Evaluation of frameworks for ecotoxicological hazard classification of waste

A new harmonized EU regulation for the classification of waste came into effect on 1st June 2015, in which the criteria and assessment methods for the classification of hazardous waste are harmonized with other internationally agreed-upon systems for hazard classification of chemicals (CLP). However, criteria and guidance for the assessment of ecotoxicological hazard (Hazard Property 14, HP14) are still lacking for waste classification. This paper have evaluated and compared two HP14 classification frameworks: (i) a calculation method (summation) for mixtures, and (ii) leaching tests. The two frameworks were evaluated by surveying and evaluating ecotoxicological data for Cu, Zn, K and Ca species in bottom ash from incinerated waste, together with geochemical speciation modelling. Classification based on the summation method proved to be highly sensitive to the choice of speciation and ecotoxicological classification. This results in a wide range of critical concentrations triggering hazardous classification (in particular for Cu and Zn). Important parameters governing the availability of toxic elements, such as transformation from one species to another and complexation on organic or inorganic sorbents, are not accounted for. Geochemical modelling revealed that a testing strategy built on CLP based leaching tests (liquid/solid ratio (L/S)⩾10,000, pH range 5.5-8.5) avoids bias and is superior to the summation method with respect to both precision and accuracy. A testing strategy built on leaching tests, designed for risk assessment purposes, (L/S ratio of 10, natural pH of the ash) severely underestimate the hazard associated with the presence of toxic compounds (Cu and Zn), while simultaneously falsely indicate a hazardousness due to the presence of non-toxic compounds (Ca and K). However, the testing methods adopted by CLP are problematic from a practical and functional point of view. To conclude, the L/S ratio and pH were found to be critical for hazard classification based on leaching test methods. Further studies are needed to develop a relevant, practical and functional testing strategy for HP14 hazardous waste classification.

Leaching of arsenic, copper and chromium from thermally treated soil

Thermal treatment, if properly performed, is an effective way of destroying organic compounds in contaminated soil, while impact on co-present inorganic contaminants varies depending on the element. Leaching of trace elements in thermally treated soil can be altered by co-combusting different types of materials. This study aimed at assessing changes in mobility of As, Cr and Cu in thermally treated soil as affected by addition of industrial by-products prior to soil combustion. Contaminated soil was mixed with either waste of gypsum boards, a steel processing residue (Fe₃O₄), fly ash from wood and coal combustion or a steel abrasive (96.5% Fe⁰). The mixes and unamended soil were thermally treated at 800 °C and divided into a fine fraction <0.125 mm and a coarse fraction >0.125 mm to simulate particle separation occurring in thermal treatment plants. The impact of the treatment on element behaviour was assessed by a batch leaching test, X-ray absorption spectroscopy and dispersive X-ray spectrometry. The results suggest that thermal treatment is highly unfavourable for As contaminated soils as it increased both the As leaching in the fine particle size fraction and the mass of the fines (up to 92%). Soil amendment with Fe-containing compounds prior to the thermal treatment reduced As leaching to the levels acceptable for hazardous waste landfills, but only in the coarse fraction, which does not justify the usefulness of such treatment. Among the amendments used, gypsum most effectively reduced leaching of Cr and Cu in thermally treated soil and could be recommended for soils that do not contain As. Fly ash was the least effective amendment as it increased leaching of both Cr and As in majority of samples.

Cellular Mutagenicity and Heavy Metal Concentrations of Leachates Extracted from the Fly and Bottom Ash Derived from Municipal Solid Waste Incineration

Two incinerators in Taiwan have recently attempted to reuse the fly and bottom ash that they produce, but the mutagenicity of these types of ash has not yet been assessed. Therefore, we evaluated the mutagenicity of the ash with the Ames mutagenicity assay using the TA98, TA100, and TA1535 bacterial strains. We obtained three leachates from three leachants of varying pH values using the toxicity characteristic leaching procedure test recommended by the Taiwan Environmental Protection Agency (Taiwan EPA). We then performed the Ames assay on the harvested leachates. To evaluate the possible relationship between the presence of heavy metals and mutagenicity, the concentrations of five heavy metals (Cd, Cr, Cu, Pb, and Zn) in the leachates were also determined. The concentrations of Cd and Cr in the most acidic leachate from the precipitator fly ash and the Cd concentration in the most acidic leachate from the boiler fly ash exceeded the recommended limits. Notably, none of the nine leachates extracted from the boiler, precipitator, or bottom ashes displayed mutagenic activity. This data partially affirms the safety of the fly and bottom ash produced by certain incinerators. Therefore, the biotoxicity of leachates from recycled ash should be routinely monitored before reusing the ash.

