Efficient Iron And Cobalt-based Fenton-like Systems And Their Applications In Degradation Of Organic Pollutants

Fenton technologies have been paid much attention in the field of water treatment,because they are simple,efficient and environment-friendly.However,conventional Fenton system requires a narrow acidic operation pH?2.5-3.5?,lacks efficient Fe³⁺/Fe²⁺cycle,and often produces a significant accumulation of iron-containing sludge.To improve the application performance of Fenton systems,modified Fenton-like systems have been developed by introducing the light,electricity,organic ligands,and supported catalysts.Besides,Fenton-like systems based on the sulfate radical(SO₄^·-)have also been developed,including the typical Co²⁺-PMS system,which can produce SO₄^·-efficiently.For the iron and cobalt-based Fenton-like systems,how to realize the efficient cycle of Fe³⁺/Fe²⁺and Co³⁺/Co²⁺,thereby reducing the addition of iron salt and cobalt salt in homogeneous systems,and further improve the catalytic activity of Fenton-like catalysts in heterogeneous systems and inhibit or even avoid the leaching of metal ions is still a key problem to be solved.Therefore,the objective of this dissertation is to develop novel homogeneous Fenton-like systems enhanced by HSO₃^-and small organic acids?SOAs?,and novel heterogeneous Fenton-like systems based on the cobalt-encapsulated and nitrogen-doped carbon nanotubes?Co/N-CNTs?and magnetic Fe₃O₄@ZIF-67 composites catalyzed activation of PMS.The main contents were summarized as follows:?1?A homogeneous Fenton-like system of Fe³⁺-HSO₃^--H₂O₂ was developed and applied for the oxidative degradation of bisphenol A?BPA?,a typical endocrine disruptor.It was confirmed that the degradation ability of Fe³⁺-H₂O₂ system was drastically enhanced by the addition of HSO₃^-.Typically,all the added BPA(0.05 mmol L^-1)was degraded in 20 min by using 0.1 mmol L^-11 Fe³⁺,0.3 mmol L^-11 HSO₃^-and 10 mmol L^-1H₂O₂ with a degradation rate constant of 2.71 min^-1,which was about 60 times higher than that(0.045 min^-1)of equivalent Fe³⁺-H₂O₂ system without HSO₃^-.This drastic enhancement effect of HSO₃^-was attributed to its complexing and reducing abilities for transforming Fe³⁺to Fe²⁺.Besides,the formation of FeSO₃⁺complex also prevented the precipitation of Fe³⁺.Electron paramagnetic resonance?EPR?and quenching experiments confirmed that several reactive species,including·OH,SO₃^·-and SO₄^·-were responsible for the degradation of BPA,among which·OH was the dominant one.?2?A homogeneous Fenton-like system of Co²⁺-SOAs-PMS was developed and applied for the degradation of organic pollutants and determination of trace-level Co²⁺in water.The addition of SOAs enhanced the catalytic activity of Co²⁺for activating PMS,thus greatly reducing the required Co²⁺addition.The cyclic voltammetry experiments and density functional theory calculation analysis showed that the enhancement of SOAs was attributed to their complexing and electron donating abilities.All the added diclofenac(30?mol L^-1)was degraded in 20 min by using 2?mol L^-11 Co²⁺,150?mol L^-11 PMS and 500?mol L^-11 acetate?Ac^-?with a degradation rate constant of 0.482 min^-1,which was about10 times higher than that(0.048 min^-1)of equivalent Co²⁺-PMS system without Ac^-.This novel system was universal for enhanced degradation of various organic pollutants.EPR and quenching experiments confirmed that SO₄^·-was the main reactive species.A sensitive spectrophotometric method was further developed for the determination of trace-level Co²⁺in water based on the Ac^-enhanced catalytic decolorization of methylene blue in Co²⁺-PMS system.Under the optimized conditions,the linear range was from 0.20to 7.0?g L^-1 and then from 7.0 to 50.0?g L^-1,with a detection limit of 0.10?g L^-1.This method possessed excellent anti-interference capability to various coexisting ions,and was used to determine the Co²⁺concentration in practical samples with satisfactory results.?3?A catalyst of cobalt-encapsulated and nitrogen-doped carbon nanotubes?Co/N-CNTs?was prepared by a simple method of calcination and acid treatment with Co₃[Co?CN?₆]₂ as a precursor,and a heterogeneous Fenton-like system was then developed by using Co/N-CNTs catalyzed activation of PMS and further applied for the degradation of organic pollutants.It was found that Co/N-CNTs were able to function as both an adsorbent and a catalyst for oxidative degradation of organic pollutants in the presence of PMS.All the added rhodamine B(10 mg L^-1)was completely degraded in 7min by using 0.1 g L^-1 Co/N-CNTs and 0.2 g L^-1 PMS.The high catalytic capacity of Co/N-CNTs was attributed to the synergistic effect between adsorption and catalysis on Co/N-CNTs,and the efficient cycle between?Co²⁺and?Co³⁺and the inhibited Co²⁺leaching induced by the encapsulated cobalt nanoparticles and the coordination interaction between Co and N.Co/N-CNTs could work well in both a wide pH range?3.3-11.2?and complex wastewater with common coexisting substances.Furthermore,Co/N-CNTs exhibited a good recyclability.?4?Magnetic Fe₃O₄@ZIF-67 composites were prepared by a wet chemical method at room temperature with Co?NO₃?₂,2-methylimidazole and Fe₃O₄ nanoparticles as raw materials,and a heterogeneous Fenton-like system of Fe₃O₄@ZIF-67-PMS was then developed and further applied for the degradation of tetrabromobisphenol A?TBBPA?,a typical brominated flame retardant.All the added TBBPA(40 mg L^-1)was completely degraded in 3 min by using 0.1 g L^-1 Fe₃O₄@ZIF-67 and 0.1 g L^-1 PMS with a degradation rate constant of 110.3 L g^-1 min^-1,being nearly 10 times higher than that(11.8L g^-1 min^-1)of ZIF-67-PMS system and 2000 times higher than that(0.06 L g^-1 min^-1)of Fe₃O₄-PMS system.This suggested the existence of synergistic catalysis between Fe₃O₄and ZIF-67 in Fe₃O₄@ZIF-67,which maybe attributed to the enhanced recycle of?Co²⁺from?Co³⁺by?Fe²⁺in Fe₃O₄.EPR and quenching experiments confirmed that several reactive species including·OH,SO₄^·-and ¹O₂ were responsible for the degradation of TBBPA,and ¹O₂ was the dominant one.Furthermore,Fe₃O₄@ZIF-67 exhibited a good recyclability and could be easily recycled by magnetic separation.