Effect Of Heavy Metals On The Crystallization Process And Purity Of Struvite

The discharge of nitrogen and phosphorus in sewage not only leads to water pollution,but also causes the loss of phosphorus resources.The magnesium ammonium phosphate precipitation method can simultaneously remove nitrogen and phosphorus from the waste water,and the crystallized product can be recovered as agricultural fertilizer.However,the crystallization process of ammonium magnesium phosphate is easily affected by heavy metal ions and reduces the quality of ammonium magnesium phosphate recovered from waste water.In order to improve the quality of the ammonium magnesium phosphate,the paper discussed the influence of cations and toxic heavy metal ions in the wastewater on the crystallization process of magnesium ammonium phosphate,and studied the influence of common organic matters on the distribution of heavy metals during magnesium ammonium phosphate crystallization,and finally studied the use of the modified Fe₃O₄ for advanced treatment of the low concentration of phosphorus in the water so that the effluent meets the national emission standards.Firstly,the effect of Mn²⁺and Ca²⁺on the crystallization of ammonium magnesium phosphate was investigated.The results showed that the presence of Mn²⁺and Ca²⁺had a significant effect on the crystallization of ammonium magnesium phosphate.The influence of Mn²⁺and Ca²⁺on the crystallinity of magnesium ammonium phosphate is positively related to the molar ratio of Mn²⁺/Ca²⁺:Mg²⁺,however,the effect of solution pH is negligible.At a solution pH of 9.5,magnesium sources were added at different molar ratios of Mg:P.It was found that approximately 97%of the phosphate can be removed.Using the step by step precipitation technique,it was found that 85%of Mn²⁺can be effectively removed,the remaining Mn²⁺concentration is controlled at about 1.2 mg/L,and the removal rate of phosphate can reach 95%.The simulation of swine wastewater shows that the heavy metal ions Cu²⁺,Zn²⁺and Cr³⁺have a significant effect on the crystallization of magnesium ammonium phosphate.It was found that heavy metal ions can not only inhibit the crystallization of ammonium magnesium phosphate,but also affect the purity of the crystallized product.Under the optimal experimental conditions,the heavy metals in the precipitate were found to be Cr³⁺?6.99 mg/g?,Zn²⁺?3.18 mg/g?,Cu²⁺?0.71 mg/g?respectively.Organic compounds have a strong complexing effect on heavy metal ions in solution.Under the optimal experimental conditions,when the concentration of EDTA in the solution is 100 mg/L,the removal rate of Cu²⁺in the solution is only 5.2%,and the removal rate of Cr³⁺is also reduced to 55%,indicating that most of the heavy metal ions are trapped by organic matter in solution.Based on the above studies,the remaining low-concentration phosphates in the wastewater of ammonium magnesium phosphate crystallization were treated with lanthanum modified Fe₃O₄.The results showed that the adsorption rate of phosphate was fast.When the reaction time was 120 min,it could reach adsorption equilibrium,and the maximum adsorption capacity could reach 65 mg/g.The adsorbent has good sedimentation performance and is easy to separate.phosphate removal rate can reach 95%or more,the effluent meet the national emission standards.