Mobility and toxicity of heavy metal(loid)s arising from contaminated wood ash application to a pasture grassland soil

Heavy metal(loid) rich ash (≤10,000 mg kg^-1 total As, Cr, Cu and Zn) originating from the combustion of contaminated wood was subjected to several experimental procedures involving its incorporation into an upland pasture soil. Ash was added to soil that had been prior amended with local cattle manure, replicating practices employed at the farm scale. Metal(loid) concentrations were measured in soil pore water and ryegrass grown on soil/manure plus ash mixtures (0.1-3.0% vol. ash) in a pot experiment; toxicity evaluation was performed on the same pore water samples by means of a bacterial luminescence biosensor assay. Thereafter a sequential extraction procedure was carried out on selected soil, manure and ash mixtures to elucidate the geochemical association of ash derived metal(loid)s with soil constituents. Predictive modelling was applied to selected data from the pot experiment to determine the risk of transfer of As to meat and milk products in cattle grazing pasture amended with ash. The inclusion of manure to soils receiving ash reduced phyto-toxicity and increased ryegrass biomass yields, compared to soil with ash, but without manure. Elevated As and Cu concentrations in pore water and ryegrass tissue resulting from ash additions were reduced furthest by the inclusion of manure due to an increase in their geochemical association with organic matter. Zinc was the only measured metal(loid) to remain uniformly soluble and bioavailable regardless of the addition of ash and manure. Risk modelling on pot experimental data highlighted that an ash addition of >1% (vol.) to this pasture soil could result in As concentrations in milk and meat products exceeding acceptable limits. The results of this study therefore suggest that even singular low doses of ash applied to soil increase the risk of leaching of metal(loid)s and intensify the risk of As transfer in the food chain.

Removal and speciation of mercury compounds in flue gas from a waste incinerator

The management and control of mercury emissions from waste incinerators have become more significant, because waste incinerators are sinks to treat mercury-containing consumer products. This study investigated the effects of mercury concentrations and waste incineration temperatures on mercury speciation using a lab-scale experimental instrument. The removal characteristics of different mercury species were also investigated using an apparatus to simulate the fabric filter with a thin layer of additives such as Ca(OH)2 and NaHCO3, activated carbon (AC), and fly ash. HgCl2 generation rates peaked at 800°C for initial Hg(0) concentrations of 0.08-3.61 mg/Nm(3) in the presence of 400 ppm HCl. A linear relationship was established between the generation rate of HgCl2 and the logarithmic value of initial mercury concentration. Fly ash proved highly efficient in mercury removal, being equal or superior to AC. On the other hand, Ca(OH)2 and NaHCO3 were shown to have no effects on mercury removal. In the dry-scrubbing process, alkali agent is often sprayed in amounts beyond those stoichiometrically required to aid acidic gas removal. The research suggests, however, that this may hinder mercury removal from the flue gas of solid waste incinerators.

Synthesis of a water-soluble thiourea-formaldehyde (WTF) resin and its application to immobilize the heavy metal in MSWI fly ash

