Preparation Of Macroporous Amidoxime Chelating Resin And Its Adsorption Behavior For Fe³⁺ From Citric Acid Solution

As one of the most important organic acid, citric acid （CA） is widely applied in food,medicine, daily chemical and other industries. With the development of industry, citric acid ismainly produced by fermentation of sucrose, starch or subsidiary agricultural products usingaspergillus niger. Traditional calcium salt precipitation and advanced simulated moving bedare the technology currently employed to extract CA from fermentation broth. However, theCA extract solution still contains Fe³⁺which affects the quality of CA products. Thus, it isimportant and urgent to research the detection and removal methods of ferric ions in CAsolution. Chelating resin is a polymer material, which could selectively chelate the specificmetal ions from the solution. Therefore, the chelating resin could be used to adsorb the Fe³⁺inCA solution. At present, there is no relevant report about the adsorption of Fe³⁺onto chelatingresin in CA solution. In this dissertation, the adsorption properties of ferric ions onto theamidoxime resin in CA solution are studied.Firstly, the detection method of ferric ions content in high concentration CA solution isestablished in this paper. Secondly, the macroporous amidoxime chelating resins are preparedby acrylonitrile-divinybenzene polymer, and the influence laws of various reaction conditionson the synthesis of amidoxime chelating resin are researched. The acrylonitrile-divinybenzenepolymer and amidoxime chelating resin are characterized by IR spectroscopy, TGA andelement analysis. Finally, in the CA solution, the static and dynamic adsorption experimentsare carried out to investigate the adsorption process of ferric ions on amidoxime chelatingresin.1. The research with the detection method of ferric ionsThe detection method of ferric ions content in high concentration CA （0.5～20%） isestablished. The buffer ratio is changed with the different concentration of CA, and they areaccordant with the following quadratic fit curve equation: y=0.003x²+0.71158x+1.80984（y,the ratio of the mola concentration of ammonia and ammonium chloride; x, the mass fractionconcentration of CA）. Then, it could detecte accurately the ferric ions in the highconcentration of CA solution.2. The study on the synthesis of macroporous amidoxime chelating resinThe adsorption properties of Fe³⁺onto the different amidoxime resin show that thenitrogen percentage is able to represent the functional group content of the amidoxime resin.The acrylonitrile-divinylbenzene polymers are selected, and its crosslinking degree is6%. Theswelling time of the acrylonitrile-divinylbenzene polymers in methanol is3h, and theconcentration of hydroxylamine hydrochloride for per gram of theacrylonitrile-divinylbenzene polymers is0.9mol·L-1. Then, the effects of each reactionconditions on the content of amidoxime group are investigated, such as pH value of thereaction medium, reaction temperature, reaction time, etc. The orthogonal optimizationexperiments show that the main factor for the nitrogen percentage is the pH value of thereaction medium, and then the reaction temperature, the volume ration of methanol and water,the reaction time. With the nitrogen percentage as the main index, the optimized synthesis conditions for amidoxime resin are obtained as the pH value of reaction medium7, thereaction temperature65°C, the volume ration of methanol and water6:4, the reaction time4～5h.3. The study on the adsorption properties of Fe³⁺on the amidoxime resin in CA solutionIt shows that the results of static adsorption experiments: the presence of citric acidaffects the mass transfer process of Fe³⁺on the amidoxime resin, so the adsorption speed ofthe resin reduces; the amidoxime resin adsorb a small amount of citric acid, thus the staticadsorption capacity decreases. The adsorption process of Fe³⁺on the amidoxime resin isendothermic, and the adsorption isotherm follows the Freundlich model well. The results ofdynamic adsorption experiments show that: the adsorption mass transfer resistance of theamidoxime resin increase with the existence of citric acid, and it is not conductive to theadsorption of Fe³⁺on the amidoxime resin; the adsorption power increases with the increaseof the concentration of Fe³⁺, and it would help to improve the adsorption rate and increase theadsorption capacity; the amidoxime resins are suitable for the system with the CAconcentration＜20%and they have higher processing capacity;0.75mol·L-1HCl is the eluent, andit need less volume of HCl and has higher desorption speed.