Study On The Adsorption Removal And Electrocatalytic Degradation Of Low-molecular-weight Phthalate Esters In Water

Phthalate esters（PAEs）are a kind of environmental hormone compounds and intensively used as important additives in industrial production of plastics,resins and rubber.Due to no chemical combination between PAEs and host plastics,PAEs can be easily released from plastic articles into the surrounding environment,and have been detected in air,water and soil.Human beings may be exposed to PAEs through inhalation,drinking,ingestion,and use of personnel care products,especially those by oral contact.Because of the homologous estrogenic activity,PAEs can combine with estrogen receptor and then cause the disorder of normal endocrine system,interfere with biological processes in humans and wildlife,potentially causing teratogenicity,mutagenicity and carcinogenicity even at very low concentration.The global production of PAEs was estimated to reach over 8 million tons,and PAEs remain in the environment for a long time due to their low degradation,and can be accumulated in various organisms and show incalculable negative effects for human health.As a result,the environmental and health hazards caused by PAEs have been paid more and more attention by government departments,academia and the public.Therefore,it is of great significance to develop a simple and effective method to remove PAEs in water for the protection of water environment quality and human health.At present,PAEs treatment technologies mainly include adsorption,biodegradation and advanced oxidation.Among these technologies,adsorption is considered as a simple and efficient method because of its easy operation,cost-effective and high regeneration ability.However,the common adsorption materials are not satisfied on the selective adsorption,and the adsorption processes need to take several hours or even longer time to attain equilibrium,and filtration or centrifugation procedure is required to separate the adsorbents from aqueous phase,which is very time-consuming and cumbersome.Biodegradation can effectively remove PAEs in water environment,but the degradation rate is very slow and specific microbes and voluminous sludge are required.The degradation efficiency of advanced oxidation technology is high,but most methods,such as（photo）chemical oxidation and photocatalytic oxidation,need high cost and special equipments,and may cause secondary pollution.At the same time,it is worth noting that PAEs are commonly present at very low concentration(from ng·L^-1toμg·L^-1)and always coexist with a variety of low toxic and biodegradable pollutants in water environment.When a particular method is applied for water treatment,the treatment of PAEs may be inefficient if it aims to remove all the pollutants at the same time.Molecularly imprinted polymers（MIPs）have high selectivity and affinity towards template molecules,they are of practical interest as tailor-designed materials to remove low-concentration PAEs.Based on the above,functionalized molecularly imprinted materials were prepared,and used for the selective adsorption and separation of three PAEs（DMP,DEP and DBP）in this thesis.In order to improve the degradation ability of PAEs,electrochemical oxidation technology has been studied due to its high effectiveness,versatility,environmental compatibility and easy control.A Ce-doped Ti/PbO₂anode was prepared by electrodeposition,and the feasibility of the simultaneously electrocatalytic degradation of the three PAEs in simulated wastewater were investigated.The main results were as follows:1.Using multi-walled carbon nanotubes（MWCNTs）as support material,Fe₃O₄magnetic nanospheres were modified on MWCNTs by hydrothermal method,and then the functional MWCNTs was obtained by sol-gel and surface grafting techniques.The four O atoms of ester bond in PAEs molecular structure can be used as acceptor to form hydrogen bond with H atom of functional monomer methacrylic acid（MAA）.Using three frequently detected low-molecular-weight PAEs（DMP,DEP and DBP）as template molecules(n _DMP:n _DEP:n _DBP=6:3:2),MAA as functional monomer,ethylene glycol dimethacrylate as cross-linking,AIBN as initiator,and acetonitrile as solvent,the molecularly imprinted composites（mag-MMIPs@MWCNTs）were prepared on the surface of functionalized MWCNTs.The composites were characterized by SEM,FTIR,VSM and BET.It was testified that Mag-MMIPs@MWCNTs possessed the composite structrue supported by MWCNTs skeleton,Fe₃O₄as magnetic core and surface imprinted layer as shell.The saturation magnetization value of mag-MMIPs@MWCNTs was detected as 25.6emu×g^-1,which was sufficient for rapid magnetic separation using an external magnet.The composites had good thermal stability,and the specific surface area and pore volume of mag-MMIP@MWCNTs were slightly higher than those of mag-NIP@MWCNTs,suggesting