Jizhi Zhou

A Heterostructure Coupling of Exfoliated Ni–Fe Hydroxide Nanosheet and Defective Graphene as a Bifunctional Electrocatalyst for Overall Water Splitting

Herein, the authors demonstrate a heterostructured NiFe LDH-NS@DG10 hybrid catalyst by coupling of exfoliated Ni-Fe layered double hydroxide (LDH) nanosheet (NS) and defective graphene (DG). The catalyst has exhibited extremely high electrocatalytic activity for oxygen evolution reaction (OER) in an alkaline solution with an overpotential of 0.21 V at a current density of 10 mA cm-2 , which is comparable to the current record (≈0.20 V in Fe-Co-Ni metal-oxide-film system) and superior to all other non-noble metal catalysts. Also, it possesses outstanding kinetics (Tafel slope of 52 mV dec-1 ) for the reaction. Interestingly, the NiFe LDH-NS@DG10 hybrid has also exhibited the high hydrogen evolution reaction (HER) performance in an alkaline solution (with an overpotential of 115 mV by 2 mg cm-2 loading at a current density of 20 mA cm-2 ) in contrast to barely HER activity for NiFe LDH-NS itself. As a result, the bifunctional catalyst the authors developed can achieve a current density of 20 mA cm-2 by a voltage of only 1.5 V, which is also a record for the overall water splitting. Density functional theory calculation reveals that the synergetic effects of highly exposed 3d transition metal atoms and carbon defects are essential for the bifunctional activity for OER and HER.

Removal efficiency of arsenate and phosphate from aqueous solution using layered double hydroxide materials: intercalation vs. precipitation

Adsorption behaviours of arsenate and phosphate over Mg-based and Ca-based layered double hydroxide (LDH) adsorbents have been examined in kinetics and thermodynamics. Removal of these anions from aqueous solution follows the Lagergren first-order and/or pseudo-second-order model, and the adsorption isotherm is well fitted with either the Langmuir or the Freundlich model. Structure analysis of used LDH adsorbents reveals that two processes, i.e. intercalation and precipitation, are responsible for the anion removal. Adsorption over the Mg-based LDH adsorbent occurs by way of intercalation into the interlayer spacing while that over the Ca-based adsorbent occurs by means of precipitation of dissolved Ca2+ with the anion. More particularly, we have found that As(V) at a concentration below 10 mg/L can be very efficiently removed through intercalation into the interlayer of reconstructed MgAl-LDHs, with less than 0.010 mg/L of As left in solution. We have also noted that phosphate at [P] up to 100 mg/L can be quickly and effectively removed through precipitation with CaAl-Cl-LDH, giving rise to ∼0.1 mg/L of P left in solution with the maximum adsorption amount up to 135 mg/g. Therefore, these two LDH materials (calcined Mg3Al-CO3-LDH and uncalcined Ca2Al-Cl-LDH) are potential cost-effective adsorbents for arsenate and phosphate, respectively.

Efficient removal of dyes by a novel magnetic Fe3O4/ZnCr-layered double hydroxide adsorbent from heavy metal wastewater

A novel magnetic Fe(3)O(4)/ZnCr-layered double hydroxide adsorbent was produced from electroplating wastewater and pickling waste liquor via a two-step microwave hydrothermal method. Adsorption of methyl orange (MO) from water was studied using this material. The effects of three variables have been investigated by a single-factor method. The response surface methodology (RSM) based on Box-Behnken design was successfully applied to the optimization of the preparation conditions. The maximum adsorption capacity of MO was found to be 240.16 mg/g, indicating that this material may be an effective adsorbent. It was shown that 99% of heavy metal ions (Fe(2+), Fe(3+), Cr(3+), and Zn(2+)) can be effectively removed into precipitates and released far less in the adsorption process. In addition, this material with adsorbed dye can be easily separated by a magnetic field and recycled after catalytic regeneration with advanced oxidation technology. Meanwhile, kinetic models, FTIR spectra and X-ray diffraction pattern were applied to the experimental data to examine uptake mechanism. The boundary layer and intra-particle diffusion played important roles in the adsorption mechanisms.

