共沈法によるシュベルトマナイトと各種陰イオン置換体の合成

FeOOH鉱物の生成における陰イオンの役割

抄録

Synthesis of schwertmannite was undertaken at 60°C and aging time for 12 mins by mixing of two equi-volume solutions of 0.02M Fe (NO₃) ₃ and 0.02M sodium salt solutions with different species of anions such as Cl, CO₃, SO4, SeO₄, CrO₄, As0₄, and PO₄ in order to elucidate the role of anions in the precipitation of ferric iron. At a given experimental condition, precipitates from S0₄, Se0₄, and Cr0₄ containing solutions were schwertmannite and that from Cl containing solution was akaganeite associated with 6L-ferrihydrite, the former two minerals whose structures are basically that of β-FeOOH. Aging of 0.01M Fe (NO₃) ₃ solution formed lepidocrocite (γ-FeOOH) via 6L-ferrihydrite. Ferrihydrite precipitated from CO₃-bearing solutions and amorphous materials from As0₄, and PO₄ solutions. The observed crystallization processes suggest that the formation of Fe polycation precursors took place early in the hydrolysis reaction, and these precursors provided nuclei to the growth and formation of different FeOOH structures by aggregation. In the course of the precursor formation, anion species such as Cl, SO₄, SeO₄, and CrO₄ that have proper values of average electronegativity could form stable complexes with Fe polycations, which resulted in the formation of β-FeOOH stmcture involving schwertmannite and akaganeite. In contrast with the seanions, NO₃ existed as a counter anion in solution because of its higher average electronegativity and consequently formed γ-FeOOH at 60°C. Since electronegativities of As0₄ and PO₄ are too small to form stable complexes with Fe polycations favorable for the formation of β-FeOOH structure, these anions led to form amorphous materials. Thus, the role of dissolved anions on the structure formation of FeOOH minerals was rationalized by taking into consideration the electron-donor characteristics of anion such as the average electronegativity and the bonding strength.