It has always been a serious challenge to design efficient, selective, and stable absorbents for heavy-metal removal. Herein, we design layered double hydroxide (LDH)-based Fe-MoS4, a highly efficient adsorbent, for selective removal of heavy metals. We initially synthesized FeMgAl-LDH and then enriched its protective layers with MoS42– anions as efficient binding sites for heavy metals. Various characterization tools, such as X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray, X-ray photoelectron spectroscopy (XPS), CHN analysis, and inductively coupled plasma analysis, were applied to confirm structural and compositional changes during the synthesis of Fe-MoS4 as final product. The prepared Fe-MoS4 offered excellent attraction for heavy metals, such as Hg2+, Ag+, Pb2+, and Cu2+, and displayed selectivity in the order Hg2+ ∼ Ag+ > Pb2+ > Cu2+ > Cr6+ > As3+ > Ni2+ ∼ Zn2+ ∼ Co2+. The immense capacities of Hg2+, Ag+, and Pb2+ (583, 565, and 346 mg/g, respectively), high distribution coefficient (Kd ∼ 107–108), and fast kinetics place Fe-MoS4 on the top of materials list known for removal of such metals. The sorption kinetics and isothermal studies conducted on Hg2+, Ag+, Pb2+, and Cu2+ suit well pseudo-second-order kinetics and Langmuir model, suggesting monolayer chemisorption mechanism through M–S linkages. XRD and FTIR studies suggested that adsorbed metals could result as coordinated complexes in LDH interlayer region. More interestingly, LDH structure offers protective space for MoS42– anions to avoid oxidation under ambient environments, as confirmed by XPS studies. These features provide Fe-MoS4 with enormous capacity, good reusability, and excellent selectivity even in the presence of huge concentration of common cations.Keywords: adsorption; complexation mechanism; Fe-MoS4 LDH; heavy-metal removal; wastewater;

Magnetic sludge-derived biochar (MSDBC) was synthetized via a one-step co-precipitation method and conducted as a novel heterogeneous catalyst of persulfate (PS) activation for the oxidative removal of acid orange 7 (AO7). The porous structure and large surface area benefits the enrichment of the pollutant, while abundant Fe3O4 species and oxygen-containing functional groups promoted the generation of oxidative radicals, thus leading to the remarkable performance of AO7 removal. MSDBC also exhibited good stability with low iron leaching and consistent efficiency in reusability experiments. Radical scavenger experiments and electron paramagnetic resonance studies identified SO4˙− and OH˙ as the dominant oxidative radicals. The magnetic properties and feasible preparation method of MSDBC guaranteed the stability, which was evidenced in detail by the satisfactory reusability performance and low iron leaching during the degradation process. Distinguished from other PS based advanced oxidation processes, acidic conditions favored AO7 removal, while two halide irons Cl− and Br− could promote AO7 removal by MSDBC/PS system. The current outcomes demonstrated our approach of converting solid waste into stable, cheap and multifunctional biochar as a feasible resource utilization method, and was highly suggestive to the treatment of both wastewater and sewage sludge.

•MSSBC is prepared from municipal sewage sludge by a feasible process.•Porous MSSBC contains copious active sites and magnetic property.•Multiple interactions between lead and MSSBC guarantee the removal efficiency.•Waste control by waste is suggestive to the treatment of both sewage sludge and wastewater.Highly efficient magnetic sewage sludge biochar (MSSBC) discloses feasible fabrication process with lower production cost, superior adsorption capacity, usage of waste sewage sludge as resource, selected by external magnetic field and exceptional regeneration property. 2 g L−1 MSSBC exhibited a high adsorption capacity of 249.00 mg g−1 in 200 ppm Pb(II) and the lead-MSSBC equilibrium was achieved within one hour, owing to the existence of the copious active sites. The adsorption kinetics was well described by the pseudo-second-order model while the adsorption isotherm could be fitted by Langmuir model. Mechanism study demonstrated the adsorption involved electrostatic attraction, ion exchange, inner-sphere complexation and formation of co-precipitates at the surface of MSSBC. Additionally, adsorption performance maintained remarkable in a broad pH window. These outcomes demonstrated the promising waste resource utilization by a feasible approach that turns the solid waste of sewage sludge into biochar adsorbent with auspicious applications in elimination of Pb(II) from wastewater.Download high-res image (110KB)Download full-size image