Effective removal and fixation of Cr(VI) from aqueous solution with Friedel's salt.

Friedels salt (3CaO x Al2O3 x CaCl2 x 10 H2O or Ca4Al2(OH)12Cl2(H2O)4) is a calcium aluminate hydrate formed by hydrating cement or concrete in seawater at a low cost. In the current study, we carefully examined the adsorption behaviors of Friedels salt for Cr(VI) from aqueous solution at different concentrations and various initial pHs. The adsorption kinetic data are well fitted with the pseudo-first-order Lageren equation at the initial Cr(VI) concentration from 0.10 to 8.00 mM. Both the experimental and modeled data indicate that Friedels salt can adsorb a large amount of Cr(VI) (up to 1.4 mmol Cr(VI)/g) very quickly (t1/2 = 2-3 min) with a very high efficiency (>99% Cr(VI) removal at [Cr] < 4.00 mM with 4.00 g/L of adsorbent) in the pH range of 4-10. In particular, the competitive adsorption tests show that the Cr(VI) removal efficiency is only slightly affected by the co-existence of Cl(-) and HCO3(-). The Cr(VI)-fixation stability tests show that only less than 0.2% adsorbed Cr(VI) is leaching out in water at pH 4-10 for 24 h because the adsorption/exchange of Cr(VI) with Friedels salt leads to the formation of a new stable phase (3CaO x Al2O3 x CaCrO4 x 10 H2O). This research thus suggests that Friedels salt is a potential cost-effective adsorbent for Cr(VI) removal in wastewater treatment.

Chemical characteristics and risk assessment of typical municipal solid waste incineration (MSWI) fly ash in China.

The release of heavy metals in municipal solid waste incineration (MSWI) fly ash has become a worrying issue while fly ash is utilized or landfilled. This work investigated the potential mobility of heavy metals in the fly ashes from 15 typical MSWI plants in Chinese mainland by the characterization of distribution, chemical speciation and leaching behavior of heavy metals. The results showed that total content of heavy metals decreased in the order Zn>Pb>Cu>Cr>Ni>Cd in samples. The toxicity characteristics leaching procedure (TCLP) of fly ash indicated that the amount of leached Cd in 67% of samples exceeded the regulated limit. Also, the excess amount of leached Zn and Pb was observed in 40% and 53% of samples, respectively. The chemical speciation analysis revealed that this excess of heavy metal leached in TCLP was contributed to the high content of acid soluble fraction (F1) and reducible fraction (F2) of heavy metal. Moreover, the great positive relevance between leaching behavior of heavy metals and F1 fraction was supported by principal component analysis (PCA). Risk assessment code (RAC) results suggested that Cd and Pb showed a very high risk class to the environment.

Enhanced removal of triphosphate by MgCaFe-Cl-LDH: Synergism of precipitation with intercalation and surface uptake

Triphosphate (TPP) is an important form of phosphate pollutants while its removal investigation has been just started now. This research examined the removal of triphosphate using Mg(2-x)Ca(x)FeCl-LDH (x = 0-2) as absorbents. We found that the removal of triphosphate over Mg(2)FeCl-LDH mainly underwent the surface adsorption and the near-edge intercalation, with the practical removal amount (9-11 mg(P)/g) corresponding to 10-15% of the theoretical one. In contrast, Ca(2)FeCl-LDH removed a higher amount of triphosphate (56.4 mg(P)/g). The comprehensive analysis of the triphosphate-uptake products with XRD/XPS/FTIR reveals that Ca(2)FeCl-LDH dissolves first and then released Ca(2+) ions react with triphosphate (TPP) to form insoluble Ca-TPP precipitate. Combination of these two different removal mechanisms enables Mg(0.5)Ca(1.5)FeCl-LDH to take up 84.2mg(P)/g from aqueous solution under similar conditions.

