A study on the chemical and mineralogical characterization of MSWI fly ash using a sequential extraction procedure

The presence of heavy metals in municipal solid waste incineration (MSWI) fly ash is of environmental concern due to their leaching potential in landfill environments. Sequential chemical extraction was performed on fly ash samples from a large-scale municipal solid waste incineration plant in East China. The transformation of the mineralogical species of fly ash during the sequential extraction was studied using X-ray fluorescence (XRF) and X-ray powder diffraction (XRD). The leaching behavior of heavy metals such as zinc, lead, cadmium and copper in MSWI fly ash was considered to have a dependency relationship with the components of calcium, such as aphthitalite, calcite, anhydrite and calcium aluminate or calcium aluminosilicate.     

Influence of flue gas SO2 on the toxicity of heavy metals in municipal solid waste incinerator fly ash after accelerated carbonation stabilization

The influence of CO₂ content and SO₂ presence on the leaching toxicity of heavy metals in municipal solid waste incinerator (MSWI) fly ash was studied by examining the carbonation reaction of MSWI fly ash with different combinations of simulated incineration flue gases. Compared with raw ash, the leaching solution pH of carbonated ash decreased by almost 1 unit and the leaching concentrations of heavy metals were generally lower, with that of Pb decreasing from 19.45 mg/L (raw ash) to 4.08 mg/L (1# carbonated ash). The presence of SO₂ in the incineration flue gas increased the leaching concentrations of heavy metals from the fly ash to different extents after the carbonation stabilization reaction. The pH of the leaching solution was the main factor influencing the leaching concentrations of heavy metals. The increase in buffer capacity with the pH of carbonated ash caused an increase in heavy metal stability after the carbonation reaction. Accelerated carbonation stabilization of MSWI fly ash could reduce its long-term leaching concentrations (toxicity) of Cu, Pb, Se, and Zn. The leaching concentrations of heavy metals from carbonated ash also likely had better long-term stability than those from raw ash. The presence of SO₂ in the incineration flue gas increased the proportion of exchangeable state species of heavy metals; slightly increased the long-term leaching toxicity of Cu, Pb, Se, and Zn; and reduced the long-term stability of these metals in the fly ash after the carbonation reaction.     

Hydrothermal synthesis of composites of well-crystallized hydroxyapatite and poly(vinyl alcohol) hydrogel

Composites of hydroxyapatite (HAp) and poly(vinyl alcohol) (PVA) hydrogel were fabricated by the hydrothermal treatment of calcium phosphate powder. Alpha-tricalcium phosphate (α-TCP) or beta-tricalcium phosphate (β-TCP) powder was dispersed in PVA hydrogel and exposed to water vapor at 120 °C, 140 °C or 160 °C for 6 h. Low crystallinity HAp was formed in specimens prepared from α-TCP and PVA hydrogel prior to hydrothermal treatment, which was caused by hydrolysis of α-TCP. This allowed specimen shape to be retained after hydrothermal treatment. β-TCP showed less reactivity in forming HAp in the PVA hydrogel, which led to the formation of large rod-shaped crystals approximately 15 μm in length. Specimens from β-TCP and PVA were too soft to retain their shape after hydrothermal treatment. HAp with controlled morphology was prepared using different types of tricalcium phosphate precursor. The application of α-TCP allowed the in situ fabrication of HAp/PVA composites.     

In‐vitro preparation of nanocrystalline calcium phosphates as bone substitution materials in surgery

A continuous preparation method for nanoscopic calcium phosphate ceramics is presented. The influence of processing parameters (temperature, time) on the properties of calcium phosphates was studied. Crystallinity, structure, and morphology are important for an application as biodegradable implant material in bone contact. The samples were studied in detail using X‐ray powder diffraction, infrared spectroscopy, atomic absorption spectroscopy, photometry, scanning electron microscopy, and transmission electron microscopy. All calcium phosphate precipitates are non‐stoichiometric, calcium‐deficient hydroxyapatites.     

