In this paper, the temperature dependence of Cr(III) oxidation in high temperature processes and the subsequent Cr(VI) leaching was studied using synthetic mixtures. It was experimentally shown that ...in the presence of alkali and alkaline earth salts, oxidation of Cr(III) takes place, consistent with thermodynamic calculations. Heating of synthetic mixtures of Cr2O3 and Na, K, or Ca salts led to elevated leaching of Cr(VI); in the presence of Na, more than 80% of the initial Cr(III) amount was converted to Cr(VI) at 600–800 °C. Kinetic experiments allowed explanation of the increase in Cr(VI) leaching for increasing temperatures up to 600–800 °C. After reaching a maximum in Cr(VI) leaching at temperatures around 600–800 °C, the leaching decreased again, which could be explained by the formation of a glassy phase that prevents leaching of the formed Cr(VI). By way of illustration, Cr(VI) formation and leaching was evaluated for a case study, the fabrication of ceramic material from contaminated sludge. Based on the proposed reaction mechanisms, countermeasures to prevent Cr oxidation (addition of NH4H2PO4, heating under inert atmosphere) were proposed and successfully tested for synthetic mixtures and for the case study.
•Traditional disposal methods for spent adsorbents have a high environmental impact.•Recycling them as raw material for ceramics production is a valuable alternative.•Mo leaching was reduced after ...production of ceramics with 3% addition of adsorbent.•Heavy metal leaching was reduced after ceramics production with spiked raw material.•Addition of spent adsorbents can also improve structural and aesthetic properties.
Spent adsorbents for oxyanion forming elements and heavy metals are classified as hazardous materials and they are typically treated by stabilization/solidification before landfilling. The use of lime or cement for stabilization/solidification entails a high environmental impact and landfilling costs are high. This paper shows that mixing spent adsorbents in the raw material for the production of ceramic materials is a valuable alternative to stabilize oxyanion forming elements and heavy metals. The produced ceramics can be used as construction material, avoiding the high economic and environmental impact of stabilization/solidification followed by landfilling. To study the stabilization of oxyanion forming elements and heavy metals during the production process, two series of experiments were performed. In the first series of experiments, the main pollutant, Mo was adsorbed onto iron-based adsorbents, which were then mixed with industrial sludge (3w/w%) and heated at 1100°C for 30min. Mo was chosen, as this element is easily adsorbed onto iron-based adsorbents and it is the element that is the most difficult to stabilize (i.e. the highest temperatures need to be reached before the concentrations in the leachate are reduced). Leaching concentration from the 97/3 sludge/adsorbent mixture before heating ranged between 85 and 154mg/kg; after the heating process they were reduced to 0.42–1.48mg/kg. Mo was actually stabilized, as the total Mo concentration after addition was not affected by the heat treatment. In the second series of experiments, the sludge was spiked with other heavy metals and oxyanion forming elements (Cr, Ni, Cu, Zn, As, Cd and Pb) in concentrations 5 times higher than the initial concentrations; after heat treatment the leachate concentrations were below the regulatory limit values. The incorporation of spent adsorbents in ceramic materials is a valuable and sustainable alternative to the existing treatment methods, saving raw materials in the ceramics production process and avoiding the use of stabilizing agents. Besides, spent adsorbents added to the raw material for ceramic products, may improve their aesthetic and structural properties.
Bottom ash from municipal solid waste incineration is an underutilized secondary resource, which currently gains large attention due to increased landfill costs and the push towards a circular ...economy. Due to the high concentrations and mobility of pollutants, bottom ash cannot readily replace virgin construction materials. Over the last decade, many research efforts have addressed these issues in view of newly developed engineering applications. However, the required quality of bottom ash varies for each application. In this review we focus on the ternary relationship between engineering applications, chemical barriers/limitations and treatment technologies for municipal solid waste incinerator bottom ash. For each intended engineering application loose (bulk) construction aggregates; sand, aggregate or cement replacement in concrete; raw material for cement or ceramics the appropriate treatment technologies are selected to overcome identified chemical barriers. This allows future top-down design decisions, starting from the most promising engineering application of bottom ash. The main chemical barrier for bottom ash recycling as loose construction aggregates is the leaching of heavy metals and/or metalloids. This can be overcome by size separation, carbonation, mild heat treatment or by using mineral additives. In structured concrete, the presence of metallic aluminum or zinc causes early cracking and a high chloride concentration causes corrosion of reinforcement steel. Therefore, recent developments in wet/semi-dry separations facilitated enhanced eddy current separation to remove non-ferrous metals. The washing of bottom ash to remove chloride, is to date the sole technology to prepare bottom ash as raw material for cement kilns. Finally, when bottom ash is used as feedstock for ceramics production, recent knowledge was generated to allow for selecting thermal process parameters in such a way that leaching of both heavy metals and metalloids is minimized.
