Investigation On The Adsorption And Co-precipitation Mechanism Of Heavy Metals On Struvite Crystals During The Process Of Phosphorus Recovery From Wastewater

The contradiction between the increasing demand for phosphorus resources in industrial production and economic development and phosphorus pollution in water bodies promotes the application and development of struvite crystallization method in wastewater phosphorus recovery.However,heavy metals and calcium ions are regarded as impurity ions in the wastewater will affect the crystallization process of struvite during the phosphorus recovery process,which in turn affects the crystal structure and content of struvite in the recovered product.This paper systematically studied the adsorption properties of water-soluble heavy metals（Zn（II）,Pb（II）,Cd（II）and Ni（II））on struvite crystals formed by spontaneous crystallization during the recovery of struvite phosphorus,and using analysis methods such as FTIR,adsorption kinetic model simulation,XRD and XPS to further analyze the effects of struvite crystals on water-soluble Zn（II）,Pb（II）,Cd（II）and Ni（II）adsorption mechanism.The results shown that:（1）Struvite had good adsorption properties for Zn（II）,Pb（II）,Cd（II）and Ni（II）,and the maximum adsorption efficiency were reached 93.99%,97.71%,94.3%and 85.5%,respectively.（2）The continuous process of struvite crystals adsorbing water-soluble Zn（II）,Pb（II）,Cd（II）and Ni（II）can be dynamically simulated with pseudo-second-order kinetic models.（3）When the concentration of heavy metals in the solution is 0-1 mg/L,the heavy metals enter the recovered products mainly through the enrichment,adsorption and heterogeneous nucleation on the surface of struvite.Among them,copper mainly forms Cu（OH）₂ minerals,and lead mainly Pb₃（PO₄）₂ minerals were formed,nickel mainly forms Ni₃（PO₄）₂ and a small amount of Ni NH₄PO₄·6H₂O（Ni-MAP）minerals enter the recovered products.（4）After adsorbed by struvite,Cd（II）and Ni（II）were mainly coordinated with-PO₄^3-groups to form the heavy metal phosphate（M-PO₄）.（5）The adsorbed heavy metals were mainly concentrated on the surface of struvite,which result in a low impact on the crystal structure of struvite.Then,pickling and other methods can be used to remove heavy metals on the surface of struvite to further purify the struvite in the recovered product.This paper studied the co-precipitation behavior of copper,lead,nickel with struvite,combines with ICP-OES,XRD,QXRD and XANES and other analysis methods,to clarify the co-precipitation mechanism of heavy metals with struvite and the effect on the recovered products.The results indicated that:（1）The presence of struvite promoted the heterogeneous nucleation of heavy metals on the surface of struvite,and the effect of heavy metals on the crystallization and purity of struvite was increased with the increase of heavy metals concentration.（2）When the concentration of heavy metals in the solution was 0-1 mg/L,the heavy metals enter into the recovered products mainly through the enrichment,adsorption and heterogeneous nucleation on the surface of struvite.Among them,copper mainly forms Cu（OH）₂mineral,lead mainly forms Pb₃（PO4）₂ mineral,nickel mainly forms Ni₃（PO₄）₂ and Ni NH₄PO₄·6H₂O mineral in the recovered products.（3）When the concentration of heavy metals in solution ranges from 10-300 mg/L,the struvite crystal formed by spontaneous crystallization promotes the heterogeneous nucleation and precipitation of heavy metal ions.At this time,copper mainly forms Cu₃（PO₄）₂ minerals,lead mainly forms Pb₃（PO₄）₂ and Pb₁₀（PO₄）₆（OH）₂ minerals,nickel mainly forms Ni₃（PO₄）₂,Ni（OH）₂ and Ni NH₄PO₄·6H₂O（Ni-MAP）minerals enter the recovered products.（4）The heterogeneous nucleation of high concentrations of heavy metals on the surface of struvite leads to the stable doping of heavy metals in the recovered products.The combined process of wastewater pretreatment and struvite crystallization was an effective method to improve the purity of struvite in the recovered product.This paper preliminarily explored the effect of the coexistence of heavy metals and calcium ions on struvite crystallization.The results shown that:（1）The co-existence of heavy metals and calcium might exist synergistic and antagonistic effects on struvite crystallization,resulting in the phosphorus content in the recovered product presents fluctuating changes,but when the concentrations of heavy metals and calcium ions were at a high level,the synergy of heavy metals and calcium ions had a greater impact on struvite crystallization.（2）The co-existence of heavy metals and calcium ions would not affect the optimal p H of struvite crystallization,which was still 9.5.（3）Using struvite crystallization process to recover phosphorus resources from electronic wastewater containing heavy metals and calcium ions,it was found that when Mg Cl₂ and NH₄Cl were used as magnesium source and nitrogen source,and the crystallization p H=9.5,the suitable molar ratio of Mg:N:P for struvite crystallization was n（Mg）:n（N）:n（P）=0.8:3:1.（4）The synergistic effect of the co-existence of high concentration of heavy metals and calcium ions on struvite crystallization leads to the stable doping of heavy metals and calcium ions in the recovered products.It is necessary to pretreat industrial wastewater to reduce the concentration of impurity ions.（5）Excessive addition of nitrogen sources can effectively reduce the impact of heavy metals and calcium ions in industrial wastewater on struvite crystallization to a certain extent,and promote the application of the recovery process of struvite phosphorus in industrial wastewater.