EVOH Nanofiber Membranes Functionalized For Chromium Ions And Dyes Adsorption

The development of economy and society results in a serious issue of the environmental pollution problem.Heavy metal ions and dyes are the crucial water pollution,and chromium(Cr)ion concentration is one of monitoring indexes of heavy metal pollution in aqueous system.The prevention and control of water pollution plays a critical role in environmental protection,which affects the quality of soil and atmospheric directly.Adsorption has been regarded as an effective method to remove heavy metal ions and dyes due to its obvious advantages in cost and performance.Polymer nanofiber membranes with high specific surface area could avoid separation process after adsorption and have high regeneration.Poly(ethylene-co-vinyl alcohol)(EVOH)nanofiber membrane contains a lot of hydroxyl groups that could complex with heavy metal ions and dyes,but it still offers limited adsorption capacity.This study is intended to improve the adsorption efficiency of EVOH nanofiber membranes by introducing amino and imine nitrogen groups which could strongly interact with Cr ions and dyes via electrostatic interaction or hydrogen bonding.In this research,three kinds of aminated-EVOH nanofiber membranes were prepared by means of simple chemical modification that L-lysine(L-Lys)or diethylenetriamine(DETA)was introduced to the surface of EVOH nanofiber membranes.PPy/EVOH and PANI/EVOH composite nanofiber membranes were obtained by in-situ oxidative polymerization.Then Cr ions and dyes adsorption experiments were carried out to test the adsorption properties,adsorption behaviour and reusability.Firstly,EVOH nanofiber membranes with controllable thickness were fabricated through the melt-blending extrusion process of immiscible blends and high-speed flow deposition.Then L-Lys/EVOH,Cy/L-Lys/EVOH and DETA/EVOH nanofiber membranes were fabricated by facile chemical modification on the surface of EVOH nanofiber membranes.Some functional groups on the surface of Cy/L-Lys/EVOH and DETA/EVOH nanofiber membrane were crosslinked.The specific surface area and water contact angle of aminated-EVOH nanofiber membranes were decreased because of the hydrogen bonding between fibers.Adsorption experiments suggested that the removal of Cr ions was highly pH dependent.The optimal pH to remove Cr ions in the solution was 2.0.Cy/L-Lys/EVOH,L-Lys/EVOH and DETA/EVOH nanofiber membranes possessed high Cr ions adsorption capacity with low dosage of 0.0125 g,and the equilibrium adsorption capacities were 234.69,202.18 and 192.96 mg/g for 30 mL 200 mg/L Cr(VI)solution,respectively.Desorption results showed that the membranes exhibited excellent reusability for removing Cr ions.The Cr ions adsorption efficiency of DETA/EVOH nanofiber membranes kept up to 98.73% of the initial adsorption after 6 successive adsorption-desorption cycles.For 50 mg/L Cr(VI)ions solution,the adsorption equilibrium time was about 500 min,and the removal of Cr ions were 75.91%,69.33% and 64.5%;the removal of Cr(VI)ions were 97.64%,96.68% and 92.04%.Adsorption kinetics study revealed that the adsorption data fitted well through Pseudo-second-order model and Freundlich isothermal model.Thermodynamic study suggested that the adsorption process was endothermic and spontaneous.The research on the adsorption of methyl orange,acid red 88,Congo red and methylene blue dyes found that Cy/L-Lys/EVOH nanofiber membrane was the best adsorbent for methyl orange,acid red 88 and Congo red dyes.L-Lys/EVOH and DETA/EVOH nanofiber membranes had good adsorption performance on Congo red dyes.Low dosage of aminated EVOH nanofiber membranes had higher Cr ions adsorption capacity and outstanding regeneration performance compared to the reported adsorbent materials.It was necessary to enhance the specific surface area and the density of effective functional groups of material to improve the adsorption rate and removal rate of EVOH nanofiber membranes for Cr ions.PPy was a prospect adsorbent for removing Cr ions and dyes due to its ability to adsorb anion via π-π,electrostatic or hydrogen bonding interaction.Then polypyrrole(PPy)self-assembled on EVOH nanofiber membranes by in-situ oxidative polymerization were prepared.PPy with small particle morphology distributed uniformly on the surface and the pores of PPy/EVOH composite nanofiber membranes,which increased the specific surface area and decreased the porosity slightly.Adsorption experiments denoted that the adsorption process was highly pH dependent.The adsorption equilibrium time was about 30 min for 50 mg/L Cr(VI)ions solution,and the removal of Cr ions was 86.6%,and the removal of Cr(VI)was 99.2% by 0.1 g/30 mL adsorbents.In addition,with coexisting ions in adsorption,PPy/EVOH composite nanofiber membranes showed obvious choice to Cr ions adsorption.The results on the adsorption of methyl orange,acid red 88,Congo red and methylene blue dyes suggested that it had good adsorption performance for anionic dyes.As the initial pH within the range of 3.12-6.98,the removal of methyl orange dyes was closed to 100%.Meanwhile the maximum adsorption capacity of methyl orange dyes for 150 mg/L solution was 78.09 mg/g.Regeneration results exhibited the limited reusability for both Cr ions and methyl orange dyes adsorption.After 6 usage cycles,the adsorption capacity could keep up to 79.48% and 78% of the first time.The adsorption data of Cr ions and methyl orange dyes fitted well with pseudo-second order-model and followed Langmuir isothermal model.The adsorption process was endothermic and spontaneous.PPy in the PPy/EVOH composite nanofiber membranes tended to get together during adsorption-desorption process,which limited the regenerability of materials.PANI could coat the nanofibers evenly though hydrogen-bonding interaction and avoid agglomeration.To improve the reusability of functionalized EVOH nanofiber membranes,subsequently,Polyaniline/EVOH(PANI/EVOH)nanofiber composite membranes were prepared through high-speed flow deposition followed by in-situ chemical oxidative polymerization of aniline on the surface of EVOH nanofibers.It was found that the PANI coating was formed on the surface of EVOH nanofibers,which increased the hydrophilicity,average diameter and pore size of nanofibers,but the surface area of membrane were decreased slightly.The adsorption experiment indicated that pH value at 2.0 was optimal for the removal of Cr ions.The equilibrium adsorption capacity was 93.09 mg/g for 200 mg/L Cr(VI)ions solution.Compared with the adsorption characters of Cu(II),Hg(II),Pb(II)and Cd(II),PANI/EVOH composite nanofiber membranes showed obvious choice to Cr ions.More importantly,regeneration experiment results demonstrated the excellent reusability for Cr ions adsorption.The removal efficiency of Cr ions could maintain up to 92.79% at the first 6 adsorption-desorption cycles and to 76.7% after 25 cycles.The removal of Cr and Cr(VI)ions were 81.63% and 99.5% for 50 mg/L Cr(VI)solution.The study on the adsorption of methyl orange,acid red 88,Congo red and methylene blue dyes suggested that PANI/EVOH composite nanofiber membranes had good adsorption performance for anionic dyes,especially for methyl orange dyes.When the pH was in the range of 2.08-6.98,the removal of methyl orange dyes was closed to 100%,and still approach to 100% after 8 usage cycles. For 150 mg/L methyl orange dyes solution,the equilibrium adsorption capacity was 86.94 mg/g.The adsorption data of Cr ions and methyl orange dyes fitted well with pseudo-second-order model and Freundlich isothermal model,and the adsorption process was endothermic and spontaneous.These results confirmed that this material with outstanding adsorption capacity,removal efficiency,and high durability could be considered as an alternative adsorbent for the removal of Cr ions and methyl orange dyes from aqueous solution.