•Removal of heavy metals and dyes by clay-based adsorbents are summarized.•Natural clays and nano-clays with 1D nanotubes and 2D nanosheets as adsorbents.•Factors affecting adsorption, mechanisms and ...adsorbent regeneration are discussed.•Exfoliate clays to 2D or 1D and reassemble as multifunctional materials of future.
Natural and synthetic clays have been used as adsorbents for removal of heavy metals and dyes from aqueous solution for many decades. In the last decade, exfoliating the natural clays to low dimensional nanosheets or nanotubes for preparation of multifunctional clay-based adsorbents has attracted considerable attention. In this review, we have summarized the removal of heavy metals and dyes by clay-based adsorbents, from natural clays to 1D clay nanotubes and 2D clay nanosheets. Firstly, the structural and adsorption performance of various natural clays (montmorillonite/bentonite/smectite, kaolinite, illite, sepiolite, palygorskite/attapulgite, halloysite) are presented. Then the adsorption of dyes and heavy metals onto modified clays including thermally treated clays, acid-washed clays, inorganic pillared clays and organic modified clays are discussed. Next, we have introduced the novel nano-clay based adsorbents with low dimensional morphologies (2D nanosheets, 1D nanotubes, 1D nanorods and clay-supported nanoparticles). The clay adsorbents with exfoliated nanosheets or nanotubes show dramatically improved performance because of the fully exposed reactive sites and very high specific surface area. More importantly, the nanosheets and nanotubes can be readily composited with coupling agents or functional polymers to prepare multifunctional adsorbents. In addition, the adsorption behaviors of clay-based adsorbents along with factors affecting adsorption, adsorption mechanisms and regeneration of adsorbents have been discussed. Inspired by the natural and modified clays, future suggestions in designing novel adsorbents for various applications have also been suggested.
Emissions from mobile sources are important contributors to both
primary and secondary organic aerosols (POA and SOA) in urban environments.
We compiled recently published data to create ...comprehensive model-ready
organic emission profiles for on- and off-road gasoline, gas-turbine, and
diesel engines. The profiles span the entire volatility range, including
volatile organic compounds (VOCs, effective saturation concentration
C*=107–1011 µg m−3),
intermediate-volatile organic compounds (IVOCs,
C*=103–106 µg m−3), semi-volatile organic compounds
(SVOCs, C*=1–102 µg m−3), low-volatile organic
compounds (LVOCs, C*≤0.1 µg m−3) and non-volatile
organic compounds (NVOCs). Although our profiles are comprehensive, this
paper focuses on the IVOC and SVOC fractions to improve predictions of SOA
formation. Organic emissions from all three source categories feature
tri-modal volatility distributions (“by-product” mode, “fuel” mode, and
“lubricant oil” mode). Despite wide variations in emission factors for
total organics, the mass fractions of IVOCs and SVOCs are relatively
consistent across sources using the same fuel type, for example, contributing
4.5 % (2.4 %–9.6 % as 10th to 90th percentiles) and
1.1 % (0.4 %–3.6 %) for a diverse fleet of light duty gasoline
vehicles tested over the cold-start unified cycle, respectively. This
consistency indicates that a limited number of profiles are needed to
construct emissions inventories. We define five distinct profiles:
(i) cold-start and off-road gasoline, (ii) hot-operation gasoline,
(iii) gas-turbine, (iv) traditional diesel and (v) diesel-particulate-filter
equipped diesel. These profiles are designed to be directly implemented into
chemical transport models and inventories. We compare emissions to unburned
fuel; gasoline and gas-turbine emissions are enriched in IVOCs relative to
unburned fuel. The new profiles predict that IVOCs and SVOC vapour will
contribute significantly to SOA production. We compare our new profiles to
traditional source profiles and various scaling approaches used previously to
estimate IVOC emissions. These comparisons reveal large errors in these
different approaches, ranging from failure to account for IVOC emissions
(traditional source profiles) to assuming source-invariant scaling ratios
(most IVOC scaling approaches).
