•Properties and mechanisms of Cr(VI) adsorption onto LDH-based materials reviewed.•Such materials: high anion exchange capacity and positively charged external surface.•Identified mechanism: anion ...exchange, adsorption-coupled reduction, electrostatic attraction.•Adsorption-coupled reduction: identified by some advanced techniques from 2016.•Thermodynamic parameters: ΔG° <0, ΔH° >0, and ΔS° >0 in the most observation cases.
An attempt has been made in this review to provide some insights into the possible adsorption mechanisms of hexavalent chromium onto layered double hydroxides-based adsorbents by critically examining the past and present literature. Layered double hydroxides (LDH) nanomaterials are typical dual-electronic adsorbents because they exhibit positively charged external surfaces and abundant interlayer anions. A high positive zeta potential value indicates that LDH has a high affinity to Cr(VI) anions in solution through electrostatic attraction. The host interlayer anions (i.e., Cl−, NO3−, SO42−, and CO32−) provide a high anion exchange capacity (53–520 meq/100 g) which is expected to have an excellent exchangeable capacity to Cr(VI) oxyanions in water. Regarding the adsorption-coupled reduction mechanism, when Cr(VI) anions make contact with the electron-donor groups in the LDH, they are partly reduced to Cr(III) cations. The reduced Cr(III) cations are then adsorbed by LDH via numerous interactions, such as isomorphic substitution and complexation. Nonetheless, the adsorption-coupled reduction mechanism is greatly dependent on: (1) the nature of divalent and trivalent salts utilized in LDH preparation, and the types of interlayer anions (i.e., guest intercalated organic anions), and (3) the adsorption experiment conditions. The low Brunauer–Emmett–Teller specific surface area of LDH (1.80–179 m2/g) suggests that pore filling played an insignificant role in Cr(VI) adsorption. The Langmuir maximum adsorption capacity of LDH (Qomax) toward Cr(VI) was significantly affected by the natures of used inorganic salts and synthetic methods of LDH. The Qomax values range from 16.3 mg/g to 726 mg/g. Almost all adsorption processes of Cr(VI) by LDH-based adsorbent occur spontaneously (ΔG° <0) and endothermically (ΔH° >0) and increase the randomness (ΔS° >0) in the system. Thus, LDH has much potential as a promising material that can effectively remove anion pollutants, especially Cr(VI) anions in industrial wastewater.
Traditional approaches to reliably transmit information over an error-prone network employ either forward error correction (FEC) or retransmission techniques. In this paper, we propose some network ...coding schemes to reduce the number of broadcast transmissions from one sender to multiple receivers. The main idea is to allow the sender to combine and retransmit the lost packets in a certain way so that with one transmission, multiple receivers are able to recover their own lost packets. For comparison, we derive a few theoretical results on the bandwidth efficiency of the proposed network coding and traditional automatic repeat-request (ARQ) schemes. Both simulations and theoretical analysis confirm the advantages of the proposed network coding schemes over the ARQ ones.
Purpose: The purpose of this paper is to outline key findings from a contemporary review of the international empirical literature focused upon teacher leadership. It synthesises what is currently ...known about the nature, practice, conditions and impact of teacher leadership and to outline patterns in the contemporary empirical research base. Design/methodology/approach: This review is based on an analysis of 150 empirical articles published in Scopus/SSCI-indexed journals between January 2003 and December 2017. Findings: The paper draws upon this contemporary knowledge base to explore: contextual and methodological patterns of teacher leadership research; definitions of teacher leadership; and evidence on the enactment of teacher leadership, factors influencing teacher leadership and impacts of teacher leadership. Originality/value: This paper highlights the progress and issues of the empirical research on teacher leadership since 2003 and identifies gaps in the knowledge base as well as areas for future scholarly enquiry.
In this study, the biosorption mechanisms of methylene blue (MB) and Cr(
iii
) onto pomelo peel collected from our local fruits are investigated by combining experimental analysis with
ab initio
...simulations. Factors that affect the adsorption such as pH, adsorption time, adsorbent dosage and initial adsorbate concentration, are fully considered. Five isotherm models-Langmuir, Freundlich, Sips, Temkin, and Dubinin-Radushkevich-are employed to estimate the capacity of pomelo peel adsorption, whereas four kinetic models-pseudo-first-order, pseudo-second-order, Elovich and intra-diffusion models-are also used to investigate the mechanisms of the uptake of MB and Cr(
iii
) onto the pomelo fruit peel. The maximum biosorption capacities calculated from the Langmuir models for MB and Cr(
iii
) at 303 K are, 218.5 mg g
−1
and 11.3 mg g
−1
, respectively. In particular, by combining, for the first time, the experimental FT-IR spectra with those obtained from
ab initio
calculations, we are able to demonstrate that the primary adsorption mechanisms of the uptake of MB onto pomelo fruit peel are electrostatic attraction and hydrogen-bond formations, whereas the adsorption mechanisms for Cr(
iii
) are electrostatic attraction and n-d interactions.
In this study, the biosorption mechanisms of methylene blue (MB) and Cr(
iii
) onto pomelo peel collected from our local fruits are investigated by combining experimental analysis with
ab initio
simulations.
