Preparation and characterization of animal bone powder impregnated fly ash catalyst for transesterification

The present work reconnoitres the feasibility of utilizing class F fly ash and calcined animal bone powder (CABP) as raw material for the synthesis of heterogeneous solid base catalyst with varying ratios (CABP of 10, 20, and 30 mass%), that is subsequently used for transesterification of mustard oil. Physicochemical characterization of CABP revealed crystalline behavior, signifying one of the components as hydroxyapatite (HAP); when calcined at 900 °C transforms to β-tricalcium phosphate having a specific surface area of 100 m² g^-1. The synthesized catalyst showed improved catalytic activity when compared to the parental species and the optimal value to achieve the highest conversion of 90.4% would be at CABP loading of 10 mass%, 5.5:1 methanol to oil molar ratio, and 10 mass% catalyst concentration for 6 h. The prepared biodiesel had a calorific value of 36.2 MJ kg^-1 with ash content < 0.01 mass%. The catalyst could be reused five times with no loss in its activity. Results indicated that calcium enriched waste materials when impregnated in fly ash might be a potential source of catalyst in biodiesel production.

Design and analysis for the removal of active pharmaceutical residues from synthetic wastewater stream

The removal of three over-the-counter pharmaceuticals from aqueous solution using four different adsorbents was analyzed. To study the effect of infused pharmaceutical and adsorbent on the adsorption system, both the concentration of drug and adsorbent dosage were varied, with constant temperature and pressure at different contact time. Adsorption kinetics, isotherm models, and ANOVA allegorized a generic trend for pharmaceutical removal efficiency of the adsorbents that varied as follows: activated carbon > fly ash > bentonite > sugar cane bagasse ash. The Tempkin model appears to fit the isotherm data better than Freundlich and Langmuir. Correspondingly, the kinetic studies implied a pseudo-second-order fit, to understand the mechanism by which the solute accumulates on the surface of a solid and gets adsorbed to the surface via intra-particle diffusion. Furthermore, some special cases of removal tendencies were noted based on sorbate-sorbent interaction. Effectively, it was observed that at an adsorbent loading of 2 g and initial concentration of 0.2 mmol L^-1, bentonite, fly ash, and activated carbon were able to strip more than 80% of all pharmaceuticals from urine. A framework for the highest significance of the experiments was obtained using response surface methodology by the combination of ciprofloxacin-bentonite followed by paracetamol-activated carbon and ibuprofen-activated carbon. Quasi-Newton and Bayesian regression methods were implemented on Langmuir isotherm by designing the neural network for the batch adsorption experiments. Based on the numerical calculations and graphical representations, the proposed model leads to the result that error is minimized and the values are optimized for different pharmaceuticals such as paracetamol, ibuprofen, ciprofloxacin that can be removed from wastewater streams by locally available adsorbents. Graphical abstract.

Geopolymers produced from drinking water treatment residue and bottom ash for the immobilization of heavy metals

Drinking water treatment residue (DWTR) and municipal waste incineration bottom ash (BA) have been traditionally considered as solid waste. With the development of urbanization, their subsequent treatment and resource regeneration need to be further researched. In this work, a composite geopolymer with BA and DWTR was successfully synthesized and applied in the immobilization of Cd, Pb and Zn. The analysis of the geopolymers with different ratios of BA and DWTR, curing times and heavy metals was performed through chemical analysis, SEM, FTIR, XRD, XPS, ICP-AES and compressive strength tests. The results show that the geopolymer samples based on BA and DWTR (BWG) presented higher compressive strength than the samples with single BA material. The sample with 20% DWTR and 80% BA (BWG20) possesses the highest compressive strength (24.10 MPa) among the materials ratios. Furthermore, the microstructure and characterization results indicate that the geopolymer matrix was successfully formed in BWG and was significantly changed by the ratio, curing time and addition of heavy metals. The immobilization efficiency for different categories and dosages of heavy metals by BWG20 were all higher than 99.43%. Moreover, the XPS results demonstrate that the heavy metals were immobilized in geopolymer mainly by divalent state forms.

Characteristics of incineration ash for sustainable treatment and reutilization

Municipal solid waste incineration (MSWI) generates bottom ash, fly ash (FA), and air pollution control (APC) residues as by-products. FA and APC residues are considered hazardous due to the presence of soluble salts and a high concentration of heavy metals, and they should be appropriately treated before disposal. Physicochemical characterization using inductively coupled plasma mass spectroscopy (ICP-MS), X-ray diffraction (XRD), and X-ray fluorescence (XRF) have shown that FA and APC have potential for reuse after treatment as these contain CaO, SiO₂, and Al₂O₃. Studies conducted on treatment of FA and APC are categorized into three groups: (i) separation processes, (ii) solidification/stabilization (S/S) processes, and (iii) thermal processes. Separation processes such as washing, leaching, and electrochemical treatment improve the quality and homogeneity of the ash. S/S processes such as chemical stabilization, accelerate carbonation, and cement solidification modify hazardous species into less toxic constituents. Thermal processes such as sintering, vitrification, and melting are effective at reducing volume and producing a more stable product. In this review paper, the treatment processes are analyzed in relation to ash characteristics. Issues concerning mixing FA and APC residues before treatment, true treatment costs, and challenges are also discussed to provide further insights on the implications and possibilities of utilizing FA and APC as secondary materials.

Levels and fingerprints of chlorinated aromatic hydrocarbons in fly ashes from the typical industrial thermal processes: Implication for the co-formation mechanism

Municipal solid waste incineration (MSWI) and iron and steel making plants are two of important sources of chlorinated aromatic hydrocarbons (CAHs). In this study, the typical CAHs including polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs), polychlorinated naphthalenes (PCNs) and chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs) in fly ash samples collected from MSWI, iron ore sintering (IOS) and steel smelting (SS) plants were simultaneously identified and quantified. The total concentrations of quantified CAHs in different fly ashes showed a large variation (5.88-4255 ng/g). Cl-PAHs were found to be predominant CAH species in most of fly ash samples, and the concentrations of mono-chlorinated PAHs in all fly ashes were obviously higher than those of di-chlorinated PAHs. The fingerprints of CAHs in MSWI fly ashes were mainly characterized by the high content of PCDDs, especially the hexa-CDD homologue. However, in IOS and SS fly ashes, tetra- to hexa-CDF homologues showed the higher abundance, and tetra-to octa-CDFs predominated over tetra- to octa-CDDs by factors of 3-26. The strong concentration correlations were observed between tetra- to octa-CDF homologues, middle-chlorinated PCN and PCB homologues, between tetra- to octa-CDD homologues, between low-chlorinated PCB and PCN homologues, between di-chlorinated PAH homologues, and between deca-CB and highly chlorinated PCN homologues. These results implied that the main formation mechanism of CAHs might be significantly varied with the change of chlorination degree and aromatic ring structure.

Residue concentrations and profiles of PCDD/Fs in ash samples from multiple thermal industrial processes in Vietnam: Formation, emission levels, and risk assessment

The residue concentrations and congener profiles of polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) were examined in fly ash and bottom ash released from different thermal industrial processes in Vietnam. PCDD/F concentrations and toxic equivalents (TEQs) in the ash samples varied greatly and decreased in the following order: steel making > aluminum recycling > medical waste incinerator > boilers > municipal waste incinerator > tin production > brick production > coal-fired power plant. Both the precursor and de novo synthesis were estimated as possible formation mechanisms of dioxins in the ash, but the latter pathway was more prevalent. The highest emission factors were estimated for the ash released from some steel-making plants, aluminum-recycling facilities, and a medical waste incinerator. The emission factors of PCDD/Fs in ash released from some steel plants of this study were two to six times higher than the UNEP Toolkit default value. The annual emission amount of ash-bound dioxins produced by 15 facilities in our study was estimated to be 26.2 to 28.4 g TEQ year^-1, which mainly contributed by 3 steel plants. Health risk related to the dioxin-containing ash was evaluated for workers at the studied facilities, indicating acceptable risk levels for almost all individuals. More comprehensive studies on the occurrence and impacts of dioxins in waste streams from incineration and industrial processes and receiving environments should be conducted, in order to promote effective waste management and health protection scheme for dioxins and related compounds in this rapidly industrializing country.

