Preparation Of Carbon Nanotubes Loading On Steel Slag And The Adsorption Properties For Heavy Metals And Organic Pollutants In Waste Water

Wastewater from textile,leather and printing industry contains a large amount of heavy metals and dyes,which are harmful to human health and the environment.Carbon nanotubes（CNTs）are a new type of adsorbent materials with high adsorption capacities for heavy metals and dyes.However,their small size and high aspect ratio makes them more prone to agglomeration.Moreover,CNTs are difficultly seperated from the water processed,which cause a secondary environmental pollution.Therefore,it has become an important research topic to maintain good adsorption performance of CNTs and,meanwhile,make it easily to be recycle after adsorption.Steel slag contains high percentage of iron and has high chemical stability.For the use of steel slag as a carrier for in-situ synthesis of CNTs,the iron oxide in steel slag can be acted as catalyst.The problems of CNTs agglomeration and high preparation cost can be solved.The utilization of steel slag,as a by-product of steel manufacture,can be realized as well.In this study,CNTs in situ loading on steel slag（SS@CNTs）were synthesized by chemical vapor deposition method.The synergistic adsorption of steel slag and CNTs were utilized to remove heavy metal ions and organic dyes from waste water.The adsorption mechanism and recyclibility of SS@CNTs were studied.The main results achieved in this study are as follows.Firstly,CNTs was synthesized directly on the steel slag particles surface by chemical vapor deposition（CVD）method with acetylene as carbon source,hydrogen as reducing gas,and nitrogen as carrier gas.The synthesis conditions were optimized.Results show that the optimal synthesis time was 45 min,optimal synthesis temperature 600℃and the optimal gas flow ratio of acetylene to nitrogen V_C2H2:V_N2=200:800.After 2 hours of heat treatment purification,the crystallinity and thermal stability of the carbon nanotubes were improved and the specific surface area and the number of oxidation groups on the surface of SS@CNTs increased.The removal efficiencies of such SS@CNTs reached 98.12%and 85.49%for Pb（II）and methylene blue,respectively.The growth pattern of carbon nanotubes on the surface of steel slag particles showed tip growth and bottom growth pattern.Secondly,SS@CNTs was used to adsorbe heavy metal ions in water,and the influences of the conditions and dyes on the heavy metals adsorption by SS@CNTs were investigated.The adsorption characteristics and mechanism were analyzed based on the adsorption isotherm,adsorption kinetics,thermodynamics and microstructure characterization.The recyclability of SS@CNTs were evaluated.Results showed that the adsorption of all heavy metals proceeded rapidly within the first 15 min and reached equilibriumat at approximately 120 min.The order of adsorption performance of SS@CNTs for the heavy metal ions was Pb（II）（?）Cd（II）≈Cu（II）>Zn（II）>Cr（VI）.The adsorption processes for all the heavy metals are in accordance with the Langmuir isothermal model and pseudo-second-order model,indicating that the adsorptions were monolayer chemisorptions.At 298K,the maximum theoretical adsorption capacities of Pb（II）,Cu（II）,Cd（II）,Zn（II）,and Cr（VI）calculated from Langmuir model were 515.46 mg·g^-1,172.41 mg·g^-1,187.97 mg·g^-1,139.67 mg·g^-1and 17.38 mg·g^-1,respectively.The inhibition effect of MB on the adsorption of Pb（II）was unobvious when the concentration of MB was low.With the increase of MB concentration,the inhibition effect was strengthened.However,when the concentration of MB increases from 100 mg·L^-1 to 120 mg·L^-1,the adsorption capacity of SS@CNTs on Pb（II）increased,resulting from the combined effect of chelation and competitive adsorption.Low concentration of MB has little effect on the adsorption of Cr（VI）.As the concentration of MB increased,the inhibition effect on the adsorption of Cr（VI）becomes stronger.With the increase of CR concentration,the adsorption capacity of SS@CNTs on Pb（II）increased first and then decreased,while the inhibition effect on the adsorption of Cr（VI）was strengthened continuously.The removal of all metals was an endothermic and spontaneous process.The adsorption mechanism of SS@CNTs for heavy metal cations includes ion exchange,surface complexation,precipitation,co-precipitation and electrostatic binding,while for Cr（VI）the adsoption mechanism mainly includes redox,precipitation and electrostatic binding.After five adsorptions–desorption cycles,the adsorption capacities of SS@CNTs can recover to more than 70%of the initial adsorption capacity,indicating that SS@CNTs have a good regeneration after adsorption of heavy metals.SS@CNTs adsorbing heavy metals were then used to prepare cement-based materials.It was found that the adsorbed heavy metal ions can be immobiled well in cement-based materials with immobile rates of over 99%for all heavy metal ions.The heavy metal ions in cement-based materials mainly exist in Fe/Mn oxides,residue,and organic agents.Although the addition of SS@CNTs leads to a slight decrease in the compressive and flexural strength of cement mortar,its fracture energy increased by 43.4%.Thirdly,SS@CNTs were used to remove oganic dyes from water,and the influences of the conditions on the adsorption were investigated.The adsorption characteristics and mechanism were clarified based on the adsorption isotherm,adsorption kinetics,and thermodynamics.The recyclability of SS@CNTs were evaluated.Results show that the adsorption of organic dyes by SS@CNTs reached equilibriumat at about 120 min of adsorption time.The adsorption capacity was negatively correlated with the dosage of SS@CNTs,while the removal rate of dyes was positively correlated with the dosage of SS@CNTs.The adsorption capacity of CR was higher than that of MB and MG at a p H below 5.0.When the value of p H was above 5.0,the adsorption capacity of dyes followed the order:MB>MG>CR.With the increase of Pb（II）concentration,the inhibition effect of Pb（II）on the adsorption of MB enhanced.When the concentration of Pb（II）increased to 120 mg·L^-1,the inhibition effect decreased.Because Pb（II）adsorbed on SS@CNTs provided new adsorption sites for MB through chelation.When the concentration of Pb（II）was above 100 mg·L^-1,Pb（II）promoted the adsorption of SS@CNTs on CR.In contrast,Cr（VI）has a great inhibitory effect on the adsorption of CR and little effect on the adsorption of MB.The adsorption of MB and MG by SS@CNTs were consistent with Freundlich model and pseudo-second-order model,indicating that the adsorptions were non-uniform multilayer chemisorptions.In contrast,the adsorption of CR followed Langmuir model and pseudo-first-order model and was a diffusion-controlled monolayer adsorption process.The adsorption processes of all organic dyes were spontaneous and endothermic,which increase the randomness of system.After five adsorption-desorption cycles,the adsorption capacities of SS@CNTs on organic dyes reached more than 50%of the initial adsorption capacity.Furthermore,after heat treatment in air,the adsorption capacity of SS@CNTs recovered by more than 90%of the initial adsorption capacity.Hence,SS@CNTs had a good recyclability.