Sorption and Speciation of Heavy Metals from Incinerator Fly Ash in a Marine Clay

Abstract

The ever-increasing popularity of waste incineration in the recent past has resulted in increasing environmental concerns on secure land disposal of incinerator fly ash. In this study, the capability of a marine clay to immobilize Zn, Pb, Cd, Cu, and Cr, which are commonly found in fly ash leachates, was investigated in a laboratory under acidic, neutral, and slightly alkaline conditions. The sorption process was carried out in batch sorption experiments in the single species system. A background electrolyte consisting of a NaCl solution was introduced. The heavy metals retained in the soil phases were fractionated into exchangeable, carbonate, reducible, organic, and residual fractions using a sequential extraction technique. The results indicated that the slightly alkaline marine clay could completely retain these heavy metals at its natural soil pH of 8.4. In acidic conditions, the mobility of heavy metals followed the Cd > Zn > Cu > Pb > Cr sequence. The heavy-metal speciation in marine clay depended significantly on soil pH, and changed with the metal elements. Between the slightly acidic and slightly alkaline conditions, metal sorption in carbonate and oxide phases was more important, whereas the exchangeable sorption could be of relatively greater importance in acidic conditions. The portion of heavy metals retained in the residual fraction could also be important, and this fraction was less influenced by soil pH. The organic component in marine clay was of the least importance in metal immobilization. Results of time-dependent sorption tests for a period of up to 65 d indicated that total metal sorption and metal speciation in marine clay changed with time. The degree of changes was dependent on soil pH.