Redox nature of organics modulates multi-pollutant photocatalysis: study of Cr(vi) reduction and degradation of organics with CuNiFe LDH/C3N4 Heterostructures

Photocatalysis has demonstrated high redox activity in decomposing organic compounds and reducing heavy metals; however, previous studies have focused primarily on enhancing these effects or treating a few pollutants simultaneously, often overlooking the impact of the chemical nature and electronic behavior of the pollutants on their performance. This study uses a CuNiFe layered double hydroxide/C₃N₄ composite to examine how different organic pollutants (4-nitrophenol (4-NP), 2-chlorophenol (2-CP), 4-aminophenol (4-AP), and levofloxacin (LEV)) influence Cr(vi) photoreduction and vice versa. Our measurements reveal that Cr(vi) reduction increased from 62.5% to 71.45% with phenolics bearing electron-withdrawing groups, indicating enhanced electron transfer. In contrast, it decreased to 58% and 41% with 4-AP and 2-CP, respectively, due to the inductive effects of their electron-donating substituents and delocalization effects, which modulate the electron density at redox-active sites and suppress overall activity. Individually, 4-NP, 2-CP, LEV, and 4-AP showed photocatalytic degradation rates of 59.15%, 57.9%, 63.01%, and 51.76%, respectively. In the presence of Cr(vi), 4-NP degradation dropped to 19.01% (competitive interaction), while 2-CP, LEV, and 4-AP removal increased to 88.37%, 68.66%, and 64.03%, respectively (synergistic effects), indicating clear redox modulation. These findings highlight the importance of understanding pollutant–photocatalyst interactions to optimize systems for multi-pollutant wastewater treatment.