Lin Chou, Jhon Shih Lun (2018-08). Impact of High Total Dissolved Solids, Manganese and Nickel Ions on the Hybrid Zero-valent Iron Treatment System. Master's Thesis.
Thesis
Wet Flue Gas Desulphurization (FGD) is an air pollution control device that scrubs sulfur oxides from the flue gas of coal-fired electric power generations. The blowdown liquid stream from the FGD process is often contaminated with heavy metals, which requires advanced treatment for compliance with environmental regulations. To reduce water usage, power plants have steadily raised water recirculation cycles in the scrubber, thus increasing the strength of the wastewater that often exceeds 30 g/L in total dissolved solids (TDS). Removing heavy metals from a complex matrix of high TDS is a challenging task. In this study, we aim to evaluate the hybrid zero-valent iron (hZVI) technology as a viable solution for managing FGD wastewater, specifically, focusing on wastewater with high TDS and/or high manganese and nickel concentrations. For the purpose, bench-top continuous-flow hZVI treatment systems were operated to treat simulated wastewater with designed compositions under controlled conditions to evaluate the impact of TDS on the hZVI system and its performance for removing Mn and Ni. Tests showed that high TDS at 62 g/L mainly consisted of Cl^-, SOv4^2-, Na^+, Ca^2+, and Mg2^+ in the simulated wastewater could initially decrease the hZVI media reactivity with respect to nitrate reduction, but the media could gradually adapt to high TDS and over time the negative impact of high TDS diminished. On a long-term operation, high TDS has no obvious impact on hZVI system performance. Dissolved silica, which was previously believed to decrease hZVI reactivity through its interactions with dissolved Fe^2+ and iron oxide surface, exhibited no additional impact on the hZVI system performance at high TDS conditions. High level of carbonate alkalinity, however, was a significant factor that could decrease the hZVI reactivity. This study showed that the hZVI system is effective for treating high TDS wastewater featuring high Cl^- and SOv4^2-, which represents a major advantage over the biological or certain physicochemical processes that are highly susceptible to background salt level. This study also demonstrated that high concentrations of Mn^2+ or Ni^2+ may be removed by hZVI, but as more Mn(II) or Ni(II) was incorporated into the FeOx structure of the hZVI media, the reactivity of the media could be negatively affected. Mn(II) or Ni(II) could substitute Fe(II) and occupy certain lattices within the FeOx structure. Such substitution could alter the chemical property of the FeOx and decrease media reactivity. Both Mn^2+ and Ni^2+ removal were greatly affected by the concentration of Fe^2+ (aq.). Residual Ni^2+ (aq.) may be dictated by Fe^2+ (aq.) following an equilibrium constant in the hZVI media system. Under typical hZVI system operation conditions at near neutral pH and in presence of aq. Fe^2+, high removal of large quantities of Mn^2+ may be achievable, but not sustainable. However, the hZVI system could be operated under a Fe^2+ deficient condition, which would allow the media to take in both Ni^2+ and Mn^2+ in substitute of Fe^2+, thus decreasing Ni and Mn to a very low level in the treated effluent. This study has expanded our knowledge of hZVI media water treatment system for industrial wastewater treatment, in particular with applications involving extremely high TDS, dissolved Mn and Ni conditions.
