Ming Kuang Wang

Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from Water: Thermodynamic and Spectroscopic Studies.

Removal of arsenic (As) from water supplies is needed to reduce As exposure through drinking water and food consumption in many regions of the world. Magnetite nanoparticles (MNPs) are promising and novel adsorbents for As removal because of their great adsorption capacity for As and easy separation. This study aimed to investigate the adsorption mechanism of arsenate, As(V), and arsenite, As(III), on MNPs by macroscopic adsorption experiments in combination with thermodynamic calculation and microspectroscopic characterization using synchrotron-radiation-based X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). Adsorption reactions are favorable endothermic processes as evidenced by increased adsorption with increasing temperatures, and high positive enthalpy change. EXAFS spectra suggested predominant formation of bidentate binuclear corner-sharing complexes ((2)C) for As(V), and tridentate hexanuclear corner-sharing ((3)C) complexes for As(III) on MNP surfaces. The macroscopic and microscopic data conclusively identified the formation of inner-sphere complexes between As and MNP surfaces. More intriguingly, XANES and XPS results revealed complex redox transformation of the adsorbed As on MNPs exposed to air: Concomitant with the oxidation of MNPs, the oxidation of As(III) and MNPs was expected, but the observed As(V) reduction was surprising because of the role played by the reactive Fe(II).

Adsorption mechanism of selenate and selenite on the binary oxide systems.

Removal of selenium oxyanions by the binary oxide systems, Al- or Fe-oxides mixed with X-ray noncrystalline SiO(2), was previously not well understood. This study evaluates the adsorption capacity and kinetics of selenium oxyanions by different metal hydroxides onto SiO(2), and uses X-ray absorption spectroscopy (XAS) to assess the interaction between selenium oxyanions and the sorbents at pH 5.0. The binary oxide systems of Al(III)- or Fe(III)-oxides mixed with SiO(2) were prepared, and were characterized for their surface area, point of zero charge (PZC), pH envelopes, X-ray diffraction analysis (XRD), and then macro-scale adsorption isotherm and kinetics of selenite and selenate, micro-scale adsorption XAS. The adsorption capacity of selenite and selenate on Al(III)/SiO(2) is greater than on Fe(III)/SiO(2). Adsorption isothermal and kinetic data of selenium can be well fitted to the Langmuir isotherm and pseudo-second-order kinetic models. Based on simple geometrical constraints, selenite on both the binary oxide systems forms bidentate inner-sphere surface complexes, and selenate on Fe(III)/SiO(2) forms stronger complexes than on Al(III)/SiO(2).

Polymerization of catechin catalyzed by Mn-, Fe- and Al-oxides

The role of short-range order (SRO) metal oxides, which are common in acid soils and associated environments, in influencing the abiotic transformations of catechin, which is common in the soil of tea plantations, still remains poorly understood. The aim of this study was to investigate the catalytic power of SRO Mn(IV)-, Fe(III)- and Al-oxides in influencing the abiotic transformations of catechin. At the end of a 90-h reaction period, the release of CO(2) in all the oxide-catechin systems is higher than that for the system with only catechin. Polymerization of catechin is catalyzed and enhanced by SRO-oxides, as is indicated by the absorbance values of the supernatants, which were obtained via visible adsorption spectroscopy, and the yields of humic polymers. The sequence of the oxides that increased the yield of total humic polymers in these systems under ambient atmosphere is: Fe(III)-oxide>Mn(IV)-oxide>Al-oxide>>no catalyst (catechin). The electron spin resonance (ESR) and Fourier transformation infrared absorption spectrometry (FT-IR) of humic polymers formed in the oxide-catechin systems were similar to the spectra obtained from the humic polymers extracted from the soil. The catalytic power of SRO-oxides in promoting the oxidative polymerization of catechin, the resultant formation of humic substances, and C turnover in acid soils thus merit attention.

Cesium and strontium sorption by selected tropical and subtropical soils around nuclear facilities.

