Application of response surface methodology with a Box–Behnken design for struvite precipitation

Struvite crystals were precipitated by the reaction of magnesium chloride hexahydrate and ammonium dihydrogen phosphate using different concentrations of citric acid as the additive (100, 300, and 500 ppm). The structure, morphology, functional groups and particle size of the crystals were evaluated experimentally by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and particle size analysis. The experimental results demonstrated that citric acid exerted a significant influence on the struvite precipitation and the crystal morphology changed from rod-like to tubular shaped with a larger size and hollow bodies. The average particle size changed from 17.60 to 33.60 μm with increasing citric acid concentration. The results of FTIR suggested that the citric acid adsorbed on the crystal surface. Following the characterization of the crystals prepared using different concentrations of citric acid, the response surface methodology coupled with Box-Behnken design were applied as a statistical tool to determine the effects of the key parameters affecting the precipitation process (temperature, pH and additive concentration) on the responses (namely, particle size and specific cake resistance of struvite). Second-order polynomial equations for both responses were improved to correlate the parameters. Analysis of variance (ANOVA) showed a significant quadratic regression model with high coefficients of the determination values. The optimum conditions for particle size were found to be 60 °C, pH 8 and 500 ppm additive concentration.     

Comparison of Amberlite IR 120 and dolomite's performances for removal of heavy metals

The presence of heavy metals in the environment is major concern due to their toxicity. Contamination of heavy metals in water supplies has steadily increased over the last years as a result of over population and expansion of industrial activities. A strong cation-exchange resin, Amberlite IR 120 and a natural zeolite, dolomite were used for the removal of lead(II) and cadmium(II). The optimum conditions were determined in a batch system as concentration range was between 5 and 100 mg/L, pH range between 1 and 8, contact time between 5 and 90 min, and the amount of adsorbent was from 0.1 to 1g. A constant stirring speed, 2000 rpm, was chosen during all of the experiments. The optimum conditions were found to be a concentration of 20 mg/L, pH of 5, contact time of 60 min and 0.5 g of adsorbent. Also, for investigation of exchange equilibria different amounts of ion exchange resin and dolomite were contacted with a fixed volume and concentration of a heavy metal bearing solutions. Sorption data have been interpreted in terms of Langmuir and Freundlich equations. The effect of adsorption temperature on the heavy metals adsorption onto dolomite was investigated at three different temperatures (20, 40 and 60 degrees C). Thermodynamic parameters were calculated. The results obtained show that the Amberlite IR 120 strong cation-exchange resin and dolomite performed well for the removal of these heavy metals. As a low cost adsorbent, dolomite can preferable for removal of heavy metals from wastewaters.     

Mechanical properties of bovine hydroxyapatite (BHA) composites doped with SiO<Subscript>2</Subscript>, MgO,Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, and ZrO<Subscript>2</Subscript>

Biologically derived hydroxyapatite from calcinated (at 850 °C) bovine bones (BHA) was doped with 5 wt% and 10 wt% of SiO₂, MgO, Al₂O₃ and ZrO₂ (stabilized with 8% Y₂O₃). The aim was to improve the sintering ability and the mechanical properties (compression strength and hardness) of the resultant BHA-composites. Cylindrical samples were sintered at several temperatures between 1,000 and 1,300 °C for 4 h in air. The experimental results showed that sintering generally occurs at 1,200 °C. The BHA–MgO composites showed the best sintering performance. In the BHA–SiO₂ composites, extended formation of glassy phase occurred at 1,300 °C, resulting in structural degradation of the resultant samples. No sound reinforcement was achieved in the case of doping with Al₂O₃ and zirconia probably due to the big gap between the optimum sintering temperatures of BHA and these two oxides.     

Influence of the orientation of the silicon substrate on the properties of avalanche Si:Er:O light-emitting structures

The influence of the orientation of silicon on the structural and luminescence properties of avalanche light-emitting diodes fabricated by the coimplantation of erbium and oxygen followed by solid-phase epitaxial (SPE) crystallization of the amorphized layer is considered. The luminescence properties are a consequence of the formation of various structural defects during the SPE crystallization: V-shaped dislocations and erbium precipitates form in (100) Si:Er:O layers, and larger structural defects, i.e, twins, are observed in (111) Si:Er:O layers along with an increase in the dislocation density by more than four orders of magnitude in comparison with the (100) orientation. The luminescence properties of avalanche and tunnel light-emitting diodes are also compared. In contrast to tunnel diodes, in avalanche diodes erbium ions are excited in the entire space-charge layer, and the effective excitation cross section of the Er³⁺ ions and their lifetime in the excited state are 3–4 times larger. Fiz. Tekh. Poluprovodn. 33, 660–663 (June 1999)     

