| Concerning the shortage of fresh water resources,producing fresh water from sea water has become an important way to solve the global water crisis.Photothermal seawater desalination technology is a newly developed method to obtain fresh water from sea water,driven by heat converted from solar radiation.Meanwhile,heavy metal ions in seawater could affect the quality of desalinated water seriously.So,how to remove the heavy metal ions from water,is another important issue to be solved urgently.Ti3C2 is a new kind of two-dimensional crystalline carbide with graphene-like structure.Due to the special lamellar structure,the carbide has a large specific surface area,excellent hydrophilicity,outstanding mechanical,electrical,and magnetic properties.Recently,it has been widely used in the fields of energy storage,adsorption,sensors and so on.However,the intrinsic multi-layer stacking structure of Ti3C2 has limited the applications in these fields.Therefore,there are still many more in-depth researches need to be carried out.In order to meet the needs of seawater desalination and removal of heavy metal ions,single layer Ti3C2(SL-Ti3C2)nanosheets with uniform size were prepared firstly in this paper.The single layer carbide was prepared by the method of ball milling using Ti3AlC2 as precursor.The parameters of ball milling process were optimized.The efficiency of photothermal conversion during the desalination of seawater was improved as well as the performance of heavy metal ions removal by means of reductive absorption.The Ti3C2/TiO2 composite was prepared by hydrothermal method,using multi-layer Ti3C2,denoted as d-Ti3C2,as the precursor.The parameters of hydrothermal process were optimized and the efficiency of heavy metal ions removal by means of reductive absorption was improved.The following studies were carried out in this paper:(1)First of all,SL-Ti3C2 was prepared by a series of procedures start from ball milling using Ti3AlC2 as precursor.The milling jars were vacuumed and the ethanol was induced as process control agent to prevent materials from oxidation during milling process.Secondly,multi-layer Ti3C2 was obtained by the method of intercalation using hydrofluoric acid.The d-Ti3C2 was then processed using ultrasonic combined with organic solvent intercalation to further expand the interlayer distance in order to produce SL-Ti3C2 nanosheets with uniform size.It was found that,the thickness of SL-Ti3C2 nanosheets obtained from procedures following a ball milling process for 5 hours is about 1～2 nm,and the length of the nanosheet was about 500 nm.In order to measure the efficiency of solar energy absorption of the carbide,a solar evaporator was prepared by combining the prepared materials with absorbent cotton.It was found that the efficiency of solar energy absorption is up to 97.18%,thanks to the special light-capture structure of SL-Ti3C2 nanosheets.Comparing to d-Ti3C2/C,the evaporator equipped with SL-Ti3C2/C can accelerate the water evaporation significantly.Because of the rapid water transportation of absorbent cotton and the excellent photothermal conversation property of single layer carbide,the evaporation rate was up to 1.32 kg·m-2h1,when exposed to one sun exposure,which is about 26.44%higher than that of evaporator equipped with d-Ti3C2/C and is about 5.28 times of the evaporation rate of pure sea water.The energy conversion efficiencies of the evaporator were as high as 90.98%and 97.64%for one sun and five sun exposure,respectively.And it shows a stable fresh water producing capacity during a long-term testing period.In addition,the quality of the treated steam was carefully checked.The concentrations of Na+,Mg2+,K+and Ca2+ions in the produced fresh water decreased sharply after desalination,as well as the concentrations of heavy metal ions such as Zn2+,Pb2+,Cd2+and Cue+.(2)In this work,SL-Ti3C2 nanosheets were prepared.Due to the characteristics of monolayer,high specific surface area and fine,uniform particle size of SL-Ti3C2 nanosheets,it can be used for the reduction and removal of multivalent heavy metal ions in wastewater.The reduction and adsorption properties of heavy metal ion,Cr(Ⅵ)were studied in detail.The specific surface area of SL-Ti3C2 nanosheets is much higher than that of d-Ti3C2,which is beneficial to the adsorption of Cr(Ⅵ).The suspension of SL-Ti3C2,which was prepared by the means of ball milling for 5 hours,with a concentration of 0.05 g L-1 and a pH value of 2 shows the best removal rate of Cr(Ⅵ).And the removal rate was up to 97.35.The mechanism of reductive adsorption is that,the positive charges distributed on the surface of SL-Ti3C2 nanosheets in an acidic solution attracted Cr2O72-firstly,then the Cr(Ⅵ)ions were reduced to Cr(Ⅲ)by H+,and Ti3C2 provides the desired electrons.Finally,the reduced ions were absorbed on the surface with the configuration of Ti-C-Cr(Ⅲ).(3)The composite material of Ti3C2/TiO2 was prepared by a simple one-step hydrothermal method,using the two-dimensional material d-Ti3C2 as the precursor.The reduction and absorption performances of the composite were promoted significantly because of the large specific surface area and abundant active sites on the surface.The process parameters of synthetization and metal ions removal were optimized,and the reductive absorption mechanism was discussed.The suspension of Ti3C2/TiO2,which was prepared by the means of ball milling for 5 hours,with a concentration of 0.05 g L-1 and a pH value of 2 shows the best removal rate of Cr(Ⅵ).And the removal rate was up to 97.35.The mechanism of reductive adsorption is that,the positive charges distributed on the surface of Ti3C2/TiO2 composite material in an acidic solutio1 attracted Cr2O72-firstly,then the Cr(Ⅵ)ions were reduced to Cr(Ⅲ)by H+,and Ti3C2 provides the desired electrons.Finally,the reduced Cr(Ⅲ)moiety is anchored to the surface of TiO2 via a covalent bond with the Ti-O bond,and finally forms Ti-O-Cr(Ⅲ),partially through the bond with Ti-C.The covalent bond is partially anchored to the surface of Ti3C2,and finally Ti-C-Cr(Ⅲ)is formed. |
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