Microcosmic Action Mechanism Of Ca/Mg And Pyrophosphate On The Surface Of Iron Phase

Pyrophosphate （PP） was precipitated by most Ca²⁺released from the dissolutionof Ca/Fe-Cl LDH, but the interaction of PP removal and remaining Ca in the solidwas not clear. To solve this problem, PP removal on iron hydroxide （AmorphousFerric Hydroxide （AFH）, goethite） in the presence of Ca²⁺was investigated, includingPP removal on Ca doped goethite. Taking into consideration big Ca ion radius andintense Ca-PP precipitation interaction, we studied the interaction of Mg and PP onthe surface of goethite. Combined with the analysis of PP removal on the ironhydroxide in the presence of Ca²⁺/Mg²⁺, we understood well surface complexationand precipitation action on iron hydroxide in order to explain the removal of PP on theCa/Fe-Cl LDH. Results in this paper are as follows:PP removal on AFH with Ca²⁺was increased, due to Ca-PP precipitation and Ca-PPcomplexation （≡FeOCa-PP） on the surface of AFH. These two processes depended onpH in solution. At pH<7.5, the formation of surface complex≡FeOCa+was supposedto absorb PP in terms of≡FeOCa-PP. At pH≥7.5, the precipitation of Ca2P2O7wasobserved remarkably. In the process of PP removal on goethite, the contribution of Cato PP removalwas similar to that in the case of AFH. Compared with Ca²⁺, Mg²⁺bonded PP in≡FeOMg-PP complex on the surface of goethite, which species wereinfluenced by pH.The structure and characterization of Ca/Mg doped iron hydroxide were related toCa/Mg content. With less than10%Ca doped, the product had the structure ofgoethite while Ca was in Ca（OH）2and adsorbed on goethite; In the case of10%-100% Ca doped iron hydroxide, lepidocrocite was observed with Ca（OH）2. Partial Ca wasin the structure of Fe-O-Ca（more stable than Ca（OH）2）；When100%-400%Ca wasdoped into iron hydroxide, Ca/Fe-LDH was formed with Ca（OH）2. The structure ofproduct doped by Mg with <4%,4%-40%and40%-400%amount was indexed asgoethite, lepidocrocite, and Mg/Fe-LDH, respectively. Correspondingly, the fractionof Mg was similar to that of Ca in the case of Ca doped iron hydroxide. Thedifference in the Mg and Ca doped product was that Mg doped iron hydroxide gavelarge surface area, different surface properties and stable structure.Compared to that on goethite, the PP removal on Ca/Mg doped goethite（Ca-/Mg-G） was increased, which was relative to solid surface properties and Ca/Mgfractions in the solid adsorbent. The mechanism of PP removal on Ca doped goethiteincluded:（1） Ca-PP complexation interaction on the surface of Ca-G at acid condition;（2） Ca-PP precipitation and Ca-PP complexation interaction on the surface of Ca-G atalkaline condition. More Ca content in solid improved the precipitation. Thisconclusion was similar to that in PP removal on AFH with Ca²⁺addition. In contrast,the increase of PP removal on Mg doped goethite depended on different adsorptionsites and large surface area, which was different from that in the removal of PP ongoethite with Mg addition.The mechanism of PP removal over Ca2FeCl-LDH at pH12was contributed toCa-PP precipitation as well as Ca-PP complex or PP intercalated into LDH. These twoprocesses were dependent on the initial PP concentration. At lower PP concentration（10-220mg/L）, the maximum PP removal amount was close to100mg/L, due to Ca-PP precipitation. Moreover, Ca over LDH was released into the solution due to theformation of Ca-PP complex. In comparison, at higher initial [PP]（220-600mg/L）, thefirst maximum PP removal amount and second one were nearly to180mg/L at320mg/L PP and230mg/L at520mg/L PP, respectively. PP removal was attributed to theformation of Ca2P2O7ï¹'2H2O crystal as well as （CaP2O7）-LDH/P2O7-LDH. Inaddition, the formation of （CaP2O7）/P2O7-LDH was related to Ca2P2O7ï¹'2H2Ocrystal.