No abstract

The effect of various sonication frequencies (28 kHz, 200 kHz, and 1.7 MHz) on the size of synthesized scorodite particles was investigated. First, the growth of the precursor with oxidation was observed in a synthesis process using stirring. The precursor size changed with the elapsed time and was <1 µm at 0 min, approximately 1 µm at 10 min, and >3 µm at 20 min. The scorodite particles were then synthesized following different procedures such that a different size of the precursor was irradiated by each ultrasound frequency for 10 min during the oxidation process with stirring for a total duration of 180 min. For each frequency, the size of the synthesized particles varied depending on the size of the precursor. Large scorodite particles can be synthesized (70 °C) by irradiating for only 30 min at various frequencies after the size of the precursor particles reaches the optimal range.

Synthesis of scorodite (FeAsO·2HO) using dynamic action agglomeration and the oxidation effect from ultrasound irradiation was investigated. The effect of different reaction temperatures (90, 70, 50, and 30°C) on the size and morphology of scorodite particles synthesized under O gas flow and ultrasound irradiation was explored because the generation of fine bubbles depends on the solution temperature. At 90°C, the size of scorodite particles was non-homogeneous (from fine particles (<1μm) to large particles (>10μm)). The oxidation-reduction potential (ORP) and yield at 90°C showed lower values than those at 70°C. The scorodite particles, including fine and non-homogeneous particles, were generated by a decrease in the oxidation of Fe(II) to Fe(III) and promotion of dissolution caused by the generation of radicals and jet flow from ultrasound irradiation. Using ultrasound irradiation in the synthesis of scorodite at low temperature (30°C) resulted in the appearance of scorodite peaks in the X-ray diffraction (XRD) pattern after a reaction time of 3h. The peaks became more intense with a reaction temperature of 50°C and crystalline scorodite was obtained. Therefore, ultrasound irradiation can enable the synthesis of scorodite at 30°C as well as the synthesis of large particles (>10μm) at higher temperature. Oxide radicals and jet flow generated by ultrasound irradiation contributed significantly to the synthesis and crystal growth of scorodite.

The synthesis of large scorodite particles (>10 µm) at low temperature (50-70 °C) with a short reaction time (3 h) using 200 kHz ultrasound irradiation was investigated. Large scorodite particles (>10 µm) were obtained using ultrasound irradiation at 70 °C. The growth of scorodite particles was confirmed by scanning electron microscopy (SEM). The precursor agglomerated at the early stage of the synthesis using ultrasound irradiation. Ultrasound irradiation was applied to agglomerate the precursor. Large scorodite particles were synthesized without fine particles using ultrasound irradiation for a short time at the early stage of the synthesis followed by oxidation using stirring and oxygen flow without ultrasound irradiation.

Ultrasound can desorb CO2 from dissolved CO2 gas [CO2(aq)] in a low-concentration monoethanolamine (MEA) solution (0.2 M) at a low temperature (25 °C). However, it is difficult to desorb CO2 using ultrasound at pH >8.2 because CO2 mainly exists as HCO3− and not as CO2(aq). We investigated the increment of the CO2 gas desorption ratio on the synthesis reaction of CaCO3 by the shift in the CO2 equilibrium in the direction of the increase of CO2(aq) by the decrease of pH from 8.2 with an increase in the H+ concentration in the MEA solution. Results indicated that a high Ca recovery ratio of 93.1% was observed by CaCl2 addition and ultrasound irradiation. Compared with the desorption ratio of CO2 gas using ultrasound irradiation without the addition of CaCl2, the CO2 gas desorption ratio increases from 2.2% to 9.8% using ultrasound with a low concentration of CaCl2 (Ca/MEA molar ratio 0.026).