Chemically-oriented Modification Of Lignin-based Adsorbent And Its Adsorption Selectivity For Heavy Metal Ions

The lignin-based adsorbents(polyphenol-type,Schiff based-type and cysteine-type)were prepared through chemical modification strategy and systematically characterized.To explore the interaction between the structures and adsorption selectivities towards metal ions of those adsorbents,the effects of pH,dosage,contact time,initial concentration and temperature on metal ions adsorption by adsorbents were studied.1.The low content of phenolic groups limits the application of lignin-based materials as biosorbents for the removal of metal ions.In this work,a novel gallic acid-grafted-lignin biosorbent(GLA)was designed by introducing gallic acid moieties.The structure of GLA was characterized by FT-IR,31 P NMR and 13 C NMR spectroscopy.The adsorption properties of GLA for Pb(II)ions were investigated under batch conditions.The grafting of polyphenolic groups onto lignin increased the maximum adsorption capacity for Pb(II)(103.69 mg/g).In addition,GLA could selectively adsorb Pb(II)with a selectivity coefficient(k)at 1.890 under the condition of coexistence of various metal ions.Kinetic and isothermal adsorption processes could be well-described by the pseudo-second order kinetic model and Langmuir isothermal model,respectively.The adsorption thermodynamics indicated that the adsorption process was endothermic and spontaneous with increasing entropy.It was found that the adsorption mechanism of Pb(II)by GLA was mainly complexation by analyzing the FT-IR spectra of before and after adsorption.2.Herein,acetic acid lignin was used in preparing Schiff base-modified-lignin adsorbent(SLA)through Mannich reaction and click reaction.The structure of SLA was characterized by FT-IR,1H NMR and EA analysis.The adsorption performance of SLA towards Pb(II)and Cu(II)were examined under different conditions,such as pH,dosage and metal ions initial concentrations.The maximum adsorption capacities of Cu(II)and Pb(II)by SLA were found to be 61.34 mg/g and 70.45 mg/g,respectively,with high selectivity under the condition of coexistence of various metal ions.The adsorption isotherms of Pb(II)adsorption isotherm could be well described by Langmuir model,showing a monolayer adsorption process.while Cu(II)adsorption by SLA could be well described by both Langmuir and Freundlich models,which suggested a multilayer adsorption behavior;Adsorption kinetics of both Pb(II)and Cu(II)adsorption were found to be in good agreement with pseudo-second model,which suggested the adsorption processes between SLA adsorbent and metal ions were chemical interactions.Thermodynamic studies exhibited that both Pb(II)and Cu(II)adsorption processes were spontaneous and endothermic with increasing entropy.It was found the reactive groups in the SLA not only had electronic sharing or transfer with Pb(II)and Cu(II),but also had a chelation with Cu(II)conforming coordination compounds.3.N-acetyl-L-cysteine functionalized lignin adsorbent(NLA)was synthesized via UV-initiated thiol-ene click reaction.The structure and morphology of NLA were characterized by FT-IR,1H NMR and SEM.The adsorption performance of NLA towards Cu(II)and Pb(II)were examined under different conditions,such as pH,dosage,metal ions initial concentrations,contact time and temperature.Due to introducing more adsorption sites,the maximum adsorption capacities of NLA for Cu(II)and Pb(II)reached 68.65 mg/g and 55.50 mg/g,with high selectivity under the condition of coexistence of various metal ions.The adsorption kinetics and isotherms indicated that Cu(II)and Pb(II)were adsorbed onto NLA by a monolayer chemisorption mechanism.Thermodynamic analysis exhibited that both Cu(II)and Pb(II)adsorption processes were spontaneous and endothermic with decreasing entropy.Results of FT-IR and XPS spectra reflected that Cu(II)and Pb(II)ions were adsorbed by active sites on NLA surface through coordination and electrostatic interactions.The feasible construction of NLA through UV-initiated reaction has been proven to be a promising “clickable” method in synthesizing lignin-based functional materials with designed modules and enhanced adsorption ability for metal ions removal.