Masakazu Niinae

A mechanistic study of arsenate removal from artificially contaminated clay soils by electrokinetic remediation

Batch desorption experiments and bench-scale electrokinetic experiments were performed to elucidate the electrokinetic remediation mechanisms of arsenate from artificially contaminated kaolinite. The electrokinetic experiments in which a constant voltage was applied demonstrated that high soil pH favored arsenate remediation with respect to both the remediation time and electricity consumption. It was also demonstrated that applying a pulse voltage (1 h ON, 1 h OFF) significantly improved the electricity consumption efficiency when the soil pH was maintained at the initial value during the experiments; this trend was not observed when the soil pH was gradually increased from the cathode side. These electrokinetic experimental results, with the support of arsenate desorption data obtained from batch experiments, indicate that the remediation rate-limiting step varied with soil pH. When the soil pH was maintained at the initial value of 7.2 during the experiments, arsenate desorption was the remediation rate-limiting step rather than the migration of dissolved arsenate toward the anode. Conversely, when the cathode pH was not controlled and the soil pH was correspondingly increased gradually from the cathode side, the migration of hydroxyl and desorbed arsenate ions toward the anode played a more important role in the control of the overall remediation efficiency.

An electrokinetic/Fe0 permeable reactive barrier system for the treatment of nitrate-contaminated subsurface soils

Effective nitrate removal by Fe(0) permeable reactive barriers (Fe(0) PRB) has been recognized as a challenging task because the iron corrosion product foamed on Fe(0) hinders effective electron transfer from Fe(0) to surface-bound nitrate. The objectives of this study were (i) to demonstrate the effectiveness of an electrokinetic/Fe(0) PRB system for remediating nitrate-contaminated low permeability soils using a bench-scale system and (ii) to deepen the understanding of the behavior and fate of nitrate in the system. Bench-scale laboratory experiments were designed to investigate the influence of the Fe(0) content in the permeable reactive barrier, the pH in the anode well, and the applied voltage on remediation efficiency. The experimental results showed that the major reaction product of nitrate reduction by Fe(0) was ammonium and that nitrate reduction efficiency was significantly influenced by the variables investigated in this study. Nitrate reduction efficiency was enhanced by either increasing the Fe(0) content in the Fe(0) reactive barrier or decreasing the initial anode pH. However, nitrate reduction efficiency was reduced by increasing the applied voltage from 10xa0V to 40xa0V due to the insufficient reaction time during nitrate migration through the Fe(0) PRB. For all experimental conditions, nearly all nitrate nitrogen was recovered in either anode or cathode wells as nitrate or ammonium within 100xa0h, demonstrating the effectiveness of the system for remediating nitrate-contaminated subsurface soils.

Recovery of Cr as Cr(III) from Cr(VI)-contaminated kaolinite clay by electrokinetics coupled with a permeable reactive barrier.

Zero-valent iron (Fe(0)) and magnetite (Fe3O4) were investigated as potential reductants in an electrokinetic/permeable reactive barrier hybrid system (EK/PRB) for the recovery of Cr as Cr(III) from Cr(VI)-contaminated kaolinite. For the EK/Fe(0) PRB, regardless of the pH in the anode well, the system facilitated the reduction of Cr(VI) into Cr(III), but the recovery of the Cr(III) in the PRB was low. Conversely, the reduction of Cr(VI) occurred only in the PRB for the EK/Fe3O4 PRB. However, when the anode pH was not controlled and the soil pH values correspondingly decreased gradually from the anode side, a greater fraction of Cr(VI) sorbed onto the kaolinite; as a result, a lower amount of Cr(VI) migrated to the Fe3O4 PRB. In addition, it was found that the majority of Cr(VI) migrating to the Fe3O4 PRB retained its oxidation state without being converted into Cr(III). These two adverse effects were mitigated by maintaining the soil pH values at 6.8, but at the same time, 18% of Cr(VI) penetrated through the Fe3O4 PRB. The penetration of Cr(VI) through the Fe3O4 PRB was successfully prevented by increasing the reaction time through the introduction of a cation exchange membrane between the Fe3O4 PRB and the anode well.

Relationship between performance deterioration of a polyamide reverse osmosis membrane used in a seawater desalination plant and changes in its physicochemical properties.

