Mechanism Of Low Molecular Weight Organic Acids On Soil Micropores

With the rapid development of global industrialization, more and more organic contaminations have been discharged into the environment. Although phytoremediation is regard as an acceptable way to remove soil contaminations, organic contaminations still could not be degraded completely. That is attributed to soil micropore (pore diameter < 2 nm) sequestration. Organic compounds sequestrated in soil micropores could not be taken up by microbes directly let alone plants and animals. This kind of organic compound is difficult to be degradation, and may be a potential source of pollution. At present, researches have shown that low molecular weight organic acids could improve bioavailability of organic contaminations. However, little research has proved that the improvement is attributed to the effects of organic acids on soil micropores, resulting in sequestrated organic pollutants release.Firstly, pore structure of 6 soils from different regions was characterized. N2 adsorption showed that the adsorption isotherm by soil is corresponded to type II, and the pore shape was plates and wedges. Soil pores were constituted by micropore, mesopore and macropore. Micropore volume was accounted for about 10% of the total measured pore volume. CO2 adsorption was nonlinear. It was an appropriate method to characterize soil micropores. Pore diameter characterized by CO2 adsorption was between 0.3 nm-1.5 nm. Results showed that black soil and red soil were two soils with rich micropores and different characteristic. Therefore, these two soils were selected as the object of further study.Secondly, 4 common low molecular weight organic acids released by plants: succinic acid, malic acid, tartaric acid and citric acid, were mixed to black soil and red soil respectively. Results showed that after 8 ds, organic acids reached equilibrium and adsorption of them in soil mainly depended on hydroxyl and carboxyl groups. All of the 4 low molecular weight organic acids could change the micropore distribution. Citric acid and malic acid could reduce the micropore surface area and pore volume significantly, resulting in the number of soil micropore reduction.Finally, effects of citric acid and malic acid on lignin were investigated, which showed that organic acids could not clog lignin micropores. Citric acid and malic acid were mixed to humin. Results showed that actiong site of organic acids was metallic mineral.This paper is the first study of mechanism of low molecular weight organic acids on soil micropores. In addition, destruction of soil micropores by organic acids and the acting sites of organic acids are proved. Low molecular weight organic acids are adsorbed by soil through hydroxyl and carboxyl. Chelation makes metal ions of mineral surface release, resulting in destruction of mineral micropores and micropores between mineral and organic matter. Meanwhile, organic matter is dissolved with the dissolution of metal ions, which makes the micropores of organic matter decreased.