Mechanism Of AOX Reduction In Pulp Bleaching Effluent By Ammonium Sulfide/Amine Inhibitor

At present most of the pulp and paper enterprises use Chlorine dioxide as the main elements of bleach(Elemental Chlorine Free,ECF)during bleaching process.However,due to the preparation technology of chlorine dioxide,the chlorine dioxide solution produced by either the synthesis method or the methanol reductant method usually contains some chlorine gas.The chlorine gas has chlorine water balance reaction in the aqueous solution,and the hypochlorous acid is formed.In addition,the contents of hypochlorous acid in chlorine dioxide solution increase because of the decomposition reaction of chlorine dioxide in aqueous solution.These hypochlorous acid and the residual lignin in the unbleached pulp react and result in the increase of the content of refractory adsorbable organic halogen(AOX),which are increasingly harmful to the environment.In order to further reduce the formation of AOX in the bleaching process,I have studied the inhibitory effect of the addition of inhibitors on the formation of AOX in the bleaching process by adding three inhibitors,such as thiourea,sulfamic acid and ammonium persulfate in this paper.Firstly,the inhibitory effect of different inhibitors on AOX in bleaching process and the effect on subsequent wastewater load(COD)were studied.Based on the different content of sulfur and nitrogen in different inhibitors,in order to study the inhibition of sulfur and nitrogen in inhibitors on AOX,the total contents of fulfur and nitrogen were investigation factor.Since chlorine dioxide bleaching(D0)in the first stage of ECF bleaching was the stage in which AOX were formed in large quantities,the inhibitory effect of thiourea,sulfamic acid and ammonium persulfate on AOX were studied by using the two factors that the addition amount of inhibitor in the D0 stage and the total amount of sulfur and nitrogen.The results showed that thiourea had the best inhibitory effect on AOX.The single factor experiment method was used to further optimize the process conditions of inhibiting AOX formation by adding thiourea in the D0 bleaching process.The optimum conditions for adding thiourea were as follows:the adding time of thiourea was 18 min and the adding amount was 0.3%.Other bleaching conditions were as follows:the pulp concentration was 10%,pH value was 4,reaction temperature was 70 ?,reaction time was 60 min and 4.4%dosage of chlorine dioxide(available chlorine).During the optimum conditions of adding thiourea,the production of AOX in bleaching wastewater decreased by 24.4%,and the contents of COD increased by 10.0%.Based on the studies above,it was found that AOX in bleaching wastewater increased rapidly in the initial stage and remained stable after 18 min.Adding thiourea when a large amount of AOX generated,the inhibition effect of AOX was the best,that is,the contents of AOX in the D0 bleaching wastewater were the lowest when adding thiourea at 18 min.Therefore,kinetic models were established for AOX formation reaction before 18 min and AOX decrement reaction after 18 min respectively.The effects of the amount of thiourea,the time of thiourea addition,the amount of chlorine dioxide,the temperature and the pH on the formation of AOX in bleaching process were studied.Finally,the AOX before 18 min was obtained to generate the reaction kinetic equation and the reduction reaction kinetic equation after 18 min of adding thiourea.dCAox/dt=0.0636·e-,925.62/T·CCIO20'864·Ch+0.331· CAOX-0.6439(0<t<18)dDAOX/dt=5685.08·e-4131.8/T·CCIO20.3189·CH+-1.3834·CCSN2H40.7181.DAOX-0.1938(18<t<26)These two dynamics equations are first-order dynamics equations and have high fitting degree.The activation energies calculated by kinetic equation of AOX generation and decrement were respectively 16.01 kJ mol-1 and 34.35 kJ mol-1.The results showed that the reaction of chlorine dioxide with lignin producing AOX and the addition of thiourea reducing AOX had strong activity.The composition of the bleaching wastewater with and without thiourea was determined by GC-MS,and the results showed that the AOX of chlorophenols,chlorobenzene and other AOX were reduced by 73.5%,35.0%and 24.6%,respectively.The degradation pathway of trichlorophenol and dichlorophenol by adding thiourea in the bleaching process was speculated based on the detection results of GC-MS.Material Studio was used to carry out transition state retrieval and calculation of reaction activation energy and reaction heat for two predicted degradation pathways.The results showed that the reaction heat of thiourea was-63.0 kJ·mol'1 and the lowest activation energy was 106.0 kJ mol-1.Therefore,the degradation reaction of thiourea to trichlorophenol was the easiest,that was,the reduction dechlorination reaction of trichlorophenol was the easiest.The inhibitory effect of thiourea on AOX was mainly achieved by degradation of trichlorophenol in bleaching wastewater.