Effective degradation of Direct Red 81 using FeS-activated persulfate process

As a burgeoning advanced oxidation process (AOP), heterogeneous activation of persulfate (PS) for synthetic refractory contaminants decontamination has recently received much attention. In this study, FeS was selected as a heterogeneous PS activator to facilitate the degradation of a typical recalcitrant contaminant of diazo dye Direct Red 81 (DR 81). The results showed that approximately 95% of 0.03 mM DR 81 was removed within 60 min with FeS and PS doses of 1.5 × 10^-3 M. The efficient decomposition of DR 81 by the FeS/PS system was assumed to be mainly attributed to the highly reactive SO₄^-• and •OH, which was related to PS cleavage by both dissolved Fe²⁺ leached from FeS and Fe²⁺ bound on the FeS surface. Except for strongly alkaline conditions, DR 81 decolorations by FeS/PS were insignificantly affected by operational parameters such as temperature, initial solution pH, and rotate speed. Meanwhile, the presence of five inorganic anions being studied had distinct impacts on DR 81 degradation and followed a strict order of NO₃^- < Cl^- < SO₄^2- < CO₃^2- < PO₄^3-. However, FeS/PS system was highly adaptable, and FeS, which is used as a PS activator was more stable. GC/MS and TOC data revealed that thorough mineralization of DR 81 by PS/FeS in an initial fast reaction phase to transform DR 81 to aromatic intermediates, followed by a slow reaction phase that mineralized these organic intermediates into carboxylic acids and carbon dioxide through further oxidation.