Tomohito Kameda

Removal of boron and fluoride in wastewater using Mg-Al layered double hydroxide and Mg-Al oxide.

Mg-Al layered double hydroxide intercalated with NO3- and Mg-Al oxide were found to remove hazardous materials such as B and As, as well as Cl- and SO42-, from artificial and real hot spring wastewater. However, compared with the mixture of Al2(SO4)3 and Ca(OH)2, both adsorbents were inferior for the removal of B from real hot spring wastewater. Both adsorbents were also found to remove F- and PO43- from artificial semiconductor plant wastewater. Both adsorbents have the same ability to remove B from landfill wastewater as the mixture of Al2(SO4)3 and Ca(OH)2; furthermore, both remove Cl-, Br-, and SO42-. The benefit of Mg-Al layered double hydroxide intercalated with NO3- is that it does not require neutralization after the treatment. Overall, it can be stated that among the materials tested, Mg-Al layered double hydroxide intercalated with NO3- is the most suitable adsorbent for the treatment of hot spring and landfill wastewater.

Adsorption isotherms and kinetics of arsenic removal from aqueous solution by Mg–Al layered double hydroxide intercalated with nitrate ions

Arsenic contamination in groundwater is an environmental problem that affects large populations on the global scale. The anion exchange material, Mg–Al layered double hydroxide (Mg–Al LDH) intercalated with NO3−, is an effective adsorbent for removing As(V) from aqueous solutions. In this study, we prepared Mg–Al LDH with a high anion exchange capacity by the co-precipitation method, and investigated its adsorption isotherm and reaction kinetics with As(V). The adsorption process is well described by the Langmuir model. The maximum adsorption capacities were determined to be 142.86 and 76.92xa0mg/g for LDHs synthesized with initial Mg/Al molar ratios of 2 and 4, respectively. The reaction kinetics of As(V) with Mg–Al LDH is demonstrated to be pseudo-second order, which indicates that chemisorption (i.e., anion exchange of HAsO42− with the intercalated NO3−) is the rate determining step. The values of the activation energy also indicate that anion exchange is the predominant adsorption mechanism.

Use of Mg–Al oxide for boron removal from an aqueous solution in rotation: Kinetics and equilibrium studies

Mg-Al oxide prepared through the thermal treatment of [Formula: see text] intercalated Mg-Al layered double hydroxides (CO3·Mg-Al LDH) was found to remove boron (B) from an aqueous solution. B was removed by the rehydration of Mg-Al oxide accompanied by combination with [Formula: see text] . When using twice the stoichiometric quantity of Mg-Al oxide for Mg/Alxa0=xa04, the residual concentration of B dropped from 100 to 2.8xa0mg/L in 480xa0min, and for Mg/Alxa0=xa02, it decreased from 100 to 2.5xa0mg/L in 240xa0min. In both cases, the residual concentration of B was highlighted to be lower than the current Japanese effluent standards (10xa0mg/L). The removal of B can be explained by way of pseudo-first-order reaction kinetics. The apparent activation energy of 63.5xa0kJxa0mol(-1), calculated from the Arrhenius plot indicating that a chemical reaction dominates the removal of B by Mg-Al oxide (Mg/Alxa0=xa02). The adsorption of B acts upon a Langmuir-type phenomena. The maximum adsorption (qm) and equilibrium adsorption constants (KL) were 7.4xa0mmolxa0g(-1) and 1.9xa0×xa010(3), respectively, for Mg-Al oxide (Mg/Alxa0=xa02). [Formula: see text] in B(OH)4·Mg-Al LDH produced by the removal of B was observed to undergo anion exchange with [Formula: see text] in solution. Following regeneration, the Mg-Al oxide maintained the ability to remove B from an aqueous solution. This study has clarified the possibility of recycling Mg-Al oxide for B removal.

