Study On The Effect Of Heavy Metal Ions In Urban Sewage On Phosphorus Recovery By HAP Crystallization Method

The shortage of phosphorus resources and the increasing demand for phosphorus resources promote the application and development of hydroxy calcium phosphate（HAP）crystallization in the field of phosphorus recovery from wastewater.However,impurities such as heavy metal ions in wastewater will enter recycled products during phosphorus crystallization,resulting in changes in HAP structure and morphology and accumulation in HAP,which affects the subsequent utilization value of recycled products.Therefore,it is very important to pay attention to the occurrence and accumulation of heavy metals in phosphorus recovery process.In this paper,the effects of reaction conditions（temperature,p H and Ca/P mole ratio）on HAP crystallization were investigated from crystallization kinetics and phosphorus removal respectively,and the experimental conditions were determined.Then,the effects of single heavy metal ions(Pb²⁺,Cd²⁺and Cr³⁺)on HAP crystallization were investigated from crystallization kinetics and calcium and phosphorus removal.The results showed that:All three heavy metal ions could reduce the nucleation rate of HAP crystallization,and the inhibition range was Cr³⁺>Cd²⁺>Pb²⁺.The kinetic effect of Pb²⁺was in two stages,and the nucleation rate increased first and then decreased with the increase of the initial concentration of Pb²⁺.The effect of Cd²⁺was mainly concentrated in the HAP nucleation induction stage,and the inhibition of Cr³⁺occurred in the whole process of HAP generation.The initial concentration of Cd²⁺and Cr³⁺were significantly positively correlated with the induction time.Pb²⁺could inhibit the phosphorus removal rate at low concentration,but promoted the phosphorus removal at high concentration.Cd²⁺promoted phosphorus removal while Cr³⁺inhibited phosphorus removal stably.During HAP crystallization,all Pb²⁺entered into the recovered product,and the removal rate of Pb² is independent of the initial concentration.A small amount of Cd²⁺and Cr³⁺remained in the solution,and the removal rate increased with the increase of the initial concentration.In this paper,the crystallization products with the participation of three heavy metal ions were analyzed by using various characterization methods and Visual MINTEQ 3.1.The results showed that:The Ca/P molar ratios of the products were decreased by the three heavy metals,but the influence modes were different.The impurities introduced by Pb²⁺mainly include lead hydroxyapatite（Pb₅（PO₄）₃OH）and phosphochlorite（Pb₅（PO₄）₃Cl）.Pb²⁺significantly reduced the purity and crystallinity of HAP by competing for HAP crystal ions(OH^-and PO₄^3-).Cd²⁺entered the product mainly through the formation of amorphous substance Cd₃（PO₄）₂,while Cr³⁺entered the product mainly in the form of amorphous precipitation of Cr₂O₃ and Cr（OH）₃.There was no significant interaction between the three heavy metals,and the corresponding product types of each heavy metal did not change in the mixed addition compared with the single addition.The precipitation composition of Pb²⁺was much more complex than that of Cr³⁺and Cd²⁺.The relationship between the precipitation-forming capacity and the effect of the three heavy metals on HAP crystallization were Pb²⁺>Cr³⁺>Cd²⁺.In this paper,the adsorption mechanism of HAP on the three heavy metal ions was further analyzed by isothermal adsorption and kinetic adsorption models combined with SEM,EDS and XRD material characterization methods.The results showed that:The Langmuir,Freundlich and D-R isothermal adsorption models could be used to describe the adsorption equilibrium data of HAP for Pb²⁺and Cd²⁺,and Freundlich and D-R models were more suitable for describing the isothermal adsorption of Cr³⁺.The adsorption of three heavy metal ions by HAP was an endothermic and favorable adsorption process.The adsorption behavior of HAP for heavy metal ions conformed to the pseudo-second-order kinetic model based on the rate-limiting step of chemical reaction.The fitting results of intra particle diffusion model indicated that the diffusion in particles was not the only controlling factor of HAP adsorption,and the adsorption rate was also affected by membrane diffusion.The results of adsorption mechanism analysis showed that the adsorption mechanism of HAP for Pb²⁺mainly included ion exchange,surface complexation and partial dissolution of HAP—precipitation of(7₅（₄）₃.The contribution of each mechanism in the adsorption process was constantly changing,but the three mechanisms always existed in the whole reaction process,and ion exchange was the main mechanism in the later stage of reaction.The possible mechanisms of HAP adsorption of Cd²⁺included surface complexation,ion exchange and partial dissolution of HAP—precipitation of(9₅₂（₄）_4.42and(9₃（₄）₂.The adsorption process of HAP on Cr³⁺was mainly chemisorption including ion exchange and precipitation of Cr₂O₃.