BOOSTING ACTIVITY OF IRON-APATITE HYBRIDIZED CATALYST FOR PHOTO-FENTON REACTION BY SURFACE MODIFICATION WITH HUMIC SUBSTANCE AS A SACRIFICE REAGENT

Abstract

Photo-Fenton reaction as an advanced oxidation process for water purification was investigated in water by using a heterogeneous iron-apatite hybridized catalyst, Fe(III)-treated hydroxyapatite (HAP-400-Fe), which was further modified with humic substance, Wako humic acid (WHA), Nordic aquatic humic acid (NHA), or Nordic aquatic fulvic acid (NFA). The substrates employed in the reaction were organic dyes, methyl orange (MO) and alizarin red S (ARS), and an herbicide, atrazine (ATZ). HAP-400-Fe catalyst modified with NHA or NFA, HAP-400-Fe-NHA or HAP-400-Fe-NFA, considerably enhanced substrate consumptions in comparison with the raw HAP-400-Fe. Particularly, HAP-400-Fe-NFA indicated complete consumptions (>99%) of all the three substrates for 24 h, and exhibited higher mineralization values of MO (41%) and ARS (68%). After the reaction, it was found that the surface NFA modifier on HAP-400-Fe had been subjected to oxidative degradation to some extent, and that catalytic activity of HAP-400-Fe-NFA had been lowered to the same activity as the raw HAP-400-Fe: The modifier can afford catalytic acceleration involving its own oxidative structural degradation, namely self-sacrifice. However, the deteriorated HAP-400-Fe-NFA could be remediated by re-adsorption treatment with NFA, and the remediated catalyst could feature the same effective catalytic activity as HAP-400-Fe. The catalyst recycling through remediation and reusing was extended to at least 4 times under the same photo-Fenton condition. A kinetic investigation based on Michaelis-Menten theory also elucidated that NFA modification improves affinity and kinetic ability of HAP-400-Fe. As a predominant oxidizing species, hydroxyl radical •OH was confirmed by a control test using a •OH scavenger, 2-propanol.