Influence Of Low Molecular Organic Acids On The Formation And Transformation Of Iron Oxides

Hematite (a-Fe2O3) is the most stable iron oxide, with n-type semiconductor properties. It has small size, large surface area, many activated centers, etc. It can be used to catalyze the degradation of organic pollutants, adsorb Cr, As and other heavy metals, also be used as biomedical materials, Low molecular organic acids in the environment play an important role in the formation of hematite. Especially reducing organic acid can generate Fe(II) by an oxidation reduction with Fe(Ⅲ), which can facilitate the formation of hematite. L-tartaric acid(L-TA), malic acid(MA), succinic acid (SA) and citric acid (CA) have different acidic, number of carboxyl and hydroxyl groups, carbon chain length, the complex stability constant of ferric with acids.They have different effects on the conversion of iron oxide.In this work, we used these four acids as additives, and applied X-ray diffraction (XRD), electron microscopes (SEM, TEM, HRTEM), Fourier transform infrared adsorption spectrum(FT-IR) and Brunauer-Emmett-Teller (BET) analysis and other modern analytical tools to discuss their impacts on the formation and transformation of iron oxides, and reveal the reaction mechanisms of hematite with different morphology and size. Based on the study, the characteristics of degradation of methylene blue (MB) by synthetic product were studied.The main results are shown as follows:1. L-TA is dicarboxylic acid, which has reducibility for its two a-OH groups. L-TA acted as both reducing agent and template agent to promote the formation of hematite, it has high promotion activity. With the amounts of L-TA increased, the intensity of peak increased first then weakened, and reached strongest at1.0%. Product morphology which was corn-rod changed unconspicuous from0.5%L-TA to1.0%L-TA, but its particle size and length to diameter ratio increased. Corn rod-like hematite with a diameter of50-100nm and a length of100-150nm were obtained at1.0%L-TA, which was the optimal condition. L-TA can selective adsorb on hematite, the surface of small hematite particles that adsorbed L-TA can form rod-like hematite through oriented attachment along (001) planes. The promoting role of L-TA was more obvious with temperature increased. The crystallinity of the hematite enhanced with the extension of reaction time. The pH of reaction system has a greater influence on reaction. Pure hematite can be obtained at pH ≦7and pH≧10. Round hematite with smooth surface whose diameter is100-200nm were obtained at10h, pH11by Ostwald ripening mechanism.2. MA is also dicarboxylic acid, which has reducibility for its only one a-OH group. It also can act as both reducing agent and template agent to promote the formation of hematite, but its promotion activity was weaker than L-TA. Pure hematite can only be obtained at pH≦7, the promoting role of MA was not obvious at pH≧8. Round hematite with rough surface whose diameter is100-150nm can be obtained through Ostwald ripening mechanism. The optimal condition is20h, initial pH7and0.5%MA.3. SA is also dicarboxylic acid, which has no reducibility for it has no a-OH groups. CA is tricarboxylic acid, which has a-OH, but its a-C has no H, so its reducibility is lower than L-TA or MA. The addition of SA and CA inhibited the formation of crystalline iron oxides. SA and CA were adsorbed on the the surface of ferrihydrite to form organic acid-ferrihydrite ligands through electrostatic interaction or specific adsorption between groups. The organic acid-ferrihydrite ligands were a stable structure, the organic acid blocking dissolution sites on surface of the iron oxide. Thereby, it would reduce direct contact between particles by spatial block effects, inhibit the formation and transformation of iron oxide.a-OH activity of CA is low, and its molecular weight is relatively large, and the stability constants of CA complexation with Fe3+is large.The formed ligand has higher stability, making it more difficult to dissolve of the iron oxide. Also their larger spatial block is not conducive to the formation of crystalline iron oxides.4. Corn rod-like hematite obtained in the presence of1.0%L-TA at pH7has higher surface activity and bigger specific surface area which performed best on the degradation of MB, and the degradation rate is higher in alkaline pH that is93.12%. The degradation rate of MB increased with the increase of the percentage of H2O2and the quality of mineral, but it decreased with the increase of MB concentration.The degradation dynamic curves on MB fitted well with the first-order dynamical equation. The mechanisms are adsorption, oxidation and flocculation.