Removal of FePO4 and Fe3(PO4)2 crystals on the surface of passive fillers in Fe0/GAC reactor using the acclimated bacteria

Highlights: ► Fe3(PO4)2 and FePO4 crystals would weaken treatment efficiency of Fe0/GAC reactor. ► Fe3(PO4)2 and FePO4 crystals could be removed by the acclimated bacteria. ► FeS and sulfur in the passive film would be removed by the sulfur-oxidizing bacteria. ► Develop a cost-effective bio-regeneration technology for the passive fillers. - Abstract: As past studies presented, there is obvious defect that the fillers in the Fe0/GAC reactor begin to be passive after about 60 d continuous running, although the complicated, toxic and refractory ABS resin wastewater can be pretreated efficiently by the Fe0/GAC reactor. During the process, the Fe3(PO4)2 and FePO4 crystals with high density in the passive film are formed by the reaction between PO43− and Fe2+/Fe3+. Meanwhile, they obstruct the formation of macroscopic galvanic cells between Fe0 and GAC, which will lower the wastewater treatment efficiency of Fe0/GAC reactor. In this study, in order to remove the Fe3(PO4)2 and FePO4 crystals on the surface of the passive fillers, the bacteria were acclimated in the passive Fe0/GAC reactor. According to the results, it can be concluded that the Fe3(PO4)2 and FePO4 crystals with high density in the passive film could be decomposed or removed by the joint action between the typical propionic acid type fermentation bacteria and sulfate reducing bacteria (SRB), whereas the PO43− ions from the decomposition of the Fe3(PO4)2 and FePO4 crystals were released into aqueous solution which would be discharged from the passive Fe0/GAC reactor. Furthermore, the remained FeS and sulfur (S) in the passive film also can be decomposed or removed easily by the oxidation of the sulfur-oxidizing bacteria. This study provides some theoretical references for the further study of a cost-effective bio-regeneration technology to solve the passive problems of the fillers in the zero-valent iron (ZVI) or Fe0/GAC reactor.