Seeking advice on the Fenton reaction phenomenon
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Debugging a Fenton system to treat oily, high-salinity wastewater from offshore drilling platforms (20g~50g). Fenton first adds hydrogen peroxide (30%) and then ferrous sulfate (10% solution), in a ratio of roughly 1:3, with a pH of around 4. Here’s the question: 1. The wastewater is orange-yellow in color; after adjusting the pH to around 9, an ink-green precipitate forms. Is this a normal phenomenon? Is there still too much ferrous ion? What should the dosage ratio be? Let’s calculate it. 2. For the polyferric iron used for flocculation prior to the Fenton reaction, if an excess amount of polyferric iron is used, will trivalent iron have an impact on the Fenton reaction? Will it react directly with the hydrogen peroxide that was added first? 3. That method is the best for measuring COD, as it is simple and easy to use. The chlorine correction method and the potassium iodide-alkaline potassium permanganate method are too complicated and not suitable for routine use; other methods have large errors. I chose the oxidation method using low-concentration potassium dichromate, but currently the COD level is too high and the dilution factor is too large, which makes the results seem inaccurate (when the Fenton process is used, the COD of the inlet water is over 2000, while that of the outlet water is over 4000).
1. After adding alkali, an dark green precipitate forms; it is definitely iron(II) hydroxide precipitate, and it turns yellow after being left for some time. This indicates that the oxidation effect is not good; **the amount of hydrogen added is insufficient, and it can be increased appropriately. 2. The flocculation method used by Fenton before? Anyway, a large amount of sludge will be generated after the Fenton process; so why not skip the flocculation step? Trivalent iron does not react with hydrogen. 3. Regarding COD testing, due to the high salt content, it is necessary to ensure that the chloride ion concentration does not exceed 2000 mg/L; the dilution factor should be increased appropriately, and the amount of mercury sulfate masking agent can also be increased. As for the analysis method, I think the rapid sealed digestion titration method is relatively accurate and fast. It is able to mask the effect of chloride ions to the greatest extent possible.
Fenton is used afterward for flocculation and precipitation; the specific dosage can be determined through pilot tests!
It is recommended to conduct a pilot test to determine the dosage ratio; flocculation can be carried out after the Fenton process, and the iron sludge also needs to be treated.
Was no pilot test conducted before the design? The dosage ratio needs to be determined on a case-by-case basis. There was previously a small-scale treatment of electroplating wastewater with a COD level of around 15,000; I can’t remember the exact dosage ratio. For the Fenton reaction, acid was added over 0.5 hours to adjust the pH to 3, while ferrous ions and hydrogen peroxide were added over separate periods of time – in total, it took 6 hours. After that, alkali was added over 0.5 hours to adjust the pH to 8
It’s likely there’s an issue with the dosage of Fenton’s reagent; the proportions were adjusted again in the pilot test, the reaction time should be over 1.5 hours, and alkali needs to be added； Adding a flocculant after the reaction is effective; adding it before the reaction has no effect.
Hydrogen peroxide can also be oxidized by potassium dichromate; if there is an excess of hydrogen peroxide, the COD level in the effluent will increase