Study On Preparation,efficiency And Mechanism Of Paper Mill-sludge Based Flocculants

Paper mill sludge is difficult to be treated with the high disposal cost,which limits the development of papermaking industry.Considering the abundant lignin content in pulp and paper sludge,cationic pulp and paper mill sludge-based flocculants with different chain architectures,molecular weights(MW)and charge densities(CD)will be synthetized via etherification or graft copolymerization by regulating and controlling cationic monomers and reaction conditions in this work.Based on the modern analytical instruments and methods,basic properties of the aforementioned flocculants will be characterized.Flocculants with different molecular structures will be used alone or used in combination with inorganic coagulants in the dye wastewater treatment to investigate its flocculation behaviors and performance,and then establish its structure-activity relationships.On-line optical determination of flocs will be real-time monitored and the relationships of the floc properties and flocculation performance will be determined The charges of the flocs flocculated by paper mill sludge-based flocculants or dual-coagulation system will be evaluated to discuss the flocculation mechanism of the flocculants.The main research contents and conclusions are displayed as follows(1)A series of paper mill sludge-based flocculants with different chain architectures,MW and CD levels were prepared from paper mill sludge.The molecular structures of the flocculants were controlled by adjusting the synthetic methods and preparation procedures.The characteristics of the target products were analyzed by fourier transform infrared spectroscopy(FTIR),transmission electron microscopy(TEM),gel permeation chromatography(GPC),X-ray photoelectron spectroscopy(XPS)and particle charge density analyzer.The results indicated that flocculants prepared by graft copolymerization were branched polymers,while those prepared b)etherification were linear ones.The flocculants with different MW and CD levels were prepared by changing the stirring rates and the dosages of potassium persulfate(KPS)and methacryloyoxyethyl trimethyl ammonium chloride(DMC)(2)Two flocculants with the similar MW and CD values but the different chain architecture were selected from the aforementioned target products.The flocculation efficiencies and floc properties of the two flocculants were investigated in disperse red(DR)and reactive blue(RB)dye wastewater treatment.The flocculation experiments indicated that branched flocculant was more efficient than linear one.Branched flocculant could remove 94.4%of DR dyes and 95.4%of RB dyes,while the color removal efficiencies of the linear flocculant in DR and RB dye removals were only 79.2%and 73.4%,respectively.Besides,the optimal dosages and costs of branched flocculant were smaller than those of linear one.Flocs formed with adding branched flocculant were large,loose,resistant,and easy to regenerate and settle.Branched flocculant was more pH-independent than linear one at the pH range of 2.0 to 10.0.Branched flocculant could exert excellent flocculation efficiencies with the coexistence of humic acid(HA)and kaolin particles,also,the UV254 could be simultaneously removed by the flocculants.In RB dye wastewater,UV254 removal efficiencies with adding branched flocculant and linear one were?90%and?60%,respectively.In DR dye wastewater,UV254 removal efficiencies were relatively low,which were?80%with adding branched or linear flocculant.Above all,the flocculation efficiencies and floc properties of branched flocculant were more excellent than linear one in DR and RB dye wastewater treatment.It is due to that branched flocculant with abundant side chains could adsorb large amounts of dye particles,thereby providing strong adsorption bridging effects.(3)A series of branched flocculants with different MW and CD levels were prepared by adjusting the stirring rates and dosages of KPS and DMC.The flocculation efficiencies of the target products were investigated in artificial RB dye wastewater treatment.The results demonstrated that flocculant CD and MW levels synergistically affected the flocculation behaviors.The flocculants with excessively high MW values but low CD values were inefficient due to the steric hindrance effects of the large amounts of side chains.There was a negative correlation between the flocculant CD level and its optimal dosage,but there was no direct relationship between flocculant CD levels and its color removal