Preparation Of Carboxylated Biosorbents Via Atom Transfer Radical Polymerization For Adsorption Of Heavy Metal Ions

Atom transfer radical polymerization?ATRP?is currently the most effective active/controllable polymerization technology.In view of the serious harm of heavy metal pollution to the ecological environment and human health,this study takes the development and utilization of agricultural waste to control heavy metal pollution in water as the starting point,uses agricultural waste corncobs as raw material,and uses ATRP technology to graft a large number of carboxyl groups?-COO^-?with strong affinity for heavy metal ions onto the surface of corncobs.Three kinds of carboxylated corncobs as Biosorbents?MC-g-PAA-Na,MC-g-PAA-K and MC-g-PGMA-g-PAA-Na?were prepared.The preparation conditions of the adsorbents,the adsorption properties and adsorption mechanism of heavy metal ions(Pb²⁺,Cd²⁺,Cu²⁺and Ni²⁺)were studied in depth.The main research work is as follows:?1?2-Bromoisobutyryl bromide is used as an initiator to react with hydroxyl on the surface of corncobs to generate ester,and the macroinitiator corncob-Br?MC-Br?with initiator activity was obtained.The polymerization of methyl acrylate?MA?was initiated on the surface of MC-Br by ATRP technology to obtain poly?methyl acrylate?grafted corncobs?MC-g-PMA?.Finally,MA grafted on corncobs surface was hydrolyzed into acrylic acid?AA?by NaOH solution to prepare MC-g-PAA-Na.The suitable hydrolysis conditions were as follows:0.3 mol L^-1 of NaOH concentration,6h of hydrolysis time,and 50 ^oC of hydrolysis temperature.Under these conditions,the hydrolysis yield of MC-g-PMA reached 26.9%,and was much lower than its theoretical hydrolysis yield?108.2%?,which indicated that some esters formed by the reaction of hydroxyl groups on the surface of corncobs with 2-bromoisobutyryl bromide were hydrolyzed during this process,resulting in the loss of all functional groups?-COO^-?grafted on these esters.All these seriously affected the adsorption performance of the adsorbent for heavy metal ions.Compared with unmodified corncobs,the maximum adsorption capacities of MC-g-PAA-Na for heavy metal ions increased by 10.26-17.68 times.?2?MC-g-PAA-K was prepared by hydrolyzing MC-g-PMA with t-BuOK solution.The appropriate hydrolysis conditions were as follows:0.4 mol L^-1 of t-BuOK concentration,8 h of hydrolysis time,and 60 ^oC of hydrolysis temperature.Under these conditions,the hydrolysis yield of MC-g-PMA was 31.2%,higher than that of NaOH solution?26.9%?,but still far lower than its theoretical hydrolysis yield?124.8%?.Compared with NaOH solution,MC-g-PMA is hydrolyzed by t-BuOK solution with larger steric hindrance,which reduces the hydrolysis of esters formed by the reaction of hydroxyl groups on corncobs surface and 2-bromoisobutyryl bromide in this process,and further increases the maximum adsorption capacities of adsorbent for heavy metal ions by 0.75-1.13 times.?3?A certain amount of glycidyl methacrylate?GMA?was grafted on the surface of MC-Br to form a protective layer for obtaining poly?glycidyl methacrylate?grafted MC-Br?MC-g-PGMA?.MC-g-PGMA has no obvious mass loss in NaOH solution.Combined with Fourier transform infrared spectroscopy?FTIR?,X-ray energy spectrum?EDS?and X-ray photoelectron spectroscopy?XPS?analysis,it is shown that the protective layer formed by grafting GMA has not been damaged by NaOH solution,but still covers the surface of MC-Br,protecting these esters formed by the reaction of hydroxyl