Kinetic and equilibrium studies of urea adsorption onto activated carbon: Adsorption mechanism

ABSTRACT We found that activated carbon effectively removed urea from solution and that urea adsorption onto activated carbon followed a pseudo-second-order kinetic model. We classified the urea adsorption on activated carbon as physical adsorption and found that it was best described by the Halsey adsorption isotherm, suggesting that the multilayer adsorption of urea molecules on the adsorption sites of activated carbon best characterized the adsorption system. The mechanism of adsorption of urea by activated carbon involved two steps. First, an amino (–NH2) group of urea interacted with a carbonyl (–C˭O) group and a hydroxyl (−OH) group on the surface of activated carbon via dipole–dipole interactions. Next, the –C˭O group of the urea molecule adsorbed to the activated carbon interacted with another –NH2 group from a second urea molecule, leading to multilayer adsorption. GRAPHICAL ABSTRACT Schematic representation of the adsorption of urea on activated carbon.

Effectiveness of Mg–Al-layered double hydroxide for heavy metal removal from mine wastewater and sludge volume reduction

Health hazards from heavy metal pollution in water systems are a global environmental problem. Of similar concern is sludge that results from wastewater treatment due to unsatisfactory sludge management technology. Therefore, the effectiveness of using Mg–Al-layered double hydroxide in the removal of heavy metals from mine wastewater was tested and compared with that of calcium hydroxide [Ca(OH)2], which is a common treatment method for heavy metal removal. Initially, the mine wastewater contained cations of the heavy metals iron (Fe), zinc (Zn), copper (Cu), and lead (Pb). The Mg–Al-layered double hydroxides were able to remove 371, 7.2, 121, and 0.4xa0mg/L of these pollutants, respectively, using the co-precipitation method. The removal of these metals is most effective using 0.5xa0gxa0Mg–Al-layered double hydroxide (Mg/Al molar ratio 4) and 20xa0min of shaking. Zn was removed by the formation of Zn(NO3)(OH)·H2O and Zn5(NO3)2(OH)8 when LDH, Mg/Al molar ratios of 4 and 2, respectively, were used. Similarly, Fe, Cu, and Pb were removed by the formation of Fe–Al-layered double hydroxide, Cu2(OH)3·NO3 and Pb4(OH)4(NO3)4, respectively. While Ca(OH)2 is also capable of reducing the heavy metal concentrations below the Japanese recommended values, this analysis shows that using 0.5xa0gxa0Mg–Al-layered double hydroxide is a better treatment condition for mine wastewater, because it generates lower sludge volumes than 0.1xa0g of Ca(OH)2. The measured sludge volume was 1.5xa0mL for Mg–Al-layered double hydroxide and 2.5xa0mL for Ca(OH)2, a nearly twofold further reduction.

Tandem μ-reactor-GC/MS for online monitoring of aromatic hydrocarbon production via CaO-catalysed PET pyrolysis

The present work demonstrates the online monitoring of aromatic hydrocarbon production via two-step CaO catalysed pyrolysis of poly(ethylene terephthalate) (PET), employing tandem μ-reactor-gas chromatography/mass spectrometry (TR-GC/MS). PET produces high-boiling terephthalic acid (TPA) during pyrolysis, which hinders the online monitoring of PET pyrolysis. In this work, TR allowed for independent control of the PET pyrolysis and CaO catalytic reaction with a very small sample loading (<1 mg) and split injection into the GC/MS (split ratio 100u2006:u20061) system; thus, fatal line clogging by TPA could be avoided. Thus, we successfully demonstrated the effect of CaO basicity on the time- and temperature-dependent dynamic production of aromatic hydrocarbons. Strongly basic CaO accelerated the decarboxylation of PET pyrolysates to afford useful aromatic hydrocarbons such as benzene, toluene, and styrene with 99.7% selectivity in the oil. In contrast, weakly basic CaO enhanced benzophenone production in preference to benzene formation. The poor deoxygenation ability of the weakly basic CaO increased the concentration of oxygen-containing compounds in the oil. Finally, the time- and temperature-dependent dynamic pathways and the mechanism involving strongly basic/weakly basic CaO were established. These findings allow for a clearer understanding of the nature of PET catalytic pyrolysis, which will be helpful for advancing PET recycling. Furthermore, the novel methodology—online monitoring of a two-step pyrolysis–catalytic upgrading process involving high-boiling compounds—will gain the highest demand in the fields of green chemistry and reaction engineering.

