| Summary: | 博士 === 國立成功大學 === 環境工程學系 === 88 === ABSTRACT
Although cement-based solidification/stabilization (S/S) technique has been advanced to treat hazardous metal-bearing wastes, many researcheres more recently studying on the long-term stability of the solidified matrix bearing hazardous heavy metal are of the opinion that metal solidified in cementitious matrix will indicate a lack of stabilizing characteristics owing to long exposure in an acidic environment. Thio-surfactants such as xanthates and dithiocarbamates are commonly used for flotation and precipitation of metals as frothers and collectors and have been applied in the industrial wastewaters for the removal of metals by forming highly insoluble metal xanthate and dithiocarbamate complexes. Some of the past studies indicate that xanthates and dithiocarbamates have the strong metal-binding properties and could strongly stabilize metal ion under acidic condition and room temperature. It is still further expected that the xanthates and dithiocarbamates will directly be used as the stabilizing agent to treat the hazardous heavy metal. However, it is still a mystery whether some significant amounts of metal ions can be leached out of insoluble metal xanthate and dithiocarbamate complexes through the various pH (acid, neutral and alkaline) of renewable synthetic extraction fluids at the different temperatures and understanding of such leaching mechanisms can be useful for evaluating the long-term stability of the insoluble metal complexes under the relevant environment situations.
Thus, the purpose of the present work was mainly to provide detailed information of the leaching mechanism and stabilizing characteristic of the synthetic copper ethyl xanthate (Cu-EtX) and copper diethyl dithiocarbamate (Cu-DDTC) complexes through the semi-dynamic leaching test (SDLT) using 1 N acetic acid (pH 2.08), 0.2 N NaHCO3 (pH 8.0) and 1 N NaOH (pH 14) extraction fluids at two different temperatures (25℃ and 60℃) to simulate the relevant environment situations. The thio-surfactants xanthate, potassium ethyl xanthate (C2H5OCS2K, abbreviated as KEtX), and dithiocarbamate, sodium diethyl dithiocarbamate (C5H10NSNa, abbreviated as NDDTC) were used as a model stabilization agents to stabilize the copper ion, which became the insoluble copper ethyl xanthate complex (Cu-EtX) and copper diethyl dithiocarbamate complex (Cu-DDTC). The leached species of the leachant (liquid phase) were characterized by using SDLT and ultraviolet-visible spectrophotometer (UV-visible) techniques to obtain the leaching mechanism of Cu-EtX and Cu-DDTC complexes under three different pH extraction fluids at two different temperatures. The crystallinity, functional group and morphology of the unleached and leached Cu-EtX and Cu-DDTC complexes (solid phase) were examined by using X-ray diffractometer (XRD), fourier-transform infrared spectrophotometer (FTIR) and scanning electron microscopy/energy- dispersive X-ray analysis (SEM/EDAX) techniques to identify the stabilizing characteristic of the Cu-EtX and Cu-DDTC complexes.
Based on the results of SDLT and UV-visible analyses of synthetic Cu-EtX and Cu-DDTC complexes through three different pH extraction fluids at two different temperatures, the leaching mechanisms are summarized as followed:
(1) Cu-EtX complex
Regardless of pH of extraction fluid and temperature of environment, the Cu-EtX complex is mostly the presence of Cu(EtX) and (EtX)2, the Cu(EtX) is the controlling factor for the dissolution of copper ion and EtX-. The (EtX)2 will constantly dissolve and leach out of the insoluble Cu-EtX complex, but the (EtX)2 still further decomposes into EtX- under 0.2 N NaHCO3 (pH 8.0) and 1 N NaOH (pH 14) extraction fluids. Then, the EtX- will decompose into EtOH and CS2 under 1 N acetic acid (pH 2.08) and 1 N NaOH (pH 14) extraction fluids.
(2) Cu-DDTC complex
Regardless of pH of extraction fluid and temperature of environment, the Cu-DDTC complex is mostly the presence of Cu(DDTC)2, the Cu(DDTC)2 is the controlling factor for the dissolution of copper ion and DDTC-. Then, the DDTC- will decompose into secondary amine (R2NH) and CS2 under 1 N NaOH (pH 14) extraction fluid.
Based on the results of XRD, FTIR and SEM/EDAX analyses of unleached and leached Cu-EtX and Cu-DDTC complexes, the long-term stabilizing characteristics of the insoluble Cu-EtX and Cu-DDTC complexes existed in three different pH extraction fluids at two different temperatures are summarized as followed:
(1) Cu-EtX complex
The raise of temperature and pH will cause the microstructural change to the leached Cu-EtX complex. The main stabilization phases existed in Cu-EtX complex leached through 1 N acetic acid (pH 2.08) and 0.2 N NaHCO3 (pH 8.0) extraction fluids at 25℃ both are Cu(EtX) and the main stabilization phases existed in Cu-EtX complex leached through 1 N NaOH (pH 14) extraction fluid at 25℃ are copper organosulfur compound and non-stoichiometric copper sulfide. The main stabilization phases of Cu-EtX complex leached through 1 N acetic acid and 0.2 N NaHCO3 extraction fluids at 60℃ are copper organosulfur compound and non-stoichiometric copper sulfide and the main stabilization phases of Cu-EtX complex leached through 1 N NaOH extraction fluid at 60℃ is copper organosulfur compound, non-stoichiometric copper sulfide and the reprecipitate of copper hydroxide.
(2) Cu-DDTC complex
The raise of pH will cause the microstructural change to the leached Cu-DDTC complex. Regardless of temperature of environment, the main stabilization phases existed in Cu-DDTC complex leached through 1 N acetic acid (pH 2.08) and 0.2 N NaHCO3 (pH 8.0) extraction fluids both are Cu(DDTC)2 and the main stabilization phases existed in Cu-DDTC complex leached through 1 N NaOH (pH 14) extraction fluid are Cu(DDTC)2 and the reprecipitate of copper hydroxide.
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