Desorption of Ammonia Adsorbed on Prussian Blue Analogs by Washing with Saturated Ammonium Hydrogen Carbonate Solution.

Prussian blue analogs (PBAs) have been reported as promising ammonia (NH3) adsorbents with a high capacity compared to activated carbon, zeolite, and ion exchange resins. The adsorbed NH3 was desorbed by heating and washing with water or acid. Recently, we demonstrated that desorption was also possible by washing with a saturated ammonium hydrogen carbonate solution (sat. NH4HCO3aq) and recovered NH3 as an NH4HCO3 solid by introducing CO2 into the washing liquid after desorption. However, this has only been proven for copper ferrocyanide and the relationship between the adsorption/desorption behavior and metal ions in PBAs has not been identified. In this study, we investigated the adsorption/desorption behavior of PBAs that are complexes of first row transition metals with hexacyanometalate anions. Six types of PBAs were tested in this study and copper ferricyanide exhibited the highest desorption/adsorption ratio. X-ray diffraction results revealed high structural stability for cobalt hexacyanocobaltate (CoHCC) and nickel ferricyanide (NiHCF). The Fourier transform infrared spectroscopy results showed that the NH3 adsorbed on the vacancy sites tended to desorb compared to the NH3 adsorbed on the interstitial sites as ammonium ions. Interestingly, the desorption/adsorption ratio exhibited the Irving-Williams order.

Single Open Sites on FeII Ions Stabilized by Coupled Metal Ions in CN-Deficient Prussian Blue Analogues for High Catalytic Activity in the Hydrolysis of Organophosphates.

CN-deficient Prussian blue analogues (PBAs), [MN(H2O)x]y[FeII(CN)5(NH3)] (MN = CuII, CoII, or GaIII), were synthesized and examined as a new class of heterogeneous catalysts for hydrolytic decomposition of organophosphates often used as pesticides. The active species of the CN-deficient PBAs were mainly C-bound FeII ions with only single open sites generated by liberation of the NH3 ligand during the catalytic reactions. [CuII(H2O)8/3]3/2[FeII(CN)5(NH3)] showed higher catalytic activity than [CoII(H2O)8/3]3/2[FeII(CN)5(NH3)] and [GaIII(H2O)][FeII(CN)5(NH3)], although N-bound CuII species has been reported as less active than CoII and GaIII species in conventional PBAs. IR measurements of a series of the CN-deficient PBAs after the catalytic reactions clarified that a part of the NH3 ligands remained on [CoII(H2O)8/3]3/2[FeII(CN)5(NH3)] and that hydrogen phosphate formed as a product strongly adsorbed on the FeII ions of [GaIII(H2O)][FeII(CN)5(NH3)]. Hydrogen phosphate also adsorbed, but weakly, on the FeII ions of [CuII(H2O)8/3]3/2[FeII(CN)5(NH3)]. These results suggest that heterogeneous catalysis of the FeII ions with single open sites were tuned by the MN ions through metal-metal interaction.

Interpretation of the Role of Composition on the Inclusion Efficiency of Monovalent Cations into Cobalt Hexacyanoferrate.

Cobalt hexacyanoferrate of various compositions was prepared in flow mode and the role of the vacancy on the structure, thermogravimetric (TG) properties, and the adsorption efficiency was studied. The material, Nay Co[Fe(CN)6 ]1-x ⋅z H2 O, with a minimum vacancy of x=0.014 to the highest x=0.47, was obtained. The TG-differential scanning calorimetry (DSC) profile showed a distinct influence of the vacancy on the water release temperature. Materials with x>0.35 showed a smooth release of water at a relatively lower temperature. However, for the materials with x<0.35, water release took place in multiple steps, suggesting the existence of various forms of water. The FTIR profiles supported the existence of free and bonded water molecules. However, the materials with multiple water peaks in the FTIR spectra showed a shift of the major XRD peaks when heated at 285 °C in N2 atmosphere. Regarding the effect of the vacancy on the adsorption behavior, for NH4 , the adsorption was found to be proportional to the number of Na atoms in the material, confirming the ion-exchange process. On the contrary, the materials with low vacancy and high Na content showed nominal Cs adsorption capacity. Interestingly, the K adsorption capacity was found to be in between that of the other two ions. This means the ionic size decides the rate of placement into the interstitial sites. For larger ions like Cs, the ease of percolation via the vacancy decides the overall adsorption efficiency.

