Morphologies and magnetic properties of α-Fe2O3 nanoparticles calcined at different temperatures.

Different morphologies and sizes of α-Fe2O3 were prepared by a coprecipitation method using polyvinylpyrrolidone as a dispersant. In the preparation process, homogeneous and dispersed nanoscale FeOOH particles were first obtained by the coprecipitation method, and then the FeOOH particles were calcined at high temperature to form α-Fe2O3. The growth and aggregation of the α-Fe2O3 particles at different calcination temperatures resulted in α-Fe2O3 powders with diversiform morphologies (nanoscale microsphere, pinecone ellipsoidal, polyhedral, and quasi-spherical structures). By analyzing the SEM images, it was inferred that the polyhedral structure of α-Fe2O3 particles was formed by the accumulation of rhomboid sheet structures and high-temperature growth. In terms of the magnetic properties, the samples belonged to the class of canted antiferromagnetic materials, and the morphology, particle size, and crystallite size of the α-Fe2O3 particles were important factors affecting the coercivity. Among these, when the calcination temperature was increased from 700 °C to 800 °C, the growth rate of the particle size was significantly faster than that of the crystallite size, and the coercivity increased substantially from 1411 Oe to 2688 Oe.

[Effects on phosphorus fraction distribution in sediment by roots of Vallisneria natans].

The present study explored phosphorus fractions in sediments with the growth of Vallisneria natans. Sediment samples in different layers were collected at 20, 50 and 80 d, and vertical change of several phosphorus fractions were measured in the samples. The root distributions and biomass of the V. natans were measured. Our results showed that roots were distributed between 0 and 14 cm in the experimental device. The average number of roots and average root length were 58 and 5.86 cm. After 80 days growth, the percentage of V. natans root biomass were 45.99%, 32.75%, 16.03% and 5.23% in the sediment with depths of 0-3, 4-6, 7-10 and 11-14 cm. Total phosphorus (TP) content, phosphorus extracted by NaOH (NaOH-P), and organic phosphorus (OP) levels remarkably decreased (P < 0.05) in the area with a high concentration of tape grass roots. The content of phosphorus extracted by HCl (HCl-P), and inorganic phosphorus (IP), showed no significant difference (P > 0.05). The results suggest that V. natans root affects the migration and transformation of phosphorus species in the sediment.

[Influence of submerged macrophytes on phosphorus transference between sediment and overlying water in the growth period].

In order to study the process of phosphorus transfer between sediment and overlying water, Hydrilla verticillata and Vallisneria natans were cultured in spring, Potamogeton crispus was cultured in winter. Changes of environmental factors and phosphorus concentrations in water and sediment were investigated. The results indicated that: submerged macrophytes could reduce all phosphorus fractions in the overlying water. Phosphorus concentrations in overlying water maintained in a relative low level in the growth period of submerged macrophytes. The concentrations of total phosphorus (TP) in overlying water of H. verticillata, V. natans and P. crispus were 0.03-0.05, 0.04-0.12, 0.02-0.11 mg x L(-1), respectively. All phosphorus fractions in sediment were reduced. The maximum value between submerged macrophyte and control of H. verticillata, V. natans and P. crispus were 35.34, 60.67 and 25.92 mg x kg(-1), respectively. Dissolved oxygen (DO), redox potential (Eh) and pH in overlying water increased (DO 10.0-14.0 mg x L(-1), Eh 185-240 mV, pH 8.0-11.0) in the submerged macrophytes groups. Submerged macrophytes increased Eh( -140 - -23 mV) and maintained pH(7.2-8.0) in neutral range. The results indicated that submerged macrophytes affected phosphorus transferring between sediment and overlying water through increasing DO, Eh and pH in overlying water, and Eh in sediment.