Fly Ash Leachate Research Articles

Leachability of elements in alkaline and acidic coal fly ash samples during batch and column leaching tests

Three acidic (HA, HB, and MA) and one alkaline (PD) fly ash samples collected from four different power plants were subjected to batch and column leaching tests to evaluate the leachability of several elements. Jar leaching tests (up to 120h), serial batch leaching tests (35days), and long-term (up to 70weeks) batch leaching tests were performed using doubly deionized (DDI) water, and column leaching tests (131–150 pore volumes) were conducted using a synthetic acid rain (SAR) solution. The results of batch and column leaching tests showed that different proportions of As, Cr, Mo, Sb, Se, and V were leached by DDI water and SAR from the four fly ash samples. The increasing leachate concentrations of As, Cr, Mo, Sb, Se (except PD fly ash), and V with time indicated that the long-term release of these elements was controlled by the dissolution of glassy fly ash particles. Mobilization of As from HA fly ash, Cr from PD fly ash, and Se from both HA and PD fly ash also showed a bimodal leaching pattern during column leaching tests, potentially indicating the association of these elements with different fly ash fractions. In general, Mo, Sb, and Se showed high mobilities, while only small amounts of Cr were released from the fly ash samples. Boron, Ca, K, Mg, Mn, and Na were released very rapidly during the initial phases of both serial and column leaching tests. These elements were likely enriched in surface-bound, highly soluble loose particles. During long-term batch leaching tests, HB and MA fly ash leachate concentrations of Ba, Co, Mn, Ni, and Zn decreased with time, indicating possible adsorption or co-precipitation with secondary minerals. Because many of the older fly ash disposal facilities are unlined, these sites are potential sources of hazardous elements to the environment. Therefore, development of effective and economic methods to contain trace elements within the disposal facility site is desirable to prevent contamination of groundwater and surface water resources.

Attenuation of trace elements in coal fly ash leachates by surfactant-modified zeolite

Potential leaching of trace elements from older, unlined fly ash disposal facilities is a serious threat to groundwater and surface water contamination. Therefore, effective methods for containing the pollutant elements within the unlined coal combustion products (CCPs) disposal facilities are required to minimize any potential impact of leachate emanating from such facilities into the nearby environment. Because surfactant-modified zeolite (SMZ) has the potential to sequester both cationic and anionic trace elements from aqueous solutions, bench-scale batch and column experiments were performed to test its ability to remediate trace elements in leachates generated from both alkaline and acidic fly ash samples. Fly ash leachate treatment results showed the potential application of SMZ as an effective permeable reactive barrier (PRB) material to control the dispersion of heavy metals and metalloids from ash disposal sites. Quantitative comparison of the elemental composition of SMZ-treated and untreated leachates indicated that SMZ was effective in decreasing the concentrations of trace elements in fly ash leachates. Similarly, SMZ treatment column experiments showed the delayed peak leaching events and overall reductions in leachate concentrations of trace elements. The effectiveness of SMZ column treatments, however, decreased with time potentially due to the saturation of sorption sites.

Alternative techniques for extracting alumina from coal fly ash

The extraction of aluminium from coal fly ash (CFA) has been carried out using two extraction techniques, acid leaching with an inorganic acid and gas phase extraction with acetylacetone, a beta-diketone. The acid leaching experiments were carried out at varying factor values of temperature, time, solid to liquid ratio, and acid concentration. Factors investigated for gas phase extraction included acetylacetone flow rate, extraction temperature and time, and aluminium (III) oxide source.Direct leaching of fly ash in 6M sulphuric acid at a solid to liquid ratio of 1:4 gave an extraction of 23.5% at 75°C after 8h 45min of leaching time. With gas phase extraction, 17.9% aluminium extraction was achieved at 250°C, 6mL/min acetylacetone flow rate, after 6h. This represents 85% and 64% extraction from the CFA amorphous phase respectively. It was also found that when a synthetic aluminium (III) oxide mixture with a similar content to CFA (31wt%) was subjected to gas phase extraction, 46.7% extraction could be achieved after 6h at 250°C. The difference in extraction levels could be explained by the fact that the alumina in CFA occurs in both mullite and amorphous phase compared with only the amorphous phase in synthetic fly ash, an indication that mineralogy has a significant effect on the extraction. Therefore the lower extraction from CFA by acetylacetone is possibly due to the non-amenability of the mullite phase to the acetylacetone beta-diketone. The results presented in this paper show that extractions using acetylacetone in the gas phase are comparatively lower than acid leaching but have advantages of reduced extraction times and the possibility of recycling the unreacted acetylacetone.

