Motoyoshi Kobayashi

Adsorption characteristics of anionic azo dye onto large α-alumina beads

Adsorption of anionic azo dye, new coccine (NC), onto large α-alumina beads in aqueous media was systematically investigated as functions of pH and NaCl concentration. Adsorption amounts of NC decrease with increasing pH of solutions due to less positive charges of α-Al2O3 surface at high pH. At a fixed pH, the NC adsorption increases with decreasing NaCl concentration, indicating that NC molecules mainly adsorb onto α-Al2O3 by electrostatic attraction. Experimental results of NC adsorption isotherms onto α-Al2O3 at different pH, and ionic strength can be represented well by two-step adsorption model. The effects of NC on surface charge and surface modification of α-Al2O3 at the plateau adsorption are evaluated by streaming potential and Fourier transform infrared spectroscopy with attenuated total reflection technique (FTIR-ATR), respectively. On the basis of adsorption isotherms, surface charge effect, and surface modification, we suggested that the formation of a bridged bidentate complex between aluminum ions of α-Al2O3 and two oxygen atoms of a sulfonic group induced the adsorption of NC onto α-Al2O3.

Structure of Colloidal Flocs in relation to the Dynamic Properties of Unstable Suspension

Dynamic behaviors of unstable colloidal dispersions are reviewed in terms of floc formation. Geometrical structure of flocs in terms of chemical conditions and formation mechanics is a key to predict macroscopic transportation properties. The rate of sedimentation and rheological properties can be described with the help of fractal dimension (D) that is the function of the number of contacts between clusters (Nc). It is also well known that the application of water soluble polymers and polyelectrolytes, which are usually used as a conditioner or flocculants in colloidal dispersions, critically affects the process of flocculation. The resulted floc structure is also influenced by the application of polymer. In order to reveal the roles of the polymers, the elementary rate process of polymer reaching to colloidal interface and subsequent reconformation process into more stable adsorption state are needed to be analyzed. The properties of permeable flocs and adsorbed polymer (polyelectrolyte) layers formed on the colloidal surfaces remain to be worked out in relation to inhomogeneous porous structure and electrokinetics in the future.

Adsorption of Polyanion onto Large Alpha Alumina Beads with Variably Charged Surface

Adsorption of strong polyelectrolyte, poly(styrenesulfonate), PSS, of different molecular weights onto large α-Al2O3 beads was systematically investigated as functions of pH and NaCl concentrations. The ultraviolet (UV) absorption spectra of PSS at different pH and salt concentrations confirmed that the structure of PSS is independent of pH. With the change of molecular weight from 70 kg/mol (PSS 70) to 1000 kg/mol (PSS 1000), adsorption amount of PSS increases and proton coadsorption on the surface of α-Al2O3 decreases at given pH and salt concentration. It suggests that higher molecular weight of PSS was less flat conformation than lower one. The adsorption density of PSS 70 and PSS 1000 decreases with decreasing salt concentrations, indicating that both electrostatic and nonelectrostatic interactions are involved. Experimental results of both PSS 70 and PSS 1000 adsorption isotherms onto α-Al2O3 at different pH and salt concentrations can be represented well by two-step adsorption model. The effects of molecular weight and salt concentration are explained by structure of adsorbed PSS onto α-Al2O3. The influence of added SDS on the isotherms is evaluated from the sequential adsorption. The SDS uptake onto α-Al2O3 in the presence of hemimicelles can prevent the adsorption of PSS at low concentration so that adsorption of PSS reduces with preadsorbed SDS.

