Research Of Carbon Nanotube Hydrogel And LiFePO₄ Three-dimensional Electrode In Wastewater Treatment

The complex compositions of dyes in wastewater is not conducive for the self-purification of water bodies,which can not only affect the growth of aquatic organisms and microorganisms,but also lead to the carcinogenic mutagenicity of human body due to their highly toxicity.According to statistics,about 1300,000 tons of dye are producted per year in the world,and over 100,000 different dyes have been widely used in various regions and industries.Among these,the annual output of dyes in China ranks in the forefront of the world's dye production.Therefore,the effective treatment of dye-containing wastewater has attracted increasing research interest of water treatment workers.Traditionally,the adsorption method and electrochemical method have been widely used to purify the dye-containing wastewater.However,the purification performances on the dye-containing wastewater by using functional carbon nanotube-doped hydrogel,as well as the three-dimensional lithium ferrous phosphate electrode have not been systematically studied.In this paper,the hydrophilic multi-wall carbon nanotubes were acidized by using mixed acid to increase the hydrophilicity.The fabrication of carbon nanotube-doped hydrogels [MWCNT/P(AA-AM)] was conducted in water bath,in which acrylamide and acrylic acid were used as monomers,ammonium persulfate as initiator,and N,N'-methylene diacrylamide as crosslinking agent.The MWCNT/P(AA-AM)hydrogels after the drying and grinding process were characterized by infrared spectroscopy and X-ray diffraction.The MWCNT/P(AA-AM)hydrogels with and without the adsorption of dyes were characterized by scanning electron microscopy.The effects of synthetic parameters,swelling temperature,swelling time,and solution p H value on the swelling properties of MWCNT/P(AA-AM)hydrogel were studied.The optimal synthetic parameters of MWCNT/P(AA-AM)hydrogels were further confirmed by orthogonal experiments,under the guidance of which the hydrogels with relatively good adsorption performance were synthesized.The adsorption experiments of the wastewater containing four dyes,including safranin-t(ST),malachite green(MG),crystal violet(CV),and methylene blue(MB),were carried out using the MWCNT/P(AA-AM)hydrogel,respectively,.The effects of hydrogel size,amount of hydrogel,adsorption temperature,and solution p H value on the dye adsorption property of MWCNT/P(AA-AM)hydrogel were investigated.The adsorption data of MWCNT/P(AA-AM)hydrogel for four dyes were fitted by Pseudo-first-order and Pseudo-second-order kinetics models,and Langmuir and Freundlich adsorption isotherm models,respectively.The desorption properties of MWCNT/P(AA-AM)under different conditions were studied,and their recycling abilities hydrogels were investigated.The results show:(1)Carboxyl hydroxyl group and other hydrophilic groups were successfully introduced into the multi-wall carbon nanotubes after the strong acid oxidation treatment.The surface area of the hydrogel was increased by the uneven porous structure of the carbon nanotubes,affording it excellent adsorption performance.Meanwhile,doped carbon nanotubes could also enhance the mechanical properties of the hydrogel.(2)The swelling performance of of MWCNT/P(AA-AM)hydrogel were influenced by various synthetic parameters.During the preparation of MWCNT/P(AA-AM)hydrogels,when the molar ratio of acrylic acid to acrylamide was 1:2,the crosslinking agent was 1.6%,and the initiator was 4%,the as-prepared MWCNT/P(AA-AM)hydrogels exhibited a relatively good swelling property.During the water absorption process,different swelling conditions also affected the swelling properties of MWCNT/P(AA-AM)hydrogel.Within the scope of the study,the maximum swelling degree could be achieved 23934%,when the p H value of the water solution is 7 and the temperature is 70 °C for 120 h.(3)The optimum synthetic parameters for MWCNT/P(AA-AM)were determined by orthogonal experiments: the amount of carbon nanotubes was 16%,the molar ratio of acrylic acid to acrylamide was 2:1,the initiator was 2%,and the crosslinking agent was 1.6%.