| Papermaking sludge is the by-product of the wastewater treatment in the pulp and papermaking industry.It has the characteristics of complex composition,high water content and high treatment cost.Up to now,the recycle methods of papermaking sludge focus on the following aspects:composting treatment,soil conditioner,activated carbon production,permutite production and building materials.Due to the complex composition of papermaking sludge,these methods cannot be widely appied,and are likely to cause secondary pollution.The biomass content in papermaking sludge is abundant.So making full use of these biomasses and producing the high value-added products are meaningful.In this paper,papermaking sludge was used as the raw biomass material to produce the cationic lignin-based flocculant(LBF)by grafting quaternary ammonium groups and acrylamide.And the synthesis process and parameters were optimized.In order to confirm the morphology and molecular structure of LBF,the physico-chemical properties were characterized by several analytical technologies.And then LBF was used with metal salts to treat simulated and actual water samples.The influence of coagualation conditions on the coagualation behavior,coagulation mechanism and floc properties was studied.To confirm application potential of LBF,the coagulation efficiency in the treatment of different water samples was compared with other common flocculants,and the economical analysis was discussed.Finally,LBF+polyaluminum chloride(PAC)dual-coagualation was combined used with ultrafiltration and chlorination.The effect of LBF addition on the membrane fouling and disinfection by-products yield was analyzed.The main conclusions are listed as follows:1.Lignin supernatant was obtained by dissolving the papermaking sludge into aqueous alkali and centrifuging at high speed.LBF was synthesized by grafting dimethyl diallyl ammonium chloride(DMDAAC)and acrylamide in the presence of potassium peroxodisulfate and EDTA-Disodium.Based on the productivity,zeta potential and the color removal ratios of reactive blue and disperse yellow,the synthetic reaction parameters were optimized as follows:Solution pH was 7.5;the mass ratio of papermaking sludge to EDTA-2Na was 2:0.05(g/g);the mass ratio of papermaking sludge to acrylamide was 2:2.5(g/g);the mass ratio of papermaking sludge to DMDAAC was 1:4(g/ml),the reaction time was 4 h;the reaction temperature was 70 ℃.Characterization results showed that DMDAAC and acrylamide were grafted onto the hydroxy of lignin.LBF was a cationic polymer.And it had hydroxyl,amido and quaternary ammonium groups.LBF with the highly branched structure had the relatively better thermostability and water solubility.Therefore,LBF could introduce the strong charge neutralization and adsorption bridging effects.2.In the treatment of simulative dye wastewater samples,LBF could offer the obvious coagulation aid effect in both ferric based coagulation and aluminium based coagulation.That is,the flocculation efficiency of LBF was not affected by metal salt types.LBF combined with polyaluminium chloride showed the high color removal ratios in the treatment of reative and disperse dye wastewater.And in the acid scarlet wastewater sample,the color removal ratios increased compared with PAC coagulation.But the color removal was not obvious in the methyl orange wastewater sample,which was related with the functional groups and molecular weight.Dual-coagulation also showed the significant removal efficiencies of humus,particle and natural organic matter.The primary coagulation mechanism was related with the structure of contaminants.Sweeping effect and charge neutralization were the main coagulation mechanisms in the treatment of disperse yellow and reactive blue wastewater,respectively.And natural organic matter and particle were easily removed by charge neutralization and sweeping effect,respectively.LBF introduced charge neutralization and absorption bridging effects to enhance the flocculating efficiency,which was not affected by water quality.Initial pH had a significant influence on the efficiency of metal salts-based coagulation.Under the faintly acid condition(pH 5-6),the coagulation efficiency was larger,and it showed a decrease under alkaline condition(pH>9).Addition of LBF could decrease the influence of pH on coagulation performance.As a result,the color removal ratios changed less during pH range of 5-9.In the treatment of natural organic matter,the optimized pH range expanded to 