Yoshiaki Umetsu

POTENTIAL OF PROTONATED ALGINATE BEADS FOR HEAVY METALS UPTAKE

Abstract The potential for uptake of several heavy metal ions by alginate in the form of protonated beads was investigated at 25 °C. The morphological characteristics of the beads and their behavior in aqueous solutions were examined as well. The ability of protonated alginate beads to remove heavy metal ions from dilute aqueous solutions was demonstrated. The uptake of trivalent chromium, copper, zinc, nickel and cobalt was found to be about 75, 77, 46, 43 and 35 mg per g of beads, respectively. The uptake increased with solution pH and acid concentration for protonation, and with decreasing ionic strength. The cross-linking agents, Ba and Ca, were not released during uptake, and protons were liberated instead. Therefore, the uptake was associated with ion exchange between protons of the free carboxylic functional groups of the alginate and metal ions from the solutions. EPMA-EDX analyses showed that heavy metals were uniformly distributed throughout the surface (external and internal) of the beads, indicating that protonated dry alginate beads can be considered as porous ion exchangers. Observation of protonated dry alginate beads by scanning electron microscopy (SEM) showed a corrugated surface having a uniform distribution of regular nodules and cavities. A mean diameter of about 1.0 mm was obtained for these beads. They were found to have chemical and structural stability in acidic and slightly acidic environments. Bead forming reaction with Ba or Ca makes it possible to use alginate as sorbent for heavy metal ions in extremely dilute aqueous solutions.

Oxidative precipitation of arsenic(III) with manganese(II) and iron(II) in dilute acidic solution by ozone

Abstract Oxidative precipitation using ozone has been examined for removal of arsenic with manganese from dilute acidic sulfate solution under the following conditions: the initial concentrations of arsenic(III) and arsenic(V)=1–100 mg/L, the initial mole ratio of Mn/As=10–100, pH=0.4–5.0 and temperature=15–80 °C. The oxidation–reduction potential (ORP) of the system was continuously measured to monitor the process of oxidation reactions. The O 3 –O 2 gas mixture (ozone partial pressure=0.010–0.031 atm, feed rate=500 mL/min) was found to produce an adequately high potential to oxidize arsenic(III) and then manganese(II). By formation of manganese(III) arsenate and uptake of arsenic by manganese dioxide formed by ozone oxidation, arsenic entities are removed to a sufficiently low concentration to meet the regulation against arsenic contaminant level for industrial waste water in Japan (0.1 mg/L) in the pH range 1.0–3.0 at 25 °C, where coprecipitation with ferric hydroxide is not very effective to remove arsenic. Furthermore, coexistence of an appropriate amount of iron(II) promotes significantly removal of arsenic with manganese at pH below 3 by ozonation.

ORP-monitored magnetite formation from aqueous solutions at low temperatures

Abstract The removal of iron and various heavy-metal ions by their incorporation into stable ferrite-type precipitates at 25°C is a promising alternative to clean up large volumes of polluted effluents. As a first step, the present work investigated the conditions to produce the iron ferrite magnetite by the aerial oxidation and neutralization of ferrous sulfate solutions under well-controlled conditions and at low temperatures. The formation of the solids was followed by monitoring the oxidation–reduction potential (ORP) and the release of protons associated with the progress of the oxidation of Fe(II) ions and subsequent hydrolysis of produced Fe(III) species. The parameters studied were: contact time at constant pH, air flow-rate, stirring intensity, aging of the solids and precipitation temperatures below 60°C. It was found that a moderate oxidation rate of ferrous entities, represented by an ORP of −120 mV and a rate of proton release of 1.60×10 −4 mol H + /l min, favored the formation of intermediate green rust-II and its conversion into magnetite at a temperature as low as 25°C, even without aging. In contrast, an extremely fast oxidation reaction favored by the enhancement of the air flow-rate and stirring intensity deteriorated or even destroyed incipient ferrite frameworks. When the formation of magnetite was incomplete, aging of the sludges in contact with their mother liquors promoted the crystallinity of the precipitates. Furthermore, the beneficial effect of stirring intensity and the only slight influence of temperature on the oxidation rate, at a level of 21 kJ/mol (5 kcal/mol), suggested that a mass-transfer step would control the oxidation of ferrous entities at low temperature. This mass-transfer step could be attributed to the transfer of oxygen into the aqueous phase. A discussion on the magnetite-forming reaction at ambient temperature is also presented.

Ambient-temperature precipitation of Zn ions from aqueous solutions as ferrite-type compounds

Abstract The magnetite-forming reaction from aqueous solutions at 25 °C was systematically studied in our earlier publications. On this basis, the present work investigated the conditions conducive to the incorporation of target metal ions into the structure of ferrites at ambient temperature. The Zn-bearing ferrite system was considered as the first case study. The formation of the solids was followed by monitoring the oxidation–reduction potential (ORP), measured against an Ag/AgCl reference electrode, and rate of proton release during the aerial oxidation of the suspensions at constant pH. Only mildly oxidizing conditions, represented by an ORP value of −150 mV and a moderate oxidation rate of Fe(II) species were conducive to the formation of crystalline Zn-bearing ferrites at 25 °C. It was also found that increasing the Fe(II)/Zn mole ratio in starting solutions enhanced the stability of the corresponding ferrite. When the formation of ferrite was incomplete, aging of the sludges in their mother liquors at 25 °C promoted the crystallinity of the precipitates. The mentioned conditions favored the progress of the ferrite-forming reaction, which involves the oxidation of Fe(II) entities, subsequent hydrolysis of produced Fe(III) species and dehydration of the intermediate. Furthermore, the formation of the ferrite permitted the elimination of Fe and Zn ions from starting solutions to sufficiently low concentration.