Wood ash effects on growth and cadmium uptake in Deschampsia flexuosa (Wavy hair-grass)

Wood ash recycling to forests is beneficial because it regains nutrients and prevents acidification, but wood ash application is restricted due to its cadmium (Cd) content. We question if Cd in wood ash represents a problem, since decreases in Cd bioavailability due to ash-induced pH changes may counteract increased total Cd concentration. We studied effects of wood ash (0, 3, 9 and 30 t ha^-1) and lime (pH increase equivalent to the wood ash treatments) on growth and Cd uptake in Deschampsia flexuosa. After four months, we measured plant biomass and Cd accumulation, and extracted Cd from the soil using three different methods; HNO₃ (total), EDTA (chelator-based) and NH₄NO₃ (salt-based). Wood ash and lime strongly stimulated plant growth. Cd concentration in the plant tissue decreased with wood ash and lime addition, and correlated positively with the NH₄NO₃ extractable fraction of Cd in the soil. In contrast, HNO₃ and EDTA extracted more Cd with increased wood ash application. We conclude that wood ash amendment increases soil pH, total Cd concentration, nutrient levels and stimulates plant growth. However, it does not increase Cd accumulation in D. flexuosa, as pH-driven decreases in Cd bioavailability leads to reduced plant Cd uptake. Finally, soil bioavailable Cd is best determined using NH₄NO₃-extraction.

Classification of coal fly ash based on pH, CaO content, glassy components, and leachability of toxic elements

Coal fly ash (CFA), a by-product generated from coal-burning power plants, readily leaches toxic elements into aquatic environments. The present study describes a classification system for CFA based on the chemical composition of CFA and leachability of toxic elements, which can promote the safe and effective utilization of CFA for uses such as fly ash cement. To classify CFA samples, the CaO content, leachate pH, leachability of toxic elements such as B, As, and Se, and the acid- and alkali-soluble Si and Al in glassy components were determined for ten types of CFA samples produced in Japan. The results indicated that the CFA samples could be grouped into three groups: group A, which was characterized by low CaO content, low leachate pH, and a relatively high amount of alkaline-soluble Al; group B, which was characterized by low CaO content, low leachate pH, and relatively low amount of alkaline-soluble Al; and group C, which was characterized by high CaO content, high leachate pH, and relatively low amount of alkaline-soluble Al. Characteristic of group A CFA was the simultaneous leaching of Al and the minor elements along with a gradual increase in pH. This type of CFA carries the risk of leaching toxic substances upon contact with alkali solutions. These results can aid the discovery and separation of safe and unsafe CFA, allowing the safe CFA to be used in cement to produce concrete under alkali conditions.

Synthetic sodalite doped with silver nanoparticles: Characterization and mercury (II) removal from aqueous solutions

In this work, a novel silver nanoparticles-doped synthetic sodalitic composite was synthesized and characterized using advanced characterization methods, namely TEM-EDS, XRD, SEM, XRF, BET, zeta potential, and particle size analysis. The synthesized nanocomposite was used for the removal of Hg²⁺ from 10 ppm aqueous solutions of initial pH equal to 2. The results showed that the sodalitic nanocomposites removed up to 98.65% of Hg²⁺, which is ∼16% and 70% higher than the removal achieved by sodalite and parent coal fly ash, respectively. The findings revealed that the Hg²⁺ removal mechanism is a multifaceted mechanism that predominantly involves adsorption, precipitation and Hg-Ag amalgamation. The study of the anions effect (Cl^-, NO₃^-, C₂H₃O₂^-, and SO₄^2-) indicated that the Hg²⁺ uptake is comparatively higher when Cl^- anions co-exist with Hg²⁺ in the solution.

Hierarchical Ag-SiO2@Fe3O4 magnetic composites for elemental mercury removal from non-ferrous metal smelting flue gas

Hierarchical Ag-SiO₂@Fe₃O₄ magnetic composites were selected for elemental mercury (Hg⁰) removal from non-ferrous metal smelting flue gas in this study. Results showed that the hierarchical Ag-SiO₂@Fe₃O₄ magnetic composites had favorable Hg⁰ removal ability at low temperature. Moreover, the adsorption capacity of hierarchical magnetic composite is much larger than that of pure Fe₃O₄ and SiO₂@Fe₃O₄. The Hg⁰ removal efficiency reached the highest value as approximately 92% under the reaction temperature of 150°C, while the removal efficiency sharply reduced in the absence of O₂. The characterization results indicated that Ag nanoparticles grew on the surface of SiO₂@Fe₃O₄ support. The large surface area of SiO₂ supplied efficient reaction room for Hg and Ag atoms. Ag-Hg amalgam is generated on the surface of the composites. In addition, this magnetic material could be easily separated from fly ashes when adopted for treating real flue gas, and the spent materials could be regenerated using a simple thermal-desorption method.

Distribution and emission characteristics of filterable and condensable particulate matter before and after a low-low temperature electrostatic precipitator

The low-low temperature electrostatic precipitator (LLT-ESP), a combination of a traditional temperature electrostatic precipitator (ESP) and a non-leakage media gas-gas exchange (MGGH), could reduce the inlet flue gas temperature below the dew point and improved the performance of the ESP. Particulate matter (PM) from the stationary sources contains filterable particulate matter (FPM) and condensable particulate matter (CPM). In this study, coal with a high ash content (coal-HA) was burned, and the emission characteristics and removal efficiencies of the particulate matter in an LLT-ESP were investigated. The standards used to test filterable and condensable PM were ISO standard 23210-2009 and U.S. EPA Method 202, respectively. The LLT-ESP was efficient in removing filterable PM, with a total filterable PM removal efficiency as high as 99.6%. The removal efficiency of filterable PM increased with increasing particulate size and decreasing imported flue gas temperature. The LLT-ESP also provided excellent removal of condensable PM with a condensable PM removal efficiency exceeding 77%. Upstream of the LLT-ESP, the concentrations of filterable PM were much higher than those of condensable PM. Downstream of the LLT-ESP, the relationship between the quantities of condensable and filterable PM reversed. To reduce the emissions of PM from coal-fired power plants, more attention should be paid to controlling condensable PM. The temperature of the flue gas upstream of the LLT-ESP played an important role in eliminating condensable PM. At lower imported flue gas temperature operation conditions, the removal efficiency of the LLT-ESP for the condensable PM and the escaping mass concentration of condensable PM increased. Among the organic fraction of the condensable PM, hydrocarbons and esters were dominant. Meanwhile, SO₄^2- was the primary component, followed by Cl^- in anions. Na⁺, Ca²⁺, and Fe³⁺ were the main components in metal ions. Particles with diameters ≥ 10 μm, which contained most of the Si and Al, were dominant in the fly ash collected from sections 1 and 2 of the LLT-ESP. The main particles in sections 3 and 4 were PM₁₀, which contained the highest concentrations of Ca and Fe.

Hydrogen generation and simultaneous removal of Cr(VI) by hydrolysis of NaBH4 using Fe-Al-Si composite as accelerator

The present study reports a novel method for hydrogen generation and simultaneous removal of Cr(VI) from synthetic wastewater using NaBH₄ as the reducing agent and Fe-Al-Si composite as accelerator. The results showed that the hydrogen generation yields of NaBH₄ occurred at low pH, high temperature and stirring speed. The presence of Cr(VI) was found to inhibit hydrogen generation at the initial pH 3.0, especially at low temperature conditions. Increasing temperature resulted in the increase of hydrogen generation, whereas a higher reduction rate of Cr(VI) was obtained at low temperature. Therefore, to alleviate the contradiction between hydrogen generation and Cr(VI) removal with respect to temperature, we introduced reduction accelerator by preparing Fe-Al-Si composite using wasted fly ash as raw materials. The resultant hydrogen generation yield could be enhanced from 32.04 to 80.70%, and total Cr removal was increased from 46.72 to 98.96% at 30 °C. First, H⁺ produced by hydrolysis of Fe-Al-Si composite improves the hydrolysis ability of NaBH₄, thus promoting its ability to reduce Cr(VI) and to produce hydrogen by itself. The Cr(VI) reduction and hydrogen generation process are competitive for H⁺, which is particularly evident at insufficient H⁺. Second, at low doses, Fe³⁺ exhibited a lower ability to promote hydrogen generation and simultaneous Cr(VI) removal than Al³⁺. One of the reasons for the low promotion ability of Fe³⁺ in the whole pH range was the formation of Fe⁰ at pH less than 3.0, and the other was the weaker hydrolysis ability of Fe³⁺ itself at pH greater than 3.0.

