Distribution And Environmental Impact Of Metal-containing Nanoparticles In Coal Fly Ashes From Coal-fired Power Plants

Nanoparticles（NPs）are ultrafine particles with an aerodynamic diameter in one dimension that is less than 100 nm and have been ubiquitous in earth system circles.They exhibited unique biochemical activities and nano-toxicities due to their surface effect,nano-sized effect,macroscopic-quantum tunnelling effect,high specific surface areas and complex chemical structures.The increasing anthropogenic NPs are continuously produced and released into the natural environment because of the widely human high-intensity activities.These NPs may be combined with other pollutants,posing potential ecological and environmental risks.NPs are important components of PM_2.5 and are the most numerous component of it.Coal combustion is an important source of atmospheric nanoparticles.At present,the identification,quantitative analysis and environmental risks of NPs are the keys and difficulties of scholars in the fields of environmental science and geochemistry.However,the various particulate emission control devices（PECDs）installed in all coal-fired power plants（CFPPs）have not yet considered NPs.There are many unknowns about the coal-sourced NPs and there is a lack of quantitative analysis of NPs in coal fly ashes（CFAs）.In addition,due to the difference in coal sources,coal ranks,combustion conditions and PECDs,NPs exhibit complex characteristics.Therefore,the distribution characteristics of NPs emitted from coal-burning activities and their environmental impact need to be studied in depth.This study takes typical CFPPs as the research object,using electron microscope technology to identify the dominant NPs in CFAs.Then based on the single-particle（SP）ICP-MS,quantitative analysis of coal-sourced NPs was carried out for the first time;and the differences of NPs with the different coal sources,coal ranks,combustion conditions and PECDs were explored.The NP emissions in the coal combustion by-products were estimated.Furthermore,the effect of NPs on human lungs was explored.This study can guide coal combustion activities.Given the above studies,the main results are as follows:（1）Based on electron microscopy technologies（SEM and TEM）,a large number of metal-containing NPs with different shapes and elemental compositions were identified in the different CFAs,of which Fe-and Ti-NPs were the dominant NP fraction.Based on the SP-ICP-MS,there are numerous metal-containing NPs（hundreds of millions of particles per milligram）in different CFAs.NPs in CFAs from the different sources have obvious spatial heterogeneity,which may be related to parameters（the combustion process and PECDs）.In addition,NPs and heavy metal ions are associated and combined,especially in the fine particle components.（2）This study found that low-rank coal combustion may be more stressful to the environment.NPs with a very low volume/mass in CFAs dominate in number.Fe-NPs are identified in low-rank coal-derived fly ashes,including hematite,magnetite,maghemite,goethite and siderite.Ti-NPs mainly exist in the form of titanium oxide.The average particle sizes of Fe-NPs in low-rank coal-derived CFAs were 52-96 nm,and the particle number concentration（PNC）range was 2.9×10⁷-2.5×10⁸ particles/mg,dominating in particle number,besides,66-90 nm and 2.5×10⁷-1.7×10⁸ particles/mg for Ti-NPs.（3）The multi-stage CFAs showed that with the PECDs stage increases,the mean particle sizes of CFAs decline gradually and the PNCs of Fe-and Ti-NPs increase significantly（p<0.05）.Specifically,PNC of final-stage CFA was 3-5 times that of the first-stage.In addition,feed coal and PECDs are the main driving factors to regulate the average particle size and PNC of metal-containing NPs in multi-stage CFAs.Compared with electrostatic precipitators（ESPs）and fabric filters（FFs）,electrostatic-fabric-integrated precipitators（EFIPs）can retain more NPs and have relatively good NP-removal efficiency.Noticeably,The NPs were mainly enriched in CFAs trapped by PECDs（accounting for 80%of total emissions）.Although the mass of fly ash escaped into/through the stack（EFA）was extremely low,it contained the highest PNCs of Fe-and Ti-NPs of all CCPs,accounting for 3.41%and 1.67%of the corresponding total NP emissions.These NPs can be released directly to the atmosphere,thus posing potential risks to human health.（4）In vitro results based on Gamble’s solution showed that bioaccessible metals included not only dissolved metals but also metal-containing NPs.Enormous quantities of metal-containing NPs（up to hundreds of billions per gram of CFA）have been ignored in the past traditional risk assessment analyses for metals,which has resulted in underestimates when evaluating the risks caused by metals.51-87%of Fe and63-89%of Ti（ratio of the mass of Fe-/Ti-containing NPs to the total mass of Fe/Ti）exist in the NP form and remain suspended in pulmonary fluid simulants.When entering lung interstitial fluids,metal-containing NPs can be agglomerated and deposited in the pulmonary interstitium at high ionic strengths,which may increase the risks of related diseases.Finally,the coexistence of toxic metal ions and metal-containing NPs in human lung fluids suggests possible synergistic effects caused by both,which calls for further studies by lung pathologists and biotoxicologists.