Recent increases in the Corporate Average Fuel Economy standards have led to widespread adoption of vehicles equipped with gasoline direct-injection (GDI) engines. Changes in engine technologies can ...alter emissions. To quantify these effects, we measured gas- and particle-phase emissions from 82 light-duty gasoline vehicles recruited from the California in-use fleet tested on a chassis dynamometer using the cold-start unified cycle. The fleet included 15 GDI vehicles, including 8 GDIs certified to the most-stringent emissions standard, superultra-low-emission vehicles (SULEV). We quantified the effects of engine technology, emission certification standards, and cold-start on emissions. For vehicles certified to the same emissions standard, there is no statistical difference of regulated gas-phase pollutant emissions between PFIs and GDIs. However, GDIs had, on average, a factor of 2 higher particulate matter (PM) mass emissions than PFIs due to higher elemental carbon (EC) emissions. SULEV certified GDIs have a factor of 2 lower PM mass emissions than GDIs certified as ultralow-emission vehicles (3.0 ± 1.1 versus 6.3 ± 1.1 mg/mi), suggesting improvements in engine design and calibration. Comprehensive organic speciation revealed no statistically significant differences in the composition of the volatile organic compounds emissions between PFI and GDIs, including benzene, toluene, ethylbenzene, and xylenes (BTEX). Therefore, the secondary organic aerosol and ozone formation potential of the exhaust does not depend on engine technology. Cold-start contributes a larger fraction of the total unified cycle emissions for vehicles meeting more-stringent emission standards. Organic gas emissions were the most sensitive to cold-start compared to the other pollutants tested here. There were no statistically significant differences in the effects of cold-start on GDIs and PFIs. For our test fleet, the measured 14.5% decrease in CO2 emissions from GDIs was much greater than the potential climate forcing associated with higher black carbon emissions. Thus, switching from PFI to GDI vehicles will likely lead to a reduction in net global warming.
In nature, the calcium carbonate shows different interactions with different metal ions. Inspiration from this natural phenomenon, in this work, the selective recovery of heavy metals from wastewater ...by mechanically activated calcium carbonate was investigated. The changes in Ca2+ concentration, pH value and metals uptake ratio of solution showed that M2+ (M = Cu, Mn, Zn and Ni) were endowed with different migration rules, resulting in the various interaction with the calcium carbonate in metal-bearing solution. The combination of XRD, SEM, and stereomicroscope affirmed that the adsorbed M2+ rarely change the lattice structure of calcium carbonate, while the adsorbed Cu2+ and Zn2+ could convert the mineral phase from calcium carbonate to posnjakite and hydrozincite, respectively. As a result of phase transition, 15% Cu2+ and 6% Zn2+ were uptaken with initial concentration of 1 mM for 100 min, however, the unsatisfactory recovery prevented the efficient recycling of metal. The mechanically activated calcium carbonate had a superior solubility at the solid/liquid interface, promoting mineral phase transformation on the premise of weak displacement adsorption. Hence, the uptake ratio of Cu2+ and Zn2+ were significantly increased to 99% and 53% at the same condition. Finally, Cu2+ was recovered from polymetallic systems from complex environment with high precision. The concept of selective recycling in this research guides the development of innovative processes from natural information.
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•CaCO3 adsorbs metals with little distortion of its lattice.•Adsorbed Cu2+ and Zn2+ convert CaCO3 to posnjakite and hydrozincite.•Mechanochemistry promotes mineral phase transition under weak adsorption.•Selective recovery of Cu2+ is achieved by mechanically activated CaCO3.
Secondary organic aerosol (SOA) is a major component of atmospheric fine particle mass. Intermediate-volatility organic compounds (IVOCs) have been proposed to be an important source of SOA. We ...present a comprehensive analysis of atmospheric IVOC concentrations and their SOA production using measurements made in Pasadena, California during the California at the Nexus of Air Quality and Climate Change (CalNex) study. The campaign-average concentration of primary IVOCs was 6.3 ± 1.9 μg m–3 (average ± standard deviation), which is comparable to the concentration of organic aerosol but only 7.4 ± 1.2% of the concentration of speciated volatile organic compounds. Only 8.6 ± 2.2% of the mass of the primary IVOCs was speciated. Almost no weekend/weekday variation in the ambient concentration of both speciated and total primary IVOCs was observed, suggesting that petroleum-related sources other than on-road diesel vehicles contribute substantially to the IVOC emissions. Primary IVOCs are estimated to produce about 30% of newly formed SOA in the afternoon during CalNex, about 5 times that from single-ring aromatics. The importance of IVOCs in SOA formation is expected to be similar in many urban environments.
