Investigation of the fate of heavy metals based on process regulation-chemical reaction-phase distribution in an A-O1-H-O2 biological coking wastewater treatment system

Regulation mechanism of typical substances including OH-, CN-, SCN- S2-, NH3 on the distribution of heavy metals was investigated in coking wastewater treatment plant with our self-designed Anaerobic-Oxic-Hydrolytic-Oxic (A-O-1-H-O-2) system through engineering data exposure and computational density functional theory (DFT) verification. The results showed that coking sludge had superior enrichment ability for heavy metals, especially for the sludge from the A and H tanks. The enrichment ratio of the 8 heavy metals including Cd, Pb, Ni, Zn, Cu, Hg, Cr and As in coking waste sludge was found to be 6232 (comparing to these in the influent wastewater of A-O1-H-O-2 system). The distribution of 8 heavy metals was closely related to their chemical (precipitation and/or complexation) and biochemical reaction potential with OH-, CN-, SCN-, S2-, NH3 in the A-O1-H-O-2 system. The regulation mechanism of these precipitation and/or complexation agents on heavy metals was confirmed by DFT calculation. The stable energy of complexes formed between typical compounds and common heavy metal ions follow the order: OH: cu(2+) > pb(2+) > zu(2+) cd(2+) > Hg2+ > Ni2+; s(2-): pb(2+) > Cu2+ > zn(2+) > cd(2+) > Hg2+ > Ni2+; CN-: zn(2+) > Cu2+ > Cd2+ > Hg2+ > Pb2+ > Ni2+; SCN-: Zn2+ > Cd2+ > Pb2+ > Hg2+ >Cu2+ > Ni2+; NH3: Cu2+ > Zn2+ > Cd2+ > Pb2+ > Hg2+ > Ni2+, providing reference for the judgement of which metal ions were preferentially combined with the typical compounds in coking wastewater. The results of this paper indicated that the enrichment of heavy metal ions in coking wastewater can be achieved by process design combined with the control of operating conditions (dissolved oxygen, hydraulic retention time, sludge retention time and pH), basing on the nature of heavy metal ions. Finally, the separation and differential management of heavy metals can be achieved.