Bacterial Mineralization Of Struvite And Its Potential Applications In Resource And Environment

Struvite precipitation from wastewater and its subsequent utilization have the significant potential to supply increasingly scarce sources of phosphorus,eliminate/reduce the danger of eutrophication,as well as remain the balance of resource utilization and its sustainable development.However,the high cost of chemical inputs significantly limited its large-scale application.The bacterial metabolic activities can provide the necessary alkaline environment for struvite precipitation,and thus alleviate the cost of alkaline reagents.Despite all that,the most common Mg sources used in bacterial mineralization of struvite are expensive water-soluble Mg salts.Due to the high input cost of Mg source,the economic feasibility of struvite recovery and large scale industrial production of struvite are still limited.To relieve the high input of production cost,the potential of struvite precipitation for N and P removal and subsequent nutrients recovery from wastewater by combining ba cterial mineralization with cheaper Mg source is indispensable to deliberate.Except for the nutrient elements of N and P,massive contaminants including heavy metals,radionuclides,organics and pathogenic bacteria are also concomitant with wastewaters.Therefore,developing facile and effective methods to remove various pollutants from wastewater is urgent and crucial.Given that the high-affinity of U(Ⅵ)to phosphates and U(Ⅵ)-phosphate solids have low solubility over a broad range of conditions,addition of phosphate-bearing amendments has been suggested as a stand-alone process to promote in situ immobilization of U(Ⅵ).As a source of phosphate,struvite may be a potential material for U(Ⅵ)immobilization in radioactive wastes.In this context,an in situ bacterial mineralization was carried out to examine the potential of low-cost,environmentally friendly MgO as Mg source in bacterial mineralization of struvite,further providing an effective and economic method for simultaneous nutrients removal and recovery as struvite from eutrophic waters.Moreover,utilizing radionuclear U(Ⅵ)as the model pollutant and combining batch experiments with thermodynamic modeling to determine the dominant mechanisms dictating this removal process,further evaluating the potential of struvite to treat radioactive wastes containing U(Ⅵ).The important results of this dissertation were summarized as follows:1.Bacterial mineralization of struvite using low-cost MgO as magnesium source,has the potential of cutting down the recovery cost of struvite,further promoting large scale industrial production.In this context,an in situ bacterial mineralization was carried out to elucidate the effect of MgO on bacterial growth and bacterial mineralization of struvite.The cheaper MgO was used as Mg source,and Shewanella oneidensis MR-1 was selected as a model microbe.Our results demonstrate that the growth and metabolism of S.oneidensis MR-1 are not influenced by the added MgO.Moreover,S.oneidensis MR-1 has not only excellent ability to mineralize well-crystallized and pure-phase struvite by using MgO as Mg source,but also enhance MgO dissolution,effectively transforming over 97%of Mg2+ from MgO into struvite.Notably,the dissolution of MgO combined with the bacterial metabolic activities can provide the necessary alkaline environment for struvite precipitation,and thus alleviate the cost of alkaline reagents.An appealing and direct finding in present study is that strain MR-1 is able to transform nitrogenous and organophosphorus compounds into struvite,while conventional abiotic P-removal technologies target orthophosphate and NH4+ removal.In this regard,current results provide an effective and economically feasible pathway for the nutrients removal and subsequent recovery from eutrophic waters,further reducing the sludge volume and eliminating/reducing the danger of eutrophication.2.Uranium(U)is a basic material for the generation of nuclear energy,and is one of the common radionuclide with chemotoxicity and radiotoxicity.Given that the high-affinity of U(Ⅵ)to phosphates and U(Ⅵ)-phosphate solids have low solubility over a broad range of conditions,struvite(MgNH4PO4·6H2O)as a source of phosphate may also be a potential material for U(Ⅵ)removal.Due to the strong chemotoxicity and radiotoxicity,uranium waste poses a severe threat on the human health and fragile ecosystem.Hence,research into developing various technologies to safely manage the uranium waste has attracted worldwide attention.In this context,radionuclide of U(VI)was chosen as the model pollutant to evaluate the potential of struvite to clean polluted waters and determine the dominant mechanism dictating this removal process.The equilibrium modelling results show that the Mg2＋,NH4+,and PO43-,which resulting from struvite dissolution over a broad range of pH conditions,would interact with the dissolved U(VⅥ),leading to the precipitation of solid uramphite[(NH4)(UO2)PO4·3H2O]and/or saleeite[Mg(UO2)2(PO4)3·9H2O]under these specific conditions.The results indicate struvite may be suitable for the purification of U(Ⅵ)-bearing wastewater.Batch experiments show that the removal processes was strongly dependent on initial pH and has better performance(nearly 100%)under acidic conditions(pH 2.0-5.0).Moreover,94.6%of dissolved U(Ⅵ)in solution can be removed at pH 3.0 within a short time of 0.5 h.Notably,the maximum removal efficiency of U(Ⅵ)reached 99.75%.The theoretical calculations and XRD analyses definitely confirmed the formation of the U(Ⅵ)-phosphates,i.e.,uramphite,saleeite,metasaleeite[Mg(UO2)2(PO4)2·8H2O]and metavanmeersscheite[U(UO2)3(PO4)2(OH)6·2H2O]during the removal process of dissolved U(Ⅵ)by struvite via the dissolution-precipitation mechanism.Specifically,the rapid dissolution of initial struvite in acidic wastewater yields a large amount of PO43’,NH4+,and Mg2+,which would interact with the dissolved U(Ⅵ),leading to the precipitation of solid U(Ⅵ)-phosphates,further promoting the removal process of dissolved U(Ⅵ).Taken together,current results suggest that the dissolution of struvite coupled to U(Ⅵ)-phosphates precipitation can provide an effective means of cleaning U(Ⅵ)-polluted waters by using struvite as a kind of efficient environmental scavenger.