Simultaneous removal of Cl− and SO42− from seawater using Mg−Al oxide: kinetics and equilibrium studies

Mg−Al oxide, obtained by the thermal decomposition of a CO32−-intercalated Mg−Al layered double hydroxide (CO3·Mg−Al LDH), simultaneously absorbed Cl− and SO42− from seawater and generated a Mg−Al LDH intercalated with Cl− and SO42−. The Mg−Al oxide with a molar ratio Mg/Alxa0=xa04 was more superior than the oxide with Mg/Alxa0=xa02 for Cl− removal, whereas a reverse phenomenon was observed for SO42− removal. The removal of Cl− and SO42− by the Mg−Al oxide with Mg/Alxa0=xa04 could be represented by first-order and pseudo second-order reactions, respectively. The removal of both Cl− and SO42− by the Mg−Al oxide with Mg/Alxa0=xa02 could be represented by a pseudo second-order reaction. The removal of both Cl− and SO42− by the Mg−Al oxides with Mg/Alxa0=xa04 and 2 was proceeded under chemical reaction control. The adsorption isotherms for Cl− and SO42− adsorbed by the Mg−Al oxides could be expressed by Langmuir-type adsorption. These reactions were derived from monolayer adsorption, indicating the intercalation of Cl− and SO42− in the interlayer space of Mg−Al LDH. The uptake of Cl− and SO42− from seawater by Mg−Al oxide was proceeded spontaneously.

Simultaneous recovery of high-purity copper and polyvinyl chloride from thin electric cables by plasticizer extraction and ball milling

Herein, we introduce a combination of plasticizer extraction from polyvinyl chloride (PVC) and ball milling for the simultaneous, effective recovery of PVC and copper (Cu) from thin electric cables. PVC coverings typically contain plasticizers for flexibility. As such, PVC cables become brittle after plasticizer extraction, causing them to be easily crushed by physical impact. Hence, we extracted the plasticizers from the PVC coverings of electric cables using organic solvents, and then crushed the obtained cable samples by ball milling. The influences of the plasticizer extraction yield and PVC morphologies before and after extraction on separation by ball milling were investigated. After a series of treatments to PVC coverings including quantitatively de-plasticizing for 5xa0h by Soxhlet-extraction in diethyl ether, 6 h ball milling and 1 h shaking in the sieve shaker, a maximum separation rate of 77% was achieved and the purity of the obtained separated Cu reached >99.8%.

Identification of number and type of cations in water-soluble Cs+ and Na+ calix[4]arene-bis-crown-6 complexes by using ESI-TOF-MS

The treatment of cesium-contaminated wastewater has become one of the biggest issues. The selective Cs+ removal from wastewater containing competitive alkali metal ions such as Na+ is desired to reduce the volume of sludge. Therefore, the present work focused on water-soluble calix[4]arene-bis-crown-6 (W-BisC6) to selectively capture Cs+. For characterization of the complex, UV-vis spectroscopy is commonly used, however, due to the limited availability of information it can be hard to quickly identify the specific structures of some complexes. In this work, the electrospray ionization time of flight spectrometry (ESI-TOF-MS) is successfully utilized to identify the number and type of cations in W-BisC6-cation complexes. ESI-TOF-MS accurately recognized 4 types of complex (W-BisC6-Na+, W-BisC6-Cs+, W-BisC6-2Na+, W-BisC6-Na+-Cs+), and the experimental and simulated results were almost perfectly matched. It also revealed the difficulty of W-BisC6-2Cs+ complex formation under the present conditions. Thus, this technique is significantly helpful for rapid identification of the specific structures of complexes during Cs+-contaminated wastewater treatment.