Preparation Of Agricultural Anionic Sorbent And The Study Of Its Adsorption Properties

Based on the related literatures and synthetizing references,a series of anionic sorbents were prepared using wheat straw and corn straw from agriculture residues. The physical and chemical characteristics of agriculture reside and modified agriculture residues were examined by lots of instruments,such as scanning electronic microscope,particle electrophoresis instrument,element analysis instrument,FT-IR spectra and X-ray diffraction instruments.According to the Static adsorption experiment,the influence of initial pH,adsorbent dose,concentration and temperature to adsorption effect were investigated.In addition,the adsorption thermodynamics and the adsorption kinetics were studied systematically.Furthmore,the technologies of desorption and reuse for the modified agriculture residues and technological applications for the waste water were also studied.The main contents and results are following;1.The modification of agriculture residues.Agriculture residues were modified by epichlorohydrin and dimethylamine with pyridine as catalyst in N,N-dimethylformamide(DMF).The modification conditions,such as material ratio, reaction temperature and reaction time,must be controlled strictly,which could affect the properties of the modified agriculture residues.2.The modification mechanism of agriculture residues.The modification mechanism was gussed as following;The synthetic reactions for the modification method utilizing cellulose as a starting material.The use of pyridine and DMF enhances the susceptibility of the epoxide ring to attack the hydroxyl groups in cellulose.Toward this end,Lewis acid is able to lower the activation energy for ring opening and permit subsequent attachment of epichlorohydrin into cellulose.Reaction with dimethylamine is believed to occur after ring opening of the epoxide group and condensation through chloromethyl group in an excess of epichlorohydrin. Epichlorohydrin attached to the cellulose in alkaline conditions has been demonstrated to be the most likely sites for the attachment of bioactive agents such as amines and imines groups 3.The changes of physical and chemical characteristics of modified agriculture residues.The results of scanning electronic microscopic measurements of the structure of modified agriculture residues and raw agriculture residues showed that the surface of modified agriculture residues was smoother than that of raw agriculture residues,and there were lots of holes in the lengthways residues.The surface areas of raw agriculture residues and modified agriculture residues were all lower than 0.1 m².g^-1,which indicated that the adsorption type of modified agriculture residues was belong to gel adsorption.The Zeta potential changed from -35 mV to +40 mV.The nitrogen content of modified agriculture residues was about 3.5%,almost ten times bigger than raw agriculture residues,and the related Total Exchange Capability enhanced about 10times.The FTIR Spectrum showed that the main structure of modified agriculture residues was not altered,but some of the associated functional groups had been modified.The appearance of a conspicuous peak at 1360cm^-1in RWS FTIR,which was associated with tertiary amino group(vC-N-stretching vibration).XRD analysis showed that the crystall degree of cellulose was changed bigger after modefication,which was because of the dissolving of part of hemi-cellulose,lignin,ash and extractives during the process of synthesis.4.The influence of initial pH,adsorbent dose,concentration and temperature on adsorption effect.The removal ration was bigger with the increasing of adsorbent dose.It was founded that the suitable adsorbent dose was at 8 g·L^-1.The higher value of pH was benefical to the adsorption efficiency of anionic ion,which could make the removal rations for nitrate and phosphate higher above 90% at the range of 5～10.At differeft initial concentrations,the adsorption curves of nitrate and phosphate had similar trends,which was a fast adsorption process that could be reached equilibrium in 30 minutes.The maximum adsorption capacity of phosphate would decrease with the temperature up,which indicated the adsorption was of exothermic reaction.From the values of E_a,the temperature did not affect the adsorption efficiency greatly, which indicated that the modifided agriculture residues could be used in different seasons and had better adsorption efficiency.5.The study of the adsorption thermodynamics.It was found that the modified agriculture residue's isotherm for nitrate and phosphate fit the Langmuir and the Freundlich isotherms,but Freundlich isotherm mode was more suitable,which indicated the adsorption types of nitrate and phosphate were in among mono-molecure layer and multi-molecure layer,and the adsorption surface was heterogeneous. The maximum adsorption capacity of modified agriculture residues enhanced greatly than raw agriculture residues.In addition,the maximum adsorption capacity of modified wheat straw for nitrate and phosphate were 2.42mmol/L and 2.38mmol/L, respectively,which higher than that of modified corn straw and other biosorbents reported in literatures.In theadsorption process,the enthalpy change(â–³H)was negative and increased with the increasing of q_e,which domenstrated that th the adsorption process was of exothermic process.The free energy change(â–³G)was negative,which indicated the adsorption process was spontaneous.The entropy change(â–³S)was also negative because of the increasing of order caused by the new ion-bonds and the decreasing of disorder in aqueous solutions.6.The study of the adsorption kinetics.It was found that the adsorption of nitrate and phosphate was a fast adsorption process that could be reached equilibrium in 30 minutes.The adsorption process was mainly fitted the the pseudo-second-order equation and intra-particle diffusion model.In addition,the intra-particle diffusion was the main rate-controlling step,and the rate constant of pseudo-second-order equation decreased with the increasing of initial concentration.When multiple anions (SO₄₂^-,H₂PO₄^-,NO₃^-,and NO₂^-)were co-present,the modified agriculture residues also had better adsorption efficiency,and the absorption order followed as SO₄₂^-＞H₂PO₄^-＞NO₃^-＞NO₂^-.The apparent activation energy(Ea)of adsorption process had smaller value between physical adsorption and chemical adsorption,which indicated the adsorption occored by strong electrostatic interactions.7.The study of the desorption and reuse technology.It was found,after using a peride time,the maximum adsorption capacity and adsorption efficiency were decreasing gradully caused by the changing of hole on the surface of adsorbent and some impuritical substances adsorbed.Thus,the modified adsorbents need to be desorbed some impuritical substances.Fromthe value of apparent activation energy (Ea),the modified adsorbent was suitable for the reuse treatment because of the properties of fast adsorption and fast desorption.Generally,the desorption ration of the modified adsorbent would be reached 90% by using 150 ml NaOH to 1.0 g adsorbent.The adsorption-desorption cycle was repeated twelve times,the results showed that the static adsorption capacities of modefied adsorbent did not change obviously,which demonstrated that the modified adsorbent had higher practical value in treatment of nitrate and phosphate pollution.8.The study of the practical treatments.The practical treatments were studied by using modified wheat straw and modified corn straw to dispose of reservoir water, municipal waste- water plant water,and chemical plant water.The influences of initial pH and adsorbent dose to adsorption effect were investigated,and the influence of other co-exist anionic ion was also studied simultaneously.It was founded that the higher value of pH was benefical to the adsorption efficiency of anionic ion,and as well as higher adsorbent dose.When the value of pH was at 5～10,it would get better removal efficiency to the practical water.As the adsorbent dose at 2 g·L^-1,the removal rations for sulfate,phosphate,and nitrate were reached about 100%,and for nitrite higher above 90%.In addition,as the adsorbent dose at 4 g·L^-1,the removal rations for different anionic ions were reached about 90%.The absorption order for co-exist anionic ions was simlar to the competive adsorption experiment.The above contents and results gave a systemical study for the preparation of agriculture residues,the adsorption properties of adsorbent,the desorption and reuse technology,and the practical treatments.Furthmore,according to the properties of agriculture and the characterizations of the adsorbent,the modification mechanism of agriculture residues synthic and the adsorption mechanism were interpreted primarily, and fitted the thermodynamics and kinetics molds,which would provid basic theory for the practical treatments.