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.

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.

Editorial for the Special Issue on "Fundamentals of Adsorbents-Synthesis, Characterization, Properties, and Application".

This Special Issue covers the widely studied topic of "Adsorbents", materials that are known to uptake ions and molecules from water or air [...].

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.

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.

Pre-concentration and separation of heavy metal ions by chemically modified waste paper gel.

Iminodiacetic acid was immobilized on waste paper by chemical modification in order to develop a new type of adsorption gel for heavy metal ions. Adsorption behavior of the gel was investigated for a number of metal ions, specifically Cu(II), Pb(II), Fe(III), Ni(II), Cd(II), and Co(II) at acidic pH. From batch adsorption tests, the order of selectivity was found to be as follows: Cu(II)>>Fe(III)>Pb(II)>Ni(II)>>Co(II)>Cd(II). Column tests were carried out for pairs of metal ions to understand the separation and pre-concentration behavior of the gel. It was found that mutual separation of Ni(II) from Co(II) and that of Pb(II) from Cd(II) can be achieved at pH 3. Similarly, selective separation of Cu(II) from Cu(II)-Fe(III) and Cu(II)-Pb(II) mixtures at pH 1.5 and 2, respectively, was observed by using this new adsorption gel. In all cases, almost complete recovery of the adsorbed metal was confirmed by elution tests with HCl.

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.

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.

Biosorbents for Removing Hazardous Metals and Metalloids.

Biosorbents for remediating aquatic environmental media polluted with hazardous heavy metals and metalloids such as Pb(II), Cr(VI), Sb(III and V), and As(III and V) were prepared from lignin waste, orange and apple juice residues, seaweed and persimmon and grape wastes using simple and cheap methods. A lignophenol gel such as lignocatechol gel was prepared by immobilizing the catechol functional groups onto lignin from sawdust, while lignosulfonate gel was prepared directly from waste liquor generated during pulp production. These gels effectively removed Pb(II). Orange and apple juice residues, which are rich in pectic acid, were easily converted using alkali (e.g., calcium hydroxide) into biosorbents that effectively removed Pb(II). These materials also effectively removed Sb(III and V) and As(III and V) when these were preloaded with multi-valent metal ions such as Zr(IV) and Fe(III). Similar biosorbents were prepared from seaweed waste, which is rich in alginic acid. Other biosorbents, which effectively removed Cr(VI), were prepared by simply treating persimmon and grape wastes with concentrated sulfuric acid.

Application of Prussian blue nanoparticles for the radioactive Cs decontamination in Fukushima region.

Cs decontamination efficiencies of the composites of iron hexacyanoferrate nanoparticles were investigated in comparison with commercial Prussian blue and natural zeolite. In pure water solution, the adsorption rate varied with sizes. In ash extract, where Cs adsorbing ability of zeolite was sharply dropped due to its poor selectivity, the impact of coexisting ions was negligible for FeHCF. FeHCF-n11, having the finest primary and secondary particle size, resulted the highest distribution coefficient, which was comparable to the high efficiency analogues, CoHCF or NiHCF. This observation suggested the possibility of preparing the high performance FeHCF by particle size and composition adjustment. FeHCF nanoparticle in bead form was tested for the removal of radioactive Cs in pilot scale. Due to larger secondary particle size, pronounced effect of solution temperature on the Cs adsorption kinetics on FeHCF bead was observed. Adjusting the mass of the adsorbent for the given solution temperature is recommended for achieving high decontamination rate.

Variation in available cesium concentration with parameters during temperature induced extraction of cesium from soil.

Cesium extraction behavior of brown forest type soil collected from paddy fields in Fukushima nuclear accident affected areas was studied. In nitric acid or sulfuric acid solutions at elevated temperature, the concentration of Cs in soil available for extraction, m0, has been estimated on the basis of modified canonical equation and the equations derived from assumed equilibria. With the variation in temperature, mixing time, and soil to solvent ratio, the observed m0 values in 0.5 M acid solution ranged between 1.5 and 2.9 mg cesium per kilogram of soil. By increasing the acid concentration to 3 M, the value of m0 could be sharply increased to 5.1 mg/kg even at 95 °C. This variation in the extractable concentration of cesium with the parameters signifies the existence of different binding sites in the soil matrix. The results observed for uncontaminated sample could be reproduced with the radioactive cesium contaminated sample belonging to the same soil group.

Recovery of palladium using chemically modified cedar wood powder.

Japanese cedar wood powder (CWP) was chemically modified to a tertiary-amine-type adsorbent and studied for the selective recovery of Pd(II) from various industrial waters. Batch adsorption tests performed from 0.1 M to 5 M HCl and HNO3 systems reveal stable performance with better results in HNO3 medium. The maximum loading capacity for Pd(II) was studied in HCl as well as in HNO3. A continuous-flow experiment taking a real industrial solution revealed the feasibility of using modified CWP for the selective uptake and preconcentration of traces of palladium contained in acidic effluents. In addition, stable adsorption performance even on long exposure to gamma-irradiation and selective recovery of palladium from simulated high-level liquid waste (HLW) are important outcomes of the study.

Chestnut pellicle for the recovery of gold.

Recovery of Au(III) from hydrochloric acid medium by using crosslinked chestnut pellicle (CCP) gel was studied. Strong selectivity was observed for Au(III) showing negligible affinity for other precious metals and some base metal ions tested. The adsorption isotherm study exhibited the maximum loading capacity of the gel as high as 10.6 mol or about 2.1 kg gold per kg dry weight of gel. The reduction of Au(III) ion to elemental form during adsorption process is expected to be the reason of high selectivity and high capacity for Au(III). Kinetic studies at various temperatures confirm an endothermic adsorption process following the pseudo-first order rate law.

Dimethylamine-modified waste paper for the recovery of precious metals.

Waste newsprint paper was modified with dimethylamine (DMA) to obtain a tertiary amine type adsorption gel called DMA-paper gel. This new derivative was investigated for adsorption, from hydrochloric acid medium, of gold, palladium, and platinum as well as some base metals. The gel exhibited selectivity only for precious metals with a remarkably high capacity for Au(III), i.e., 4.6 mol/kg dry gel and comparable capacities, i.e., 2.1 and 0.9 mol/kg for Pd(II) and Pt(IV), respectively. Also, Au(III) was reduced to the elemental form during adsorption. Furthermore, column adsorption and subsequent elution of the adsorbed metal ions by acidic thiourea revealed encouraging recoveries (approximately 90%), thus enhancing the scope of the gel for effective preconcentration, separation, and recovery of precious metals. The effectiveness of recovery of precious metals from real industrial liquor was also tested, and it showed highly encouraging results with respect to the stability of the gel in the harsh medium, and selectivity for the targeted metal ions in the presence of excess of other metal ions.