Preparation Of Biomass Based Cationic Adsorbent And Its Adsorption, Regeneration Properties For Different Anions

Based on the comprehensive analysis of numerous literatures, three kinds of methods (pyridine catalyst method, two-stage method and ethylenediamine crosslinking method) for modification of biomaterials based adsorbents were first determined in this work. The physicochemical properties and active functional groups of the potential biosorbents were explored and characterised by different techniques. The binding between the adsorbed anions and the functional groups were also evaluated. The adsorption of NO3-,PO43-, C1O4-and Cr(VI) by the biosorbents at different adsorption conditions (temperature, dosage and pH) were conducted; the parameters of adsorption kinetics and thermodynamics were intensively investigated. The adsorption parameters of fixed-bed columns were obtained at different influent conditions (flow rate, pH, influent concentrations and bed heights). Chemical desorption properties were then evaluated based on the column adsorption parameters. In addition, the loaded perchlorate on surface of biosorbents were reduced by the mixed perchlorate-reduction bacteria at different conditions and the reduction discussed in this chapter. The main conclusions were summarized as follows:1. In the pyridine catalyst method, the stage of catalyst (including the conditions of catalyst temperature and catalyst time) was the most important process for the biosorbents preparation. The reaction temperature was the essential factor that controlled the preparation of epoxy propyl triethyl ammonium chloride intermediation in two-stage method. In the ethylenediamine crosslinking method, the ethylenediamine crosslinking process was the control factor for the biosorbents preparation. It was observed that the removal efficiencies of NO3-and PO43-by biosorbents modified from thylenediamine crosslinking method was higher (5-15%) than those obtained from pyridine catalyst method, two-stage method. This indicated that the biosorbents modified from thylenediamine crosslinking method had shown higher adsorption capacities for anions. Result also indicated that the biosorbent originated from wheat stalk showed higher NO3-and PO43-uptake capacity as compared with those originrated from corn stalk, cotton stalk and reed.2. Biosorbents obtained from ethylenediamine crosslinking method showed the highest weight growth rate and yeild as compared with those of pyridine catalyst method and two-stage method. As a result, the ethylenediamine crosslinking method would be more commercial for preparation of biosorbents. Biosorbents obtained from ethylenediamine crosslinking method showed higher nitrogen content; this indicated that more amine functional groups were grafted into the biomaterials. The zeta potentials of the virgin wheat stalk, modified wheat stalks prepared by pyridine catalyst method, two-stage method and ethylenediamine crosslinking method were+2.1,+39,+31and+42mV, respectively. The results illustrated that a large amounts of positive charged amine groups were attached onto the surface of the virgin wheat stalk, so as the zeta potentials were increased. FTIR spectra indicated that the surface of modified wheat stalk was attached with amine groups and chloroalkane, forming the available adsorptive site for anions. Raman spectra indicated that the adsorption of nitrate, phosphate and perchlorate was mainly based on electrostatic attraction between the adsorbed anions and functional amine groups, and no chemical adsorption was involved. However, chemical bond was observed between the attached Cr(Ⅵ) and the biosorbent. As a result, electrostatic attraction, complexation reaction as well as other chemical binding might be involved for the Cr(Ⅵ) uptake by biosorbents.3. The adsorption of NO3-, PO43-, C1O4-and Cr(Ⅵ) by the wheat stalk based biosorbent was a quick adsorption process. The adsorption reached the equilibrium within10-20min. The pH was an essential factor that affected the adsorption process. The optimal adsorption was obtained at4.0-9.0for NO3-, PO43-, and C1O4-and2.0-4.0for Cr(VI). The adsorption isotherms were fit well with Langmuir model, and the maximum adsorption capacities (Qmax) obtained from this model were65.4,54.6,156.3and201.3mg/g for NO3-,PO43-, C1O4-and Cr(Ⅵ), respectively. Parameters of adsorption thermodynamics indicated that the adsorption of NO3-, PO43-, and C1O4-by the wheat stalk based biosorbent was a spontaneous and exothermic adsorption process. A decreased temperature would be in favour of the adsorption process. However, the negative values of△G,△H and△S obtained from Cr(Ⅵ) adsorption illustrated a endothenmic and spontaneous adsorption reaction between the Cr(VI) and the biosorbent. The adsorption was enhanced at higher adsorption temperature.4. The adsorption kinetics indicated that the adsorption of NO3-, PO43-, ClO4-and Cr(VI) by the wheat stalk based biosorbent followed pseudo-second-order kinetics equation. The activation energy calculated from the Arrhenius equation were in range of7.4-24.5kJ/mol, which implied that the adsorption was based on ion exchange. Results also indicated the linear relationship between the amouts of adsorbed anions and eluted C;-; this also confirmed the ion exchange mechanism for the anions adsorption by the biosorbent. A small amount of Cr (Ⅲ) was found in the Cr(Ⅵ) adsorption solution. In addition to this, XPS spectra also detected the Cr (Ⅲ) on surface of the biosorbent, which validated the Cr(VI) reduction reaction occurred on surface of the biosorbent.5. The fixed-bed column adsorption of various anions were controlled by the influent conditions such as influent concentration, flow rate, influent pH and bed depth. It was found that fixed-bed systems achieved a better uptake of NO3-, PO43-, ClO4and Cr(VI) by the biosorbent at a lower concentration, lower feed flow rate and biosorbent bed-depth. The suitable column adsorption conditions would be helpful for improving the efficiency of the column. As a result, the column adsorption conditions for the NO3-, PO43-, and ClO4-were determined as:bed depth2.7cm, influent concentration200mg L-1, feed flow rate5mL min-1and influent pH6.0. The column adsorption conditions for the Cr(VI) followed as:bed depth2.7cm, influent concentration200mg L-1, feed flow rate5mL min-1and influent pH3.0.。6. The chemical regeneration tests were conducted in the fixed-bed column with the0.1M HC1,0.1M NaOH and0.1M NaCl solutions. It was observed that the adsorbed anions could be desorbed in a short time. The desorption mechanism of HC1and NaCl was based on the reverse ion exchange between the high concentration of Cl-and adsorbed anions. After adsorpion of NO3-, PO43-and C1O4-, the spent bisorbent could be reused for several times with less capacity loss. However, the adsorption capacity of the Cr(VI) loaded biosorbent was significantly decreased after four cycles of adsorption-desorption tests. Although the bio-regeneration could reduce the loaded perchlorate to nontoxic chlorate, the biosorbent performance after bio-regeneration with mixed bacteria seemed to be inefficient as compared with the brine desorption technique. The enhanced bio-regeneration method was observed more promising in regeneration of the spent biosorbent as compared with that of direct bio-regeneration method.