Effective removal of selenate from aqueous solutions by the Friedel phase

This research has demonstrated that the Friedel phase, e.g. a chloride-containing hydrocalumite (Ca(2)Al(OH)(6)Cl(H(2)O)(2) x mH(2)O), can rapidly adsorb large amounts of SeO(4)(2-) (up to 1.37 mmol/g). SeO(4)(2-) is removed via anionic exchange, as evidenced by the expansion of the d-spacing from 0.78 nm of Cl-hydrocalumite to 0.97-0.98 nm of SeO(4)-hydrocalumite. The newly formed SeO(4)-adsorbed hydrocalumite is stable in water at pH 4-13, indicating the strong fixation of selenate within the phase. In contrast, intercalated selenate in the Freidel phase can be recovered by desorbing in the NaCl solution, which can also regenerate and recycle the used adsorbent. The findings in this research strongly suggest that the Friedel phase is a new, environmentally friendly and cost-effective adsorbent to adsorb selenate from wastewater streams and dilute solutions.

Inhibitory effect of high-strength ammonia nitrogen on bio-treatment of landfill leachate using EGSB reactor under mesophilic and atmospheric conditions

The inhibitory effect of high-strength NH(3)-N on anaerobic biodegradation of landfill leachates in an EGSB bioreactor has been investigated. The research compared start-up performance of the reactor treating the landfill leachate with NH(3)-N in 242-1200 mg/l to that treating the compost leachate with NH(3)-N in 38-410 mg/l. The observations showed that the performance of the reactor treating the landfill leachate was only marginally worse than that treating the compost leachate at the mesophilic temperature when NH(3)-N concentration was under 1500 mg/l. We also noted that NH(3)-N at the concentration of 1500-3000 mg/l inhibited the biodegradation. The comparative biodegradation performance at the mesophilic and atmospheric temperature demonstrated that the maximal OLR of atmospheric digestion was only reduced to 44 kg COD/m(3)d. These findings indicate that landfill leachates with NH(3)-N less than 1500 mg/l could be efficiently treated in the EGSB bioreactor even under the atmospheric condition with methane generated.

Characteristics of dioxins content in fly ash from municipal solid waste incinerators in China

MSWI fly ashes sampled from 15 large-scale commercial municipal solid waste incineration plants in China were analyzed for seventeen polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/PCDFs) as well as twelve dioxin-like polychlorinated biphenyls (dl-PCBs). The concentration of PCDD/PCDFs and dl-PCBs in fly ash samples ranged from 2.8 to 190ngg(-1), and 59.6ngg(-1) on average. The toxic equivalent (TEQ) ranged from 34 to 2500ng WHO(2005)-PCDD/PCDF-PCB-TEQkg(-1), and 790ng WHO(2005)-PCDD/PCDF-PCB-TEQkg(-1) on average. For PCDDs, hexa-chlorinated homolog was the dominant compound except two fly ash samples. Tetra-chlorinated homolog was dominant for PCDFs except one sample. The ratio of PCDDs/PCDFs ranged from 0.32 to 2.44 (average 0.97). The contribution of dl-PCBs to total concentration and TEQ was relatively minimal. Correlation between the concentration of three congeners and total TEQ values of fly ashes was also established. The findings obtained in this work provided overview information on the PCDD/PCDF-PCB content characterization of MSWI fly ash in China, which can be available for MSWI fly ash management in the environment.

Stabilization of chromium-bearing electroplating sludge with MSWI fly ash-based Friedel matrices.

This work investigated the feasibility and effectiveness of MSWI fly ash-based Friedel matrices on stabilizing/solidifying industrial chromium-bearing electroplating sludge using MSWI fly ash as the main raw material with a small addition of active aluminum. The compressive strength, leaching behavior and chemical speciation of heavy metals and hydration phases of matrices were characterized by TCLP, XRD, FTIR and other experimental methods. The results revealed that MSWI fly ash-based Friedel matrices could effectively stabilize chromium-bearing electroplating sludge, the formed ettringite and Friedel phases played a significant role in the fixation of heavy metals in electroplating sludge. The co-disposal of chromium-bearing electroplating sludge and MSWI fly ash-based Friedel matrices with a small addition of active aluminum is promising to be an effective way of stabilizing chromium-bearing electroplating sludge.