Layered double hydroxides (LDH) and its magnetic composites have potential as recyclable high capacity sorbents for removal of oxyanions such as phosphate (P) from wastewater. The question is if these sorbents are sufficiently stable for use and regeneration at relevant waste water pH between 7 and 8? Despite a high number of publications in the field there are very few studies on the stability of the LDH phase when used as sorbent. We have synthesized Fe3O4—Mg3Fe-LDH composites and investigated their phosphate sorption properties and stability versus pH. However, the LDH component was highly unstable at pH below 9, with up to 50 % of Mg in LDH released to solution during 1 h. Elevated P concentrations also stimulated LDH dissolution even at high pH. SEM, TEM, Mössbauer spectrosopy and XRD data demonstrate that various sorption reactions take place depending on pH and phosphate concentration. At acid pH, P is adsorbed to the amorphous poorly crystalized ferrihydrite phase produced when the LDH dissolves. At neutral pH range, the LDH dissolves to a less extent and the release of Mg2+ is partially suppressed. Hence, P is sorbed on the remaining Mg3Fe LDH as well as the poorly crystalized ferrihydrite phase that has formed. Finally, at alkaline pH, Mg3Fe LDH is the primary sorbent of P at low P concentration. At higher P concentration, the Mg3Fe LDH phase is not stable and magnesium phosphates precipitate. Thus, P retention in the LDH system takes place via precipitation, and adsorption to Mg3Fe LDH and the poorly crystalized ferrihydrite phase - depneding on pH. This study illustrates the instability of the LDH phase that strongly limits their application as recyclable sorbents.

Magnetic LDH composites for phosphate sorption - is the LDH sufficiently stable?

ACS Spring Meeting 2021

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