Preparation Of Reusable Monolithic Nanomaterials And Profiling Their Dye Removal Mechanisms

Recently,increase level of organic contamination in water bodies has become a global concern.Among several treatment techniques,adsorption and advance oxidation process（AOPs）are most attractive options to alleviate the current environmental pollution.The progress in nanotechnology has shown remarkable contribution to develop innovative nanomaterial designs for these treatment technologies.However,the industrial adaptability of nanomaterials can mainly dictate by an inexpensive material designs with high-efficiency,and easiness in operation upon reuse.Within this frame work,the nanomaterials of graphene（a-b）,and cobalt oxide（c）rationally design into a monolithic material（i.e.,a single piece of interconnected nanostructures in a column-like macroscopic dimensions）for adsorption and advance oxidation process applications.（a）Adsorption is very promising and efficient treatment technology to remove dyes form contaminated water.Among several classical adsorbent designs,nanomaterials of graphene with unique dimensions and excellent physicochemical features are emerging adsorbent materials which offer scalable and cost-effective alternatives to traditional activated carbon and carbon nanotubes（CNTs）designs.However,engineering concerns associated with an inconvenient recollection of nanoscale graphene and environmental concerns associated with bio-toxicity to human cells significantly hampered its use in environmental protection.To address this,nanomaterial of graphene was assembled into a monolithic material via hydrothermal method using an inexpensive chemical precursor of urea and graphite.The monolithic graphene employed as a reusable adsorbent to remove methylene blue,MB dye from water.The monolithic material was characterized by a Scanning electron microscopy,Fourier transmission infrared spectroscopy（FTIR）,Raman spectroscopy,X-ray photoelectron spectroscopy（XPS）,Thermogravimetric curves analysis and Nitrogen adsorption-desorption isotherms.The characterization results show that a graphene monolithic forms of graphene with a property of macroscopic size,three-dimensional（3D）pores network and favorable surface properties（e.g.,functional groups and aromatic domains）can allow facile mass transfer between monolith and pollutant molecules and simple recollection.The adsorption of MB best suited to Langmuir isotherm,pseudo-second-order kinetics,and intraparticle diffusion kinetics models.The monolith adsorbent could remove>98%of MB dye even in alkaline pH and saline environment.The removal of MB using graphene monolith was favorable,spontaneous and exothermic adsorption process.Electrostatic and?-?staking interactions were identified as a key interaction process.Overall results demonstrate that removal of MB using graphene monolith provides a promising alternative of graphene nanosheets with no environmental concerns.（b）Although,monolithic graphene designs with combine properties of an affordable reuse and 3D-pores morphology showed better potential compare to powder-type graphene nano-adsorbents to remove dyes from contaminated water.However,the mechanical instability associated with large pores dimensions in micrometer range and insufficient dye removal capacities associated with surface hydrophobicity upon the use of organic amines calls for further innovation in monolithic designs.Among several attempts,the use of nano-guest（e.g.,CNTs）widely approached to improve the pores morphologies and mechanical stability of graphene monolith.However,hydrophobic nature and expensive preparation of CNTs usually unattracted to remove hydrophilic-type organic pollutant（e.g.,dyes）from water.Within this frame work,we employed an inexpensive,mechanically strong and hydrophilic nano-gust of TEMPO-oxidized cellulose nanofibers（CNFs）to assembled a hybrid design（GO/CNFs）of monolithic graphene via hydrothermal method.The hybrid monolith was characterized by various advanced techniques and employed as adsorbent to remove MB dye form water.The hybrid monolith of GO/CNFS showed remarkable improvements in surface area（486 m²/g）and mechanical stability（380 Kpa）which was four-folds and five-folds higher than surface area（128 m²/g）and mechanical stability（75Kpa）of graphene monolith alone,respectively.It is proposed that,strong chemical interaction mainly hydrogen bonding was primary driving force for the formation of hybrid monolith.Incorporation of nanocellulose exhibits synergy improvement in dye adsorption performance.The GO/CNFs monolith could completely remove trace to moderate concentrations of MB dye.Adsorption isotherm behaviors can be ranked as:Langmuir isotherm>Freundlich isotherm>Temkin isotherm model.Maximum adsorption capacity of227.27 mg/g was achieved that was much higher than several reported monolithic graphene designs and functional composites of magnetically recyclable adsorbents.Incorporation of nanocellulose showed an exponential relationship with dye adsorption capacities.The high surface charge density and surface area were identified as the main dye adsorption mechanism.Hybrid design showed excellent reusability and regeneration performance with zero recontamination of treated water.（c）Oxidative decomposition of organic pollutants into a harmless product（i.e.,CO₂ and H₂O）via sulfate radical（SO₄·-）produced by activation of peroxymonosulfate（PMS）emerges as an attractive advanced oxidation techniques（AOPs）.Among which,the combination of PMS and solid cobalt oxide（Co₃O₄）catalyst（i.e.,nanoscale heterostructures）are most preferable due to limited energy inputs,simplicity of process（i.e.,operation at normal temperature and pressure conditions）,and desirable degradation and mineralization performance.However,most of these designs（i.e.,Co₃O₄）usually showed complicated reusability and sluggish rate kinetics due to ultrafine powder-forms and limited surface area associated with agglomeration in water flowing systems.To address this,a variety of sophisticated designs have been proposed to produces porous nanostructures of Co₃O₄ with enhanced surface area,and employing Co₃O₄ on to support materials for facile reuse.However,the cobalt heterostructures with rational control on synthesis affordability,tunable catalytic properties and water phase functionality still highly desirable for practical advance oxidation applications.In this study,we demonstrate a new and scalable approach that based on thermal-induced phase separation chemistry,and allow a controllable in-situ growth of Co₃O₄ nanoflowers onto a self-standing sponge-like monolithic polymer.The monolithic sponge catalyst was characterized by several advance techniques such as Field emission scanning electron microscope（FESEM）,transmission electron microscope（TEM）,X-ray photoelectron spectroscopy（XPS）,nitrogen adsorption-desorption isotherms analysis,temperature-programmed hydrogen reduction analysis（TPR-H₂）,cyclic voltammetry（CV）curves,and elastic and mechanical dynamics to obtained the morphology,physicochemical and catalytic properties of monolithic catalyst.Detailed characterization revealed that the Co₃O₄ nanoflowers were uniformly and tightly attached on a mesoporous polymeric network with a surface area of 230.0 m²/g and excellent mechanical stability,elastic properties,tunable geometric shape.The formation mechanism of the monolithic sponge catalyst was systematically studied in detail.The catalytic performance of monolithic sponge catalyst（SP-50）was evaluated by degradation of model organic compounds（i.e.,acid orange 7,AO7）under various operational conditions.The results indicate that complete color removal of AO7 can be achieved in<2 min,with a>84%TOC mineralization and negligible cobalt leaching under optimal conditions.The SP-50/PMS system could also effectively remove a broad range of compounds like antibiotic tetracycline,acid orange 52（typical fluorescent azo-dye）,and rhodamine B（a typical basic dye）.Time-resolved electron paramagnetic resonance（EPR）data revealed that SO₄·-and OH·-were the primary oxidative species.Furthermore,tailored macroscopic shapes with excellent mechanical stability allowed facile reusability of the monolithic catalyst over multiple runs.The excellent catalytic performance can be attributed to the formation of abundant Co-OH bonds on the monolithic sponge surface.Overall results exemplified the advantages of high-performance reusable SP-50catalyst for the remediation of organic pollutants from water environment.