| Around 2.7 billion tons of solid waste will be generated by the year 2050,most of which will be disposed of in landfills,producing high-salt,toxic leachate.Sodium bentonite-polymer composites have been developed globally for leachate containment.In China,the reserve of sodium bentonite is low,with 80%of natural bentonite being Ca-bentonite（Ca B）.However,it is difficult to exfoliate Ca-montmorillonite in water,due to the strong binding of the montmorillonite platelets by divalent Ca2+.In order to prepare bentonite-based composites,the Ca B has to be converted into sodium bentonite first,which is a complex and costly process that is commonly used in most industrial processes.To these ends,this thesis work focuses on the rational design of monomer intercalation,aiming to achieve in-situ intercalation polymerization of Ca B,and directly fabricate Ca B composite barrier materials.As prepared composite materials showed exceptional barrier properties against high-salt leachates,combined with good resistance to polymer elution.The modified Ca B materials were scaled up to a 50 kg level,and subjected to large-scale cutoff wall tests.The main contents are as follows:（1）Anionic monomers,i.e.,acrylic acid（AA）and 2-acrylamido-2-methylpropanesulfonic acid（AMPS）,were selected to modify Ca B.AA and AMPS intercalated into montmorillonite interlayers through the electrostatic attraction with interlayer Ca2+,whereby they were polymerized in-situ to form polymer modified bentonite nanocomposites:Ca B/P（AA-AMPS-MBA）.The swell index of the Ca B/P（AA-AMPS-MBA）in the coal combustion product leachate（I=178 m M）is 11.6 m L/2g,which is 2.1times as high as that of pristine Ca B.Hydraulic conductivity of the composite in coal combustion product leachate is 6×10-12 m/s,which is four orders of magnitude lower than that of pristine Ca B,meeting the standard for geosynthetic clay liners(hydraulic conductivity<5×10-11 m/s).The strong sulfonic acid groups in P（AA-AMPS-MBA）confers good hydrophilicity to the composite material,thus improving the swelling properties of Ca B.The Ca B/P（AA-AMPS-MBA）particles were encapsulated in the geosynthetic clay liners,swelled and interlocked with each other in coal combustion product leachate,closing the inter-particle voids and improving barrier properties by making the water conduction path more tortuous.（2）A cationic monomer（N,N-diethylacrylamide,DEA）,and an anionic monomer（AA）were designed,which intercalated Ca B synergistically and allowed for the preparation of polymer-modified bentonite material named Ca B/P（AA-DEA）.Anionic AA monomers first intercalate into montmorillonite interlayers through electrostatic interaction with Ca2+,leaving a negatively charged montmorillonite surface,which further attracts cationic DEA monomer.Attributed to this synergistic effect,the exfoliation efficiency of montmorillonite in Ca B/P（AA-DEA）is higher than that in Ca B/P（AA-AMPS-MBA）.Meanwhile,the molecular conformation of the amphiphilic P（AA-DEA）polymer is more extended in high-salt solutions,resulting in stronger salt resistance compared to that of the anionic polymer such as P（AA-AMPS-MBA）.The swell index of Ca B/P（AA-DEA）in coal combustion product leachate is 15.8 m L/2g,and its hydraulic conductivity is 1.6×10-12 m/s,which means,the barrier properties of Ca B/P（AA-DEA）is 3.6 times as high as that of Ca B/P（AA-AMPS-MBA）.The hydraulic conductivity of Ca B/P（AA-DEA）in high-salt trona ash leachate（I=746 m M）is 3.4×10-11 m/s,which meets the standard for geosynthetic clay liners.（3）On basis of the anionic monomers for Ca B modification,the vinyltriethoxysilane（VTES）monomer was introduced to replace MBA monomer,aiming to facilitate the Ca B/polymer interfacial crosslinking and enhance its polymer elusion resistance.The montmorillonite surfaces are rich in silicon hydroxyl functional groups,which reacted with VTES repeating units in P（AA-AMPS-VTES）and formed interface cross-linked structures that enhanced the bentonite/polymer interface interaction.Polymer elution rate of the interface cross-linked composite in coal combustion product leachate was 3.2×10-3%/d,indicating that polymer elution resistance performance of the interface-crosslinked composite was 5.6 and 6.6 times higher than that of Ca B/P（AA-AMPS-MBA）and Ca B/P（AA-DEA）,respectively.The introduction of VTES also increased the compactness of Ca B/P（AA-AMPS-VTES）filter cakes.Thus,the hydraulic conductivity of Ca B/P（AA-AMPS-VTES）in trona ash leachate is 1.2×10-11 m/s,which means,the barrier properties of Ca B/P（AA-AMPS-VTES）is 9.8 and 1.8 times as high as that of Ca B/P（AA-AMPS-MBA）and Ca B/P（AA-DEA）,respectively.（4）In this chapter,the scaled preparation（from 0.1 to 50 kg/batch level）of three types of aforementioned polymer-modified Ca B barrier materials were studied.For the pilot-scale reaction,the volume of the reaction mixture was～1000 times that of a bench-scale reaction,resulting poor heat dissipation,inhomogeneous heating,and inefficient mixing of monomers during the modification reaction.To these ends,a constant-temperature feeding method was presented to maintain a low temperature during monomer feeding.Stirring speed was increased by reducing the reaction mixture viscosity to promote heat and mass transfer.These approaches increased the single-pot yield of Ca B barrier materials from 0.1to 50 kg level.The montmorillonite exfoliation and polymer surface modification of the resulting Ca B composite materials are lower than that of the bench-scale reaction products.Nevertheless,the Ca B composites prepared in pilot-scale have hydraulic conductivities of2.5～4.2×10-11 m/s in the coal combustion product leachate,which meet the requirement for synthetic clay liners.A total of 100 kg Ca B barrier materials were produced to construct compacted clay liner at a gold mine ore landfill in Suichang County.The predicted hydraulic conductivity of the compacted clay liner in deionized water,obtained from permeability test,was 3.7×10-10 m/s,which meet the standard for compacted clay liners(<10-9 m/s). |
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