The dynamics of Cs and Sr sorption by soils, especially in the subtropics and tropics, as influenced by soil components are not fully understood. The rates and capacities of Cs and Sr sorption by selected subtropical and tropical soils in Taiwan were investigated to facilitate our understanding of the transformation and dynamics of Cs and Sr in soils developed under highly weathering intensity. The Langmuir isotherms and kinetic rates of Cs and Sr sorption on the Ap1 and Bt1 horizons of the Long-Tan (Lt) and the A and Bt1 horizons of the Kuan-Shan (Kt), Mao-Lin (Tml) and Chi-Lo (Cl) soils were selected for this study. Air-dried soil (<2mm) samples were reacted with of 7.5 x 10(-5) to 1.88 x 10(-3)M of CsCl (pH 4.0) or 1.14 x 10(-4) to 2.85 x 10(-3)M of SrCl(2) (pH 4.0) solutions at 25 degrees C. The sorption maximum capacity (q(m)) of Cs by the Ap1 and Bt1 horizons of the Lt soil (62.24 and 70.70 mmol Cs kg(-1) soil) were significantly (p<0.05) higher than those by the A and Bt1 horizons of the Kt and Cl soils (26.46 and 27.49 mmol Cs kg(-1) soil in Kt soil and 34.83 and 29.96 mmol Cs kg(-1) soil in Cl soil, respectively), however, the sorption maximum capacity values of the Lt and Tml soils did not show significant differences. The amounts of pyrophosphate extractable Fe (Fe(p)) were correlated significantly with the Cs and Sr sorption capacities (for Cs sorption, r(2)=0.97, p<1.0 x 10(-4); for Sr sorption, r(2)=0.82, p<2.0 x 10(-3)). The partition coefficient of radiocesium sorbed on soil showed the following order: Cl soil>>Kt soil>Tml soil>Lt soil. It was due to clay minerals. The second-order kinetic model was applied to the Cs and Sr sorption data. The rate constant of Cs or Sr sorption on the four soils was substantiality increased with increasing temperature. This is attributable to the availability of more energy for bond breaking and bond formation brought about by the higher temperatures. The rate constant of Cs sorption at 308 K was 1.39-2.09 times higher than that at 278K in the four soils. The activation energy of Cs and Sr sorbed by the four soils ranged from 7.2 to 16.7 kJ mol(-1) and from 15.2 to 22.4 kJ mol(-1), respectively. Therefore, the limiting step of the Cs(+) or Sr(2+) sorption on the soils was diffusion-controlled processes. The reactive components, which are significantly correlated with the Langmuir sorption maxima of Cs and Sr by these soils, substantially influenced their kinetic rates of Cs and Sr sorption. The data indicate that among components of the subtropical and tropical soils studied, short-range ordered sesquioxides especially Al- and Fe-oxides complexed with organics play important roles in influencing their capacity and dynamics of Cs and Sr sorption.

Low-molecular-weight Organic Acids Exuded by Millet ( setaria italica (l.) Beauv.) Roots and Their Effect on the Remediation of Cadmium-contaminated Soil

Cadmium (Cd)-contaminated soils are of concern because of their possible effects on ecosystems and human health. Millet (Setaria italica (L.) Beauv.) is moderately tolerant to salinity and poor soil fertility as well as pristine soils contaminated with trace elements. It has been hypothesized that low-molecular-weight organic acids (LMWOA) exuded into the rhizosphere may play important roles in Cd uptake by millet. Thus the objectives of this study were to assess the LMWOA exudations of millet in Cd-contaminated soils and to evaluate the capacity of millet to remove Cd for phytoremediation. Millet was grown for 3 weeks in soils containing between 0 and 1,000 mg Cd kg−1. These soils and plant tissues were analyzed for Cd content after harvest. The Cd accumulated in millet tissues was significantly increased with increasing Cd-amended rate. The total LMWOA concentrations in root exudates were significantly correlated with the amount of Cd accumulated in millet shoots and roots. High levels of Cd amendment stimulated secretion of LMWOA, a process capable of reducing Cd toxicity via the formation of Cd-organic complexes (Cd-LMWOA), thereby detoxifying the contaminated soil.

