(600at) Application of Noble Catalytic System for Nitrate Removal in Wastewater

Application of
Noble Catalytic System for Nitrate Removal in Wastewater

Min-Sung
Kim^a, Sang-Ho
Chung^a and Kwan-Young Lee^a,b,*

^a Department of
Chemical and Biological Engineering, Korea University, 5-1, Anam-dong,
Sungbuk-gu, Seoul, 136-701, Republic of Korea

^b Green School, Korea
University, 5-1, Anam-dong, Sungbuk-gu, Seoul, 136-701, Republic of Korea

Nitrates (NO₃^-)
have no detectable color and taste materials, and have been increasing everywhere
in the world because of overusing of man-made fertilizers. Nitrate can occur
fatal disease to infants under 6 months of age, called blue baby syndrome. Also
high concentration of nitrate may cause some cancers and teratogenic effects. Biological
and physicochemical treatments such as sedimentation, filtration, chlorination
or pH adjustment, ion exchange and reverse osmosis, can effectively remove
nitrates but have several economical disadvantages. Catalytic nitrate removal has
been promising method for the reduction of nitrate in water. The method is
based on the catalytic hydrogenation of nitrate to nitrogen. The reaction
scheme is shown in Fig. 1. Several combinations between a noble and a non-noble
metal catalyst have been studied for reduction of nitrate in water. This study
evaluated the effectiveness of the catalysts with specific combination,
characterized features of the catalysts. 

Fig.
1. Reaction
scheme of the catalytic nitrate hydrogenation.

BET isotherms showed that silaca gel support had
mesostructure with type ¥³.
Measured BET surface area are 465 m²/g, while pore sizes are 7.34
nm.

Fig.
2.
(Left) Nitrogen adsorption-desorption isotherms of commercial silica-gel;
(Right) Pore size distribution of commercial silica-gel.

When nitrate is reduced to nitrogen, hydroxide ions
are formed, which resulted in increasing pH value of the reactant. Therefore,
many studies reported that increasing pH value of the solution was unfavorable
to the nitrate removal and nitrogen formation. As can be seen in Fig. 3, the
reduction of nitrates is quite different depending on reactant pH. Among the
catalysts, Rh-Cu presented relatively maintained nitrate conversion.

Fig.
3. Nitrate conversion in
dependence of the pH value for the hydrogenation of nitrate to nitrogen.
Initial concentration of nitrate(C(NO₃^-)_i): 2
mM; catalyst: 0.15 g; amount of gaseous hydrogen: 30 ml/min; HCl: 0.05 M;
reaction pressure: 1 atm; reaction temperature: 25°C; Reaction time: 5 h.

Fig. 4 represented nitrate
conversion and nitrogen selectivity of prepared catalysts. The order of
activity of the catalysts: Rh-Cu > Pd-Cu > Pt-Cu, with nitrate conversion
values of 65%, 47.5% and 68%, respectively, while Pt-Cu had the highest
nitrogen selectivity.

Fig.
4. Nitrate conversion and
selectivity to nitrogen of the prepared catalysts. Initial concentration of
nitrate(C(NO₃^-)_i): 2 mM; catalyst: 0.15 g;
amount of gaseous hydrogen: 30 ml/min; HCl: 0.05 M; reaction pressure: 1 atm;
reaction temperature: 25°C; Reaction time: 5 h.

Consequently, the performance of
catalytic nitrate reduction was different according to reactant pH and supported
metal. Furthermore, Rh-Cu catalysts had higher activity than other catalysts,
retained nitrate conversion notwithstanding pH difference.