Research On Syntheses And Performances Of Lignin-based Water Absorbent And Heavy Metal Ion Adsorbent Materials

Lignin is an abundant and renewable biomass resource and its development and utilization is of great value. The main source of industrial lignin is pulping process & paper making industry. Most of the lignin from pulping process & paper making industry is incinerated and discharged directly without any treatment, which makes great resource waste and severe pollution. Thus, rationally utilize the natural renewable resource lignin and decrease the environment pollution is of great significance. Increasing researches on lignin show that many kinds of industrial lignin and their modified products feature high adsorption for many heavy metal ions. Superabsorbent resin is a new functional material with high water absorbency and water retention capability and can also be used to adsorb heavy metal ions. To combine the features and functions of lignin-based adsorbents and superabsorbent resins and the development tendency of wastewater treatment, three lignin-based water absorbent and heavy metal ion adsorbent materials were synthesized and their water absorbency, water retention and adsorption for Pb2+, Zn2+, Cd2+ were studied.The lignin-based water absorbent and heavy metal ion adsorbent material (LS-g-P(AA-co-AM)) was synthesized by graft-copolymerization of renewable biomass resource lignin with acrylamide and partially neutralized acrylic acid through solution polymerization. The optimized synthesis conditions, acquired through Taguchi experimental design and miniTab software analysis were as follows. Crosslinker N,N'-methylene-bisacrylamide CNMBA 7.5×10-4 mol/L, initiator potassium persulfate CKPS 4.0×10-3 mol/L, magnesium lignosulfonate CMgLS 2.5 g/L, neutralization degree of acrylic acid NAA 60 and mole ratio of acrylamide to acrylic acid RAM/AA 1:1. Reaction duration and temperature were programmed as 55℃for 2.5 hrs, then 65℃for 2.5 hrs and followed by 75℃for 3 hrs to guarantee complete copolymerization and therefore, good products. The optimized final (LS-g-P(AA-co-AM)) features good water absorption, repeat water absorption and water retention under natural condition and/or high temperature. The confirmed maximum water absorbency,0.09% saline absorbency and 0.9% saline absorbency of the optimized final (LS-g-P(AA-co-AM)) is 1156 g/g,451 g/g and 122 g/g, respectively.On the basis of synthesis of (LS-g-P(AA-co-AM)), the cheap inorganic Bentonite (BT) was employed to in-situ react with (LS-g-P(AA-co-AM)) to prepare LS-g-P(AA-co-AM)/BT composite water absorbent and heavy metal adsorbent material. The optimal dosage of Bentonite is 15g/L. The maximum water absorbency,0.09% saline absorbency and 0.9% saline absorbency of the optimized final (LS-g-P(AA-co-AM)/BT) is 1020 g/g,397 g/g and 109 g/g, respectively. After composition, the material's strength after water absorbing, repeat water absorption, water retention and thermal stability are improved while its cost is lowered.Again, on the basis of synthesis of (LS-g-P(AA-co-AM)), the cheap, renewable biomass resource-starch (St) was introduced into the lignin-based water absorbent and heavy metal ion adsorbent material to synthesized lignin/starch-graft-acrylic acid-co-acrylamide (LS/St)-g-P(AA-co-AM). The optimal dosage of starch is 5 g/L. The maximum water absorbency,0.09% saline absorbency and 0.9% saline absorbency of the optimized final (LS-g-P(AA-co-AM)/BT) is 1299.6 g/g,296.9 g/g and 85 g/g, respectively. After the introduction of starch, the material's equilibrium water absorbency, repeat water absorption, water retention are also improved while its cost is lowered.SEM