efficiencies.Flocculants with low CD levels could provide excellent color removal efficiencies by enlarging their dosages when their MW levels exceeded 5×105 g/mol.The MW levels of the flocculants were not directly relevant to their optimal dosages,application costs or flocculation efficienciesThe floc properties were simultaneously affected by the MW and CD levels of flocculants.When CD<25000 ?eq/L,flocculant CD levels played a dominant role in floc structures,the higher the CD values,the looser the flocs.However,when CD>25000 ?eq/L,the higher the MW values,the looser the flocs.It was worth noting that the MW or CD levels of the flocculants could not directly affect the floc size,resistance or recovery abilities.The study on the corrections among flocculant characteristics and the flocculation efficiencies was important to the design and development of high-efficiency flocculants.(4)A high-performance paper mill sludge-based flocculant,named as PS-g-DMC,was selected according to the results above.A commercial lignin-based flocculant(LBF)was prepared by the same synthetic methods as PS-g-DMC,but the raw material was replaced by the commercial lignin.The flocculation efficiencies of PS-g-DMC and LBF were investigated in DR and RB dye wastewater treatment.The results demonstrated that PS-g-DMC and LBF were branched flocculants,and the extension state of PS-g-DMC was better than that of LBF.PS-g-DMC and LBF exerted the similar flocculation efficiencies,it demonstrated that the impurities of paper mill sludge did not weakened the performance of PS-g-DMC.The performance of PS-g-DMC was also investigated compared with commercial polyaluminium chloride(PAC)or cationic polyacrylamide(CPAM).The results indicated that flocs formed by PS-g-DMC were large,loose,resistant,easy to regenerate and settle.Accordingly,the flocculation efficiencies of PS-g-DMC were more excellent than PAC or PAC+CPAM coagulation systems.Besides,PS-g-DMC system was more pH-independent than PAC or PAC+CPAM coagulation systems,the color removals could attain 90%at the pH range of 2.0 to 10.0 in DR dye removals However,the performances of PAC and PAC+CPAM coagulation systems were affected by the initial pH,which exerted good color removals at pH=4.0?6.0(5)PAC&PS-g-DMC composites and PAC+PS-g-DMC dual-coagulation systems with different PAC basicities and PAC/PS-g-DMC mass ratios were discussed in DY dye wastewater treatment.The results indicated that PAC&PS-g-DMC composites were new products instead of the mixture of PAC and PS-g-DMC solutions.There were some reactions between PS-g-DMC and PAC,which weakened the efficiencies of PAC and PS-g-DMC.Therefore,although PAC&PS-g-DMC composites were more efficient than PAC,the flocculation efficiencies of the composites were not as effective as PAC+PS-g-DMC dual-coagulation systems.The lower the PAC basicities and PAC/PS-g-DMC mass ratios,the more excellent the flocculation efficiencies of the PAC&PS-g-DMC composites.However,PAC+PS-g-DMC dual-coagulation systems with high PAC/PS-g-DMC mass ratios performed well.Compared with single PAC system,the addition of PS-g-DMC reduced the dosages of PAC,broadened the pH application range,improved floc properties and enhanced the sedimentation abilities of flocs.(6)The efficiencies of PS-g-DMC in practical printing and dyeing wastewater treatment were investigated.The results indicated that PAC mainly removed humic acid-like pollutants and dye particles,while PS-g-DMC could also remove tryptophan-like pollutants.Therefore,PS-g-DMC could reduce the concentrations of chemical oxygen demand(CODCr)more effectively than PAC or PAC+CPAM coagulation systems.Although PAC+PS-g-DMC coagulation system was more efficient than PAC system,the effluent quality and floc properties were not as effective as PAC+CPAM dual-coagulation system.PS-g-DMC was used to treat the effluent of PAC system,the color and CODcr removals were significantly enhanced compared with single PAC system,the enhancements were 16.2%and 13.3%,respectively.Considering the pH requirement of the subsequent anaerobic biological treatment in practice application,the pH of the test water was adjusted to 7.0.The color removal,turbidity removal and CODCr removal of PAC-PS-g-DMC secondary coagulation system were 90.2%,82.5%and 32.6%,respectively.Compared with PAC system,the enhancements in color,turbidity and CODCr removals were 23.0%,27.9%and 16.6%.Besides,the effluent pH of the PAC?PS-g-DMC secondary coagulation system maintained at 7.10±0.03,which met the pH requirement of the subsequent biological treatment.