Design And Preparation Of Core-Brush Structured Chitosan-based Composites And Adsorption Mechanism

Water is an essential source of the lives on the earth.However,in the past few decades,the rapid development of industrialization and the increase of human activities have caused serious water pollution.There are kinds of pollutants which are difficult to be removed by traditional water treatment process,including dyes,antibiotics,heavy metal ions,etc.The existence of these pollutants in water has a negative impact on aquatic organisms,ecosystems,even on human health.At present,various methods are used to remove pollutants from water bodies.Among them,adsorption is a simple,environmentally friendly,and effective method,and it also has low-cost property,thus,it is widely utilized in the field of water treatment.Therefore,more and more attention has been paid on developing an efficient adsorbent with environmental friendliness and economical efficiency.Chitosan is one of the most abundant and lowest-cost biopolymers in the world.It has many characteristics such as wide applicability,high abundance,antibacteria,non-toxicity,biocompatibility,and biodegradability,thus it has been considered an ideal adsorbent for removing pollutants.Aiming at the disadvantages of its complicated preparation process,poor mechanical property and acid stability,the?core-shell‘structural magnetic chitosan composites as carriers were synthesized using a simple inverse emulsion cross-linking method.Four chitosan-based composites with?core-brush‘structure were designed and prepared by grafting different polymer brushes including anionic,cationic,and amphoteric polymer brushes as well as polymer brushes with specific structure via free radical polymerization,and utilized to adsorb organic dyes,ciprofloxacin,copper ions(Cu(?)).The adsorption performance of the prepared adsorbents toward targeted pollutants and corresponding mechanisms were systematically studied.The contents and conclusions of this work can be listed as follows:Firstly,using magnetic chitosan as a carrier,anionic polymer brushes—poly[2-acrylamido-2-methylpropane sulfonic acid(AMPS)and acrylic acid(AA))]were designed and grafted onto the surface of magnetic chitosan(MCS)by graft copolymerization.The morphology,chemical structure and magnetic properties of the prepared MCS–g–P(AMPS–AA)were characterized by SEM,FTIR,XPS,¹³C NMR,XRD,TG,VSM,etc.Using cationic dye—methylene blue(MB)as a targeted pollutant,its adsorption on MCS–g–P(AMPS–AA)and the influence of external conditions on its adsorption process were systematically explored.Based on the above-mentioned results,the adsorption kinetics and isotherm models were established.The results showed that MCS–g–P(AMPS–AA)had good magnetic separation property and excellent adsorption performance for MB(q_max was 925.9 mg/g).The adsorption process of MCS–g–P(AMPS–AA)for MB was monolayer adsorption on a homogeneous surface,and the rate-controlling step was chemical adsorption.The result of p H effect and XPS analysis confirmed that both electrostatic interaction and hydrophobic interaction were involved in MB adsorption on MCS–g–P(AMPS–AA).The reusability of MCS–g–P(AMPS–AA)was explored.After five cycles,the recovery efficiency of 86.1%indicated the ease of regeneration and good reusability.As for the anionic pollutants in the wastewater,poly([2-(methacryloxy)ethyl]trimethy-lammonium chloride)(PDMC)chosen as cationic polymer brushes was grafted on MCS,and MCS–g–PDMC was prepared.The SEM,BET,FTIR,XPS,TG,VSM,etc.were used to characterize the synthesized adsorbents,and based on the elemental analysis,the grafted quaternary ammonium groups were quantified to 3.64mmol/g.Besides,the influence of p H and other factors on the adsorption performance of MCS–g–PDMC for food yellow 3(FY3)and acid yellow 23(AY23)were explored.Evidently,MCS–g–PDMC showed excellent adsorption performance for both dyes in a wide p H range,and its adsorption capacities(q_max(_FY3)was 833.33 mg/g,q_max(_AY23)was666.67 mg/g)were considerably higher than unmodified adsorbents,which could be attributed to the electrostatic interaction and anion exchange.Moreover,the adsorption process was also explored by establishing kinetic and isotherm models,and the conclusion