Perchlorate Destruction In Aqueous Solution Using Chitosan-stabilized Nanoscle Zero-Valent Iron

Perchlorate(4ClO), the dissociated anion of perchlorate salts such as sodium, potassium, andammonium perchlorate, has been recognized as a persistent contaminantin the environment. Perchlorate is not easilyabsorbed, nonvolatileand highly soluble in aqueous solution, and it isextremely stable and kinetically inert due to the high activation energy associated with its reduction. Perchlorate is a potent competitive inhibitor of the sodium-iodide symporter(NIS) found on the basolateral membrane of thyroid cells. In pharmacological doses, perchlorate decreases the active transport of iodine into the thyroid, result in potential adverse effects on human health. The U.S.EPA added perchlorate to its Contaminant Candidate List(CCL1) for drinking water in 1998, and added it to CCL2 and CCL3 in 2005, 2009, respectively.Perchlorate has beenclassified as a endocrine-disrupting chemicals(EDCs).Due to its uniquephysic-chemical properties, Perchlorate cannot be removed easily by conventional water treatment processes. Compared with other reduction methods(Biodegradation, Electrochemical reduction), Chemical reduction of perchlorate using stabilized zero-valent iron nanoparticles(n ZVI) offers a promising alternative to biodegradation and electrochemical reduction.Nanoscale zero-valent iron(nZVI) has been frequently used as a reactive nanomaterial for treatingvarious contaminants in wastewater and contaminated groundwater. NZVI have been shown to exhibit large surface area, high reactivity and distinguishedreductioncapacity, it can remove pollutantsby reduction and sorption. However, because n ZVI particles are easily oxidizedwhenexposure to oxygen and agglomerate rapidlydue to van der Waals and magnetic forces, which greatly reduces its stability and reactivity. Based on our prior work and the published literatures about strategise forstabilizing n ZVI particles. To overcome the drawbacks of non-stabilized n ZVI, chitosan was used as a stabilizer in preparing nanoparticles. Chitosan is a low-cost and environmentally friendly polyelectrolyte and carries carboxylate groups and hydroxyl groups. Thus, chitosan is expected to interact with n ZVI particles to preventthe nanoparticles from agglomeration and maximize its reactivity. Consideringthe good adsorption ability of chitosan for perchlorate and the dispersibility characteristicsstabilized n ZVI, chitosan stabilized nanoscale zero-valent iron(CS-n ZVI)wassynthesized to test its feasibility for perchloratedestruction in water. This work aimed to(1) investigate the optimized preparation conditions of CS-n ZVI for perchlorate removal,(2) to examine the influences of CS-n ZVI dosage, temperature, salinity, initial perchlorate concentration and p H on the reactivity of the CS-n ZVI,(3) to discuss the degradation mechanism of perchlorate by CS-n ZVI.The well-dispersed CS-n ZVI particles were synthesized using chitosan as astabilizer, the size of the freshly prepared CS-n ZVI particlesranging from 100 to 200 nm. The CS/Fe2+mass ratio and ethanol concentration in the system will have an impact on perchlorate removal efficiency, the optimizedpreparationconditionsasfollows: the mass ratio CS/Fe2+ was 0.5/1 and the volume fraction of ethanol was 50% in the system. The ambient temperature of the synthesis of CS-n ZVI has little influence on perchlorate removalefficiency. Different kinds of stabilizers(sodium carboxymethyl cellulose, starch, quaternary chitosan salt, carboxymethyl chitosan)were used in the preparation of stabilized n ZVI particles and compared their removal efficiency for perchlorate with CS-n ZVI,The result showed that CS-n ZVI was more conducive toperchlorateremoval.Theremoval rate of ?4Cl O by CS-n ZVI was significantly greater than non-stabilized n ZVI at the same conditions, and ZVI almost had no degradation on ?4Cl O in aqueous solutionat room temperature. The ?4Cl O removal efficiency can be facilitated by increasing the CS-n ZVI dosage and raising reaction temperature, the observed pseudo-first-order rate constant(kobs) was improved by 6.41 times when the temperature was increased from 50 to 92℃, half-life of the reaction decreased from 13.24 h to 2.06 haccordingly. CS-n ZVI hasexcellentadaptability to the initialp H value, salinityand initial perchlorate concentration in ?4Cl O aqueous solution. The most significant perchlorate removal occurred in solutions with slightly acidic or near-neutral initial p H values. The removal rate of ?4Cl O by CS-n ZVI were more than 77% with the initial p H values at 3.1-11.2, ?4Cl O can be effectively degraded by CS-n ZVI when the initial concentration of perchlorate within 200mg/L at moderatelyelevated temperatures, and the remaining ?4Cl O in solution is low, ranging from 0.21 to 1.8mg/L after the reaction. The concentration of salinity(?Cl in the aqueous solution from 0 to 30g/L) has little effects on ?4Cl O removal. The degradation efficiency of ?4Cl O in the presence of co-existingions decreases when compared with that without salts, the anions slowed the ?4Cl O removal, to differentdegrees, with the following order in terms of their inhibition effects:-23 CO >-24 SO >-3HCO.Theobserved pseudo first rate constant(kobs)wascalculated in differentambient factorsby fitting the pseudo-first rate expression from equation(4-6) to the experimental data. The observed rate constants at different temperatures fit well toapseudo-first-order rate model, reaction rates were found to increase with increasing temperature. Activation energy, Ea, computed from the Arrhenius relationship with the rate constants obtained at the four temperatures was42.30 k J·mol-1. The stabilization mechanism of CS-n ZVI was discussed based on its characterization data, maybe both carboxymethyl groups and hydroxyl groups of chitosan can interact with n ZVI particles, in addition, complexation maybe formed by ferrous ionscomplexing with chitosan. dispersion(or reduce agglomeration) of n ZVI particles can be enhanced through electrostatic repulsion and steric hindrance. The amount of Fe(Ⅲ) on the surface of CS-n ZVI significantly increase after reacted with?4Cl O, thepresence of Cl 2pdetected by X-ray photoelectron spectroscopy(XPS) illustrates ?4Cl O is reduced by CS-n ZVI in the reaction.Perchlorate can be removed byadsorption and reduction of CS-n ZVI. The rapid adsorption and reduction of perchlorate is concurrent, CS-n ZVI can rapidly adsorb ?4Cl O from water, resulting in the observed rapid drop in perchlorate concentration in the solution phase,almostsimultaneously, ?Cl wasdetected in the solution. The decrease of the concentration of?4Cl O, at the same time, was accompanied by the accumulation of ?Cl. During the reaction, only a small amount of ?3Cl O and no ?Cl O, ?2Cl O was detectedin the aqueous solution. ?4Cl O can be rapidly and almost completely reduced by CS-n ZVI at moderately elevated temperatures without other by-products generated. The adsorption of chitosan and adsorption/coprecipitation of chloride ion by the iron oxidesformed during the reaction are the main reasons forslightlydecreaseof the total chlorine species.