Mercury removal from MSW incineration flue gas by mineral-based sorbents.

Three samples of commercially available mineral-based sorbents (zeolite, bentonite and diatomaceous earth) were selected and evaluated for Hg capture under conditions of simulated dry flue gas atmosphere typical in Municipal Solid Waste Incineration (MSWI). The experiments were carried out in a lab-scale fixed-bed device at temperatures between 120 and 200 °C. Two samples of activated carbons (AC) (raw-AC and sulphur impregnated AC) were tested under the same conditions. The mineral-based sorbents were chemically promoted by sulphur, FeCl3 and CaBr2, achieving an improvement in the overall reduction percentage of Hg0out (g) up to 85%, which was comparable to that obtained using a commercial activated carbon for Hg capture (sulphur impregnated AC). The study demonstrates that sorbents with a matrix relatively richer in TiO2, Fe2O3 and Al2O3, as bentonite, favour Hg heterogeneous oxidation. The best Hg capture capacity was achieved with a zeolite sorbent sample characterized by high specific surface (132 m2/g) and impregnated with elemental sulphur. The final form of mercury retained in this sorbent was HgS with proved long-term stability in disposal and landfilling. The higher the temperature, the lower the efficiency of Hg capture being the optimum temperature for Hg-capture in the range of 120-150 °C. This study provides a basis for the development of new efficient non-carbon sorbents for mercury removal in the air pollution control lines of MSWI facilities considering the non-hazardous final form of mercury and its long-term landfilling/sequestration.

Mitigation of gaseous mercury emissions from waste-to-energy facilities: Homogeneous and heterogeneous Hg-oxidation pathways in presence of fly ashes.

This study describes the main mechanisms that take part in the mercury homogeneous oxidation pathway in presence of some of the main reactive compounds formed during waste incineration processes (O2, HCl, SO2 and NO). Series of model, synthetic dry flue gases were used to elucidate the effects of HCl, SO2, NO and their proportions in the gas on mercury behaviour. Three samples of fly ash collected from a MSWI facility were characterized and evaluated both for Hg heterogeneous oxidation and Hg removal in a laboratory scale device. The results obtained in this study showed that homogeneous mercury oxidation in the models MSWI and coal combustion flue gas atmospheres was 52 ± 5% and 25%, respectively. SO2, NO and HCl have a synergetic effect in Hg oxidation in presence of oxygen, but the main differences found are mainly caused by the strong influence of HCl and the likely inhibitory oxidation effects of SO2. Surface area together with carbon and chloride content of the fly ashes were correlated with their capacity for Hg-heterogeneous oxidation and adsorption. The sample of fly ash with relatively high content of unburnt carbon and chlorine, and with BET surface (2.42 m2/g) was able to remove up to 100% of Hg0 (g) during 300 min. The results obtained in this study provide a complete overview of the behaviour of mercury during MSWI processes and may help to clarify the fate/behaviour of mercury in a filter (e.g. electrostatic precipitator) providing a deeper knowledge about the impacts of fly ash properties on mercury fate in waste incineration.

Effects of flooding on lead and cadmium speciation in sediments from a drinking water reservoir.

Rímov water reservoir on the river Malse is the main source of drinking water for the town of Ceské Budejovice and for the majority of inhabitants in the South Bohemian region, Czech Republic. Changes in cadmium and lead contents in bottom sediments before and after an extensive flood on the river Malse in August, 2002 were therefore determined. A five-step sequential extraction procedure was used in order to obtain more detailed information about the influence of the flood on heavy metal retention. In order to determine the mobility of lead and cadmium, the mobility factor (MF) for these heavy metals was calculated. The mobility factor of cadmium showed a significant decrease in the upper parts of the sediment profiles after the flood (e.g., from 59.4% to 49.1%) caused by a release of cadmium especially from the exchangeable fraction. There were no significant changes in the lead mobility factor after the flood, but a decrease of lead concentration in the exchangeable fraction was observed. Presented results show that the flood led to a leaching of the heavy metals present in bottom sediments into the environment.