Treatment of acid orange 7 solution by sunlight-Fenton process applying natural iron-bearing minerals

• Visible band contributed about 74% of the treatment effect of sunlight-Fenton. • Treatment efficiency strongly correlated with the contents of dissolved iron ions from minerals. • Photocatalytic oxidation had a certain contribution at low contents of dissolved iron ions. • Flocculation had obvious influence on the removal effect at high contents of dissolved iron ions. Natural pyrite, magnetite, ilmenite, V-Ti magnetite, magnesioferrite, and iron tourmaline were used as catalysts in sunlight-Fenton system for the treatment of azo dye acid orange 7 (AO7) solution. About 74 % of the treatment effect was contributed by the 400–780 nm visible band of simulated sunlight. There was a significant correlation between the treatment effect and the contents of total iron ions dissolved from minerals. By comparing with the homogeneous sunlight-Fenton system with equal contents of dissolved iron ions, it was found that the treatment effect of the mineral-catalyzed sunlight-Fenton mainly came from the homogeneous reaction of dissolved iron ions. In addition, the effect of direct sunlight photolysis and photo-assisted decomposition of H 2 O 2 was weak, while the photocatalytic oxidation of pyrite, magnetite and magnesioferrite after repeated use for 1 to 3 times had a certain contribution. The flocculation process due to the dissolved iron ions of pyrite, V-Ti magnetite and iron tourmaline had obvious influence on the sunlight-Fenton treatment effect. Iron tourmaline showed the most stable and efficient removal performance with about 30 % TOC removal rate in the system, which was related to a high quantity of leached iron ions rather than its ability to heterogeneously catalyze Fenton process.