Dembowski Mateusz, Graham Trent R, Reynolds Jacob G, Clark Sue B, Rosso Kevin M, Pearce Carolyn I
Pacific Northwest National Laboratory, Richland, Washington, 99352, USA.
Phys Chem Chem Phys. 2020 Nov 14;22(42):24677-24685. doi: 10.1039/d0cp04820h. Epub 2020 Oct 26.
The role of oligomeric aluminate species in the precipitation of aluminum (Al) phases such as gibbsite (α-Al(OH)) from aqueous hydroxide solutions remains unclear and difficult to probe directly, despite its importance for developing accurate predictions of Al solubility in highly alkaline systems. Precipitation in this system entails a transition from predominantly tetrahedrally coordinated aluminate (Al(OH)) species in solution to octahedrally coordinated Al in gibbsite. Here we report a quantitative study of dissolved Al in the Al-KOH-HO system using a combination of molecular spectroscopies. We establish a relationship between changes in Al NMR chemical shifts and the relative intensity of Raman vibrational bands, indicative of variations in the ensemble speciation of Al in solution, and the formation of unique contact ion pair interactions with the aluminate dimer, AlO(OH). A strong correlation between the extent of Al oligomerization and the amount of solvated Al was demonstrated by systematically varying the KOH : Al molar ratio. The concentration of dissolved oligomeric Al in solution also directly impacted the particle size and morphology of the precipitated gibbsite. High concentrations of dimeric AlO(OH) yielded smaller and more numerous anhedral to subhedral gibbsite particles, while low concentrations yielded fewer and larger euhedral gibbsite platelets. The collective observations suggest a key role for the AlO(OH) dimer in promoting gibbsite precipitation from solution, with the potassium ion-paired dimer catalyzing a more rapid transformation of Al from tetrahedral coordination in solution to octahedral coordination in gibbsite.
尽管低聚铝酸盐物种对于准确预测铝在高碱性体系中的溶解度很重要，但它在从氢氧化盐水溶液中沉淀出诸如三水铝石（α - Al(OH)₃）等铝（Al）相过程中的作用仍不明确且难以直接探究。该体系中的沉淀过程涉及从溶液中主要以四面体配位的铝酸盐（Al(OH)₄⁻）物种向三水铝石中八面体配位的铝的转变。在此，我们结合分子光谱法对Al - KOH - H₂O体系中溶解态铝进行了定量研究。我们建立了铝核磁共振化学位移的变化与拉曼振动带相对强度之间的关系，这表明溶液中铝的整体形态变化，以及与铝酸二聚体Al₂O(OH)₂²⁻形成独特的接触离子对相互作用。通过系统改变KOH与Al的摩尔比，证明了铝低聚程度与溶剂化铝量之间存在强相关性。溶液中溶解的低聚铝浓度也直接影响了沉淀三水铝石的粒径和形态。高浓度的二聚体Al₂O(OH)₂²⁻产生更小且数量更多的从无定形到次晶形的三水铝石颗粒，而低浓度则产生更少且更大的自形三水铝石片晶。这些综合观察结果表明，Al₂O(OH)₂²⁻二聚体在促进三水铝石从溶液中沉淀方面起关键作用，钾离子配对的二聚体催化铝从溶液中的四面体配位更快地转变为三水铝石中的八面体配位。
