The Reduction Of Cr(VI) In Contamintated Water Using CMC-stabilized Zero Valent Iron Nanoparticles

Groundwater which contains hexvalent chromium (Cr(â…¥)) remediation by iron nanoparticles has received increasing interest in recent years. Agglomeration of magnetic metal nanoparticles takes place, and then reduces the specific surface area and the interfacial free energy, thereby diminishing particle reactivity. Laboratory batch experiments were conducted to investigate the feasibility of using a new class of stabilized zero-valent iron (ZVI) nanoparticles for in situ reductive immobilization of Cr(â…¥) in water. Carboxymethyl Cellulose (CMC), a nontoxic and biodegradable stabilizer, is used in the synthesis of the nanoscale zero-valent iron as a stabilizer.In the present paper is reported an efficient method of synthesizing CMC-stabilized zero-valent nanoparticles. They were observed by environmental scanning electron microscope (ESEM) and transmission electronic microscopy (TEM). The results of using the CMC-stabilized nanoscale particles for the treatment of Cr(â…¥) are showed. And the kinetics and mechanism of Cr(â…¥) removal is elucidated. Furthermore the effect of humic acid or calcium, sodium to the removal efficiency of Cr(â…¥) by iron nanoparticles is also researched. The results are as following:1. Because the reaction of chromium removal by Fe⁰ nanoparticles is a surface reaction, so the smaller the Fe⁰ particles, the higher the surface area, the faster the reaction rate. The addition of CMC can enhance dispersion (or reduce agglomeration) of nanoparticles through (a) electrostatic repulsion and (b) steric hindrance. In addition to weakening the physical interactions, encapsulating nanoparticles with select stabilizers can also passivate the highly reactive surface from reacting with the surrounding media such as dissolved oxygen (DO) and water. In this study, the optimal ratio of CMC: Fe⁰ is 5:1. And CMC-stabilized Fe⁰ nanoparticles' removal rate was 4.3 times as well as that by Fe⁰ nanoparticles and 12 times as well as that by Fe fillings.2. The effect factors of Cr(â…¥) removal contain CMC addition, Fe⁰ addition, initial Cr(â…¥) concentration, pH value, reaction temperature and the cations concentration. The removal efficiency increased with the increasing of Fe addition and reaction temperature. Low pH value could accelerate the rate of Cr(â…¥) removal.3. Reduction of Cr(â…¥) by CMC-stabilized zero-valent nanoparticles was pseudo-first-order reaction. First-order rate coefficient k_obs increased with increasing Fe⁰ addition and reaction temperature but inversely with Cr(â…¥) concentration and initial pH. According to the Arrehenius equation, the superficial active energy E=29.3 kJ/mol and k₀=2341.6 min^-1 was obtained in this research4. Humic acid has an obvious inhibitory effect towards the chromium removal by Fe⁰. The experimental results showed reaction rate decreased as the humic acid concentration increased. When the HA concentration increased to 40 mg/L, the efficiency was only half of not adding HA. The addition of CMC could accelerate the removal rate by Fe⁰ nanoparticles and eliminate the inhibition effect by humic acid.5. The mechanism of Cr(â…¥) reduction by Fe°was as following: Cr(VI) reacted with atomic H, the product of Fe corroding. And Cr(â…¥) was also removed directly by Fe⁰ and Fe(â…¡). The product was Cr(â…¢)&Fe(â…¢) hydroxides, which cover the surface of Fe to slow down the reaction rate.