# Copper metallurgical slag as a sustainable precursor of iron oxide photocatalysts to remove indigo carmine dye from water using the photo-Fenton process

## TL;DR

Copper slag can be transformed into a sustainable catalyst to remove indigo carmine dye from water using the photo-Fenton process.

## Contribution

A sustainable, efficient nanocatalyst derived from copper slag using edible citric acid is proposed for dye removal.

## Key findings

- Copper slag leached with citric acid produced a maghemite-rich nanocomposite effective in dye degradation.
- The catalyst achieved 96.1% efficiency in removing indigo carmine dye within 60 minutes.
- Using edible citric acid reduced energy and reagent requirements compared to analytical-grade reagents.

## Abstract

The synthesis of an efficient composite photo-Fenton nanocatalyst was investigated using acid leaching processes of copper slag (CS) with two types of citric acid (EC = edible and AR = analytical-grade reagent). For this purpose, citric acid at room temperature or at 40 °C was used to lixiviate the elements from CS, and then, the leachates were subjected to a thermal treatment (350 °C). The formation of maghemite (γ-Fe2O3) accompanied with calcite as the main mineralogical phase was observed by X-ray diffraction (XRD) studies. The development of nanospheres dispersed in the calcite matrix and the proportion of γ-Fe2O3 in the catalyst or composite material were revealed by scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively. The catalyst performance in the Fenton and photo-Fenton processes was investigated in the degradation of the indigo carmine dye (IC, 10 ppm), achieving an efficiency of 96.1 ± 2.1% in 60 min, using 5 mM of H2O2 (30% weight) and UV-light (λ = 365 nm). The activity of these materials was analyzed and compared as a function of the specific surface area and the % γ-Fe2O3 in the composites. The preparation of maghemite-rich nanocomposite using the CS leachate, generated with edible citric acid, is an excellent alternative to substitute materials produced with analytical grade reagents because in the removal of IC dye, it showed similar efficiency, kinetics, and stability, but with lesser requirements of lamp energy radiation and doses of photocatalyst and oxidant, with respect to data reported in the literature.

The online version contains supplementary material available at 10.1007/s11356-025-36072-5.