Preparation Of Carboxylated Lignin-based Adsorbent And Its Adsorption Mechanisms For Fluoroquinolone Antibiotics

Antibiotics are a common class of drugs for humans and livestock.Among them,fluoroquinolones(FQs)are one of the main representatives of antibiotics,which are widely used in the fields of medicine,animal husbandry and aquaculture.However,due to the abuse of antibiotics and its improper treatment methods,those drugs such as FQs seriously polluted the natural water bodies.Among various water treatment methods,adsorption has the advantages of simple operation and good treatment effect.Natural polymeric adsorbents are widely used because of their low price and environmental friendliness.Lignin(LN)is a wide-source natural polymer with a three-dimensional network structure formed by the interconnection of various phenylpropane units through ether bonds and carbon-carbon bonds.Because of containing a large number of oxygen-containing functional groups on the backbone such as hydroxyl groups,LN has been used as an adsorbent for the removal of various pollutants such as dyes and heavy metals from water.However,few study related to lignin-based adsorbents for the removal of FQs pollutants was reported,but FQs and LN contain the similar functional groups such as aromatic rings and hydroxyl groups,and they can effectively combine with each other through various interactions.So LN is considered as a potential and environmentally-friendly material as an efficient adsorbent for removal of FQs from water.In this paper,aiming at efficient removal of FQs such as ofloxacin(OFL)from water,several lignin-based adsorbents have been prepared.Firstly,LN was chemically cross-linked by using epichlorohydrin(EPI)as a crosslinking agent to obtain a crosslinked LN(LNEs)for effective improvement of the physical and chemical stability.Further,chloroacetic acid(CA)and acrylic acid(AA)were used as modifiers to introduce carboxylic groups onto the LN backbone by etherification and grafting copolymerization,respectively,to obtain two lignin-based carboxylated adsorbents,etherified carboxymethyl LN(LNECs)and poly(acrylic acid)(PAA)grafted LN(LNAEs)to increase the adsorption capacities of LN adsorbents for FQs.Due to the different reaction mechanisms of etherification and grafting copolymerization,the linkage mode and distribution of the carboxylic groups on the surface of LNECs and LNAEs are far different,which leads to different mechanisms for adsorption of FQs.The specific research results are as follows:(1)The adsorption and removal performance of LNEs and LNECs on FQs such as OFL have been compared at the beginning.Both LNEs and LNECs could effectively adsorb OFL,but LNECs exhibited better performance due to the introduction of carboxylic groups.The maximum adsorption capacity of LNECs was 0.828mmol/g.The crosslink density and the degree of carboxymethyl substitution were two important structural parameters of the above lignin-based adsorbent.Appropriately reducing the crosslink density or increasing the degree of carboxymethyl substitution could increase the adsorption capacity.LNEs and LNECs had good anti-interference to inorganic salts and humic acids in adsorption of OFL;after reaching the saturated adsorption,the above lignin-based adsorbents could be effectively regenerated and reused.In addition,because of both containing multiple functional groups,the adsorbent and adsorbate could be combined by various forces,such as hydrogen bonding,electrostatic attraction,π-π electron donor-acceptor interaction(π-π EDA),and negative charge assisted hydrogen bonding(CAHB).pH could change the surface charge distribution of the adsorbent and the adsorbate,so the contribution of different adsorption mechanisms to adsorption at varied pH levels was different,resulting in a strong pH dependence of the final adsorption properties of LNEs and LNECs.According to the dissociation constant(pKa)values of OFL and the actual adsorption effects,the pH can be divided into three regions to investigate its multiple adsorption mechanism.Hydrogen bonding was dominant;under weakly acidic and neutral conditions,the adsorption capacity of LINEs and LINECs reached a maximum,and the adsorption was dominated by the synergistic effects of electrostatic attraction and π-π interaction.Under alkaline conditions,it was dominated by negative charge-assisted hydrogen bonding.In addition,five FQs with similar structures:OFL,ciprofloxacin(CIP),norfloxacin(NOR),enrofloxacin(ENR)and fleroxacin(FLE)and their two substructure analogs:Fluoromethine(FUL)and 1(2-fluorophenyl)piperazine(FPP),which were quantified by density functional theory,and their electron cloud density(ECD)distribution and electrostatic potential(ESP)values were obtained.The corresponding relationship between the ESP and their adsorption capacities of the above lignin-based adsorbents was investigated.It was confirmed that FQs with secondary amino groups and low π electron cloud density were more easily combined with the lignin-based adsorbents.(2)As for the PAA grafted lignin-based adsorbents(LNAEs),two different series of LNAEs were first obtained by controlling the dosages of monomer,AA,and the initiator,potassium persulfate(KPS):(a)LNAEsB with the same number of PAA graftchains but different length,and(b)LNAE2s with different number of PAA graft-chains but the same graft-chain length.The performance and mechanisms of LNAEs of adsorption of FQs,especially the effects of structural factors such as the length and number of PAA graft-chain,have been explored in detail.In the single-component adsorption systems of OFL and CIP,respectively,LNAEs exhibited good adsorption properties for both FQs.Hydrogen bonding and electrostatic interaction were the two main mechanisms involved to promote the adsorption.For the functional group of the lignin-based adsorbent as a whole,the contribution of hydrogen bonding to the adsorption capacity of OFL and CIP was greater than that of electrostatic interaction.However,for the grafted carboxylic acid on the surface of LNAEs,the hydrogen bond interaction with the OFL contributed to the adsorption capacity in much the same way as the electrostatic interaction,but the hydrogen bond interaction with CIP contributed more to the adsorption capacity than the electrostatic interaction.By normalizing the total amount of grafted carboxylic groups,it was found that,at the constant of the grafted carboxylic group content,the higher number of grafted chains(N)was,the higher FQs uptakes were,indicating the "dense" distribution of carboxylic groups on the surface of LNAEs were benifcial to adsorption of both OFL and CIP.This may be due to the fact that the number of widely distributed graft chains would expose more active sites.In the study of two-component adsorption system of OFL and CIP,it was found that LNAEs preferentially adsorbed CIP,and the selectivity coefficient βOFLCIP of LNAE2C with the most number of grafted chains exhibted the highest selectivity,indicating that the density of graft-chain distribution was an important structural factor for the selective adsorption of CIP over OFL by LNAEs.Besides,by comparing the inhibition effects of inorganic salts and humic acids on the adsorption of FQs in this two-component system,the selective adsorption of CIP by LNAEs may be mainly ascribed to the hydrogen bonding effect.In summary,the physicochemical stability and adsorption performance of the lignin-based adsorbents for FQs have been both significantly improved after suitable chemical modification.The experimental results show that these lignin-based adsorbents are a kind of high-efficient and environmentally friendly natural polymeric adsorbents,which has wide application potenitals in the water treatment.