A study on the effects of initial pH and feed flowrate in the dechlorination of trichloroethylene (TCE) using zero-valent iron

• Main Authors: Abuton, Maria Victoria., Reyes, Christian Nicole., Sule, Kelvin James.
• Format: text
• Language: English
• Published: Animo Repository 2005
• Subjects: Trichlorecthylene > Toxicity testing; Trichlorechtylene > Physiological effect; Chemical Engineering
• Online Access: https://animorepository.dlsu.edu.ph/etd_bachelors/9375
• Institution: De La Salle University

Purification of groundwater contaminated with Trichloroethylene (TCE), a pollutant with adverse effects to human health and a suspected carcinogen, is a major concern in different industries and military bases today. Hence, a constant search for an effective and economical way of treating contaminated groundwater with this pollutant is prevalent among researchers. The objective of this study is to investigate further the applicability of using scrap iron shavings in the Dechlorination of TCE. Dechlorination of TCE involves the step-wise reduction of TCE into less chlorinated compounds such as dichloroethylene and vinyl chloride to ethylene. The reduction is caused by the oxidation of iron thereby releasing electrons, producing energy for the degradation of TCE. Cold-rolled untreated iron shavings of specific amount and size were packed in a glass column. TCE passed through the system at flowrates varied at 20 mL/min. 40 mL/min, and 60 mL/min. Using the best flowrate among these three, the initial pH of the TCE solution was then varied at 2, 4, 6 and 7. Analyses were confined to the determination of the decrease of TCE concentration, and to the increase in chloride ion concentration. Feed flowrate of 60 mL/min gave the highest degradation with almost 90% degradation and 28.45% dechlorination. In addition, percent degradation and percent dechlorination decreased as flowrate decreased for the three flowrates tested. This increase in chloride concentration and decrease in TCE concentration as flowrate increased may be attributed to the decrease in the diffusion layer that hindered mass transport of TCE to the iron. In addition, 60mL/min flowrate had more capability of carrying off precipitates from the metal, than that of 40mL/min and most especially of the 20 mL/min, thereby exposing more metal surfaces available for the reaction. The highest degradation of 92.38% and dechlorination of 30.58% was obtained when the initial pH of the solution was 6. On the other hand, initial pH 7 trailed only by a small percentage from pH 6, with percentage degradation 89.052% and percentage dechlorination of 28.45%. However, at initial pH levels 4 and 2, lower percent degradation and dechlorination were observed. On such low pH levels, low dechlorination and degradation of TCE may be attibuted to the competition between the organic molecule and water as electron acceptors. It is known that the rate of dissociation of water is quite fast at low pH values thus affecting the extent of the dechlorination process. It was observed that the pH of the TCE-water solution fluctuated while dechlorination was taking place. The fluctuations could be the result of the instability of HCI in the solution, a product of the dechlorination process, wherein it completely dissociated into H⁺ and C1⁻ ions.