Highly Sensitive and Exceptionally Wide Dynamic Range Detection of Ammonia Gas by Indium Hexacyanoferrate Nanoparticles Using FTIR Spectroscopy.

Ammonia gas is useful but caustic; thus, its concentration is monitored depending on applications. We prepared indium hexacyanoferrate nanoparticles (InHCF-NPs, HCF = [FeII(CN)6]4-) with average diameter around 8 nm by simple reaction at room temperature between In cations and HCF anions, and we found the unique functionality of InHCF-NPs which were capable of highly sensitive (16 ppb) and exceptionally wide range (190000, 16 ppb -0.3%) detection of ammonia gas within 6 min by IR measurements. Slope changes of the IR peak ratio of adsorbed ammonium over CN moieties in the InHCF framework indicated a good log-log linear correlation along gas concentrations, in which a wide dynamic range over 105 was realized for the first time in the field of ammonia gas detection.

Historical Pigment Exhibiting Ammonia Gas Capture beyond Standard Adsorbents with Adsorption Sites of Two Kinds.

Prussian blue is a historical pigment synthesized for the first time at the beginning of 18th century. Here we demonstrate that the historical pigment exhibits surprising adsorption properties of gaseous ammonia. Prussian blue shows 12.5 mmol/g of ammonia capacity at 0.1 MPa, whereas standard ammonia adsorbents show only 5.08-11.3 mmol/g. Dense adsorption was also observed for trace contamination in atmosphere. Results also show higher adsorption by Prussian blue analogues with the optimization of chemical composition. The respective capacities of cobalt hexacyanocobaltate (CoHCC) and copper hexacyanoferrate (CuHCF) were raised to 21.9 and 20.2 mmol/g, the highest value among the recyclable adsorbents. Also, CoHCC showed repeated adsorption in vacuum. CuHCF showed regeneration by acid washing. The chemical state of the adsorbed ammonia depends on the presence of the water in atmosphere: NH3, which was stored as in the dehydrated case, was converted into NH4(+) in the hydrated case.

Dealing with the aftermath of Fukushima Daiichi nuclear accident: decontamination of radioactive cesium enriched ash.

Environmental radioactivity, mainly in the Tohoku and Kanto areas, due to the long living radioisotopes of cesium is an obstacle to speedy recovery from the impacts of the Fukushima Daiichi Nuclear Power Plant accident. Although incineration of the contaminated wastes is encouraged, safe disposal of the Cs enriched ash is the big challenge. To address this issue, safe incineration of contaminated wastes while restricting the release of volatile Cs to the atmosphere was studied. Detailed study on effective removal of Cs from ash samples generated from wood bark, household garbage, and municipal sewage sludge was performed. For wood ash and garbage ash, washing only with water at ambient conditions removed radioactivity due to (134)Cs and (137)Cs, retaining most of the components other than the alkali metals with the residue. However, removing Cs from sludge ash needed acid treatment at high temperature. This difference in Cs solubility is due to the presence of soil particle originated clay minerals in the sludge ash. Because only removing the contaminated vegetation is found to sharply decrease the environmental radioactivity, volume reduction of contaminated biomass by incineration makes great sense. In addition, need for a long-term leachate monitoring system in the landfill can be avoided by washing the ash with water. Once the Cs in solids is extracted to the solution, it can be loaded to Cs selective adsorbents such as Prussian blue and safely stored in a small volume.

Recovery of Pure Methanol from Humid Gas Using Mn-Co Prussian Blue Analogue.

Conventional methanol recovery and purification processes are highly energy-intensive; processes using selective adsorbents that consume low energy are preferable. However, conventional adsorbents have low methanol selectivity under humid conditions. In this study, we develop a selective methanol adsorbent, manganese hexacyanocobaltate (MnHCC), which enables the efficient removal of methanol from waste gas and its subsequent reuse. MnHCC adsorbs 4.8 mmol-methanol/g-adsorbent at 25 °C in a humid gas containing 5000 ppmv of methanol, which is five times higher than the adsorption capacity of activated carbon (0.86 mmol/g). Although MnHCC exhibits the simultaneous adsorption of methanol and water, it has a higher adsorption enthalpy for methanol. Thus, pure methanol (95%) was recovered via thermal desorption at 150 °C after dehydration. The estimated energy of this recovery was 18.9 MJ/kg-methanol, approximately half that of existing mass production methods. MnHCC is reusable and stable even after 10 cyclic experiments. Consequently, MnHCC has the potential to contribute to both the recycling of methanol from waste gas and its low-cost purification.

