Study On The Removal Of Heavy Metals From Water By Hydrogel Adsorbents Derived From Chinese Medicine Residues

Heavy metal pollution in water matrix and solid waste discharge are two major environmental issues that arouse public concerns worldwide.The average emissions of Chinese medicine residues are 30 million tons annually,and is considered as one of the biggest categories solid wastes in China.The traditional disposal methods such as incineration and landfill will not only cause waste of resources（biomass）but also bring secondary pollution;on the other hand,the increase of heavy metal concentration in water matrixes from natural sources and human activities not only causes serious drinking water safety problems but also raises food safety concerns.Therefore,it is of far-reaching significance to seek a technology that can bridge the Chinese medicine solid waste disposal issue and heavy metal removal from water.The most common existing technology is the carbonization of herbal residues into biochar adsorbent under high-temperature anaerobic conditions,however,this method is energy intensive,environmentally unfriendly,and the performance of the derived biochar adsorbent is far below expectations.Therefore,it is urgent to develop a cost-effective and environmentally friendly technology.Polymer hydrogels are widely used as a low-cost,high-efficiency and environmentally friendly adsorbent material for the removal of heavy metals from water matrices.In this thesis,four double-network hydrogel adsorbents derived from the solid waste of Chinese medicine residues were prepared,and the removal performance and mechanism were investigated thoroughly by combining batch experiments and fixed-bed process.The main findings are as follows:（1）Cellulose（CNFs）was successfully extracted from Astragalus herbal residue biomass using bleaching and alkaline wet-up method and well dispersed in aqueous system by high-power ultrasonication,and cellulose/polyacrylic acid（CNFs/PAA）double network hydrogel adsorbent was prepared by free radical polymerization method.The composite of cellulose nanofibers and acrylic acid exhibited a porous network structure.The gel exhibited excellent mechanical properties（～0.60 MPa）thanks to the interaction between the hydroxyl groups on the cellulose surface and the acrylic acid.The adsorption experiments of Pb（II）,Cu（II）and Cd（II）in wastewater showed that the adsorption capacities of CNFs/PAA gel for Pb（II）,Cu（II）and Cd（II）were 202.9,45.2 and 37.2 mg g^-1,respectively.For the initial concentration of about10 mg L^-1 of Pb（II）,Cu（II）and Cd（II）solutions,CNFs/PAA showed good anti-interference ability（p H effect,ionic strength effect,etc.）.In addition,for divalent heavy metal ions such as Pb（II）,Cu（II）,Cd（II）,Zn（II）,Mn（II）and Ni（II）,CNFs/PAA showed good selectivity for the first three kind of metal ions.（2）The amino-modified functionalized cellulose/polyacrylic acid（NH₂-CNFs/PAA）double network hydrogel adsorbent was prepared from cellulose extracted from the Astragali residues,using a free radical polymerization-joining amination modification method.The theoretical maximum adsorption capacity of NH₂-CNFs/PAA modified by amination was as high as 994.5 mg g^-1 for Pb（II）due to its abundant functional groups,which was much higher than unmodified ones;in addition,due to the excellent water permeability and three-dimensional network structure of NH₂-CNFs/PAA,Pb（II）could be rapidly transported into the inner part of the gel,showing a fast kinetic behavior(～10 min to reach the adsorption equilibrium,k_f=2.01×0^-5 m s^-1);meanwhile,NH₂-CNFs/PAA showed strong resistance to the aquatic environment,with～100%removal of Pb（II）in a wide p H range from 2 to 6;excellent cycling performance（the removal rate is still close to 100%after～15 cycles）.Moreover,the NH₂-CNFs/PAA had a very high selectivity for Pb（II）(distribution coefficient K_d～2.4×0⁶ m L g^-1).The fixed-bed process showed that NH₂-CNFs/PAA could effectively remove 114.6 BV of Pb（II）-containing wastewater with a feed concentration～5000μg L^-1,and the enrichment coefficient could reach 10.9.The accuracy of the external mass transfer coefficient（k_f）and pore diffusion coefficient（D_p）was verified by fitting the fixed-bed column experimental data with the pore surface diffusion model（PSDM）,and the treatment capacity of lead-containing wastewater under different operating conditions（initial Pb（II）concentration,EBCT,etc.）was successfully predicted by the PSDM,which can save operating time and operating costs.（3）NCF/P（AA-co-AM）hydrogel beads with nanocellulose/acrylic acid-acrylamide copolymerization was prepared using an interfacial polymerization method.The nanocellulose was extracted from the waste residue of the Chinese traditional medicine red peony.The material has superb mechanical properties（～12 MPa）and excellent water permeability（each gram of beads can absorb 40 times its own weight of water）,which endows the material with excellent recycling and kinetic properties.Sequential batch adsorption experiments have shown that NCF/P（AA-co-AM）can reduce 22.8 mg L^-1 Pb（II）to below～0.002 mg L^-1(below the EPA standard of～0.015 mg L^-1),high theoretical adsorption capacity(q_m=468.0 mg g^-1),wide working p H range（4-6）,excellent resistance to cation interference,excellent selectivity（K_d=1.76×0⁶）and good cycling stability（removal rate can still be close to 80%after 8 cycles）.The continuous production method adopted in this work could not only enhance the performance of the gel adsorbent,but also meets the industrial production requirements better.（4）The iron-based metal organic framework（NH₂-MIL-101（Fe））-loaded double network hydrogel（NH₂-MIL-101@CNFs/PAA）adsorbent was prepared using hydrothermal and sol-gel methods.NH₂-MIL-101（Fe）was uniformly dispersed on the CNFs/PAA gel to ensure uniform distribution and adequate exposure of the adsorption sites.For an initial concentration of about20 mg L^-1 of Pb（II）,the adsorption equilibrium was achieved within 10 min.The isothermal adsorption experiments fitted NH₂-MIL-101@CNFs/PAA for Pb（II）up to 695.2 mg g^-1.In addition,the materials showed good resistance to environmental interference,selectivity and cycling stability.The experiments of Pb（II）removal in real water matrices showed that it has potential for application in real water matrices.Fixed-bed column experiments showed that NH₂-MIL-101@CNFs/PAA could treat simulated wastewater containing Pb（II）at an initial concentration of 10 mg L^-1 to a volume of 108 BV（～6.1 L）below the discharge standard(1.0mg L^-1).Finally,the intrinsic mechanism of Pb（II）adsorption by NH₂-MIL-101@CNFs/PAA was revealed in combination with XPS analysis.The problems of difficult recovery of MOF and low adsorption capacity of cellulose-based hydrogel adsorbent were solved synergistically by the protocol of MOF loaded on hydrogel substrate.