Transport phenomena in high Reynolds number wall-bounded stratified flows are dominated by the interplay between the turbulence structures generated at the wall and the buoyancy-induced large-scale ...waves populating the channel core. In this study, we want to investigate the flow physics of wall-bounded stratified turbulence at relatively high shear Reynolds number $Re_\tau$ and for mild to moderate stratification level – quantified here by the shear Richardson number varying in the range $0\leqslant Ri_{\tau } \leqslant 300$. By increasing stratification, active turbulence is sustained only in the near-wall region, whereas intermittent turbulence, modulated by the presence of non-turbulent wavy structures (internal gravity waves), is observed at the channel core. In such conditions, the wall-normal transport of momentum and heat is considerably reduced compared with the case of non-stratified turbulence. A careful characterization of the flow-field statistics shows that, despite temperature and wall-normal velocity fluctuations being very large at the channel centre, the mean value of their product – the buoyancy flux – vanishes for $Ri_{\tau } \geqslant 200$. We show that this behaviour is due to the presence of a $\sim {\rm \pi}/2$ phase delay between the temperature and the wall-normal velocity signals: when wall-normal velocity fluctuations are large (in magnitude), temperature fluctuations are almost zero, and vice versa. This constitutes a blockage effect to the wall-normal exchange of energy. In addition, we show that the friction factor scales as $C_f \sim Ri_{\tau }^{-1/3}$, and we propose a new scaling for the Nusselt number, $Nu \cdot Re_{\tau }^{-2/3} \sim Ri_{\tau }^{-1/3}$. These scaling laws, which seem to be robust over the explored range of parameters, complement and extend previous experimental and numerical data, and are expected to help the development of improved models and parametrizations of stratified flows at large $Re_{\tau }$.
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•Review of the activated carbon production from sludge.•Physical and chemical techniques for activation.•Adsorption of metals and dyes by sludge-based adsorbents.•Effect of various ...factors on the uptake capacity of the adsorbents.•Critical views on the activated carbon preparation and adsorption processes.
Sludge, a byproduct produced from numerous industrial activities, has been recognized as an ecological burden for the society. However, viewing the sludge, as a carbon-rich material, has stimulated new gateways for the production of porous activated carbons for water treatment applications. Herein, various textural properties of the sludge-based activated carbons have been compiled and critically reviewed with the focus on surface area, pore size and pore size distribution for both physically and chemically activated carbons. It has been found that chemical activation using various activating agents yields more superior adsorbents with high specific surface areas than physical activation methods. Moreover, the potential of sludge-derived activated carbons for the sequestration of metals and dyes from aqueous media has been discussed. Furthermore, the adsorption mechanism in several adsorbent–adsorbate systems and the effect of various parameters on the adsorption behavior of different dyes and metals on sludge-based activated carbons have been reviewed. It has been shown that the pollutant uptake capacities of the adsorbents derived from sewage sludge are not only governed by the textural properties of the adsorbents, but also by their surface properties, such as the functional groups and the surface charge and thus it is proposed that both of these crucial factors be considered concurrently.
•A novel developed adsorbent material derived from waste printed circuit boards’ component.•The innovative adsorbent material can effectively remove cadmium ions from aqueous solutions.•The maximum ...capacity for cadmium ion removal is 2.1mmol/g.•Cadmium removal capacity is either equivalent or better than commercial resins.
Cadmium is a highly toxic heavy metal even at a trace level. In this study, a novel material derived from waste PCBs has been applied as an adsorbent to remove cadmium ions from aqueous solutions. The effects of various factors including contact time, initial cadmium ion concentration, pH and adsorbent dosage have been evaluated. The maximum uptake capacity of the newly derived material for cadmium ions has reached 2.1mmol/g at an initial pH 4. This value shows that this material can effectively remove cadmium ions from effluent. The equilibrium isotherm has been analyzed using several isotherm equations and is best described by the Redlich–Peterson model. Furthermore, different commercial adsorbent resins have been studied for comparison purposes. The results further confirm that this activated material is highly competitive with its commercial counterparts.
