| Summary: | 碩士 === 國立中山大學 === 環境工程研究所 === 104 === Perchlorate (ClO4-) is a high oxidizing, high water solubility and low viscosity material, which is commonly used as an oxidizer in fireworks, explosives rocket propellants, and other manufacturing. In addition, perchlorate can be stably present in groundwater for a long term, and can easily diffuse with the groundwater flow and cause a large-scale pollution. Because perchlorate has been linked to its negative influence on the thyroid gland, human will be inhibit their ability to absorb iodine if intaking excessive perchlorate. To solve this problem, a hybrid electrodialysis and reversal-reverse osmosis process (EDR + RO) was carried out to treat and recycle perchlorate-contaminated groundwater in this study. The results show that voltage (V) played an important role in EDR performance, which enhanced the migration of ions in water with increased voltage. Up to 94.5% of ClO4- could be removed in 2.5 hours of operation at 40. Effluent from the EDR unit was passed through the RO unit and results show that the perchlorate was desalinated to below 0.02 mg/L after the RO treatment. Although the water quality was significantly improved, however, it could barely meet the standard of irrigation water quality.
In this study, site groundwater was used for the feasibility study. Up to 99.9% of ClO4- can be removed via the two stage system (EDR + RO) system. More than 95% of other ions in water (e.g., Cl-, NO3-, SO42-, ClO4-, heavy metals) were also concentrated and removed via the EDR system. Effluents from the EDR system could be further treated via the followed RO system. More than 98% of ClO4- and other ions were removed after the RO unit. This results also indicate that perchlorate and other ions in water accumulated in the membrane pores and formed fouling. To solve perchlorate concentrate after EDR + RO desalination, zero-valent iron (Fe0) and zero-valent aluminum (Al0) were used to reduce the concentrates via the chemical reduction process. Results show that approximately 13.7% and 30.8% of ClO4- could be reduced by Fe0 and Al0, respectively. It indicates that chemical reduction process is a potential method to treat perchlorate concentrate.
Results from scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analyses show that the strong acidic functional group [sulfonic acid group (SO3H)] on the EDR cation exchange membrane was electrolyzed after EDR+RO treatment. This demonstrates that the contents of sulfur accumulated on the ion-exchange membrane. Moreover, calcium carbonate (CaCO3) sediments were observed on the membrane pores. The SEM-EDS analysis results show that heavy metals (such as molybdenum, magnesium, arsenic, potassium) deposited on the RO membrane. Results from the 3-D excitation-emission fluorescence matrix (EEFM) show that the most proportion of organic matter in groundwater was the type II of aromatic protein (AP) and the type IV of soluble microbial by-product-like (SMP). This indicates that the EDR treatment had less effect on organic compound separation. After the treatment of EDR and RO units, AP and SMP contents in filtrate were lower than feed water (the desalination water of EDR). Therefore, the RO membrane can adsorb and block the organics with low molecular weights.
Results indicate that the two two-stage system could effectively treat perchlorate-contaminated groundwater to below 0.02 mg/L, and the estimated cost for groundwater treatment was about NT$23.74/ton of groundwater (with initial perchlorate concentration of 10 mg/L).
The treated groundwater could meet the irrigation standard and subsurface injection standard. Moreover, the system could electrolyzed perchlorate and generate hydrochloric acid (HCl) and chlorine ions (Cl-), which resulted in the acidified effluents. This could inhibit the hydroxide production and prevent the fouling on the RO membrane. Results indicate that using the hybrid (EDR+RO) system is a feasible and effective method for the treatment and reuse of perchlorate-contaminated groundwater.
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