Characteristics of industrial and laboratory meat and bone meal ashes and their potential applications

This paper reports the characterization of four meat and bone meal (MBM) ashes obtained from specific incineration (laboratory) and from co-incineration (industrial process). Three out of the four MBM ashes were mainly composed of calcium phosphates (hydroxyapatite and whitlockite). Their compositions (major and trace) were in the range for natural phosphate rocks. Trace element contents, including heavy metals, were below 0.6% and industrial ashes contained much more heavy metals than laboratory ash. The amounts of leached elements were low, especially for laboratory ash. According to the European classification of waste to be landfilled, the laboratory ash can be classified as an inert waste. Two industrial ashes are mostly inert. Only one ash is highly leachable and needs a stabilization treatment to be classified at least in the category of hazardous waste. It seems, from these results, that possibilities other than landfilling could be considered to give economic value to these ashes.     

The possibility to recycle solid residues of the municipal waste incineration into a ceramic tile body

The feasibility of utilising solid residues of the municipal wastes incineration such as grate (bottom) ash and electrofilter and sleeve filter fly ash, in combination with a porcelain stoneware body, in the preparation of tiles has been investigated. While the chemical, mineralogical, thermal and rheological characterisation of the waste raw materials carries out some problems arising by using fly ash, these seem to be overcome with the bottom ash. The introduction of up to 20 wt% of this powder into the ceramic body does not substantially change the mineralogical and thermal behaviour of the product.     

SEM/EDS and XRD characterization of raw and washed MSWI fly ash sintered at different temperatures

The disposal of fly ash generated during municipal solid waste incineration (MSWI) may pose a significant risk to the environment due to the possible leaching of hazardous pollutants, such as toxic metals. Sintering technology attracted more attention than the vitrification process because of its low energy needed. Generally, a preliminary washing treatment of raw fly ash with water was necessary for this sintering technology. This study investigated the composition and morphology of raw fly ash (RFA) and washed fly ash (WFA) at different sintering temperatures, and examined the newly formed minerals during sintering. Toxicity characteristic leaching procedure (TCLP) tests were carried out to investigate the effect of the washing treatment and sintering process on the leaching performance of heavy metals in fly ash. Results showed that, with an increase of sintering temperature more complex aluminosilicates were formed; the incorporation of Mg, Fe and Pb into the aluminosilicates occurred during the sintering process at higher temperatures (800 and 900 degrees C). The washing treatment reduced the leachable concentration of Cd, Pb and Ni, but increased that of Cr. A CaCrO(4) compound was considered as a potential soluble species.     

Melting characteristics during the vitrification of MSWI fly ash with a pilot-scale diesel oil furnace

Treatment of municipal solid waste incineration (MSWI) fly ash is becoming an important issue in China. A pilot-scale experiment was carried out to treat MSWI fly ash by using a diesel oil furnace (DOF) for more than 6 months. The effects of melting temperature on volume reduction, weight loss, compositional changes, and toxicity of leach water for molten slag have been investigated and reported. Results indicated that the volume reduction fraction of raw fly ash (RFA) and washed-fly ash (WFA) was 75–80% and the weight loss fraction was 23.8–30% at 1260–1350 °C. During the vitrification, CaO, A1₂O₃, and SiO₂ percentages in fly ash increased as the temperature increased, especially for SiO₂, which was caused by both the decomposition of carbonates or sulfates and the volatilization of metal chlorides because the main components in secondary fly ash collected from fabric filter bags were NaCl and KCl. The leaching concentrations of heavy metals in molten slag were lower than the standard values of TCLP. The releasing levels of dioxin and other pollutants (such as SO₂, HCl, CO, NO_x, etc.) in flue gas were all lower than the Chinese standard.     

Preparation and evaluation of osteoinductive porous biphasic calcium phosphate granules obtained from eggshell for bone tissue engineering

The synthetic bone graft substitutes currently used clinically are osteoconductive but not osteoinductive; their low success rate is thus their biggest disadvantage. The use of biomass raw materials for synthesizing calcium phosphate has gradually attracted increased research attention. In this study, hydroxyapatite powder was prepared through liquid-phase precipitation from eggshell, and porous biphasic calcium phosphate granules (EBGs) were then obtained using a pore former and sintering procedure. The EBGs were discovered to have high biocompatibility and no cytotoxicity. The results of animal experiments showed that the area of new bone growth was high, numerous Haversian canals could be observed, and almost all EBGs were surrounded by new bone tissue, which proved that the EBGs had excellent osteoinductivity. By contrast, numerous fat cells were found in the femoral defect area when a commercial bone graft was employed. Various biological inorganic ions (Mg, Sr, Na, and Fe) originally in the eggshell raw materials were incorporated into the EBGs, and the EBGs exhibited excellent osteogenic abilities. The developed approach provides an economical and feasible solution for the treatment of bone defects.     