Altering the textural properties of activated carbons (ACs) via physicochemical techniques to increase their specific surface area and/or to manipulate their pore size is a common practice to enhance ...their adsorption capacity. Instead, this study proposes the utilization of the vacant sites remaining unoccupied after dye uptake saturation by removing the steric hindrance and same-charge repulsion phenomena via multilayer adsorption. Herein, it has been shown that the adsorption capacity of the fresh AC is a direct function of the dye molecular size. As the cross-sectional area of the dye molecule increases, the steric hindrance effect exerted on the neighboring adsorbed molecules increases, and the geometrical packing efficiency is constrained. Thus, ACs saturated with larger dye molecules render higher concentrations of vacant adsorption sites which can accommodate an additional layer of dye molecules on the exhausted adsorbent through interlayer attractive forces. The second layer adsorption capacity (60–200 mg·g–1) has been demonstrated to have a linear relationship with the uncovered surface area of the exhausted AC, which is, in turn, inversely proportional to the adsorbate molecular size. Unlike the second layer adsorption, the third layer adsorption is a direct function of the charge density of the second layer.
•There is a major environmental issue about the printed circuit boards throughout the world.•Different physical and chemical recycling techniques have been reviewed.•Nonmetallic fraction of PCBs is ...the unwanted face of this waste stream.•Several applications of the nonmetallic fraction of waste PCBs have been introduced.
E-waste, in particular waste PCBs, represents a rapidly growing disposal problem worldwide. The vast diversity of highly toxic materials for landfill disposal and the potential of heavy metal vapors and brominated dioxin emissions in the case of incineration render these two waste management technologies inappropriate. Also, the shipment of these toxic wastes to certain areas of the world for eco-unfriendly “recycling” has recently generated a major public outcry. Consequently, waste PCB recycling should be adopted by the environmental communities as an ultimate goal.
This article reviews the recent trends and developments in PCB waste recycling techniques, including both physical and chemical recycling. It is concluded that the physical recycling techniques, which efficiently separate the metallic and nonmetallic fractions of waste PCBs, offer the most promising gateways for the environmentally-benign recycling of this waste. Moreover, although the reclaimed metallic fraction has gained more attention due to its high value, the application of the nonmetallic fraction has been neglected in most cases. Hence, several proposed applications of this fraction have been comprehensively examined.
•Polyaniline-coated PVDF-HFP nanofibrous membranes were fabricated.•Chromium was effectively removed from wastewater using the adsorbent membranes.•Cr(VI) was first adsorbed on the imine groups and ...then reduced to Cr(III).•PAni-coated membranes were successfully recycled and reused for at least 5 cycles.
Polyaniline (PAni)-coated polyvinylidenefluoride-co-hexafluoropropilene (PVDF-HFP) nanofibrous membranes were fabricated by electrospinning of PVDF-HFP followed by in-situ polymerization of PAni on the nanofiber surface. The resulting membranes with PAni coating were applied for chromium removal, where the efficiency was evaluated as functions of the pH value and adsorption time. Dynamic adsorption tests were also carried out at different flow rates and volumes of chromium in water. It was found that the PAni coating greatly enhanced the chromium removal efficiency with the maximum adsorption capacity being 15.08 mg/g at pH = 4.5. The desorption study further confirmed the recyclability of the PAni coated PVDF-HFP membrane showing an efficiency over 70% even after 5 cycles of usage. The structure and property relationships of these membranes were also characterized by FTIR spectroscopy, capillary flow porometer, water contact angle, scanning electron microscopy and X-ray photoelectron spectroscopy.
