M.H. Liu*, S.H. Tang*
*168, Jifeng E. Rd., Wufeng District, Taichung 41349, Taiwan
Keywords: Hexavalent chromium; Supercritical fluid; Nanoscale zero-valentiron
Chromium, as one of the widely used heavy metals anda commonraw materialin many industrial applications such as electroplating, leather tanning, pharmaceutical and others will certainly produce Chromium contaminated wastewater. Thus, hexavalent chromium has been frequently identified in the wastewater generated by factories and in the groundwater nearby. Zero-valentmetals,with its remarkable reducing capability like zero-valent iron, have been widely introduced as an agent of removing contaminants found in wastewater and groundwater. Supercritical fluid, with its outstanding characteristics of high extraction capacity, low surface tension, relatively low pollution, high transmittance, high diffusion, has been put to work on extraction of food ingredients, herb medicine extraction, waste treatment and preparation of nano-materials in recent years.
This study was to utilize supercritical fluid with supplement of sodium carboxymethyl cellulose (CMC) dispersant to prepare zero-valent iron. The zero-valent iron was made by using FeCl3·6(H2O) and NaBH4. Under the supercritical condition of varied pressure and temperature combination (100 bar, 40°C; 100 bar, 50°C; 150 bar, 50°C; and 150 bar, 60°C) and with carbon dioxide as fluid, variable weight ratio of 1%, 5%, 15% and 30% of CMC were introduced to prepare zero-valent iron. The characteristics of zero-valent iron were then examined by reducing hexavalent Chromium under parameters such as varied pH value, dosage of iron powder, concentration of pollutants, and reaction molar ratio. To characterize the property of zero-valent iron, the analysis conducted by specific BET surface area analyzer and Multi-functional Field-Emission Scanning Electron Microscope (FE-SEM) were further implemented. The results suggested, with participation of CMC, the specific surface area of zero-valent iron can be effectively improved up to 6 or 7 times. And the study also found chloride ion in the water does not render impact on the reduction of hexavalent Chromium. The parameter of pH value was set as 3 for all the remaining studies due to the understanding of better performance by lower pH value. In the experiments of hexavalent Chromium reduction, 0.361 mg chromium was removed with varied concentration of chromium (10, 20, 30, 40, 50, 100 mg/L) mixed with 0.2 g commercial iron powder under the condition of volume 50 mL and 30 minutes reaction time. And 0.171 mg chromium was removed with varied volume (10, 20, 50, 100 mL), fixed concentration 100 mg/L of chromium and 30 minutes reaction time. To study the effect of varied CMC dosage, zero-valent iron prepared by wet chemical synthesis was then mixed with concentration 3% and 5% of CMC, 70% reduction of chromium was achieved by 0.05 g of zero-valent iron with reaction time 4 hours. For experiments of adding 1% CMC in supercritical fluid synthesis, the reduction of chromium was around 50% owing to relatively identical particle size synthesized in the condition of varied pressure and temperature which were higher than supercritical parameters of carbon dioxide. The efficiency of zero-valent iron-nickel bimetallic particles prepared by wet chemical synthesis showed better performance without CMC than with because product was coagulated when CMC was introduced into the solution of iron-nickel bimetal. If acid washing was conducted onto zero-valent iron prepared by wet chemical synthesis with 5% CMC, the molar ratio of iron powder to hexavalent chromium was raised from 4.37 (0.893 mmole:0.204 mmole) to 17.5 (3.571 mmole:0.204 mmole), the reduction of hexavalent chromium can be achieved from 50% to 100% within 10 minutes. Likewise, zero-valent prepared by supercritical fluid method, under the same increasing rate of molar ratio as aforementioned, the reduction of hexavalent chromium can reach 100%. Under the condition of 25°C, 0.05 g of zero-valent iron prepared by wet chemical synthesis reacted with varied concentration of hexavalent chromium (25, 50, 100, 200 mg/L) in solution volume 500 mL, the rate of reduction can be defined as a second order reaction with respect to kinetic calculation. The constant of reaction rate was 97.63, 2.5x10-2, 3.6x10-3, 2.3x10-4 (mM．min)-1 for varied concentration of 25, 50, 100, 200 mg/L, respectively.
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