The Reaction Mechanism Between Phenolic Lignin And Chlorine Dioxide

The lignin structure can undergo a series of oxidative degradation reactionsunder the action of chlorine dioxide.This process mainly involves the transformation of lignin structure and functional groups in lignin structure.Phenolic lignin structure is one of the most representative lignin structures.It is important to study the reaction mechanism of phenol lignin and chlorine dioxide for the analysis of lignin degradation mechanism and the formation of adsorbable organic chloride?AOX?in bleaching process.In this paper,the reaction mechanism of phenolic lignin and chlorine dioxide was studied by using phenol lignin model compound.Three different phenol lignin models were selected.The names of the models are 4-hydroxy-3-methoxyacetophenone?APO,G-type monomer model?,3,5-dimethoxy-4-hydroxyacetophenone?DHP,S-type monomer model?and Guaiacylglycerol-?-guaiacyl Ether?G-?-g,?-o-4-type dimer model?,respectively.The reaction mechanism of APO and DHP with chlorine dioxide was studied at room temperature?25??and high concentration of chlorine dioxide.Firstly,the change of characteristic absorbance of each substance in the reaction system was characterized by UV,and it was speculated that APO was activated by chlorine dioxide during the reaction process.In the early stage of the reaction,phenol oxygen radicals were rapidly generated,and then p-benzoquinone,o-benzoquinone or o-benzoic acid intermediates were produced by a series of oxidation reactions.And then the relationship between the absorbance of free chlorine dioxide at 360 nm and the intermediate product at 260 nm and the concentration of APO and chlorine dioxide in the reaction system was studied.The molar ratio of APO to chlorine dioxide is 1:3,and that of DHP to chlorine dioxide is 1:4.The internal relationship between the absorbance at 260 nm and the concentration of APO was found,and the linear regression equation between the absorbance of intermediate products and the concentration of APO was established,which was y=5x.The existence of the linear equation further confirmed the existence of p-benzoquinone,o-benzoquinone or o-benzoic acid intermediates.The results showed that in the presence of room temperature and high concentration of chlorine dioxide,chlorine dioxide could oxidize and degrade APO rapidly,and produce stable products of p-benzoquinone,o-benzoquinone or o-benzoic acid.The reaction mechanism of APO and chlorine dioxide was analyzed by studying the absorbance changes of reactants and products,and the reaction kinetic equation of chlorine dioxide and APO was established.Based on the study of the formation and degradation of p-benzoquinone,o-benzoquinone or o-phthalic acid intermediate?AC?in the reaction of APO with chlorine dioxide.Under the condition of simulated bleaching,the steady state assumption[APO]=-?k₂[AC]?/k₁ is established,and a new complete second-order rate equation is derived,which is d[Cl O₂]/dt=[AC][Cl O₂](0.0028ln^[H]++0.0666).The fitting results of the kinetic equation are good,the fitting constant is higher than 0.90,which has a very high correlation.The reaction accords with the first-order reaction characteristics.When the ratio of chlorine dioxide to lignin structure unit reaches about 1:3,the amount of AOX in bleaching wastewater will decreases to30%.This experiment provides a new way for AOX reduction in bleaching process.The oxidation reaction process of chlorine dioxide with three phenol lignin models was analyzed at the 6-31+g*group level by using the B3LYP method of DFT and the gausion 09 software package.The transition state of chlorine dioxide was searched and the reaction energy barrier was compared.The results show that the intermediate products are all formed rapidly and the energy needed is very low.The reaction energy barrier of DHP and chlorine dioxide is 12.6kcal/mol,that of G-?-g and chlorine dioxide is 12.8 kcal/mol,and that of APO and chlorine dioxide is 15.0 kcal/mol.The results show that the reaction of three different phenol lignin models with chlorine dioxide has different degrees of difficulty,S-type structure is more easily oxidized by chlorine dioxide to form intermediate products,G-type lignin model is the most difficult,and?-O-4-type lignin dimer model is similar to S-type structure,it also shows that the structure of lignin and the connection between units also affect the reactivity of lignin and chlorine dioxide.