Understanding how specific functional groups in humic acid affect the sorption mechanisms of different calcinated layered double hydroxides

Humic acid (HA) can affect the adsorption modes of CLDHs, and change the form of pollutants, making it a key factor when considering CLDHs for practical applications. However, the complicated and uncertain structures of HA make it difficult to explore how they will influence the mechanism, so using simple compounds to simulate the role of different reactive groups in HA necessary to understand how they will interact with the application of CLDHs. It was found that both CLDHs strongly adsorb phthalic acid and catechol. All hydroxyl and carboxyl compounds had a similar promoting effect on the adsorption rate and regeneration rate of Mg-CLDH, which were not affected by the number of functional groups, the length of the aliphatic chain, or the number of the aromatic ring. However, their effects were more significant than that of the HA. While they could make the structure of Mg-LDH stacked and incomplete, just like the HA, they did not damage the crystallinity. In particular, catechol inhibited the regeneration of Mg-CLDH to a certain degree. Since the regeneration process was different from Mg-CLDH, different types of hydroxyl or carboxyl compounds degraded the removal of borate by Ca-CLDH, but promoted the formation of ettringite to some extent, especially benzoic acid and 4-phenylbutyric acid. However, catechol severely inhibited the structural transformation of Ca-CLDH into ettringite. Due to the strong interaction of the HA with the surface of Ca-LDH, the formation of ettringite was hindered, which deteriorated the removal of borate. HA did change the adsorption rate and the regeneration process of CLDHs via different reactive groups, and their effects on various CLDHs were different.