Rob N. J. Comans

Carbonation processes in municipal solid waste incinerator bottom ash and their effect on the leaching of copper and molybdenum

The interaction of CO2 with municipal solid waste incinerator (MSWI) bottom ash was studied in order to investigate the resulting changes in pH and bottom ash mineralogy and the impact that these changes have on the mobility of Cu and Mo. Carefully controlled carbonation experiments were performed on bottom ash suspensions and on filtered bottom ash leachates. Changes in leachate composition were interpreted with the geochemical model MINTEQA2, and neoformed minerals were investigated by means of chemical and spectroscopic analysis. The leaching of Cu and Mo during artificial carbonation is compared to the leachability of Cu and Mo from a sample of naturally carbonated bottom ash from the same incinerator. During carbonation in the laboratory, a precipitate was formed that consisted mainly of Al-rich amorphous material, calcite, and possibly gibbsite. Carbonation to pH ≈8.3 resulted in a reduction of more than 50% in Cu leaching, and a reduction of less than 3% in Mo leaching. The reduction in Cu leaching is attributed to sorption to the neoformed amorphous Al-minerals. During natural weathering/carbonation of bottom ash, additional sorption sites are formed which further reduce the leaching of Cu and Mo on a time scale of months to years.

Characterisation of major component leaching and buffering capacity of RDF incineration and gasification bottom ash in relation to reuse or disposal scenarios

Thermal treatment of refuse derived fuel (RDF) in waste-to-energy (WtE) plants is considered a promising solution to reduce waste volumes for disposal, while improving material and energy recovery from waste. Incineration is commonly applied for the energetic valorisation of RDF, although RDF gasification has also gained acceptance in recent years. In this study we focused on the environmental properties of bottom ash (BA) from an RDF incineration (RDF-I, operating temperature 850-1000°C) and a RDF gasification plant (RDF-G, operating temperature 1200-1400°C), by evaluating the total composition, mineralogy, buffering capacity, leaching behaviour (both at the materials own pH and as a function of pH) of both types of slag. In addition, buffering capacity results and pH-dependence leaching concentrations of major components obtained for both types of BA were analysed by geochemical modelling. Experimental results showed that the total content of major components for the two types of BA was fairly similar and possibly related to the characteristics of the RDF feedstock. However, significant differences in the contents of trace metals and salts were observed for the two BA samples as a result of the different operating conditions (i.e. temperature) adopted by the two RDF thermal treatment plants. Mineralogy analysis showed in fact that the RDF-I slag consisted of an assemblage of several crystalline phases while the RDF-G slag was mainly made up by amorphous glassy phases. The leached concentrations of major components (e.g. Ca, Si) at the natural pH of each type of slag did not reflect their total contents as a result of the partial solubility of the minerals in which these components were chemically bound. In addition, comparison of total contents with leached concentrations of minor elements (e.g. Pb, Cu) showed no obvious relationship for the two types of BA. According to the compliance leaching test results, the RDF-G BA would meet the limits of the Italian legislation for reuse and the European acceptance criteria for inert waste landfilling. RDF-I BA instead would meet the European acceptance criteria for non hazardous waste landfilling. A new geochemical modelling approach was followed in order to predict the leaching behaviour of major components and the pH buffering capacity of the two types of slags on the basis of independent mineralogical information obtained by XRD analysis and the bulk composition of the slag. It was found that the combined use of data regarding the mineralogical characterization and the buffering capacity of the slag material can provide an independent estimate of both the identity and the amount of minerals that contribute to the leaching process. This new modelling approach suggests that only a limited amount of the mineral phases that control the pH, buffering capacity and major component leaching from the solid samples is available for leaching, at least on the time scale of the applied standard leaching tests. As such, the presented approach can contribute to gain insights for the identification of the types and amounts of minerals that control the leaching properties and pH buffering capacity of solid residues such as RDF incineration and gasification bottom ash.

