|M.Sc Student||Dagan Chen|
|Subject||Hydrogenation of Nitrate in Groundwater using|
Microtubular CNT-made Catalytic Membrane Contactor
|Department||Department of Civil and Environmental Engineering||Supervisor||Dr. Youri Gendel|
|Full Thesis text|
Contamination of groundwater with nitrate has become a major problem in Israel and worldwide. According to the European Environment Agency (EEA), about a quarter of the groundwater is in a poor chemical state primarily due to nitrate pollution. Electro-dialysis, reverse osmosis, and ion exchange are the main treatment methods currently used for the removal of nitrate from groundwater. The main drawback of the above listed processes is a generation of brines that require further treatment prior to the disposal.
Catalytic hydrogenation of nitrate is a highly promising technology for nitrate removal from water as no concentrate stream is formed. This study is dedicated to the development and investigation of the catalytic process for hydrogenation of nitrate into nitrogen gas using catalytic membrane contactor based on microtubes made of carbon nanotubes (CNT) loaded with bimetallic Pd-Cu catalysts.
The microtubes were produced using infiltration of MWCNT solution via hollow fiber microfiltration membrane, whereas the bimetallic catalyst was placed on the microtube via wet co-impregnation method Creating a catalytic membrane contactor (CMC) based on CNT microtube. The CMC characterized using BET, TGA, SEM and TEM analysis.
The CNT-based CMC was tested for nitrate hydrogenation at varied pH (5-9) and hydrogen pressures (0.1-2.5 bar). The results have shown that low pH values and high H2 pressures have increased nitrate reduction rate. Nitrate hydrogenation activity reached 2.6 (mgN/min/g Pd) at pH=5.0 and H2 pressure of 0.5 bar. Catalytic membrane contactor at a catalytic diffuser configuration allows high utilization of hydrogen. Measurements have shown up to 93.4% of hydrogen efficiency during a 5-hour experiment.
The CNT-based CMC performance was tested for nitrate hydrogenation in groundwater. The system was able to reduce nitrate during a 20-hour experiment from 100 mgN/l to meet the levels of drinking water according to the WHO standard (7 mgN/l), that is 93% nitrate removal. Beside nitrogen gas, hydrogenation of nitrate in groundwater resulted in the formation of both ammonium and nitrite ions.
With the understanding that the CNT CMC is easily broken, making it difficult to operate outside the laboratory walls, further research was dedicated to impart mechanical strength to the CNT microtubes. The composite CNT-polymer microtubes prepared from CNT-made microtubes and polyvinyl alcohol (PVA) polymer cross-linked by suberic acid (SA) were studied and characterized. Properties relevant to their function both as CMC and as an electrode in electrochemical process were tested. The polymer’s presence stabilized the microtube and gave it the necessary mechanical strength. Maximal mass loading raised from 44.2 g of the CNT-made microtubes to 118-187.6 g for CNT-PVA composite, depending on the PVA-SA loading.