Deintercalation of Li/Al LDH and its application to recover adsorbed chromate from used adsorbent

Layered double hydroxides (LDHs) have been shown to be potential adsorbents for anionic contaminants, but less studies have focused on how to treat the used adsorbents to recover adsorbed contaminants. In this study, we investigated the chromate (HCrO4−) adsorption on Li/Al LDH and the recovery of adsorbed chromate, achieved through the Li deintercalation reaction of Li/Al LDH. The adsorption of chromate on LDH was a fast process that could be accomplished in minutes at any temperature, ranging from 10 °C–90 °C. On the other hand, the Li deintercalation rate of Li/Al LDH was relatively slow at a temperature ≤ 25 °C and increased as the reaction temperature was increased. More than 95% of structural Li+ could be deintercalated from the LDH structure within 30 min at a temperature ≥ 60 °C. The Li deintercalation resulted in the loss of positive charges in the hydroxide layers and consequently the releases of counterbalancing chromate and Cl− in the interlayer. Therefore, practically, the material can be used as an effective adsorbent for chromate when the deintercalation reaction is inhibited at low temperature. After chromate is adsorbed, it can be released from the used material into the solution through deintercalating Li+ from the structure. This process can be achieved by simply using hot water to treat the used material and no additional chemicals are required. The final products of the deintercalation reaction include gibbsite and a Cr-containing solution. Gibbsite may be reused to synthesize Li/Al LDH and the solution containing Cr may be further treated to recover Cr. An innovative approach to recover adsorbed contaminant and regenerate the used adsorbent was proposed in this study.