Because of the high concentrations of heavy metals, municipal solid waste incineration (MSWI) fly ash is classified as a hazardous waste, which need to be treated to avoid damaging the environment. A novel water-soluble thiourea-formaldehyde (WTF) resin was synthesized by two step reactions (hydroxymethylation reaction and condensation reaction) in the laboratory. Synthetic conditions, removal of free formaldehyde in the resin and the ability of immobilization heavy metals in the MSWI fly ash were studied. The possible molecular structure of the resin was also discussed by elemental analysis and FTIR spectra. Experimental results showed that the synthesis conditions of WTF resin were the formaldehyde/thiourea (T/F) mole ratio of 2.5:1, hydroxymethylation at pH 7.0-8.0 and 60 °C for 30min, and condensation of at pH 4.5-5.0 and 80 °C. In addition, the end point of condensation reaction was measured by turbidity point method. The result of elemental analysis and FTIR spectra indicated that thiourea functional group in the WTF resin chelated the heavy metal ions. Melamine can efficiently reduce the free formaldehyde content in the resin from 8.5% to 2%. The leaching test showed that the immobilization rates of Cr, Pb and Cd were 96.5%, 92.0% and 85.8%, respectively. Leaching concentrations of Cr, Pb and Cd in the treated fly ash were decreased to 0.08 mg/L, 2.44 mg/L and 0.23 mg/L, respectively. The MSWI fly ash treated by WTF resin has no harm to the environment.

Producing a synthetic zeolite from secondary coal fly ash

Secondary coal fly ash is known as a by-product produced by the extracting alumina industry from high-alumina fly ash, which is always considered to be solid waste. Zeolitization of secondary coal fly ash offers an opportunity to create value-added products from this industrial solid waste. The influence of synthesis parameters on zeolite NaA such as alkalinity, the molar ratio of SiO2/Al2O3, crystallization time and temperature was investigated in this paper. It was found that the types of synthetic zeolites produced were to be highly dependent on the conditions of the crystallization process. Calcium ion exchange capacity and whiteness measurements revealed that the synthesized product meets the standard for being used as detergent, indicating a promising use as a builder in detergent, ion-exchangers or selective adsorbents. Yield of up to a maximum of 1.54 g/g of ash was produced for zeolite NaA from the secondary coal fly ash residue. This result presents a potential use of the secondary coal fly ash to obtain a high value-added product by a cheap and alternative zeolitization procedure.

Potassium-based sorbents from fly ash for high-temperature CO2 capture

Potassium-fly ash (K-FA) sorbents were investigated for high-temperature CO₂ sorption. K-FAs were synthesised using coal fly ash as source of silica and aluminium. The synthesised materials were also mixed with Li₂CO₃ and Ca(OH)₂ to evaluate their effect on CO₂ capture. Temperature strongly affected the performance of the K-FA sorbents, resulting in a CO₂ uptake of 1.45 mmol CO₂/g sorbent for K-FA 1:1 at 700 °C. The CO₂ sorption was enhanced by the presence of Li₂CO₃ (10 wt%), with the K-FA 1:1 capturing 2.38 mmol CO₂/g sorbent at 700 °C in 5 min. This sorption was found to be similar to previously developed Li-Na-FA (2.54 mmol/g) and Li-FA (2.4 mmol/g) sorbents. The presence of 10 % Li₂CO₃ also accelerated sorption and desorption. The results suggest that the increased uptake of CO₂ and faster reaction rates in presence of K-FA can be ascribed to the formation of K-Li eutectic phase, which favours the diffusion of potassium and CO₂ in the material matrix. The cyclic experiments showed that the K-FA materials maintained stable CO₂ uptake and reaction rates over 10 cycles.

Speciation and mobility of volatile heavy metals (Cd, Pb, and Tl) in fly ashes

Speciation of volatile metals Cd, Pb, and Tl in fly ashes (FAs) produced from burning of hard coal in stocker-fired boilers (SFBs) was studied. Two grain fractions of fly ash collected in a multicyclone and battery cyclone of the systems of dust separation from three SFB units operating in various urban heating plants were analyzed. The characteristic feature of speciation of the three metals was a large share of labile fractions: Cd (av. 46.1 %), Pb (av. 39.8 %), and Tl (av. 21.6 %). The fraction which most clearly reflected the different chemical properties of the investigated metals was the oxidizable fraction: F(4)-Cd-0 %, F(4)-Pb-av. 10.0 %, and F(4)-Tl-av. 30.2 %. The importance of condensation of the volatile metal species on FA particles for shaping speciation of these metals was characterized using the normalized enrichment factor (NEF): Pb (2.3 ± 0.8) > Tl (1.8 ± 0.9) ≈ Cd (1.7 ± 0.6). Speciation of heavy metals may also be important economically, because the level of mobility coefficients (K _Cd = 0.46, K _Pb = 0.40, and K _Tl = 0.22) in the case of fly ashes considerably enriched with toxic metals (Cd 4.8 ± 3.4 mg/kg, Pb 293 ± 210 mg/kg, and Tl 6.3 ± 4.5 mg/kg) may limit their utilization range.