Adsorptive removal of ammonium ion from aqueous solution using surfactant-modified alumina

Environmental context Ammonium ion, an inorganic pollutant in agricultural land, can induce eutrophication, impacting on water quality. We investigate the adsorption of ammonium ion on surfactant-modified alumina and demonstrate highly efficient removal of ammonium ions by the alumina from two agricultural water samples. Adsorption mechanisms are also proposed based on adsorption isotherms, surface modification and the change in surface charge. Abstract The adsorptive removal of ammonium ions (NH4+) from aqueous solution using surfactant-modified alumina (SMA) was investigated. The optimum NH4+ adsorption removal conditions on SMA were systematically studied and found to be pH 4, contact time 180min, adsorbent dosage 30mgmL–1 and ionic strength 1mM NaCl. The equilibrium concentration of NH4+ was measured by capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C4D) and spectrophotometry. Surface modification of α-Al2O3 with the anionic surfactant sodium dodecyl sulfate (SDS) at high salt concentration induced a significant increase of removal efficiency. The change in surface charge and surface modification of α-Al2O3 by pre-adsorption of SDS and subsequent adsorption of NH4+ were evaluated by zeta potential measurements and Fourier-transform infrared spectroscopy. Under optimum adsorption conditions, NH4+ removal from two agricultural water samples achieved very high removal efficiencies of 99.5 and 96.5%. The adsorption of NH4+ onto SMA increases with decreasing NaCl concentration because desorption of SDS from the α-Al2O3 surface is minimised. Experimental results of NH4+–SMA adsorption isotherms at different ionic strengths can be represented well by a two-step adsorption model. Based on adsorption isotherms, surface charge effect and surface modification, we suggest that the adsorption mechanism of NH4+ onto SMA was mainly electrostatic attraction between cationic NH4+ and the negatively charged SMA surface.

Charge reversal of sulfate latex induced by hydrophobic counterion: effects of surface charge density

We studied experimentally and theoretically the charge reversal of sulfate latex colloid in the presence of monovalent hydrophobic counterion TPP+ (tetraphenylphosphonium). The intrinsic or chemical energy of adsorption of TPP+ on the latex was evaluated from the concentration at charge reversal. The isoelectric point (IEP) increases with increasing the surface or electrokinetic charge density of sulfate latex spheres. That is, at low surface or electrokinetic charge density, the charge inversion concentration is low, and IEP shifts to higher values with the increase of surface or electrokinetic charge density. The intrinsic energy of adsorption decreases with increasing the surface or electrokinetic charge density. Finally, our experimental and theoretical results suggest that the hydrophobicity is a determining factor for the charge inversion of hydrophobic colloids, and the intrinsic energy of adsorption also varies with the variations of surface or electrokinetic charge density.

Charging and aggregation behavior of silica particles in the presence of lysozymes

To gain insight into the colloidal stability in the presence of proteins, we measured the electrophoretic mobility and aggregation rate constant of silica particles coated with lysozymes, and the adsorbed amount of lysozymes on the silica. We also examined model analyses, which are based on the Derjaguin, Landau, Verwey, and Overbeek theory with the effect of charge heterogeneity, to discuss the aggregation of lysozyme-coated silica. Our results show that lysozymes enhance the aggregation of silica when the lysozyme-coated silica is near the isoelectric point. When the adsorbed amount of lysozyme is low, the effect of charge-patch attractive force promotes the aggregation of silica. The effect of charge heterogeneity weakens with the increase of adsorbed amount of lysozyme. Our model which takes account of the effect of charge heterogeneity can capture the trend of the aggregation of silica in the presence of lysozyme qualitatively, but there are also large quantitative discrepancies between the theoretical prediction and experimental results. Further improvement is required to describe realistic charge heterogeneity and the effect of the surface coverage of lysozyme on the silica.

Transport of colloidal silica in unsaturated sand: Effect of charging properties of sand and silica particles.

We have studied the transport of colloidal silica in various degrees of a water-saturated Toyoura sand column, because silica particles are widely used as catalyst carriers and abrasive agents, and their toxicity is reported recently. Since water-silica, water-sand, and air-water interfaces have pH-dependent negative charges, the magnitude of surface charge was controlled by changing the solution pH. The results show that, at high pH conditions (pH 7.4), the deposition of colloidal silica to the sand surface is interrupted and the silica concentration at the column outlet immediately reaches the input concentration in saturated conditions. In addition, the relative concentration of silica at the column outlet only slightly decreases to 0.9 with decreasing degrees of water saturation to 38%, because silica particles are trapped in straining regions in the soil pore and air-water interface. On the other hand, at pH 5 conditions (low pH), where sand and colloid have less charge, reduced repulsive forces result in colloidal silica attaching onto the sand in saturated conditions. The deposition amount of silica particles remarkably increases with decreasing degrees of water saturation to 37%, which is explained by more particles being retained in the sand column associated with the air-water interface. In conclusion, at higher pH, the mobility of silica particles is high, and the air-water interface is inactive for the deposition of silica. On the other hand, at low pH, the deposition amount increases with decreasing water saturation, and the particle transport is inhibited.

