In highly alkaline solution, aluminum speciates as the tetrahedrally coordinated aluminate monomer, Al(OH)₄⁻ and/or dimer Al₂O(OH)₆²⁻, yet precipitates as octahedrally coordinated gibbsite (Al(OH)₃). This tetrahedral to octahedral transformation governs Al precipitation, which is crucial to worldwide aluminum (Al) production, and to the retrieval and processing of Al-containing caustic high-level radioactive wastes. Despite its significance, the transformation pathway remains unknown. Here we explore the roles of atmospheric water and carbon dioxide in mediating the transformation of the tetrahedrally coordinated potassium aluminate dimer salt (K₂Al₂O(OH)₆) to gibbsite versus potassium dawsonite (KAl(CO₃)(OH)₂). A combination of in situ attenuated total reflection infrared spectroscopy, ex situ micro X-ray diffraction, and multivariate curve resolution-alternating least squares chemometrics analysis reveals that humidity plays a key role in the transformation by limiting the amount of alkalinity neutralization by dissolved CO₂. Lower humidity favors higher alkalinity and incorporation of carbonate species in the final Al product to form KAl(CO₃)(OH)₂. Higher humidity enables more acid generation that destabilizes dawsonite and favors gibbsite as the solubility limiting phase. This indicates that the transition from tetra- to octahedrally coordinated Al does not have to occur in bulk solution, as has often been hypothesized, but may instead occur in thin water films present on mineral surfaces in humid environments. Our findings suggest that phase selection can be controlled by humidity, which could enable new pathways to Al transformations useful to the Al processing industry, as well as improved understanding of phases that appear in caustic Al-bearing solutions exposed to atmospheric conditions.

中文翻译：

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大气在片钠铝石与四面体铝酸盐类三水铝石沉淀中的控制作用

在强碱性溶液中，铝形成四面体配位的铝酸盐单体 Al(OH) ₄ ^-和/或二聚体 Al ₂ O(OH) ₆ ^2-，但沉淀为八面体配位的三水铝石 (Al(OH) ₃ )。这种四面体到八面体的转变控制着铝沉淀，这对全球铝 (Al) 生产以及含铝腐蚀性高放射性废物的回收和处理至关重要。尽管具有重要意义，但转化途径仍然未知。在这里，我们探讨了大气水和二氧化碳在介导四面体配位铝酸钾二聚体盐 (K ₂ Al ₂ O(OH)₆ ) 三水铝石与钾片钠铝石 (KAl(CO ₃ )(OH) ₂ )。的组合原位衰减全反射红外光谱法，易地微X射线衍射，和多元曲线分辨率交替最小二乘化学计量学分析表明，湿度由溶解的CO限制性碱度中和的量起着变换关键作用₂ . 较低的湿度有利于较高的碱度和最终铝产品中碳酸盐物质的结合形成 KAl(CO ₃ )(OH) ₂. 较高的湿度会产生更多的酸，从而使片钠铝石不稳定并有利于三水铝石作为溶解度限制相。这表明从四面体配位铝到八面体配位铝的转变不一定发生在本体溶液中，正如通常假设的那样，而是可能发生在潮湿环境中矿物表面的薄水膜中。我们的研究结果表明，相选择可以通过湿度控制，这可以为铝加工行业提供新的铝转化途径，以及更好地了解暴露于大气条件下的含腐蚀性铝溶液中的相。