| Nanostructured mineral ecomaterials(NMEMs)are promising materials for wastewater treatment because of their unique environmental,resource,and application advantages,such as environmental friendliness,simple and abundant raw materials,and extensive usages.Compared with natural and modified NMEMs,the synthetic ones are more tunable in morphology,size,composition,and structure,and possess higher specific surface areas and various assemblies,thus exhibiting superior removal performances for aqueous pollutants.This dissertation focuses on syntheses of NMEMs to remove conventional pollutants,including radionuclides and heavy metals,as well as emerging pollutants,such as engineered nanoparticles.By ascertaining the physicochemical properties of these pollutants and the bottlenecks affecting their removals,the potential NMEMs were targeted at nanostructured metal oxides,carbonates,hydroxides,sulfides,and their nanocomposites.The removal performances of the synthetic NMEMs to several typical pollutants were systematically investigated by batch adsorption experiments,and the removal mechanisms were also comprehensively explored by various characterization techniques.This dissertation can provide insight into the preparations and applications of different types of NMEMs for various aqueous pollutants.The main contents are summarized as follows:1.Nuclear power is one of the energy drivers to address climate change and to achieve carbon peaking and carbon neutrality goals.With the development of nuclear power,however,the excess discharge and leakage of nuclear wastewaters can cause environmental and health risks.Uranium(U)and iodine(I)are two predominant elements producing radioactivity in nuclear wastewaters,and usually dissolved as U(Ⅵ)cations and I-anions,respectively.Despite that various types of adsorbents have been synthesized for single U(Ⅵ)or I-,little information is available on their simultaneous removal.Moreover,designing bifunctional adsorbents for U(Ⅵ)and I-is highly challenging,due to their distinct physicochemical properties.To this end,in Chapter 2 of this dissertation,an Mn3O4@PANI nanocomposite was successfully prepared by loading Mn3O4 nanoparticles onto polyaniline(PANI)nanofibers,and used as an adsorbent for U(Ⅵ)and I-.Batch adsorption experiments show that the component Mn3O4 is mainly responsible for U(Ⅵ)removal,but PANI for I-.The dispersed loading of Mn3O4 enhances the removal efficiency of Mn3O4 to U(Ⅵ)and maintains the high removal performance of PANI to I-.The nanocomposite not only exhibits superior performance to many reported inorganic and organic adsorbents in both adsorption kinetics and capacities,but also shows selective adsorption to U(VI)and I-at high concentrations of competing ions.X-ray photoelectron spectroscopy(XPS)results confirm that the strong coordination bonds of U(Ⅵ)with the Mn-O,-NH-,and=Nare responsible for the U(Ⅵ)adsorption,but the I-adsorption occurs via Cl-exchange in PANI chains,anion-π interactions with PANI skeletons,and electrostatic attraction to Mn3O4 surface.In summary,the Mn3O4@PANI nanocomposite could be a promising adsorbent for practical U(Ⅵ)and I-pollution,and the related results can pave the way to develop more adsorbents for coexisting pollutants with distinct properties.2.In addition to radionuclides,the contamination of heavy metals is also a global environmental and public health concern.Pb(Ⅱ)and Cd(Ⅱ)are of great concerns due to their high toxicities and wide applications in industry.Of the mineral materials for Pb(Ⅱ)or Cd(Ⅱ)removal,metal hydroxides can induce appreciable Pb(OH)2 precipitate on the adsorbent surfaces and thereby generally exhibit high removal capacity for Pb(Ⅱ).However,the removal of Cd(Ⅱ)on metal hydroxides can be severely inhibited by the coexisting Pb(Ⅱ),due to the strong competitive adsorption of Pb(Ⅱ)with Cd(Ⅱ)onto the metal hydroxide adsorbents.By contrast,carbonate mineral materials exhibit higher removal capacities for Cd(Ⅱ)by inducing the surface precipitation of CdCO3.In this regard,basic carbonate mineral materials should be suitable for the removal of coexisting Pb(Ⅱ)and Cd(Ⅱ),since their abundant OH-and CO32-can firmly bind the heavy metals.Nevertheless,previous investigations on the removal of Pb(Ⅱ)or Cd(Ⅱ)by hydroxide and carbonate adsorbents have demonstrated that the Pb(Ⅱ)and Cd(Ⅱ)residual concentrations are too high to meet the maximum sale levels(MSLs)of 10μg/L Pb and 3 μg/L Cd set by the World Health Organization(WHO),even if the dosages of the adsorbents are elevated.This indicates that the basic carbonates may also be ineffective for the MSLs.To these problems,we proposed that the nanocomposites of basic carbonates and transition metal sulfides could be efficient for enhanced removal of coexisting Pb(Ⅱ)and Cd(Ⅱ),synergistically by the high affinities of the transition metal sulfides to heavy metal ions.In Chapter 3,an MnS@HM nanocomposite was