Removal of arsenic from water by zero-valent iron

Batch and column experiments were conducted to investigate the effect of dissolved oxygen (DO) and pH on arsenic removal with zero-valent iron [Fe(0)]. Arsenic removal was dramatically affected by the DO content and the pH of the solution. Under oxic conditions, arsenate [As(V)] removal by Fe(0) filings was faster than arsenite [As(III)]. Greater than 99.8% of the As(V) was removed whereas 82.6% of the As(III) was removed at pH 6 after 9h of mixing. When the solution was purged with nitrogen gas to remove DO, less than 10% of the As(III) and As(V) was removed. High DO content and low solution pH also increased the rate of iron corrosion. The removal of arsenic by Fe(0) was attributed to adsorption by iron hydroxides generated from the oxic corrosion of Fe(0). The column results indicated that a filtration system consisting of an iron column and a sand filter could be used for treatment of arsenic in drinking water.     

Synthesis of nanometric-size magnesium nitride by the nitriding of pre-activated magnesium powder

Magnesium nitride (Mg₃N₂) was synthesized by the reaction of magnesium in the highly reactive form (Mg^*) with nitrogen at 450°C under normal pressure. The effect of doping with nickel dichloride on the nitridation of Mg^* was investigated. Differential thermal analysis (DTA) of Mg^* systems and transmission electron microscopy (TEM) measurement of the product formed were carried out. TEM measurement showed that the particle size of the Mg₃N₂ synthesized was in the nanometric range. The dependence of nitridation of the NiCl₂-doped Mg^* on temperature was investigated at temperatures ranging from 300 to 500°C. The nitridation of NiCl₂-doped Mg^* could occur even at temperature as low as 300°C.     

Properties of porous magnesium prepared by powder metallurgy

Porous magnesium-based materials are biodegradable and promising for use in orthopaedic applications, but their applications are hampered by their difficult fabrication. This work reports the preparation of porous magnesium materials by a powder metallurgy technique using ammonium bicarbonate as spacer particles. The porosity of the materials depended on the amount of ammonium bicarbonate and was found to have strong negative effects on flexural strength and corrosion behaviour. However, the flexural strength of materials with porosities of up to 28 vol.% was higher than the flexural strength of non-metallic biomaterials and comparable with that of natural bone.     

Chemical characterization of melt-textured YBCO produced by hybrid powder metallurgy

Monolithic YBCO samples were made by traditional top-seeded melt-texturing processes from cold-milled mixtures of Y123 (YBa₂Cu₃O_7−δ) and elemental Y. The bulk composition does not lie on the Y211 (Y₂BaCuO₅)–Y123 join so, formation of Y123 from liquid and Y211 is not an essentially isothermal process on cooling. The reaction liquid + Y211 = Y123 is a ternary reaction and occurs over several tens of degrees. The Y123 thus produced has a range in compositions – particularly in Cu:(Y + Ba) – which may reflect crystallization over the thermal interval. The liquid migrates to an invariant point at which CuO also crystallizes with complete loss of liquid. The presence of trains of CuO grains in the YBCO indicates the locations of the last liquids to be preserved in the sample. These trains are dominantly in an annulus 1–3 mm from the edge of the 20-mm diameter sample. Mapping the compositional variation in Y123 may allow mapping the path of crystallization of these monolithic YBCO samples.     

镁铝尖晶石粉体的制备与表征

以Mg、Al、TiO_2和B_2O_3粉体为原料,采用自蔓延高温合成法合成了MgAl_2O_4和TiB_2复合粉体,用稀硝酸除去产物中的TiB2及残余反应物后,获得了较高纯度的MgAl_2O_4。然后用MgO部分取代Mg研究了不同Mg源对合成产物的影响,用XRD和SEM检测了两组反应获得的MgAl_2O_4粉体,用激光粒度测试仪分析了产物的粒径大小及分布,用红外光谱仪测试了粉体性能较佳的反应酸洗产物的红外透光率。结果表明,以单一Mg粉为Mg源时,MgAl_2O_4的相对含量为85.96%,且颗粒较小,平均粒径为5.36μm,粒径分布较集中,0.1~10μm的颗粒占80.68%;以(Mg+MgO)为Mg源时,MgAl_2O_4的相对含量为71.55%,且颗粒较大,平均粒径为11.18μm,粒径分布较分散,0.1~10μm的颗粒占54.96%;综合考虑,以Mg为镁源获得的MgAl_2O_4粉体性能好,经红外光谱分析,可得,MgAl_2O_4透过率高,透过波段范围宽,在1 100~2 978cm~(-1)之间的红外透过率达到50%以上,高透光率的镁铝尖晶石具有低的辐射率,可应用于红外隐身材料。     

Low temperature fabrication of magnesium phosphate cement scaffolds by 3D powder printing

Synthetic bone replacement materials are of great interest because they offer certain advantages compared with organic bone grafts. Biodegradability and preoperative manufacturing of patient specific implants are further desirable features in various clinical situations. Both can be realised by 3D powder printing. In this study, we introduce powder-printed magnesium ammonium phosphate (struvite) structures, accompanied by a neutral setting reaction by printing farringtonite (Mg₃(PO₄)₂) powder with ammonium phosphate solution as binder. Suitable powders were obtained after sintering at 1100°C for 5 h following 20–40 min dry grinding in a ball mill. Depending on the post-treatment of the samples, compressive strengths were found to be in the range 2–7 MPa. Cytocompatibility was demonstrated in vitro using the human osteoblastic cell line MG63.     