Initial studies of the recovery of Cu from MSWI fly ash leachates using solvent extraction

Large volumes of ash from combustion of municipal solid waste are produced and most of it is landfilled. As this type of ash contains significant amounts of metal compounds the landfilling strategy is not optimal when considered from a resource conservation perspective. A better situation would be created if metals were recovered from the ash. In the present study leaching and solvent extraction was applied for release and separation of copper from municipal solid waste combustion fly ashes. The results showed promising results with Cu yields of 50-95%. The yield was heavily dependent on the efficiency of the initial leaching of Cu from the ash.

Basic characteristics of leachate produced by various washing processes for MSWI ashes in Taiwan

Approximately 19.2% of Taiwan's municipal solid waste (MSW) that passes through incineration disposal is converted into ashes (including bottom ash and fly ash). Although bottom ash can pass nearly all of the standards of the toxicity characteristic leaching procedure (TCLP), its high chloride content makes its reuse limited; it generally cannot be used as a fine aggregate material in concrete applications. This research examined washing four types of bottom ash (BA) and fly ash (FA) with water to reduce their chloride content. The optimal water intensity for washing pretreated bottom ash was found to be 7-8L of water per kg of bottom ash, and the optimal water intensity for washing untreated fly ash was found to be 20-25 L of water per kg of fly ash. Based on regression analyses of the chloride concentrations of the leachates and their electrical conductivity (EC) values, each MSW incineration plant has its own ash characteristics as well as a specific regression line in bottom or fly ash leachate. Clearly, it is possible to monitor the EC values of the leachates online by estimation from regression equations to determine the chloride concentrations in the leachates.

Fate and abatement of mercury and other trace elements in a coal fluidised bed oxy combustion pilot plant

The fate of trace elements and the properties of oxy-combustion residues were evaluated in a 90kWth bubbling fluidized bed (BFB) oxy-combustion pilot plant fed with coal and limestone (bed material). Sampling of solid streams (coal, limestone, bottom ash, cyclone fly ash, and bag-filter fly ash) and exhaust gas (SOx, NOx, CO2, CO, and Hg) was undertaken through 4h operating at 70% load, 70:30 CO2/O2 ratio of raw gas and 800–820°C in a single (without re-circulation) oxy-combustion test run. The retention of SO2 (98.2 %), NOx (99.9%), and Hg (92.5%) and remaining elements (99.9%) attained high efficiencies while low retention for Cl is achieved (19%). Speciation of gaseous Hg revealed that most of the gaseous Hg occurs in the exhaust gas as Hg0. The proportions of Cl and Hg in the exhaust gas may be considered of concern with respect to subsequent CO2-rich gas treatment and oxy-combustion cycles by partial re-circulation of exhaust gas. The high abatement capacity for most of elements (Hg, S, NH4+, F, Se, As, B, Se, Cd, and Sn) is mostly due to condensation in bag filters. The low gas temperature (45°C), and high-Ca content promotes condensation on fly ash collected in the bag filters. The Hg gaseous speciation and the enrichment in bag filters is indicative that most of the Hg retained is Hg2+, most probably as Hg-sulphate species. Calcite and lime are the main crystalline phases in bottom ash, quartz, illite, and calcite in cyclone fly ash and gypsum–bassanite in bag-filter fly ash. The dissolution of these species produces a moderate alkalinity (pH=10.7–11.1) in bottom ash and cyclone fly ash leachates and slight (pH=8) in bag-filter fly ash leachates. The leaching potential of most of elements in oxy-combustion residues is relatively low, falling in the range of non-hazardous materials according to Decision 2003/33/EC. Nevertheless, the high condensation of elements of environmental concern coupled with the use of a fuel rich in the aforementioned elements would increase the leachable potential, and consequently, may affect the disposal of this type of fly ash.

