Precipitation Characteristics Of Heavy Metals In Highly Alkaline Wastewater By Industrial Exhaust Gas Bubbling

At precent, the aplication of municipal solid waste incineration?MSWI? has a rapid expansion, MSWI fly ash is a kind of potential resources for substitute some virgin material, such as cement raw materials, but the high content of chlorine salts in fly ash can limit this potential, the use of a water-based washing process to remove the soluble salts from MSWI fly ash was recognized, meanwhile heavy metals were also leached in washing process. Fly ash wastewater, which is highly alkalinity and contains a lot Ca²⁺ ions, has a potential to sequester CO₂, the removal of heavy metals could be accomplished by bubbling industrial exhaust gas through the resulting wastewater and the emission of greenhouse gas CO₂ was also reduced.The impacts of water-washing on MSWI fly physicochemical properties and heavy metal leaching was discussed in this paper, and the precipitation characteristics of heavy metals in fly ash wastewater by industrial exhaust gas bubbing was investigated. A geochemical model program PHREEQC was used to help explain the mechanism of heavy metals precipitation.XRD analysis showed after washing the chemical composition of MSWI fly ash was changed, the peaks of KCl and NaCl disappeared, while the peaks of Ca?OH?2 became intense, the SEM-EDS analysis confirmed the lose of chlorine salts and showed the origin fly ash contained more pores. The pH values of wastewater increased with the increase of L/S ratio, while decreased as raising the washing temperature. Leached concentrations of Pb, Zn, Cu decreased as the increase of L/S ratio, however As concentrations shows more variable when L/S ratio has increased. The optimum washing condition was an L/S ratio of 8, and the pH of wastewater was 12.4. Pb, Zn, Cu and As leached in high concentrations?20.24 mg/L, 0.136 mg/L, 0.59 mg/L and 0.109 mg/L, respectively?.Different temperature?25??50? and 80 ?, respectively? of industrial exhaust gas were used in the bubbing experiment, the capacity of remove these heavy metals was limited in high temperature. Pb, Zn, Cu and As Leached concentrations shows a V or L-shaped dependency on pH. Pb, Zn, Cu and As concentrations were considerably below the comprehensive sewage discharge standard at pH values between 8 and 9. The geochemica simulation shows that these elements were effectively removed in the calcium carbonate?CaCO₃? or ferrum/aluminum-hydroxides that precipitated when CO₂ was bubbled into the fly ash wash wastewater; meanwhile the pH of the wastewater was effectively reduced. Heavy metals also can precitation as pH change. Pb, Zn and Cu mainly reaction with ions to form precitation in high pH zone, but gradually sorption by CaCO₃ and ferrum/aluminum-hydroxides when pH decresed, however As concentration was mainly inflened by the sorption process with CaCO₃ and ferrum/aluminum-hydroxides.