While it is known that the performance of reverse osmosis membranes is dependent on their physicochemical properties, the existing literature studying membranes used in treatment facilities generally focuses on foulant layers or performance changes due to fouling, not on the performance and physicochemical changes that occur to the membranes themselves. In this study, the performance and physicochemical properties of a polyamide reverse osmosis membrane used for three years in a seawater desalination plant were compared to those of a corresponding unused membrane. The relationship between performance changes during long-term use and changes in physicochemical properties was evaluated. The results showed that membrane performance deterioration (i.e., reduced water flux, reduced contaminant rejection, and increased fouling propensity) occurred as a result of membrane use in the desalination facility, and that the main physicochemical changes responsible for performance deterioration were reduction in PVA coating coverage and bromine uptake by polyamide. The latter was likely promoted by oxidant residual in the membrane feed water. Our findings indicate that the optimization of membrane materials and processes towards maximizing the stability of the PVA coating and ensuring complete removal of oxidants in feed waters would minimize membrane performance deterioration in water purification facilities.

Immobilization of arsenate in kaolinite by the addition of magnesium oxide: An experimental and modeling investigation

MgO was chosen as an As(V) immobilization agent and a series of immobilization experiments was performed to obtain insights into the behavior of As(V) and MgO during leaching tests. Our experimental and modeling results demonstrated that As(V) immobilization by MgO consists of the following steps: (i) an increase in sample pH, (ii) desorption of As(V) from the samples, and (iii) the re-immobilization of As(V) by MgO/Mg(OH)2 particles. Regarding the behavior of MgO, the modeling results showed that when the MgO dosage was 25 mgMgO/4 g-drysample or less, the majority of MgO was used to increase pH, and less than 1% of MgO was used to sorb As(V), which was consistent with the result of leaching tests showing that a high level of As(V) was leached at the MgO dosages. On the other hand, when the MgO dosage was above 25 mgMgO/4 g-drysample, the percentage of MgO used for As(V) sorption increased up to 35%, and correspondingly, the As(V) leaching level decreased to below 0.01 mgAs/L at an MgO dosage of 75 mgMgO/4 g-drysample. Additionally, when the MgO dosage was 50 mgMgO/4 g-drysample or more, it was found that more than 40% of MgO remained as fresh MgO without undergoing chemical reactions.

Recovery of tungsten and cobalt from tungsten carbide tool waste by hydrometallurgical method

In a present process to treat the tungsten carbide (WC) tool waste, the wastes are roasted in air and then an alkali leaching is carried out in an autoclave. An environmentally friendly process is required to recover rare metals (Co and W) from the wastes. The effect of mechano-chemical (MC) treatment on leaching of rare metals was investigated in this study. The solvent extraction and crystallization-stripping methods were applied to separate and recover tungsten and cobalt in the leached solutions. The MC treatment for the rare metal leaching is effective to dissolve rare metals from the wastes due to the change in crystalline structure of WC and oxidation of WC with KMnO4. Cobalt ions are extracted with di-2-ethylhexyl phosphoric acid by a cation-exchange reaction. Tungsten in the leachate can be extracted by tri-octyl amine as an extractant, because tungsten species exist as anionic species in acidic solution. The rare metals in organic phase are recovered as insoluble salts such as oxalates and ammonium salts in the crystallization-stripping process.

Preferential adsorption and surface precipitation of lead(II) ions onto anatase in artificially contaminated Dixie clay

During TEM-EDS (transmission electron microscopy coupled with an X-ray energy dispersive spectrometer) analysis of Dixie clay artificially contaminated with Pb(II), we observed that Pb(II) was preferentially adsorbed and precipitated on the surface of TiO2. To deepen the understanding of the mechanism and importance of this phenomenon, batch sorption experiments, XANES (X-ray absorption near edge spectroscopy) analysis, and sequential extraction analysis were performed. The TiO2 in Dixie clay was found to be anatase, and anatase showed a higher Pb(II) sorption propensity than rutile, α-FeOOH, and one of two MnO2 investigated in this study. Our experimental results indicated that the Pb precipitates preferentially formed on the surface of anatase was Pb(II) hydroxide or Pb(II) oxide. Additionally, sequential extraction analysis showed that at least 32% and 42% of Pb(II) was sorbed onto anatase in the Dixie clay contaminated with a Pb content of 736mg Pb/kg and 1,958mg Pb/kg, respectively. These results demonstrated that in addition to Fe and Mn oxides that are well-known metal oxides that serve as sinks for Pb(II) in the soil environment, TiO2 is also a metal oxide that controls the behavior and fate of Pb(II) in soils.