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.