5-8.Dual-coagulation could be applied water samples with the broad pH range.In the treatment of three kinds of water samples,dosing sequence also had an influence on the coagulation behavior.Dosing metal salts firstly showed the optimal coagulation performance.And it was related with the mechanism of floc formation.Moderate hardness could enhance the coagulation efficiency.But when hardness concentration was too high,the coagulation efficiency would decrease due to the competition between Ca2+ and coagulants.3.Compared with other common flocculants,flocs formed by PAC+LBF dual-coagulant in the treatment of disperse yellow and reactive blue water samples had the relatively larger size,the stronger anti-crush ability,the larger recovery factor,the faster sedimentation rate and the more open structure.The optimal floc properties were acquired at pH 5-6,where charge neutralization was the primary mechanism.Addition of LBF could enchance floc size and growth rate under alkaline condition.When the hardness concentration was low,floc size,recovery ability and compact structure showed an enhancement in the PAC and polyferric chloride(PFC)coagulation.That was because the chelation and charge neutralization of Ca(II).When hardness concentration was too high,recovey factor of floc showed a decrease.When stirring force was introduced,floc showed the weak recovery ability.Addition of LBF could decrease the influence of stirring speed on floc properties.And the floc strength and recovery factor increased accordingly.When the stirring speed was larger than 300 rpm,the recovery factor of flocs formed by PAC+LBF increased by 5-18%.In the treatment of humic acid-kaolin water sample,PAC+LBF or PFC+LBF dual-coagulants could enhance floc properties.The optimal floc properties was formed at pH 6.Dosing sequence also had an influence on the floc properties.Flocs coagulated by PFC+LBF had the better properties,which was because flocs were formed by bridging and charge neutralization of LBF.In the treatment of Xiaoqinghe River,the properties of flocs formed by LBF were better than that of PDMDAAC.And dual-coagulation could decrease the influence of pH on floc property.4.Compared with other common anionic,cationic and neutral flocculants,PAC+LBF dual-coagulant showed the best coagulation efficiency in the treatment of disperse yellow wastewater.And the sedimentation time could save 5 min.As for reactive blue wastewater,the coagulation aid of LBF and cationic polyacrylamide(CPAM)was more obvious.And And the sedimentation time could save 10 min.When the same color remova ratio was acquired,LBF combined with metal salts could decrease the chemical cost and sludge yield.In the treatment of humic acid-kaolin and xiaoqinghe water samples,LBF offered the optimal flocculation efficiency compared with other common cationic flocculants.That was because LBF introduced the relatively stronger charge neutralization and bridging effect.5.LBF+PAC dual-coagualant was used in the coagulation-ultrafiltration combined process to treat xiaoqinghe River.Strongly-attached external fouling was the primary fouling mechanism.The slighter fouling was achieved after dosing coagulant aids due to the introduction of polymer bridging and electrostatic patch effects.And the coagulant aid of LBF was more significant compared with PDMDAAC.Addition of LBF could accelerate the formation of cake layer with more pores and further control internal fouling.PAC+LBF mainly decreased external fouling resistance and reversible internal fouling to enhance permeation flux.And the total resistance was lowered by more than 50%compared with PAC.Initial pH showed a significant effect on membrane fouling.Under different pH condition,fouling index was co-determined by floc size and fractal dimension.The slightest membrane fouling was acquired at pH 5.5 and the relatively severe membrane fouling was acquired at pH 9.5.And the least interception action of cake layer occurred at pH 3.5.Addition of LBF could decrease the influence of pH on the fouling index and distribution.During the pH range of 3.5-7.5,the variance of membrane fouling changed less.Coagulation pretreatment and filtration process all could be used to decrease the yield of trihalomethanes(THMs),and dual-coagualtion played the most significant role.Compared with raw water,the THMs yield decreased by 6.8%and 49.6%in the filtered water sample and unfiltered water sample after treated by PAC+LBF,respectively. |
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