Research On Structural Evolution Of Thermally Treated Al-substituted Goethite And Its Response Of Surface Reactivity

Iron oxides are important mineral component of the earth’s surface system. Thereare sixteen kinds of iron oxides were found in natural environments, in which commoniron oxides take up5types, namely hematite, goethite, magnetite, lepidocrocite,ferrihydrite. Moreover, goethite is the most crucial iron oxides in the supergeneoxidation environment as well as the one of the most important components havingstrong surface activity and chemical-biological activities in soils and deposits. As anatural nano-micron mineral, goethite plays an important role in transfer, transforamtionand enrichment of organic pollutions, inorgainc anions and heavy metals in theenvironments. Therefore, the structure and surface reactvity of goethite became the hotpoint in the field of mineralogy and environmental geochemistry. However, the currentstudy mianly concentrates on the bulk structure and its physicochemical properties. Thestudy on the relationship between the surface stucture and reactivity especially for theeffect of the struture of atomic local area was scarely reported, which limited theunderstanding of the role of mineral in environmental self-purification. On the otherhand, the study on the structure and physicochemical properties of pure goethite wasreported extensively. However, the researchs on the effect of isomorphrous substitutionby Al on the local structure, crystal chemical properties and surface reactivity wasrarely reported, although the existence of Al substitution for Fe in the structure ofgoethite has been demonstrated.Therefore, in this present thesis, the techniques of micro-beam and micro district(FESEM, TEM, etc), modern spectroscopy techniques (FTIR, Raman, etc) andcharacterization techniques of mineral phase and crystal chemistry were used tocharacterize the synthetic Al-substituted goethite and natural goethite. The objective isto illuminate the effect of Al substitution for Fe in the structure of goethite on the localstructure, surface physicochemical properties and thermal stability of goethite.Eventually, the functionized natural goethite by annealing in air or hydrogen was usedas catalysts or catalysts carrier to catalytic cracking biomass tar, decompose nitrate andPNP, remove phosphate and heavy metals ions from aqueous solution. The maincreative results are list as fellows. Four kinds of hydroxyl and three kinds of surface water were identified accordingto the results of FT-ATR and FT-IES. The four hydroxyl groups contained one structurehydroxyl and three surface hydroxyls.The synthesis of Al-substituted goethite indicated that the high pH favored theformation of Al-substituted goethite. The occurrence of Al susbtitution not onlyhampered the growth of goethite and decreased the crystallinity, but also affected thecrystal morphology reflecting in the decrease of long-width ratio with the increase of Alsubstitution amount. The substitution of Al for Fe increased the thermal stability ofgoethite. The starting temperature of the transforamtion of goethite to hematiteincreased from225oC to275oC and the active energy increased from117.8kJ/mol to126.7kJ/mol as the Al substitution amount increased from0to9.1mol%. In addition,the Al substitution still hindered the transformation of goethite to metallic iron.Limonite obtained from Xinqiao was mainly composed of acicular goethites withopen nano pore structure, in which the content of goethite was over95%. In addition,the existence of Al-substituted goethite was proved in the samples. The transforamtionof goethite to hematite occured at225oC, newly formed hamatite kept the originalshape of goethite and much nanopore was formed due to the dehydroxyl of goethite.The particles of hematite and pore size increased with the increase of annealingtemperature resulting in the reduce of specific surface area. The formed hematite had alargest surface area after annealing at the range of250-350oC. Comparing with thegothite, the surface area of annealed products increased from12.7to111.6m~2/g, whichprovided a crucial predomination for the utilizaiton of annealed goethite as a catalyst orcatalyst carrier.Based on the understanding to the structural evolution of Al-substituted goethite,the annealed products involving hematite, magnetite and zero valent iron were utilizedto catalytic cracking of biomass tar, decompose nitrate and PNP, remove phosphate andheavy metals compared with commercial iron powder. The results showed that naturalgoethite had better property as catalyst carrier than synthetic goethite. In addition, nanozero valent iron (NZVI) prepared by reducing natural goethite had a better surfacereactivity in decomposition of nitrate and PNP, and removal of heavy metals fromaqueous solution than that of commercial iron powder. It was speculated that the strongsurface reactivity of newly formed hematie and this NZVI contributed to the crystaldefects and large surface area. This indicated that natural goethite possesses promisingapplication potential in purifying pollutions. Besides, a new approach to detect theconcentration of carbonaous gases was exploited. The TOC/N was proved to be a goodinstrument for detecting organic carbon, inorganic carbon and methane, etc.