Graphical Abstract
► Calcium antimonate or romeite may control antimonate solubility in alkaline matrices. ► The effect of vacancies in the romeite structure has been poorly examined. ► The CaSb(OH)6+ association ...affects antimonate solubility. ► Vacancies in romeite fill up at high pH and high calcium availability. ► Romeite causes lower antimonate solubility than previously assumed.
The stability of pyrochlores as a function of solution composition is relevant in the context of hazardous waste immobilisation as this mineral family comprises minerals that can control actinide solubility in ceramic formulations as well as romeites or calcium antimonates that possibly control the solubility of Sb in cement pastes. However, to date, no thermodynamic model exists that can be used for long-term risk assessment of pyrochlores as stabilising minerals. In this work synthetic romeites were precipitated at pH 6.5 and pH 12 and at varying molar Ca:Sb ratio and analysed by XRD and Rietveld refinement which showed that, like naturally occurring romeites, synthesized romeites have a Ca1+xSb2O6OH2−2x structure with less vacancies and a higher Ca-content as pH and Ca-availability were higher during synthesis. These romeites dissolve incongruently with preferential Ca leaching and antimonate is less soluble from romeites synthesized in alkaline conditions. Batch extractions of romeite in equilibrium with increasing Ca(NO3)2 additions up to 1molL−1 showed a minimum equilibrium Sb concentration at Ca=0.01molL−1 that could only be explained by assuming the formation of a CaSb(OH)6+ complex. Increasing the Ca concentration in romeite suspensions also resulted in a pH decline in the equilibrium solution, which may be explained by the increasing concomitant uptake of Ca2+ and OH− in the romeite structure as a function of increasing Ca(NO3)2 additions. Thermodynamic modelling supported these assumptions that may explain the incongruent dissolution behaviour of romeite, but the fitting of two equilibrium constants was required. Although the validity of these constants needs more experimental confirmation, this study suggests that despite possible CaSb(OH)6+ formation, the solubility of Ca antimonate in alkaline conditions can be responsible for lower equilibrium Sb concentrations than previously assumed.
One of the main risks of CCS (Carbon Capture and Storage) is CO₂ leakage from a storage site. The influence of CO₂ leakage on trace metals leaching from contaminated marine sediment in a potential ...storage area (Northern Spain) is addressed using standardized leaching tests. The influence of the pH of the leaching solution on the leachates is evaluated using deionized water, natural seawater and acidified seawater at pH = 5, 6 and 7, obtained by CO₂ bubbling. Equilibrium leaching tests (EN 12457) were performed at different liquid–solid ratios and the results of ANC/BNC leaching test (CEN/TS 15364) were modeled using Visual Minteq. Equilibrium tests gave values of the final pH for all seawater leachates between 7 and 8 due to the high acid neutralization capacity of the sediment. Combining leaching test results and geochemical modeling provided insight in the mechanisms and prediction of trace metals leaching in acidified seawater environment.
Mo, Sb and Se form oxyanions in solution, and are therefore difficult to remove by traditional wastewater treatment methods (e.g. alkaline precipitation). In this paper, a method for the simultaneous ...removal of these three elements from wastewater by adsorption, zeolite-supported magnetite is developed. The adsorbent consists of finely divided magnetite particles on a zeolite substrate as carrier material. Basic adsorption parameters such as ideal pH, maximum adsorption capacity and equilibration time, are determined for the oxyanions separately. Much attention is paid to the study of interferences that can limit adsorption. Anions like sulphate and chloride, which often occur in large amounts in wastewaters, do not really compete for adsorption places on magnetite, but oxyanions largely interfere with each other. The reason for this competition is a similar adsorption mechanism (inner-sphere complex formation) for all studied oxyanions, except for selenate, that forms outer-sphere complexes, as was confirmed by geochemical modeling. The adsorption of Mo, Sb and Se oxyanions from an aqueous solution containing the most important detected interferences and from a real wastewater containing also cations is compared, showing that the most important interferences are identified. The order of adsorption is Mo(VI) > Sb(V) > Se(VI). As a case study, Mo, Sb and Se oxyanions are removed by adsorption from an industrial wastewater, the flue gas cleaning effluent of a waste incinerator. For an adsorbent concentration of 20 g/l, removal efficiencies of 99, 97 and 77 % are obtained for Mo, Sb and Se.
