Sustainable LDH-based Zn–Fe and Co–Fe systems and derived oxides for photocatalytic removal of wastewater pollutants

Significant global efforts are being made to enhance living standards and life expectancy; however, insufficient attention is directed towards the chemical waste generated by associated chemical processes. Municipal wastewater increasingly contains residues such as dyes and pharmaceuticals, many of which are resistant to conventional oxidation processes. Advanced oxidation processes (AOPs), including photocatalysis, can be a sustainable solution.

Our research focuses on developing photocatalytic systems using layered/oxide microscopic materials synthesized through conventional methods (co-precipitation, CP) or unconventional methods (mechanochemical, MC) and thermally induced in air. These systems are based on inexpensive transition elements M²⁺(Co or Zn) and M³⁺(Fe) and are synthetized in the presence of organic alkali (TMAH) or inorganic alkali (IA- NaOH and Na₂CO₃), maintaining the molar ratio M²⁺_xM³⁺ (x=1;3;5;7). The physicochemical characterization of the materials (XRD, DRIFT, ATR, DR-UV-Vis, BET, TEM/SEM) confirms layered and oxide structures that are responsible for the catalytic performance.

To evaluate the photocatalytic properties (1 mg photocatalyst/mL pollutant solution), the azo dye Orange G (4×10^-5 M) and the anti-cancer drug Epirubicin (3×10^-4 M) were selected as target pollutants, utilizing sunlight, sunlight simulated by an LED lamp (17-60.5 klx), and UV light from a low-pressure mercury lamp (254 nm). The degree of discoloration of the solution was assessed by measuring the variation in absorbance of the pollutant solutions using a Jasco Carry 50 spectrophotometer.

The most effective results were η = 57.64% for solar Orange G degradation using LDH Zn₇Fe TMAH MC in 180 minutes, η = 69.79% for simulated-solar (60.5 klx) Orange G degradation using LDH Co₇Fe IA CP in 150 minutes, and η = 95.66% for simulated-solar (35 klx) Epirubicin degradation using LDH Co₃Fe IA CP in 80 minutes. The Langmuir-Hinshelwood mechanism offers the most precise explanation for the bimolecular reaction between pollutant molecules and reactive oxygen species produced on the photocatalyst surface.