LOW-MOLECULAR-WEIGHT ORGANIC ACID EXUDATION OF RAPE (BRASSICA CAMPESTRIS) ROOTS IN CESIUM-CONTAMINATED SOILS

Cesium is an emission element from nuclear energy generation easily transferred to food chain. More evidence has been found that Brassica crops take up pollutants from soils. The objective of this study was to investigate the correlation between soil bioavailability of cesium and low-molecular-weight organic acids (LMWOAs) in rape (Brassica campestris) root exudates. Longtan (LT) red (Typic Hapladox) and Kuanshan (KS) iron-rich calcareous soils (Typic Paleudalf) were collected for this study. The pot experiments of rape were conducted with cesium-amended soils and plants grown in the soils (4 weeks). Cesium concentration in shoots and roots correlated well with Cs concentration in the amended soils. Within the amended range of 50 to 300 mg Cs kg−1 soil, Cs did not inhibit rape growth. The bioaccumulation ratio ([Cs]root/[Cs]soil] or [Cs]shoot/[Cs]soil) for Cs in shoots of pot grown plants ranged between 9 and 31 and showed significant differences (P < 0.05). Plant roots can exude LMWOAs, which are important components in root exudation. The total amounts of volatile and nonvolatile LMWOAs in all Cs-amended soils were higher than those in nonamended soils. Meanwhile, the LMWOA concentrations of the rape root exudates showed good correlation with Cs concentrations in the applied range of 50 to 300 mg Cs kg−1 soil.

Effects of low molecular weight organic acids on 137Cs release from contaminated soils

Radio pollutant removal is one of several priority restoration strategies for the environment. This study assessed the effect of low molecular weight organic acid on the lability and mechanisms for release of (137)Cs from contaminated soils. The amount of (137)Cs radioactivity released from contaminated soils reacting with 0.02 M low molecular weight organic acids (LMWOAs) specifically acetic, succinic, oxalic, tartaric, and citric acid over 48 h were 265, 370, 760, 850, and 1002 Bq kg(-1), respectively. The kinetic results indicate that (137)Cs exhibits a two-step parabolic diffusion equation and a good linear relationship, indicating that the parabolic diffusion equation describes the data quite well, as shown by low p and high r(2) values. The fast stage, which was found to occur within a short period of time (0.083-3 h), corresponds to the interaction of LMWOAs with the surface of clay minerals; meanwhile, during the slow stage, which occurs over a much longer time period (3-24 h), desorption primarily is attributed to inter-particle or intra-particle diffusion. After a fifth renewal of the LMWOAs, the total levels of (137)Cs radioactivity released by acetic, succinic, oxalic, tartaric, and citric acid were equivalent to 390, 520, 3949, 2061, and 4422 Bq kg(-1) soil, respectively. H(+) can protonate the hydroxyl groups and oxygen atoms at the broken edges or surfaces of the minerals, thereby weakening Fe-O and Al-O bonds. After protonation of H(+), organic ligands can attack the OH and OH(2) groups in the minerals easily, to form complexes with surface structure cations, such as Al and Fe. The amounts of (137)Cs released from contaminated soil treated with LMWOAs were substantially increased, indicating that the LMWOAs excreted by the roots of plants play a critical role in (137)Cs release.