Mechanochemical degradation of PCDD/Fs in fly ash within different milling systems

Two distinct mechanochemical degradation (MCD) methods are adopted to eliminate the polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) from fly ash in municipal solid waste incinerators. First, experiments are conducted in a planetary ball mill for selecting suitable additives, and an additive system of SiO₂-Al is chosen for its high-efficiency, low-price, and good practicability. The I-TEQ value of PCDD/Fs in washed fly ash decreases dramatically from 6.75 to 0.64 ng I-TEQ/g, after 14 h of milling with 10 wt % SiO₂-Al, and dechlorination is identified as the major degradation pathway. Then, this additive is applied in a horizontal ball mill, and the results indicate that the degradation of PCDD/Fs follows the kinetic model established in planetary ball mills. However, longer milling time is required for the same supplied-energy because of the lower energy density of horizontal ball mills, resulting in partial loss of Al reactivity and a lower degradation efficiency of PCDD/Fs. During MCD, the evolution of PCDD/F-signatures is analogous, indicating a similar acting mechanism of all additives in both the two milling systems. Finally, a major dechlorination pathway of PCDD-congeners is proposed based on the signature analysis of congeners synthesized from chlorophenols.

Evaluation of the phytotoxicity of coal ash on lettuce (Lactuca sativa L.) germination, growth and metal uptake

Land application of coal ash is considered an environmentally friendly option to improve soil quality, but limited information exists on metal bioavailability and phytotoxicity of coal ash to sensitive plant species such as lettuce (Lactuca sativa L.). Germination and pot bioassay experiments were conducted at six coal application rates (0% (control), 5%, 15%, 25%, 50% and 75% v/v) to investigate the hypothesis that, coal ash will have a hormetic effect on germination, growth, metal uptake and biomass yield of lettuce, characterized by stimulatory and phytotoxicity effects at low and high application rates, respectively. Total concentrations (mg/kg) of metals in coal ash spanned several orders of magnitude, and decreased in the order: Fe (5150.5), Mn (326.0), Zn (102.6), Cu (94.7), Ni (74.7) and Pb (11.6). Bioavailable concentrations of metals were very low (0.0-14.1 mg/kg), accounting for less than 2% of the total concentrations. Coal ash had no significant effect on germination indices, but had hormetic effects on radicle elongation, evidenced by stimulatory and phytotoxicity effects at low (5-25%) and high (50-75%) application rates, respectively. Coal ash application at 50% and 75% significantly (p < 0.05) reduced lettuce growth and edible biomass yield, but lower application rates (5-25%) were similar to the unamended soil (control). Fe, Mn, Zn, Cu and Ni bioavailability and plant uptake generally decreased with increasing coal ash application rates particularly at 50% and 75%. Soil pH significantly increased (p < 0.05) from 6.5 for the control to about 8 for 75% coal ash, while electrical conductivity (EC) increased by 2-7 times to about 0.9 and 1.5 dS/m at 50% and 75% coal ash, respectively. Significant inverse linear relationship (p < 0.05; r2 = 0.80) were observed between edible and total biomass yields and EC, suggesting that increased salinity at high coal ash application rates could account for reduced growth and biomass. Partial elemental balances showed that plant uptake of metals was very low, accounting for just less than 2% of the bioavailable concentrations, while the bulk of the metals (98-99%) remained in the soil. In conclusion, the current findings show that coal ash may have hormetic and phytotoxic effects on sensitive plant species, an observation contrary to the bulk of earlier literature documenting beneficial effects of coal ash application to soils. Long-term field studies are required to confirm the current findings based on laboratory and pot bioassay experiments.

Affordable and efficient adsorbent for arsenic removal from rural water supply systems in Newfoundland

The fly ash from the Corner Brook Pulp and Paper (CBPP) mill was used in this study as the raw material for the preparation of a low-cost adsorbent for arsenic removal from the well water of Bell Island. The CBPP fly ash was physically activated in two different ways: (a) activation with pure CO₂ (CAC) with the iodine number and methylene value of 704.53 mg/g and 292.32 mg/g, respectively; and (b) activation with a mixture of CO₂ and steam (CSAC) with the iodine number and methylene value of 1119.98 mg/g and 358.95 mg/g, respectively, at the optimized temperature of 850 °C and the time of 2 h for both activations. The BET surface areas of the CAC and CSAC at the optimized conditions were 847.26 m²/g and 1146.25 m²/g, respectively. The optimized CSAC was used for impregnation with iron (III) chloride (FeCl₃) with different concentrations (0.01 M to 1 M). The study shows that the adsorbent impregnated with 0.1 M FeCl₃ is the most efficient adsorbent for arsenic removal. Isotherm analysis shows that the Langmuir model better describes the equilibrium behavior of the arsenic adsorption from both local well water and synthesized water compared to the other models. The maximum arsenic adsorption capacity was 35.6 μg/g of carbon for local well water and 1428.6 μg/g of carbon for synthesized water. Furthermore, the kinetic behavior of arsenic adsorption from synthesized and local well water was well depicted by the pseudo-second order kinetic model.

Measurement of Dioxin Emissions from a Small-Scale Waste Incinerator in the Absence of Air Pollution Controls

Polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/Fs) emissions from basic small-scale waste incinerators (SWI) may cause health risks in nearby people and are thus subject to stringent regulations. The aim of this study was to evaluate PCDD/F emission and reduction of a basic SWI in the absence of air pollution controls (APCs). The results indicated that the stack gas and fly ash presented average PCDD/F levels and emission factors of 3.6 ng international toxic equivalent (I-TEQ)/Nm³ and 189.31µg I-TEQ/t and 6.89 ng I-TEQ/g and 137.85µg I-TEQ/t, respectively, much higher than those from large municipal solid waste incinerators (MSWI). PCDD/Fs congener fingerprints indicated that de novo synthesis played a dominant role in the low-temperature post-combustion zone and increased the presence of high-chlorine substituted congeners. On the basis of the emission factor 327.24 µg I-TEQ/t-waste, approximately 3000 g I-TEQ dioxins might be generated in total through basic SWIs and open burning. After refitting an SWI by adding activated carbon injection with a bag filter (ACI+BG), the PCDD/F emissions decreased to mean values of 0.042 ng I-TEQ/Nm³, far below the standard of 0.1 ng I-TEQ/Nm³, and the removal efficiency reached 99.13% in terms of I-TEQ. Therefore, it is entirely feasible to considerably reduce PCDD/F emissions by refitting basic SWI, which is positive for the future development of rural solid waste (RSW (RSW) disposal by SWI.

Emission characteristics for co-combustion of leather wastes, sewage sludge, and coal in a laboratory-scale entrained flow tube furnace

Four different mixed fuels consisted of leather waste, coal, and sewage sludge were combusted in a lab-scale entrained flow fluidized bed furnace. The influence of blending ratio on emission characteristics of SO₂, NO_x, HCl, particulate matter (PM), heavy metals, and polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) was studied. Results showed that the mixing of coal with sewage sludge had a complex effect on the emission characteristics. On the one hand, with more sewage sludge blending in the mixed fuel, the acid gas pollutant (SO₂, NO_x) decreased a lot, and the recovery of volatile heavy metals (Cd, Pb) increased at the same time. Furthermore, the leaching toxicity of Cr in the fly ash and bottom ash went down below the national standard with the adding of sewage sludge. On the other hand, the mixing of sewage sludge which consisted of more ash content resulted in the increase of the PM emission. Moreover, the high content of Cu and chlorine in the sewage sludge can promote the formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) when the fuel 3 and 4 were combusted. Most importantly, the concentration of toxic PCDD/Fs in the flue gas produced from fuel 3 and fuel 4 was successfully controlled down below 0.20 ng I-TEQ/Nm³ by the active carbon.

