Study On New Type Of Polyacrylamide Modified Bentonite Impermeable Materials

Bentonite is a kind of clay mineral. It has high economic value and has been applied widely today. With bentonite as the main raw material, using its multi-layered structure, water swellability and low permeability, and by improving its microstructure and particle surface properties with modifiers, its practical application efficiency as a functional material can be enhanced. Therefore, it has high application value and a wide prospect in purification, remediation and control of environmental pollution.In this paper, with the Na-base bentonite from JiLin Liufangzi as raw material and with polyacrylamide as modifier, the polyacrylamide modified bentontites are synthesized by aqueous solution intercalation polymerization. Firstly, bentontite is modified by the intercalation polymerization with acrylamide as monomer to prepare polyacrylamide modified bentonite material. The preparation conditions are optimized by simple and orthogonal experiments; its water swelling, thermal stability and salt resistance are measured, and the structural characteristics and formation mechanism of the modified bentonite are researched with analytical instruments. Based on this research and according to the present research situation of sanitary landfill impermeable materials, a new type of polyacrylamide modified bentonite impermeable material is synthesized by direct intercalation polymerization of modified polyacrylamide. The focal point of study is synthesis of modified polyacrylamide, the processing conditions of modified polyacrylamide are discussed and the effect of quantity of the modified polyacrylamide on structure of the modified bentonite impermeable material is investigated. Finally, with leachate of Shenyang Daxin landfill as the treatment object and with new type polyacrylamide modified bentonite as impermeable material, the adsorption mechanism of the modified bentonite impermeable material for COD of leachate is investigated at the static adsorption conditions, and the effectiveness of removing pollutants and feasibility of controlling penetration of the modified bentonite impermeable materials for the leachate are investigated. Main contents and the results are as follows:1 With Na-base bentonite as raw material, by solution intercalation polymerization of acrylamide monomer, a type of polyacrylamid modified bentonite was prepared.(1) Simple experimental results show that the amounts of crosslinker, initiator, dissolved water and bentonite all affect the swelling effect of polyacrylamide modified bentonite. Optimal preparation conditions obtained by the orthogonal experiment are:the amount of crosslinker (mC/mM) is 2.16×10-4, the amount of initiator (mI/mM) is 0.04, the amount of dissolved water (mW/mM) is 2.3, the amount of sodium bentonite (mB/mM) is 1.2, and the various factors affecting the swelling of modified bentonite in turn are:initiator> bentonite> dissolved water> crosslinker.(2) Under experimental conditions, water absorbency of modified bentonite material is up to 35.98(g/g), rising by nearly one order of magnitude than that of the original bentonite; in a certain concentration solution of NaCl, CaCl2, FeCl3, water absorbency (g/g) of modified bentonite material is 7.491～12.22, which is larger than water absorbency (g/g) 3.595 of original bentonite, and the difference of their water absorbency rate in different kinds of salt solution with various concentration degrees is small; the water absorbency test of modified bentonite material at 20～80℃for 1h show that water absorbency changes little, it show that polyacrylamide modified bentonite has good water swellability, salt tolerance and thermal stability.(3) Formation mechanism of polyacrylamide modified bentonite. Polymerization mechanism of polyacrylamide is a kind of free radical reaction. When the crosslinking agent N, N-methylene bisacrylamide is present, crosslinking reaction occurs, so that polymer has a moderate crosslinking degree, and together with bentonite forms polyacrylamide modified bentonite. (4) Structural analysis of polyacrylamide modified bentonite. X-ray diffraction (XRD) characterization results indicate that the insertion of reactive monomers among the layers of bentonite leads to the significantly increased spacing of layers after polymerization. However, except that the spacing of crystal faced layer 001 in modified bentonite slightly increases than that of Na-base bentonite, the positions of other diffraction peaks hardly change, which indicate that basic structure of bentonite does not change; FT-IR characterization results show that polyacrylamide modified bentonite also has the characteristic IR peaks of polyacrylamide and bentonite, which indicate that there is polyacrylamide among layers of bentonite. Scanning electron microscopy (SEM) and electron microscopy characterization results show that there is a good compatibility between bentonite and polyacrylamide in modified bentonite, and the modified bentonite has a three-dimensional cross-linked network structure. All characterization results indicate that the structure of the modified bentonite is as follows:a part of acrylamide (AM)are embedded into the layers of bentonite and bonded by van der Waals force and hydrogen bond at the layers of bentonite. After cross-linking polymerization, bentonite was mainly responsible for the framework support of modified bentonite while another part of acrylamide (AM) forms hydrogen bonds by exchanging cations with bentonite surface and the last part of acrylamide generates "free" polymer network among bentonite particles and bonded with bentonite by polymer chains.2. Based on study on modified bentonite by intercalation polymerization with acrylamide monomer, it is the first time a new type of modified bentonite impermeable material is prepared by direct intercalation polymerization of modified polyacrylamide at room temperature with original bentonite as raw. The synthesis process of modified polyacrylamide is researched, and the effect of quantity of the modified polyacrylamide on structure of the modified bentonite impermeabile material is discussed through the analysis of characterization results. The basic performances of the modified bentonite are also measured. Main research results are as follows:(1) Modification of polyacrylamide is achieved in two steps, firstly with acrylamide as monomer, ammonium persulfate as the initiator, sodium formate as chain transfer agent, and then N-methylolated polyacrylamide was prepared by reacting low molecular weight polyacrylamide with formaldehyde under alkaline conditions; and finally the modificated polyacrylamide is synthesized by reacting N-methylolated polyacrylamide and acrylamide in the condensation reaction under alkaline conditions.