Mechanism And Synergic Effect On Capture Of Polyphosphate By Polynary Layered Double Hydroxide (LDH)

Polyphosphate is important inorgainc species of phosphate and has widely used in industrial application. Recently, its removal attracts much attention due to the slow conversion of aqueous polyphosphate to orthophosphate (OP) which is nutrient for many aqueous plants, causing a fast growth of aquatic vegetation and leading to severe aquatic ecosystem destroy. Moreover, polyphosphate is also revealed to improve the growth of plant like the orthophosphate. Thus, the explore of the physical-chemistry principle, method and technology is necessary for the efficient polyphosphate removal from the wastewaters. In current work, a Ca-based layered double hydroxide (Mg_2-xCa_x-LDH , x=0.0-2.0) with tunable components was prepared by a co-precipitation method and at first time used in the removal of pyrophosphate (PP) and triphosphate (TPP) which are common species of polyphosphate in the industral application. The uptake behavior of PP and TPP over LDH was investigated by isotherm and kinetics removal experiment and the determination of components in both solution and solid. Subsequently, the characterizations of XRD, FTIR, TEM, XPS and NMR were empolyed to reveal PP and TPP uptake mechanism over LDH. Moreover, the thermal activation of as-synthesized LDH was carried out while its product was used in the removal of TPP as well in which the mechanism for TPP removal was also studied. The results reveal:The removal of PP over binary Ca₂FeCl-LDH underwent predominantly dissolution-precipitation process, in which the dissolved Ca²⁺ precipitated with PP in form of Ca₂P₂O₇·2H₂O. The element analysis and physical characterization revealed that there is partial Ca retained in Fe oxide framework after LDH dissolution. With the increasing of initial [TP], the residual Ca was further released to precipitate PP in which the removed Ca/P was close to 1.0 that is Ca/P ratio in Ca₂P₂O₇·2H₂O. XPS results also suggested the dissolution-precipitation process probably took place on the Fe oxide framework surface, similar to the process in the formation of Ca₂P₂O₇·2H₂O over hydroxyapaptite surface.For TPP removal, the removal amount of 56.4 mg/g over Ca₂FeCl-LDH and 107.8 mg/g over Ca₂AlCl-LDH was higher than that over Mg based LDH (9-11 mg/g over Mg₂FeCl-LDH and 35.4 mg/g over Mg2AlCl-LDH). The comprehensive analysis of the TPP-uptake products reveals that the the removal of TPP over Mg-based LDH mainly underwent the surface adsorption/near-edge intercalation and partial ion exchange. In contrast, the TPP removal over Ca-based LDH was attributed to the dissolution-precipitation process in which Ca-based LDH dissolved first and then released Ca²⁺ ions react with TPP to form insoluble Ca₅(P₃O₁₀)₂·nH₂O precipitate.Combined with these two different removal mechanisms, a synergic effect on the removal of TPP was observed in the case of Mg_0.5Ca_1.5FeCl-LDH with the TPP removal amount of 55.2 mg/g from aqueous solution under similar conditions. On the contrary, there is no synergic effect in the case of ternary Mg_2-xCa_xAlCl-LDH due to the release of Al in the form of Al(OH)₄^- at high pH. The dissolution of Al resulted in the releasing of Ca during the dissolution of (CaAl)O oxide framework that enhanced the precipitation of TPP with the removal amount of 84.4 mg/g.The TPP removed product was found stable in both P free water and 0.5 M NaCl solution. In water, no more than 20% of TPP was dissolved. The ion strength improves the releasing of TPP which resulted in the slight higher leaching rate of TPP in NaCl solution. The experiments indicated the releasing of TPP is predominantly due to the desorption of TPP and dissolution of Ca₅(P₃O₁₀)₂·nH₂O.After thermal activation at 450 oC, LDH was converted to Mg_2-xCa_x-LDO composed of MgCaM(III)-O (M=Al or Fe) composite. For TPP capture, LDO can remove more TPP than pristine LDH as more Ca dissolved from LDO to improve the precipitation of TPP. Furthermore, the removal amount of TPP over Mg_2-xCa_xAl-LDO is higher than that over Mg_2-xCa_xFe-LDO. The release of Al in the form of Al(OH)₄^- at high pH (pH=9.9-10.8) is responsible for the enhanced TPP removal as the releasing of Ca in the dissolved (MgCaAl)O composite.