Al-Doped magnetite encapsulated in mesoporous carbon: a long-lasting Fenton catalyst for CWPO of phenol in a fixed-bed reactor under mild conditions

Al-Doped magnetite spinel nanoparticles encapsulated in mesoporous carbon (MC) were identified as a promising heterogeneous Fenton catalyst towards phenol degradation in a continuous system for practical applications. Under the working conditions inside the fixed-bed reactor hercynite in the fabricated 21%γ-Fe 2 O 3 /28%FeAl 2 O 4 @MC material reacts with H 2 O 2 . In this reaction Al ions occupied the vacant octahedral cationic sites in the framework of the γ-Fe 2 O 3 component converting it to Al substituted magnetite spinel. The Al in the obtained Fe 3+ 0.66 Fe 2+ 0.33 (Fe 2+ 0.33 Fe 3+ 0.33 Al 3+ 0.33 ) 2 O 4 @MC polarizes electrons of iron ions through its Lewis acid properties imparting more positive charge on iron ions (Fe n +( δ +) ). This expedites the rate of the challenging reduction reaction Fe 3+ → Fe 2+ with H 2 O 2 to produce HOO&z.rad; and reinforces the bonding of iron ions in the spinel improving their activity and stability. Hence, under mild operating conditions (pH 5, 40 °C, 8.6 ml water per ml cat h −1 , 0.036 mol H 2 O 2 , 200 ppm phenol) the in situ produced catalyst Fe(Fe 0.66 Al 0.33 ) 2 O 4 @MC of 35 nm containing 19.9%Fe and 2.4%Al with a surface area of 335 m 2 g −1 exhibited long-lasting high catalytic activity and stability during a 500 h run. 80% TOC conversion and ∼1 ppm of leached Fe in treated water were obtained without a visible change of catalytic performance. Al substituted magnetite was identified as a promising heterogeneous Fenton catalyst for CWPO of 200 ppm phenol in a continuous system under mild conditions (pH 5, 40 °C, 0.1 ml min −1 , 1.2 S H 2 O 2 ) for 500 h with 80% TOC conversion, 1 ppm Fe leaching.