Removal of pollutant ultrafine particles from low concentrated suspensions using a solid waste

Abstract Removal of pollutant ultrafine silica, hematite and cadmium sulfide particles from low concentrated suspensions was investigated by collecting onto the surface of a solid waste material, fibers of a slag from ferro-nickel smelting, placed in a column bed. The removal efficiency was found to be dependent on the pH of the suspensions. In an acidic environment the removal efficiency was 100% for silica and 98% for hematite and cadmium sulfide. Differences in the collection behavior of the three types of particles suggest that surface interaction is playing an important role in the particle collection process. For unfavorable removal conditions, a surface treatment made to the slag fiber collector significantly improved the removal efficiency of the three different ultrafme particles. This surface treatment increased the removal efficiency from 3% to 93% for silica, from 15% to 100% for hematite and from 60% to 100% for cadmium sulfide. Due to the extremely small size of the ultrafine particles (≤ 0.10 μm) and the small fiber/space ratio within the collector bed (0.176), the experimental results were discussed by considering the total interaction potential model based on the DLVO theory. This was complemented by a second mechanism regarding coagulation by metal cations dissolved from the collector.

Ambient-temperature synthesis of metal-bearing ferrites: how and why?

Abstract Various metal-bearing ferrites were produced directly from aqueous solutions at 25°C by simultaneous control of the oxidizing conditions and pH. In order to make clear the involved mechanisms, this work investigated the correspondence between the reaction conditions of formation of ferrite-type compounds and their crystallinity, nature of incorporated water and developed magnetic characteristics. The formation of a Zn-bearing ferrite was considered as a first-case study. X-ray diffractometry (XRD), Fourier transform infrared (FT-IR) and extended X-ray absorption fine structure spectroscopy (EXAFS) measurements were undertaken. It was found that the crystallinity, the dehydration of the intermediate and the diminution of the sulfate contents in the ferrite-type precipitates could be promoted by: (i) increasing the Fe/Zn mole ratios; (ii) a suitable duration of the aeration of the suspensions at pH 11.0 (contact stage) or, (iii) by aging of the precipitates in their mother liquors at 25°C. These effects were attributed to the suitable progress of the oxidation–hydrolysis reactions of Fe(II) entities and the loss of molecular water from the intermediate compound, which also explained the observed enhancement in the saturation magnetization (Ms) of the precipitates. Furthermore, the analysis of the local structure of Fe and Zn atoms by EXAFS evidenced that the ambient temperature ferrite exhibited a similar structure than the one produced by the ceramic method (above 1000°C) and Zn atoms were fully incorporated into the ferrite frameworks occupying the most stable position; i.e. the tetrahedral sites.

Separation of cobalt and nickel by ozone oxidation

The utilization of ozone for the separation of cobalt from nickel sulfate was investigated by determining the oxidation rate for Co(II) and Ni(II) ions under various ozonation conditions at 60°C. The oxidation reaction was observed to follow a first order rate with respect to the ozone partial pressure of the O3-O2 mixture gas and to be promoted considerably by vigorous agitation. The oxidation rates were virtually constant down to a fairly low concentration of the oxidizable ions. Nickel ion was found to be oxidized more easily at lower pH in the mixed sulfate solutions than in solutions of a single sulfate. At pH 2.5–5.0, ozone oxidation seems to be effective to separate cobalt ions selectively from nickel sulfate solutions, due to the extremely slow oxidation of the nickel ion in comparison with cobalt.

Changes in the Characteristics of Manganese Dioxide Produced by Ozone Oxidation during Heat Treatment.

The manganese dioxide prepared by ozone oxidation in acidified solutions of MnSO4 at different temperatures (OMD) was characterized by thermal analysis (DTA and TGA). The changes detected in the thermal analysis were further investigated by X-ray diffraction and oxidation capacity measurement through oxalate-reduction for the OMD samples heat-treated at various temperatures. The discharge behavior was also determined for the heat-treated OMD samples.The ozone oxidation conditions, temperature and sulfuric acid concentration, remarkably affect the changes appearing in thermal analysis due to the crystal transformation of MnO2 from γ-or α-type to β-type and the first thermal decomposition of MnO2 to Mn2O3 as well as due to the liberation of the water in a temperature range below 200°C.The oxidation capacity of OMD sample, expressed in MnO2 equivalent, was observed to decrease in duration of heat treatment, more pronouncedly for OMD precipitated at higher acid concentration and low temperature.Discharge characteristics of OMD were significantly changed by heating for 2 hours at temperatures lower than 200°C. The heat treatment at around 100°C improved the discharge behavior of OMD prepared at 60 to 80°C and acid concentration up to 1.0 M, the discharge capacity being comparable to or better than that of EMD. The OMD samples heated to above 150°C showed poor discharge behavior.It is suggested that the crystal lattice space and activity as an oxide of OMD are changed in close corelation with water liberation during heating at fairly low temperatures.