Sorption and sorption reversibility of cadmium on calcite in the presence of phosphate and sulfate

The influence of phosphate and sulfate on the sorption of cadmium on calcite was studied in laboratory experiments at I = 0.7, pH 7.4–9.5 and PCO2 = 10−3.5 atm. The fast initial sorption of cadmium decreased with increasing phosphate and sulfate concentrations. Since total phosphate and total sulfate sorption increase with decreasing pH, due to lower carbonate concentrations, the effects on the sorption of cadmium are most pronounced in the pH range 8.3–7.4. Phosphate and sulfate also affect the sorption reversibility of cadmium. The short term (t 144 h) sorption reversibility of cadmium increases in the pH range 7.4–8.3 and with increasing phosphate (or sulfate) concentrations. The main cause for this is a decreased recrystallization rate of calcite and therefore a smaller amount of cadmium is lattice bound. Based on the reduction of the CdCa ratios at the calcite surface and in the ripened calcite exposed to increasing phosphate concentrations in our sorption experiments, the relationship between CdCa ratios in benthic foraminifera and phosphate concentrations in deep ocean water appears to result from biochemical, rather than chemical processes.

Sorption and sorption reversibility of Cd on calcite under simulated riverine, estuarine and marine conditions

Laboratory experiments using 109Cd and 45Ca tracers were carried out to study sorption and sorption reversibility of cadmium on calcite under simulated riverine, estuarine and marine conditions. n nSorption of Cd on calcite increased with increasing sorption time, calcite surface area, aqueous Cd2+ activity, and decreasing aqueous Ca2+, Mg2+ and Cl− activity. Electrostatic factors which could be influenced by the ionic strength, such as surface charge or double layer thickness, had little effect on the sorption behavior of Cd. Therefore, Cd is considered to be chemisorbed and sorption data can be described with a Ca-Cd exchange model. n nDesorption experiments showed that Cd sorption on calcite is partly irreversible. The extent of reversibility depends on sorption time and pH. After short sorption times (<1 week) desorption and rapid isotopic exchange experiments with 109Cd showed that sorption reversibility decreases from pH 8.3 to 7.9. At lower pH, Cd may merely be sorbed at the energetically most favorable sites like kinks and steps, because of the increased competition with Ca. After long equilibration times (2–6 months) the sorption reversibility decreases as a result of recrystallization. 45Ca exchange experiments suggest that the recrystallization rate decreases from pH 7.9 to 8.2. As a result thereof, the slow removal rate of Cd decreases as well. n nThis study indicates that although at a pH of 8.3 more Cd is rapidly adsorbed, a pH of 7.9 seems favorable for more permanent uptake of Cd by calcite. Assuming residence times of suspended calcite in rivers (lower pH) are 24 h, the release of Cd in the estuary (higher pH) is expected to be limited.

Comparing in situ distribution coefficients and exchangeability of radiocaesium in freshwater sediments with laboratory predictions.

Two short-term (24 h) laboratory procedures, based on measurement of the affinity and capacity of the specific binding sites for radiocaesium on illitic clay minerals, are used to predict the in situ 137Cs distribution coefficients (KD) in two Dutch freshwater sediments. Differences in the measured amount of specific binding sites between the sediments are shown to be related to differences in the particle size and illite content. A procedure based on measurement of the combined parameter KD[NH4+] predicts the measured in situ 137Cs distribution in both sediments very well. This is remarkable because the in situ data are based on 137Cs/sediment contact times of > 1 year, while previous experiments have shown that increasing contact times between Cs+ and sediments decreases the short-term exchangeability of caesium, as measured by extraction with a high ammonium concentration. The good predictions of the 37Cs solid/water distribution by short-term ion-exchange experiments, therefore, indicate that more 137Cs in the sediments is exchangeable than is measured by the commonly used extraction with high ammonium concentrations.

Reference Spectra for Environmentally Important Secondary Minerals: Ettringite (Ca6Al2(SO4)3(OH)12⋅26H2O) by XPS

Ettringite, found in metamorphosed limestone and also as an important secondary mineral in concrete and certain combustion waste environments, plays an important role in concretion strength development, and in the control of pH and immobilization of trace elements in alkaline hazardous wastes. Consequently, the ability to identify ettringite via surface-sensitive spectroscopies is important for understanding cementitious mechanisms and the environmental behavior of wastes. We have used x-ray photoelectron spectroscopy (XPS) to study the binding energy of principal photoelectrons from an air-exposed external crystal surface of a well-formed needle-shaped ettringite sample from a bituminous limestone deposit in Asad, Israel. General survey and high resolution spectra were collected under standard operating conditions using a Perkin Elmer Physical Electronics 5200C spectrometer. Adventitious carbon was used as the energy referencing method. Charge-corrected binding energies were identified for the Ca 2p3/2, ...