Expansion of d-spacing of boehmite for enhanced phosphate adsorption via hydrogen bond network

Abstract

The optimization of the interlayer water content to increase the d-spacing of layered materials has recently attracted interest in pollutant removal. Several studies have explored phosphate adsorption onto boehmite considering its high surface area; however, the influence of its d-spacing on the adsorption performance remains unexplored. Therefore, our study aimed to investigate the impact of the synthesis pH of boehmite on the d-spacing and adsorption performance of phosphate ions. For this purpose, boehmite samples were synthesized under various pH conditions at 100°C to be tested for phosphate adsorption. As a result of this mild synthesis condition, the number of reactive hydroxyl groups in boehmite samples ranged from 7.02 – 9.02mmol/g, nearly four times higher than the reported boehmites. Boehmite prepared under acidic conditions exhibits a relatively high interlayer water content and an enlarged d-spacing from 0.640 to 0.996nm, resulting in superior phosphate adsorption (215mg/g) within one minute. We also confirmed that phosphate adsorption increased the binding energy states of aluminum and oxygen in boehmite, indicating a robust hydrogen bond network with phosphate ions in the inner space. In contrast, the adsorption decreased the binding energy for other types of boehmite, indicating charge interaction. Competitive adsorption tests also showed the strong affinity of boehmite for phosphate ions, even in the presence of fluoride ions. These findings highlight the importance of the interlayer water molecules in controlling the d-spacing of boehmite, indicating their critical role in removing anions from water.