Tuning the stability and phosphate sorption of novel MnII/IVFeII/III layered double hydroxides

Layered double hydroxides (LDHs) have been intensively studied for phosphate (P-i) removal but suffer from poor stability and low sorption affinity under ambient conditions. In this paper, well crystallized (MnFeFeIII)-Fe-II-Fe-II-Cl, (MnFeIII)-Fe-II-CO3 and novel (MnFeIII)-Fe-IV-CO3 LDHs were synthesized. The LDHs show fast P-i sorption with 90 % uptake within 20 min, and high P-i sorption capacity of 11 mg P/g at low solution P-i concenrations of 0.1 mg P/L, corresponding to a very high Pi sorption affinity (K-d 1.1 x 10(5) L/kg). Fast MnII dissolution from the (MnFeFeCl)-Fe-II-Fe-II-Cl-III_ LDHs and formation of MnFe2O4 at pH 7 were observed in aqueous suspensions of non-oxidized material where up to 70% of total Mn was released within 2 h. However, when interlayer Cl- was exchanged with CO32-, much lower Mn dissolution (5.4%) was observed. Furthermore, after oxidation of MnII to MnIV, the obtained (MnFeIII)-Fe-IV-CO3 LDH maintained the layered structure of LDH and the particles were surrounded by birnessite nanorods. The (MnFeIII)-Fe-IV-CO3 LDH showed excellent stability but lower P-i sorption capacity. However, a high sorption affinity was maintained which is attributed to more positively charged Fe-centered sorption sites. XPS and ATR-FTIR data together with DFT calculations demonstrated that P-i was mainly sorbed via the formation of mononuclear mono- and bidentate P-i surface complexes on planar LDH particle surfaces.

Automated summary

Three different LDHs were synthesized and studied for phosphate removal, showing fast and efficient sorption properties. The exchange of interlayer Cl- with CO32- significantly affected the dissolution of Mn. The phosphate sorption mechanism was mainly through the formation of mononuclear mono- and bidentate surface complexes on the LDH particles.