Surface modification enhances the bulk proton conductivity of Prussian blue.

Surface-modified Prussian blue shows 102 times higher bulk proton conductivity (0.018 S cm-1) than that of unmodified Prussian blue. This enhancement is attributed to the monolayer adsorption of Na4[Fe(CN)6] on the nanoparticle surface, which reduces the surface resistance. Surface modification is an effective strategy for improving bulk proton conductivity.

Thermal Decomposition Behavior of Prussian Blue in Various Conditions.

Prussian blue analogs (PBA) are widely studied for radioactive cesium decontamination. However, there are fewer works related to their post use storage. Considering the oxidative stabilization of the material after the selective uptake of Cs, the thermogravimetric properties in powder and bead form, with various Cs and other alkali metal ions adsorbed, and various heating rates were studied. TG-DTA taken in dry air condition shows an exothermic decomposition at ~270 °C. This temperature varied with the heating rate, mass, and the proportion of adsorbed ions. The best condition for complete oxidation of Prussian blue (PB) is found to be a gradual oxidative decomposition by heating in the temperature range of 200-220 °C until the total mass is decreased by >35%. After this, the temperature could be safely increased to >300 °C for the complete oxidative decomposition of PB that formed iron oxide and salt of the adsorbed Cs. A pilot scale test conducted using the radioactive Cs adsorbed Prussian blue microbeads (PB-b) confirmed that no Cs was released in the effluent air during the process.

Ammonium salt production in NH3-CO2-H2O system using a highly selective adsorbent, copper hexacyanoferrate.

Ammonia is a beneficial material that is widely used in agriculture, but its emission into the atmosphere causes air pollution. Recently, Prussian blue (PB) and its analogs (PBA) were found to be ammonia adsorbents with high selectivity and capacity. In this study, we utilized a highly potent PBA adsorbent, copper hexacyanoferrate (CuHCF), to desorb ammonia and turn it into a reusable form. Because the reported NH3-CO2-H2O system phase diagram suggests the possibility of the recovery of solid NH4HCO3, we examined whether adsorbed ammonia desorbs into the saturated ammonium hydrogencarbonate solution (sat. NH4HCO3aq). We demonstrated that 40% of adsorbed ammonia desorbed into sat. NH4HCO3aq. After the desorption, CO2 was blown into the washing liquid, and NH4HCO3 precipitated, which was confirmed by Fourier transform infrared spectroscopy. The molar amount of solid NH4HCO3 was almost equal to that of desorbed ammonia. Our findings pave the way for recovery of ammonia as a valuable product from waste gas.

Selective Adsorption of Potassium in Seawater by CoHCF Thin Film Electrode and Its Electrochemical Desorption/Regeneration.

Cobalt Hexacyanoferrate (CoHCF) was tested for the selective uptake of K from seawater and the electrochemical method was adopted for the desorption and regeneration of the material. Powder form CoHCF could adsorb about 6.5 mmol/g of K from the seawater. For the ease of the electrochemical desorption and regeneration, CoHCF thin film was coated onto the Indium Tin Oxide (ITO) glass to obtain a CoHCF electrode. K adsorption kinetics on CoHCF thin film was found to be well fitted with the intraparticle diffusion model, which was a two-step process. Five consecutive adsorption-desorption-regeneration cycles were carried out to know the gradual decrease in the adsorption capacity owing to changes in the redox states of two metals, Co and Fe, in the material. Fourier Transform Infrared Spectroscopy (FT-IR) and Ultraviolet-Visible (UV-Vis) measurement results corresponded to the color change of CoHCF thin film, indicating the valence change of transition metals and the exchange of alkali metal cations happened on the CoHCF at different operation stages. In order to elucidate the reaction mechanism, composition of the material was analysis in the following steps: adsorption, desorption, and regeneration. It was proved that the system based on CoHCF thin film modified electrode had the potential of recovering potassium from seawater.

Ammonium removal and recovery from sewage water using column-system packed highly selective ammonium adsorbent.

One of the strategies to realize a nitrogen cycle society, we attempted to recover ammonium ions from industrial wastewater, especially sewage water with adsorbent materials. We have developed an adsorbent with high ammonium selectivity based on copper hexacyanoferrate and granulated it as pellets. Using a compact column system filled with this granule adsorbent, ammonium ions were recovered from sewage containing 1000-1500 mg-NH4+/L ammonium ions. Despite the coexistence of many metal ions, the adsorbent selectively and stably adsorbed ammonium ions. Furthermore, it was shown that the saturated adsorbent can be regenerated by flowing a potassium ion solution through a column adsorbent to desorb ammonium ions. In other words, the column can be used repeatedly, and there was almost little deterioration in adsorption even after 250 cycles. In addition, it was shown that by increasing the number of stages of this column, it is possible to sufficiently reduce the ammonium in the adsorbent solution and recover the concentrated ammonium solution.

