Role of catalytic effect and components of volcanic ash in the synthesis of humic acids from phenolic compounds

Abstract

The role of the catalytic effect and components of volcanic ash in the synthesis of humic acids from phenolic compounds was investigated. The presence of volcanic ash in the system increased the yields of humic acids to various levels, depending on the chemical structure of the phenolic compounds. The yields were in the order of: pyrogallol>catechol>protocatechuic acid>gallic acid> hydroquinone>resorcinol>phloroglucinol systems. The synthetic humic acids generally showed a high degree of humification.

The citrate-bicarbonate-dithionite (CBD) treatment removed much larger amounts of Al, Fe, and Si oxides from the ash compared to the sodium acetate-hydrogen peroxide (NaOAc-H₂O₂) treatment. For the removal of Mn oxides, there was no appreciable difference between the two treatments. The CBD treatment and the NaOAc-H₂O₂ treatment of the ash remarkably decreased the yields of hydroquinone and resorcinol humic acids. On the other hand, the formation of catechol humic acid was considerably suppressed by the CBD treatment, but not by the NaOAc-H₂O₂ treatment. Furthermore, short-range ordered Mn(IV) oxide remarkably promoted the synthesis of hydroquinone and resorcinol humic acids, and short-range ordered Fe(III) oxide significantly accelerated the formation of catechol humic acid. These results suggest that the major catalytic components of volcanic ash responsible for the synthesis of phenol-derived humic acids are Mn oxides for the m- and p-phenolic compounds and Fe oxides and Fe-bearing silicates for the o-phenolic compounds.

Key Words:

• catalytic power and component
• humic acid
• phenolic compound
• volcanic ash