Effect of ammonia stripping and influence of contaminants in zinc plating wastewater.

Because many heavy metals, such as zinc, are used in plating plants, it is important for wastewater treatment technologies to comply with wastewater regulations. In zinc wastewater treatment, zinc is separated from wastewater via hydroxide precipitation. However, ammonia tends to enter the wastewater produced from the plating process and inhibits hydroxide precipitation, which causes difficulties in treating wastewater. In this study, we investigated the effect of ammonia stripping on plating wastewater to remove ammonia, which inhibits wastewater treatment. Based on the results of this study, it was observed that the effectiveness of ammonia removal depends on the pH and air flow rate. It was also found that the presence of zinc increased the ammonia removal effect in most cases. This increase was due to the transformation of some of the precipitated zinc hydroxide in the wastewater into zinc oxide, which afforded an ammonia adsorption effect. After the addition of ethylenediamine-N,N,N',N'-tetraacetic acid disodium salt dihydrate (EDTA・2Na), which is a complexing agent, the ammonia removal effect was slightly decreased. This result is attributed to the decrease in the amount of zinc oxide, which occurs due to the complex formation of EDTA with zinc precipitates in the wastewater. Furthermore, the addition of an excess amount of EDTA・2Na increased the concentration of free ammonia in the wastewater and increased the removal effect. Moreover, the effect of ammonia stripping was successfully applied to wastewater obtained from a zinc plating plant, and zinc wastewater treatment was improved with the removal of ammonia. These results indicate that ammonia stripping is effective in zinc wastewater treatment.

Structure and dynamics of water in mixed solutions including laponite and PEO.

To investigate the structure and dynamics of water in mixed solutions including laponite clay particles and poly(ethylene oxide) (PEO), we measured the Raman spectra of the mixed solutions in the temperature range 283-313 K. The results show that the vibrational energies of the O-H stretching modes in the mixed solutions depend on the water content and temperature. The energy shifts of the O-H stretching modes are attributed to changes in the water structure. By applying a structural model of bulk water to the spectra in the O-H stretching region, the local structures of water in the solutions were analyzed. The result shows that the formation probability of hydrogen bonds in the solutions decreases as the water content decreases. Laponite and PEO have effects to disrupt the network structure of hydrogen bonds between water molecules. Further, it was found that laponite and PEO cause increase in the strength of hydrogen bonds of surrounding water,although the strength of the hydrogen bonds increases with the order water-laponite < water-water < water-PEO. It is concluded that water in laponite-PEO mixed solutions has a less-networked structure with strong hydrogen bonds compared with bulk water.