•Lewis and Brønsted acids can sharply improve the oxygen transfer efficiency of a manganese(II) catalyst with non-heme ligand.•The catalytic activity improvement is acidity strength or net charge dependent.•The promotional effect by either Lewis acid or Brønsted acid was originated from the dissociating sluggish di-μ-oxo core.•Distinctions of reactive intermediate were also demonstrated for Lewis acid or Brønsted acid.This work demonstrates that certain Lewis and Brønsted acids can sharply improve the oxygen transfer efficiency of a manganese(II) catalyst bearing non-heme ligand. In the absence of Lewis and Brønsted acids, oxidation of manganese(II) complex will generate di-μ-oxo-bridged dinuclear Mn2(III,IV) core which is very sluggish for olefin epoxidation. Adding non-redox metal ions as Lewis acid or Brønsted acid will both improve the catalytic epoxidation of olefin, and this improvement is dependent on the pKa of Brønsted acid, or the net charge of non-redox metals of Lewis acid. Mechanism study revealed that similar promotional effect by either Lewis or Brønsted acids was originated from a similar reaction pathway by dissociating aforementioned sluggish di-μ-oxo core. However, distinctions of reactive intermediate were also demonstrated for Lewis or Brønsted acids.Download full-size image

Non-redox metal ions can affect the reactivity of active redox metal ions in versatile biological and heterogeneous oxidation processes; however, the intrinsic roles of these non-redox ions still remain elusive. This work demonstrates the first example of the use of non-redox metal ions as Lewis acids to sharply improve the catalytic oxygen atom transfer efficiency of a ruthenium complex bearing the classic 2,2′-bipyridine ligand. In the absence of Lewis acid, the oxidation of ruthenium(II) complex by PhI(OAc)2 generates the Ru(IV)O species, which is very sluggish for olefin epoxidation. When Ru(bpy)2Cl2 was tested as a catalyst alone, only 21.2% of cyclooctene was converted, and the yield of 1,2-epoxycyclooctane was only 6.7%. As evidenced by electronic absorption spectra and EPR studies, both the oxidation of Ru(II) by PhI(OAc)2 and the reduction of Ru(IV)O by olefin are kinetically slow. However, adding non-redox metal ions such as Al(III) can sharply improve the oxygen transfer efficiency of the catalyst to 100% conversion with 89.9% yield of epoxide under identical conditions. Through various spectroscopic characterizations, an adduct of Ru(IV)O with Al(III), Ru(IV)O/Al(III), was proposed to serve as the active species for epoxidation, which in turn generated a Ru(III)–O–Ru(III) dimer as the reduced form. In particular, both the oxygen transfer from Ru(IV)O/Al(III) to olefin and the oxidation of Ru(III)–O–Ru(III) back to the active Ru(IV)O/Al(III) species in the catalytic cycle can be remarkably accelerated by adding a non-redox metal, such as Al(III). These results have important implications for the role played by non-redox metal ions in catalytic oxidation at redox metal centers as well as for the understanding of the redox mechanism of ruthenium catalysts in the oxygen atom transfer reaction.

Redox-inactive metal ions can modulate the reactivity of redox-active metal ions in a variety of biological and chemical oxidations. Many synthetic models have been developed to help address the elusive roles of these redox-inactive metal ions. Using a non-heme manganese(II) complex as the model, the influence of redox-inactive metal ions as a Lewis acid on its catalytic efficiency in oxygen atom transfer was investigated. In the absence of redox-inactive metal ions, the manganese(II) catalyst is very sluggish, for example, in cyclooctene epoxidation, providing only 9.9% conversion with 4.1% yield of epoxide. However, addition of 2 equiv. of Al3+ to the manganese(II) catalyst sharply improves the epoxidation, providing up to 97.8% conversion with 91.4% yield of epoxide. EPR studies of the manganese(II) catalyst in the presence of an oxidant reveal a 16-line hyperfine structure centered at g = 2.0, clearly indicating the formation of a mixed valent di-μ-oxo-bridged diamond core, MnIII-(μ-O)2-MnIV. The presence of a Lewis acid like Al3+ causes the dissociation of this diamond MnIII-(μ-O)2-MnIV core to form monomeric manganese(IV) species which is responsible for improved epoxidation efficiency. This promotional effect has also been observed in other manganese complexes bearing various non-heme ligands. The findings presented here have provided a promising strategy to explore the catalytic reactivity of some di-μ-oxo-bridged complexes by adding non-redox metal ions to in situ dissociate those dimeric cores and may also provide clues to understand the mechanism of methane monooxygenase which has a similar diiron diamond core as the intermediate.