Characterization and valorization of biomass ashes

In India, farming is the primary source of income for many families. Following each harvest, a huge amount of biomass is generated. These are generally discarded as "agrowaste," but recent reports have indicated several beneficial uses for these biomasses and their ashes. However, before the utilization of biomass ashes (BMAs), their chemical and physical properties need to be investigated (characterized) so as to utilize their potential benefit to the fullest. In this paper, eight different biomass ashes (soybean plant ash, mustard plant ash, maize ash, groundnut plant ash, cotton plant ash, wheat plant ash, pigeon peas ash, and groundnut shell ash) were characterized, and their chemical properties are discussed. Surface chemical composition analysis, proximate analysis, and ultimate analysis were performed on all BMA samples, and properties such as porosity, particle density, bulk density, point of zero charge, BET surface area, water-absorption capacity, and bulk parameters such as surface pH and surface charges were determined. BMAs were characterized by SEM and FTIR. The surface areas of biomass ashes vary from 1.9 to 46 m²/g, and point of zero charge for all BMAs exceed 9.8, which confirmed the alkaline nature of these samples. Based on the chemical composition, BMAs are categorized into four types (S, C, K, and CK), and their utilization is proposed based on the type. BMAs find applications in agriculture and construction industries; glass, rubber, and zeolite manufacturing; and in adsorption (as a source of silica/zeolites). The paper also discusses the research challenges and opportunities in utilization of BMAs.

The impact of thermal treatment and cooling methods on municipal solid waste incineration bottom ash with an emphasis on Cl

Municipal solid waste incineration (MSWI) bottom-ash products possess qualifications to be utilized in cement production. However, the instant use of bottom ash is inhibited by a number of factors, among which the chlorine (Cl) content is always strictly restricted. In this paper, the unquenched MSWI bottom ash was used as the experimental substance, and the influences of thermal treatment and cooling methods on the content and existence of Cl in the ash residues were investigated. The characterization of the MSWI bottom-ash samples examined by utilizing X-ray diffraction, optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy. The experimental results show that as a function of thermal treatment, the reduction rate of Cl is slight below 15.0%, which is relatively low compared with water washing process. Different cooling methods had impacts on the existing forms of Cl. It was understood that most of Cl existed in the glass phase if the bottom ash was air cooled. Contrarily in case of water-quenched bottom ash, Cl could also be accumulated in the newly-formed quench products as chloride salts or hydrate substances such as Friedel's salt.

Geopolymerisation of fly ashes with waste aluminium anodising etching solutions

Combined management of coal combustion fly ash and waste aluminium anodising etching solutions using geopolymerisation presents economic and environmental benefits. The possibility of using waste aluminium anodising etching solution (AES) as activator to produce fly ash geopolymers in place of the commonly used silicate solutions was explored in this study. Geopolymerisation capacities of five European fly ashes with AES and the leaching of elements from their corresponding geopolymers were studied. Conventional commercial potassium silicate activator-based geopolymers were used as a reference. The geopolymers produced were subjected to physical, mechanical and leaching tests. The leaching of elements was tested on 28 days cured and crushed geopolymers using NEN 12457-4, NEN 7375, SPLP and TCLP leaching tests. After 28 days ambient curing, the geopolymers based on the etching solution activator showed compressive strength values between 51 and 84 MPa, whereas the commercial potassium silicate based geopolymers gave compressive strength values between 89 and 115 MPa. Based on the regulatory limits currently associated with the used leaching tests, all except one of the produced geopolymers (with above threshold leaching of As and Se) passed the recommended limits. The AES-geopolymer geopolymers demonstrated excellent compressive strength, although less than geopolymers made from commercial activator. Additionally, they demonstrated low element leaching potentials and therefore can be suitable for use in construction works.