•Cl and S are linked by the sulphation reaction in the early stages of a WtE boiler.•Elemental and mineralogical analysis provides evidence for theoretical findings.•Corrosive species form when O2 is ...unavailable for sulphation reactions to proceed.•High SO2 concentrations convert sulphates to low-melting pyrosulphates.
Corrosion of heat-exchanging components is one of the main operational problems in Waste-to-Energy plants, limiting the electrical efficiency that can be reached. Corrosion is mainly related to the devolatilization and/or formation of chlorides, sulphates and mixtures thereof on the heat-exchanging surfaces. Theoretical considerations on this corrosion were already put forward in literature, but this paper now for the first time combines theory with a large scale sampling campaign of several Waste-to-Energy plants. Based on the outcome of elemental and mineralogical analysis, the distribution of Cl and S in ashes sampled throughout the plant during normal operation is explained. Cl concentrations are high (15–20%) in the first empty pass, decrease in the second and third empty pass, but increase again in the convective part, whereas the S concentrations show an inverse behavior, with the highest concentrations (30%) observed in the second and third empty pass. Sampling of deposits on specific places where corrosion possibly occurred, gives a better insight in the mechanisms related to corrosion phenomena in real-scale WtE plants and provides practical evidence for some phenomena that were only assumed on the basis of theory or lab scale experiments before. More specific, it confirms the role of oxygen content, temperatures in the different stages of the boiler, the presence of polysulphates, Pb and Zb, and the concentrations of HCl and SO2 in the flue gas for different types of boiler corrosion.
Most studies on oxyanion adsorption focus on their removal from synthetic solutions. It is often claimed that the considered adsorbents can be used to treat real (industrial) wastewaters, but this is ...seldom tested. Perlite-supported magnetite was characterized first by determining its specific surface area, magnetite content and by examining the coating. Tests on a synthetic solution showed that at the ideal pH values (pH 3 to 5), the order of adsorption is Mo(VI) > As(V) > Sb(V) > Cr(VI) > Se(VI). Most oxyanions can be removed for more than 75% with an adsorbent dosage of 1 g/1. Furthermore, perlitesupported magnetite has a higher removal efficiency for oxyanions than commercially available adsorbents and comparable adsorbents described in literature. Perlite-supported magnetite is suitable for treating real wastewaters: it can remove several oxyanions simultaneously from the considered industrial wastewater, but the adsorption order changes due to the presence of interfering anions.
Elevated Cr and Mo concentrations are often found in leachates of thermally treated solid waste, but there is no general explanation for this so far. Therefore, we studied the leaching behaviour ...after thermal treatment as a function of heating temperature and residence time for two types of solid waste: contaminated sludge and bottom ash from municipal solid waste incineration. The leaching behaviour of both waste streams was compared with experiments on synthetic samples, allowing deduction of a general mechanism for Cr and Mo leaching. Cr and Mo showed a similar leaching behaviour: after an initial increase, the leaching decreased again at higher temperatures. Oxidation of these elements from their lower oxidation states to chromate and molybdate at temperatures up to 600 °C was responsible for the increased leaching. At higher temperatures, both Mo and Cr leaching decreased again owing to the formation of an amorphous phase, incorporating the newly formed chromate and molybdate salts, which prevents them from leaching.
One of the main risks of CCS (Carbon Capture and Storage) is CO
2 leakage from a storage site. The influence of CO
2 leakage on trace metals leaching from contaminated marine sediment in a potential ...storage area (Northern Spain) is addressed using standardized leaching tests. The influence of the pH of the leaching solution on the leachates is evaluated using deionized water, natural seawater and acidified seawater at pH = 5, 6 and 7, obtained by CO
2 bubbling. Equilibrium leaching tests (
EN 12457) were performed at different liquid–solid ratios and the results of ANC/BNC leaching test (
CEN/TS 15364) were modeled using Visual Minteq. Equilibrium tests gave values of the final pH for all seawater leachates between 7 and 8 due to the high acid neutralization capacity of the sediment. Combining leaching test results and geochemical modeling provided insight in the mechanisms and prediction of trace metals leaching in acidified seawater environment.
► Tier structured approach for assessment of the release of metals from sediment. ► Leaching test as useful tool to evaluate risks of a CCS site due to CO
2 leakages. ► Prediction of metal release from marine sediment in contact to acidified seawater. ► Metal and DOC release in equilibrium leaching tests as function of L/S ratio and pH. ► Geochemical modeling of chemical species release from contaminated sediment.
Metal mobility from contaminated sediment under CO
2 leakages in CCS sites using leaching tests.
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