In this study, we developed a hierarchical thin-film nanofibrous composite (TFNC) membrane with electrospun mat as substrate and hydrophilic nanocellulose as the antifouling barrier layer. We found ...that, due to the super-hydrophilic nature of the nanocellulose, the contact angle of the barrier layer (20−28°) decreased rapidly with time and reached nearly zero after a few seconds, whereby the membrane flux were remarkably higher (52L.m−2.h−1) than conventional polymeric membranes (4−14L.m−2.h−1) at a very low transmembrane pressure of 0.5 psi. In addition, the membrane surface was considerably more negatively-charged due to the high concentration of carboxylate groups, resulting in higher repulsive electrostatic forces between the barrier layer and the model foulant. As a result, the nanocellulose-based hierarchical membranes exhibited a lower fouling tendency (<10%) and a higher degree of protein rejection ratio compared with the conventional membranes (fouling tendency >30%). The effect of the nanocellulose layer thickness on the membrane fouling was also examined and it was demonstrated that the nanocellulose barrier layer thickness had a significant effect on the membrane fouling. The higher flux, lower fouling, and good rejection properties of this membrane system suggest nanocellulose is a promising barrier material for filtration membranes for water purification and other separation processes.
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•Nanocellulose as antifouling nanolayer was coated on the electrospun membranes.•Effect of nanocellulose layer thickness on membrane fouling and flux was evaluated.•Superhydrophilic nanocellulose-coated membranes outperformed commercial membranes.•Nanocellulose membranes exhibited fouling-resistance and 5 times higher flux values.
•Systematic analysis of all emission streams and established mass balance of reaction reagents, bromide and carbon.•Limited difficulties in emission treatment due to sole destination of ...pollutants.•Excellent environmental sustainability by bromide recovery from the Washing Liquid.•Provide evaluation criteria for waste recycling process development.
The short life span of electrical and electronic equipments (EEE) produces a tremendous amount of waste electrical and electronic equipments (WEEE) globally at an overwhelming rate. Currently, the recycling of waste printed circuit board (WPCB) is an emerging and sensitive environmental issue since traditional treatment methods have been proved to be inappropriate. Also, the recovery of valuable components from WPCB is limited by the environmental emissions from the recycling process, which have been seriously neglected by the research community before. To analyze the environmental emission impacts and set evaluation criteria, a newly developed aluminosilicate adsorbent production process is discussed in this work. It is proved to be capable to recycle the NMF of WPCB as the feedstock of the functionalization process. Besides, the mass balance of reaction reagents, bromide and carbon were established with clear pathways and destinations by various analytical chemistry methods. The debromination process in functionalization significantly reduced the total toxicity by giving inorganic bromide and bisphenol A (BPA) as two main products. The results confirm this process to be an environmentally friendly process which can provide a basis for evaluating recycling processes.
Despite the advantages of membrane processes, their high energy requirement remains a major challenge. Fabrication of nanocomposite membranes by incorporating various nanomaterials in the polymer ...matrix has shown promise for enhancing membrane flux. In this study, we embed functionalized cellulose nanofibers (CNFs) with high aspect ratios in the polymer matrix to create hydrophilic nanochannels that reduce membrane resistance and facilitate the facile transport of water molecules through the membrane. The results showed that the incorporation of 0.1 wt % CNF into the polymer matrix did not change the membrane flux (~15 L · m - 2 · h - 1 ) and Bovine Serum Albumin (BSA) Fraction V rejection, while increasing the CNF content to 0.3 wt % significantly enhanced the flux by seven times to ~100 L · m - 2 · h - 1 , but the rejection was decreased to 60-70%. Such a change in membrane performance was due to the formation of hydrophilic nanochannels by the incorporation of CNF (corroborated by the SEM images), decreasing the membrane resistance, and thus enhancing the flux. When the concentration of the CNF in the membrane matrix was further increased to 0.6 wt %, no further increase in the membrane flux was observed, however, the BSA rejection was found to increase to 85%. Such an increase in the rejection was related to the electrostatic repulsion between the negatively-charged CNF-loaded nanochannels and the BSA, as demonstrated by zeta potential measurements. SEM images showed the bridging effect of the CNF in the nanochannels with high CNF contents.
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