Removal mechanism of phosphate from aqueous solution by fly ash

This work studied the effectiveness of fly ash in removing phosphate from aqueous solution and its related removal mechanism. The adsorption and precipitation of phosphate by fly ash were investigated separately in order to evaluate their role in the removal of phosphate. Results showed that the removal of phosphate by fly ash was rapid. The removal percentage of phosphate in the first 5min reached 68-96% of the maximum removal of phosphate by fly ash. The removal processes of phosphate by fly ash included a fast and large removal representing precipitation, then a slower and longer removal due to adsorption. The adsorption of phosphate on fly ash could be described well by Freundlich isotherm equation. The pH and Ca2+ concentration of fly ash suspension were decreased with the addition of phosphate, which suggests that calcium phosphate precipitation is a major mechanism of the phosphate removal. Comparison of the relative contribution of the adsorption and precipitation to the total removal of phosphate by fly ash showed that the adsorption accounted for 30-34% of the total removal of phosphate, depending on the content of CaO in fly ash. XRD patterns of the fly ash before and after phosphate adsorption revealed that phosphate salt (CaHPO4 x 2H2O) was formed in the adsorption process. Therefore, the removal of phosphate by fly ash can be attributed to the formation of phosphate precipitation as a brushite and the adsorption on hydroxylated oxides. The results suggested that the use of fly ash could be a promising solution to the removal of phosphate in the wastewater treatment and pollution control.     

Preparation of alinite cement from municipal solid waste incineration fly ash

The feasibility of partially substituting ordinary raw materials with municipal solid waste incineration (MSWI) fly ash in alinite cement production was investigated by X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF) and scanning electron microscopy (SEM). The physical properties and leaching behavior of the produced cement were also evaluated. Experimental results show that good quality clinkers can be obtained by firing the raw mixes, in which the replacement of MSWI fly ash reaches to 30%, at 1200 °C for 2 h. Alinite cements have higher early strengths at all gypsum additions, while the best result having acceptable early and 28-day strengths is obtained at 5% of gypsum addition. Results also show that the leaching toxicity of heavy metals is far lower than that of the regulatory limit at all testing ages. Based on this study, MSWI fly ash is viable as an effective, alternative raw material in alinite cement production.     

Organic carbon leaching behavior from incinerator bottom ash

The understanding of the leaching behavior of organic carbon from incinerator bottom ash is an important aspect for the control of organic carbon emissions from landfills in order to minimize their potential risk to the environment. The leaching behavior of organic carbon from incinerator bottom ash samples, obtained from two different solid waste sources, as well as the effects of calcium (Ca) on organic carbon (DOC) leaching associated with pH were therefore investigated in this paper. The effect of pH on the leaching of DOC and other major elements from bottom ash was ascertained through pH-stat leaching experiments. Column leaching experiments were carried out to evaluate the dependence of the leached amount of DOC on Ca leaching. It was found that the bottom ash produced by incineration of municipal solid waste (MSW) was composed of two DOC fractions: DOC leached independent (fraction I) of Ca leaching, observed at alkaline-neutral pH, and DOC leached dependent (fraction II) on Ca leaching, observed at acid pH. However, the bottom ash produced by incineration of industrial solid waste (ISW) was composed of only DOC fraction I. The addition of calcium oxide during incineration and the differences in the paper to plastic ratio in the physical composition of the solid wastes incinerated would explain the distinct organic carbon leaching behaviors of bottom ash samples.     