Characterizations and mechanisms for synthesis of chitosan-coated Na-X zeolite from fly ash and As(V) adsorption study

Solid waste fly ash with low aluminum of Yunnan Province in China was used as pristine material to prepared chitosan-coated Na-X zeolite, and the obtained composite material was employed as As(V) adsorbent. Then, the prepared materials were characterized by XRD, FT-IR, and XPS. And the results suggested that the low aluminum fly ash was successfully convert into Na-X zeolite, and the mineralization between Si-OH of the obtained Na-X zeolite and C-OH of chitosan was the dominated mechanism for coated chitosan over the surface of Na-X zeolite. From the batch experiments of As(V) removal, it has been found that the coated chitosan could significantly improve As(V) performance of Na-X zeolite. The optimal working pH for removal As(V) by chitosan-coated Na-X zeolite was attained at pH 2.1 ± 0.1, and the maximum adsorption capacity was 63.23 mg/g. And the adsorption data at different interval time was excellent fitted by pseudo-second-order kinetic model. From the analyze of XPS, the results suggested that As(V) uptake over adsorbent by the bond of As-N and As-O and the surface hydroxyl group of Al-OH and -NH₂ were involved in uptake As(V) from acid wastewater.

Effects of typical modified passivators on speciation of heavy metals in protein extracted from sewage sludge

The sewage sludge contains abundant organic substances as well as a complex variety of inorganic substances (such as heavy metals). The extraction of protein from sludge is a new treatment approach to promote the utilization of sludge as a resource. However, heavy metals in sludge are extracted together with organic matter during the extraction process. In this study, the amounts of protein and heavy metal in the supernatant extracted from sewage sludge were investigated, and the effects of different passivator (modified fly ash and modified sepiolite) on the speciation of different heavy metals in the sludge were examined. Both materials reduced the contents of protein and heavy metal in the supernatant. When the dosage of sepiolite was 0.10 g/g total suspended solids of sludge, the content of heavy metals was the lowest and the protein content had little change. It can be deduced by analysis of specific area that sepiolite can complex with heavy metal ions and the fly ash adsorb the metals by physical adsorption. The modified sepiolite can be seen as an ideal passivator due to higher protein content and less heavy metals in the supernatant, as well as more stable heavy metals in the sediments.

Synthesis of fly ash based zeolite-reduced graphene oxide composite and its evaluation as an adsorbent for arsenic removal

A zeolite-reduced graphene oxide (ZrGO) based composite was synthesized to remove arsenic from water. To make a low-cost adsorbent, zeolite was synthesized using an inexpensive waste material; fly ash, which was further used to produce the ZrGO composite. Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), and Raman spectra were used to characterize the morphology and surface composition of the synthesized materials. Synthesized materials: zeolite, rGO and ZrGO were evaluated as an adsorbent to remove arsenic from water. The results indicated that all three were able to adsorb arsenic from water but the removal efficiency of ZrGO was the best as it was able to bring down the arsenic concentration within the WHO permissible limits. The maximum adsorption capacity for 100 μg/L of initial arsenic concentration was found to be 49.23 μg/g. Results indicate that pseudo second order kinetics describes the arsenic adsorption on ZrGO. Adsorption isotherm study for ZrGO shows best fit for Redlich-Peterson model of adsorption.

Migration characteristics and toxicity evaluation of heavy metals during the preparation of lightweight aggregate from sewage sludge

A lightweight aggregate (LWA) was manufactured from municipal sewage sludge, gangue, and coal ash. The product performance and environmental safety of the sintered material were evaluated by changing the sludge blending ratio and sintering temperature. The distribution and migratory transformation characteristics of heavy metals in LWA were examined by BCR sequential extraction in combination with inductively coupled plasma optical emission spectrometry (ICP-OES). The environmental safety performance of LWA was comprehensively evaluated by the OPTI index for the first time. The leaching concentration of the heavy metals Pb, Ni, Cu, and Zn in raw materials without sintering reached 1.17, 1.6, 7.84, and 7.56 mg/L, respectively, far exceeding the regulatory threshold value. At 1250 °C, sintering with 60% sludge content resulted in Cu and Zn leaching concentrations of only 0.41 mg/L and 0.37 mg/L, respectively. Furthermore, a big portion of heavy metals were in the residual fraction of sintered LWA. The proportion of comprehensive pollutant toxicity index is only 199.17. Additionally, the mechanical properties of sintered LWA exceed the standards stipulated in the GB/T1743.1-2010 standard. Using sewage sludge to manufacture lightweight aggregate is not only environmentally safe but also produces LWA with good engineering characteristics.

Research on the electrostatic characteristic of coal-fired fly ash

China is the biggest consumer of coal. Every year, half or more of China's coal is used in the power industry. Most thermal power plants in China use electrostatic precipitators to treat solid particulate matter in flue gas. The efficiency of the electrostatic precipitator in removing dust has a considerable influence on atmospheric pollutants. However, the most important factor affecting the efficiency of the electrostatic precipitator in removing dust is the dielectric properties of the fly ash. Through the study of volt-ampere characteristics, electrical and capacitive properties, and electrical breakdown characteristics of coal-fired fly ash, it is found that the V-I characteristics of ash samples in thermal power plant do not strictly follow the formula ([Formula: see text]), the type of coal, and the chemistry of coal. Ingredients are related; disparate types of ash samples have disparate capacitances, and the measurement of capacitive contrast resistance has a certain influence.

Effects of the co-disposal of lignite fly ash and coal mine waste rocks on AMD and leachate quality

Lignite fly ash (FA) and waste rocks (WRs) were mixed in three different ratios (1:1, 1:3 and 1:5) and studied to compare the effects of adding FA on acid mine drainage generation from coal mining WRs, leachability of elements and the potential occurrence of the secondary minerals. FA mixed with WRs showed significant differences in pH levels compared to previous research. The 1:1 mixture performed best of all the three mixtures in terms of pH and leachability of elements, mainly due to the higher proportion of FA in the mixture. The pH in the 1:1 mixtures varied between 3.3 and 5.1 compared to other mixtures (2.3-3.5). Iron and SO₄^2- leached considerably less from the 1:1 mixture compared to the others, indicating that the oxidation of sulphides was weaker in this mixture. Aluminium leached to a high degree from all mixtures, with concentrations varying from mg L^-1 to g L^-1. The reason behind this increase is probably the addition of FA which, due to acidic conditions and the composition of the FA, increases the availability of Al. For the same reason, high concentrations of Mn and Zn were also measured. Geochemical modelling indicates that the 1:1 mixture performs better in terms of precipitation of Al³⁺ minerals, whereas Fe³⁺ minerals precipitated more in mixtures containing less FA. These results suggest that, with time, the pores could possibly be filled with these secondary minerals and sulphate salts (followed by a decrease in sulphide oxidation), improving the pore water pH and decreasing the leachability of elements. Since grain size plays a crucial role in the reactivity of sulphides, there is a risk that the results from the leaching tests may have been influenced by crushing and milling of the WR samples.

Synthesis of coal fly ash zeolite for the catalytic wet peroxide oxidation of Orange II

Fly ash, a coal combustion residue produced by Termotasajero in Colombia, has been hydrothermally treated after an alkaline fusion to produce zeolite without addition of silicon or aluminum. The starting material was thoroughly mixed with NaOH, in a 1:1.2 mass ratio, to obtain a homogeneous mixture that was heated to 100 °C during different times (6, 8, and 10 h) and three zeolite samples were produced. The samples were characterized by XRD, SEM, XRF, Mössbauer spectroscopy, and N₂ physisorption. According to characterization results (high surface area and appropriate morphological properties including crystallinity) and synthesis time, zeolitic catalyst synthesized with 8 h of hydrothermal treatment was selected to perform further analysis. This sample consisted of a mixture of zeolite X and zeolite A of high surface area (301 m² g^-1) and a Fe content of 6% wt/wt. The zeolite was used as a catalyst for the Fenton oxidation of Orange II. Experiments were performed in a laboratory batch reactor at 70 °C and constant pH = 3, using different concentrations of H₂O₂. When the stoichiometric amount of H₂O₂ was used, good mineralization (X_TOC = 45%), complete discoloration, and oxidant consumption were obtained after 240 min of reaction. The sample retained activity after 16 h of usage. The presence of Fe in the reaction media was always detected and a homogeneous Fenton mechanism induced by surface-leached iron is suggested.