that the adsorption of FY3 and AY23 by MCS–g–PDMC was monolayer adsorption,and the rate-controlling step was chemisorption can be arrived at.In addition,the spent MCS–g–PDMC had a great magnetic separation property.With the help of magnet,it can be easily separated from water and can be effectively regenerated.Therefore,MCS–g–PDMC had a great potential for practical application.However,the above-mentioned pollutants often coexist in water bodies.Considering this condition,the amphoteric polymer brush—poly(dimethylaminoethyl methacrylate(DMAEMA)and acrylic acid(AA))was further designed and synthesized on MCS,and thus,the effective removal of pollutants with different charge properties through p H adjustment was expected to be achieved.The prepared adsorbent not only exhibited good magnetic separation performance and acid stability,but also achieved effective adsorption for methylene blue(MB)and acid orange(AO7)(the q_max of MB was 627.4 mg/g,and the q_max of AO7 was 1146.8 mg/g).MCS–g–P(DMAEMA–AA)exhibited high p H-responsive behavior.Through tuning p H,the surface charge and hydrophobicity/hydrophilicity of MCS–g–P(DMAEMA–AA)were changed,thereby realizing selective decolorization for targeted dyes.Based on the analyses of p H effect and the characterization results of FTIR,XPS as well as Zeta potential,the adsorption mechanisms for MB adsorption were electrostatic and hydrophobic interactions,while for AO7 sorption,the electrostatic attraction played the essential role.The regeneration performance of the adsorbent in the concentrated desorbing agent was explored,and the result showed that MCS–g–P(DMAEMA–AA)still exhibited more than 80%recovery efficiency after five regeneration cycles even in a small volume of desorbing agents.Therefore,the good reusability and dye enrichment suggested the great potential of MCS–g–P(DMAEMA–AA)in the field of water treatment.There are some pollutants which exist as a cation,zwitterion or anion at different p H values,such as antibiotic ciprofloxacin(CIP).Aiming at effective removal of this kind of pollutants and exploring the interaction between them and the adsorbents with core-brush structure,a magnetic bioadsorbent with specific morphology and molecular structure—poly(sodium p-styrenesulfonate hydrate)(PPSS)grafted magnetic carboxymethyl chitosan(MCC–g–PPSS)was prepared utilizing magnetic carboxymethyl chitosan(MCC)as the carrier.The prepared adsorbents were used to remove the antibiotic ciprofloxacin(CIP)and Cu(?)that often co-exists with CIP from the aqueous solution.The prepared MCC–g–PPSS had excellent adsorption performance toward CIP,with q_max of 527.93 mg/g,which presented great adsorption potential in comparison with the reported adsorbents.MCC–g–PPSS also showed good adsorption capacity of 41.03 mg/g for Cu(?).Besides,using XPS and FTIR,the adsorption behaviors of CIP and Cu(?)on MCC–g–PPSS surface were evaluated.The results presented that CIP molecules were captured by MCC–g–PPSS owing to electrostatic interaction,cation exchange,?–?interaction,and hydrogen bonding,while for Cu(?),the driving force mainly included chelation interaction and electrostatic interaction.The simultaneous adsorption experiments showed that MCC–g–PPSS can effectively remove CIP and Cu(?)in the binary system.More than 80%of the recovery efficieny after five adsorption-desorption cycles confirmed the reusability of MCC–g–PPSS.This study provided a new dimension for structural design of surface-modified adsorbents that can more effectively bind targeted contaminants.To explore the practical application potential of the prepared adsorbents,the removal rates of MCS–g–P(AMPS–AA),MCS–g–PDMC,MCS–g–P(DMAEMA–AA)and MCC–g–PPSS for organic dyes,CIP and Cu(?)in different water bodies were evaluated.The removal ratios of TOC during organic dyes and CIP adsorption in surface water(Minzhu Lake)were also studied.The results suggested that MB and CIP removal was inhibited to some extent due to the co-existing ions and DOC in water body,while the decrease in the removal of anionic dyes and Cu(?)was insignificant.More than 70%removal ratios of TOC from Minzhu Lake indicated that the adsorbents with designed?core-brush‘structure have great potential to remove contaminants from complex water conditions.