Research On The Removal Characteristics And Mechanisms Of Hexavalent Chromium Adsorbed On Miex Resin From Aqueous Solution

Hexavalent chromium Cr (VI), with high toxicity, is recognized as carcinogenic,teratogenic, and mutagenic metal material. Raw water pollution, especially thesudden chromium pollution, caused great trouble in China. This threatens the safetyof drinking water seriously.The objective of this study is determining the use of magnetic ion exchangeresin (MIEX) as an adsorbent for the removal of Cr (VI). The batch experiment isconducted in this work. The independent factor, orthogonal experiment, responsesurface experiments, adsorption equilibrium, adsorption kinetics, adsorptionthermodynamics and adsorbent regeneration are considered to analyze theexperimental data. The modern analytical methods of materials are used to determinethe removal mechanism. And the effects of humic acid on MIEX adsorption forremoval of Cr(VI) are further studied preliminarily.The results show that the initial pH, initial concentration, contact time,adsorbent dosage, temperature, stirring intensity, coexisting aions and naturalorganic matter (NOM) have different effects on Cr (VI) removal. The effect ofsolution temperature is small, and the NOM represented by humic acid improves theremoval of Cr (VI) on MIEX obviously.Orthogonal experiments indicate that the impact degree of primary andsecondary factors influencing the removal of Cr (VI) is in the order of resin dosage＞initial Cr (VI) concentration＞pH of solution＞conexisting anions＞adsorption time. According to the response surface methodology theory, thesecond-order polynomial regression equation including MIEX resin dosage, initialconcentration, pH of solution and adsorption time was developed. Response surfaceand contour plots by fitting the equation display the interactions of resin dosage andinitial concentration, the initial concentration and adsorption time are significant onthe effect of Cr (VI) removal on MIEX resin.The Redlich-Peterson isotherm model can describe Cr(VI) adsorptionequilibrium on MIEX resin well at different temperature. For a given temperature, as the Cr (VI) concentration increases, adsorption potential decreases. And the additionof HA does not significantly change the law of Cr (VI) adsorption equilibrium.Pseudo-first order kinetic model can describe the adsorption kinetics process ofCr (VI) well at low concentrations (≤5mg/L), and the liquid film diffusion is therate limiting step. While the pseudo-second order model conduct well at highconcentrations (≥7mg/L). The kinetics process changed at low concentrations whenHA added in the solution, and the chemisorption is the rate controlling step.The adsorption of Cr (VI) on MIEX resin is spontaneous and exothermic, andthe adsorption will improved by the decrease of temperature. On he other hand, theadded HA caused the adsorption thermodynamic properties from exothermic toendothermic.Regeneration tests showed that2.60mmol/L of NaCl solution have better effectto regenerate the saturated resin, and about30min can complete the process.Cr (VI) removal mechanism on MIEX resin can mainly explained by ionexchange in single component system. The electrostatic adsorption also affects theCr (VI) removal. In addition, there are some functional groups,-OH,-CH2-,-C=Oand–C-O, incolved in the removal mechanism. Adding HA, the Cr (VI) is reducedto Cr (III) as a result of complexation between functional groups of humic acid andadsorbate.