Adsorption mechanisms of chromate and phosphate on hydrotalcite: A combination of macroscopic and spectroscopic studies

Abstract Hydrotalcite (HT) is a layered double hydroxide (LDH), which is considered as a potential adsorbent to remove anion contaminants. In this study, adsorption of chromate (CrO 4 ) and phosphate (PO 4 ) on HT was conducted at various pH and temperatures. Related adsorption mechanisms were determined via the isotherm, kinetic, and competitive adsorption studies as well as the Cr K -edge X-ray absorption fine-structure (XAFS) spectroscopy. The maximum adsorption capacities for CrO 4 and PO 4 on HT were 0.16 and 0.23 mmol g −1 . Regarding adsorption kinetics, CrO 4 and PO 4 adsorption on HT could be well described by the second order model, and the rate coefficient of CrO 4 and PO 4 on HT decreased significantly with the increasing pH from 5 to 9. The adsorption kinetics for CrO 4 and PO 4 were divided into fast and slow stages with the boundary at 15 min. This biphasic adsorption behavior might be partially attributed to multiple reactive pathways including anion exchange and surface complexation. Fitting results of Cr K -edge EXAFS analysis showed a direct bonding between CrO 4 and Al on HT surfaces. Such a surface complexation appeared to be the rate-limiting step for CrO 4 adsorption on HT. By contrast, the diffusion through the hydrated interlayer space of HT was the major rate-limiting step for PO 4 . This study determined the adsorption behaviors of CrO 4 and PO 4 on HT, including the initial transfer process and the subsequent adsorption mechanisms. Such information could improve the strategy to use HT as the potential adsorbent for the remediation of anionic pollutants.