Degradation Of Organic Pollutants By Photocatalytic Fibers Based On Carboxyl-modified Graphitic Carbon Nitride In Wastewater

In recent years,with the rapid development of agriculture,industry,and medical industries,and the continuous enrichment of urban life,the discharge of various types of wastewater is increasing,and the composition of organic pollutants in the corresponding wastewater is also becoming more and more complex,which pose a huge threat to human being.The traditional sewage treatment processes are difficult to remove organic pollutants with complex and stable structure.Therefore,it is urgent to develop a wastewater treatment technology with fast degradation rate,complete removal,green and harmless,and wide application range.The advanced oxidation process based on peroxymonosulfate（PMS-AOP）has emerged due to the low cost and high oxidizing ability.Currently,the commonly used PMS activation method is the activation of transition metal ions.However,the metal ions released into the solution are difficult to recover and cause heavy metal pollution.In order to solve this problem,researchers have turned their attention to non-metal activation.As a metal-free semiconductor photocatalyst,graphitic carbon nitride（g-C₃N₄）exhibits excellent physicochemical properties,and can be used as a green PMS activator.However,g-C₃N₄ still suffers from the shortcomings of fast photogenerated electron-hole pair recombination,narrow absorption range for visible light,and poor catalytic degradation performance.At the same time,photocatalysts often exist in the form of powders,which are very easy to agglomerate and are difficult to recycle.As fiber has the advantage of large surface area and excellent chemical stability,it is commonly used as carriers of powder catalysts.However,the most of catalysts attached to fibers are not strong and easy to fall off.Therefore,it is urgent to develop a mean to firmly support the catalysts on the fibers.In this research,in view of high photogenerated charge recombination rate of g-C₃N₄ and its weak ability to activate PMS,g-C₃N₄ was modified with metal-free carboxyl groups,and the photocatalyst with high activity was prepared.The modified photocatalyst was stably supported on the surface of both cotton and fiberglass through a padding-bonding process to prepare a highly active photocatalytic fiber in order to solve the problem of powder catalyst recycling.The specific research results obtained in this paper are as follows:In this paper,potassium persulfate（KPS）was used to carboxylate g-C₃N₄ to prepare carboxyl-modified graphitic carbon nitride（g-C₃N₄（KPS））.The modified photocatalyst were investigated by means of field scanning electron microscope（SEM）,Fourier transform infrared spectroscopy（FTIR）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,Photoluminescence（PL）,photocurrent response and Zeta potential.The results showed that we successfully introduced the carboxyl group on g-C₃N₄,which improved the dispersion of g-C₃N₄.This also accelerated the transfer of electrons,therefore,enhanced the separation efficiency of photogenerated electrons and holes.CBZ was selected as a model compound for photocatalytic degradation experiments.The results showed that under solar radiation,g-C₃N₄（KPS）had high catalytic activity with95.6%degradation rate within 20 mins.And at the same time,the g-C₃N₄（KPS）/PMS catalytic system was effective under acidic,neutral and alkaline conditions.Particularly,in the environment with high concentration of Cl^-and NO3^-,it still had the ability to efficiently activate PMS and maintain excellent catalytic activity.In addition,the g-C₃N₄（KPS）had excellent recycling performance and chemical stability,which could degrade a variety of sulfonamide antibiotics within 30 mins with 100%removal rate including sulfachloropyridazine（SCP）and sulfamethoxazole（SMX）.In addition,combined with the results of electron paramagnetic resonance（EPR）test and trapping agent experiment,it was shown that there were active species·OH,·O2^-,SO4·^-,h⁺and¹O₂ in the catalytic system.And·OH and ¹O₂ radicals played the main role in pollutants degradation.The degradation products and degradation process of CBZ by g-C₃N₄（KPS）photocatalyst were inferred by ultra-performance liquid chromatography and high-definition mass spectrometry（UPLC/HDMS）.CBZ was ring-opened and oxidized under the joint action of various active species.Finally,all kinds of intermediate products were eventually further degraded.In order to solve the recycling problem of g-C₃N₄（KPS）powder photocatalyst,methacrylic acid and neutral silica sol were used as binders,and g-C₃N₄（KPS）was supported on the surface of cotton and fiberglass cloth by padding-bonding method.Two kinds of catalytic fibers were prepared.The photocatalytic fibers were characterized by means of SEM,FTIR,XRD and XPS.The results showed that the g-C₃N₄（KPS）photocatalyst was firmly bound to the fibers,and both catalytic fibers maintained the excellent visible light response of the g-C₃N₄（KPS）photocatalyst.The PL test results showed that the recombination of photogenerated electron-hole pairs of g-C₃N₄（KPS）@cotton and g-C₃N₄（KPS）@fiberglass was suppressed to a certain extent.Under the solar radiation,the two catalytic fibers showed excellent photocatalytic degradation performance and recycle stability for CBZ.After five cycles,the degradation rates of CBZ by g-C₃N₄（KPS）@cotton and g-C₃N₄（KPS）@fiberglass were 98%and 92%,respectively.The catalytic fibers after recycling were analyzed by SEM,and it was found that the surface morphology and structure did not change significantly.At the same time,the catalytic fibers could activate PMS and degrade organic pollutants efficiently in the water environment where various anions existed,indicating that they had good environmental adaptability,and the above two catalytic fibers had a similar reaction mechanism to that of the powder catalyst.In addition,the degradation products and degradation process of CBZ by catalytic fibers were also analyzed and speculated.This paper provided a feasible idea for the activation of PMS by metal-free modified photocatalytic fibers,and provided a new way for wastewater treatment.