Study On Synthesis Of Titanium-bearing Blast Furnace Slag Based Photocatalytic Materials And Degradation For Tetracycline In Water

Faced with the problems of current global energy shortage and environmental pollution,it is the key of resource development to realize the recyclable and efficient utilization of resources.The iron and steel industry produces a large amount of slag waste,which is accompanied by a considerable number of valuable elements or materials.How to recover or utilize these valuable components has attracted more and more researchers’attention,which not only saves metal and mineral resources,but also has important significance for environmental protection.At present,there are problems of low utilization rate and secondary pollution in utilization and disposal of titanium-bearing blast furnace slag（TB）in China.It is urgent to develop efficient and environmentally friendly reuse approaches.Therefore,this thesis focuses on the effective reuse of TB and carries out studies on the preparation of slag-based photocatalytic materials and their application in water treatment.The conclusions are as follows:The photocatalyst Ti O₂@LDH（TL）was prepared by acid dissociation-coprecipitation method with TB.Crystal regulation and photo-absorption enhancement were achieved by controlling reaction conditions.By SEM,TEM and XRD,etc.,TL was confirmed to be a kind of nano-Ti O₂ particle composite supported by hydrotalcite（LDH）,the 2D structure of LDH and the positively charged matrix layer contributed to the improvement of the dispersion and adsorption performance of nano Ti O₂ particles.The homojunction of anatase phase Ti O₂ and rutile phase Ti O₂ was favorable for photo-induced electron/hole transport to surface along the exposed anisotropic crystal plane.Meanwhile,TL had Ti³⁺/O_V lattice defect structure and good electron separation and migration performance.The dark adsorption and UV photocatalytic removal rates of TL to tetracycline（TC）were 19.51%and 94.43%,respectively,showing good recycling and p H adaptability.The study of photocatalytic reaction mechanism showed that the UV absorption capacity of TL was significantly enhanced by slag material modification.The valence band（VB）position of TL（2.86 e V）was more positive than E（OH^-/·OH）（1.99 e V）,and h⁺concentrated at the bottom of VB can oxidize OH^-on the surface of the catalyst to·OH.h⁺and·OH,as the main active species in photocatalytic degradation system,attacked TC molecules on TL surface.With addition,demethylation,hydroxylation,ring-opening,denitrification and other reactions,TC degraded into a series of low molecular by-products,eventually mineralized into CO₂,H₂O,NH₄⁺and other inorganic compounds,reducing the environmental risk.To solve the problems of poor visible light response and limited adsorption capacity of TL,cerium（Ce）was doped to modify the synthesis catalyst Ce-Ti O₂@LDH（Ce TL）.The results showed that the introduction of Ce promoted the formation of nanopore structure,significantly improved the specific surface area,enhanced the adsorption performance of the material,and facilitated the transformation of Ti O₂ from crystal to amorphous phase.Abundant Ti³⁺/O_V defect sites were formed on Ce TL surface,which effectively improved the transfer rate of photo-induced electron/hole and reduced energy loss during electron transfer.The dark adsorption and visible light catalysis（90 min）removal rates of TL to TC were 19.51%and 94.43%,respectively,and the apparent rate constant k was 0.0331 min^-1,14.39 times higher than the k value of TL(0.0023 min^-1).The ammonia-nitrogen with low concentration,phosphorus with high concentration,and the dissolved organic matter（DOM）with molecular weight of1 k Da～3 k Da was the most significant inhibition on degradation rate.When UV lighted,photog-induced electrons can transfer to Ce 4f band at a lower energy level and be captured by Ce⁴⁺to achieve effective separation of photo-induced carriers.Then,through the reversible transformation of Ce⁴⁺and Ce³⁺,O₂ on Ce TL surface was provided with e^-to generate·O₂^-,which inhibited the recombination of photoinduced electron hole pairs.In addition,photo-induced h⁺on VB reacted with OH^-in water to produce·OH,and·O₂^-and·OH synergically degrade TC.The VB and CB of Ce TL was located at 2.06 e V and-0.50 e V respectively,so the production of·O₂^-（-0.33 e V）was more advantageous than that of·OH（1.99 e V）,which may explain that·O₂^-played a major role in TC degradation.Ce doping changed crystal structure,strengthened adsorption capacity,reduced band gap width,improved the visible light response capacity,photo-induced e^-/h⁺separation and transmission efficiency,then synergically enhanced the visible light catalytic performance of Ce TL in multiple aspects.In order to strengthen the visible light catalytic performance of the material,break through the limitation of crystal intrinsic separation,and enhance the carrier separation and transport efficiency by using the interface media,the graphite phase carbon nitride（g-C₃N₄）and TL were calcined to synthesize the heterojunction system g-C₃N₄/Ti O₂@LDH（CNTL）.It was found that g-C₃N₄ molecule was coupled to TL surface in 2D nanosheet form through chemical bond to form a stable heterojunction structure,which shortened the distance of electron transport and effectively promoted the separation and migration of photocarriers between interfaces.At the same time,the formation of mesoporous structures increased the specific surface area and surface-active sites.CNTL showed higher reaction rate and activity than TL and Ce TL,which also improved the yield of active species and enhanced the antibacterial performance.CNTL was Z-type heterojunction,when excited by visible light,the VB of g-C₃N₄ acted as a photo-induced h⁺trap to capture e^-from the CB top of TL,which reduced the recombination probability of e^-on CB of g-C₃N₄ and h⁺on VB of TL,and promoted the accumulation of e^-on CB of g-C₃N₄,and reacted with O₂ on the catalyst surface to generate·O₂^-.while more h⁺with high oxidation potential gathered under VB of TL and reacted with OH^-on the catalyst surface to generate·OH.CNTL presented excellent redox ability and efficiently removed TC from water.For the purpose to understand the feasibility of the application of slag-based photocatalytic materials,CNTL was used to carry out photocatalytic degradation experiments on the residual water of dewatering sediment,analyzed the photocatalytic degradation effect of TC in the residual water,and revealed the influence of photocatalysis on the morphological transformation of nitrogen,phosphorus and dissolved organic matter（DOM）.The mechanism of photocatalytic reaction in residual water was further explored by two-dimensional correlation analysis.The results showed that the degradation effect of TC in residual water was slightly lower than that in laboratory simulated wastewater.Photocatalysis promoted the transformation of ON and NO₃^--N to NH₄⁺-N and NO₂^--N,and promoted the transformation of OP to IP and small phosphorus.The removal effect of humic acid substance was better than that of aromatic protein like substance,the removal of humic acid and aromatic protein like substance was mainly affected by adsorption and photocatalytic,respectively.DOM with 20 k Da molecular weight was greatly reduced,and the degradation product was mainly located below 5 k Da.The sensitivity sequence of groups in residual water components to photocatalytic was as follows:For p H=3,C-O-C in humic acid>C=C>C-N>C-OH in TC>BCD-ring of TC>C=O in A-ring of TC>benzene carboxy/phenolic group of humic acid>aromatic protein like substance.For p H=7,C-O-C in humic acid>C-N>C-OH in TC>BCD-ring of TC>benzene carboxy/phenolic group of humic acid>C=O in A-ring of TC>aromatic protein like substance;For p H=11,C-O-C in humic acid>C-N>BCD-ring of TC>benzene carboxy/phenolic group of humic acid>aromatic protein like substance>C=O in A-ring of TC.