Preparation Of Chitosan Nano-iron And Restoration Of Surface Water Hexavalent Chromium Pollution

Surface water is the most important water source for human beings.However, with the development of industry and economy,Cr(â…¥) is widely detected in surface water.Cr(â…¥) anions,including chromate(CrO₄^2-) and dichromate(Cr₂O₇^2-),are highly soluble in aquatic systems and are severe contaminants to environment.Much concern has been paid on the technology for Cr(â…¥) contamination remediation.Recently,due to large specific surface area and more active sites the use of zero valent iron(Fe⁰),especially Fe⁰ nanoparticles,as reactive media for in situ subsurface environment remediation has been extensively investigated.Applications to in situ remediation require the Fe⁰ nanoparticles to be stable in water.However,due to van der Waals forces and magnetic interactions,these Fe⁰ nanoparticles tend to agglomerate and grow rapidly to micrometer or millimeter scale particles,thereby diminishing their mobility and chemical reactivity.On the other hand,Fe⁰ nanoparticles show high activity with large specific surface area,but are easily oxidized by air or ignite spontaneously when exposed to air.Therefore,intensive efforts have been made to coat and protect Fe⁰ nanoparticles from agglomeration and air oxidation.This research work was supported in part by Tianjin Natural Science Foundation of China under grant No.07JCZDJC01800.The objectives of this research are to:(1) to prepare chitosan-Fe⁰(chitosan-Fe⁰) by modification of solution method.(2) to evaluate the capacity of chitosan-Fe⁰ to remove Cr(â…¥) from surface water through batch experiment and column experiment.(3) to investigate the kinetics and mechanisms of Cr(â…¥) removal from water by chitosan-Fe⁰.There are four main parts in this research work:1.The condition for preparation of chitosan-Fe⁰ was optimized:chitosan was dissolved in 0.05 mol/L HNO₃ to make the final concentration of 0.5%by weight. Finally,chitosan solution was filtered through 0.22μm syringe filters to remove any suspensions.Chitosan-Fe⁰ was prepared in situ by reducing Fe(â…¡) with KBH₄ in the presence of chitosan as a stabilizer.To ensure all the Fe(â…¡) were reduced,excess of KBH₄ over the Fe(â…¡) was used.The detailed procedure was as follows:10 mL of solution containing 0.2978 g of FeSO₄·7H₂O was first mixed with 3 mL of 0.5% chitosan solution.The mixture was stirred for 30 min under nitrogen gas.Then,to the mixture,10 ml of freshly prepared aqueous solution containing 0.2889 g of KBH₄ was added dropwise.At this stage,gas was evolved vigorously and black precipitation was formed.Again,the mixture was stirred for another 90 min.The resulted black precipitate was collected and washed by deoxygenated water for three times to get rid of the excess chemicals.2.The results of characterization indicated that the distribution of particle size of chitosan-Fe⁰ is widespread ranging from 20 to 150 nm with a mean diameter of 82.4 nm and has good stability against oxidation.Compared with EW-Fe⁰,the chitosan-Fe⁰ can stay stable in water.3.The capacity of chitosan-Fe⁰ to remove Cr(â…¥) from surface water was examined through batch experiment and column experiment.The ability of chitosan-Fe⁰ to remove Cr(â…¥) was far greater than that of EW-Fe⁰ and 200 mesh iron powder with a Cr(â…¥) removal capacity at approximately 148.08 mg Cr(â…¥) per gram of Fe⁰ nanoparticles.Ca²⁺,Mg²⁺,CO₃^2-,HCO₃^-,organic matters and dissolved oxygen (DO) in the surface water have negative effect on the Cr(â…¥) removal capacity of chitosan-Fe⁰.However,chitosan-Fe⁰ still show the superiority over bulk material.4.Due to the adsorption and reduction,chitosan-Fe⁰ can remove Cr(â…¥) from water rapidly.The rate of reduction of Cr(â…¥) to Cr(â…¢) can be expressed by a pseudo-first-order reaction kinetics.The rate constants increase with the increase in temperature and iron loading but decrease with the increase in initial Cr(â…¥) concentration and pH.It is found that the amount of Cr(â…¥) been adsorbed is proportional to the aqueous Cr(â…¥) concentration.The apparent activation energy is found to be 33 kJ mol^-1,which is characteristic of a chemically controlled reaction. Chitosan can hinder the formation of Fe(â…¢)-Cr(â…¢) precipitate by virtue of its coordinating capability consequently enhances the reduction of Cr(â…¥) to Cr(â…¢).