•Synergetic effect led to remarkably improved performances in various dehydrogenation reactions.•The catalytic activity improvement is Lewis acidity strength dependent.•Dioxygen was used as the solely terminal oxidant under mild conditions.•An adduct of Ru(IV) = O/Sc(III) was proposed as the key active species to improve the catalytic efficiency.Dioxygen activation as the solely terminal oxidant in organic synthesis and catalytic oxidation is particularly attractive from the point of economic and environmental view. In our previous study, we have displayed that the introducing of non-redox metal ions can sharply improve the olefin epoxidation catalyzed by ruthenium complex with PhI(OAc)2 as the organic oxidation. Inspired by the successful strategy and dioxygen activation, in this study, we demonstrate an alternative protocol that adding non-redox metal ions to cis-Ru(bpy)2Cl2 catalyst can remarkably improve the oxidation from Ru(II) to Ru(IV) = O with dioxygen as the oxidant and promote oxidative dehydrogenation of saturated C–C bond whereas Ru(bpy)2Cl2 alone is very sluggish. Through UV–Vis, NMR, cyclic voltammogram and EPR characterization, it has been testified that oxidation from Ru(II) to Ru(IV) = O can be realized under the condition of 1 atm of dioxygen and 323 K within 25 min while extreme tough condition of 20 atm of dioxygen, 343 K and 8 h is necessary for Ru(bpy)2Cl2 alone. Combined the reaction data with characterization results, an adduct of Ru(IV) = O/Sc(III) is proposed as the key active species to improve the catalytic efficiency. The activity improvement in oxidative dehydrogenation illustrated a novel strategy of that adding non-redox metal ions to sluggish catalysts can remarkably improve its efficiency for dehydrogenation of saturated C–C bond, especially through dioxygen as ultimate oxidant.Download full-size image

•A mesoporous CuCo@MnO2 nano-rod catalyst was prepared and characterized.•A synergistic degradation was investigated from Cu and Co oxides.•Activation pathway of PMS and oxidative radicals were proposed.•Redox cycle of M2+ ↔ M3+ ↔ M2+ and Cu enhanced generation of Co(II)-OH are critical for the synergistic effect.The development of transition metal based heterogeneous catalysts with efficient reactivity and intensive stability is of great demand in peroxymonosulfate based AOPs in water treatment. Herein, we present a novel approach of creating stable and effective nano-rod catalyst of CuCo@MnO2 with tetragonal structure. A remarkable synergetic effect was found between bi-metallic oxides of Cu and Co: 0.5%Cu-2%Co-MnO2 can efficiently degrade 100% of 30 ppm phenol, while 0.5%Cu@MnO2 or 2%Co@MnO2 alone is apparently sluggish for the degradation of organic contaminants. The nanocatalyst retained good stability in recycling tests, during which little leaching of Co and Cu ions can be detected and crystallinity of support α˗MnO2 remained unchanged. Mechanism study indicated that SO4− and OH are accounted to participate the degradation, and the generation of radicals is originated from the interaction of CuCo@MnO2 and PMS through metal site with peroxo species bond. The redox cycle among the active metals (M2+ ↔ M3+ ↔ M2+) and Cu enhanced generation of Co(II)–OH complex are critical for the remarkable performance in CuCo@MnO2/PMS system. Both the synergetic acceleration of catalyst activity and instinct mechanism are highly suggestive to the design of heterogeneous catalysts for the degradation of organic contaminants in PMS based advanced oxidation processes.Download high-res image (273KB)Download full-size image

