Synthesis and phosphate adsorption behaviour of Mg/Al-pillared montmorillonite loaded with La(OH)3

Abstract

Magnesium/aluminum-pillared montmorillonite loaded with lanthanum hydroxide (Mg/Al-MMT-La(OH)₃) was synthesized using non-toxic raw materials by ion-exchange and co-precipitation for phosphate removal from wastewater. Some adsorbents were fabricated in different molar ratios of Mg to Al, analyzed using various characterizations, and investigated in batch adsorption experiments. The determined adsorption kinetics of the 1:4 Mg/Al-MMT-La(OH)₃ composite fitted well with the Elovich model. In addition, the Langmuir model revealed the high adsorption efficiency of phosphate by the adsorbent with a maximum adsorption capacity of 79.33 mg/g. The negative value of ΔG° and positive value of ΔH° (64.25 kJ/mol) demonstrated that phosphate adsorption onto 1:4 Mg/Al-MMT-La(OH)₃ was spontaneous and endothermic in nature. Moreover, when the molar ratio of ${\mathrm{PO}}_4^{3-}$ to ${\mathrm{CO}}_3^{2-}$ was 1:5, the phosphate adsorption capacity reduced by 53.5%, far exceeding the effect of ${\mathrm{NO}}_2^{-}$, ${\mathrm{NO}}_3^{-}$, Cl^-, and ${\mathrm{SO}}_4^{2-}$. The addition of Al³⁺ caused a sharp decline in phosphate removal property by 81.9% when ${\mathrm{PO}}_4^{3-}$/Al³⁺ molar ratio was 1:5, however, the presence of other cations showed the negligible impact on it. The adsorption mechanism primarily involved ion exchange with intercalated anions and surface coordination with loaded hydroxides. Results proved that 1:4 Mg/Al-MMT-La(OH)₃ material has a favourable application potential in the surface water remediation.

Keywords: La(OH)3; Mg/Al mixed solution; Phosphate; adsorption; montmorillonite.