Preparation Of Amino Modified Kapok Fiber And Its Adsorption Performance For Anionic Dyes

With the rapid development of social economy,the demand for dyes is on the rise in various industries,and the discharge of dyes wastewater has caused serious pollution.Dyes are extremely stable in environment and difficult to degrade.It will reduce the transparency of water,thus affecting the activities of aquatic animals and plants,destroy the ecological balance.Dyes are usually polyphenyl ring substituted compounds with strong biological toxicity.The aromatic amines produced by their decomposition have carcinogenicity and teratogenicity,which caused serious threats to human health.Therefore,the treatment of dye wastewater has important environmental significance.Adsorption is widely used in the treatment of refractory wastewater due to its advantages of simple operation,wide range of application,low cost and high efficiency.The selection of adsorbents has an important effect on the cost of the water treatment.The preparation of adsorbents with cheap and easily available natural cellulose materials has always been a research hots-pot.Kapok fiber（KF）is hydrophobic and oleophilic due to its waxy surface and hollow structure,and has been used to adsorb oils,organic solvents and heavy metals.However,studies on the application of kapok fiber to dye adsorption are relatively rare so far.Kapok fiber is an ideal carrier material with large specific surface area and good chemical reactivity.After modification,it can be used as a green and efficient dye adsorbent.In this study,a new adsorption material KF-PEI was prepared from low-cost natural kapok fiber.It’s properties were characterized and it’s preparation process were optimized.The adsorption performance of KF-PEI to the acid dye Sunset Yellow（SY）and the direct dye Congo Red（CR）were studied.The main research contents and conclusions are as follows:（1）After pretreatment with Na Cl O₂,an epoxy group was introduced into the cellulose by epichlorohydrin,and then polyethylenimine was grafted into the cellulose by ring-opening reaction to prepare an amino-modified KF-PEI adsorbent.Scanning electron microscopy（SEM）,infrared spectroscopy（FTIR）and thermogravimetric analysis（TGA）were used to demonstrate that PEI monomer was grafted onto the kapok fiber successfully,and the cellulose structure was not affected,and various impurities were effectively removed.KF-PEI has good thermal stability,and the specific surface area has improved compared with that before modification.（2）The conditions of grafting reaction were optimized.The orthogonal experiment showed that the amount of PEI,p H and molecular weight of PEI had significant effects on the grafting efficiency.The optimal reaction conditions were as follows:the amount of PEI was 2 m L,the molecular weight was 10000 Da,the temperature was 50℃,p H was11,and the reaction time was 8 h.The optimal amino content of KF-PEI adsorbent was5.23 mmol/g.（3）The adsorption performance of KF-PEI to acidic dye Sunset Yellow（SY）was studied.The equilibrium adsorption capacity of KF-PEI on SY decreased with the increase of p H value,and the maximum adsorption capacity was 223.55 mg/g,which was14.9 times higher than that before modification.SO₄^2-,Cl^-and NO₃^-in the solution would reduce the adsorption capacity,and the effect is obvious at high concentration.Langmuir isotherm model fits better than Freundlich model in SY adsorption,indicating that the reaction was monolayer adsorption.PSO kinetic model has the highest correlation with the reaction,indicating that chemisorption is the main rate-limiting step of the reaction.Adsorption reaction is a spontaneous exothermic process and the adsorption capacity decreases with the increase of temperature.After 5times of adsorption-desorption cycles,the adsorption capacity of SY can still reach84.69%of the initial adsorption capacity.According to the analysis of the experimental results,the adsorption mechanism is electrostatic action.（4）The adsorption capacity of KF-PEI to direct dye Congo Red（CR）was studied.The adsorption capacity of KF-PEI to CR increased firstly and then decreased with the increase of p H value.The optimal p H value was 6,and the maximum adsorption capacity was 725.20 mg/g.SO₄^2-,Cl^-and NO₃^-in the solution reduced the adsorption capacity.The results of isotherm model fitting showed that Langmuir model had better correlation than Freundlich model,indicating that the reaction was monolayer adsorption.The PSO kinetic model has the highest correlation with the adsorption reaction,and chemisorption is the rate-limiting step of the reaction.The adsorption reaction is a spontaneous exothermic process,and increasing temperature is not conducive to the reaction.After five adsorption-desorption cycles,the adsorption capacity of CR was 450.53 mg/g,which was 37.88%lower than the initial adsorption capacity.According to the analysis of the experimental results,the adsorption mechanisms are electrostatic action and hydrogen bonding.This paper has proposed a simple technology,with green and cheap natural kapok fiber as raw materials,polyethyleneimine（PEI）as a functional monomer,to prepare aminated kapok fiber（KF-PEI）by chemical crosslinking.KF-PEI shows strong dye adsorption capacity and good regeneration efficiency.This technology is expected to be applied on a large scale,and provides new ideas and new directions for the development of cellulose adsorption materials.