Charging and Aggregation Behavior of Cellulose Nanofibers in Aqueous Solution

To understand the charging and aggregation of cellulose nanofibers (CNFs), we performed the following experimental and theoretical studies. The charging behavior of CNFs was characterized by potentiometric acid-base titration measuring the density of deprotonated carboxyl groups at different KCl concentrations. The charging behavior from the titration was quantitatively described by the 1-pK Poisson-Boltzmann (PB) model for a cylinder. The electrophoretic mobility of CNFs was measured as a function of pH by electrophoretic light scattering. The mobility was analyzed with the equation for an infinitely long cylinder considering the relaxation of the electric double layer. Good agreement between experimental mobilities and theoretical calculation was obtained by assuming a reasonable distance from the surface to the slipping plane. The result demonstrated that the negative charge of CNFs originates from the deprotonation of β(1-4)-d-glucuronan on the surface. The aggregation behavior of CNFs was studied by measuring the hydrodynamic diameter of CNFs at different pH and KCl concentrations. Also, we calculated the capture efficiencies of aggregation, using interaction energies of perpendicularly and parallelly oriented cylinders. The interaction energies between cylinders in both orientations were obtained by the Derjaguin, Landau, Verwey, and Overbeek theory, where the electrostatic repulsion was calculated from the surface potential obtained by the 1-pK PB model. From comparison of the theoretical capture efficiency with the measured hydrodynamic diameter, we suggest that CNFs can be aggregated in perpendicular orientation at low pH and low salt concentration, and the fast aggregation regime of CNFs is realized by the reduction of electric repulsion for both perpendicularly and parallelly interacting CNFs. Meanwhile, the application of Smoluchowskis equation to the mobility of CNFs results in the underestimation of the zeta potential.

Enhanced bioconversion of hydrogen and carbon dioxide to methane using a micro-nano sparger system: mass balance and energy consumption

Simultaneous CO2 removal with renewable biofuel production can be achieved by methanogens through conversion of CO2 and H2 into CH4. However, the low gas–liquid mass transfer (kLa) of H2 limits the commercial application of this bioconversion. This study tested and compared the gas–liquid mass transfer of H2 by using two stirred tank reactors (STRs) equipped with a micro-nano sparger (MNS) and common micro sparger (CMS), respectively. MNS was found to display superiority to CMS in methane production with the maximum methane evolution rate (MER) of 171.40 mmol/LR/d and 136.10 mmol/LR/d, along with a specific biomass growth rate of 0.15 d−1 and 0.09 d−1, respectively. Energy analysis indicated that the energy-productivity ratio for MNS was higher than that for CMS. This work suggests that MNS can be used as an applicable resolution to the limited kLa of H2 and thus enhance the bioconversion of H2 and CO2 to CH4.

Characteristics of ultra-fine bubble water and its trials on enhanced methane production from waste activated sludge

In this study biogas production efficiency was evaluated by adding ultra-fine bubble water (UFBW) into waste activated sludge (WAS) through anaerobic digestion (AD). Four kinds of gases, i.e. Air, N2, CO2 and H2 were introduced into tap water (TW) to prepare the UFBW with their properties being first investigated. Results show that hundreds of millions of nanoscale bubbles with the negative zeta potential could be stable in the UFBW for longer than two weeks whereas almost no nanometer bubbles could be detected in the raw TW. As for their impact on subsequent AD of WAS, the cumulative methane production with the addition of UFBW was 14-21% higher than that from the raw TW addition group. Interestingly, the Air-UFBW also could promote the biogas production in this study, which is different from the common understanding of AD, an obligate anaerobic process.