Study On The Degradation Of Salicylic Acid In Aqueous Solution Catalytic Ozone Oxidation By Supported Catalyst

SA is a kind of important industrial raw material. In the pharmaceutical industry, it used to make aspirin and sodium salicylate, etc. In the rubber industry, it used in the manufacture of scorch and foaming agent, etc. In addition, SA also has an important role in the field of cosmetics and spices, etc. Therefore, the production of SA increased year by year in our country and our country has become the world’s largest SA producer. However, the mass production and application of SA makes SA content in water environment of China rising and result with pollution, then, pose a threat to people’s health and ecological balance.As the electron withdrawing groups, the electronegativity of carboxyl and hydroxyl groups on the benzene ring of SA molecular is larger, and it lead to the electron cloud density of the main structure of SA decreased. It is difficult for ozone directly react with SA, beacuse reaction rate constant is600L/(mol-s) only. However, as a commonly-OH scavenger, SA easily react with OH for the reaction constant is as high as2.2×1010L/(mol·s).Catalytic ozonation is a kind of advanced oxidation technology in recent years the rise of, its principle is promoting ozone generate more OH to degradate refractory organic matter by adding catalyst. Supported catalyst catalytic ozone oxidation belongs to the heterogeneous catalytic oxidation of catalytic ozone oxidation, it combines the strong oxidation ability of ozone with the adsorption and oxidation of metal oxide, then, the organic matter will removal by mineralization at room temperature. In addition, not only the preparation of supported catalyst is simple, sort is various, but also, supported catalyst can easily separated from the wastewater, its activation and operational continuity is strong and there is no subsequent metal ions removal problems. All in all, supported catalyst has incomparable advantages in the degradation of S A.By impregnation method, this paper chooses Mg-Al(2:1) hydrotalcite, Mg-Al(3:l) hydrotalcite, the Ni-Mg-Al(36:3:13) hydrotalcite like compounds which obtains by homemading and γ-Al2O3particles and nano γ-Al2O3which obtains by purchasing, as the carrier to support nickel oxide, copper oxide, cobalt oxide, manganese oxide, then catalytic ozonation to degradate salicylic acid. The affecting factors and degradation effect ware studied, the results are as follows: (1) The phase composition of the catalysts, metal oxide content, specific surface area, zero charge and adsorption ability of SA was investigated. By XRD figure, nickel metal oxides, copper metal oxides, cobalt metal oxides and manganese metal oxides can better dispersed on the surface of carrier, and load effect is good; The determination of ICP-AES reveals the actual metal oxide content of catalyst is greater than9%, compared with the theory of metal content10%, relative deviation are controlled within10%; The determination of nitrogen adsorption, the specific surface area of catalysts around30%to40%lower than the carrier nano y-Al2O3’s; Drawing zero charge curve of catalyst, we know that except manganese catalyst’s zero charge is5.35, other support catalyst and the carrier nano y-Al2O3’s zero charge are greater than8.00; According the determination of the adsorption performance of SA, we known that the SA adsorption ability of catalyst is different in different pH, the greater pH value, the poorer adsorption, and the catalyst adsorption data of SA logarithmic form is in line with the Freundlich isothermal equation, and the correlation coefficient decreases with increasing pH. Besides, we also find the adsorption data of catalysts in the logarithmic form consistent with the Freundlich isotherm equation and the correlation coefficient decreases with pH increase.(2) Influencing factors on the ozonation of SA were studied. It showed that pH value had a great influence on SA degradation effect. Within the range of the pH=2-10, with the increasing of pH, SA and COD degradation rate is firstly increased and then decreased. When pH=6, SA degradation effect is best, and the value is67.23%. When pH=8, COD degradation effect is best and the value is45.44%.(3) Influencing factors on the catalytic ozonation of SA were studied. With γ-Al2O3as carrier, respectively supported nickel, copper and cobalt, manganese four kinds of metal oxide catalysts (C1, C2, C3, C4). It showed that the pH, catalyst dosing quantity, type of catalyst and catalytic carrier had a great influence on catalytic ozonation degradation of SA.Within the range of pH=4-10, the influence of pH value is greater for C1, C2and C3than C4. According to the SA degradation rate from high to low order, for C1、C2、C3, the order is:pH8>pH6> pH4>pH10; And for C4, the order is:pH4≈pH6≈pH8> pH10. Within the range of catalyst dosage is2.5-15g/L, with the increase of dosage, the removal rate of SA and COD rise gradually, when it increased to10g/L, the removal rate both reached the maximum value. Continue to increase the dosage to15g/L, the removal rate both dropped. The effects of C1, C2, C3, C4four kinds of catalyst and carrier γ-Al2O3catalytic ozonation on SA degradation was found that compared with separate ozone oxidation, the addition of catalysts promoted the ozone generating more·OH, and the degradation rate of SA and COD increased significantly. Among them, C4catalytic own the most significant effect. With Mg-Al(2:1), Mg-Al(3:1) hydrotalcite, the Ni-Mg-Al(36:3:13) hydrotalcite like compounds and particle Al2O3, nano γ-Al2O3as the carrier to support MnO2catalytic ozone to removal SA, and the results showed that the catalytic effect order is nano y-Al2O3>particle y-Al2O3>Mg-Al(2:1)>Mg-Al(3:1)-Ni-Mg-Al(36:3:13).(4) Under the condition of SA’s initial concentration3mmol/L, catalyst dosage2.5g/L, use the C4which own the excellent catalytic effect as catalyst to carry the experiment in the pH=8and pH=4, respectively. And the results showed that the catalyst effect has a better stability, catalytic effect is excellent.(5) The mechanism of catalytic ozonation degradation of SA was analyzed. It showed that there is no direct relationship for the catalytic performance of catalyst with SA molecular adsorption ability. The catalytic performance associated with the active center M-OH2+which can be produced under certain conditions. Therefore, in the experiment, the catalyst is not simple as adsorbent, but play a role in catalyst, and it can catalytic ozone to produce more·OH, then achieved the good degradation of SA.