Phosphate immobilization from aqueous solution by fly ashes in relation to their composition

Phosphate sorption capacities of 15 Chinese fly ashes were determined and related to their composition. The data of P sorption were best fitted to Langmuir equation, and the calculated sorption maxima of phosphate (Qm) ranged from 5.51 to 42.55 mg/g. The Qm value showed a significantly positive correlation with total Ca content (r=0.9836**) and total Fe content (r=0.8049**), but negative correlation with total Si and total Al content. Correlation coefficients of CaO (r=0.9647**) and CaSO4 (r=0.9399**) were much greater than that of CaCO3 (r=0.6361*). Correlation coefficients of Qm with Fe2O3d and Al2O3d were much higher than those of total Fe and total Al contents, respectively. Fractionation of P sorbed by fly ash revealed that loosely bound P fraction and/or Ca+Mg-P fraction were the dominant form of immobilized phosphate. Ca content was strongly correlated with the Ca+Mg-P fraction instead of Mg content, whereas Fe content was highly correlated with Fe-Al-P fraction compared with Al content. The loosely bound P was correlated well with both Ca and Fe content. The greatest removal of phosphate occurred at alkaline conditions for high calcium fly ash, at neutral pH levels for medium calcium fly ash, while low calcium fly ash immobilized little phosphate at all pH values. This behavior was explained by the reaction of phosphate with Ca and Fe related components. It was concluded that P immobilization by fly ash was governed by Ca ingredient (especially CaO and CaSO4) and Fe ingredient (especially Fe2O3d).     

磷酸钙涂覆炭织物体外细胞毒性的评价

医用植人体的成功与否常常取决于器件植入后细胞与材料表面间的相互作用.采用生物体外测试法考察了声电化学法制备的磷酸钙涂层对炭织物的骨细胞附着、增殖能力的影响.借助MTS检测技术、扫描电子显微镜,选择人类成骨细胞(MG63)作为细胞模型,通过测定细胞与炭织物、磷酸钙涂覆炭织物、以及其各自的提取液作用后的存活能力,研究了细胞/材料的相互作用,并对基底材料的细胞毒性进行了评价.结果表明,炭织物、磷酸钙涂覆炭织物均不具有细胞毒性,且磷酸钙涂层可提高成骨细胞的附着和增殖.SEM图像显示,细胞形貌正常,与对照组相比较生长增殖情况相似.     

On the development of an apatitic calcium phosphate bone cement

Development of an apatitic calcium phosphate bone cement is reported. 100 μ Particles of tetracalcium phosphate (TTCP) and dicalcium phosphate dihydrate (DCPD) were mixed in equimolar ratio to form the cement powder. The wetting medium used was distilled water with Na₂HPO₄ as accelerator to manipulate the setting time. The cement powder, on wetting with the medium, formed a workable putty. The setting times of the putty were measured using a Vicat type apparatus and the compressive strength was determined with a Universal Testing Machine. The nature of the precipitated cement was analyzed through X-ray diffraction (XRD), fourier transform infrared spectrometry (FTIR) and energy dispersive electron microprobe (EDAX). The results showed the phase to be apatitic with a calcium-to-phosphorous ratio close to that of hydroxyapatite. The microstructure analysis using scanning electron microscopy (SEM) showed hydroxyapatite nano-crystallite growth over particulate matrix surface. The structure has an apparent porosity of ∼ 52%. There were no appreciable dimensional or thermal changes during setting. The cement passed the in vitro toxicological screening (cytotoxicity and haemolysis) tests. Optimization of the cement was done by manipulating the accelerator concentration so that the setting time, hardening time and the compressive strength had clinically relevant values.     

Development of a new calcium phosphate powder-binder system for the 3D printing of patient specific implants

A key requirement for three-dimensional printing (3-DP) of medical implants is the availability of printable and biocompatible powder-binder systems. In this study we developed a powder mixture comprising tetracalcium phosphate (TTCP) as reactive component and β-tricalcium phosphate (β-TCP) or calcium sulfate as biodegradable fillers, which can be printed with an aqueous citric acid solution. The potential of this material combination was demonstrated printing various devices with intersecting channels and filigree structures. Two post-processing procedures, a sintering and a polymer infiltration process were established to substantially improve the mechanical properties of the printed devices. Preliminary examinations on relevant application properties including in vitro cytocompatibility testing indicate that the new powder-binder system represents an efficient approach to patient specific ceramic bone substitutes and scaffolds for bone tissue engineering.     

