| The combination of Pd/Fe bimetal reductive dechlorination and biological oxidation for the removal of chlorinated endocrine disruptors in water has received increasing interest in recent years. However, due to the limitation of Pd/Fe nanoparticles (easily agglomerate leading to decreased reactivity), Carboxymethyl Cellulose (CMC) is used in the synthesis of Pd/Fe nanoparticles to maintain high reactivity as a stabilizer. Then the dechlorinated pollutants are easily biodegraded by activated sludge to achieve effective removal on account of lower toxicity.In the present paper we studied the effects of CMC as a stabilizer on the CMC-stabilized Pd/Fe nanoparticles. And the effects of experimental parameters on the2,4-D and2,4-DCP reductive dechlorinated such as CMC concentration, stirring speed and initial pH value were also investigated. Then the resulted solution of2,4-D dechlorination was used for biological oxidation by activated sludge and the effects of initial pH value, activated sludge volume, initial contaminant concentration and temperature on the removal of PA were studied as well as microbial community structure. Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Zeta potential instrument, Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and high performance liquid chromatography (HPLC) were employed to determine and analyze the degradation process of the target pollutant on the combination of reductive dechlorination and biological oxidation. Results showed that:1. The introduction of CMC prevented aggregation of Pd/Fe nanoparticles, made them appear to be well dispersed and thus maintained the high reactivity. Moreover, CMC and the nanoparticles were connected with the chemical bond resulting in the surface of the nanoparticles wrapped up by CMC. The reactivity of CMC-stabilized Pd/Fe nanoparticles was higher than non-stabilized and micro-sized counterparts. The removal of2,4-D was achieved up to97%,70%and48%within180min, respectively.2. Compared to Pd/Fe nanoparticles, CMC-stabilized Pd/Fe nanoparticles had a greater electrostatic repulsion. The addition of CMC decreased the isoelectric point of Pd/Fe nanoparticles. At lower pH values (pH≤4.1),2,4-D was more easily adsorbed on surface of CMC-stabilized Pd/Fe nanoparticles and then quickly reduced.3. Lower pH values and suitable CMC-to-Fe mass ratio (CMC/Fe=5/1) favored the removal of2,4-D. Furthermore,2,4-D was first adsorbed by the nanoparticles then reduced to2-chlorophenoxyacetic acid (2-CPA), and later quickly converted to the sole final organic product PA.4. The reductive dechlorination of2,4-DCP was a pseudo first order reaction. According to the Arrehenius Equation, the apparent activation energy98.64kJ/mol and pre-exponential factor6.24×1015min-1was obtained in this research.2,4-DCP may be first reduced to chlorinated intermediates2-chlorophenol and4-chlorophenol by the nanoparticles, and then converted to the sole final organic product phenol.The removal efficiency of2,4-DCP increased with the higher Pd/Fe nanoparticles dosage, Pd loading, CMC concentration, temperature and the lower pH value. The appropriate experimental parameters for2,4-DCP dechlorination were CFe0.6g/L, Pd loading0.50%, CMC/Fe=2/1,250rpm, pH6.0and25℃within171min.5. PA could be easily biodegraded by the domesticated activated sludge to achieve effective removal than2,4-D. And the analysis of similarities showed that the microbial community structure of domesticated activated sludge with2,4-D was stable, while PA counterparts had a complicated change.6. There were several effect factors of PA removal in the activated sludge system. They included initial pH value, activated sludge volume, initial PA concentration and temperature. In this study, pH=7,50mL activated sludge/200mL solution, PA14.6mg/L and30℃favored the removal of PA. Besides, a certain amount of pollutants were adsorbed by activated sludge. |
PDF Full Text Request