Stabilization/solidification of municipal solid waste incineration fly ash via co-sintering with waste-derived vitrified amorphous slag

Municipal solid waste incineration (MSWI) fly ash (FA) is classified as hazardous waste and requires special treatment prior to landfilling due to its high levels of alkali chlorides and heavy metals. In this paper we presented and discussed a novel method of converting FA into an inert and non-hazardous material, by using the metastable state of vitrified amorphous slag (VAS). XRD results showed that VAS remained in the amorphous state when sintered at 700 and 800°C and were in the crystalline state at 900 and 1000°C. Heavy metals- and Cl-containing phases appeared during phase transformation process. The residual rates of heavy metals and Cl increased with the decrease of FA:VAS ratios. The prolonged leaching test and potential ecological risk assessment of heavy metals showed that the heavy metals were well immobilized into the sintered samples and presented no immediate threat to the environment. The results indicated that the immobilization of heavy metals was due to the reaction with silicate or aluminosilicate matrices within VAS and/or the incorporation into the new generated crystals. The proposed method can be considered as a potential promising technique for the stabilization/solidification of MSWI fly ash with high Cl content.

Development of Nano-Sulfide Sorbent for Efficient Removal of Elemental Mercury from Coal Combustion Fuel Gas

The surface area of zinc sulfide (ZnS) was successfully enlarged using nanostructure particles synthesized by a liquid-phase precipitation method. The ZnS with the highest surface area (named Nano-ZnS) of 196.1 m(2)·g(-1) was then used to remove gas-phase elemental mercury (Hg(0)) from simulated coal combustion fuel gas at relatively high temperatures (140 to 260 °C). The Nano-ZnS exhibited far greater Hg(0) adsorption capacity than the conventional bulk ZnS sorbent due to the abundance of surface sulfur sites, which have a high binding affinity for Hg(0). Hg(0) was first physically adsorbed on the sorbent surface and then reacted with the adjacent surface sulfur to form the most stable mercury compound, HgS, which was confirmed by X-ray photoelectron spectroscopy analysis and a temperature-programmed desorption test. At the optimal temperature of 180 °C, the equilibrium Hg(0) adsorption capacity of the Nano-ZnS (inlet Hg(0) concentration of 65.0 μg·m(-3)) was greater than 497.84 μg·g(-1). Compared with several commercial activated carbons used exclusively for gas-phase mercury removal, the Nano-ZnS was superior in both Hg(0) adsorption capacity and adsorption rate. With this excellent Hg(0) removal performance, noncarbon Nano-ZnS may prove to be an advantageous alternative to activated carbon for Hg(0) removal in power plants equipped with particulate matter control devices, while also offering a means of reusing fly ash as a valuable resource, for example as a concrete additive.

Leaching Behavior of Selected Trace and Toxic Metals in Coal Fly Ash Samples Collected from Two Thermal Power Plants, India

Studies on leaching behavior of metals associated with coal fly ash (FA) are of great concern because of possible contamination of the aquatic environment. In the present study, leaching behavior of metals (As, Se, Cr, Pb, V, Zn, etc.) in two different FA samples (FA1 and FA2) was investigated at various pH (2-12), temperatures of leachate solution and using TCLP. At pH 2, the highest leaching was observed for Fe (21.6 and 32.8 µg/g), whereas at pH 12, Arsenic was found to have the highest leaching (1.5 and 2.4 µg/g) in FA1 and FA2. Leachate solution temperature showed a positive effect on the metal's leachability. In TCLP, most of the metal's leachability was observed to be higher than that of batch leaching tests. The present study suggests that, leaching of As and Se from FA samples can moderately affect ground/surface water quality at the study locations.

