drainage from approximately 80 abandoned or closed Japanese mines has been treated by neutralization over the last forty years. A more efficient and cost-effective treatment process is required, as the national government spends billions of yen each year to protect the environment from acid mine drainage discharge. Surface complexation at the interface between hydroxides such as ferrihydrite or aluminum hydroxide and wastewater is the most important mechanism for the removal of dilute toxic ions. We have categorized the immobilization mechanism of inorganic toxic elements at the hydroxide solid/liquid interface into surface complexation and surface precipitation. We have introduced this concept using experimental methods to understand their mechanism, for example arsenate co-precipitation with ferrihydrite, fluorine co-precipitation with aluminum hydroxide, and boron co-precipitation with magnesium hydroxide. From detailed investigations using isotherm formation, X-ray diffractometry, and Fourier-transform infrared spectroscopy analysis, we found that co-precipitation achieved a more efficient removal of toxic anions than simple adsorption, because of surface precipitation or surface complexation of multiple complexes.

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After precise dismantling of 49 scrap cellular phone models released between 1999 and 2007, it was found that the average number of tantalum condensers used in the cellular phone models decreased significantly after 2005. The automatic sorting technique which combines a weight meter with a laser three-dimensional shape-detection system was used to identify whether a scrap cellular phone was released before 2004 or after 2005. As the result, the automatic sorting technique was adequate for separating 191 different scrap cellular phone models into those two groups. In addition, automatic sorting of various other small electronic devices into three groups based on resource value was achieved with high accuracy.

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Polyacrylamide (PAA) cryogels were prepared by freezing monomer solution of acrylamide with cross-linker N,N-methylenebis(acrylamide) and polymerizing initiated by N,N,Nʹ,Nʹ-tetramethyl ethylene diamine and ammonium peroxodisulfate at sub-zero temperatures. Iron hydroxide oxide particles were formed by stepwise penetration of iron(III) chloride solution (FeCl₃) and alkaline aqueous solution into cryogel wall. The particles formed were observed on the surface of the cryogel wall. The amount of the particles was changed by the concentration of FeCl₃ and the kind of alkaline aqueous solution. The polyacrylamide cryogel containing iron hydroxide oxide particles successfully adsorbed a large amount of As(V) at both low and high concentrations. The adsorption of As(V) with the cryogel containing iron particles was not affected by the pH value of the aqueous solution. The cryogels containing iron particles prepared using sodium carbonate had larger amount of iron than that prepared using sodium hydroxide. The adsorption amount of As(V) on the cryogel had a tendency with the contents of iron in the cryogels.

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Layered double hydroxides (LDHs) were synthesized by a co-precipitation method using steelmaking slag as a raw material. The adsorption of toxic anions including As(III), B, Cr(VI) and Se(IV) in aqueous solution was investigated in order to clarify the removal property of anionic species by the obtained products. The order of removal for four anionic species with the steelmaking slag-derived LDHs (slag-LDHs) is as follows; Se(IV) > Cr(VI) > As(III) > B in the low equilibrium concentration below 10 mg/dm³. The order implies that the valency of anionic species is mainly related to the anion adsorption order in the low concentration, that is, an anion with larger valency is easily captured in the LDHs structure. On the other hand, the saturated adsorption amount is considered to be the following order; As(III) > Se(IV) > Cr(VI) > B as a standard of mass. No remarkable difference in the crystalline materials is found before and after the As(III) adsorption operation. It is found that the steelmaking slag-derived LDHs are applicable as an anion removal agent for the dilute aqueous solution containing anionic species.

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