Green synthesis of octahedral layered-type material from Mn mine tailings as Fenton-like process catalyst for 4-Nitrophenol water remediation

Abstract

A novel iron(II)-birnessite catalyst was synthesized using manganese-ore beneficiation waste from a decommissioned tailings dam in the Amazon rainforest. The catalyst was produced through a simple, two-step procedure based on Na-birnessite preparation by hydrothermal treatment of annealed tailings, followed by sodium/iron (II) (Na⁺/Fe²⁺) cation exchange at room temperature. Such an approach allows for the synthesis of valuable Mn-based nanostructured materials minimizing costs and reducing the environmental impact stemming from mining operations. XRD, SEM, and TEM characterization confirmed a well-defined lamellar structure typical of birnessite, with an interplanar spacing of ∼7.22 Å after ion-exchange. Raman, FTIR, and EDS analyses further verified Fe²⁺ incorporation within the layered structure. The material was tested as a catalyst in a heterogeneous Fenton-like process for 4-nitrophenol (4NP) degradation under batch conditions. A complete parametric study assessed the influence of catalyst load, initial pH, H₂O₂ dosage and temperature. 4NP complete degradation was achieved. Besides, most of the phenolic compound was removed in less than 60 min using a 7.5 mM H₂O₂ solution (H₂O₂:catalyst ∼1.13). 4NP mineralization was additionally improved using a solar-assisted process capable of reaching a 90 % TOC (Total Organic Carbon) removal and decreasing effluent toxicity, as demonstrated by Artemia sp. mortality tests. Besides, a possible pollutant degradation mechanism was proposed by considering the role of Mn species in material's catalytic activity. Thus, the present material can be considered a step toward enhancing environmental sustainability. Indeed, the Fe-modified birnessite can be regarded as a “low-cost/low-end” catalyst for environmental remediation, joining together mining tailings reuse and organic pollutant removal.