that mag-MMIP@MWCNTs was more suitable for the binding of the three PAEs.2.As adsorbents,mag-MMIPs@MWCNTs showed a stable adsorption ablity to DMP,DEP and DBP in the p H range of 5 to 9.Mag-MMIP@MWCNTs with nano-sized imprinting layer could effectively improve mass transfer and thus greatly shorten the adsorption time,and so the adsorption equilibrium of DMP,DEP and DBP(5 mg L^-1)could be reached within 40 min.The adsorption processes of mag-MMIP@MWCNTs to DMP,DEP and DBP followed pseudo second-order kinetic model,and the observed rate constants were 2.86,1.28 and 4.41 mg·（μg·min）^-1,respectively.mag-MMIP@MWCNTs was more effective to adsorb DMP and DEP not exceeding 30 mg·L^-1,and had a good applicability for DBP from low to high concentrations.The adsorption thermodynamics followed Langmuir isothermal adsorption model very well,and the maximum adsorption capacities(Q_max)of DMP,DEP and DBP were obtained as 0.95,1.38 and7.09 mg·g^-1,respectively.The adsorption thermodynamic studies indicated that the adsorption processes were exothermic and spontaneous in the temperature range of10-40°C,and dominated by physical adsorption relying on hydrogen bond,hydrophobic interaction and van der Waals force.The Scatchard analysis revealed that the template-polymer system had a two-site binding behavior.Because of the existence of specific binding sites,mag-MMIPs@MWCNTs showed specific adsorption for DMP,DEP and DBP,and the adsorption capacities of the three PAEs on mag-MIP@MWCNTs were much higher than those on mag-NIP@MWCNTs under the same conditions.Using DEHP and H₂PA as analogues,the imprinting factors of DMP,DEP and DBP on mag-MMIP@MWCNTs were calaulated as 2.79,2.53 and 1.54,respectively,suggesting that mag-MMIP@MWCNTs had selectivity for the template molecules.mag-MMIP@MWCNTs also showed good reproducibility and reusability for simultaneous adsorption of the three PAEs.The potential application of mag-MMIP@MWCNTs was proved by the removal of DMP,DEP and DBP spiked in environmental water samples.3.Electrocatalytic oxidation was investigated to remove the three PAEs.Ce doped Ti/PbO₂electrode was prepared by electrodeposition using low-cost PbO₂as electrode material and Ce doping.The surface morphology,crystal structure and elemental states were characterized by SEM,XRD and XPS,respectively.When the mole ratio of Ce and Pb was 1:5,Ce could enter into PbO₂lattice through filling or substitution to form local energy levels,which changed the deposition mode of PbO₂active layer and the microstructure of electrode surface,and further improved the electrocatalytic activity of the electrode.The linear polarization measurement showed that the current density of Ce-doped Ti/PbO₂electrode was significantly higher than that of Ti/PbO₂electrode at the same voltage,indicating that there were more electrochemical active sites on the surface of Ce-doped Ti/PbO₂electrode.In the electrodeposition process,the voltage of the cell was higher due to the doping of Ce,which could accelerate the deposition of PbO₂crystal particles on Ti substrate,and thus refined the particles and increased the specific surface area of the electrode.Therefore,Ce doping improved the electrocatalytic activity of Ti/PbO₂electrode.According to the results of cyclic voltammetry,it was speculated that the electrochemical degradation of PAEs in this system was indirect oxidation by active species（such as·OH）produced in electrolysis process.4.The effects of supporting electrolyte and current density on the electrocatalytic oxidation of DMP,DEP and DBP were investigated.The removal efficiencies of 0.05-5mg·L^-1DBP,DMP and DEP in 0.05 M Na₂SO₄（p H 7）were 99.3%-98.2%,97.2%-95.8%and 88.4%-81.1%at current density of 25 m A·cm^-2after 10 h degradation,respectively.Because of the longer carbon chain（two butyls）,DBP is more vulnerable to the attack of·OH,and so the fastest degradation rate and the highest degradation efficiency were obtained for DBP.The degradation processes followed pseudo first-order kinetic model very well,and the observed rate constants of DBP,DEP and DMP were 0.42,0.40 and 0.29 h^-1,respectively.LC-MS/MS analysis showed that the three PAEs had similar degradation products and degradation pathways.Hydroxyl radicals produced in the electrode process first attacked the ester bonds of PAEs to form monomethyl phthalate and phthalic acid,and then attacked 4-position carbon atom of phthalic acid to form 4-hydroxyphthalic acid.Meanwhile,·OH could also induce the decarboxylation and hydroxylation of phthalic acid,and thus formed salicylic acid and dihydroxybenzene,which could be further transformed into low-molecular-weight organic acids（e.g.,succinic acid and oxalic acid）through ring opening of the aromatic group.In summary,the main degradation pathways of the three PAEs may include dealkylation,hydroxyl addition,decarboxylation and benzene ring cleavage.