Investigations On Adsorptive Property Of Anionic Pollutants Onto The Modified Tobacco Straw

Agricultural straw has a great storage and a low price, and modified it into adsorbent has become a kind of way of efficient utilization of it. But adsorbents on the market belonged to hybrid applied adsorbents mostly at present, were not targeted for a specific pollutant, could not achieved the ideal adsorption effect of specific pollutants. Therefore, study on the exclusive adsorbents to corresponding same type of pollutants were necessarily. In this research, tobacco straw as raw material, the lignin and hemicellulose in straw were removed by NaOH+H2O2 alkaline pretreatment firstily, and then, the straw fiber was cationization modified by dimethyl diallyl ammonium chloride ?DADMAC? and Al2?SO4?3, for preparation of the exclusive adsorbent for anionic pollutants. The best modification conditions were optimized by single factor experiment and orthogonal experiment. The adsorption effects of two kinds of anionic contaminants(Congo red and PO₄^3-) and two kinds of cationic contaminants(methylene blue and Cu²⁺) in water onto AD-MTS under controllable adsorption conditions were investigated to confirm the exclusive adsorbing property of AD-MTS to anionic pollutants. Characterization?IR and SEM?, adsorption kinetics and adsorption isotherm were investigated to estimate the adsorption behavior and adsorption mechanism of Congo red and PO₄^3- on AD-MTS. The main research results were as follows.?1? the best modification conditions of AD-MTS were as follows:TS:DADMAC ?g/ mL?was 1:2, TS:Al2?SO4?3?g/g? was 1:2, the reaction temperature was 80?, reaction tim ewas 2 h, TS:CAN ?g/g? was 6:1.?2? The specific surface area of AD-MTS was 90.76 m2/g, increased approximately 1 times than TS, which provided more active adsorption sites. In addition, the zeta potential of TS and AD-MTS was-10.9 mV and 24.7 mV, respectively. Also, electropositivity strengthened the charge neutralization between AD-MTS and anionic pollutants.?3? The results of IR of AD-MTS and TS showed that:lignin and hemicelluloses in TS has been removed in pretreatment process, and in modified reaction, DADMAC and Al?or poly aluminum group? has been introduced in AD-MTS successfully, generated a new cationic polymer which has Al salt characteristics. During the adsorption process of anion pollutants(Congo red and PO₄^3-) onto AD-MTS, the adsorption mechanism may mainly relied on charge neutralization, polymer bridging, hydrogen-bond interaction of cationic polymer, and the trap effect and complexation of poly aluminum group. SEM photos showed that, on the surface of AD-MTS, a lot of flakes crisscrossed overlap orderly and evenly, and embraced each other like a flower, which formed many tiny pores, whereas TS appeared irregular locks overlap structure, which was more density than AD-MTS. Modification brought AD-MTS more porosity and adsorption sites, enhanced its adsorbability. Comparied IR and SEM of AD-MTS to IR and SEM of the adsorbed Congo red saturated modified straw?G-MTS? and the adsorbed PO₄^3- saturated modified straw?P-MTS?, the results showed that the IR differences were due to various physical and chemical actions in the process of adsorption affecting the infrared absorption of corresponding functional groups in AD-MTS. And the structures of G-MTS and P-MTS were more density than AD-MTS.?4? The removal rate of Congo red and PO₄^3- were inversely proportional to solution pH, while the removal rate of methylene blue and Cu²⁺ were negatively consistent with pH. The removal effects of AD-MTS on anionic pollutants(Congo red and PO₄^3-) were obviously better than that of cation pollutants (ethylene blue and Cu²⁺) with any adsorbing conditions. The above was proved that AD-MTS has realized the efficient removal of anionic pollutants in water, it could be used as exclusive adsorbent for anionic pollutants, and the charge neutralization is a main adsorption mechanism.?5? The adsorption kinetics of Congo red onto AD-MTS followed pseudo second order kinetic model, which indicated that the adsorption process was dominated by chemical adsorption. The adsorption kinetics of PO₄^3- showed that the adsorption rate of PO₄^3- onto AD-MTS was controlled by the chemical adsorption and physical adsorption, and the former was the leading one. Intra-particle diffusion model fitting results showed that the intraparticle diffusion in pore played key roles in Congo red adsorption.while it could be neglected in PO₄^3- adsorption. Both Congo red and PO₄^3-'s adsorption behavior could be described well by Langmuir model in the 20?40? temperature range, and the maximum sorption capacity ?qm? were 537.63?549.45 mg/g and 26.10?27.16 mg/g, respectively, which indicated that the adsorption process of Congo red and PO₄^3- onto AD-MTS belonged to monolayer adsorption, and temperature had little influence on the adsorption process, the value of n got from Freundilch were all at around 5, indicated that the process of Congo red and PO₄^3- onto AD-MTS belonged to favorable adsorption.?6? Cl^-, NO₃^- and SO42- showed a inhibitory to the adsorption of PO₄^3- onto AD-MTS in acid environment, while showed a promoting effect in alkaline environment. The promoting effect of NO₃^- were the best of the three ions above. In the practical application of mixed wastewater, the adsorption rate of P could be improved by adjusting the pH val ue accordi ng to pol I utant sped es i n water.?7? At the same adsorbing condition, AD-MTS, compared with tobacco straw, pretreatmented tobacco straw, and commonly used adsorbent ?activated carbon?, has the best adsorbing efficiency on Congo red and PO₄^3-?8? In practical application, the appropriate desorbing conditions were as follows:at room temperature,0.05 mol/L NaOH for desorping Congo red and 0.1 mol/L NaOH for desorping FO₄^3-,30?60 min's desorption time. The removal rates of Congo red and PO₄^3- were 30?40% after adsorption-desorption recycling for three times, indicated that AD-MTS has certain regeneration recycling performance.