Solid phase characteristics and leachability of heavy metals and metalloids from bottom ash in Southern Vietnam

Understanding the chemical and mineralogical characteristics including the potential release of heavy metals (HMs) and metalloids from bottom ash (BA) is necessary because it provides essential information for environmental management options for this waste. This study was conducted with the aim to investigate the leaching of HMs (Cd, Co, Cu, Cr, Mo, Ni, Pb, and Zn) and metalloids (As and Sb) from BA, in combination with a detailed mineralogical characterization. Two BA samples collected in southern Vietnam were subjected to pHstat leaching test with emphasis on the influence of moderately acidification (pH = 4, 96 h). In addition, bulk chemical composition (ICP-OES) and solid phase characteristics (X-ray diffraction (XRD) and Electron Probe Micro Analysis (EPMA)) were determined. The studied BA were composed of oxides (corundum, hematite, magnetite, manganese oxide, rutile, and quartz) and Ca-minerals (calcite and gypsum). The concentrations of HMs and metalloids varied considerably: Co (63-928 mg/kg), Cr (572-804 mg/kg), Cu (818-1126 mg/kg), Mo (42-278 mg/kg), Ni (232-1372 mg/kg), Pb (63-817 mg/kg), Sb (30-140 mg/kg) and Zn (940-1461 mg/kg). However, no discrete mineral phases composed of HMs or metalloids were detected by XRD. Leaching tests showed a low release of As, Pb and Mo (0.1-1 % relative to total concentrations) while a higher release was observed for Cd, Co, Cu, Cr, Ni and Zn (1-9 %). EPMA result revealed the co-existence of Co, Mo and Ni in Fe-rich phase in both samples, while As, Pb, and Zn were found in a Si-rich phase in one of the samples. This shows that EPMA can complement XRD results because it identifies some phases related to HMs and metalloids that cannot be detected by XRD. Similar release patterns during leaching tests were observed for Co, Ni, and Fe in one BA sample. Hence, this may suggest that Co and Ni are released from Fe-rich phases in that sample. Result from this work demonstrated that the application a comprehensive multidisciplinary approach can be helpful to identify the solid phases that retain HMs and metalloids, as well as to understand the long term release of these elements under acidification.status: publishe