A new algorithm for ground wave propagation based on a hybrid ray-mode approach

An efficient novel algorithm is introduced for ground wave propagation problems. First, ground wave propagation characteristics for a vertically polarized short electric dipole over a smooth spherical earth are reviewed, reducing the vector electromagnetic problem for the three-dimensional spherical geometry to an equivalent two-dimensional rectilinear scalar potential problem which is solved by spectral analysis and synthesis. Alternative evaluations of the spectral integral yield ray optical and normal mode solutions, which are conventionally referred to as the Norton and Wait formulations, respectively. Combining these formulations in an efficient manner yields a hybrid algorithm which is constructed so as to account adaptively for the characteristics of ground wave propagation in interference, intermediate and diffraction regions (including mixed paths) for various source and/or receiver heights. Numerical comparisons are made with reference results obtained via the parabolic equation (PE) method, in parametric ranges where PE is reliable; this permits assessment of the effectiveness of the hybrid approach. © 1998 John Wiley & Sons, Ltd.     

Local spectroscopy of band gaps in ferroelectric photonic crystals

We have measured visible to near-UV reflection spectra of opal photonic crystals infiltrated with ferroelectrics: barium titanate, sodium nitrite, potassium iodate, and triglycine sulfate. An experimental procedure has been developed for the infiltration of various ferroelectrics into opal pores through laser ablation and laser implantation. Using a fiber-optic probe, we were able to analyze surface reflection spectra of photonic crystals with a 0.2-mm resolution. A deuterium lamp was used as a broadband UV source, which allowed us to observe both the first and second [111] photonic band gaps in the reflection spectrum of opal crystals.     

Effect of soft segment molecular weight on tensile properties of poly(propylene oxide) based polyurethaneureas

Influence of soft segment molecular weight and hard segment content on the morphology, thermomechanical and tensile properties of homologous polyurethaneurea copolymers based on narrow molecular weight poly(propylene oxide)glycol (PPG) oligomers were investigated. A series of polyurethaneureas with hard segment contents of 12–45% by weight and PPG number average molecular weights <M_n> of 2000 to 11,800 g/mol were synthesized and characterized structurally by SAXS and mechanically by DMA and stress strain analysis. Bis(4-isocyanatocyclohexyl)methane and 2-methyl-1,5-diaminopentane were used as the diisocyanate and the chain extender respectively. All copolymers displayed microphase separation by SAXS and DMA. The critical entanglement molecular weight (M_e) of PPG is reported to be around 7700 g/mol. Our mechanical results suggest that when copolymers possess similar hard segment contents and are compared to those based on soft segments with number average molecular weights (M_n) greater than M_e, they generally displayed higher tensile strengths and particularly lower hysteresis and creep than those having soft segment molecular weights below M_e. These results imply that soft segment entanglements in thermoplastic polyurethaneureas may provide a critical contribution to the tensile properties of these copolymers – particularly in the range where the soft segment content is dominant.     

Polyacrylamide–gelatine carrier system used for invertase immobilization

Invertase was immobilized into polyacrylamide–gelatin carrier system by chemical cross-linking with chromium (III) acetate, chromium (III) sulphate, and potassium chromium (III) sulphate. The optimum conditions, namely substrate concentration, temperature, and pH were determined. The effect of polyacrylamide–gelatin ratio and cross-linker concentration on immobilized enzyme activity were analysed. Maximum immobilized enzyme activities were obtained with chromium (III) acetate (0.01 mol dm⁻³), chromium (III) sulphate (0.004 mol dm⁻³) and potassium chromium (III) sulphate (0.001 mol dm⁻³) for 0.177 (w/w) polyacrylamide–gelatin carrier ratio as 79%, 72% and 79%, respectively. The K_m values were 86 and 166 mM for free and immobilized enzyme, respectively. All immobilized samples were used 20 times over a period of 2 months without a considerable loss of activity.     

Influence of soft segment molecular weight on the mechanical hysteresis and set behavior of silicone-urea copolymers with low hard segment contents

Effect of polydimethylsiloxane (PDMS) soft segment molecular weight (M_n = 3200, 10,800 and 31,500 g/mol) and urea hard segment content (2.0-11.4% by weight) on the hysteresis and permanent set behavior of segmented silicone-urea (TPSU) copolymers were investigated. In spite of very low hard segment contents, all copolymers formed self-supporting films and displayed good mechanical properties. When the mechanical hysteresis and set behavior of the silicone-urea copolymers with similar hard segment contents (around 7.5% by weight) but based on PDMS-3K, PDMS-11K and PDMS-32K were compared, it was very clear that as the PDMS molecular weight increased, hysteresis and instantaneous set values decreased significantly. Copolymers based on the same silicone soft segment (PDMS-11K or PDMS-32K) but with different hard segment contents showed a linear increase in hysteresis and a slight decrease in the instantaneous set as a function of hard segment content. Constant initial stress creep experiments also showed lower creep as the PDMS segment molecular weight increased for copolymers with similar urea contents. Since the critical entanglement molecular weight (M_e) of PDMS is stated to be 24,500 g/mol, our results tend to suggest important contribution of chain entanglements on the hysteresis and instantaneous set of these silicone-urea copolymers.