Green fabrication of a complementary electrochromic device using water-based ink containing nanoparticles of WO3 and Prussian blue.

We fabricated a complementary electrochromic device (ECD) by using water-dispersible nanoparticles (NP) of Prussian blue (PB) and WO3 by using a wet process, which involved just coating. Although the ECD had a thick WO3 film, it showed much higher contrast compared to other techniques. In addition, the ECD also showed fast optical switching speed and high durability over 100 cycles because of wettability control of NP inks.

Trace Ammonia Removal from Air by Selective Adsorbents Reusable with Water.

Ammonia adsorbents effective even in trace concentrations are key to the countermeasure for air pollution of particulate matter caused by ammonia emission from agriculture sectors. We revealed that Prussian blue (PB) and its analogues (PBAs), one of the porous coordination polymers, have higher ammonia adsorption capacity in 10 ppmv of ammonia (parts per million in volume, 10 ppmv = 0.0001 volume percent), ≥8 times that of conventional adsorbents. Moreover, these compounds can be recycled only through water flushing. The adsorption capacity of PBA was restricted to 10 cycles of adsorption/desorption, and the air sample for the experiment was collected from the composting equipment present in a swine farm. Despite the presence of saturated water vapor in the exhaust gas, the adsorbents showed excellent selectivity in the removal of ammonia from the sample.

Electrochemical control of the elution property of Prussian blue nanoparticle thin films: mechanism and applications.

We found a method for controlling the elution property of a thin film of water-dispersible Prussian blue (w-PB) nanoparticles using a simple electrochemical treatment. The thin film of the w-PB nanoparticles fabricated by the spin-coating method was unstable in water and eluted easily. However, after electrochemical reduction and oxidation, the film did not elute in water. To clarify the mechanism, we investigated films using ultraviolet-visible absorption spectroscopy, X-ray photoemission spectroscopy, and atomic force microscopy. We propose herein that the change in the elution property occurs because of alkali cation exchange from Na(+) to K(+). We also demonstrated that this elution-control technique is effective for the fabrication of w-PB nanoparticle nanostructures such as multilayered thin films.

One million cyclable blue/colourless electrochromic device using K2Zn3[Fe(CN)6]2 nanoparticles synthesized with a micromixer.

Well-defined K2Zn3[Fe(CN)6]2-nanoparticles (NPs) synthesized with a micromixer showed robust redox reaction cyclability. Crystal structure analysis revealed that the robustness results from the maintenance of the original rhombohedral crystal structure in the oxidation state. The blue/colourless electrochromic device with the K2Zn3[Fe(CN)6]2-NPs and Prussian blue NPs showed recyclability over 1 million redox reactions.

Pre-enrichment of radioactive cesium in muddy water separated into suspended and dissolved substances for trace analysis.

A new in-situ pre-enrichment system was developed for trace analysis of radioactive Cs in environmental water such as rivers, ponds, and seas, where the radioactive Cs is separated into suspended substances (SS) and dissolved substances (DS). The SS component was collected as the enrichment slurry by cross-flow filtration, thereby compact systems were realized. The DS component was collected in a small cartridge filled with a non-woven fiber with immobilized adsorbent nanoparticles. The recovery rate was estimated at around 95%. The size distribution of the SS and the concentration of radioactive Cs after enrichment were the same as that of the raw water before enrichment. The condensed SS can be used for other evaluations such as ignition loss. From the field test at a pond in the Fukushima-area, the Cs concentration of the SS was found to be 4-5 times higher than that of the sediment in the same pond. The organic content estimated by ignition loss was also three times higher.

Adsorption of ng L-1-level arsenic by ZIF-8 nanoparticles: application to the monitoring of environmental water.

