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TECHNICAL REPORTS

Heavy Metals in the Environment

Leaching of Carbonated Air Pollution Control Residues Using Compliance Leaching Tests

^a State Key Laboratory of Pollution Control and Resources Reuse, Tongji University, Shanghai 200092, China
^b Chemical Engineering Department, National Taiwan University, Taipei, Taiwan, 10617

^* Corresponding author (solidwaste{at}mail.tongji.edu.cn, xhpjk{at}mail.tongji.edu.cn)

Received for publication August 14, 2005. The leaching characteristics of air pollution control (APC) residues collected in Shanghai, China, were compared by performing three compliance leaching tests. These were the standard Chinese method for determining the leaching toxicity of solid waste (GB 5086.1-1997), the USEPA's Toxicity Characteristic Leaching Procedure (TCLP), and the new European shake test (EN 12457-3). In particular, behaviors of raw samples and samples that had been subjected to natural aging were compared. Both the leaching tests and natural aging substantially affected the leaching results concerning the APC residue samples. Most importantly, EN and GB tests classified the raw APC residues as hazardous, but the residues passed the TCLP test as nonhazardous. After it had been naturally aged for 720 h, however, the aged sample was classified as hazardous by the TCLP and EN tests, but as nonhazardous by the GB test. Metals that are thought to have been immobilized by carbonation were released at pH 6.3. Model calculations based on the geochemical thermodynamic equilibrium model MINTEQA2 revealed that the formation of metal carbonates did not correspond to the noted change in the leaching behaviors in the three leaching tests. Rather, the partial neutralization of alkaline ash by dissolved CO₂ changing the final pH of the leachate dominated the leaching characteristics. The leaching results showed a change in leachate pH.

Abbreviations: AAS, atomic absorption spectrophotometer • AFS, atomic fluorescence spectrometer • APC, air pollution control • EC, electrical conductivity • EN, European shake test (EN 12457-3) • GB, Chinese method for determining the leaching toxicity of solid waste (GB 5086.1-1997) • L/S, liquid to solid ratio • MSW, municipal solid waste • ORP, oxidation–reduction potential • TCLP, Toxicity Characteristic Leaching Procedure