Enhanced formation of formate by freezing in solutions of hydrated formaldehyde-metal-hydrogen peroxide

We detected the formation of formate (HCOOH/HCOO-) in solutions containing hydrated formaldehyde (CH2(OH)(2)),HOOH, and Fe(III) or Cu(II). Formate formation increased significantly (ca. 5 times) when the solution underwent freezing and thawing. Although the reaction mechanisms are not clearly understood, we believe that the concentration effect of freezing enhanced the catalytic reactions between HOOH and Fe(III) or Cu(II) and the reduction of transition metals, i.e., Fe(III) to Fe(II) and Cu(II) to Cu(I). The concentration effect also enhanced reactions between Fe(II) and HOOH or Cu(I) and HOOH, which generated OH radical ("freeze-Fenton reaction"). We suspect that hydroxyl radical formed by the freeze-Fenton reaction probably oxidized CH2(OH)(2) and resulted in the formation of formate. Samples of identical compositions kept in the refrigerator did not form any formate except for the CH2(OH)(2)-Fe(III)-HOOH solution that formed a small amount of formate. Our study of the effects of pH on the CH2(OH)(2)-Fe(III)-HOOH solution (pHs between 1.5-5.0) showed that formate formation was the highest at pH = 4.0, indicating that the speciation of Fe(III) affected the formation of formate. Concentration-dependent experiments demonstrated that Fe is probably the limiting agent under typical environmental conditions.