Physical separation is a technology to produce valuable metal powder from metals included in waste products. Many separation technologies are already used in conventional recycling processes for the recovery of common and precious metals. However, there are few separation technologies for the recovery of expensive metals such as tantalum and rare earth metals, which are called “rare-metals” in Japan, from small domestic appliances (SDAs). Advanced grinding, sorting and separation techniques have been investigated in our research group in order to recycle these metals from SDAs. Our representative studies for advanced physical separation processes are introduced, and the future picture of physical separation which we have painted is described in this report.

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Arsenic removal from contaminated groundwater using microorganisms is a desirable alternative to conventional methods. In this study, Bacillus cereus strain W2, which was isolated from soil in Miyazaki Prefecture, Japan, was examined for its potential to remove arsenic. Strain W2 was capable of removing arsenic from culture medium supplemented with either As(III) or As(V). When grown in medium containing 50.0 mg As (III)/l, strain W2 removed 1.870 mg As per gram of dry cell weight in 12 h. In experiments of arsenic adsorption by dried cells of strain W2, the ratio of As(III) adsorption by strain W2 ranged from 97.3 to 99.1% in solutions containing up to 1.0 mg As/l. Arsenic concentrations in solution decreased to 0.01 mg As/l or lower with initial concentrations of As(III) up to 0.5 mg As/l. The arsenic-removing capacity of strain W2 is higher than marine bacteria and genetically-modified Escherichia coli reported to date. These promising results indicate that strain W2 can be utilized for the bioremediation of arsenic-contaminated water.

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The sol-gel synthesized powder was a single phase Li₂ZrO₃. This synthesized powder reacted with CO₂ at temperatures from the ambient temperature to a high temperature. The reaction products reacted reversibly to form Li₂ZrO₃ at temperatures above 925 K. The degree of absorption was defined as the value obtained by dividing the fractional mass gain of Li₂ZrO₃ after absorption by the fractional mass gain corresponding to a 100% reaction. Consequently, the degree of absorption was determined to be 95.2% under the absorption condition of 773 K and 7.2 ks. When CO₂ absorption at 773 K and release at 993 K were repeated five times to examine the cyclic behavior, the degree of absorption was almost the same as the initial amount. The degree of absorption of commercial powder prepared by a solid-solid reaction of Li₂CO₃ and ZrO₂ was 10.6% under the same absorption condition. The degrees of absorption with the sol-gel synthesized powder were determined to be 26, 31 and 43% under the absorption conditions of 293, 313 and 333 K for an exposure time of 115.2 ks. The absorption behavior could be best explained by an intraparticle diffusion mechanism. The diffusion of CO₂ gas through the reaction product with an apparent activation energy of 24 kJ·mol⁻¹ was a rate-determining step of the absorption reaction.

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In Application of electrokinetic soil remediation to the heavy metals contaminated soils, the acidification might cause dissolution of part of the solid matrix, and so it is desirable to perform the soil treatment at neutral pH. However, the solubility of most heavy metals is significantly reduced at elevated pH values. The solubility of metals can be enhanced by adding reagents that form metal complexes. Apart from efficacy as metal extractants, complexing agents need also to be rated for safety of use. Biodegradability is of importance because treated soils always contain residual agents that may, upon soil reuse in the field, result in actual metal mobilization and transport to groundwater. It appears therefore essential for these residual agents to be rapidly biodegraded. This is even more important for in-situ soil flushing operations. In this research, the biodegradable chelating agents [S,S]-ethylenediaminedisuccinic acid (EDDS) and N,N-dicarboxymethyl glutamic acid (GLDA) were investigated for their applicability for the removal of lead from soils by electrokinetic processing. The removal efficiency of lead from soils was improved with increase in the concentration of EDDS and the applied electric field strength. It was also found that the concentration of EDDS has a strong effect on the removal efficiency of lead from soils compared with the electrical field strength at the range from 10 V and 20 V. And also, the removal efficiency of lead using platinum-coated titanium used as electrodes increased compared with graphite and iridium dioxide-coated titanium as electrode. Meanwhile, the removal efficiency of lead with GLDA was superior to that with EDDS. However, the high viscosity of GLDA significantly affected the removal efficiency of lead.

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