Preparation Of Magnesium Doped Tricalcium Aluminate And The Simultaneous Removal Of Ammonium And Phosphorus In Digestted Piggery Wastewater

Digested piggery wastewater with the characteristics of high ammonium concentration,high ammonium to phosphorus?N/P?ratio,and neutral environment would damage to the environment.Until now,it is of great urgency to seek an economical and facultized treatment to efficiently remove ammonium and phosphorus up to the emissions standards.The struvite crystallization method that can remove ammonium and phosphorus with the molar ratio of 1:1 by adding magnesium source in alkaline environment has attracted great attention.However,the high N/P ratio and the neutral water environment in the digested piggery wastewater hinder the application of struvite crystallization method.The limitation of this method is that it cannot remove ammonium efficiently in the digested piggery wastewater.Calcium aluminum layered double hydroxides?LDHs?,anionic layered materials,can self-dissolve and release a large amount of Ca2+,Al?OH?₄^-,and OH^-ions.The released OH^-can provide an alkaline environment,thereby favoring part of ammonium removal via neutralization and promoting the formation of struvite.Additionally,the intermediates CaAl-LDH,Ca2+and Al?OH?₄^-can combine with phosphate to form phosphorus-containing compounds.Thus,it can be expected that the combination of CaAl-LDHs and struvite crystallization can co-remove ammonium and phosphorus from digested piggery wastewater efficiently.Tricalcium aluminate?Ca3 Al2O6,C3 A?is the main components of cement clinker,with the unique characteristics of hydration and lattice substitution.During it hydration,CaAl-LDH can be formed and Ca2+,Al?OH?₄^-and OH^-are released.Besides,the calcium in the C₃A' lattice can be substituted by magnesium and the doped magnesium does not affect its hydration behavior.Thus,magnesium is doped into the C₃A lattice via isomorphous substitution to fabricate magnesium doped C₃A?Mg-C₃A?.When Mg-C₃A is added into the digested piggery wastewater,the released OH^-can enhance the solution alkalinity and neutralize part of ammonium;Mg²⁺can combine with ammonium and phosphorus to form struvite;Ca2+,and Al?OH?₄^-can combine with phosphate to form phosphorus-containing compounds.Therefore,Mg-C₃A can effectively remove and recover ammonium and phosphorus from wastewater.In addition,the products that contain pollutants can be recovered as slow-release fertilizers of ammonium and phosphorus to achieve high value-added utilization of resources without secondary pollution.Mg-C₃A was synthesized via solid-state reaction method.The effect of Mg doping on the crystal structure,morphology and hydration behaviors of Mg-C₃A were investigated by XRD,Raman,HRTEM,SEM,and ICP.The simutaneous removal and reaction mechanism of ammonium and phosphate on Mg-C₃A was investigated under the effects of Mg doping amount,Mg-C₃A dosage,initial ammonium and phosphate concentration,and reaction time with the laboratory simulated digested piggery wastewater.The removal performance of Mg-C₃A on ammonium and phosphate in authentic wastewater was also studied.The detailed results are as follows:?1?Mg-C₃A with different Mg contents from 2.wt%in Mg2-C₃A to 18 wt%in Mg18-C₃A were successfully prepared through a solid-state reaction method.The results show that the Mg doping does not affect the cubic symmetric structure and morphology of C₃A.Mg-C₃A exhibits irregular block structure with small spherical particles uniformly distributed on the rough surface.Mg element is not only doped into the C₃A crystal by isomorphous substitution of Ca,but also exists in the form of spherical particles MgO.The Mg doping is saturated with the Mg doping amount of 15%,and its stoichiometric formula is Ca2.84Mg0.16Al2O6·0.72MgO.After hydration,the released Mg²⁺,Ca2+,and Al?OH?₄^-were 0.8,4.0,and 1.0 mmol·g-1 under the Mg doping amount of 15%,respectively.The pH value was 9.0.?2?When the Mg doping amount is 15%,Mg-C₃A dosage is 4 g·L-1,and initial ammonium and phosphate concentration is 1000 and 80 mg·L-1,the maximum removal capacity of ammonium and phosphate is 42.2 and 20.2 mg·g-1 at 8 h,respectively.The experimental data are better fitted with the pseudo-second-order kinetic equation,indicating that the removal process is dominated by chemical reaction.In addition,compared with pure Mg source?represented by MgO?,Mg-C₃A exhibit better capacity on ammonium and phosphate removal.?3?The combination of characterization of products after reaction and the solution component analysis show that the co-removal of ammonium and phosphate by Mg-C₃A is dominated by the released ions(Mg²⁺?Ca2+?Al?OH?₄^-,and OH^-)during its hydration.The removal mechanism of ammonium is mainly the neutralization and the precipitation;phosphate is removed via the precipitation 1ombining with rg2+.The solid products after the reaction is affected by various factors,among which the initial ammonium concentration has the greatest influence.?4?The performance of Mg-C₃A on the removal of ammonium and phosphate in actual wastewaters was investigated.The results show that Mg-C₃A can remove ammonium to an acceptable level?standard for livestock and poultry wastewater?from authentic wastewaters at low concentration??600 mg·L-1?,or remove?50%ammonium from authentic wastewaters at high concentration??1100 mg·L-1?.The residual phosphate concentration in the three authentic wastewaters all meet the standard for livestock and poultry wastewater.The solid products after Mg-C₃A reacted with actual wastewater are CaAl-LDH,struvite and Ca?NH4?2?HPO4?2·2H2O,which achieves the goal of recycling ammonium and phosphate in actual wastewater.Mg-C3 A has potential application in actual wastewater treatment.