Study On The Preparation And Defluoridation Properties Of Modified Chitosan Using New Crosslinking Agent

Fluoride pollution in drinking water is the major cause of endemic fluorosis spreading in China. Various technologies are adopted to reduce the excessive fluoride concentrations. Due to its simplicity and low cost, adsorption is one of the most mature and widely used defluoridation technologies, especially for distributed drinking water treatment. The key technique is the preparation and application of an efficient, safe and economic fluoride agent. In this study, a new absorbent which can remove fluoride from drinking water is prepared using Lanthanum incorporated chitosan beads which have been cross-linked with ethylene glycol diglycidyl ether (EGDE) previously. The CEB-La's preparation, adsorption conditions, physical and chemical characteristics, preparation and adsorption mechanism etc., are discussed. Drawing conclusions as follows:(1)The synthesis conditions of CEB-La are optimized by using orthogonal experiment: chitosan beads are cross-linked with EGDE for 4 hours at the temperature of 20℃under the condition that the molar ratio of EGDE and amino is 1:2. Then 2g·L^-1 cross-linked chitosan beads (CEB) are chelated with La³⁺(Concentration 0.020mol·L^-1) under solution temperature 40℃for 3 hours. The order of factors that affect defluoridation rate is: Crosslinking temperature> Dosage of CEB> Chelating time> Crosslinking time> Dosage of EGDE. Meanwhile, stability test shows that adsorbent is undissolved in water at pH between 7.0 and 8.5. Its solubility slightly increased with lower pH value.(2)Static and dynamic adsorption of fluoride onto absorbents (CEB-La) was respectively investigated by means of single factor experiments. The effects of various physico-chemical parameters are studied. Static experiment results shows that the optimal adsorption conditions are: pH value is 7.0, water temperature is 50℃, contact time is 30 min, oscillation frequency is 120 rpm and CEB-La dose is 3g·L^-1. Under this condition, the defluoridation rate is up to 92.9% for water that contains 10mg·L^-1 F^-. Besides, the residual concentration of F^- is 0.71mg·L^-1 which is lower than 1.0mg·L^-1 that restricted by Standards for Drinking Water Quality. Furthermore, the sorption equilibrium data fits well both to Langmuir and Freundlich models; the maximum adsorption capacity for F^- is 25.7 mg·L^-1. Sorption dynamics study reveals that CEB-La's adsorption follows pseudo-second-order and its adsorption process can be mainly ascribed to the chemical nature of adsorption (chemisorptions). The sorption process is complex: liquid film boundary and intra-particle diffusion both contribute to the rate-determining step. The removal efficiencies of F^- are related to the co-existing anions: Cl- and NO₃^- has not obvious negative effect; SO42– has great influence; the presences of CO₃^2- and HCO3- lead to a markedly decrease adsorption capacity of CEB-La for F^-.Micro-column test (column internal diameter 20mm and column length 80mm) is used to investigate dynamic adsorption performance. The results demonstrate that: CEB-La is applied to defluorinating water containing 2¹5mg·L^-1 F^- when the volumetric flow rate is 3ml·min^-1. If the concentration of F^- is 2 mg·L^-1 and 5 mg·L^-1, respectively , the breakthrough time is 240min and 120min, corresponding breakthrough volume is 720ml and 360ml. The dynamic adsorption capacity derived from column kinetics described by Thomas model is 3.67mg·g^-1. Simultaneously, the Thomas rate constant calculated from this model is between 3.26⁶.20ml·mg^-1·min^-1.(3)The used adsorbents can be regenerated by chelating with lanthanum after soaking in 0.5mol·L?1 of sodium hydroxide for 6h. The adsorption capacity is almost maintained after regenerated ten times. Thus, CEB-La has a commendable reusability. Combining with the dynamic test results, this absorbents is practical that the calculated application cost is (about) $0.30⁰.60 per day.(4)The preparation and adsorption mechanism is discussed by FT-IR and XRD. The XRD spectra shows the crystallinity of CEB and CEB-La decrease distinctly compare to that of chitosan beads which in favor of loading Lanthanum on CEB and Fluoride adsorption to CEB-La. The FT-IR spectra indicates that EGDE reacts with amino and primary hydroxyl groups in chitosan, Lanthanum coordinates with second hydroxyl group in CEB. Sorption mechanism analysis demonstrates that CEB-La removes fluoride by means of not only chemical adsorption but also hydrogen bonding. Coordination reaction between second hydroxyl group in CEB and La³⁺, ion-exchange reaction between CEB-La and F^- is confirmed by phase identifying results of XRD.