| Water resource is a prerequisite for the survival of human beings with a self-evident importance.Nowadays,however,water environment continues to deteriorate and water pollution is becoming more and more severe.The discharges of industrial wastewater,agricultural wastewater,and domestic wastewater have been threats to the sustainable development of the society.On the other hand,oil pollution,among all the pollutants,is considered to be the most urgent threat to ecological environment.Oil pollution happens not only in the fresh water system,but also causes damages to the oceans.The solutions to deal with the oil pollution can be divided into three categories:chemical method,biological method and physical method.Adsorption method takes the unique advantages over the other methods in dealing with the oil pollution.For the existing adsorption materials include expanded perlite,zeolites,wool fiber,activated carbon,sawdust,microporous polymer,expanded graphite,carbon nanotubes,etc.They have more or less the following drawbacks:low adsorption capacity,poor adsorption selectivity for oils and water separation,secondary pollution,low recyclibility,and high cost,etc.These adsorptive materials are yet to meet the requirements for dealing with oil pollutions.Therefore,it is urgent to develop new adsorption materials.The motivation of present thesis,on the one hand,uses the ultrastrong mechanical property and ultrahigh specific surface area of graphene as building units to fabricate spongy graphene(SG)with an excellent adsorption performance by applying various fabrication methods.Compared with the conventional adsorbents,the SG possesses many advantages including ultrahigh adsorption capacity,high recyclibility,etc.On the other hand,this thesis also presents the fabrication of carbon sponge by using cottons or waste paper as starting materials.These carbon sponges also show good adsorption capacity.In addition,the fabrication strategy for carbon sponges is simplest and the precursor material,i.e.raw cotton or waste paper,is the cheapest among all the commercially available sorbents.The main results can be summarized as follows:1.The concept of metal casting was introduced into the fabrication process of wet graphene gel.Typically,wet graphene gels with different shapes could be produced through the combination of hydrothermal method and special-designed autoclaves,then these graphene gels were taken out from the autoclaves and dried for 15 h at 32?,after which various graphene castings have been obtained.In addition,the sol-gel transformation was suggested to explain the mechanism of shaping of graphene sheets.Moreover,the effect of pH values of graphene oxide dispersion on the structure and mechanical and electrical properties of graphene castings was studied,which showed that,at pH?10,graphene castings had the smoothest surface and the most compact,uniform internal structure.This endowed the cast with high compressive strength of?360 MPa(6 times higher than that of conventional graphite productions)and excellent electrical conductivity of?7.8 S/cm(comparable to the graphene foams by CVD method).When the pH is around 10,the graphene oxide sheets have the maximal charge density,which causes the graphene sheets to aggregate simultaneously as a whole during the hydrothermal reduction process and finally to form the castings with a compact uniform structure.2.The as-prepared wet graphene gels were subjected to the freeze drying to produce spongy graphene.The freezing temperature was found key factor during freeze drying.Based on the home-made freezing platform(-170 ? to-0 ?),we have fabricated spongy graphene under a series of freezing temperatures.Characterizations and tests for these SG shown that the pores became denser,the pore size decreased,and the pore wall became thinner as the freezing temperature went down(For example,pore size 10 to 15 ?m,wall thickness 20 um at-170 ?,while at-10 ?,the pore size 700 to 800?m,the wall thickness 80?m can be obtained,respectively).When the freezing temperature was above-20 ?,the SG was water resistant.By contrast,the temperature lower than-50 ?,the SG was a water adsorbent.In addition,lower freezing temperatures also caused better mechanical properties such as compressive strength,and Young's Modulus.Different freezing temperatures control ice nucleation rate and growth rate.This competitive mechanism between ice nucleation rate and growth rate leads to the large-range control of microstructures in SGs.3.Based on these researches,we have investigated the applications of hydrophobic SG in dealing with the organic liquid pollutions.The SG possessed strong hydrophobicity,high specific surface area,high porosity,etc.,showing highly efficient absorption of not only petroleum products and fats,but also toxic solvents such as toluene and chloroform.For example,1 g of SG can adsorb 86 g of chloroform,which is higher than any of conventional adsorbents.In addition,based on its chemical and thermal stability,SG can be regenerated(>10 times)by heat treatment,yielding the full release of adsorbates(>99%).Therefore,SG has a widespread potential for applications in industry as well as environmental protection.4.In order to further improve the adsorption capacity of SG,an improved method was adopted to fabricate SG.Typically,graphene oxide dispersion was direct subjected to the freeze drying and produced graphene oxide sponge.The sponge was heated with up to 1000 ? in an inert atmosphere enabling the reduction of graphene oxide sponge to SG.Finally,SG bulks were placed on the flame of wax candles for several seconds to modify the SG with carbon nanoparticles and obtain the final form of SG,namely SG-F.The density of SG-F was measured to be?0.9 mg/cm3 based on Archimede's theory,and it can be controlled through adjustment of the concentration of the graphene oxide dispersion.The absorption capacity of SG-F to chloroform reached 616 times of its own weight,which is approximately 7 times higher than the very first SG.In addition,the hydrophobicity of SG-F has been further enhanced.For example,the contact angle of SG-F is 155°,while 114° for SG in the last section.5.Both SG and SG-F have a common drawback of fragility.Therefore we discovered a new fabrication method including three steps:low temperature(90 ?)reduction of graphene oxide,freeze drying,and high temperature(200 ?)reduction.The density and size of the sponge can be controlled easily.The as-prepared sponge demonstrated an excellent compressibility.Under compressive strain over 90%,the sponge can easily recover back to its original shape.Meanwhile,after two cycles,the stress-strain curves tend to overlapp each other,which also indicated a good compressibility.Based on this unique property,this sponge can be used not only as adsorbents,but also as the building block in strain sensors,or devices for energy storage.6.Graphene-based adsorbents have better performances when compared with other adsorbents,but the cost is indeed a concern.Therefore we have developed twisted carbon fiber(TCF)sponges and carbon microbelt(CMB)sponges by using raw cotton and waste paper as starting materials,respectively.On the one hand,due to the lower cost,abundant sources,nontoxicity to humans of raw cotton,the TCF sponges produced by pyrolysis of raw cotton is a cost-effective and promising sorbent for the removal of pollutants.CMB sponge,on the one hand,is significant for the richnes of waste paper sources,which can further lower the cost.In fact,effective use of the waste papers not only reduces their pollution for the cities,but also cuts down the high cost for disposal sand recycling.Both of TCF sponges and CMB sponges have high adsorption capacity,as they can uptake organic pollutants at 50 to 190 times its own weight.In addition,the distillation,combustion and squeezing processes can be used for the recycling of both sponges.The pollutants can be recovered for reusage,or centralized disposal,which depends on their properties and economic value.In conclusion,compared to the conventional adsorbents(zeolite,expandable graphite,activated carbon,etc.)and novel adsorbents including carbon nanotube sponge,graphene sponge,polymers,and so on,both TCF sponges and CMB sponges have the following features:1)rich source,nontoxicity,and low cost;2)simple and environmental-friendly process;3)high selectivity for oils and water separation;4)high sorption capacity,and 5)excellent recyclability.It was believed that an ideal adsorbent should possess all the above features.The conventional or some novel adsorbents just have two or three features,which hindered their commercial applications.Therefore,the carbon sponges which have been studied meet the business requirements not only from their fabrication process but also from their utility,which lead to huge potential applications. |
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