photographs shows that the three kinds of lignin-based water absorbent and heavy metal ion adsorbent materials, LS-g-P(AA-co-AM), LS-g-P(AA-co-AM)/BT and (LS/St)-g-P(AA-co-AM) have formed porous network structure. LS-g-P(AA-co-AM) shows a porous sponge net structure. For LS-g-P(AA-co-AM)/BT, Bentonite is stripped and dispersed in the porous sponge net structure of LS-g-P(AA-co-AM). However, (LS/St)-g-P(AA-co-AM) has formed honeycomb three-dimensional net structure.Thermogravimetric analysis indicates that LS-g-P(AA-co-AM), LS-g-P(AA-co-AM)/BT and (LS/St)-g-P(AA-co-AM) feature good thermal stability. Their initial weight-loss temperatures at the maximum mass-change stage are higher than 330℃and their maximum weight-loss temperatures are higher than 360℃.The introduction of lignosulfonate into the superabsorbent resin has increased the ion strength in the network of superabsorbent resin. High ion strengths and porous network structures are the foundations for good performances of the three lignin-based water absorbent and heavy metal ion adsorbent materials.The kinetic behaviors of LS-g-P(AA-co-AM), LS-g-P(AA-co-AM)/BT and (LS/St)-g-P(AA-co-AM) absorbing distilled water and 0.09% saline can be simulated by equation dQ/dt=k(Q-Qt)2.LS-g-P(AA-co-AM), LS-g-P(AA-co-AM)/BT and (LS/St)-g-P(AA-co-AM) all shows good adsorption for Zn2+, Pb2+ and Cd2+ cations with fast adsorption rates and high adsorption capabilities.As the adsorbent dosage is 1.000 g/L and initial ion concentration is 2.000 mmol/L, the equilibrium adsorption density of LS-g-P(AA-co-AM) for Zn2+, Pb2+ and Cd2+ is 1.3552 mmol/g,1.6021 mmol/g and 1.1902 mmol/g, respectively. Under the same conditions, the equilibrium adsorption density of LS-g-P(AA-co-AM)/BT for Zn2+, Pb2+ and Cd2+ is 1.1239 mmol/g,1.5612 mmol/g and 1.4413 mmol/g, respectively, and the equilibrium adsorption density of (LS/St)-g-P(AA-co-AM) for Zn2+, Pb2+ and Cd2+ is 1.2339 mmol/g,1.4456 mmol/g and 1.4690 mmol/g, respectively. The adsorption mechanism for lignin-based water absorbent and heavy metal ion adsorbent materials-heavy metal ion adsorption systems is not unique and may include chemical adsorption, ion-exchange adsorption, physical adsorption and other mechanisms.The kinetic behaviors for LS-g-P(AA-co-AM), LS-g-P(AA-co-AM)/BT and (LS/St)-g-P(AA-co-AM) adsorbing Zn2+, Pb2+ and Cd2+ can be well simulated by Lagergren quasi-secondary kinetic equation. Both the adsorbent concentration and adsorbate concentration show influences on the adsorption of lignin-based water absorbent and heavy metal ion adsorbent materials-heavy metal ion adsorption systems. Equilibrium adsorption density increases with initial ion concentration (adsorbate concentration) and decreases with adsorbent concentration.Except Freundlich adsorption isotherm equation can be barely used to simulate the adsorption of LS-g-P(AA-co-AM)-Zn2+ adsorption system, Langmuir adsorption isotherm equation and Freundlich adsorption isothermal equation can not be employed to describe the adsorption of lignin-based water absorbent and heavy metal ion adsorbent materials-heavy metal ion adsorption systems.Since the three lignin-based water absorbent and heavy metal ion adsorbent materials, LS-g-P(AA-co-AM), LS-g-P(AA-co-AM)/BT and (LS/St)-g-P(AA-co-AM) feature good comprehensive performances, i.e., high water absorbency, good water retention, high salt resistance and good adsorption for Zn2+, Pb2+, Cd2+ ions, these three lignin-based water absorbent and heavy metal ion adsorbent materials may be hopefully used in water absorption & retention, saline and alkaline land treatment & improvement, fertilizer absorption & retention, wastewater treatment and other fields.