Preparation Of Cassava Stalk Into Anionic Adsorbent And Study Of Its Nitrate Adsorption Property

In China, due to the rapid development of urbanization and the growth of population, large amount of waste-water containing the elements of N and P was discharged, it caused a serious pollution-Eutrophication. Eutrophication will deteriorate the quality of water and the development of the aquaculture industry. So it is a serious threat to socio-economic development and people’s health. In the present technology of nitrogen and phosphorus removal, there are some shortcomings, such as high cost, and it is not suitable for processing large amount of waste water. Guangxi Zhuang autonomous region is the largest base of cassava (Manihot esculenta) production. Much of the cassava stalk were burned and discarded without proper utilization, resulting in waste of resources and serious environmental pollution. Recent years, researches on preparing functional polymers from agriculture residues have been investigated. Cassava stalk (CS) as a lignocellulosic agricultural residue was chemically converted into a weak basic anion exchanger and evaluated for their exchanger capacity for nitrate in this studyBased on the related literatures and synthetizing referenes, an optimal synthesis condition of the modified adsorbent was determined. Factors like material ratio, reaction time, reaction temperature were used in single-factor experiments and orthogonal experiments for determining optimal conditions, with nitrate removal efficiency as indicators. The results indicated that the optimal conditions obtained in the orthogonal experiments were consistent with those in the single-factor experiments, the time of catalysis was considered as the key. The optimal synthesis conditions obtained are as follows:2grams of the cassava stalk which was prepared with NaOH (4mol/L) for4hours was reacted with epichlorohydrin (100ml) in100ml of DMF in a500ml3-neck round bottom flask at100℃for1.5h.30ml of pyridine was added to the solution and stirring continued for1h at100℃. Amine groups were then introduced by adding30ml of33%(w/w) solution. The mixture was stirred for4h at100℃.The result of SEM measurements in the structure of modified cassava residues and raw cassava residues showed that the surface of modified residues was soother than raw residues, and a lot of holes were observed in the lengthways in modified residues. The changers of surface area between raw residue and modified residue were not clear.The Zeta potential changed from-20.5mV to+37.3mV. The nitrogen content of modified residues was much more than in raw residues. XRD analysis showed that the crystall degree of cellulose was changed bigger after modification. The IR spectrum showed that the modified residue was associated with tertiary amino group with C-N.In the experiment of removal of nitrogen, the best nitrate removal result was reached at pH range of4.0～10.0, it would get the best effiency when pH was nearly neutralized. The modified cassava straw adsorbed nitrate quickly, reaching equilibrium within30minutes. The kinetics of nitrate adsorption at different initial concentrations (25mg/L,50mg/L, and75mg/L) all fit a second order reaction. The adsorption process was fitted to pseudo-second-order equation.The adsorption rates were controlled by both membrane diffusion and intra-particle diffusion, but the intra-particle diffusion is not only the main rate-controlling step. The maximum adsorption capacity was decreasing when the temperature increasing. The values of parameters such as free energy (△G), enthalpy (△H) and entropy (△S) were also negative, and indicated that the adsorption was a spontaneous and exothermic reaction in nature, and the order of system was enhanced. On kinetic adsorption, the process will get a good efficency in the condition that the pH of the solution was6, and the velocity was5mL/min. The0.05mol/L NaCl、HCl、and NaOH solutions had good desorption capacities.