Potential of fly ash for neutralisation of acid mine drainage

Lignite (PK), bituminous (FI) and biomass (SE) fly ashes (FAs) were mineralogically and geochemically characterised, and their element leachability was studied with batch leaching tests. The potential for acid neutralisation (ANP) was quantified by their buffering capacity, reflecting their potential for neutralisation of acid mine drainage. Quartz was the common mineral in FAs detected by XRD with iron oxide, anhydrite, and magnesioferrite in PK, mullite and lime in FI, and calcite and anorthite in SE. All the FAs had high contents of major elements such as Fe, Si, Al and Ca. The Ca content in SE was six and eight times higher compared to PK and FI, respectively. Sulphur content in PK and SE was one magnitude higher than FI. Iron concentrations were higher in PK. The trace element concentrations varied between the FAs. SE had the highest ANP (corresponding to 275 kg CaCO3 tonne(-1)) which was 15 and 10 times higher than PK and FI, respectively. The concentrations of Ca(2+), SO4 (2-), Na(+) and Cl(-) in the leachates were much higher compared to other elements from all FA samples. Iron, Cu and Hg were not detected in any of the FA leachates because of their mild to strong alkaline nature with pH ranging from 9 to 13. Potassium leached in much higher quantity from SE than from the other ashes. Arsenic, Mn and Ni leached from PK only, while Co and Pb from SE only. The concentrations of Zn were higher in the leachates from SE. The FAs used in this study have strong potential for the neutralisation of AMD due to their alkaline nature. However, on the other hand, FAs must be further investigated, with scaled-up experiments before full-scale application, because they might leach pronounced concentrations of elements of concern with decreasing pH while neutralising AMD.

Combustion characteristics and arsenic retention during co-combustion of agricultural biomass and bituminous coal

A combination of thermogravimetric analysis (TG) and laboratory-scale circulated fluidized bed combustion experiment was conducted to investigate the thermochemical, kinetic and arsenic retention behavior during co-combustion bituminous coal with typical agricultural biomass. Results shown that ignition performance and thermal reactivity of coal could be enhanced by adding biomass in suitable proportion. Arsenic was enriched in fly ash and associated with fine particles during combustion of coal/biomass blends. The emission of arsenic decreased with increasing proportion of biomass in blends. The retention of arsenic may be attributed to the interaction between arsenic and fly ash components. The positive correlation between calcium content and arsenic concentration in ash suggesting that the arsenic-calcium interaction may be regarded as the primary mechanism for arsenic retention.

Mix design and pollution control potential of pervious concrete with non-compliant waste fly ash

Pervious concrete mix was optimized for the maximum compressive strength and the desired permeability at 7 mm/s with varying percentages of water-to-binder (W/B), fly ash-to-binder (FA/B), nano-iron oxide-to-binder (NI/B) and water reducer-to-binder (WR/B). The mass ratio of coarse aggregates in sizes of 4.75-9.5 mm to the binder was fixed at 4:1. Waste FA used in the study was not compliant with a standard specification for use as a mineral admixture in concrete. One optimum pervious concrete (Opt A) targeting high volume FA utilization had a 28-day compressive strength of 22.8 MPa and a permeability of 5.6 mm/s with a mix design at 36% W/B, 35% FA/B, 6% NI/B and 1.2% WR/B. The other (Opt B) targeting a less use of admixtures had a 28-day compressive strength and a permeability of 21.4 MPa and 7.6 mm/s, respectively, at 32% W/B, 10% FA/B, 0.5% NI/B and 0.8% WR/B. During 10 loads at a 2-h contact time each, the Opt A and Opt B achieved the average fecal coliform removals of 72.4% and 77.9% and phosphorus removals of 49.8% and 40.5%, respectively. Therefore, non-compliant waste FA could be utilized for a cleaner production of pervious concrete possessing a greater structural strength and compatible hydrological property and pollution control potential, compared to the ordinary pervious concrete.

Hepatic and renal trace element concentrations in American alligators (Alligator mississippiensis) following chronic dietary exposure to coal fly ash contaminated prey

Little is known about the propensity of crocodilians to bioaccumulate trace elements as a result of chronic dietary exposure. We exposed 36 juvenile alligators (Alligator mississippiensis) to one of four dietary treatments that varied in the relative frequency of meals containing prey from coal combustion waste (CCW)-contaminated habitats vs. prey from uncontaminated sites, and evaluated tissue residues and growth rates after 12 mo and 25 mo of exposure. Hepatic and renal concentrations of arsenic (As), cadmium (Cd) and selenium (Se) varied significantly among dietary treatment groups in a dose-dependent manner and were higher in kidneys than in livers. Exposure period did not affect Se or As levels but Cd levels were significantly higher after 25 mo than 12 mo of exposure. Kidney As and Se levels were negatively correlated with body size but neither growth rates nor body condition varied significantly among dietary treatment groups. Our study is among the first to experimentally examine bioaccumulation of trace element contaminants in crocodilians as a result of chronic dietary exposure. A combination of field surveys and laboratory experiments will be required to understand the effects of different exposure scenarios on tissue residues, and ultimately link these concentrations with effects on individual health.