Environmental geochemistry of the feed coals and their combustion by-products from two coal-fired power plants in Xinjiang Province, Northwest China

The mineralogical and geochemical characteristics of feed coals and their combustion by-products (fly ash and slag) from Hongyanchi (HONG) and Weihuliang (WEI) coal-fired power plants in Xinjiang Province, Northwest China, were investigated. The enrichment and partitioning behaviour, as well as the leaching potential of the major and trace elements in these combustion by-products were evaluated. Three feed coals from the two power plants (HC, WC1, and WC2) were characterised by medium-moisture contents, low- to medium-ash yields, high volatile matters, and low-S contents as well as low-mineral contents. Except B, Sr, and Ba in WC1 feed coal; P, Ti, V, Cr, Mn, Cu, Zn, Rb, and Ba in WC2 feed coal; and Ti, Mn, Cu, Zn, Rb, Sr, and Ba in the HC feed coal, most major and trace element concentrations were low compared with other Chinese coals and coals worldwide. Most trace element concentrations in the studied fly ashes (HA, WAf, and WAc) were even lower than their minimum and/or 25th percentile (P25) values reported for EU fly ashes, as were the element concentrations in the fly ash leachates. The studied fly ashes were all classified as type F fly ashes, with high glass contents as well as high SiO2/Al2O3 ratios in the glass matrix. Through enrichment factors, mass balance, and partitioning of elements, S, F, Cl, Hg, and B were highly volatile in both PCC plants. Tin, As, Se, Cu, and Zn were also moderately volatile, and they were enriched in fly ashes mainly by volatilisation/condensation. Iron and Mn were slightly enriched in slags, probably due to the high density of Fe–Mn-bearing phases. In the light of the above-mentioned properties, fly ashes and slags from the two power plants in Xinjiang will have a very high potential for a variety of uses, with very low environmental limitations. In summary, this study is demonstrating the high quality of Xinjiang coals with enormous coal reserves, and also the high quality of the combustion by-products that will be produced also in very high amounts in the near future. Major focus is on trace elements.

Comprehensive characterization of anthracite fly ash from a thermo-electric power plant and its potential environmental impact

Abstract Anthracite fly ash from a deactivated Portuguese thermo-electric power plant has been characterized to assess its relationship to the mineral matter in typical feed coal and the potential environmental impacts that might be caused by ash disposal. Several different aspects of the ash were characterized, including the petrographic and mineralogical composition, as well as the chemical composition of both the ash samples and their water leachates. Petrographic analysis demonstrates that the fly ash consists predominantly of glass, followed by char and crystalline mineral particles. SEM-EDX studies were used to complement this approach, revealing a mixture of different inorganic phases and unburnt organic matter. Quantitative XRD analysis further showed that amorphous material (or glass) is the main constituent (60–70%), followed by mullite, quartz, and traces of maghemite. Small proportions of kaolinite and illite also occur in some of the fly ash samples. According to their chemical composition, the fly ashes would be classified as sialic and are within the range of values shown by other European fly ash samples. Relatively low concentrations of elements in leachates from the fly ash could be due to enclosure of most of the elements within the amorphous material, together, possibly, with mobilization of some of the material (e.g. adsorbed ions) from the ashes with storage. The fly ash leachates developed very acidic pH values (between 2.5 and 2.8) under test conditions, attributed to the low-CaO content and the co-firing with fuel oil.

Geochemical Controls of Coal Fly Ash Leachate pH

When coal fly ash is initially mixed with water, the initial pH of resulting extract or leachate may be strongly acidic (pH 4) or alkaline (pH12). With time, however, this pH range tends to narrow because of geochemical buffering reactions. Because pH is the major variable that controls the leaching of many potential groundwater contaminants, understanding the long-term pH behavior of fly ash leachate is crucial to evaluating the environmental impacts of fly ash management. Using laboratory extract data, kinetic-geochemical models were created to gain a better understanding of the potential buffering that influences the long-term pH of ash leachate. The initially low pH of acidic fly ashes may be short-lived because the acidity is neutralized by the dissolution of calcium and magnesium oxides, is ultimately buffered by carbon dioxide yielding a pH of 7 to 8. Alkaline fly ash leachate (pH \> 10) tends to absorb carbon dioxide, and the resulting pH of the liquid phase will decrease with time to a pH between 8 and 9. Kinetic modeling suggests that the chemical composition of short-term laboratory extracts of coal fly ash will not be representative of long-term leachate after equilibrating with the atmosphere.