The provisional contamination level of arsenic in drinking water is 10 µg L-1. For decreasing this value to a safer level, a more precise method for analyzing dissolved arsenic is required. With this aim, we synthesized zeolitic imidazolate framework-8 (ZIF-8) in the aqueous phase and characterized its potential application for monitoring the trace arsenic in fresh water. In this regard, we report following three notable outcomes. First, we demonstrate the excellent performance of ZIF-8 nanoparticles (nZIF-8) for the adsorption of ng L-1 levels of AsO4 3-. nZIF-8 is able to adsorb over 99% of arsenic from as low as 10 ng L-1 AsO4 3- solutions. This performance was maintained even in the presence of commonly coexisting anions, for example, >90% adsorption from a 0.1 µg L-1 arsenic solution was observed in the presence of 10 mg L-1 of Cl-, NO3 -, CO3 2-, or SO4 2-, or 1 mg L-1 of PO4 3-. Second, we clarified that the mechanism of arsenic adsorption by ZIF-8 is simply a ligand exchange process, in which the As(v) oxide anion replaces the imidazolate unit in the framework. Third, we propose a handy scheme for the analysis of ng L-1 levels of arsenic in drinking water, in which nZIF-8 is used for the concentration of trace level AsO4 3-. By doing this, as low as 100 ng L-1 arsenate in drinking water can be quantified by colorimetric analysis, the detection limit of which is 5 µg L-1 in pure water. The application of this scheme is expected to highly enhance AsO4 3- detection first by concentrating it to an easily detectable range, and second by excluding the majority of interfering ions present in the system. Therefore, a reduction in the minimum quantifying limit of arsenic in fresh water to as low as 1 ng L-1 can be expected if the method is coupled with ICP-MS.

Unveiling Cs-adsorption mechanism of Prussian blue analogs: Cs+-percolation via vacancies to complete dehydrated state.

Metal hexacyanoferrates (MHCF) or Prussian blue analogs are excellent Cs+-adsorbents used for radioactive Cs-decontamination. However, the adsorption mechanism is controversial. To clarify the issue, we quantitatively investigated the Cs-adsorption behaviors of potassium copper hexacyanoferrate (KCuHCF) and A y Cu[Fe(CN)6]1-x ·zH2O. To obtain samples having homogeneous chemical composition and particle size, flow systems were used for both synthesis and purification. After sufficient rinsing with water, the range of x stable in aqueous solution in time appropriate for Cs-adsorption was 0.25 < x < 0.50. The relations y = 4 - 2x and z = 10x were also found independent of x, indicating complete dehydration of K+ in the crystal. We concluded that the excellent Cs-selectivity of MHCF was not due to difference in free energy of the adsorbed state between K+ and Cs+ but because of the hydrated state in aqueous solution. We also found that the guiding principle for determining the maximum capacity depended on the chemical composition. In particular, for the range 0.25 < x < 0.35, we propose a new model to understand the suppression of the maximum capacity. In our model, we hypothesize that Cs+ could migrate in the crystal only through [Fe(CN)6]4- vacancies. The model reproduced the observed maximum capacity without fitting parameters. The model would also be applicable to other MHCFs, e.g. a little adsorption by soluble Prussian blue. The ion exchange between Cs+ and H+ occurred only when the implemented K+ was small.

High-capacity and selective ammonium removal from water using sodium cobalt hexacyanoferrate.

A new NH4 + adsorbent with high capacity and selectivity, sodium cobalt(ii) hexacyanoferrate(ii) (NaCoHCF, Na y Co(ii) [Fe2+(CN)6] x ·zH2O), was prepared. The adsorption performance was investigated by varying the mixing ratio of [Fe(CN)6]4- to Co2+ during synthesis, R mix. The ammonia capacity was found to be proportional to R mix, indicating that the NH4 + capacity can be increased by increasing the Na+-ion content in NaCoHCF. To conduct a detailed study, we prepared homogeneous nanoparticles by flow synthesis using a micromixer with R mix = 1.00. Even on the addition of a saline solution (NaCl) with an Na+-ion concentration of 9350 mg L-1, the capacity was maintained: q max = 4.28 mol kg-1. Using Markham-Benton analysis, the selectivity factor, defined by the ratio of equilibrium constants for NH4 + to that for Na+, was calculated to be α = 96.2, and 4.36 mol kg-1 was found to be the maximum capacity. The high selectivity of NaCoHCF results in good NH4 +-adsorption performance, even from seawater. In comparison with other adsorbents under the same conditions and even for a NH4Cl solution, NaCoHCF showed the highest capacity. Moreover, the coexisting Na+ caused no interference with the adsorption of ammonium by NaCoHCF, whereas the other adsorbents adsorbed ammonia only slightly from the saline solution. We also found that the pores for NH4 + adsorption changed their sizes and shapes after adsorption.