groups on corncobs surface with 2-bromoisobutyryl bromide to avoid hydrolysis in NaOH solution.The polymerization of MA was initiated on the surface of MC-g-PGMA to obtain poly?methyl acrylate?grafted MC-g-PGMA?MC-g-PGMA-g-PMA?.Finally,MA grafted on corncobs surface was hydrolyzed into AA by NaOH solution to prepare MC-g-PGMA-g-PAA-Na.The preparation process was verified by FTIR,scanning electron microscope?SEM?,EDS and XPS technologies.The suitable hydrolysis conditions were as follows:1.0 mol L^-1 of NaOH concentration,20 h of hydrolysis time,and 60 ^oC of hydrolysis temperature.Under these conditions,the hydrolysis yield of MC-g-PGMA-g-PMA was 76.4%,which was higher than that of NaOH solution and t-BuOK solution.Compared with MC-g-PAA-Na,the maximum adsorption capacities of MC-g-PGMA-g-PAA-Na added with protective layer to heavy metal ions increased by 2.20-3.00 times.?4?FTIR,SEM,EDS and XPS were used to characterize MC-g-PAA-Na and MC-g-PGMA-g-PAA-Na.It was proved that MA undergoing polymerization was hydrolyzed into AA by NaOH,and these AA reacted with NaOH to generate sodium acrylate?AA-Na?,making Na exist in the form of sodium carboxylate?-COO-Na?.The higher percentage of Na indicates that a large number of carboxyl groups?-COO^-?exist in the carboxylated corncob biosorbent,and the carboxyl content in MC-g-PGMA-g-PAA-Na is 6.02 mmol g^-1.Compared with adsorbents reported in recent years,MC-g-PGMA-g-PAA-Na has higher adsorption capacities for Pb²⁺,Cd²⁺,Cu²⁺and Ni²⁺,and is a new type of high-efficiency adsorption material.?5?The optimal pH values of the solutions for adsorption of Pb²⁺,Cd²⁺and Cu²⁺by the three carboxylated corncobs are all 5.00,and the optimal pH values for adsorption of Ni²⁺are all 7.00.The removal rates of Pb²⁺,Cd²⁺,Cu²⁺and Ni²⁺are?98.08%,indicating that the three adsorbents can effectively remove Pb²⁺,Cd²⁺,Cu²⁺and Ni²⁺in aqueous solution.The adsorptions of Pb²⁺,Cd²⁺,Cu²⁺and Ni²⁺by three carboxylated corncobs are spontaneous,endothermic and accompanied by enthalpy increase.They are more in line with Langmuir model,belong to single-layer adsorption,have strong adsorption stability;following the pseudo-second-order reaction mechanism,they belong to chemical adsorption and have faster adsorption.?6?MC-g-PGMA-g-PAA-Na has good application stability,the static desorption rate of Pb²⁺is 95.18%,and has little influence on adsorption performance after 5times of reuse.In the early stage of dynamic adsorption of Pb²⁺by MC-g-PGMA-g-PAA-Na,the mass concentration of Pb²⁺in the effluent solution was less than 0.2 mg L^-1,which reached the standard of direct emission.Moreover,dynamic adsorption was fast.These showed that MC-g-PGMA-g-PAA-Na had strong practicability.?7?Thermogravimetry,FTIR,SEM,EDS and XPS were used to characterize MC-g-PGMA-g-PAA-Na before and after adsorption.It was proved that when MC-g-PGMA-g-PAA-Na was contacted with a solution containing heavy metal ions(Pb²⁺,Cd²⁺,Cu²⁺and Ni²⁺),the carboxyl groups?-COO^-?contained in MC-g-PGMA-g-PAA-Na acted on the heavy metal ions to adsorb the heavy metal ions in the solution and form carboxylate salts.The coordination mode of carboxyl and heavy metal ions was mainly a bidentate bridge.At the same time,all Na⁺contained in MC-g-PGMA-g-PAA-Na were released into the solution,indicating that the process of adsorption of heavy metal ions in the solution was accompanied by cation exchange between Na⁺and heavy metal ions.There were 127 figures,28 tables,and 215 references in this thesis.