Effect of liquid property on adsorption and catalytic reduction of nitrate over hydrotalcite-supported Pd-Cu catalyst

Abstract

This study focused on investigating the effect of liquid property on adsorption and catalytic reduction of nitrate over hydrotalcite-supported Pd-Cu catalyst. Batch experiments were conducted under specific operating conditions. It was found that nitrate ions were adsorbed apparently by hydrotalcite-supported Pd-Cu catalyst at different reaction temperature (10, 25 and 35 °C). Adsorption isotherms for NO 3 − were described by Langmuir's equation. Higher reaction temperature accelerated nitrate adsorption and reduction, and simultaneously decreased the accumulation of NO 2 − and NH 4 +. Nitrate adsorption and reduction were independent of the initial pH because of the high buffering capacity of hydrotalcite when the initial pH of solution was from 5.25 to 7.4. However, the dissolve of hydrotalcite-supported Pd-Cu catalyst at low initial pH (3.77) and higher concentration of OH − ions at high initial pH (10.74) decreased nitrate adsorption and reduction. When using various nitrate salts as a source of nitrate ions, nitrate adsorption and reduction removal efficiency decreased in the order: Fe 3+ > Mn 2+ > Mg 2+ > Ca 2+ > Na +, which was attributed to different affinity toward OH −. Co-existed cations in water with higher valence and lower crystal ionic radius and solubility constant of hydroxide showed stronger affinity toward OH − and consequently increased the concentration of active sites accessible to NO 3 − and NO 2 −. The competitive adsorption of Cl −, SO 4 2− and HCO 3 − with nitrate on the catalyst surface suppressed the adsorption and reduction of NO 3 −. Besides, few metals dissolved in neutral and alkaline solution. The activity of the hydrotalcite-supported Pd-Cu catalyst for nitrate reduction kept steady after repeated use.