①By changing the initiator, chain transfer agent concentration and reaction time, this research studies their effect on molecular weight polyacrylamide. The results show that effect of initiator concentration on molecular weight polyacrylamide is more direct, and effect of chain transfer agent concentration and the reaction time on molecular weight polyacrylamide is not very obvious. The optimal conditions for synthesis of low molecular weight polyacrylamide are: initiator concentration is 2.5×10-2 mol/L; chain transfer agent concentration is 8.0×10-2 mol/L; reaction time is 3h. An appropriate viscosity of low molecular weight polyacrylamide solution is favorable to the following modification operation of low molecular weight polyacrylamide.②The suitable hydroxymethylation conditions of low molecular weight polyacrylamide with formaldehyde are:n(PAM):n(F)=1:0.5～1:1, reaction time 2 h, the reaction temperature 40℃, pH=10. The appropriate dehydration condensation conditions of the hydroxymethylated polyacrylamide and acrylamide are:reaction temperature 55℃, n(AM):n(F)=1:1, reaction time 3 h, pH=10. When n(PAM):n(F):n(AM)=1:0.6:0.6, hydroxymethylation rate was 99 % and the conversion rate of acrylamide was 95%.③At room temperature and with N, N’-methylene bisacrylamide as crosslinker, the modified polyacrylamide was crosslinked rapidly by redox system, so a new type of polyacrylamide modified bentonite impermeable material was prepared by direct intercalation polymerization of modificated polyacrylamide and bentonite. The crosslinking points of the crosslinked polyacrylamide are at the side chain while the main chain structure is relatively stable. Because amide groups partly are transformed at crosslinked side chains, which means primary amides are transformed to secondary amides, the activity of the groups are decreased, so resistance to hydrolysis is increased. The crosslinking degree of crosslinked product of modified polyacrylamide (n(PAM):n(F):n(AM)=1:0.6:0.6) at room temperature is 67%, it meets the practical application demand for the modified bentonite materials, and ensures effective bonding of crosslinked modified polyacrylamide with bentonite. Water absorbency of crosslinked product of modified polyacrylamide and polyacrylamide modified bentonite impermeable material is 3.584,3.621 g/g respectively, and it is similar to the water absorbency of bentonite (3.595 g/g) used in this experiment, it ensures stability of the new type of modified bentonite impermeable material and avoids the cracking phenomenon after absorbing water expansion.(2) The structure of the modified bentonite is characterized with analytical instruments. The results show that 2#(the amount of modified polyacrylamide is 30%) specific surface area of the modified bentonite is bigger than specific surface area of the original bentonite, distribution of its particle size is better. X-ray diffraction (XRD) results show that spacing of layers of polyacrylamide modified bentonite changed little and the structure and morphology of other parts are not affected, and peak of d001 is sharp, which show that dispersion of polyacrylamide in bentonite is good. SEM characterization results show that the irregularity of surface of modified bentonite significantly increased. IR and TG analysis results show that there is polyacrylamide among layers of bentonite and that the intercalated polyacrylamide made structure water among layers disappear.(3) The results from the immersion experiment of modified bentonite show that under half-limit state, if the modified bentonite is immersed in tap water for 1 d, the volume expansion rate will reach 182%, and then curve will show a general rising trend, but then the increasing of volume expansion rate becomes very slow, the maximum is 190%. After 22 d immersion, modified bentonite impermeable material is still in the state of no segregation and no fragments, and the material has good durability and stability.3. With Shenyang Daxin landfill leachate as treatment object and a static adsorption test, this research studies adsorption mechanism of the new type polyacrylamide modified bentonite for leachate. The test results show that adsorption efficiency of the modified bentonite for organic pollutants is obviously better than that of the original bentonite. The maximum COD removal rate of leachate containing high concentrations of organic compounds could reach 30.73%.The isothermal adsorption curve show that adsorption process of modified bentonite for COD can be described appropriately with Freundlich model. The equation of the adsorption curve is y=0.4211x-1.3787, R2=0.9921 and the adsorption of modified bentonite for COD at static adsorption is mainly physical adsorption. The dynamic tests show that anti-seepage capability of modified bentonite material is good, the permeability coefficient is 1.04×10-7 cm/s, which meets national standard (1.04×10-7 cm/s) of impermeable materials for landfill. During the dynamic test, the removal effect of modified bentonite for COD, ammonia nitrogen and heavy metallic ions is better, and the adsorption effect is stronger, with the maximum removal rate of COD and ammonia nitrogen being 81% and 91% respectively while the maximum removal rate of TFe, Zn2+ and TCr being 71%,58%, and 73% respectively. The test results indicate that the new type polyacrylamide modified bentonite impermeable material holds excellent impermeability and decontamination performance.