Transport and retention of positively charged zinc oxide nanoparticles in saturated porous media: Effects of metal oxides and clays.

The effects of metal oxides and clays on the transport of zinc oxide nanoparticles (ZnO-NPs) in saturated porous media were investigated under different ionic strength (IS) conditions. We studied the transport and retention behavior of ZnO-NPs for different types of porous media (untreated, acid treated, and acid-salt treated sand). The selected untreated sand was used as a representative sand, coated with both metal oxide and clay. The acid treated and acid-salt-treated sands were used and compared to investigate the effects of clays on the surface of the sand. In addition, the effects of clay particles in bulk solutions on the mobility and retention of ZnO-NPs were observed using bentonite as a representative clay particle. We found that the increased mobility of positively charged ZnO-NPs can be attributed to increasing charge heterogeneity of silica sand with metal oxides (mainly, iron oxide) and clays in untreated sand. No breakthrough of ZnO-NP was observed for acid-treated (presence of clays and absence of metal oxides) and acid-salt-treated sand (absence of both metal oxide and clays). Most of the injected ZnO-NPs were deposited on the surface of the sand near the column inlet. The transport of bentonite-facilitated ZnO-NPs was improved at the lowest IS (0.1 mM) (∼20%), whereas there was no difference in the mobility of ZnO-NPs at high IS solutions (1 mM and 10 mM). In particular, the breakthrough amount improved with increasing bentonite concentration. Classical Derjaguin-Landau-Verwey-Overbeek interactions help explain observed interactions between ZnO-NPs and sand as well as bentonite and sand.

A CFD study of the transport and fate of airborne droplets in a ventilated office: The role of droplet-droplet interactions.

Previous studies reported that specially designed ventilation systems provide good air quality and safe environment by removing airborne droplets that contain viruses expelled by infected people. These water droplets can be stable in the environment and remain suspended in air for prolonged periods. Encounters between droplets may occur and droplet interactions should be considered. However, the previous studies focused on other physical phenomena (air flow, drag force, evaporation) for droplet transport and neglected droplet interactions. In this work, we used computational fluid dynamics (CFD) to simulate the transport and fate of airborne droplets expelled by an asymptomatic person and considered droplet interactions. Droplet drag with turbulence for prediction of transport and fate of droplets indicated that the turbulence increased the transport of 1 µm droplets, whereas it decreased the transport of 50 µm droplets. In contrast to only considering drag and turbulence, consideration of droplet interactions tended to increase both the transport and fate. Although the length scale of the office is much larger than the droplet sizes, the droplet interactions, which occurred at the initial stages of release when droplet separation distances were shorter, had a significant effect in droplet fate by considerably manipulating the final locations on surfaces where droplets adhered. Therefore, it is proposed that when an exact prediction of transport and fate is required, especially for high droplet concentrations, the effects of droplet interactions should not be ignored. ELECTRONIC SUPPLEMENTARY MATERIAL: Supplementary material is available in the online version of this article at 10.1007/s11783-021-1465-8 and is accessible for authorized users.

Sustainable treatment of bimetallic (Ag-Pd/α-Al2O3) catalyst waste from naptha cracking process: An innovative waste-to-value recycling of precious metals.

Bimetallic (Ag-Pd/α-Al2O3) catalysts are essentially applied to naptha-cracking process with a controlled CO2 emission. After losing the catalytic properties in long run, the landfilling disposal of spent catalysts poses severe stress to the environment and deprivation of precious metals. Therefore, an innovative solvo-chemical recycling approach that involving the solid-liquid and liquid-liquid mass transfer phenomena was studied. The parametric variations for dissolving precious metals yielded >98% efficiency at a lixiviant concentration, 2.0 mol L-1 HCl; pulp density, 20% (wt./vol.); agitation speed, 300 rpm, temperature, 90 °C, and duration, 60 min. The activation energy of silver (6.9 kJ mol-1) and palladium (11.9 kJ mol-1) leaching indicated that the process was governed by a diffusion-controlled mechanism. Subsequently, silver and palladium were separated using 0.15 mol L-1 LIX 84-I at different acid concentration that yielding the maximum separation factor (ß(Ag/Pd) = 12,501) at 2.0 mol L-1 HCl. Stripping of separately (Ag/Pd)-loaded organic solutions with different solutions of HNO3, (NH4)2SO4, and CH4N2S showed higher affinity for thiourea, yielding 56%, 38%, and 87% efficiency, respectively. Thus the counter-current extraction at an organic-to-aqueous (O:A) ratio of 1:2.5 and stripping with 0.5 mol L-1 CH4N2S at an O:A ratio of 2:1 yielded a five-fold enrich solutions of precious metals (75.2 mg L-1 Ag and 188.5 mg L-1 Pd) with a purity of >99.9%. The process essentially aims to Goal 12 under the United Nations' Sustainable Development Goals for sustainable recycling of industrial wastes consequently conserving the natural mineral reserves.

