Selective oxidation decontamination in cobalt molybdate activated Fenton-like oxidation via synergic effect of cobalt and molybdenum.

In this study, cobalt molybdate (CoMoO4) activated peracetic acid (PAA) was developed for water purification. CoMoO4/PAA system could remove 95% SMX with pseudo-first-order reaction rate constant of 0.15410 min-1, which was much higher than CoFe2O4/PAA, FeMoO4/PAA, and CoMoO4/persulfate systems. CoMoO4/PAA system follows a non-radical species pathway dominated by the high-valent cobalt (Co(IV)), and CH3C(O)OO• shows a minor contribution to decontamination. Density functional theory (DFT) calculation indicates that the generation of Co(IV) is thermodynamically more favorable than CH3C(O)OO• generation. The abundant Co(IV) generation was attributed to the special structure of CoMoO4 and effect of molybdenum on redox cycle of Co(II)/Co(III). DFT calculation showed that the atoms of SMX with higher ƒ0 and ƒ- values are the main attack sites, which are in accordance with the results of degradation byproducts. CoMoO4/PAA system can effectively reduce biological toxicity after the reaction. Benefiting from the selective of Co(IV) and CH3C(O)OO•, the established CoMoO4/PAA system exhibits excellent anti-interference capacity and satisfactory decontamination performance under actual water conditions. Furthermore, the system was capable of good potential practical application for efficient removal of various organics and favorable reuse. Overall, this study provides a new strategy by CoMoO4 activated PAA for decontamination with high efficiency, high selectivity and favorable anti-interference.

Effective oxidation and adsorption of As(III) in water by nanoconfined Ce-Mn binary oxides with excellent reusability.

Herein, a highly efficient As(III) purifier Ce-Mn@N201 with excellent reusability was developed by stepwise precipitating hydrated cerium(IV) oxides (HCO) and hydrated manganese(IV) oxides (HMO) inside N201, a widely-used gel-type anion exchange resin. Owing to confinement of unique nanopores in N201, the in-situ generated nanoparticles (NPs) inside Ce-Mn@N201 were highly dispersed with ultra-small sizes of around 2.6 nm. Results demonstrated that HMO NPs effectively oxidized As(III) to As(V) with the conversion of Mn(IV) to Mn(II), while the generated Mn2+ was mostly re-adsorbed onto the negatively-charged surface of HMO NPs. During the regeneration process by simple alkaline treatment, the re-adsorbed Mn2+ was firstly precipitated as (hydr)oxides of Mn(II) and then oxidized to HMO NPs by dissolved oxygen to fully refresh its oxidation capacity. Though HCO NPs mainly served as adsorbent for arsenic, they could partially oxidize As(III) to As(V) at the beginning, while the oxidation capacities continuously diminished with the irreversible conversion of Ce(IV) to Ce(III). In 10 consecutive adsorption-regeneration cycle, Ce-Mn@N201 efficiently decontaminated As(III) from 500 µg/L to below 5 µg/L with Mn2+ leaching less than 0.3% per batch. During 3 cyclic fixed-bed adsorptions, Ce-Mn@N201 steadily produced 8500-9150 bed volume (BV) and 3150-3350 BV drinkable water from the synthesized and real groundwater, respectively, with Mn leaching in effluent constantly < 100 µg/L.

Mechanistic insight into selective adsorption and easy regeneration of carboxyl-functionalized MOFs towards heavy metals.

The development of heavy metal adsorbents with high selectivity has become a research hotspot due to the interference of coexisting ions (e.g., Na+, Ca2+) in the actual wastewater, but the more difficult regeneration caused by high adsorption selectivity severely limits its practical applications. Herein, a carboxyl adsorbent, MIL-121, demonstrated high adsorption selectivity for heavy metals at 10,000 mg/L of Na+ (removal > 99% for Cu2+) as well as unexpected easy regeneration (desorption > 99%) at low H+ concentration (10-3.5-10-3.0 M), which is hundreds of times lower than that of ever reported selective adsorbents (> 10-1 M H+). X-ray photoelectron spectrometry (XPS), extended X-ray absorption fine structure (EXAFS) coupled with Density functional theory (DFT) simulation unveil that the -COOH groups in MIL-121 for heavy metals adsorption is specific inner-sphere coordination with higher binding energy (1.31 eV for Cu), and less energy required for regeneration (0.26 eV for H). Similar high selectivity and easy regeneration were also satisfied with other heavy metals (e.g., Pb2+, Ni2+), and removal of heavy metals remained > 99% in 10 consecutive adsorption-desorption cycles. For actual copper electroplating wastewater treatment, MIL-121 could produce ~ 3600 mL clean water/g sample, outperforming 300 mL that of the benchmark commercial adsorbent D-113. This study shows the potential of MIL-121 for heavy metal wastewater treatment and provides mechanistic insight for developing adsorbents with high selective adsorption and easy regeneration.

