Removal Behavior Research Of Orthophosphate And Pyrophosphate By CaFe-Cl-LCDs And Calcined Dolomite Powders

With the rapid development of social economy, environmental pollution isincreasing severely in the same time. Eutrophication occurrs frequently all around theworld. The main source of eutrophication is the industrial sewage and domesticwastewater which caused serious water pollution or potential pollution risk.Phosphorus chemical pollution prevention and control, environmental protection andreasonable use of limited and non-renewable resources are the urgent tasks andimportant subjects of the environment and sustainable development in our country.Control of the phosphate emissions such as the orthophosphate and pyrophosphateplays an important role in the study of the water pollution.Layered Double Hydroxides （LDHs） has a unique interlayer anionicexchangeability. LDHs has attracted much attention because of the good adsorptionperformance. In consideration for the adjustability of layer board metal ions and theinterlayer anions as well as the important role of Ca and Fe in the traditionalphosphorus removal, we synthesized the CaFe-LDHs in this study. In our research,the purpose is to remove orthophosphate and pyrophosphate pollutants efficiently. Onthe basis of the different removal mechanisms of the removal process oforthophosphate and pyrophosphate and the combination system by CaFe-LDHs, westudied the dynamic adsorption and isothermal adsorption in detailed. According tothe composition analysis of reaction solution and characterization of solid product, theresearch focused on the contrast of the removal mechanism and the competitive effectin the combination system. Moreover，on the grounds of the existing studies ofCaFe-LDHs and MgFe-LDH, we builded the CaMgFe-LDH system applied in theorthophosphate and pyrophosphate remove. And we also studied the removalefficiency of phosphate by the CaMgFe-LDH system. We get some preliminaryconclusions as follows:1. CaFe-LDHs can be synthesized successfully through coprecipitation method. Theformula for product is Ca1.9Fe（OH）5.8Cl0.92（CO3）0.081.8H2O. Ca and Fe exists in II and III oxidation state respectively. We use calcined dolomite powders andFeCl36H2O to synthetize layered double hydroxide through coprecipitation method.LDHs was successfully synthesized in M²⁺:M³⁺=3,4without adding NaOH. But, inthe cause of adding NaOH LDHs was successfully appeared in M²⁺:M³⁺=2,3,4. Itcan be speculated that synthetic products mainly contained a mixture of MgFe-LDHsand CaFe-LDHs by solid characterization.2.When the initial orthophosphate concentration are0.1613mmolP/0.1g LDH,0.3226mmolP/0.1gLDH,0.4839mmolP/0.1gLDH,0.6452mmolP/0.1gLDH,0.8065mmolP/0.1g LDH, the removal process reached equilibrium at5h,24h,32h,36h and40h. While under the same condition, the removal process of pyrophosphatereached equilibrium at2h,6h,24h,36h and40h. The kinetics fitting parametersshow that both of the removal process conformed to the secondary dynamic equation.It changed from the homogeneous reaction to heterogeneous reaction.3.Freundlich isothermal adsorption equation described the removal process oforthophosphate preferably. The removal process is a complicated process includingphysical adsorption and chemical adsorption. The theory adsorption capacity is3.27mmol/g. However, the removal process of pyrophosphate neither corresponds toFreundlich isothermal adsorption equation nor Langmuir isothermal adsorptionequation. The process mainly depends on the reaction process between Ca²⁺andP₂O₇^4-.4.When the initial total phosphorus concentration are0.1613mmolTP/0.1g LDH,0.3226mmolTP/0.1gLDH,0.4839mmolTP/0.1gLDH,0.6452mmolTP/0.1gLDH,0.8065mmolTP/0.1gLDH, we configure the ratio of orthophosphate andpyrophosphate to1:1. There is no competition between orthophosphate andpyrophosphate when initial total phosphorus concentration is low. This is because theamount of Ca²⁺is enough to precipitate with PO43-and P₂O₇^4-. Removal rate appearedobvious difference when the total phosphorus concentration came to0.4839mmolTP/0.1gLDH. Pyrophosphate removed faster relatively.5.Orthophosphate concentration is:0.6452mmolP/gLDH,1.290mmolP/gLDH,1.935mmolP/gLDH,2.581mmolP/gLDH,3.226mmolP/gLDH. With pyrophosp hate concentration increasing, the orthophosphate removal rate declines whenthe initial concentration of orthophosphate is1.290mmolP/gLDH. Competitionbegan to appear under the condition of lack of Ca²⁺. Competition is more andmore obvious with the increasing of pyrophosphate concentration.6.The CaMgFe-LDH system composed of calcined dolomite powders and ferric-chloride is a high-efficient material for orthophosphate removal attribute to chemicalprecipitation for orthophosphate. The experimental results showed that the addition ofFeCl36H2O can promote the release of Ca and Mg in the dolomite powders. Thiseffect contributes to the remove of orthophosphate which gets bigger with the increaseof dosage of FeCl36H2O.7. Calcium precipitation predominate the removal process in the low concentration ofpyrophosphate by the CaMgFe-LDH system composed of calcined dolomite powdersand ferric chloride. In high pyrophosphate concentration, P₂O₇^4-complexed withMg²⁺, Fe³⁺and Ca²⁺result in the decrease of removal efficiency.8.The reaction mechanism of the removal process of orthophosphate andpyrophosphate by CaFe-LDHs is dissolution–precipitation. Furthermore, Ca²⁺complexed with P₂O₇^4-when the initial concentration of pyrophosphate get to4.5mmolPP/gLDH. Ca²⁺which dissolved from LDHs precipitated with PO43-and P₂O₇^4-to form hydroxyapatite or calcium pyrophosphate. This is the main mechanism. Andferrihydrite formed surface complexes with phosphate which strengthened theremoval effect.