Kinetics of fly ash leaching in strongly alkaline solutions

We have leached fly ash samples from six power stations in potassium hydroxide solutions at a water-to-solid mass ratio of 40 g/g. A wet chemical method was developed which provides for a detailed characterization of the reactivity of fly ash. The leaching process could be divided into three stages. In stage one, reaction progress measured by the relative mass of fly ash reacted (α) was controlled by the rate of glass network dissolution while very little gel formed (α 0.45). The data were modeled using a modified Jander equation and rate constants were calculated for each process. The rate constants for stage one reflect an intrinsic glass property, chemical durability, which increased linearly with increasing concentration of network formers in the glass phase of a fly ash.

Recovery of germanium from real fly ash leachates by ion-exchange extraction

Germanium recovery from real fly ash (FA) leachates was studied using ion-exchange procedures. The procedure was based on germanium complexation with catechol (CAT) in an aqueous solution followed by the retention of the Ge–CAT complex onto a conventional strongly basic anionic resin (IRA-900). A centred composite rotatable design (CCRD) with two factors was employed for experimental design and analysis of the results. The factors examined during sorption tests were CAT amount and resin dosage. Optimum values of these parameters were: a CAT/Ge molar ratio of 9 and resin/Ge dosage of 3, obtaining 96.1% germanium retention. The obtained response surface and mathematical model are significant with high correlation coefficients ( R 2 = 0.8794). The maximum amount of retained germanium was calculated to be 215.5 mg/g resin. The elution of the germanium complex was achieved with acid solutions in an ethanol matrix.

TREATMENT OF ASH LEACHATE USING DIFFERENT FUNCTIONAL GROUP ANION EXCHANGE RESINS

Fly ash leachate wastewater containing a wide variety of toxic heavy metals either in cationic or oxyanionic forms. Treatment of such water represents a challenge for all researchers due to the higher concentrations and competitive effect of all the existing cations and anions. The process of treatment was investigated using different functional groups anion exchangers to reach the most suitable materials and conditions for lowering the level of contamination for the wastewater reuse. The effects of most abundant anionic species in leachate wastewater such as sulfate on the adsorption of boron, molybdate, chromate, arsenate and selenate oxyanions were studied at different experimental conditions. The data obtained indicate that LAYNAERT is the most suitable anion exchanger for the removal of arsenate and selenate, whereas, IRA743 and A400 are good for the removal of boron and molybdate respectively.

A technique for sequential leaching of coal and fly ash resulting in good recovery of trace elements

Coal and fly ash contain many elements. These elements exist in different forms which may change throughout the coal combustion process. There are several processes, including X-ray techniques and leaching techniques by which studies have attempted to assess the form of a particular element in a sample. This work focuses on determining the leachability of selected elements sequentially leached in four extraction solutions: water, 1 M ammonium acetate, 3 M hydrochloric acid and 50% hydrofluoric acid. The emphasis is on evaluating the steps involved in the leaching process with the mass recovery for each element being the basis for evaluation. The total amount of each element that will leach out under the given extraction condition is presented as a fraction of the total present in the material. The materials evaluated were NIST coal and fly ash standards. The elements measured in this study include aluminum, barium, beryllium, calcium, chromium, cobalt, iron, magnesium, manganese, nickel, potassium, sodium, strontium, vanadium and zinc.