Chemical changes in powder reference materials examined by accelerated ageing methods

A method is proposed for planning and executing research involving accelerated ageing in studying the chemical stability of powder reference materials (RM) for spectral analysis. It is necessary to perform a series of experiments having various degrees of extremity in the instability factor in order to draw correct conclusions on the stability. Translated from Izmeritel'naya Tekhnika, No. 11, pp. 64–66, November, 1999.     

Chemical preparation of spherical lithium tantalate powder

Spherical, micron size lithium tantalate powder was prepared by spray drying a lithium tantalate precursor using a mini spray dryer. Three chemical routes were used to prepare the lithium tantalate precursors, using lithium acetate and tantalum ethoxide as starting materials. The affects of processing parameters, such as solvent, drying temperature, and water, on the properties of the resulting powders were investigated. The as-dried powders were amorphous and became crystalline at a temperature as low as 450° C.     

Influence of texture on the mechanical properties of commercially pure magnesium prepared by powder metallurgy

In the present work the influence of texture on the mechanical properties up to 500 °C of commercially pure magnesium prepared by PM was determined. Extrusion of magnesium powders was carried out between 250 and 450 °C. All extruded materials exhibited an intense fibre texture with the basal planes parallel to the extrusion direction whose intensity increased in line with the extrusion temperature. The microstructure consisted of highly elongated magnesium powder particles. All the materials presented a heterogeneous grain size resulting from the size distribution of the original magnesium powder particles. In addition, small MgO particles were found mainly decorating the original powder boundaries. The best mechanical properties corresponded to the materials extruded at 400 and 450 °C. This behaviour was associated particularly with the intense fibre texture of these materials.     

Biological denitrification of drinking water in a slow sand filter

Biological removal of nitrate from drinking water was studied in a slow sand filter. Optimum carbon to nitrogen ratio (C/N) was found to be 1.8 when using acetic acid in batch tests. The filtration rates impact on NO(3)-N removal through the sand filter was assessed for 22.6 mgNO(3)-N/l concentrations while keeping C/N ratio as 1.8 for acetic acid. The filtration rates varied from 0.015, 0.02, 0.03, 0.04, 0.05, and 0.06 m/h, respectively, corresponding to an overall average NO(3)-N removal efficiency of 94%. Although increasing filtration rates decreased NO(3)-N removal, effluent NO(3)-N concentrations at the effluent port were lower than the limit value. The slow sand filter process was unable to provide NO(3)-N removal rate more than 27.1 gN/(m(2)day) (0.05 m/h flow rate). The NO(3)-N removal efficiency slightly dropped from 99% to 94% when the loading rate increased from 27.1 to 32.5 g/(m(2)day), but the effluent water contained higher concentration of NO(2)-N than the standard value.     

Chemical synthesis and stabilization of magnesium substituted brushite

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) is the most ubiquitous calcium phosphate phase used in implant coatings and more recently in gene/drug delivery applications due to its chemical stability under normal physiological conditions (37 °C, pH ～ 7.5, 1 atm.). However, different calcium phosphate phases, such as brushite (CaH(PO₄)?2(H₂O)) and tricalcium phosphate (Ca₃(PO₄)₂) which are thermodynamically unstable under physiological conditions are also being explored for biomedical applications. One way of stabilizing these phases under physiological conditions is to introduce magnesium to substitute for calcium in the brushite lattice. The role of magnesium as a stabilizing agent for synthesizing brushite under physiological conditions at room temperature has been studied. Chemical analysis, Fourier transform infrared spectroscopy and X-ray diffraction have also been conducted to validate the formation of magnesium substituted brushite under physiological conditions.     

Effect of starting particle size on hot-pressing of magnesium oxide powder prepared by vapour-phase oxidation process

Effect of starting particle size on hot-pressing of magnesium oxide (MgO) powder was examined using seven kinds of MgO powders prepared by a vapour-phase oxidation process; the average primary particle sizes were 11, 25, 32, 44, 57, 107 and 261 nm. These compressed powders (compacts) were hot-pressed at a temperature between 900 and 1300°C. The densifications of these compacts during the hot-pressing proceeded via (i) the sintering of primary particles within secondary particles and the rearrangement of secondary particles/grains (900°C), (ii) the gradual grain growth controlled by the pore migration (900∼1100°C) and (iii) the rapid grain growth due to the active mass transfer (1300°C); the grain sizes of MgO compacts hot-pressed at and below 1100°C were <1 μm, while those at 1300°C attained 20∼30 μm. The transluscent compact with the relative density of 99.7% could be obtained when the compressed powder with the average primary particle size of 44 nm was hot-pressed at a temperature as low as 1100°C for 1 h. This revised version was published online in November 2006 with corrections to the Cover Date.     

