Improving the mechanical characteristics and restraining heavy metal evaporation from sintered municipal solid waste incinerator fly ash by wet milling.

The milling process has a verified stabilizing effect on the leaching of heavy metals into the environment from municipal solid waste incinerator (MSWI) fly ash. The aim of this current study is to further improve and confirm the effectiveness of the process by exploring its effects on the evaporation of heavy metals and on the mechanical characteristics of the sintered MSWI fly ash. The chemical composition of the MSWI fly ash is first altered by the addition of water treatment plant sludge (WTS) and cullet, and then processed to produce sintered specimens suitable for reuse as an aggregate. In the experiments, fly ash, WTS and cullet (40%: 30%: 30%, respectively) were mixed and milled for 1h. Samples were sintered for 60 min at temperatures of 850, 900, 950 and 1000°C. Test results confirm that milling increased the compressive strength of the sintered specimens. The compressive strength of unmilled specimens sintered at 900°C was only 90 kg/cm(2), but that of milled specimens was 298 kg/cm(2) when sintered at only 850°C. There was also an improvement in the soundness ranging from 11.04% to 0.02% and a reduction in the evaporation rates of Pb, Cd, Cu, Cr and Zn from 54-64%, 43-49%, 38-43%, 30-40% and 14-35% (900-1000°C) to 19-21%, 19-21%, 14-19%, 12-19% and 14-17% (850-1000°C), respectively. The improvement in compressive strength was attained by the combination in the liquid sintering stage of powdered ash with the amorphous material. The amorphousness of the material also helped to seal the surface of the fly ash, thereby reducing the evaporation of heavy metals during the heating process.

Adsorption of Se species on crushed granite: a direct linkage with its internal iron-related minerals.

The adsorption of selenium species on crushed granite is investigated directly linking to its internal iron-related minerals. Experimental results demonstrated that granite has higher affinity toward Se(IV) adsorption than Se(VI) adsorption. Se(IV) adsorption on granite is insensitive to background electrolytes while the effect of ionic strength on Se(VI) adsorption is not observed, which is attributed to the overloading of Se(VI) ions. Results of chemical sequential extraction showed that the removal of crystalline iron oxides dramatically reduces Se(IV) adsorption, which corresponds to the disappearance of goethite signal within XRD pattern. Based on our results, it is proposed that goethite within granite dominates Se adsorption in crushed granite. Although these goethites probably stem from some sample preparation processes including drilling in situ, crushing, washing and drying granite samples in laboratory, the formation of goethite enhances the granite affinity toward Se species adsorption. Images of SEM/EDS furthermore revealed that goethite is embedded within the fractures. In addition, quantification by standard addition method by spiking goethite suspension indicates that only around 20% of goethite minerals are available during Se(IV) adsorption.

Evaluation of buffer materials by associating engineering and sorption properties.

To provide an overall functional evaluation of buffer materials, this study attempted to investigate the relationships among the engineering properties, plastic index (PI), compaction efficiency, sorption properties, and distribution ratio (Rd) for some buffer materials composed of quartz sand and bentonite. Th and U were nuclides of interest, and both synthetic groundwater (GW) and seawater (SW) were used for batch sorption experiments, while the deionized water (DIW) was used for engineering property tests. SW and GW were also used to evaluate the effects on PI. The results show that the maximum dry density was reached when bentonite content was 30% with the same compaction energy by the ASTM D698 method. PI and bentonite content of tested buffer materials consisting of bentonite and quartz sand demonstrated a linearly proportional relationship regardless of the solution used. The following sequence of PIDIW > PIGW > PISW is due to coagulation and flocculation effects. The buffer materials of lower PI value could decrease swelling potential and increase permeability. The Rd observed in GW and SW of U increased linearly with PI measured in DIW, although the Rd of Th remained relatively constant above a PI of 88. From the viewpoints of associated engineering and sorption properties, the buffer materials containing 30-50% bentonite are probably the most favorable choice. Another result shows that U has a better additivity with respect to Rd than Th in both synthetic GW and synthetic SW. These results will allow a determination of more effective buffer material composition, and improved estimates of the overall Rd of the buffer material mixture from the Rd of each mineral component.