Iron-based metal-organic framework derived pyrolytic materials for effective Fenton-like catalysis: Performance, mechanisms and practicability.

In this study, a new catalyst was fabricated by pyrolysis under nitrogen atmosphere with MIL-53(Fe) as the precursor, and was applied to catalyze Fenton-like process. Effects of calcination temperature and pH on decontamination performance, and stability of materials were investigated. Under optimal conditions (calcination temperature of 500 °C and pH of 5.0), the new Fenton-like system remained low iron leaching, and achieved high pseudo-first-order rate constant of 0.0251 min-1 for bisphenol S (BPS) removal, which is much higher than those in MIL-53(Fe), and nano-Fe3O4 catalyzed Fenton-like systems. The superiority of the new catalyst for Fenton-like catalysis was attributed to high specific surface area, as well as formed Fe(II), coordinatively unsaturated iron center and the Fe-O/Fe-C compounds based on the analyses of characterizations. Furthermore, main active species for BPS degradation was identified as hydroxyl radicals, and total hydroxyl radical generation was determined by trapping experiments. The degradation pathways of BPS were also proposed by intermediates monitoring. Moreover, this catalyst showed good potential for practical application, according to the evaluation of reuse, different pollutants degradation, and BPS removal in real wastewater. We believe this study developed a new catalyst with high catalytic activity, high stability and wide application scope, and also sheds light on further development of metal-organic frameworks for Fenton-like catalysis.

High-Efficiency and Sustainable Desalination Using Thermo-regenerable MOF-808-EDTA: Temperature-Regulated Proton Transfer.

Adsorption as a desalination approach has the advantages of energy efficiency, low cost, and operational convenience, but its practical application is limited by low desalination capacity, consumption/disposal of strong acids/bases as regeneration reagents, and poor reusability. Herein, we synthesized a thermo-regenerable salt absorbent by grafting ethylenediaminetetraacetic acid (EDTA) onto a metal-organic framework (MOF), MOF-808-EDTA, which could rapidly adsorb NaCl within 30 min from saline water at 25 °C with a desalination capacity as high as 9.4 mmol/g. Moreover, the saturated adsorbent could be facilely regenerated in 80 °C water. Fourier transform infrared spectroscopy and derivative thermogravimetry revealed that temperature-regulated proton transfer between amino and carboxyl groups was the mechanism of thermo-regeneration. EDTA on MOF-808-EDTA appears in a zwitterionic state in water at room temperature, which allowed simultaneous adsorption of Na+ and Cl-. At elevated temperature, it returned to a nonionic state accompanied by the desorption of ions. A similar temperature-dependent adsorption-regeneration process was also observed for other salts, including LiCl, KCl, CaCl2, and MgCl2. Column experiments of brackish groundwater showed that 1 g of MOF-808-EDTA could produce ∼106 mL of fresh water (total dissolved solids < 600 mg/L) without significant capacity loss after 10 successive adsorption-regeneration cycles. This study is the first to propose an EDTA-based MOF for desalination and indicates the potential of MOF-808-EDTA as a green adsorbent for sustainable water desalination.

Temperature regulated adsorption and desorption of heavy metals to A-MIL-121: Mechanisms and the role of exchangeable protons.