Chemical preparation of PLZT powder from aqueous solution

A new method for preparing homogeneous lanthanum-modified lead zirconate-titanate (PLZT) powder from aqueous salt solution containing hydrogen peroxide by precipitation is described. Inorganic salts such as zirconium oxychloride and titanium tetrachloride instead of organo-metallic salts can be available as raw materials which attain well reactioned and fine-grained PLZT powder processed by decomposing the dried product at 650°C. PLZT ceramics formed by hot-pressing the powder show considerable improvements in the qualities such as optical transparency and homogeneity.     

Evolution of alloying elements during outgassing in an aluminium zinc magnesium alloy made by powder metallurgy

To obtain high-quality produces from aluminium alloys powders, degassing is usually carried out before consolidation. But, in the case of Al-Zn-Mg alloys an evaporation of alloying elements may occur during outgassing. The aim of this work was to determine the influence of degassing conditions (temperature, pressure, nature of the gaseous atmosphere) on the stability of the alloy composition. These studies were performed on X7091 alloy by means of thermogravimetry and temperature-programmed desorption technique. The results show that liberation of alloying elements is closely connected with the evolution of the surface oxide layer which itself depends on various thermodynamic and kinetic parameters. The role of water vapour in the gaseous environment was particularly emphasized.     

Degradation of perchloroethylene in cosolvent solutions by zero-valent iron

Remediation of sites contaminated by chlorinated organic compounds is a significant priority in the environmental field. Subsequently, the addition of cosolvent solutions for in situ flushing of contaminated source zones has been successfully field tested. However, the treatment of effluent fluids in such cleanup efforts is an often overlooked component of this technology implementation. The purpose of this research was to evaluate the effectiveness of zero-valent iron (Fe(0)) in treating perchloroethylene (PCE) in an aqueous solution, and how the presence of a cosolvent (ethanol) and modification of the iron surface altered dechlorination. The modified iron surfaces included in this study were nickel-plated iron, acid-treated iron, and untreated iron surfaces. PCE dechlorination in the presence of each of the iron surfaces displayed pseudo first-order kinetics. The highest degradation rate of PCE occurred on the nickel-plated iron surface, 5.83 x 10(-3)h(-1), followed by the acid-treated iron, 4.92 x 10(-3)h(-1), and the untreated iron, 3.34 x 10(-3)h(-1). Dechlorination on each of the surfaces decreased with increasing cosolvent fractions. It was shown that as cosolvent fractions increased, PCE adsorption decreased and resulted in a concomitant decrease in PCE degradation rates.     

Morphology of nickel powder contactly precipitated on magnesium in aqueous solutions

We analyze the morphology of nickel powders obtained by contact precipitation on magnesium and establish the hierarchy of the shape of particles. We show that in the concentration range of nickel salt of 0.2–2.0 M and at the temperature range 20–95°C, elementary precipitated round particles form rings, arcs, and polygonal conglomerations of the second level of formation. The reproduction of the magnesium surface occurs at the third level. The greater the specific yield of nickel, the better this reproduction. We establish that the value of the specific yield of nickel increases with increase in the temperature and concentration of nickel ions in the solution.Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 3, pp. 118–122, May–June, 2004.     

Effects of pH on dechlorination of trichloroethylene by zero-valent iron

The surface normalized reaction rate constants (k(sa)) of trichloroethylene (TCE) and zero-valent iron (ZVI) were quantified in batch reactors at pH values between 1.7 and 10. The k(sa) of TCE linearly decreased from 0.044 to 0.009l/hm(2) between pH 3.8 and 8.0, whereas the k(sa) at pH 1.7 was more than an order higher than that at pH 3.8. The degradation of TCE was not observed at pH values of 9 and 10. The k(sa) of iron corrosion linearly decreased from 0.092 to 0.018l/hm(2) between pH 4.9 and 9.8, whereas it is significantly higher at pH 1.7 and 3.8. The k(sa) of TCE was 30-300 times higher than those reported in literature. The difference can be attributed to the pH effects and precipitation of iron hydroxide. The k(sa) of TCE degradation and iron corrosion at a head space of 6 and 10ml were about twice of those at zero head space. The effect was attributed to the formation of hydrogen bubbles on ZVI, which hindered the transport the TCE between the solution and reaction sites on ZVI. The optimal TCE degradation rate was achieved at a pH of 4.9. This suggests that lowering solution pH might not expedite the degradation rate of TCE by ZVI as it also caused faster disappearance of ZVI, and hence decreased the ZVI surface concentration.