Study On Adsorption Of Modified Chitosan-based Adsorbents To Cr(?) In Water

Heavy metal Cr?VI?is internationally recognized as one of the three carcinogenic metals.The environmental pollution caused by Cr?VI?poses a serious threat to human health and survival,and has aroused widespread concern from all walks of life.Because of its high efficiency,simplicity,flexibility,and low cost,the adsorption method has been considered as hotspot technology for the removal of Cr?VI?from water.High adsorption capacity,good selectivity,and rapid and simple separation is the key to choice of adsorbent.The chitosan chain has an amino group and a hydroxyl group which can act as chelate metal ion sites,and therefore,it is considered to be a natural biomaterial adsorbent suitable for adsorbing heavy metal ions.In this study,using chitosan as raw material,a Fe₃O₄@CS@Hc adsorbent with high adsorption capacity and rapid magnetic separation was prepared by chemical cross-linking;the chitosan was carboxyl modified,and dithiocarbamate?DTC?was used in the modification of surface functional group of the chitosan,and a new adsorbent O-CMCS-DTC with high adsorption capacity and rapid precipitating separation ability can be prepared.With Cr?VI?as the target pollutant,the adsorption kinetics and adsorption isotherms of Cr?VI?in water were studied by two kinds of adsorbents,the influence factors of adsorbent performance such as adsorbent dosage,initial pH of the solution,initial temperature,co-existing cation,kinetics,and isotherm were systematically investigated.And the regeneration of adsorbents was studied.The surface morphology and functional groups of O-CMCS-DTC were characterized by SEM,FT-IR and other methods.The surface composition,functional groups and magnetic properties of Fe₃O₄@CS@Hc were characterized and analyzed by BET,FT-IR,XPS and VSM,such as characterization analysis.The main conclusions of this study are as follows:?1?The result of kinetic experiments showed that both Fe₃O₄@CS@Hc and O-CMCS-DTC followed pseudo-secondary kinetics model,the adsorption process was controlled by chemical adsorption.Adsorption isotherm experimental results demonstrated that the adsorption of Cr?VI?from water by Fe₃O₄@CS@Hc and O-CMCS-DTC followed the Langmuir adsorption isotherm model and the maximum adsorption capacity was142.65 mg/g and 270.27 mg/g.?2?The result of influencing factors showed that,pH was the most critical factor affecting the adsorption reaction.Under acidic condition,the adsorption of Cr?VI?by Fe₃O₄@CS@Hc and O-CMCS-DTC adsorbents had better effect and the optimum pH was4.0 and 3.0 respectively.With the increase of adsorbent dosage,the adsorption capacity decreased,and the increase of temperature favored the adsorption of O-CMCS-DTC.This adsorption was a spontaneous endothermic process and thetemperature had no effect on the adsorption of Fe₃O₄@CS@Hc.Coexisting cations in water,in addition to Mg²⁺inhibiting the adsorption reaction,other co-existing cations had little effect on the adsorption capacity of Cr?VI?adsorbed by these two adsorbents.?3?The regeneration experiment indicated that the best analytical performance for saturated Fe₃O₄@CS@Hc was obtained with 0.1 mol/L NaOH solution.The resolution reached 97.6%in 5 to 10 min,and the removal rate could still reach 93.7%after five cycles of regeneration.It showed that Fe₃O₄@CS@Hc had good regeneration performance.?4?SEM analysis showed that O-CMCS-DTC surface was slightly rough due to grafting reaction.FT-IR analysis showed that,the new characteristic peaks emerged at1454 cm^-1?NC=S?and 1002 cm^-1?CS?after surface modification of dithiocarbamate,and the functional group participated in the adsorption reaction.The BET analysis of modified Fe₃O₄@CS@Hc showed that its specific surface area decreased,but the pore volume increased.FT-IR indicated that the characteristic peak of humic acid was found at1577cm^-1.It showed that humic acid was successfully complexed on magnetic chitosan,and the increase of carboxyl groups was conducive to the adsorption of Cr?VI?.According to XPS analysis,iron oxides were involved in the adsorption reaction.The Cr element was present in the adsorbent adsorption product.About 70.4%of the Cr elements on the surface of the adsorbent exist as Cr?III?,and about 29.6%exist as Cr?VI?.the adsorption of Cr?VI?by adsorbents were mainly adsorption and reduction.VSM showed that the saturation magnetization of Fe₃O₄ and Fe₃O₄@CS@Hc were 46.5 emu/g and 19.5 emu/g,both of which had a certain superparamagnetism and could achieve rapid magnetic separation.