(4)The experimental results of the treatment of dye-containing wastewater by MWCNT/P(AA-AM)hydrogel under different conditions showed that the maximum adsorption of the hydrogel wan achieved when the particle size was less than 0.088 mm,the dosage was 1 g/L,and the p H value of the dye solution was 7.Suitable adsorption temperatures were 40 °C for ST and CV,10 °C for MG and 20 °C for MB.The adsorption capacities for ST/CV/MG/MB under the optimal experimental conditions were 940.91 mg/g,959.38 mg/g,821.28 mg/g,and 525.41 mg/g,respectively,which improved 41.82%,71.69%,54.8%,48.09% than P(AA-AM).(5)The adsorption kinetic results showed that the adsorption processes of ST,CV,MG and MB on MWCNT/P(AA-AM)hydrogels were more in line with the Pseudo-second-order kinetics reaction process,and the correlation coefficient was better than the Pseudo-First-Order kinetics model.Therefore,the adsorption kinetics of four dyes on MWCNT/P(AA-AM)hydrogels were dominated by chemical adsorption.The fitting results of adsorption isotherms of MWCNT/P(AA-AM)hydrogels for dyes showed that under the same conditions,RL in Langmuir model was greater than 0.9,which could better describe the adsorption behavior of four dyes than Freundlich model.Therefore,the dyes were adsorbed as a monolayer onto MWCNT/P(AA-AM)hydrogel and and did not interact with the adjacent adsorbates.(6)The desorption of dyes from MWCNT/P(AA-AM)hydrogel could be achieved by using HCl solution with concentration of 0.1 mol/L at 70 °C for 24 h.After five adsorption-regeneration cycles,MWCNT/P(AA-AM)hydrogel still possessed good adsorption properties for dyes.The three-dimensional electrode is a new environment-friendly electrochemical catalytic oxidation method,which is formed by adding conductive particles to the electrolytic cell on the basis of the traditional two-dimensional electrode.The three-dimensional electrode method hold promsing potential application in the treatment of organic wastewater due to the simple operation,highly controllability,small occupied area,strong oxidative degradation,high energy efficiency and no secondary pollution.In this paper,the titanium-based electrode was selected as the anode material,and the lanthanum doped lead dioxide intermediate layer and surface-active layer were generated by electrodeposition to enhance the stability and conductivity of the electrode.The common graphite electrode was selected as the cathode material,and the lithium ferrous phosphate(LiFePO~4)three-dimensional electrode was prepared to improve the efficiency of the electrode in wastewater treatment.ST,CV,MG were selected as the research object to study the oxidative degradation ability of the LiFePO~4 three-dimensional electrode.The effect of LiFePO~4 three-dimensional electrode on the removal of dye solution was determined by using a single variable method under different experimental conditions,such as the size and dosage of Li Fe PO~4,electrode spacing,current intensity,initial p H value of solution,and initial concentration of solution,and the degradation kinetics of dyes was studied.The as-prepared titanium-based lead dioxide electrode doped with lanthanum and the LiFePO~4 three-dimensional electrode were characterized by SEM and XRD.The results showed that the titanium substrate was completely covered by the outer layer of the lead dioxide after the electrodeposition process.This polygon spatial structure greatly increased the specific surface area and the active adsorption sites of the electrode,which could improve the mass transfer speed and facilitate the oxidation reaction.The experimental results of dye treatment showed that the LiFePO~4 three-dimensional electrode with size of 5 cm~2,the addition amount of LiFePO~4 three-dimensional electrode is 20 g/L,the spacing between the main electrode plates is 2 cm,the working current intensity is 0.8 A,the initial p H value of the solution is 5,the electrolyte concentration is 5 g/L,and the initial concentration of the CV solution is 800 mg/L,ST and MG solution is 600 mg/L,LiFePO~4 three-dimensional electrode had the most appropriate removal effect on the dye solution.Under the optimal conditions,the removal rate of ST,CV,MG reached to 91.6%,93.7%,93.8%,respectively,more than 23% higher than two-dimensional electrode method.The degradation kinetics of dyes by LiFePO~4 three-dimensional electrode was more in line with the first-order kinetics model.