COSMOS-rice technology abrogates the biotoxic effects of municipal solid waste incinerator residues

Fly ashes generated by municipal solid waste incinerator (MSWI) are classified as hazardous waste and usually landfilled. For the sustainable reuse of these materials is necessary to reduce the resulting impact on human health and environment. The COSMOS-rice technology has been recently proposed for the treatment of fly ashes mixed with rice husk ash, to obtain a low-cost composite material with significant performances. Here, aquatic biotoxicity assays, including daphnidae and zebrafish embryo-based tests, were used to assess the biosafety efficacy of this technology. Exposure to lixiviated MSWI fly ash caused dose-dependent biotoxic effects on daphnidae and zebrafish embryos with alterations of embryonic development, teratogenous defects and apoptotic events. On the contrary, no biotoxic effects were observed in daphnidae and zebrafish embryos exposed to lixiviated COSMOS-rice material. Accordingly, whole-mount in situ hybridization analysis of the expression of various tissue-specific genes in zebrafish embryos provided genetic evidence about the ability of COSMOS-rice stabilization process to minimize the biotoxic effects of MSWI fly ash. These results demonstrate at the biological level that the newly developed COSMOS-rice technology is an efficient and cost-effective method to process MSWI fly ash, producing a biologically safe and reusable material.

Organic and inorganic amendment application on mercury-polluted soils: effects on soil chemical and biochemical properties

On the basis of a previous study performed in our laboratory, the use of organic and inorganic amendments can significantly modify the Hg mobility in soil. We have compared the effectiveness of organic and inorganic amendments such as digestate and fly ash, respectively, reducing the Hg mobility in Chernozem and Luvisol soils differing in their physicochemical properties. Hence, the aim of this work was to compare the impact of digestate and fly ash application on the chemical and biochemical parameters in these two mercury-contaminated soils in a model batch experiment. Chernozem and Luvisol soils were artificially contaminated with Hg and then incubated under controlled conditions for 21 days. Digestate and fly ash were applied to both soils in a dose of 10 and 1.5 %, respectively, and soil samples were collected after 1, 7, 14, and 21 days of incubation. The presence of Hg in both soils negatively affected to processes such as nitrification, provoked a decline in the soil microbial biomass C (soil microbial biomass C (MBC)), and the microbial activities (arylsulfatase, and β-glucosaminidase) in both soils. Meanwhile, the digestate addition to Chernozem and Luvisol soils contaminated with Hg improved the soil chemical properties (pH, dissolved organic carbon (DOC), N (Ntot), inorganic-N forms (N-NH4 (+) and N-NO3 (-))), as consequence of high content in C and N contained in digestate. Likewise, the soil MBC and soil microbial activities (dehydrogenase, arylsulfatase, and β-glucosaminidase) were greatly enhanced by the digestate application in both soils. In contrast, fly ash application did not have a remarkable positive effect when compared to digestate in Chernozem and Luvisol soil contaminated with mercury. These results may indicate that the use of organic amendments such as digestate considerably improved the soil health in Chernozem and Luvisol compared with fly ash, alleviating the detrimental impact of Hg. Probably, the chemical properties present in digestate may determine its use as a suitable amendment for the assisted-natural attenuation of mercury-polluted soils.

Arsenic Speciation in Bituminous Coal Fly Ash and Transformations in Response to Redox Conditions

The risk of the mobilization of coal ash into the environment has highlighted the need for the assessment of the environmental behavior of coal ash, particularly with respect to toxic trace elements such as arsenic (As). Here, we examined As speciation in coal fly ash samples and transformations in response to aquatic redox conditions. X-ray absorption spectroscopy indicated that 92-97% of total As occurred as As(V), with the remainder present as As(III). Major As-bearing hosts in unamended ashes were glass, iron (oxyhydr)oxides, and calcium arsenate. Oxic leaching resulted in immediate As mobilization to the aqueous phase, reprecipitation of As-iron ferrihydrite, and As adsorption to mineral surfaces. Under anoxic conditions, the (reductive) dissolution of As-bearing phases such as iron ferrihydrite resulted in increased dissolved As compared to oxic conditions and reprecipitation of iron arsenate. Overall, As in coal ash is not environmentally stable and can participate in local biogeochemical cycles.

