Phosphorus Transformation and Vivianite Recovery Enhancement from Acid Leaching Pig Manure

Pig manure has been recognized as a potent source for phosphorus recovery due to its substantial phosphorus content. To optimize the efficiency of phosphorus extraction, the effects of various acid types (sulfuric acid-H₂SO₄, hydrochloric acid-HCl, oxalic acid-H₂C₂O₄, and ascorbic acid-C₆H₈O₆), concentration of acids, duration of leaching, and the liquid-to-solid ratio were thoroughly examined. Additionally, the leaching process was monitored for the release behavior of heavy metals such as As, Cr, Cu, and Zn. The transformation mechanisms of phosphorus speciation before and after treatment were analyzed using the C-J extraction method. The potential of Fe²⁺-induced crystallization to produce vivianite, a phosphorus recovery method, was also explored. Results indicate that H₂C₂O₄ was superior in phosphorus release efficiency compared to H₂SO₄, HCl, and C₆H₈O₆. The sequence of heavy metal release during the acid leaching was Zn > Cu > As > Cr, with the H₂C₂O₄ group showing a significantly lower heavy metal release rate than the H₂SO₄ and HCl groups. In terms of phosphorus conversion in solid forms (Al-P and Ca-P), all acid treatments achieved over 80% conversion efficiency. Notably, the H₂C₂O₄ treatment showed enhanced conversion rates for Ex-P in solid phosphorus, as well as for particulate phosphorus (PP) and readily dispersible phosphorus (RDP) in the liquid phase. The optimal conditions for phosphorus extraction using H₂C₂O₄ were determined to be a concentration of 0.10 mol·L^-1, a leaching duration of 60 minutes, and a liquid-to-solid ratio of 50 mL·g^-1. Under these conditions, the soluble orthophosphate (Ortho-P) concentration in the liquid phase soared from 188 mg·L^-1 to 882 mg·L^-1, while the phosphorus release content dramatically increased from 9.42 mg·g^-1 to 44.1 mg·g^-1, marking a 369% enhancement in phosphorus release efficiency. The method of employing Fe²⁺-induced crystallization to recover phosphorus from the leachate treated with H₂C₂O₄ yielded high-purity vivianite has been confirmed by comprehensive analyses including X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). These findings lay a solid foundation for improving both the phosphorus recovery rate and the value of products derived from pig manure.