Transport of citrate-coated silver nanoparticles in saturated porous media.

In this study, the influences of physical and chemical factors [e.g., ionic strength (IS), pH, and flow rate] on the fate and transport of citrate-coated silver nanoparticles (AgNPs) were investigated through experiments using saturated columns. For the transport behavior of AgNPs under various conditions, retardation was confirmed with an increase in ionic strength (IS) while early elution developed with an increase in pH and flow rate. These transport experiment outcomes were simulated through Hydrus-1D, and the observed breakthrough curves were confirmed to have a significant correlation with the fitted results. Interestingly, the AgNPs and quartz sand used in this study showed a negative charge in the investigated experimental conditions. Although the reaction between AgNPs and quartz sand was expected to be unfavorable, AgNPs were observed to have been deposited onto the sand surface during the column test. To clarify the mechanism of the deposition of AgNPs even in unfavorable conditions, the interaction energy profiles were calculated based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. From the results, unfavorable interactions were expected in the NP-NP and NP-sand interactions in every condition. It was concluded that the deposition of AgNPs onto the sand surface under the unfavorable conditions in this study was mainly because of the physical roughness of the sand surface. Moreover, this hypothesis was supported by the zone of influence calculation in accordance with IS, the interpretation results of the fractional sand surface coverage in accordance with concentration changes of AgNPs, and series column tests.

Transport, retention, and long-term release behavior of ZnO nanoparticle aggregates in saturated quartz sand: Role of solution pH and biofilm coating.

The transport, retention, and long-term release of zinc oxide nanoparticle aggregates (denoted below as ZnO-NPs) were investigated in saturated, bare and biofilm (Pseudomonas putida) coated sand packed columns. Almost complete retention of ZnO-NPs occurred in bare and biofilm coated sand when the influent solution pH was 9 and the ionic strength (IS) was 0.1 or 10 mM NaCl, and the retention profiles were always hyper-exponential. Increasing the solution IS and biofilm coating produced enhanced retention of ZnO-NPs near the column inlet. The enhanced NPs retention at high IS was attributed to more favorable NP-silica and NP-NP interactions; this was consistent with the interaction energy calculations. Meanwhile, the greater NPs retention in the presence of biofilm was attributed to larger roughness heights which alter the mass transfer rate, the interaction energy profile, and lever arms associated with the torque balance; e.g., scanning electron and atomic force microscopy was used to determine roughness heights of 33.4 nm and 97.8 nm for bare sand and biofilm-coated sand, respectively. Interactions between NPs and extracellular polymeric substances may have also contributed to enhanced NP retention in biofilm-coated sand at low IS. The long-term release of retained ZnO-NPs was subsequently investigated by continuously injecting NP-free solution at pH 6, 9, or 10 and keeping the IS constant at 10 mM. The amount and rate of retained ZnO-NP removal was strongly dependent on the solution pH. Specifically, almost complete removal of retained ZnO-NPs was observed after 627 pore volumes when the solution pH was 6, whereas much less Zn was recovered when the eluting solution pH was buffered to pH = 9 and especially 10. This long-term removal was attributed to pH-dependent dissolution of retained ZnO-NPs because: (i) the solubility of ZnO-NPs increases with decreasing pH; and (ii) ZnO-NPs were not detected in the effluent. The presence of biofilm also decreased the initial rate and amount of dissolution and the subsequent transport of Zn(2+) due to the strong Zn(2+) re-adsorption to the biofilm. Our study indicates that dissolution will eventually lead to the complete removal of retained ZnO-NPs and the transport of toxic Zn(2+) ions in groundwater environments with pH ranges of 5-9.

Porous Ca-based bead sorbents for simultaneous removal of SO2, fine particulate matters, and heavy metals from pilot plant sewage sludge incineration.

In this study, a porous calcium-based sorbent was prepared for simultaneous removal of SO2, particulate matter (PM), and heavy metals generated during incineration of sewage sludge. The prepared sorbent was confirmed to have a 3-dimensional-network pore structure, a high specific surface area of 68.5m(2)/g, and gas permeability of 1.12 × 10(-10)m(2). Laboratory-scale tests indicated that there was an improvement in the performance of SO2 removal as the porosity and the specific surface area of the sorbent increased. Additionally, increasing reaction temperature led to greater SO2 removal. Meanwhile, the SL-4 and LS-3 sorbents prepared in this study were installed for operation during pilot tests treating the sewage sludge combustion gas generated by a fluidized incinerator in order to compare and evaluate their feasibility for use in industrial applications. The results showed that the reactivity between SO2 and the starting material of the sorbent (Ca(OH)2>CaCO3), as well as the high specific surface area of the sorbent, were confirmed to be critical factors that improved the performance of SO2 removal. Notably, the results confirmed that both fine PM (≤ 1 µm) and heavy metals were simultaneously removed with increasing efficiency over the time of operation.