Removal of Mn(II) and Zn(II) ions from flue gas desulfurization wastewater with water-soluble chitosan

Heavy metals are the most important pollutants in wastewater from dual-alkali flue gas desulfurization (FGD) system. Mn(II) and Zn(II) could predominate in the catalyzed oxidation of sulfite. So the feasibility of precipitation of heavy metal ion (Mn(II) and Zn(II)) by water-soluble chitosan was studied in a lab scale experiment. The association between chitosan and metal ions was verified through FT-IR. The pH investigation revealed that at the pH ranged from 5 to 9, there were three stages for different actions: chelation of chitosan for metal ion, precipitation of metal hydroxide and coprecipitation of metal hydroxide and chitosan–metal complex. The selective chelation of chitosan for Mn(II) and Zn(II) mixture solution was also studied. The results showed that the chelation of chitosan for Mn(II) was prior to Zn(II) in multiple component solution. Compared with the settling of metal hydroxide, the chitosan–metal complex had better separating performance. Application of chitosan solution for chelation could remove Mn(II) and Zn(II) efficiently and make it easily to separate sediment from dual-alkali FGD wastewater. On the other hand co-precipitation of the complicate heavy metal in the FGD wastewater enhanced the heavy metal removal of chitosan chelation.