Distribution of mercury in the combustion products from coal-fired power plants in Guizhou, southwest China

Method 30B and the Ontario Hydro Method (OHM) were used to sample the mercury in the flue gas discharged from the seven power plants in Guizhou Province, southwest China. In order to investigate the mercury migration and transformation during coal combustion and pollution control process, the contents of mercury in coal samples, bottom ash, fly ash, and gypsum were measured. The mercury in the flue gas released into the atmosphere mainly existed in the form of Hg°. The precipitator shows a superior ability to remove Hg^p (particulate mercury) from flue gas. The removal efficiency of Hg²⁺ by wet flue gas desulfurization (WFGD) was significantly higher than that for the other two forms of mercury. The synergistic removal efficiency of mercury by the air pollution control devices (APCDs) installed in the studied power plants is 66.69-97.56%. The Hg mass balance for the tested seven coal-fired power plants varied from 72.87% to 109.67% during the sampling time. After flue gas flowing through APCDs, most of the mercury in coal was enriched in fly ash and gypsum, with only a small portion released into the atmosphere with the flue gas. The maximum discharge source of Hg for power plants was fly ash and gypsum instead of Hg emitted with flue gas through the chimney into the atmosphere. With the continuous upgrading of APCDs, more and more mercury will be enriched in fly ash and gypsum. Extra attention should be paid to the re-release of mercury from the reutilization of by-products from APCDs. Implications: Method 30B and the Ontario Hydro Method (OHM) were used to test the mercury concentration in the flue gas discharged from seven power plants in Guizhou Province, China. The concentrations of mercury in coal samples, bottom ash, fly ash, and gypsum were also measured. By comparison of the mercury content of different products, we found that the maximum discharge source of Hg for power plants was fly ash and gypsum, instead of Hg emitted with flue gas through the chimney into the atmosphere. With the continuous upgrading of APCDs, more and more mercury will be enriched in fly ash and gypsum. Extra attention should be paid to the re-release of mercury from the reutilization of by-products from APCDs.

Volatilization of toxic elements from coal samples of Thar coal field, after burning at different temperature and their mobility from ash: Risk assessment

In present study, the volatilization of arsenic (As), cadmium (Cd), mercury (Hg) and lead (Pb) by the burning of coal samples in the electric furnace was evaluated. The coal samples were obtained from different blocks of newly developed Thar coal field, Pakistan. The replicate coal samples were heated/burned in an electric furnace at laboratory scale for three temperature intervals (200, 400 and 900 °C). The ash obtained after each temperature intervals were analyzed for selected toxic elements. The resulted data indicated that the total contents of As, Cd, and Pb in coal samples of block X and XI were found in the range of 16.8-18.5, 4.21-4.72, and 14.2-18.8 mg/kg, respectively. Whereas, 67.8-79.7%, 34.3-36.8% and 9.89-10.8% of As, Cd, and Pb, respectively, were volatized out after combustion of coal samples at 900 °C. The total contents of Hg in selected coal samples of block X and XI were observed in the range of 0.985-1.46 and 0.992-1.41 mg/kg, respectively. The contents of Hg volatilized out via burning in a furnace at 200 °C and 400 °C, were observed in the range of 32.0-36.5% and >91.0% of its total contents, respectively, whereas at 900 °C, it was not detected in ash. The partitioning and mobility of studied toxic elements in residue termed as (laboratory made ash) obtained after burning at 900 °C, was carried out by three-step sequential extraction scheme. The resulted data of present study will give a better knowledge about the quality of coal and its burning product, which may help to take measure to reduce the adverse effects on the environment in future.

Emission of Unintentionally Produced Persistent Organic Pollutants from Some Industrial Processes in Northern Vietnam

Concentrations of PCDD/Fs, dioxin-like PCBs (dl-PCBs), PeCB and HCB were determined in flue gas, fly ash and bottom ash samples collected from brick production, steel production, and zinc production plants, an industrial waste incinerator and a medical waste incinerator in northern Vietnam to understand the contamination levels, accumulation patterns and extent of emission. Total TEQs concentrations of PCDD/Fs and dl-PCBs in flue gas and ash samples from these industrial plants ranged from 0.304 to 50.55 pg/Nm³ and 1.43 to 440 pg/g, respectively. PeCB and HCB residues in flue gas samples ranged from 0.839 to 46.59 ng/Nm³ and 1.16 to 60.5 ng/Nm³, respectively. The emission factors of 4.8-740 ngTEQs/tonne for PCDD/Fs and dl-PCBs, 67.12-240.7 µg/ton for PeCB and 11.64-889.3 µg/ton for HCB were obtained in flue gas samples. This is among the first reports on the emission factor of PCDD/Fs, dl-PCBs, PeCB, HCB in brick production, zinc production and waste incineration in Vietnam.

Fly-ash-incorporated electrospun zinc oxide nanofibers: Potential material for environmental remediation

Fly ash (FA), a solid waste generated in thermal power plants, is considered an environmental pollutant. Therefore, measures must be taken to dispose of FA in an environmentally friendly manner. In this paper, an electrospinning technique was employed to incorporate FA particles onto zinc oxide nanofibers (ZnO NFs), and the product (FA/ZnO composite) was used for the removal of methylene blue (MB) from the water. Herein, ZnO NFs may serve as effective semiconductor photocatalysts and provide sufficient surface area for FA, while the FA particles serve as an effective adsorbent. The adsorption capacity and photocatalytic efficiency of the as-synthesized nanocomposite fibers were enhanced compared to those of the pristine ZnO NFs, and this result is attributed to the uniform distribution of FA on the surface of the ZnO NFs. The as-synthesized nanocomposite could have great significance in wastewater treatment.

Processing and uses of fly ash addressing radioactivity (critical review)

Fly ash is the residue of coal combustion collected by electrostatic or cyclone separator. It is one of the largest quantities of waste disposed in the world. Fly ash represents mostly less than 100 micron in size spherical particles with pozzolanic and hydraulic properties depending on its composition. Utilization of fly ash depends on its chemical, mineralogical composition and morphology. Because of coal nature, fly ash represents a significant drawback with presence of radionuclides such as ²²⁶Ra, ²³²Th and ⁴⁰K. The fly ash can be used for various applications. The main amount of the fly ash is used for building materials production as cement additive and concrete production. Therefore, the determination of radiological properties both in the fly ash and final products are important parameters to consider. Radioactive isotopes cause release of alpha, beta, particles gamma rays and radon exhalation. However, fly ash addition doesn't increase the gamma dose substantially. Moreover, radioactive elements are generally immobilized within glass phase and therefore, radon emanation is not high. In this review the latest development of utilization of the various fly ashes with a different level of radioactive elements content for value added application are presented and a possible new direction of applications are discussed.

Adsorption kinetics and modeling of H2S by treated waste oil fly ash

Waste oil fly ash (OFA) collected from disposal of power generation plants was treated by physicochemical activation technique to improve the surface properties of OFA. This synthesized material was further used for potential hydrogen sulfide (H₂S) adsorption from synthetic natural gas. The raw OFA was basically modified with a mixture of acids (20% nitric acid [HNO₃] and 80% phosphoric acid [H₃PO₄]), and it was further treated with 2 M potassium hydroxide (KOH) to enhance the surface affinity as well as surface area of synthesized activated carbon. Correspondingly, it enhanced the adsorption of H₂S. Crystallinity, surface morphology, and pore volume distribution of prepared activated carbon were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) analyses. Fourier transform infrared (FTIR) study was also performed to identify the functional groups during different synthesis stages of modified activated carbon. The Langmuir, Freundlich, Sips, and dual-site Langmuir (DSL) models were used to study the kinetic and breakthrough behavior of H₂S adsorption over alkali-modified activated carbon. Modeling results of isotherms indicated that OFA has dual sites with high and low affinity for H₂S adsorption. The Clark model, Thomas model, and Yoon-Nelson model were used to examine the effects of flow rate and inlet concentration on the adsorption of H₂S. Maximum uptake capacity of 8.5 mg/g was achieved at 100 ppm inlet concentration and flow rate of 0.2 L/min. Implications: Utilization of worthless oil fly ash from power plant is important not only for cleaning the environment but also for solid waste minimization. This research scope is to eradicate one pollutant by using another pollutant (waste ash) as a raw material. Chemical functionalization of synthesized activated carbon from oil fly ash would lead to attachment of functional groups of basic nature to attract the acidic H₂S. Such type of treatment can enhance the uptake capacity of sorbent several times.