Precipitation of Germanium from Coal Fly Ash Leachates

The valorization of a coal gasification fly ash was studied by leaching the fly ash using different aqueous solutions followed by the subsequent precipitation of the germanium solubilized from the ash. Experimental investigations were focused on the precipitation of a germanium-catechol (CAT) complex with cetyl trimethyl ammonium bromide (CTAB). The influences of pH and amounts of CTAB and CAT on the precipitation yield were investigated. To this aim, a central composite rotatable design and ANOVA Design Expert 7.0.3 Wiley software were employed for experimental design and analysis of the results. Thus, the independent and combined effects of pH, CAT/Ge and CTAB/Ge molar ratios were investigated and optimized using a quadratic mathematical model. The optimum values of these factors were found to be 10, 12, and 4, respectively (in this case, the germanium precipitation yield was 98.8% for water leachates). The precipitation of germanium as a complex compound with CAT and CTAB was found to be selective towards germanium and this element can be effectively separated from As, Mo, Sb, V, or Zn. Total organic carbon in solution was measured to estimate the amounts of CAT and CTAB precipitated with germanium and with the interferences present in aqueous leachates. In addition, thermogravimetric analyses have been performed on the germanium-complex solids as a result of which 600°C was determined as the minimum temperature to completely remove the organic content of the precipitate.

Determination of lead(II) in fly ash leachate using a newly developed simple spectrophotometric method

A new simple method for the spectrophotometric determination of Pb(II) in fly ash leachates was developed. These leachates tend to contain a large amount of Ca(II) and Zn(II); this interferes with spectrophotometric determination of Pb(II) when conventional colorimetric agents are used. A copolymer consisting of protoporphyrin IX disodium salt and acrylamide was synthesized as a colorimetric agent. A measuring reagent containing ethylenediamine-N,N′-dipropionic acid (EDDP) as a masking agent for Zn(II) and an appropriate amount of Ca(II) together with the copolymer was applied to determine Pb(II). The temporal change in the absorption spectrum of the measuring reagent was acquired with a newly developed portable spectrophotometer for this method. The composition of EDDP and Ca(II) in the measuring reagent was optimized to measure leachates contaminated with Ca(II) and Zn(II). The detection limit and relative standard deviation of Pb(II) measured using the optimized method were 0.05 mg L −1 and 2.3%, respectively. The tolerance limits for Ca(II) and Zn(II) contaminants, where errors of less than 10% were allowed at a concentration of 0.5 mg L −1 Pb(II), were 4000 and 4 mg L −1, respectively. The determination of Pb(II) in various samples of actual leachates from incinerator fly ash was examined with this method. The obtained values correlated well with those obtained by flame atomic absorption spectroscopy.

Metals removal from soil, fly ash and sewage sludge leachates by precipitation and dewatering properties of the generated sludge

This study concerns the treatment by precipitation of three acidic and metal-rich leachates by using various reagents. Two treatment modes (simple and combined precipitation) have been performed to evaluate the metals removal efficiency and the dewatering ability of the generated sludge. It appears that for the three leachates used, the Ca(OH) 2 addition gave better metals removal at pH 10.0 than the use of NaOH at the same pH. Moreover, the combination of NaOH and Na 2S allows better removal for Pb 2+, Zn 2+, Cu 2+ and Mn 2+ ions than with NaOH/Na 2CO 3 or NaOH/Na 2HPO 4. The dewaterability (vacuum filtration) of precipitates produced during the treatment of soil leachate was established as follows on the basis of the specific resistance to filtration (SRF) values: sulphides (4.3 × 10 12 m kg −1) < phosphates (6.4 × 10 12 m kg −1) < hydroxides [Ca(OH) 2] (14.2 × 10 12 m kg −1) < hydroxides [NaOH] (19.7 × 10 12 m kg −1) < carbonates (26.5 × 10 12 m kg −1). For the fly ash leachate treatment, the obtained results were quite similar no matter the treatment mode used; the SRF values were in a range of 5–7 × 10 12 m kg −1, while sludge volume index (SVI) ranged from 39 to 50 mL g −1. The sludge generated during the treatment of sewage sludge filtrate using phosphate salt gave the best results regarding to SVI and SRF parameters (180 mL g −1 and 7.6 × 10 12 m kg −1, respectively). Experimental results show that whatever precipitating agent is used, vacuum filtration is more efficient in water elimination [total solids: 11–73% (w w −1)] than in centrifugation [5–11% (w w −1)] from sludge.

