Preparation Of ZnO NPs/BC Through Plant Enrichment-pyrolysis Method And Its Mechanism Study Of Tetracycline Degradation

The large-scale application of Zn O nanoparticles（Zn O NPs）has made it inevitably enter the aqueous environment.As an invasive species,water hyacinth propagated very fast and consumed a lot of dissolved oxygen in water when growing,which has caused water quality deterioration and eutrophication in recent years.In order to provide an effective method for the resource utilization of Zn O NPs and water hyacinth,this study used water hyacinth to enrich Zn O NPs at different concentrations in the solution.At the end of enrichment,the roots of water hyacinth were pyrolyzed into biochar containing Zn O NPs（Zn O NPs/BC）.The prepared Zn O NPs/BC in combination with persulfate（PS）and UV lamp degraded tetracycline hydrochloride（TC）,a typical antibiotic with complex molecular structure and difficult to remove.It was not only the sustainable utilization of Zn O NPs and water hyacinth,but also a new method for the treatment of antibiotic wastewater such as TC.The main findings of this study were as follows:（1）Analysis of the enrichment effect and mechanism of water hyacinth on Zn O NPs.The roots of water hyacinth uptook Zn O NPs by secreting mucus,and the stems-leaves absorbed Zn O NPs through the submerged part directly and the transport from roots via phloem and xylem.The Zn content in roots was about 3.5～9.0 times of that in stems-leaves,and the highest content reached 2.68%.After being prepared into Zn O NPs/BC,the maximum Zn level was concentrated to 7.55%.（2）The microscopic characterization of Zn O NPs/BC.The Zn concentration significantly affected TC removal,and Zn O NPs200/BC was optimum for TC degradation,which was selected for FESEM,XRD and BET characterization.The results exhibited that Zn O NPs were uniformly dispersed on the surface of Zn O NPs200/BC with an average particle size of 22.3nm.Moreover,compared with the original BC,Zn O NPs200/BC possessed a larger specific surface area and more micropores.（3）Exploration of the optimal experimental conditions.It was determined that 0-50min was dark reaction and 50-120min was light reaction.Considering the degradation rate of TC and the energy consumption,the best reaction conditions were0.1g/L Zn O NPs/BC,1.0m M PS,30W UV and p H=7.（4）The effects of common anions and organics on TC degradation.The eight common anions such as Cl^-、SO₄^2-、NO₂^-、NO₃^-、H₂PO₄^-、HPO₄^2-、CO₃^2-、HCO₃^-and two natural organic compounds EDTA and HA could inhibit the reaction through hindering PS decomposition,masking the critical reaction sites on the surface of Zn O NPs/BC or generating less active free radicals.（5）Analysis of the contribution and generation mechanism of free radicals.The results of ESR spectrum and quencher test showed that TC decomposition mainly relied onSO₄^·-and HO^·attached to the surface of Zn O NPs/BC.The contributions ofSO₄^·-and HO^·to TC removal were 50.73%and 7.3%at 50 min,and 76.74%and19.81%at 120 min,respectively.The formation mechanism of the two free radicals was varied in the dark reaction and light reaction.During the dark reaction of 0- 50min,PS directly received single electron from persistent free radicals（PFRs）contained in Zn O NPs/BC to produceSO₄^·-,and then reacted with O₂ or H₂O to HO^·.In the 50-120min photoreaction stage,Zn O NPs was excited by UV to produce hole-electron pairs,and then reacted with H₂O or O₂ to generate HO^·,while PS was directly stimulated by UV to formSO₄^·-.At the same time,PS could also accept the photogenerated electrons from Zn O NPs to generateSO₄^·-,promoting both PS activation and the generation and separation of photogenerated hole-electron pairs of Zn O NPs,and the the calculated synergy between PS and Zn O NPs was up to 0.35.（6）Study on the enhancement of water hyacinth enrichment method on the reaction.Stimulated by Zn O NPs,the roots of water hyacinth secreted more total phenols and flavonoids.These phenolic compounds coud acted as precursors of PFRs,which was beneficial to the formation of more PFRs.In addition,the formed PFRs owned a half-life of up to 95 days due to their close combination with the biochar substrate.The entry of Zn O NPs resulted in the formation of more interconnected micropores in the water hyacinth biochar,reducing the resistance of electron transfer of the biochar matrix.Compared with the original BC,the electrochemical impedance of Zn O NPs200/BC decreased significantly.On the one hand,this property was conducive to the electron transfer of PFRs to activate PS to form moreSO₄^·- participation in the reaction.On the other hand,it could also strengthen the photolysis reaction.The reason could be explained that with the decrease of impedance,the electrons generated during PFRs formation would more easily enter the Zn O NPs crystal conduction band,and enhance its absorption ability to UV and visible light, thus forming more hole-electron pairs under UV excitation.Moreover,the hole-electron pairs were more quickly accepted by PS and O₂ in the system to generate more HO^·,further enhancing TC degradation.（7）The TOC was degraded evidently with the removal rate was 47.53% at 120 min.Sixteen main intermediates were detected via HPLC/MS test.It was revealed thatSO₄^·-and HO^·could oxidize the TC molecule through attacking the carbon-carbon double bond,phenol hydroxyl and amino groups.TC gradually broke the chain and was mineralized into CO₂ and H₂O eventually.