Effect of bio-additives on physico-chemical properties of fly ash-ground granulated blast furnace slag based self cured geopolymer mortars

Heat curing catalyzed the geopolymerization reaction in geopolymers whereas the practical challenges in applying heat curing process limited the applications of geopolymers to precast elements alone. Bio-additives inclusion could facilitate ambient temperature curing that lead to environment friendly self cured geopolymers. Natural sugars (molasses/palm jaggery/honey) and terminalia chebula were used as bio-additives in fly ash (FA) and ground granulated blast furnace slag (GGBS) based geopolymer mortars. X-ray diffraction (XRD), Fourier transform infrared spectral (FTIR), thermogravimetric (TG/DTG) and scanning electron microscopic (SEM) studies were used for the characterization for all geopolymer mortar samples. Physico-chemical test results of bio-additives added FA-GGBS based self cured geopolymer mortar samples showed XRD peaks at 2θ = 20°, 26°, 39°, 48°, 66° and 75°, FTIR bands at 3430, 2922, 1390, 1270, 1120 and 881cm^-1, DTG peaks at 120 °C, 126 °C and 134 °C that led to the conclusion that development of differences in geopolymer reaction products, intense structural reorganization leading to stable geopolymer matrix and more ordered geopolymer gels. Further SEM observations revealed compact and dense microstructure development in bio-additives added FA-GGBS based self cured geopolymer mortar samples.

Chemical and mineral composition of fly ashes from home furnaces, and health and environmental risk related to their presence in the environment

The study presents the results of an analysis of fly ashes produced from burning of solid fuels mixed with municipal waste and assesses the environmental and health risk associated with infiltration of the selected metals to the environment. The phase composition suggests that the material is extremely mixed and diverse. Low-temperature components were mixed with substances formed in high temperatures. The variable composition of waste from different home furnaces with high content of the amorphous phase (which dissolves in water more easily than its crystalline equivalents) may be harmful to the environment and for the people. The dominant elements were silicates and aluminosilicates, such as: quartz, feldspar and plagioclase (albite). Clay minerals (kaolinite and mullite), carbonates (calcite), oxides/oxidehydroxides of iron and sulfate minerals (gypsum and anhydrite) were also abundant. The particles' major constituents were Si, Al, Ca and Fe oxides (85.5%), while S, Mg, Na, K and Ti oxides accounted for 12.6% of the total content. The risk assessment code suggested: Low Risk for As, Co, Cr, Ni, Medium Risk for Cd, Cu and Pb, and High Risk for Zn. Hazard Index (HI) calculated for non-cancerogenic substances for children was 2.35E+00. The total Risk index for children was 4.88E-05. As for adults, HI was 2.42E-01 for women, and 2.89E-01 for men, while the Risk index value was 6.85E-05 for women, and 8.48E-05 for men. The value HI > 1 points to the risk of adverse health impact on children exposed to fly ashes.

Quantitative cesium speciation and leaching properties in alkali-activated municipal solid waste incineration fly ash and pyrophyllite-based systems

There is an urgent need to dispose safely of the municipal solid waste incineration fly ash (MSWIFA) contaminated by cesium (Cs) released from the Fukushima Daiichi nuclear power plant accidents. This study investigated the relationship between quantitative Cs speciation and Cs immobilization in composite product from alkali-activated pyrophyllite and MSWIFA, with added stable cesium chloride and cesium carbonate. Cesium speciation was determined using X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS), while Cs immobilization was evaluated by a leaching method. Twenty-four composite products, with different Cs leaching properties, were prepared under various conditions. The XRD analysis of six composite products with different Cs leaching properties qualitatively identified a crystalline pollucite in the two products with the least Cs leaching. The quantitative speciation determined using XAFS revealed a strong negative correlation between the pollucite content in the 24 products and their Cs leaching ratio. Cesium species with a CsO bond were found in all products. These were at least two Cs species with different leaching properties. These results indicate that the change from a Cs species to pollucite in the product increased the Cs immobilization level in the system.

Uranium speciation in coal bottom ash investigated via X-ray absorption fine structure and X-ray photoelectron spectra

Similar to chromium contamination, the environmental contamination caused by uranium in radioactive coal bottom ash (CBA) is primarily dependent on the chemical speciation of uranium. However, the relationship between uranium speciation and environmental contamination has not been adequately studied. To determine the relationship between uranium speciation and environmental contamination, X-ray absorption fine structure (XAFS) and X-ray photoelectron spectra (XPS) analyses were performed to determine the uranium speciation in CBA exposed to different chemical environments and simulated natural environments. The leachability of the different forms of uranium in the CBA was studied via a simulated acid rain leaching experiment, and the results showed that 57.0% of the total uranium was leached out as U(VI). The results of a linear combination fit (LCF) of the X-ray absorption near edge structure (XANES) spectrum revealed that in the raw CBA, the uranium mainly occurred as U₃O₈ (71.8%). However, in the iron-rich particles, the uranium mainly occurred as UO₂ (91.9%) after magnetic separation. Magnetite is a ubiquitous ferrous-bearing oxide, and it was effective for the sorption of U(IV). The result of FeSO₄ leaching experiment indicated that 96.57% of total uranium was reduced from U(VI) to U(IV) when infiltrated with the FeSO₄ solution for 6months. This result clearly demonstrated the changes in chemical valence of uranium in the coal ash and provided a conceptual principle for preventing uranium migration from ash to the surrounding soil and plants.

Influence of coal ash on potassium retention and ash melting characteristics during gasification of corn stalk coke

This study experimentally investigated the potassium fixation ability and ash fusion characteristics during the gasification of corn stalk coke blended with coal ash in CO₂ atmosphere. The ash samples were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). FactSage was also employed to calculate the components and quantities of solid and liquid phase. The results show that the ratio of potassium retained in the biomass ash decreases with the increase in the gasification temperature. In addition, the retention ratio increases with more coal ash added. The improvement of potassium retention ratio is due to the reaction of potassium with alumina/silica to form the solid/slag phase of the potassium aluminosilicates. On the other hand, the ash fusion temperatures of the blended ash are increased by adding the coal ash, compared with the biomass ash. The study confirms that coal ash is a potential additive for not only fixing potassium, but also increasing the ash fusion temperatures of easy-to-slagging biomass.

Accelerated carbonation of wood combustion ash for CO2 removal from gaseous streams and storage in solid form

In this work, ash generated by the combustion of wood in a central heating plant was used to remove and permanently store by accelerated carbonation CO₂ contained in a gas mixture simulating biogas. The process was studied as an alternative treatment to the ones currently available on the market for biogas upgrading. The process was investigated at laboratory scale by setting up a facility for directly contacting the wood ash and the synthetic biogas in a fixed bed reactor. The process was able to completely remove CO₂ during its initial phase. After about 30 h, CO₂ started to appear again in the outlet stream and its concentration rapidly increased. The specific CO₂ uptake achieved in solid carbonate form was of about 200 g/kg of dry wood ash. This value is an order of magnitude higher than the ones found for waste incineration bottom ash carrying out similar experiments. The difference was ascribed to the physicochemical properties of the ash, characterized by a fine particle size (d₅₀ < 0.2 mm) and high content of reactive phases with CO₂ (e.g., Ca hydroxides). The leaching behavior of the wood ash was examined before and after the accelerated carbonation process showing that the release of several elements was lower after the treatment; Ba leaching in particular decreased by over two orders of magnitude. However, the release of the critical elements for the management of this type of residues (especially Cr and sulfates) appeared not to be significantly affected, while V leaching increased.

Cr(VI) removal using different reducing agents combined with fly ash leachate: A comparative study of their efficiency and potential mechanisms

Remediation of high concentrations of Cr(VI) in wastewater involves its chemical reduction to Cr(III), a product with low toxicity that can be easily removed. To date, NaBH₄ has rarely been used to reduce Cr(VI). This article reports a comparative study of Cr(VI) removal by NaBH₄ and five sulfur-based reducing agents (FeSO₄, Na₂S₂O₅, NaHSO₃, Na₂S₂O₃, and Na₂SO₃). The potential mechanisms of Cr(VI) removal by these six reducing agents with and without fly ash leachate (FAL) are also discussed. The results revealed that the reduction and subsequent removal of Cr(VI) are influenced by the hydrolysis and ionization of the reducing agents in solution. Thus, the reduction reaction was significantly enhanced when Na₂S₂O₅ and NaHSO₃ were added in excess of 600 mg L^-1. Combined with FAL, smaller amounts of NaBH₄ were required to reduce Cr(VI) to Cr(III) at pH 3.0 compared to those with the other reducing agents. NaBH₄ combined with FAL at a dose of 100 mg L^-1 afforded a total Cr (Cr_T) removal of 96.32% within 20 min, a value much higher than that obtained with the other reducing agents. The catalytic mechanism of NaBH₄ for such a FAL-catalyzed Cr(VI) reduction system is similar to that of acid catalysis via the hydrolysis of the Fe(III) and Al(III) species in FAL. Improvement of the Cr_T removal was also observed via Cr(VI) entrapment in the structure of Fe(III) and Al(III) metal hydroxides. These results indicate that relatively low loadings of NaBH₄ combined with FAL show great promise for Cr(VI) pollution remediation.

