Study On The Removal Of Low-concentration Antibiotics In Water With Moderately Hydrophobically Modified Chitosan Flocculant

Water contamination caused by antibiotics has attracted much concern.Due to their very low concentration in water,it is difficult to remove antibiotics by conventional flocculants.Therefore,new flocculants with specific functional groups aiming at targeted antibiotics are needed to be designed.In this paper,chitosan was modified by hydrophobic phenylalanine and polyacrylpiperidine(PNAPD)to obtain two sorts of flocculants with different chain structures.Two typical antibiotics(norfloxacin NOR and tylosin TYL)at trace level were removed in turbid water with the coexistence of natural organic matter(NOMs)and suspended particles.Given the hydrophilicity/hydrophobicity and structural properties of NOR and TYL,two kinds of phenylalanine(Phe)modified chitosan flocculants with different molecular structures(linear CHS-Phe and comb-like CHS-PPhe)were synthesized.Response surface methodology(RSM)was used to evaluate effects of various parameters(pH,flocculant dosage,kaolin and HA concentrations)on flocculation performance and then to optimize these parameters.The results showed that,CHS-Phe with linear molecular architecture displayed higher removal efficiencies of antibiotics than commercial flocculants,and the removal efficiencies of NOR and TYL under the optimal conditions were 76.19%and 82.01%,respectively.By contrast,CHS-PPhe,owning the same Phe functional groups but comb-like architecture(polyPhe polymer branches grafted on chitosan backbone),showed poor performance.And the maximum removal rates of NOR and TYL were 36.43%and 40.46%,respectively.Deep mechanism investigations indicated that,for hydrophilic and flexible CHS-Phe linear molecules,bridging and sweeping flocculation for antibiotics were enhanced by electrostatic attraction,?-electron-containing interaction,and H-bond between introduced Phe groups and antibiotic molecules.However,for comb-like CHS-PPhe,excessively hydrophobic polyPhe branches aggregated to coils.This phenomenon caused fewer exposed flocculation sites outwards,although the interactions of Phe-antibiotic still existed.3D response surfaces of CHS-Phe generated by RSM exhibited that,concentrations of coexisting kaolin and HA,as well as pH and flocculant dosage,could be controlled to obtain desired performance.In the previous part,the phenylalanine modified chitosan flocculant CHS-Phe showed good flocculation performance on the removal of NOR and TYL,but the chain length was not easy to control during the synthesis process.Then,the synthesis of flocculants with different chain length structures was optimized by a "graft-to" method.Chitosan was modified with PNAPD to obtain three flocculants with different chain lengths:CS-g-PNAPD 10,CS-g-PNAPD 20 and CS-g-PNAPD 30.Effects of pH,flocculant dosage and chain lengths on the flocculation performance were investigated.For NOR removal,the acidic conditions were in favor of charge neutralization and H-bond between the flocculant and the contaminants,while hydrophobic association also promoted the removal of antibiotics.The maximum removal rate of NOR by three different chain lengths could reach 75%,80%and 85%.For TYL removal,due to its molecular morphological characteristics,near-neutral conditions were preferred because of the electrical neutralization,H-bond and hydrophobic association between the flocculant and the contaminants.The maximum removal rate of TYL by three different chain length flocculants could reach 70%,75%and 70%.With the increase of chain length,the flocculation performance gradually increased,but when the chain length reacheed a certain level,the enhanced hydrophobicity would agglomerate the chain length to shield a part of the action site,and the flocculation performance was no longer enhanced.The modification of chitosan by phenylalanine not only enriches the understanding of the effect of molecular structure of polymer flocculant on the removal of pollutants,but also provides practical guidance for the regulation of flocculation parameters in practical operation.In practical applications,according to the characteristic structure of the contaminants,chitosan can be modified by selecting other biodegradable,non-secondary and rich functional groups,in order to achieve the desired water purification performance.Modification of chitosan by PNAPD enriches the molecular design method and can design specific hydrophilic/hydrophobic flocculants by controlling the chain length of PNAPD.At the same time,the application of flocculants in actual water bodies requires more exploration.