18:15 〜 19:30
[U05-P14] A novel remediation method for nickel-bearing wastewater at neutral conditions
キーワード：Remediation, Layered double hydroxide, Nickel
Advances in technology such as the electronics and metal plating industries have increased the demand of Ni year by year. On the other hand, the resources are unevenly distributed in a few countries and the supply is highly dependent on strategic policies. Moreover, since Ni is not produced in Japan and it is dependent on imports from foreign countries, the supply structures of Ni are vulnerable. Because of this, the necessity of recovering Ni from wastewaters and other waste forms has been increasing. Some industrial wastewaters contain large amounts of Ni. Generally, the removal of Ni from contaminated wastewater by adding antalkaline and flocculants to increase the pH to 10 or above would result in the generation of Ni-hydroxides after treatment. After that, it is necessary to adjust the pH below the effluent standard (pH 5.8〜8.6). However, this method suffers from some disadvantages, such as the high cost for chemical reagents, problems in the disposal of alkali sludge and inefficient treatment system. Therefore, a more sustainable remediation method must be developed to achieve sustainable wastewater treatment operations. This study focused on natural attenuation processes which are safer, cost-effective and more environmentally friendly than traditional methods. For example, at Dougamaru abandoned mine in Japan, high concentrations of Cu and Zn in wastewater are naturally incorporated in the structure of layered double hydroxides (LDH), which forms in the presence of Al ions, hence, natural attenuation of Cu and Zn occurs (Okamoto et al., 2010). Because LDH has the hydroxide structure, six-coordinated heavy metals such as Cu, Ni and Co can be incorporated into the structure during the formation process. Therefore the objective of this study is to develop a remediation method for Ni-bearing wastewaters at neutral conditions, and to clarify the behavior of Ni in the neutralization and precipitation process.
In this context, to check the applicability of LDH in the treatment of Ni-bearing wastewaters, synthesis experiments were carried out by co-precipitation of Ni-bearing LDHs containing SO42- as the interlayer anion with different concentrations of dissolved Al ions. Analysis of water chemistry before and after the co-precipitation show that the removal efficiencies of Ni from the synthetic wastewaters increased with increasing dissolved Al concentration. The results further show that the presence of Al in the formation of LDH removed Ni at pH values lower than previous methods which precipitated Ni-hydroxides. It is expected that treatment costs will be reduced in actual wastewater treatment systems because Al addition leads to the reduction of antalkaline use and the neutralization process.
Ni adsorption experiments and extraction experiments were conducted to investigate the sorption behavior of Ni. Only a small amount of Ni was adsorbed to LDH and basaluminite (major minerals in coprecipitation experiments) as inner- and outer-sphere complexes. From the result of XAFS analysis, Ni was incorporated into the structure by being able to precipitate LDH selectively. This shows Ni is fixed securely in the structure of LDH and that the mobility of Ni will be governed by the solubility of LDH. Thermodynamic modeling suggests that the precipitation of LDHs with the optimum Al/Ni molar ratio (0.25〜0.50) is determined by the initial conditions (e.g. pH, Al, Ni concentrations). Furthermore, modeling results reproduce the experimental results such as removal efficiency and mineral species well, opening the possibility of its application in actual wastewater treatment operations.
In this context, to check the applicability of LDH in the treatment of Ni-bearing wastewaters, synthesis experiments were carried out by co-precipitation of Ni-bearing LDHs containing SO42- as the interlayer anion with different concentrations of dissolved Al ions. Analysis of water chemistry before and after the co-precipitation show that the removal efficiencies of Ni from the synthetic wastewaters increased with increasing dissolved Al concentration. The results further show that the presence of Al in the formation of LDH removed Ni at pH values lower than previous methods which precipitated Ni-hydroxides. It is expected that treatment costs will be reduced in actual wastewater treatment systems because Al addition leads to the reduction of antalkaline use and the neutralization process.
Ni adsorption experiments and extraction experiments were conducted to investigate the sorption behavior of Ni. Only a small amount of Ni was adsorbed to LDH and basaluminite (major minerals in coprecipitation experiments) as inner- and outer-sphere complexes. From the result of XAFS analysis, Ni was incorporated into the structure by being able to precipitate LDH selectively. This shows Ni is fixed securely in the structure of LDH and that the mobility of Ni will be governed by the solubility of LDH. Thermodynamic modeling suggests that the precipitation of LDHs with the optimum Al/Ni molar ratio (0.25〜0.50) is determined by the initial conditions (e.g. pH, Al, Ni concentrations). Furthermore, modeling results reproduce the experimental results such as removal efficiency and mineral species well, opening the possibility of its application in actual wastewater treatment operations.