RSM-driven enhancement of catalytic nitrate reduction: Mechanism, kinetics, and strategic N ₂ selectivity control

Nitrate contamination of aquatic systems represents a pressing global environmental challenge. This investigation centered on the catalytic reduction of aqueous nitrate through synergistic application of zero-valent iron (Fe
0
) and Pd-Cu bimetallic catalysts. Key objectives included operational parameter optimization via response surface methodology (RSM), mechanistic elucidation of reaction pathways, strategic enhancement of N
2
selectivity, and comprehensive kinetic analysis. Under RSM-derived optimal conditions (20 mg L
-1
NaNO
3
, 3 g L
-1
Fe
0
, pH 5.2, 127-min duration, Pd:Cu mass ratio 3.4, 3.2 g L
-1
catalyst), Pd-Cu/graphene achieved a remarkable 71.6% N
2
selectivity. Three enhancement strategies were identified: (1) transition to composite supports, exemplified by Pd-Cu/γ-Al
2
O
3
-diatomite (68% N
2
selectivity) outperforming single-component counterparts (Pd-Cu/γ-Al
2
O
3
: 66%; Pd-Cu/diatomite: 57%); (2) acid-washing pretreatment of carrier with 1 mol L
-1
HCl, elevating Pd-Cu/diatomite N
2
selectivity from 56% to 62% while marginally reducing Pd-Cu/γ-Al
2
O
3
from 67% to 64%; and (3) active-site modification with sodium bis(2-ethylhexyl) sulfosuccinate (AOT), boosting N
2
selectivity by 4-8% in Pd
AOT
-Cu systems. Based on the catalytic denitrification kinetic studies of 16 different supported catalysts, kinetic analysis confirmed first-order behavior governing the nitrate reduction process, which proceeds through a multistep surface-mediated mechanism.