successfully prepared by loading MnS nanoparticles on hydromagnesite(HM,Mg5(CO3)4(OH)2·4H2O)hierarchical microspheres,and used as a paradigm material for enhanced removal of Pb(Ⅱ)and Cd(Ⅱ).The results show that the MnS@HM nanocomposite has excellent removal performance for Pb(Ⅱ)and Cd(Ⅱ).Specifically,with the MnS loading,the residual concentrations of Pb(Ⅱ)and Cd(Ⅱ)are significantly decreased and lower than the MSLs of WHO for drinking water.When both the initial concentrations of coexisting Pb(Ⅱ)and Cd(Ⅱ)increase to 1000 mg/L,their removal capacities are up to 1345 and 433 mg/g,respectively,much higher than the values obtained by many previous nanocomposite adsorbents.Furthermore,systematic investigations into the removal mechanism reveal that Pb(Ⅱ)and Cd(Ⅱ)are immobilized on the nanocomposite in the forms of Pb(Ⅱ)or Cd(Ⅱ)(basic)carbonates,hydroxides,and sulfides.As a result,this work donates a potential ecomaterial for efficient treatment of the wastewater containing Pb(Ⅱ)and Cd(Ⅱ),and provides guidance on designing other high-performance adsorbents for heavy metal cocontamination.3.Besides the conventional pollutants,emerging pollutants are of increasing concerns,such as engineered nanoparticles.Of them,silver nanoparticles(AgNPs)are a typical nanopollutant.Many studies have reported that the production,use,and disposal of AgNPs inevitably lead to their discharge into the environment,thereby posing threats to the environment and public health.It has also been confirmed that AgNPs can not only cause cell membrane damage,metabolic pathway disruption,and gene expression alteration,but also carry other pollutants to promote the migration of the carried pollutants.On the other hand,AgNPs are regarded as profitable and recyclable materials in wastewaters,as silver is a noble metal with increasing demand.In this context,the removal and recovery of AgNPs have considerable significance for ecological safety and sustainable development.Therefore,in Chapter 4,a hierarchical calcite(HC)was successfully prepared by a facile microwave-assisted ethylene glycol process combined with a calcination treatment,and further used as an adsorbent to remove polyvinylpyrrolidone-coated AgNPs(PVP-AgNPs),one kind of typical engineered AgNPs.To evaluate the potential sustainable application of the recovered AgNPs,the AgNPs adsorbed on the HC were also harvested along with the HC and used as a catalyst for the reduction removal of the organic pollutant 4-nitrophenol by NaBH4.The results show that the HC has high removal percentages for PVP-AgNPs over a wide pH range of 6-10.In addition,the removal performance is almost not impaired by the coexisting PVP and anions Cl-,NO3-,SO42-,or CO32-that may interfere the adsorption removal.Furthermore,the AgNPs adsorbed on HC show high catalytic activity and good reusability for the reduction removal of 4-nitrophenol.In conclusion,this work not only develops a potential new adsorbent for AgNPs in practical scenarios,but also provides insight into the conversion of metal nanoparticles from pollutants to functional materials.4.Nano-Mg(OH)2(brucite)may be another promising candidate for removal of AgNPs,due to its large specific surface area and simplicity in industrial production,such as MgO hydrolysis.In water treatment,endowing nano-Mg(OH)2 with magnetically separable properties can facilitate the separation and recovery of the postadsorbents,thus reducing the treatment cost and the risk of secondary pollution caused by residual post-adsorbents.Therefore,in Chapter 5,an Fe3O4@Mg(OH)2 magnetic nanocomposite was successfully prepared via a facile one-pot method integrating the hydrolysis of MgO into Mg(OH)2 nanoplates with the coprecipitation of Fe2+and Fe3+into Fe3O4 nanoparticles.Batch adsorption experiments were also conducted to investigate the removal performance of the nanocomposite to PVP-AgNPs,and the removal mechanism was also systemically explored.The results show that the nanocomposite is the best adsorbents for PVP-AgNPs in both adsorption capacity and partition coefficient compared with the previously reported adsorbents.In addition,the nanocomposite exhibits efficient magnetic separation after the adsorption treatment,good reusability within five adsorption-desorption cycles,and excellent removal performance in real water matrices such as lake water and seawater.Both the Fe3O4 and Mg(OH)2 nanocomponents contribute to the removal,and their nanocomposite exhibits an enhanced performance because of the higher specific surface area and larger pore volume.Chemical adsorption and electrostatic attraction between the coatings of AgNPs and the two nanocomponents are responsible for the removal.Overall,the facile synthesis,convenient magnetic separation,and high removal performance highlight the great potential of the Fe3O4@Mg(OH)2 magnetic nanocomposite for practical applications. |
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