Effect Of Hazardous Metals On Crystallization Of Precipitated Struvite During Phosphorus Recovery

Recovered phosphorus?P?from wastewater by struvite as slow-release phosphate fertilizer to is an important means to solve the shortage of phosphorus resources and reduce eutrophication of water.During phosphorus crystallization,the heavy metals in wastewater will incorporate into struvite precipitation,causing the variation of struvite structure and morphology as well as accumulating in struvite products,which may result in potential hazards to soil and crops.Recent researchs focused on the optimization of equipment conditions and experimental parameters(such as pH value,precipitation time,the influence of Ca²⁺,Fe³⁺and organic matter)etc,but less attention has been paid to the accumulation of heavy metals in the products during phosphorus recovery from wastewater.This paper is focus on the influence of the content,crystal shape,morphology and chemical morphology of struvite during struvite crystallizating with heavy metals?Cu,Zn,Pb,Cd,Ni?added,explaining the mechanisms of heavy metals incorporate into struvite products.Besides,nvestigated the effects of coexistence of various heavy metals on the content and phase of struvite products.And understand the influence of different Mg sources and the molar ratio of N/P/Mg on struvite crystallization during struvite recover from real waste water.The precipitation exprements were conducted at simulation wastewater with0.1-500 mg/L heavy metals added.The products were characterized by XRD?ICP-OES?SEM?TEM-Mapping and XPS.The results indicate that Cu formed the components as Cu-NH₃,Cu-PO₄ and Cu-OH attached to the surface of struvite,incorporating into products.With the formation of Zn-OH and Zn-PO₄,Zn precipitates attached to the surface of struvite and incorporated into the product,which had the most significant influence on the content of struvite for struvite vanishing with 500 mg/L Zn added.With 0.1-50 mg/L Pb added,it mainly shaped Pb-PO₄ and/or Pb-OH groups attached to the superficial of struvite and entered into the product.While at 100-500 mg/L,Pb accumulated in the product by the formation of lead hydroxyphosphate crystals.At 0.1-10 mg/L,Ni²⁺replaces Mg²⁺for the formation of NiPO₄NH₄·6H₂O;at 50-500 mg/L Ni adhered to the surface of struvite by the shape of Ni-PO₄ and Ni-OH components.Cd mainly generates the phases with Cd-PO₄ and Cd-OH components attached to the surface of the struvite.When Cu/Zn or Cu/Ni or Zn/Ni coexisted,struvite is the crystal in the products and its content decreases with the increase of heavy metal concentration ratio.At Cu/Zn=50:5-50:25,Cu/Ni=50:5-50:300 and Zn/Ni=50:5-50:300,the struvite content of the product was higher than the products with only single Cu or Zn or Ni added,which indicated that at these concentration ratios,the interaction between heavy metals will make their influence on struvite yield less than Cu or Zn or Ni added individually.However,at Cu/Zn=50:50-50:300,the interaction between Cu and Zn can significantly reduce the content of struvite in the product,making greater effect on the content of struvite than that when copper or zinc added alone.During struvite recovery from actual wastewater,not only struvite but Na₃PO₄·12H₂O?Na₂HPO₄·2H₂O and NiAP precipitated in the recovery product.Using Mg?OH?₂ as magnesium source is more favorable for struvite production,but the amount of Cu and Zn in the product also increases.By studying the influence of heavy metals on the quality of struvite products and the mechanism of their incorporate into struvite products,can provided theoretical guidance for the recovery of high-purity struvite products.