Phosphate-activated high-calcium fly ash acid-base cements

Some fly ashes are used in the concrete industry but some are deemed unsuitable owing to their chemical compositions. This study investigated the use of such a high-calcium fly ash containing large amounts of anhydrite, free lime, and calcite, to produce room-temperature acid-base cements by activation with phosphate sources. Orthophosphoric acid solutions and potassium dihydrogen phosphate were used as activators. Paste microstructures were studied using x-ray diffraction, scanning electron microscopy, isothermal calorimetry, and pH measurements. These findings were related to strength development up to 28 d. Room-temperature cured pastes activated with a 60% H₃PO₄ solution and a solution-to-powder ratio of 1.0 gave the highest 1-d strength of 15 MPa and 28-d strength of 22 MPa. Partial replacement of the ash with glass powder further increased the 28-d strength. Crystalline calcium phosphates, Brushite and Monetite, were among the products of the solution-activated pastes, as well as some amorphous phases. Potassium salt-activated pastes did not contain the calcium phosphate crystals and gave lower strengths. The ultimate pH of well-reacted mixtures were close to neutral.     

磷酸钙骨粘合剂的研究

在柠檬酸中添加壳聚糖配成的固化液与磷酸钙骨水泥(CPC)调和制备的骨修复材料具有类似口香糖的胶状特性, 可应用于碎骨粘结, 称之为磷酸钙骨粘合剂(CPCBA)。本研究考察了柠檬酸的含量对抗压强度、固化时间、水化产物和粘结强度的影响, 同时对该体系进行了初步的体外生物学评价。结果显示, 加入柠檬酸可以缩短固化时间并且时间可以通过柠檬酸的含量进行调控, 同时也改善了抗水性能。壳聚糖可以与骨水泥中的钙离子发生螯合作用, 可以增加界面的粘结强度。小鼠原成骨细胞(MC3T3-E1)在其表面粘附良好, 该体系骨水泥有望取代PMMA成为新的骨粘结剂。     

Utilization of municipal solid waste incineration ash in Portland cement clinker

Municipal solid waste incineration (MSWI) ash is used in part as raw materials for cement clinker production by taking advantage of the high contents of SiO₂, Al₂O₃, and CaO. It is necessary for environmental reasons to establish a material utilization system for the incineration waste ash residue instead of disposing these ashes into landfill. The aim of this paper is to study the feasibility of replacing clinker raw materials by waste ash residue for cement clinker production. MSWI bottom ash and MSWI fly ash are the main types of ashes being evaluated. The ashes were mixed into raw mixture with different portions of ash residue to produce cement clinker in a laboratory furnace at approximately 1400°C. X-ray diffraction and X-ray florescence techniques were used to analyze the phase chemistry and chemical composition of clinkers in order to compare these ash-based clinkers with commercial Portland cement clinker.     

Chemical activation of hybrid binders based on siliceous fly ash and Portland cement

The hydration of Portland cement (PC) blended with a high amount of a siliceous fly ash (70% fly ash, 30% PC) has been examined. The fly ash contributes significantly to the long-term strength development, when compared to a reference sample with quartz powder. However the long setting time and the poor early strength prevent the use of such binders. Therefore the effect of different activators (sodium carbonate, potassium sodium silicate, potassium citrate and sodium oxalate) on the setting, the hydration kinetics and the strength development of the fly ash-PC blend has been investigated.The addition of the activators increases the pH and decreases thus the calcium concentrations in the pore solution, which leads to a faster reaction of alite and thus to early setting and increased early strength. On the long term, the high alkali concentrations lower the compressive strength and lead to a (partial) destabilization of ettringite.Sodium oxalate and potassium sodium silicate accelerate both the setting of the fly ash-PC blend and increase the early compressive strength. Furthermore, they show better compressive strengths at later ages compared to the other activators. Based on these findings, they can be considered as the most suitable accelerators among the investigated activators.