Leaching of valuable elements from thermal power plant bottom ash using a thermo-hydrometallurgical process

The solid industrial wastes generated from thermal power plants (TPPs) can be considered as renewable secondary sources for recovery of valuable metals. This study presents the results from investigations that integrated a thermo-hydro-metallurgical method for treatment of bottom ash obtained from the Enel Maritsa East 3 TPP in Bulgaria. Leaching was performed with 20, 30 and 40 wt% sulphuric acid, respectively, in an autoclave at 100(o)C, 120(o)C and 140(o)C for 120, 240, 360 and 480 min, at a constant value of the liquid/solid ratio. After autoclaving, the samples (suspensions) were diluted with a constant value of water and stirring at 50(o)C for 60 min. On the basis of the experimental data the leaching efficiency (α) of the elements in the liquid phase after filtration was estimated. The leaching of aluminium increases significantly with increasing of the temperature, reaching the maximum value of 70 wt%. The highest leaching efficiency values for the other elements are as follows: Fe (86.4%), Ca (86.6%), Na (86.6%), Ni (83.3%) and Zn (83.3%). The maximum value of leaching for Mg, K, Mn, Cu and Cr is in the interval of 46-70%.

Geochemical modeling and assessment of leaching from carbonated municipal solid waste incinerator (MSWI) fly ash

Municipal solid waste incinerator (MSWI) fly ashes are characterized by high calcium oxide (CaO) content. Carbon dioxide (CO2) adsorption by MSWI fly ash was discussed based on thermogravimetry (TG)/differential thermal analysis (DTA), minerology analysis, and adapting the Stenoir equation. TG/DTA analysis showed that the weight gain of the fly ash below 440 °C was as high as 5.70 %. An adapted Stenoir equation for MSWI fly ash was discussed. The chloride in MSWI fly ash has a major impact on CO2 adsorption by MSWI fly ash or air pollution control (APC) residues. Geochemical modeling of the critical trace elements copper (Cu), cadmium (Cd), zinc (Zn), lead (Pb), and antimony (Sb) before and after carbonation was performed using a thermodynamic equilibrium model for solubility and a surface complexation model for metal sorption. Leaching of critical trace elements was generally found to be strongly dependent on the degree of carbonation attained, and their solubility appeared to be controlled by several minerals. Adsorption on ferrum (Fe) and aluminum (Al) colloids was also responsible for removal of the trace elements Cd, Pb, and Sb. We used Hakanson's potential ecological risk index (HPERI) to evaluate the risk of trace element leaching in general. The results demonstrate that the ecological risk showed a V-shaped dependency on pH; the optimum pH of the carbonated fly ash was found to be 10.3-11, resulting from the optimum carbonation (liquid-to-solid (L/S) ratio = 0.25, carbonation duration = ∼30-48 h). The dataset and modeling results presented here provide a contribution to assessing the leaching behavior of MSWI fly ash under a wide range of conditions.

Recovery of combustibles from electrostatic precipitator discharge

Coal is the main fuel for the direct reduced iron (DRI) plants of India, which are one of the major sources of fly ash generation. The generation of fly ash and its disposal has raised concern because of the environmental impacts. In the present study, two different fly ash samples were investigated to explore the scope of recovery of combustibles. One did not show any recovery potential. The second sample indicated that about 40% of material could be recovered, with 35% fixed carbon and 10,841 kJ kg(-1) gross calorific value. This can be used as a fuel blend in standard fluidised bed combustion boilers efficiently burning inferior coal. A process flowsheet has been suggested for the recovery purpose. It was estimated that for a small 0.2 million tonnes per year (Mtpy) capacity DRI plant, approximately 3.4 ha of land could thus annually be saved from dumping of the generated fly ash, while recovering 1.83 MW of electrical energy.