Adsorption is a viable technology to remove trace heavy metals from wastewater, but regeneration of adsorbents in an economic and environmentally friendly manner often represents a limiting factor of its application. Compared with traditional strong acid desorption, developing a chemical-free method is of great significance to both economic and the environmental welfare. Herein, we synthesized a novel thermoresponsive absorbent, A-MIL-121, which could effectively remove trace Cu(II) (> 95 %) from a high-salinity ([Na+]/[Cu2+] = 20000) water at normal temperature. At elevated temperature, A-MIL-121 could quickly and efficiently desorb Cu(II), with over 90% desorption rate at 80°C within 3 h. Fourier transform infrared spectroscopy (FTIR) analysis revealed that two types of -COOH groups existed in the material. One was in free form and acted as the sites for Cu(II) adsorption; the other was in dimer connected by two H-bonds, which cleaved at elevated temperature. As a result, massive exchangeable protons were released to the solution, which caused the desorption of Cu(II). Similar temperature dependent adsorption-desorption behavior was also found to other heavy metals, such as Cd2+, Pb2+, Ni2+. No significant capacity loss was observed after 10 successive adsorption-desorption cycles. Finally, Column experiments using a real copper electroplating wastewater showed that a total of ~ 1650 mL of clean water was generated before breakthrough (Cu2+ < 0.5 mg/L), while less than 45 mL of 80°C water was used for regeneration. This study indicates the potential of A-MIL-121 as a novel green adsorbent to address trace heavy metals in wastewater.

A novel water-stable two-dimensional zeolitic imidazolate frameworks thin-film composite membrane for enhancements in water permeability and nanofiltration performance.

Polymer membranes for water treatment are constrained by the permeability-separation trade-off. Herein, two-dimensional (2D) zeolitic imidazolate frameworks (ZIFs) made of benzimidazole interconnected with Zn ions are used to create 2D Zn2(Bim)4 molecular sieve nanosheets, which is explored as an asymmetric, thin-film composite (TFC) nanofiltration (NF) membrane for removing organic dyes and salts from water with a high water permeability under a low operating pressure (1 bar). The 2D Zn2(Bim)4 TFC NF membrane is synthesized via ionic bonds between polycations and the peripheral hydroxy groups of 2D Zn2(Bim)4 nanosheets, regulating the assembly of 2D Zn2(Bim)4 to create a novel crack-free functional layer on top of a polyvinylidene fluoride (PVDF) ultrafiltration membrane. FESEM and XPS confirmed the presence of a polycations-regulated ultrathin functional layer with a thickness of ∼37 nm on the PVDF support. Benefiting from its structural feature, our 2D Zn2(Bim)4 TFC NF membrane could achieve an ultra-high flux of ∼290 L/(m2·h·bar) (5-10-fold higher than that of graphene-based membranes), good anti-fouling properties and high rejection rates (above 98%) for organic dyes. Moreover, the desalinization rate is 50-75%. That is, our membrane is endowed with NF capability, and its intrinsic ultrafiltration features (high water permeance, ultrafast, and energy-saving) are also well maintained.

Nitrogen doped hierarchically structured porous carbon fibers with an ultrahigh specific surface area for removal of organic dyes.

Recently, tremendous efforts have been devoted to creating inexpensive porous carbon materials with a high specific surface area (SSA) as adsorbents or catalysts for the efficient removal of organic pollutants. Here, activated porous carbon fibers with hierarchical structures were designed and constructed by an electrospinning technique, in situ polymerization, and activation and carbonization processes. Benefiting from the precursor fiber design and subsequent activation techniques, the activated porous carbon fibers (APCFs) derived from a benzoxazine/polyacrylonitrile (BA-a/PAN) precursor exhibited an ultrahigh SSA of 2337.16 m2 g-1 and a pore volume of 1.24 cm3 g-1, showing excellent adsorption capacity toward methylene blue (MeB, 2020 mg g-1). Interestingly, the APCFs after pre-adsorption of MeB also display robust activation of peroxymonosulfate (PMS) with singlet oxygen for the ultrafast removal of MeB. Meanwhile, the synergistic effect of adsorption and a catalytic oxidation reaction using APCFs can realize outstanding total organic carbon (TOC) removal in a comparatively short time. Moreover, a synergistic adsorption-oxidation mechanism for promoting the removal of MeB using APCFs was proposed. This study is useful for the design and development of novel metal-free carbon adsorbents, catalysts or catalyst carriers with an ultrahigh SSA for various applications.