改性磁性荔枝皮对水及果汁中砷的吸附效果及动力学分析

为探索安全高效实用的果汁类食品砷吸附材料,以天然植物副产物荔枝皮为材料,通过共沉法制备砷吸附材料磁性荔枝皮(magnetic litchi peel,MLP),采用电感耦合等离子体质谱法测定砷含量,以砷清除率为判断指标,研究了MLP对三价砷和五价砷吸附效果及机理。结果表明:改性MLP表面含有粉末状的Fe3O4,与未改性荔枝皮相比部分官能团增加,表面粗糙、更加凹凸不平,结构疏松,微孔数量增加,有利于吸附作用的进行,在砷浓度为0.20 mg/L水溶液中三价砷和五价砷吸附率为88%和90%,对苹果汁中的三价砷和五价砷吸附率为83%和93%;MLP对三价砷和五价砷溶液吸附等温线符合Langmuir等温线模型,拟合决定系数R2均大于0.99,为单分子层吸附,在298 K条件下饱和吸附量三价砷为0.181 mg/g,五价砷为0.225 mg/g;MLP对砷溶液吸附动力学可以通过准二级吸附动力学模型描述,R2均高于0.99;MLP对三价砷和五价砷的吸附过程中吉布斯自由能小于0,吸附反应是自发进行的,焓变小于0,MLP对两种价态砷的吸附都是放热反应。研究结果可为改性荔枝皮在果汁类食品中砷脱除方法和工艺的应用提供理论与数据参考。

Adsorption effect of modified magnetic litchi peel for arsenic in water and fruit juice

Abstract: Natural plant by-product litchi peel was chosen as experimental material. Magnetic litchi peel, a kind of arsenic adsorption material, was prepared by co-precipitation technique, and inductively coupled plasma mass spectrometry was applied to measure arsenic content. Arsenic clearance rate was the judgment index. The adsorption effect and mechanism of magnetic litchi peel on trivalent arsenic and pentavalent arsenic were studied. The magnetic litchi peel and litchi peel were characterized by scanning electron microscopy and infrared spectroscopy. The results showed that the modified magnetic litchi peel surface contains powdery Fe3O4, some functional groups are increased, rough and loose structure makes the surface is more uneven, and the number of micropores is also increased. This structure is favorable for the adsorption. The clear rate of arsenic from magnetic litchi peel decreases with the increase of arsenic concentration. The clear rate of trivalent arsenic from magnetic litchi peel is 88% in aqueous solution with arsenic concentration of 0.20 mg/L. The clear rate of pentavalent arsenic from magnetic litchi peel is 90% in aqueous solution with arsenic concentration of 0.20 mg/L. The effect of magnetic litchi peel on the clear of arsenic is better because the loose and porous structure of the modified magnetic litchi peel increases the contact chance of arsenic ions and the adsorption site, and the adsorption rate is faster. After the magnetization of litchi peel, the active site is increased, and the adsorption activity of arsenic is enhanced. The clear rate of trivalent arsenic from magnetic litchi peel is 83% in apple juice with arsenic concentration of 0.20 mg/L. The clear rate of pentavalent arsenic from magnetic litchi peel is 93% in apple juice with arsenic concentration of 0.20 mg/L. The effect of pentavalent arsenic and trivalent arsenic adsorption in fruit juice is weaker than that in aqueous solution, due to the presence of organic matter and some ions such as phosphate ion in fruit juice, which are competitive for binding sites with pentavalent arsenic and trivalent arsenic. The correlation coefficient of Langmuir isotherm model of pentavalent arsenic and trivalent arsenic adsorption process is larger than that of Freundlich adsorption model. The R2 value of isotherm model is more than 0.99, which means it can be described by Langmuir isotherm model. The pentavalent arsenic and trivalent arsenic from magnetic litchi peel are adsorbed on the monomolecular layer from the adsorption state. The adsorption kinetics of magnetic litchi peel on arsenic solution can be described by the pseudo second order adsorption kinetics model, R2 is higher than 0.99, and the theoretical and actual values approximate. In the process of the adsorption of trivalent arsenic and pentavalent arsenic by magnetic litchi peel, Gibbs free energy is less than zero, and the adsorption reaction is spontaneous. The Gibbs free energy of trivalent arsenic and pentavalent arsenic increases with temperature rising, and the adsorption process is easier to be carried out at low temperatures. In the process of the adsorption of trivalent arsenic and pentavalent arsenic by magnetic litchi peel, enthalpy change is less than zero, and the adsorption of 2 kinds of valence arsenic is exothermic reaction. The results can provide theoretical and data reference for the process of removing arsenic in fruit juice using modified litchi peel.