•Pd catalyst based AOP is utilized for landfill leachate degradation.•H2O2 is generated in-situ by H2 and O2 by Pd catalyst.•Fe2+ consumption is reduced while COD removal is maintained compared with traditional Fenton technology.•High stability of Pd catalyst ensures the efficiency and economic advantages of this technology comparing with other AOPs.Municipal solid waste (MSW) leachate contains various refractory pollutants that pose potential threats to both surface water and groundwater. This paper established a novel catalytic oxidation process for leachate treatment, in which OH is generated in situ by pumping both H2 and O2 in the presence of Pd catalyst and Fe2+. Volatile fatty acids in the leachate were removed almost completely by aeration and/or mechanical mixing. In this approach, a maximum COD removal of 56.7% can be achieved after 4 h when 200 mg/L Fe2+ and 1250 mg/L Pd/Al2O3 (pH 3.0) are used as catalysts. After oxidation, the BOD/COD ratio in the proposed process increased from 0.03 to 0.25, indicating that the biodegradability of the leachate was improved. By comparing the efficiency on COD removal and economical aspect of the proposed Pd-based in-situ process with traditional Fenton, electro-Fenton and UV-Fenton for leachate treatments, the proposed Pd-based in-situ process has potential economic advantages over other advanced oxidation processes while the COD removal efficiency was maintained.

Redox-inactive metal ions can modulate the reactivity of redox-active metal ions in a variety of biological and chemical oxidations. Many synthetic models have been developed to help address the elusive roles of these redox-inactive metal ions. Using a non-heme manganese(II) complex as the model, the influence of redox-inactive metal ions as a Lewis acid on its catalytic efficiency in oxygen atom transfer was investigated. In the absence of redox-inactive metal ions, the manganese(II) catalyst is very sluggish, for example, in cyclooctene epoxidation, providing only 9.9% conversion with 4.1% yield of epoxide. However, addition of 2 equiv. of Al3+ to the manganese(II) catalyst sharply improves the epoxidation, providing up to 97.8% conversion with 91.4% yield of epoxide. EPR studies of the manganese(II) catalyst in the presence of an oxidant reveal a 16-line hyperfine structure centered at g = 2.0, clearly indicating the formation of a mixed valent di-μ-oxo-bridged diamond core, MnIII-(μ-O)2-MnIV. The presence of a Lewis acid like Al3+ causes the dissociation of this diamond MnIII-(μ-O)2-MnIV core to form monomeric manganese(IV) species which is responsible for improved epoxidation efficiency. This promotional effect has also been observed in other manganese complexes bearing various non-heme ligands. The findings presented here have provided a promising strategy to explore the catalytic reactivity of some di-μ-oxo-bridged complexes by adding non-redox metal ions to in situ dissociate those dimeric cores and may also provide clues to understand the mechanism of methane monooxygenase which has a similar diiron diamond core as the intermediate.

Non-redox metal ions can affect the reactivity of active redox metal ions in versatile biological and heterogeneous oxidation processes; however, the intrinsic roles of these non-redox ions still remain elusive. This work demonstrates the first example of the use of non-redox metal ions as Lewis acids to sharply improve the catalytic oxygen atom transfer efficiency of a ruthenium complex bearing the classic 2,2′-bipyridine ligand. In the absence of Lewis acid, the oxidation of ruthenium(II) complex by PhI(OAc)2 generates the Ru(IV)O species, which is very sluggish for olefin epoxidation. When Ru(bpy)2Cl2 was tested as a catalyst alone, only 21.2% of cyclooctene was converted, and the yield of 1,2-epoxycyclooctane was only 6.7%. As evidenced by electronic absorption spectra and EPR studies, both the oxidation of Ru(II) by PhI(OAc)2 and the reduction of Ru(IV)O by olefin are kinetically slow. However, adding non-redox metal ions such as Al(III) can sharply improve the oxygen transfer efficiency of the catalyst to 100% conversion with 89.9% yield of epoxide under identical conditions. Through various spectroscopic characterizations, an adduct of Ru(IV)O with Al(III), Ru(IV)O/Al(III), was proposed to serve as the active species for epoxidation, which in turn generated a Ru(III)–O–Ru(III) dimer as the reduced form. In particular, both the oxygen transfer from Ru(IV)O/Al(III) to olefin and the oxidation of Ru(III)–O–Ru(III) back to the active Ru(IV)O/Al(III) species in the catalytic cycle can be remarkably accelerated by adding a non-redox metal, such as Al(III). These results have important implications for the role played by non-redox metal ions in catalytic oxidation at redox metal centers as well as for the understanding of the redox mechanism of ruthenium catalysts in the oxygen atom transfer reaction.