Characteristics of the cement-solidified municipal solid waste incineration fly ash

Cement solidification is an important pre-treatment technology for municipal solid waste incineration (MSWI) fly ash into landfill. The physicochemical properties and leaching characteristics are the foundation for assessing the long-term stability of the fly ash solidified with benchmark cement in landfills. The leaching performances of bulk components (Na, K, Ca, Cl, CO₃^2-, and SO₄^2-) and heavy metals (Cu, Zn, Cr, Pb, and Zn) were analyzed based on the percolation column test and pH dependent test respectively. The research showed that in the cement-solidified fly ash, Na and K were mainly in the form of soluble chloride salts and would be washed out severely at the initial leaching stage due to the weak fixation effect of cement. Moreover, a considerable amount of Ca was washed out simultaneously with Na and K, causing a temporary increase in pH value, and then Ca leaching was controlled by the solubility of minerals, mainly calcium carbonate, ettringite formed with CO₃^2- and SO₄^2-. Cement solidification reduced the cumulative release of mobile Cu, Zn, Cr, Pb, and Cd contained in MSWI fly ash. In the cement-solidified fly ash, the leaching of Cu and Zn was controlled by mineral solubility under alkaline conditions, Cr was dependent on the redox conditions, and Pb was related to the complex structures formed with Si-O bonds of silicates. A further research on the long-term stability of the cement-solidified fly ash in landfills was needed.

Two-step modification towards enhancing the adsorption capacity of fly ash for both inorganic Cu(II) and organic methylene blue from aqueous solution

A new adsorption material from fly ash (FA) was prepared by a two-step surface modification process, which showed higher ability for the removal of both inorganic and organic cationic pollutants from aqueous solution, i.e., Cu²⁺ and methylene blue (MB). Firstly, FA was modified by hydrothermal method in alkaline solution at 80 °C (FA80) to have a larger BET surface area. Afterwards, FA80 was further modified by sodium dodecyl benzene sulfonate (SDBS), of which a layer of anionic functional groups were grafted on the surface. The adsorption performance of SDBS@FA80 for removal of Cu²⁺ and MB were detailedly investigated. The results showed that SDBS@FA80 presented the optimal adsorption capacity at pH 7.0. Additionally, the maximum adsorption capacities of SDBS@FA80 for the removal Cu²⁺ and MB were up to 227.3 and 50.76 mg g^-1 at 70 °C, respectively, as well as being about three times higher than that of FA. When the initial concentrations of Cu²⁺ and MB were lower than those of 20 and 10 ppm, their removal efficiencies were as high as 99.75 and 96.4%, respectively. The pseudo-second-order model was well applied to describe the adsorption kinetics, indicating that chemisorption was taking place. Furthermore, a plausible mechanism is proposed by XPS studies, where the high adsorption capacity is mainly contributed to the electrostatic attraction and π-π stacking interaction between the cationic Cu²⁺/MB and anionic functional groups of SDBS. Due to the low-cost and high adsorption capacity, SDBS@FA80 is regarded as a promising adsorbent for the removal of cationic pollutants.

Ag-TON nanospheres coupled with fly ash cenospheres for wastewater treatment under visible light irradiation

Using tetra-n-butyl titanate as raw material and fly ash cenospheres (FAC) as carrier, the photocatalysts of Ag-TON/FAC were successfully prepared by solvothermal and in-situ hydrolysis method. These visible light photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), fluorescence spectroscopy (FL) and UV-vis diffuse reflectance spectra (DRS). In this study, methyl orange and ciprofloxacin were used as wastewater degradation targets to investigate the effect of the amount of titanium dioxide and the amount of Ag doping on the activity of photocatalysts. On the basis of this, the optimal ratio of TiO₂ to FAC was 2:1 and the optimum doping ratio of Ag was determined to be 15 wt.%. The composite photocatalysts dispersed uniformly and were easy to recycle and reuse, which were benefits in fully utilizing the solar energy. The degradation efficiency remained at more than 60% after being renewed five times for MO and ciprofloxacin. The photocatalysts of Ag-TON/FAC can reduce the environmental burden caused by FAC also.

Immobilization and characterization of Fe(0) catalyst on NaOH-treated coal fly ash for catalytic reduction of p-nitrophenol

In this study, coal fly ash (CFA), i.e., an industrial waste product created in large quantities by thermoelectric power plants, was treated with sodium hydroxide to afford a novel Fe (0) catalyst supported on alkaline-treated CFA. The NaOH-treated CFA (NCFA) exhibited a morphological change from slick spheres to pointed, leaf-like spheres, which was accompanied by a noticeable increase in specific surface area from 1.2 to 7.5 m²/g. Sequential addition of an Fe(III) precursor and NaBH₄ solution to a suspension of NCFA resulted in the formation of Fe (0) particles on the surface of NCFA (Fe/NCFA). The catalytic activity of Fe/NCFA toward the reduction of p-nitrophenol (p-NP) was examined; among the Fe/NCFAs synthesized from different NCFAs (1, 3, and 7 M NaOH), the Fe/3 M NCFA sample displayed the highest activity owing to the highest Fe content on its surface, without leaching any toxic heavy metals. In addition, the effects of NaBH₄ concentration, Fe loading, and catalyst dosage on the catalytic reduction of p-NP by Fe/NCFA were comprehensively investigated. Finally, the recyclability and stability of Fe/NCFA were examined, demonstrating the complete reduction of p-NP over four continuous recycling cycles. The present results demonstrate the marked potential of CFA as a component in reactive catalysts for the removal of environmental pollutants from wastewater.

Fly ash toxicity, emerging issues and possible implications for its exploitation in agriculture; Indian scenario: A review

Fly ash is considered as an environmental hazard worldwide, since it generally contain organic pollutants, probable toxic metals like Se, As, B, V, Al, Pb, Hg, Cr and radionuclide's Uranium, Thorium. Although fly ash contains toxic substances, it also contains most of the oxides and trace elements. Presence of oxides contributes to its alkaline pH while trace elements provides nutrients for plant growth hence, it is suggested that it can be used in low concentration in agriculture sector as well as a soil conditioner as fly ash improves the physico-chemical and biological properties of contaminated soils. This article presents a review on causes of fly ash toxicities due to organic pollutants, heavy metals, radioactive elements and environmental issues related to its utilization and possibilities of fly ash exploitation in agriculture sector such as phytoremediation, bioremediation, reclamation of wasteland and forestry.

The effect of fly ash on sunflower growth and human health

One of the challenges brought by the circular economy requires a reconsideration of waste, which may under certain circumstances turn into genuine resources. By extension, soil pollution with heavy metal is a major concern since it directly affects the health of the population. The goal of the present research work is to analyze the impact of the use of waste from other technological processes in agriculture: fly ash (resulting ash from thermal power plants), zeolite bush (resulting from the processing of rock from zeolite quarries), and manure (garbage from zoo technical farms). In this respect, complex treatments based on inorganic substances (fly ash and volcanic indigenous tuff with 70% clinoptilolite) were applied to less-favored agricultural soils in the absence and in the presence of an organic fertilizer (manure), respectively. After cultivating sunflower (Helianthus annuus L.), a semi-early hybrid grown in the type of soil on which fly ash has been applied, there have been obtained seed crops 15.8% higher than the seed crops grown in the soil on which no fertilizer has been applied. The results obtained when combining fly ash and manure tend to amount to those obtained when combining manure with indigenous volcanic tuff with 70% clinoptilolite. The quality of the seed crops, obtained in the case of the three types of soil on which amendments were added in the absence/presence of the fertilizer, corresponds to the requirements of the national rules and allows their food processing.

