Fenton-Like Oxidation of 4-Chlorophenol: Homogeneous or Heterogeneous?

Abstract

Heterogeneous Fenton-like catalysts have received considerable research attention because they could potentially be attractive for oxidative removal of organic contaminants from tertiary wastewater.However, process design is still hampered by insufficient understanding of the chemical pathways involved, and especially whether oxidation activity stems from heterogeneous surface chemistry or minute concentrations of dissolved metal ions in the homogeneous phase. Using inductively coupled plasma-optical emission spectroscopy (ICP-OES) in combination with pH monitoring and ultravioletvisible spectroscopy (UV-Vis) we have monitored the degradation of 4-chlorophenol (4-CP) over two Fenton-like heterogeneous systems, namely FeOx supported on TiO2 and CuFe2O4. We show conclusively that these systems proceed predominantly through a homogeneous route via dissolved metal ions from the solid phase catalysts. Control experiments with homogeneous Fe 3+ or Cu 2+ systems reveal that even minute concentrations ( M/sub-ppm) of dissolved metal ions leached from the solid phases account for the observed 4-CP degradation rates in the heterogeneous systems. ICP-OES revealed that metal leaching was time-dependent and variable because of pH variations associated with changing acid release rates. Buffering solutions at pH 7.4 suppressed metal leaching (and hence 4-CP degradation) in the FeOx/TiO2 system, but not in others. For example, pH buffering did not entirely suppress metal leaching from CuFe2O4, for which 4-CP degradation was retained through small concentrations of Fe and Cu ions in solution. Our results highlight the importance of careful monitoring of metal content in the aqueous phase, certainly with analytical sensitivity below ppm concentrations of the dissolved metals, and also the crucial influence of time-dependent pH variations on the reaction process. Recyclability of catalysts, pH buffering of solutions or monitoring of metal content in the solid phase by less sensitive analytical methods, e.g., chemical analysis, gravimetry, X-ray fluorescence, or energy dispersive X-ray analysis in electron microscopes, cannot exclude the homogeneous Fenton route in the presence of solid catalysts. 2Keywords CuFe2O4, leaching, homogeneous, heterogeneous, catalysis IntroductionFenton's reaction is an efficient way for treating organic contaminants, e.g. for soil remediation, 1 oil spill clean-up, 2 and industrial wastewater treatment. 3 Through the addition of Fe 2+ , H2O2 and acid to wastewater streams OH radicals ( • OH) can be generated through the oxidation of Fe 2+ (Fenton's reaction, eq. 1).• OH is the active species that degrades organic contaminants. Fenton's reaction proceeds optimally around pH 3, thus wastewater streams need to be acidified. 5 Subsequent neutralisation leads to Fe(OH)3 precipitates as a sludge, requiring an additional separation step. 6 This complexity of the homogeneous Fenton route motivated investigations of heterogeneous Fenton-like systems, seeking to localise the Fenton process at the su...

A Short Review of Techniques for Phenol Removal from Wastewater

“…In other research, Kuan et al, using FeO x /TiO 2 as catalyst for 4-chlorophenol (4-CP) treatment, suggested that although the catalyst was in heterogeneous form, the reaction actually occurred in homogeneous solution. After a 6.5-h reaction in acidic environment, the concentration of Fe ions increased from 0.8 to 30 μM, the concentration of 4-CP decreased from 0.4 mM to nearly 0 [51].…”

“…The Fenton-like process uses iron (III) as catalyst to turn the reaction from homogeneous to heterogeneous and is cheaper and more efficient compared to the classic Fenton process while having a similar mechanism [50,51]. According to Pariente et al, with a Santa Barbara Amorphous-15 (SBA-15) silica-supported iron oxide (Fe 2 O 3 /SBA-15) as catalyst, nearly 99 % of phenol can be removed using this process at 160°C in an acidic environment.…”

“…However, they indicated that in the buffer system used, this leaching was not a problem for the effluent since the metal ion concentration would decrease after the degradation of 4-CP. They suggested that the H + generated from 4-CP degradation was the cause for Fe and Cu ions' dissolution; when 4-CP was fully degraded, the H + concentration decreased and caused a reverse dissolution reaction of CuFeO 2 , which in an unbuffered system cannot happen [51].…”

Novel highly stable β-cyclodextrin fullerene mixed valent Fe-metal framework for quick Fenton degradation of alizarin

“…[9][10][11] In the recent years, magnetic nanoparticles with low iron dissolution have gained attention as a catalyst for the degradation of organic dyes due to their remarkable features such as a large specic surface area and easy recovery of catalyst. 12 The application of magnetic Fe 3 O 4 particles are limited in different media as it has less number of surface active functional groups, tend to aggregate, formation of clusters and iron leaching which reduce their activity and stability. 13 Thus, surface modication of Fe 3 O 4 is essential and choosing the right candidate to enhanced stability of the Fe 3 O 4 is a challenge, depending on the type of application.…”

Catalytic oxidative degradation of 4‐chlorophenol: Effect of operating parameters and prediction of intermediates/by‐products

“…The degradation process occurred in two stages: (i) a slow degradation stage attributed to heterogeneous reaction occurring on iron surface followed by (ii) rapid degradation stage due to homogenous Fenton reaction. However, the application of such processes is restricted due to mass transfer limitations, leaching of metal ions into the solution and long reaction time [12]. For example, Fenton-like process using zero valent aluminium (ZVAl) as catalyst showed 95% 4-CP degradation after 10 h of reaction [13].…”