Aluminium and nutrients induce changes in the profiles of phenolic substances in tea plants (Camellia sinensis CV TTES, No. 12 (TTE))

BACKGROUNDnTea plants are always cultivated in acid soils in hilly regions and their growth can be dependent on to soluble aluminium (Al). The mechanism of Al detoxification and the influence of Al on phenolic compounds (i.e. catechin) in the roots of tea plants has remained obscure. This study aimed to investigate the influence of Al changes on the concentrations of phenolic substances in tea plants through hydroponic experiments.nnnRESULTSnTea plants were cultivated in nutrient solution containing 1.5 and 2.5 mmol L(-1) Al, and these treatments enhanced the growth of new buds and roots. Aluminium stimulated the uptake of Ca, Mg, K and Mn, whereas the uptake of Fe, Cu and Zn was retarded. Moreover, total phenol concentrations in tea plant tissues increased with increasing Al concentrations. In general, catechin concentrations in leaves increased with increasing Al concentrations in the hydroponic experiments. High correlation coefficients were obtained between Al and (-)-ECG (r(2) = 0.85, P < 0.01) and between Al and total phenols (r(2) = 0.92, P < 0.01).nnnCONCLUSIONSnThe Al concentration in tea plants indeed increases catechin concentrations and plays an important role in the growth of tea plants.

ELECTRO-OSMOTIC CHEMICAL TREATMENTS: EFFECTS OF Ca2+ CONCENTRATION ON THE MECHANICAL STRENGTH AND pH OF KAOLIN

Electro-osmotic chemical treatment is an innovative method to improve the strength of soft clays for geotechnical engineering purposes; the effectiveness of the treatment may be related to treatment time, the concentration of the solutions injected, and to variation of pH in the clay. The objective of this study was to investigate the relationship between the above-mentioned factors and the improvement in strength when calcium chloride solution was used as an injection material. A series of tests was carried out by injecting different concentrations of calcium chloride solution into a kaolin suspension, for different treatment times, during electro-osmosis. After the tests, the pH, cone resistance, water content, and concentration of Ca2+ in the kaolin at different locations were measured and analyzed. The results show that the concentration of Ca2+ in the kaolin, the pH, and the strength were increased near the cathode with increases in concentration of CaCl2 and treatment time. An insignificant increase in strength, due to ion exchange over the entire specimen, for short treatment times of 2 to 24 h, was observed because of a small increase in concentration of Ca2+ and in pH. During long-term treatment (120 h), a considerable increase in concentration of Ca2+ (137.0 mg/g) and pH (pH = 10) was observed near the cathode. This led to a pozzolanic reaction, which in turn caused a significant increase in the mechanical strength of the kaolin.

Red soils developed from Quaternary deposits on the Linkuo terrace, northern Taiwan

Abstract The Linkuo terrace, situated to the west of the Taipei Basin in NW Taiwan, has thick red soils that have been little studied. This paper aims to interpret the development of these soils through chemical and micromorphological investigations, and relate the soils to their palaeo-environments. The soil samples were air dried, crushed and passed through a 2 mm sieve, and then subjected to conventional soil chemical and physical analyses, together with clay mineralogical and morphological characterization. Pedons I and II were clay with low base saturation (BS), cationexchange capacity (CEC), and exchangeable cations. The pH of pedons I and II ranged from 3.80 to 5.26. The low magnetic susceptibility of these soils indicates that no magnetite (Fe3O4) or maghemite (γ-Fe2O3) are present. X-ray diffraction patterns of the magnetic clay fraction showed lepidocrocite, goethite and hematite, the amounts of which may relate to water fluctuation in the soil environments. Illite, kaolinite and quartz are major clay minerals in the red soil clay fractions. The micromorphology of all horizons showed a great accumulation of Fe-oxides. The upper horizon showed darkened isotropic Fe-oxide materials, and lower horizon showed a black to reddish dense plasma with soil matrix. The groundmass of the oxic horizon is generally characterized by a homogeneous distribution of the different coarse and fine constituents. The thick (4 m depth) and homogeneous red soils of the Linkuo terrace were developed from fine sediments after the gradual subsidence of the Taipei Basin. Alarge amount of gravel was flushed from the Xindian River before the Taipei Basin subsided. The Linkuo red soils can be classified as mesic, Typic Kandiudox. From the chemical compositions of clay fractions and the red soil features, these red soils can be considered as lateritic red earths or red earths that do not reach the criteria for laterite.