In this work, direct As(III) removal without pre-oxidation by the in-situ generated Fe(III) hydroxide derived from ferrous sulfate on the surface of calcium carbonate was investigated. The ...coagulation sediments were characterized by XPS, Zeta potential, AFM and SEM, respectively and the mechanism of the high As(III) removal efficiency was discussed and compared with the traditional neutralization of Fe2(SO4)3 at the same pH around 6. In the FeSO4 + CaCO3 system, Fe(II) ions dispersedly absorb on the surface of the CaCO3 particles. Accompanied with the slowly hydrolyzed OH− from CaCO3 and the O2 in air, Fe(III) hydroxide is in-situ generated, which maintains nanosized particles without evident agglomeration into larger ones as usually observed with the alkali neutralization and therefore presents high activity to As(III). Although Fe arsenite has higher solubility than the arsenate, the in situ formed Fe(III) hydroxide demonstrates high reactivity to form basic Fe arsenite with Fe/As molar ratio at 4.5, resulting in almost complete arsenic precipitation. By this pathway, it is possible to remove directly arsenic at the unchanged trivalent state, without the usual requirement to oxidize As(III) to As(V), verified by the measurements of arsenic valence and hydroxyl radicals (OH) formation during the conversion of Fe(II) into Fe(III).
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•In-situ generated Fe(III) hydroxide achieves great As(III) removal effect.•Fe(II) ions dispersedly absorb on the surface of the CaCO3 particles.•Fe(III) hydroxide maintains a nanosized state without evident agglomeration.•As(III) directly removed as basic iron arsenite without usual oxidization to As(V).
The removal of lead in water and disposal of tailings are important environmental issues that need to be addressed urgently. This work explored the feasibility of utilizing the carbonate-based ...tailings (CBT) for removing lead from the simulated wastewater with the aid of wet stirred ball milling (mechanical activation). Batch experiments were performed to evaluate the influences of various experimental parameters like dosage of CBT, milling balls addition and initial concentration of lead. Under the action of mechanical activated CBT, the lead removal in the solution could reach more than 99% in 2 h, and the lead removal capacity reached 832 mg/g. The results of X-Ray Diffraction (XRD), Fourier Transform infrared spectroscopy (FTIR) and Scanning Electron Microscope-Energy Dispersive Spectra (SEM-EDS) revealed that the calcite (CaCO3) in CBT played a major role in removing lead and the lead in the solution was transferred to the precipitate as cerussite (PbCO3). The mechanical activation promoted the dissolution of calcite, reduced the particle size of CBT and peeled off the lead carbonate precipitation on the surface of calcite, thereby enabling the reaction to be efficiently and thoroughly completed. The lead content in the precipitate after the reaction reached 38.4 wt%, which made it possible for lead recovery.
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•A new method using carbonate-based tailings to treat lead wastewater was studied.•The mechanical activation of tailings enhanced the lead removal by three ways.•The lead removal capacity of the tailings reached 832 mg/g.•The calcite in tailings played a major role in triggering a chemical precipitation.