Acidic processing of fly ash: chemical characterization, morphology, and immersion freezing

Fly ash can undergo aging in the atmosphere through interactions with sulfuric acid and water. These reactions could result in chemical and physical changes that could affect the cloud condensation or ice nucleation activity of fly ash particles. To explore this process, different water and acid treated fly ash types were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM), electron dispersive spectroscopy (EDS), selected area diffraction (SAED), and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Then, their immersion freezing activity was assessed. With water and acid treatment, a wide variety of metals were leached, depending on the starting composition of the fly ash. Acid treatment resulted in the formation of gypsum, Ca(SO4)·2H2O, for fly ash containing Ca as well as morphological changes. The immersion freezing activity was also assessed for each fly ash system to compare the effects of water and acid processing. Our results support the assertion that fly ash can serve as a cloud condensation or ice nucleus to affect climate.

The use of zeolite-based additives for immobilising iron during pressure filtration of coal refuse slurry

Pressure filtration of coal refuse slurry has the potential to provide a concentrated solids stream that can be stacked, thereby offering multiple environmental benefits. However, potential leachates from the solids stream can impact the environment. In that context, this study performed preliminary investigations of the application of zeolite-based additives to adsorb metals leaching from coal refuse slurry at low pH. Additives were added to the coal refuse slurry, which was filtered using bench- and lab-scale pressure filtration units. Results indicated that the overall filtrate flux and cake moisture characteristics were not significantly affected by the addition of additives up to 20% (by weight of solids). It was shown that adsorption as high as 80% was achieved by using the additives to capture iron. It was concluded that the finer additive with less silicon content was more effective in capturing iron. The results showed that the thickener feed stream leached out less iron than the thickener underflow stream. The adsorption process was not significantly affected by slight variations in initial iron concentration in the solution. The use of lower pH water on the filter cakes treated with the additive showed minimal release of iron and manganese into the aqueous phase.

Rapid ultrasound assisted hydrothermal synthesis of highly pure nanozeolite X from fly ash for efficient treatment of industrial effluent

Nanoporous green materials like nanozeolite has been in focus for their superb efficiency to treat industrial wastewater. The study reports ultrasound assisted hydrothermal method as a very fast method to convert industrial fly ash from different sources of eastern India to nanozeolite X for efficient removal of toxic dyes and metals from industrial effluent. The pure nanosized zeolite X was synthesized rapidly at 20 min of ultrasound treatment after alkali fusion. The efficiency of nanozeolite X was determined in terms of the adsorption capacity towards various divalent ions such as Zn²⁺, Cu²⁺, Cd²⁺, Pb²⁺, Ni²⁺, Ca²⁺, and Mg²⁺ as well as various dyes such as methylene blue, crystal violet, indigo carmine, and Congo red. In comparison to commercially available microporous Zeolite X (a maximum of 120.43 mg g^-1 for Pb²⁺ and 134.62 mg g^-1 by for methylene blue), all synthesized nanozeolite X shows high adsorption capacity for metals (a maximum of 196.24 mg g^-1 for Pb²⁺) as well as dyes (193.45 mg g^-1 for methylene blue). Highly pure nanozeolite X has shown immense potential for treatment of real time industrial wastewater.

Adsorptive treatment of coking wastewater using raw coal fly ash: Adsorption kinetic, thermodynamics and regeneration by Fenton process

Raw coal fly ash (RCFA) was used directly as adsorbent to treat coking wastewater. The results show that the RCFA can introduce COD (14, 4, and 11 mg L^-1 at pH = 1.0, 7.0, and 14.0, respectively) into water due to the dissolution of reductive components from RCFA, this can be avoided by washing (6 times) using distilled water. The concentration of leached metal elements in wastewater is lower than the standard in GB18918-2002, China. The adsorption process accords with the pseudo-second order adsorption kinetic model and Langmuir thermodynamics model better than other ones, and it belongs to a physical and exothermic process. More loading of RCFA (10-60 mg L^-1) means less adsorption capacity, and 40 g L^-1 is the optimal value. The variation of regeneration rate of SRCFA for the first time (RR₁) with regeneration time accords with the behavior of hyperbolic function (1RR₁=0.986+53.913t), and the [H₂O₂], [Fe²⁺] and regeneration temperature can affect the RR₁ at the manner of exponential function ( [Formula: see text] , [Formula: see text] , and RR₁=2.064·e^-251.048T). At the optimal regeneration condition of SRCFA ([H₂O₂] = 5 mM, [Fe²⁺] = 8 mM and temperature = 293 K), the RR₁ can reach 87.1% after 270 min. The stability of RCFA shows two different stages, i.e., within the first 4 regenerations the RR increases from 87.1% to 89.7% and then decreases gradually and always. This variation trend can be confirmed by the results of SEM and BET tests.

Influence of the type of furnace on behavior of radioactive cesium in municipal solid waste thermal treatment

Municipal solid waste (MSW) contaminated by radioactive cesium (r-Cs) has been incinerated since the Fukushima Daiichi Nuclear Power Plant accident. Eight thermal treatment plants with four different types of furnaces were comprehensively investigated to provide fundamental data to improve our understanding of the behavior of r-Cs in various types of MSW thermal treatment facilities. R-Cs tended to distribute to the fly ash (FA) more than to the residue from the bottom of the furnace (bottom ash, incombustibles or slag). The r-Cs concentrations in the FA depended on the type of furnace and followed the order; fluidized-bed incinerator < stoker type incinerator < gasification melting furnaces. Shaft-type gasification melting furnace separated r-Cs selectively into FA and simultaneously discharged decontaminated slag. The leaching rate of r-Cs from FA was high, 30-100%, and independent of the type of furnace, whereas r-Cs in the residue from the bottom of the furnace scarcely dissolved in water. Heat recovery ash e.g. gas cooler ash was characterized by intermediate r-Cs concentrations and leachabilities compared with bottom residue and FA in stoker type and fluidized-bed incinerator. In the case of shaft-type gasification melting furnace, however, heat recovery ash showed similar property to FA due to a cyclone followed by heat recovery process. We evaluated whether baghouses (air- pollution control equipment) successfully removed r-Cs from flue gas. In all cases, r-Cs in flue gas was below the limit of detection after baghouse. We concluded that different types of furnaces affected r-Cs distributions, but flue gases from baghouse systems of all types of furnaces were safe.

Effects of coal blending in electrostatic precipitation efficiency-Inner Mongolia, China

Based on this study, the Al₂O₃ content of Jungar coal ash is over 45%, and the resistivity of high-Al₂O₃ ash in Jungar reaches up to 10¹² Ω​·cm. These results seriously influenced the electric characteristics of fly ash, and the collection efficiency of electrostatic precipitators (ESPs) evidently decreased. To facilitate the effective collection of fine particle in the flue gas generated before and after coal blending via ESP, the fly ash obtained from a power plant electrostatic precipitation was analyzed in terms of resistivity, size distribution, and cohesive force through a portable dust electrical resistivity test instrument, Bahco centrifuge, and a cohesive force test apparatus invented by the researchers. The mixed ratio of else coal is higher than 50%, the resistivity of the fly ash in the flue gas was lowered to approximately two orders of magnitude, and the size distribution showed an evident decrease in the PM2.5 and PM10 content in fly ash. In addition, the adhesive force and efficiency increase from 95.9 to 99.5% in the electrostatic precipitation. Therefore, the combustion of blending coal is an effective approach to improve the efficiency of ESP used to collect high-Al₂O₃ fly ash.

Screening of heavy metal containing waste types for use as raw material in Arctic clay-based bricks

In the vulnerable Arctic environment, the impact of especially hazardous wastes can have severe consequences and the reduction and safe handling of these waste types are therefore an important issue. In this study, two groups of heavy metal containing particulate waste materials, municipal solid waste incineration (MSWI) fly and bottom ashes and mine tailings (i.e., residues from the mineral resource industry) from Greenland were screened in order to determine their suitability as secondary resources in clay-based brick production. Small clay discs, containing 20 or 40% of the different particulate waste materials, were fired and material properties and heavy metal leaching tests were conducted before and after firing. Remediation techniques (washing in distilled water and electrodialytical treatment) applied to the fly ash reduced leaching before firing. The mine tailings and bottom ash brick discs obtained satisfactory densities (1669-2007 kg/m³) and open porosities (27.9-39.9%). In contrast, the fly ash brick discs had low densities (1313-1578 kg/m³) and high open porosities (42.1-51. %). However, leaching tests on crushed brick discs revealed that heavy metals generally became more available after firing for all the investigated materials and that further optimisation is therefore necessary prior to incorporation in bricks.