Study On Treatment Of PO₄^3--containing Wastewater By Carrier Induced HAP Precipitation And Crystallization

Phosphorus is one of the main factors causing water eutrophication.With the development of social economy and human activities,efficient phosphorus control in wastewater and phosphorus resource recovery become particularly important.Chemical precipitation method is a common treatment method for phosphorus containing wastewater,but it has the disadvantages of cumbersome process,slow removal speed and difficult recycling.Carrier induced precipitation crystallization method is its improved technology,which can simplify the process and improve the reaction speed.Using a variety of natural minerals as carriers,the process of inducing HAP(Ca₁₀（PO₄）₆（OH）₂)precipitation and crystallization to remove PO₄^3-was studied.Taking PO₄^3-containing water as the treatment object,Ca Cl₂as the precipitant,10 kinds of natural minerals as the induction carrier and PMB method（ammonium molybdate spectrophotometry）as the determination means,the study was carried out:the effects of different pretreatment methods on the determination of PO₄^3-were compared to obtain the best pretreatment conditions;The best conditions of PO₄^3-removal without carrier were obtained through single factor test.Under this condition,the removal effect and mineral composition of HAP precipitation crystallization induced by each carrier were compared,and the qualitative relationship between them and removal rate was analyzed;The Zeta potential of each carrier in the process of induced precipitation crystallization and its isoelectric point in different systems were measured,the relationship between PO₄^3-removal rate and the isoelectric point of the carrier was analyzed,the effects of different factors on Zeta potential were investigated,and the optimal control factors in the operation of induced precipitation crystallization were determined;Quantitative analysis was carried out on the correlation between the removal kinetics of different carriers and the removal kinetics of different carriers;The surface dissolution and adsorption experiments of the best carrier were carried out,and the reaction process of inducing HAP precipitation and crystallization to remove PO₄^3-was preliminarily analyzed in combination with Zeta potential data,electric double layer theory and kinetic conclusions.The results show that:（1）The determination results of each pretreatment method are consistent,the relative standard deviation is very small,and the test precision is high.The pretreatment method of standing for 2 minutes after sampling is selected to determine PO₄^3-;The optimum conditions of HAP precipitation without carrier:p H 10.0,Ca/P molar ratio 4:1,reaction equilibrium time 80 min;After adding carriers,the precipitation equilibrium time of 10 carriers was significantly shortened.Except magnetite and ilmenite,the removal rate of PO₄^3-was improved,and the induction effect of dolomite was the best;The carrier containing Ca O,Mg O,Si O₂and other components has good induction effect,which is consistent with the preliminary test results of the research group.It preliminarily shows that the mineral composition should be paid attention to in the selection of carrier.（2）In the process of induced precipitation crystallization,the carrier Zeta potential is closely related to its induction effect.The smaller the absolute value of Zeta potential,the better the induction effect.Its law is similar to the previous research of the group.Controlling Zeta potential can be used to optimize the parameters of induced precipitation process and reduce the workload of process control;In the precipitation equilibrium system,water distribution system containing PO₄^3-and deionized water system,the larger the isoelectric point of the carrier,the better the effect of inducing HAP precipitation crystallization.The isoelectric point determination of deionized water system is the simplest,which can be used for carrier selection.It is verified that the isoelectric point determination is universal for the carrier selection of inducing anion precipitation crystallization.（3）Hebei dolomite induced precipitation and crystallization effect is the best,the best experimental conditions are:p H value 10.0,Ca/P molar ratio 3:1,carrier dosage40 g/L,carrier particle size 300-400 mesh,reaction equilibrium time 20 min,removal rate 97.73%;The precipitation of Ni S and Pb S induced by dolomite is similar to that of HAP.The best carrier can be selected according to the influence of different components of dolomite on the equilibrium removal rate;Compared with no carrier,the reaction activation energy is significantly reduced after adding the carrier.In the process of carrier induced HAP precipitation crystallization,the reaction rate can be improved by increasing the amount of carrier in a certain range.（4）In the water distribution system containing PO₄^3-and deionized water system,the dissolution rate of dolomite mineral surface is different,and its dissolution process conforms to the quasi second-order kinetic model;The decrease of PO₄^3-concentration is mainly due to the induced precipitation crystallization,and occurs on the surface of the carrier;In the process of inducing HAP precipitation crystallization to remove PO₄^3-,dolomite is positively charged in the form of Ca OH₂⁺、Mg OH₂⁺,and the positively charged surface ions attract the negatively charged PO₄^3-to form a double layer,resulting in the concentration of PO₄^3-in the adsorption layer is much higher than that in the solution body.After adding Ca Cl₂,the HAP precipitation potential of the adsorption layer increases,the removal rate of PO₄^3-increases,the activation energy of HAP precipitation crystallization reaction decreases and the reaction rate increases.