Nanoscale zero-valent iron (nZVI) is highly useful to remediate contaminants such as arsenic but it is currently difficult to be widely applied because of the complicated and costly synthesis process ...of nZVI. In this work, a novel production method by simple grinding of microscale ZVI with calcium carbonate (CaCO3) is proposed to enhance the reactivity of ZVI for the removal of As(III) from water. The mechanically activated ZVI/CaCO3 by the above grinding process was characterized by XRD, SEM-EDS, XPS, etc. The modified ZVI/CaCO3 by grinding shows excellent As(III) removal performance and good regeneration property. The mechanisms of co-grinding ZVI with CaCO3 leading to the increased reactivity of ZVI and resulting in enhanced As(III) removal are deciphered. The process of grinding helps to strip the original passivation layer from the surface of micro ZVI while the CaCO3 prevents the agglomeration of ZVI during the grinding process and protects the fresh ZVI from forming a passivation layer. During the process of arsenic removal, the CaCO3 covering on the surface of ZVI is continuously removed into the solution to expose fresh surface of ZVI simultaneously. In addition, the presence of CaCO3 facilitates the corrosion of ZVI and the formation of Fe(II) and H2O2 to trigger more highly reactive intermediates by Fenton reaction to promote the oxidation of As(III) to As(V) and Fe(II) to Fe(III), which results in dramatically enhanced As(III) removal. The presently developed simple grinding method using cost-effective CaCO3 remarkably improved the reactivity of ZVI and thus, offers an alternative for using microscale ZVI to deal with environmental pollution of arsenic more effectively and economically.
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•Simple co-grinding of ZVI with CaCO3 improves its reactivity remarkably.•ZVI/CaCO3 composite shows excellent As(III) removal and cycling stability.•CaCO3 prevents agglomeration of ZVI and passivation layer formation on ZVI surface.•Formation of Fe(II) and H2O2 may have promoted oxidation of As(III) to As(V).
Secondary organic aerosol (SOA) formation from in-use vehicle emissions was investigated using a potential aerosol mass (PAM) flow reactor deployed in a highway tunnel in Pittsburgh, Pennsylvania. ...Experiments consisted of passing exhaust-dominated tunnel air through a PAM reactor over integrated hydroxyl radical (OH) exposures ranging from ∼0.3 to 9.3 days of equivalent atmospheric oxidation. Experiments were performed during heavy traffic periods when the fleet was at least 80% light-duty gasoline vehicles on a fuel-consumption basis. The peak SOA production occurred after 2–3 days of equivalent atmospheric oxidation. Additional OH exposure decreased the SOA production presumably due to a shift from functionalization to fragmentation dominated reaction mechanisms. Photo-oxidation also produced substantial ammonium nitrate, often exceeding the mass of SOA. Analysis with an SOA model highlight that unspeciated organics (i.e., unresolved complex mixture) are a very important class of precursors and that multigenerational processing of both gases and particles is important at longer time scales. The chemical evolution of the organic aerosol inside the PAM reactor appears to be similar to that observed in the atmosphere. The mass spectrum of the unoxidized primary organic aerosol closely resembles ambient hydrocarbon-like organic aerosol (HOA). After aging the exhaust equivalent to a few hours of atmospheric oxidation, the organic aerosol most closely resembles semivolatile oxygenated organic aerosol (SV-OOA) and then low-volatility organic aerosol (LV-OOA) at higher OH exposures. Scaling the data suggests that mobile sources contribute ∼2.9 ± 1.6 Tg SOA yr–1 in the United States, which is a factor of 6 greater than all mobile source particulate matter emissions reported by the National Emissions Inventory. This highlights the important contribution of SOA formation from vehicle exhaust to ambient particulate matter concentrations in urban areas.
Food cooking can be a significant source of atmospheric particulate organic matter. In this study, the chemical composition of particulate organic matter (POM) in PM2.5 emitted from four different ...Chinese cooking styles were examined by gas chromotography−mass spectrometry (GC−MS). The identified species are consistent in the emissions from different Chinese cooking styles and the quantified compounds account for 5∼10% of total POM in PM2.5. The dominant homologue is fatty acids, constituting 73∼85% of the quantified compounds. The pattern of n-alkanes and the presence of β-sitosterol and levoglucosan indicate that vegetables are consumed during Chinese cooking operations. Furthermore, the emissions of different compounds are impacted significantly by the cooking ingredients. The candidates of organic tracers used to describe and distinguish emissions from Chinese cooking in Guangzhou are tetradecanoic acid, hexadecanoic acid, octadecanoic acid, oleic acid, levoglucosan, mannosan, galactosan, nonanal, and lactones. During the sampling period, the relative contribution of Chinese cooking to the mass concentration of atmospheric hexadecanoic acid should be less than 1.3% in Guangzhou.
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