Facet-dependent phosphate adsorptive reactivity by lanthanum hydroxides of different crystal structure: Role of surface hydroxyl groups

Highlights: • Three La(OH)₃ materials of different crystal structure were fabricated. • Different dominating facets were identified for different La(OH)₃ crystals. • The density and type of –OH groups were determined and correlated with affinity. • Single La atom coordinated –OH groups -LaOH were dominating for phosphate affinity. • La(OH)₃ of smaller crystal grain size with -LaOH also promoted adsorption kinetics. Lanthanum hydroxides (La(OH)₃) materials play an important role in managing eutrophication. The crystal structure-reactivity relationship of La(OH)₃ nanoparticles deserves in-depth investigation. Herein, three La(OH)₃ nanomaterials of different crystal structures denoted as LH-1, LH-2 and LH-3 were freshly synthesized. LH-1 and LH-3 were in the form of rod-like crystallite with a dominant exposure of {1 0 0} facets which was occupied by singly La atom coordinated (-LaOH), doubly La atom coordinated (-La₂OH) and triply La atom coordinated (-La₃OH) hydroxyl groups. LH-2 presented as nano-plates and was majorly enclosed by {1 0 1} facets which was dominated by -La₂OH hydroxyl groups. The quantity of -LaOH groups on three materials was positively correlated with the Langmuir constants derived from the adsorption isotherm, which suggested they were the predominant species for enhanced phosphate affinity. In comparison, the contribution of -La₂OH to phosphate affinity was weaker, and function of -La₃OH was negligible. The high affinity and fast kinetic of LH-3 towards phosphate could be attributed to the abundance of -LaOH groups on its main exposed crystal planes and relatively small grain size. These results provided new insights into the understanding of functions of facets on La(OH)₃ crystals which could generate new strategies for engineering novel materials for water treatment.