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I've been having trouble with reducing COD (Chemical Oxygen Demand) below approx. 500 ppm by using Fentons Reagent.

For 100 gm of the sample (initial COD 3000 ppm) I use 5 gm of 30% $\ce{H2O2}$ & 0.1 gm of $\ce{FeSO4}$. pH is adjusted to between 3 to 5. Temp. is approx. Room Temp. & I've tried reaction times up to 6 hrs.

Do these sound reasonable choices?

Any advice?

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I'm not an expert in Fenton chemistry but it appears that difficulty in getting the Fenton reagent to reduce COD to below 500 ppm is not unusual.

  1. Does your particular wastewater have chelants such as phosphates, EDTA, formaldehyde, and/or citric/oxalic acids?

  2. Perhaps at long reaction times all the iron you add is being precipitated as iron (hydr)oxides. Could you lower the pH further?

Reaction rates with Fenton’s Reagent are generally limited by the rate of $\ce{.OH}$ generation (i.e., concentration of iron catalyst) and less so by the specific wastewater being treated. Typical $\ce{Fe:H2O2}$ ratios are 1:5-10 wt/wt, though iron levels < 25-50 mg/L can require excessive reaction times (10-24 hours). This is particularly true where the oxidation products (organic acids) sequester the iron and remove it from the catalytic cycle. Fenton’s Reagent is most effective as a pretreatment tool, where CODs are >500 mg/L. This is due to the loss in selectivity as pollutant levels decrease...In addition to free radical scavengers, the process is inhibited by (iron) chelants such as phosphates, EDTA, formaldehyde, and citric/oxalic acids. Because of the sensitivity of Fenton’s Reagent to different wastewaters, it is recommended that the reaction always be characterized through laboratory treatability tests before proceeding to plant scale.

My tip would be to increase the concentration of both $\ce{H2O2}$ and $\ce{FeSO4}$, and, if possible, to make sure the pH is 3 (or maybe even lower). If that doesn't work, do you think iron chelators could be present in your particular wastewater sample(s)?

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  • $\begingroup$ Thanks for the tips. My water sample is expected to have NaCl as an inorganic and only C H O Cl (i.e. no P N S or other elements). In particular it very likely has aromatics, aromatics with sidechains (e.g. Styrene) & -OH groups / Chlorohydrins / Epoxides. No citric nor oxalic acid nor EDTA nor formaldehyde is expected. $\endgroup$ Mar 8, 2015 at 14:09
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    $\begingroup$ I will try increasing both concentrations but I think I'm already considerably above the conc. given at the link you mentioned. About pH I thought 3 to 5 was the optimum so I was keeping it there. Should I keep it lower? e.g. I read on your link that " it may be necessary to perform the oxidation in steps, readjusting the pH upwards to pH 4-5 after each step so as to prevent low pH from inhibiting the reaction." Apparantly reaction rate is too slow at low pH? $\endgroup$ Mar 8, 2015 at 14:10

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