Behavior of B, Cr, Se, As, Pb, Cd, and Mo present in waste leachates generated from combustion residues during the formation of ettringite.

The behavior of B, Cr, Se, As, Pb, Cd, and Mo in the leachates generated from two combustion residues, coal-fired power plant fly ash and municipal solid waste incineration ash, during precipitation of ettringite is presented. Experiments also were performed using modeled waste leachates as well as controlled solutions containing all the investigated elements. Moreover, to determine the possible effect of pH, lime treatment was conducted using the waste and modeled leachates. Results indicated the removal of B, Se, and Cr from the leachates because of incorporation of their oxyanions in the ettringite structure. The removal of B could further be explained by considering the structure of ettringite and monosulfoaluminate. The removal of Pb also occurred, probably because of lime-induced precipitation of Pb(OH)2. Similarly, the removal of As was observed as a result of lime-induced precipitation of Ca3(AsO4)2 and incorporation into the ettringite structure. On the other hand, the precipitation of ettringite did not reduce the concentrations of Mo and Cd because of the complex nature of these elements in the leachates. The results also suggest that the presence of high amounts of constituents such as Cl-, an alkali metal in the solution, also affected ettringite precipitation behavior. Moreover, elemental speciations as well as the presence of other constituents in the solution affected the incorporation by ettringite.