Recovery of phosphorus from desorption solution via vivianite crystallization: Influence mechanism of coexisting...

Adsorption has been widely used for phosphorus (P) removal, but few studies have focused on recovering P from desorption solutions. In this study, the adsorption and desorption conditions of adsorbent La-N201 were optimized, achieving an adsorbed amount of 45.15 ± 0.19 mg-P/g and desorption efficiency of 78.27 ± 0.19 %. The effects of coexisting anions (SO₄^2-, NO₃^-, SiO₃^2-, HCO₃^-) on adsorption-desorption-crystallization process were systematically evaluated. Results showed that all four anions reduced P adsorption, following the order: SiO₃^2- > SO₄^2- > NO₃^- > HCO₃^-. During desorption, SO₄^2-, NO₃^- and HCO₃^- inhibited P release, while SiO₃^2-enhanced it. Following the addition of an Fe(II) salt to the desorption solution, the resulting formation of vivianite via crystallization at pH of 7.0 and Fe/P of 1.5 achieved a phosphorus recovery efficiency of 88.38 ± 0.30 % in the absence of anions. This efficiency, however, decreased in the presence of competing anions: SO₄^2- stabilized recovery at approximately 63.5 %, NO₃^- reduced it to about 70.7 % at high concentrations, while both SiO₃^2- and HCO₃^- exhibited concentration-dependent suppression. SEM analysis revealed that anion competition led to the deposition of secondary phases on vivianite surfaces: SO₄^2- promoted the formation of Fe(OH)₂ and goethite; NO₃^- enhanced Fe(II) oxidation to Fe(III) and FePO₄ precipitation; SiO₃^2- and HCO₃^- led to the formation of FeSiO₃ and FeCO₃, respectively. These findings provide critical mechanistic insights into anion-specific interference during P crystallization, facilitating P recovery from secondary effluents via adsorption-coupled crystallization.