Removal And Destruction Of N-Nitrosodimethylamine (NDMA) From Water By Microporous Mineral Sorption Followed By Microwave-induced Degradation

N-nitrosodimethylamine (NDMA), which is typically formed during chlorination/chloramination and ozonation processes in drinking water and wastewater treatment, is a disinfection byproduct of significant concern. Because of its toxicity and potential carcinogenicity, many countries and organizations establish drinking water standards or regulations for NDMA. However, NDMA in water is difficult to be removed by physical treatment, chemical method, and biodegradation, due to its small molecular size, high polarity and water solubility. There has been continued effort on improving the performance of the conventional drinking treatment processes at NDMA removal and developing alternative technologies.This research investigates the sorptive removal of NDMA from aqueous solution by porous minerals followed by destruction of the sorbed NDMA by microwave irradiation, with the ultimate goal of developing an alternative method for its control in water and wastewater treatment. The main tasks and findings are as follows:1. Sorption of NDMA on porous minerals with different pore sizes and surface hydrophobicity from water and microwave-induced degradation of the sorbed NDMA. A series of porous minerals, including ZSM-5 (Si/Al=12.5,25,40,130), Cu-ZSM-5 (Si/Al=25), H-ZSM-5 (Si/Al=25), Mg-ZSM-5 (Si/Al=25), CBV-780, CBV-10A,4A, diatomite, and MCM-41, were evlauted as sorbents. The sesults indicate that the pore sizes and surface cation density play important roles in NDMA sorption and microwave-induced degradation of the sorbed NDMA. Dealuminated Na-ZSM-5 (Si/Al=25) exhibited a high NDMA sorption capacity and the NDMA sorbed in its micropores degraded quickly under microwave irradiation..2. Effect of microwave energy and bulk sorbent temperature on the degradation of the sorbed NDMA. The degradation rate of NDMA under microwave irradiation at different power levels and at different bulk sorbent temperatures (adjusted by cooling a stream of compressed air) was studied. The results suggest that the degradation rate was strongly correlated with the bulk temperature of ZSM-5 and microwave power, which can be attributed to formation of more "hot spots" and/or "hot spots" of higher temperatures at higher microwave energy levels or higher bulk sorbent temperatures.3. The changes in C/N ratio and infrared spectra of Na-ZSM-5 (Si/Al=25) with sorbed NDMA (and DMA) during microwave irradiation. The changes in the total carbon and nitrogen contents of ZSM-5 with sorbed NDMA (and DMA) under microwave irradiation were studied. The results show that the C/N ratio gradually reduced during microwave irradation, which indicates NDMA could be fully mineralized with continued irradiation. The changes in the infrared specta are in agreements with the degradation intermediates formed, and support degradation of NDMA under microwave irradiation.4. Mechanism of microwave-induced degradation of NDMA and its precursor dimethylamine (DMA) sorbed in mineral micropores. The degradation intermediates of NDMA (e.g., HCOOH, CH3COOH) when sorbed on Na-ZSM-5 (Si/Al=25) under microwave irradation were analyzed. NDMA degraded rapidly under microwave irradiation and its degradation intermediates also decomposed with further irradiation. The degradation intermediates suggests that NDMA degradation occurs via the mechanism of pyrolysis caused by formation of micro-scale "hot spots" within the mineral micropores under microwave irradiation.5. The apparent activation energy for microwave-induced degradation of NDMA. The microwave-induced degradation rate of NDMA sorbed on of Na-ZSM-5 (Si/Al=25) as a function of the bulk sorbent temperature was measured. The results indicate that the apparent activation energy (46 kJ/mol) is lower compared to typical thermal reactions (60-250 kJ/mol), suggesting non-thermal effect of microwave irradiation also contributes to the degradation.The above experimental results demonstrate that microporous mineral sorption coupled with microwave-induced degradation could efficiently remove NDMA and its precursor DMA from water, and eventually cause their full mineralization without formation of toxic secondary products. Compared to existing treatment options for NDMA removal, microporous mineral sorption coupled with microwave-induced degradation has the unique advantages of being efficient and energy-savinge, and thus could serve as a promising alternative method.