Influence of NaOH solution on the synthesis of fly ash geopolymer

A study was conducted on leaching of fly ash mixed with NaOH solution and on mixing procedure for preparing geopolymer. Leaching of SiO 2 and Al 2O 3 was investigated by mixing fly ash with NaOH solution for different time intervals and leachates were analyzed in terms of silica and alumina contents. To make geopolymer paste, separate mixing and normal mixing were used. For separate mixing, NaOH solution was mixed with fly ash for the first 10 min; subsequently sodium silicate solution was added into the mixture. For normal mixing, fly ash, sodium hydroxide and sodium silicate solution were incorporated and mixed at the same time. Geopolymers were cured at 65 °C for 48 h. Microstructure of paste and compressive strength of mortar were investigated. Results revealed that solubility of fly ash depended on concentration of NaOH and duration of mixing with NaOH. For mixing procedure, separate mixing gave slightly better strength mortar than normal mixing. High strength geopolymer mortar up to 70.0 MPa was obtained when the mixture was formulated with 10 M NaOH and sodium silicate to NaOH ratio of 1.0, and the separate mixing sequence was used.

Chemical stabilization of medical waste fly ash using chelating agent and phosphates: Heavy metals and ecotoxicity evaluation.

Chemical stabilization of heavy metals in medical waste fly ash has been carried out using the following compounds: a chelating agent (Ashnite S803), a commercial acidic phosphoric acid solution (Ashnite R303) as well as basic one (Ashnite R201). In order to predict the leachability of heavy metals, Japanese Leaching Test (JLT-13) procedure was applied to the stabilized fly ash products. An ecotoxicity assessment of the stabilized fly ash products leachate and the unstabilized fly ash leachate was conducted using a battery of bioassays based on lettuce root elongation inhibition, Daphnia magna mortality and Vibrio fischeri photoinhibition. The results showed that the three stabilizing agents were able to significantly decrease (ANOVA, P < 0.05) the concentration of heavy metals in the leachates. Although the leachate from both stabilized and unstabilized fly ash were very toxic to lettuce and daphnids, the incorporation of these stabilizing agents diminished significantly (ANOVA, P < 0.05) the toxicity of the leachates towards the three tested organisms. Pearson correlation analysis was used to analyze the strength of the relationship between chemical elements concentration in the leachate and bioassays results. Most of the heavy metals in the leachate were significantly correlated (ANOVA, P < 0.05) with the toxicity values of the bioassays. However, the correlation was not found between the concentration of dissolved organic carbon (DOC) and the toxicity effect of the leachate to the tested organisms.

Biotoxicity evaluation of fly ash and bottom ash from different municipal solid waste incinerators

Different types of municipal solid waste incinerator (MSWI) fly and bottom ash were extracted by TCLP and PBET procedures. The biotoxicity of the leachate of fly ash and bottom ash was evaluated by Vibrio fischeri light inhibition test. The results indicate the following: (1) The optimal solid/liquid ratio was 1:100 for PBET extraction because it had the highest Pb and Cu extractable mass from MSWI fly ash. (2) The extractable metal mass from both fly ash and bottom ash by PBET procedure was significantly higher than that by TCLP procedure. (3) The metal concentrations of fly ash leachate from a fluidized bed incinerator was lower than that from mass-burning and mass-burning combined with rotary kiln incinerator. (4) The TCLP and PBET leachate from all MSWI fly ash samples showed biotoxicity. Even though bottom ash is regarded as a non-hazardous material, its TCLP and PBET leachate also showed biotoxicity. The pH significantly influenced the biotoxicity of leachate.

Comparison of fly ash leaching methods

Five leaching methods (serial batch leaching procedure (SBLP), synthetic groundwater leaching procedure (SGLP), mine water leaching procedure (MWLP), 3 TIER integrated framework leaching protocol (3TIER), and toxicity characteristic leaching procedure (TCLP)) were compared using two samples, a class F fly and a class C fly ash. Each method was tested in triplicate, according to the published procedure, and leachate was analyzed for 23 elements. The procedures were compared on the basis of elemental availability, defined as the cumulative elemental release per kg of sample, and the variation in extraction with increased L/S as a surrogate for long term release. Higher availability values were obtained with the SBLP and MWLP, which cover a range of pH’s and 3TIER, which uses EDTA to form soluble complexes. SGLP, a high pH procedure, and TCLP, mildly acidic, generally produced lower availability values. Cumulative elemental extraction as a function of L/S in SBLP, MWLP and 3TIER generally agreed within an order of magnitude.