Catalytic effect of volcanic ash on the formation of humic polymers in ando soils

Abstract

The catalytic effect of volcanic ash as a whole and of its components in promoting the synthesis of diphenol-derived humic acids (HAs) in ando soils was investigated. The presence of volcanic ash in the system increased the yields of HAs to a varying extent, depending on the chemical structure of the diphenols. The yields were in the order: catechol > hydroquinone > resorcinol systems. 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-H 2O 2) treatment. For the removal of Mn oxides, there was no appreciable difference between the two treatments. The CBD treatment and the NaOAc-H 2O 2 treatment significantly decreased the yields of hydroquinone and resorcinol HAs. On the other hand, the formation of catechol HA was markedly suppressed by the CBD treatment, but not by the NaOAc-H 2O 2 treatment. Furthermore, short-range ordered Mn(IV) oxide significantly promoted the synthesis of hydroquinone and resorcinol HAs and short-range ordered Fe(III) oxide significantly accelerated the formation of catechol HA. These results suggested that the major catalytic components of volcanic ash involved in the synthesis of hydroquinone and resorcinol HAs and catechol HA were Mn oxides and Fe oxides, respectively.