Preparation Of The O-rich Lignin Biochars And Their Adsorption Properties

Lignin is the most abundant natural aromatic polymer in nature.It contains abundant functional groups such as benzene ring,hydroxyl group,carbonyl group and sulfonic group.It is considered to be a suitable raw material for the production of carbon based adsorption materials.However,the adsorption capacity of unmodified lignin biochar is low,and the modified lignin biochar can improve its adsorption capacity.In this paper,a method of one-step preparation of oxygen enriched lignin-biochars adsorbent by sulfuric acid oxidation carbonization was developed.The adsorption of heavy metal ions and cationic dyes was studied.（1）The preparation of oxygen rich lignin biochar by sulfuric acid oxidation carbonization one-step method was studied.The lignin-biochars were investigated by FTIR,XPS and ¹³C CP-MAS NMR.The results showed that some aromatic side chains could be oxidized by sulfuric acid,introducing oxygenated functional groups,especially carboxyl groups.The oxygen-containing functional groups-COOH and Ar-OH of lignin biochar reached 8.64 mmol/g.The adsorption study showed that the maximum adsorption capacity of lignin biochar for Pb²⁺was 679 mg/g,which was significantly higher than other lignin based adsorbents,and the removal rate was still high at high adsorption capacity.The adsorption isotherm was fitted with Freundlich model（R²=0.9832）,and the adsorption kinetics was fitted with quasi second order kinetic equation（R²=0.9946）,which indicated that the adsorption was mainly multi-layer heterogeneous chemical adsorption.Through the establishment of lignin biochar adsorption capacity and carboxyl correlation model,it shows that the adsorption capacity is directly proportional to the contact carboxyl.（2）Functional lignin-biochars with rich sulfonic and carboxyl groups were produced via a facile simultaneous oxidization-carbonization of lignosulfonate by using 96%sulfuric acid at 60℃and 210℃,signed as SLBC-60 and SLBC-210.The structure and acidic oxygen-containing groups of the biochars were investigated by Boehm titration,elemental analysis,FT-IR,¹³C CP-MAS NMR,XPS.SLBC-60 contained 1.66 mmol/g of sulfonic groups,1.40 mmol/g of carboxyl groups and 4.41 mmol/g of phenolic hydroxyl groups,while SLBC-210 contained 0.34 mmol/g of sulfonic groups,3.22 mmol/g of carboxyl groups and 5.41 mmol/g of phenolic hydroxyl groups.Their adsorption capabilities of methylene blue（MB）were separately estimated in the aqueous solution at p H from 1 to 10.The results showed that SLBC-60 had higher adsorption capability for MB（adsorption capacity>463.9mg·g-1,removal efficiency>91.9%）than that of SLBC-210（adsorption capacity<231.5mg·g-1,removal efficiency<45.8%）.Combined with their structures and Zeta potentials,the high adsorption capacity of SLBC-60 may be related to its rich sulfonic acid groups and the enrichment of negative charges on the surface in the solution of p H=1-10.The saturated adsorption capacity of SLBC-60 for MB is 755.1 mg/g after optimizing the conditions of p H,adsorbent addition,adsorption time.The adsorption capacity is not affected by mono-valent metal ions.In addition,the study of the adsorption mechanism of MB on SLBC-60 showed that the adsorption isotherm fitted the Langmuir model（R²=0.9989）and the adsorption kinetic fitted the pseudo-second order kinetic equation（R²=0.9995）,indicating that the adsorption was dominated by mono-layer chemical adsorption.Therefore,these sulfonic functional lignin-biochars can be used as efficient adsorbents for cationic dyes and are expected for the treatment of dyeing wastewater.（3）The pyrolytic lignosulfonate（LPBC）was prepared by traditional pyrolysis method in a tube furnace at 15℃/min to 550℃and maintained at 550℃for 2 h.The commercial activated carbon（PAC）was purchased from the market and SLBC-60 adsorbents prepared in the previous chapter.The structure of the biochars were investigated by elemental analysis,FTIR,XRD and BET.The results show that LPBC has less oxygen functional groups on the surface and only has 3.13 m²/g specific surface area,so it has no adsorption effect on dye opportunities.Although PAC has less oxygen functional groups on the surface,its specific surface area reaches 1937.71 m²/g,and has better adsorption effect on anion dyes,neutral dyes and cation dyes.SLBC-60 has a small specific surface area,but it has many oxygen-containing functional groups on its surface.It has a good adsorption effect on cationic dyes and can selectively adsorb cations.This is related to the rich oxygen-containing functional groups on the surface.From the previous chapter,we know that the zeta potential of SLBC-60 is low,which may be the reason for its selectivity.N-rich lignin biochar was prepared by introducing nitrogen-containing functional groups to achieve the selective adsorption of anionic dyes.