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•Zn-S catalyst improves DB22 decolonization and mineralization rate by heterogeneous catalytic ozonation.•Zn, Ca constituents in Zn-S improve OH* generation in heterogeneous catalytic ...ozonation of DB22.•DB22′s mineralization by Zn-S catalytic ozonation was highest in alkaline condition.•Mineralization kinetic of DB22 follows pseudo-first order kinetic model.
This study developed a low cost catalyst, namely, zinc slag (Zn-S) for the ozonation process of Direct Black 22 (DB22) from aqueous solutions. Among five different kind of low cost metal slags including Fe-S, Cu-S, Cd-S, Pb-S and Zn-S, the Zn-S slag was selected as an efficient catalyst in this study. Zn-S contained mainly zinc (Zn) and calcium (Ca) discharged from zinc slag waste in Vietnam. It was found that Zn-S could effectively decolonize and mineralize DB22 through heterogeneous catalytic ozonation. The degradation kinetic of DB22 followed the pseudo-first order model. The best removal efficiency of DB22 (Zn-S/O3/H2O2 (76%) > Zn-S/O3 (69%) > O3/H2O2 (66%) > O3 (55% for COD) occurred at pH 11 for heterogeneous catalytic ozonation processes with Zn-S as the catalyst as well as ozone alone and perozone processes due to fast decomposition of O3 in alkaline solution to generate powerful and non-selective OH radicals. An increase in decolonization and mineralization rate was observed when increasing the Zn-S dosage from 0.125 g/L to 0.75 g/L for Zn-S/O3 and 0.125 g/L to 1.0 g/L for Zn-S/O3/H2O2. The K values of the pseudo-first order model followed the same sequence as mineralization rates of DB22 in term of COD removal. Ca and Zn constituents in the Zn-S catalyst contributed to the increase in O3 decomposition and improvement of reaction rate with H2O2. Subsequently, the degradation of DB22 by the ozonation process with Zn-S catalyst was enhanced through the enrichment mechanism of hydroxyl radicals (*OH) and surface adsorption. The degradation mechanism of DB22 by hydroxyl radicals was surely affirmed by tests with the decrease in degradation percentage of DB22 in case of the presence t-butanol, Cl− and CO32−.
Fundamental qualitative properties of the minimum sum-of-squares clustering problem are established in this paper. We prove that the problem always has a global solution and, under a mild condition, ...the global solution set is finite. Moreover, the components of each global solution can be computed by an explicit formula. Based on a new concept of non-trivial local solution, we get necessary conditions for a system of centroids to be such a local solution. Interestingly, these necessary conditions are also sufficient ones. Finally, it is proved that the optimal value function is locally Lipschitz, the global solution map is locally upper Lipschitz, and the local solution map has the Aubin property, provided that the original data points are distinct. The obtained complete characterizations of the non-trivial local solutions allow one to understand better the performance of not only the k-means algorithm, but also of other solution methods for the problem in question.
Red mud modified by chitosan (RM/CS) was utilized as an adsorbent to effectively remove Pb(II) from aqueous solution. The surface area of RM/CS was found to significantly increase by more than 50% ...compared to that of original red mud. Different factors that affected the Pb(II) removal on this material, such as initial Pb(II) concentration, pH, and contact time, were investigated. The pseudo-first-order, pseudo-second-order, and intra-diffusion models were used to fit the experimental data to investigate the Pb(II)'s removal kinetics. The Pb(II) removal followed the intra-diffusion model. Additionally, the non-zero C value obtained from this model indicates that the removal was controlled by many different mechanisms. We also found that the interaction of Pb(II) and carbonate group on the material's surface played a primary role once the adsorption equilibrium was reached. Finally, the maximum adsorptive capacity was found to be about 209 mg/g. This obtained value is higher than those obtained for some other materials. Therefore, the present RM/CS should be a potential material for removing Pb(II) from aqueous solution.
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•Red mud modified by chitosan (RM/CS), a novel adsorbent, was used to remove Pb(II).•Characteristics of RM/CS were investigated by XRD, SEM, BET, FT-IR, and TGA-DSC.•The adsorption capacity is 208.48 mg g−1 higher than some other materials.•Several different mechanisms of the uptake of Pb(II) onto RM/CS were proposed.
