H2O2 and Formaldehyde Oxidation
Introduction
H2O2 will oxidize HCHO in either acidic or alkaline media. Acidic medium would be needed to mineralize HCHO to CO2.
| Acidic pH |
| Step 1: | HCHO + H2O2 → HCOOH + H2O |
|---|
| Step 2: | HCOOH + H2O2 → 2H2O + CO2 |
|---|
HCHO will be oxidized to formic acid and then to CO2. Complete conversion to CO2 is favored by strongly acidic media (< pH 2 - 3), elevated temperatures (40 - 60 deg-C), the presence of Fe2+ ions (25 - 100 mg/L, known as Fenton's Reagent), and an increased H2O2 dose. The reaction will take 15 - 60 minutes depending on how stressing the reaction conditions are. The theoretical H2O2 requirement for conversion to CO2 is 2.3 parts H2O2 per part HCHO, which translates into an H2O2 cost of about $2.00 USD per lb-HCHO1.
| Alkaline Reaction |
| 2HCHO + 2 NaOH + H2O2 → 2HCOONa + 2 H2O + H2 |
HCHO will be oxidized to formate / formic acid. The reaction will take 10 - 180 minutes depending on reaction temperature, pH, and H2O2 charge. With moderate temperatures (< 40 deg C) and a pH of 10 - 11, the H2O2 demand should be about 0.8 parts H2O2 per part HCHO, which translates into an H2O2 cost of about $0.75 USD per lb-HCHO. It is possible to reduce the H2O2 demand by increasing the NaOH charge (to 2 - 3 g/L) and elevating the temperature (to 60 - 80 deg C) to affect aldol condensation. In this case, the H2O2 demand should be about 0.3 parts H2O2 per part HCHO, which translates into an H2O2 cost of about $0.30 USD per lb-HCHO. However, these savings are partially offset by the higher NaOH charge (about $0.25 USD per lb - HCHO).
It is also noted that hydrogen gas will be produced by the alkaline reaction. However, the risk can be easily managed if the flow is small and HCHO concentrations are low - i.e., for a 3,500 mg/L HCHO stream, the level of hydrogen that would be generated would be 117 mg/L. Nonetheless, the treatment should be done in an open vessel with reasonable headspace turnover - since the solubility of H2 in water is about 1.5 mg/L, virtually all the H2 will be evolved.
1Based on a cost of $4.00 USD per gallon (50% H2O2), or $0.40 USD per lb-50%.
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