Degradation of dechloro-diphenyl-trichloroethane (DDT) by sequential nano-zero valent iron-biofilm process

• Main Author: Tran, Ngoc Minh Quyen
• Format: text
• Language: English
• Published: Animo Repository 2012
• Online Access: https://animorepository.dlsu.edu.ph/etd_masteral/4348
• Institution: De La Salle University

This study involved the degradation of dichloro-diphenyl-trichloroetane (DDT) using nano-zero valent iron and microbial biofilm as separate and combined processes. The nano-zero valent iron with size of less than 20nm and specific surface area of 45.2 m2 g-1 was synthesized using NaBH4 and FeCl3.6H2O. On the other hand, the biofilm having a thickness of 195.56 µm was developed on sand particles with the inoculum obtained from the sludge of the wastewater treatment facility of De La Salle University and a reactor that was used to treat polychlorinated biphenyls (PCBs). The NZVI treatment step was investigated to obtain the optimal conditions by response surface methodology following Box-Behnken design, with three factors such as initial DDT concentration, NZVI loading and cobalt loading. The response expressed as percent DDT degraded, a second order model with good agreement between experimental results and predicted values. The model terms showed that there is interaction between initial DDT concentration and cobalt loading due to their competition in receiving electron from iron. In addition, the increase in NZVI loading resulted in increased degradation rate, but only up to certain level because of iron agglomeration. The optimal conditions were found to be 20 ppm DDT concentration and 1g/L NZVI loading to obtain 57% DDT degraded. The agglomeration of NZVI was addressed by encapsulated NZVI, which degraded almost completely 20 ppm of DDT by using 1g l-1 NZVI. The kinetic study was conducted on both NZVI and ENZVI. Results showed that experimental data fit with both the power law of a homogeneous system and Michaelis-Menten equation for the heterogeneous system. The sequential treatment was conducted on 20 ppm DDT. Under NZVI process, only 57% of DDT was degraded by 1g l-1 NZVI. The decrease in DDT concentration resulted in the formation of dechlorinated metabolites, whose amounts increased during the NZVI treatment step. However, all DDT residues and its metabolites were degraded in the biofilm step. At the end of the sequential process, 20 ppm DDT was decomposed almost completely at 96.96%. The dechlorinated -iv- De La Salle University products were degraded by the biofilm. The biofilm process also resulted in the formation of metabolites believed to be polar compounds which did not appear under NZVI process. These metabolites could be ring-fission products which may contain hydroxyl (-OH) groups or carboxyl (-COOH) groups. The sequential NZVI-Biofilm process proved to be effective in treating DDT. This process could be applied to other chlorinated pesticides.