Evaluating the prognosis of patients with aneurysmal subarachnoid hemorrhage (aSAH) who may be at risk of poor outcomes using grading systems is one way to make a better decision on treatment for ...these patients. This study aimed to compare the accuracy of the modified World Federation of Neurosurgical Societies (WFNS), WFNS, and Hunt and Hess (H&H) Grading Scales in predicting the outcomes of patients with aSAH. From August 2019 to June 2021, we conducted a multicenter prospective cohort study on adult patients with aSAH in three central hospitals in Hanoi, Vietnam. The primary outcome was the 90-day poor outcome, measured by a score of 4 (moderately severe disability) to 6 (death) on the modified Rankin Scale (mRS). We calculated the areas under the receiver operator characteristic (ROC) curve (AUROCs) to determine how well the grading scales could predict patient prognosis upon admission. We also used ROC curve analysis to find the best cut-off value for each scale. We compared AUROCs using Z-statistics and compared 90-day mean mRS scores among intergrades using the pairwise multiple-comparison test. Finally, we used logistic regression to identify factors associated with the 90-day poor outcome. Of 415 patients, 32% had a 90-day poor outcome. The modified WFNS (AUROC: 0.839 95% confidence interval, CI: 0.795-0.883; cut-off valuegreater than or equal to2.50; P.sub.AUROC <0.001), WFNS (AUROC: 0.837 95% CI: 0.793-0.881; cut-off valuegreater than or equal to3.5; P.sub.AUROC <0.001), and H&H scales (AUROC: 0.836 95% CI: 0.791-0.881; cut-off valuegreater than or equal to3.5; P.sub.AUROC <0.001) were all good at predicting patient prognosis on day 90.sup.th after ictus. However, there were no significant differences between the AUROCs of these scales. Only grades IV and V of the modified WFNS (3.75 standard deviation, SD: 2.46 vs 5.24 SD: 1.68, p = 0.026, respectively), WFNS (3.75 SD: 2.46 vs 5.24 SD: 1.68, p = 0.026, respectively), and H&H scales (2.96 SD: 2.60 vs 4.97 SD: 1.87, p<0.001, respectively) showed a significant difference in the 90-day mean mRS scores. In multivariable models, with the same set of confounding variables, the modified WFNS grade of III to V (adjusted odds ratio, AOR: 9.090; 95% CI: 3.494-23.648; P<0.001) was more strongly associated with the increased risk of the 90-day poor outcome compared to the WFNS grade of IV to V (AOR: 6.383; 95% CI: 2.661-15.310; P<0.001) or the H&H grade of IV to V (AOR: 6.146; 95% CI: 2.584-14.620; P<0.001). In this study, the modified WFNS, WFNS, and H&H scales all had good discriminatory abilities for the prognosis of patients with aSAH. Because of the better effect size in predicting poor outcomes, the modified WFNS scale seems preferable to the WFNS and H&H scales.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Potential induced degradation (PID) has been affecting the performance, durability, and reliability of crystalline Si solar cells/modules. The authors demonstrate that micro‐cracks also act as the ...additional recombination centers, which reduce short‐circuit current density (Jsc), open‐circuit voltage (Voc), and the effective lifetime of carriers in solar cells, in PID condition. This hypothesis was confirmed by external quantum efficiency measurements and microwave photo‐conductance decay method at the micro‐cracked areas before and after PID stress tests. In addition, the PID recovery was made significantly but non‐fully for the laminated micro‐cracked modules owing to the electrical PID recovery method. In addition, the authors showed and discussed the challenging problems in recovering the performance of the PID‐affected micro‐cracked solar modules without lamination. The achieved results from 11 cycles of PID stress/recovery tests with the same duration of PID stress and recovery processes showed that an exponentially decreasing function fits the correlation between the relative losses of Pmax, Voc, fill factor (FF), and Jsc due to the PID stress process and the number of the PID stress/recovery cycles. This provides a model for the estimation of the relative losses of Pmax, Voc, FF, and Jsc due to the PID stress process versus the number of the PID stress/recovery cycles. Finally, based on the decreasing tendency in the performance loss of the PID‐affected laminated micro‐cracked solar modules after PID stress/recovery cycles, the authors suggest that cyclic PID tests with suitable conditions might be a reasonable approach to control PID.
PID recovery was made significantly but incompletely for both laminated intact and micro‐cracked solar cells. A model for the estimation of the relative losses of Pmax, Voc, FF, and Jsc due to the PID stress process versus the number of PID stress/recovery cycles was established based on an exponentially decreasing function. Also, the model was supported by the micro‐PCD response and the effective carrier's lifetime value.
Two experimental configurations of a hybrid K-band (25 GHz) microwave photonic link (MPL) are investigated for seamless broadband wireless access networks. Experimental configurations consist of ...optical fiber, free-space optics (FSO) and radio frequency (RF) wireless channels. We analyze in detail the effects of channel impairments, namely fiber chromatic dispersion, atmospheric turbulence and multipath-induced fading on the transmission performance. In the first configuration, transmission of the 64-quadrature amplitude modulation (QAM) signal with 5, 20 and 50 MHz bandwidths over 5 km standard single-mode fiber (SSMF), 2 m turbulent FSO and 3 m RF wireless channels is investigated. We show that, for QAM with a high bandwidth, the link performance is being affected more by atmospheric turbulence. In the second configuration, the 20 MHz 4/16/64-QAM signals over a 50 km SSMF and 40 m FSO/RF wireless links are successfully transmitted with the measured error vector magnitude (EVM) values of 12, 9 and 7.9%, respectively. It is shown that, for all transmitted microwave vector signals, the bit error rate is lower than the hard-decision forward-error-correction limit of 3.8×10
. Moreover, an extended FSO link span of 500 m for 25 GHz hybrid MPL with 16-QAM at 10 Gb/s under the weak and strong turbulence regimes is evaluated via simulation analysis to mimic a practical outdoor system.
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