The Effects Of Humic Acid, Iron Oxides And Their Combinations On The Occurrence State And Biological Activity Of Mercury In Soils

At present, the terrestrial ecosystem has become to be a sink of mercury (Hg). In order to understand Hg biogeochemistry cycle and find a useful pathway to control of Hg pollution, it is imperative to control the cycling of Hg in soil and its accumulation in the crops and vegetables so as to prevent Hg in soil from getting into the food chain. It has been proved that humic acid and its components can inhibit the bioactivities of Hg effectively and reduce Hg contents in plants. Meanwhile, iron oxides can complex and redox reactions with humic acid respectively, which influences the environmental behaviors not only of humic acid, but also of iron oxides. The existing studies on "iron oxides-humic acid-Hg" coexistence system focus on the adsorption-desorption characteristics, but rare studies related to speciation transformation and the biological activity of Hg as influenced by humic acids are still insufficient. Meanwhile, the distribution and transportation of Hg in plants at different growth stages in relation to the Hg states in soils are still not clear. It will be easier for us to understand the mechanism for Hg tolerance and the nature of Hg biotoxicity in plants if we can reveal the chemical speciations of Hg in plants and its relationship with Hg species in soil and plants. Thus pot experiments were carried out (1) to understand the transformation processes of Hg in soils as influenced by humic acids and iron oxides singly or jointly existence; (2) to reveal the bioactivity of various Hg forms in soils combining with the successive extraction method under different experimental conditions. The main results were summarized as follows:(1) Organic-bound Hg and residual-Hg are the main forms of Hg in natural soil (account for above 78%). After a month, exogenetic mercury (the final concentration of Hg was 7mg/kg) in soil was in the forms of organic-bound Hg mainly, and then followed by exchange-Hg and acid soluble-Hg, and the percentage of residual-Hg was very small,<7.56%.The percentages of organic-bound Hg in soils was:grey-brown-purple soil(48.47%)>mineral yellow soil(41.77%)>purple soil(36.69%); The percentages of acid soluble-Hg in soils was:purple soil (25.88%)>grey-brown-purple soil(15.80%)>mineral yellow soil (15.18%); The percentages of exchange-Hg in mineral yellow soil, purple soil and grey-brown-purple soil were 29.41%、25.41% and 16.57% respectively. Residual-Hg acoounted for 3.88% at least in purple soil and its percentage in grey-brown-purple soil was largest.(2) The effects of different additives on the speciations of Hg in grey-brown-purple soil were different, and there was no obvious synergy or antagonism between humic acid and iron oxides. All additives had little influence on residual-Hg. In the first month, all additives were helpful for the growth of water soluble-Hg within a narrow range, and the effect of humic acid+hematite mixture was the strongest (the increasing amplitude was 4.99% larger than that of the control group); In addition to the influence of the absorption by plants, all the additives were effective in reducing the percentage of exchange-Hg, and the effect of humic acid+goethite mixture was strongest and goethite was the minimum; Humic acid+hematite mixture restrained the growth of acid soluble-Hg, and the rest of additives could speed up its formation, and effect of humic acid was most significant (the increasing degree was 23.38% larger than that of the control group); Humic acid-goethite complex promoted the formation of humic acids-bound Hg, and the rest of additives were helpful for its reduction, including humic acid+goethite mixture, humic acid, humic acid-hematite complex which had more obvious effect and weakened in turn; humic acid+hematite mixture, hematite, humic acid+goethite mixture were helpful for the accumulation of organic-bound Hg and the effect weakened in turn, but other additives inhibited its formation and the effects of humic acid, goethite were strongest, their increasing amplitudes were 5.89%、5.73% smaller than that of the control group respectively. In the second month, the percentage of water soluble-Hg declined by 4.54%,3.74% and 4.02% under the influence of hematite, humic acid+goethite mixture and humic acid+hematite mixture respectively; All additives restrained the formation of acid soluble-Hg, and effect intensity:humic acid-hematite complex>goethite>humic acid> humic acid+hematite mixture>humic acid+goethite mixture>humic acid-goethite complex> hematite; Goethite, humic acid-goethite complex promoted the reduction of humic acids-bound Hg and the effects of the other additives were the opposite; All additives were conducive to the decrease of organic-bound Hg, and effect of goethite was strongest (decreasing amplitude was 18.42% larger than that of the control group).(3) The effects of humic acid on forms of Hg in grey-brown-purple soil and purple soil were much obviser than that in mineral yellow soil. And the effects of goethite on the speciation of Hg in different kinds of soil were different also. (4)The effects of different additives on the species transformation of Hg in plants had different features. Residual-Hg, ethanol soluble-Hg and hydrochloric acid soluble-Hg were the main forms of Hg in stem, leaf of the control group (accounted for 63.57%-88.58%). Two months later, the proportion of water soluble-Hg rose by 40.87% at most. As for stem and leaf, all additive restrained the growth of ethanol soluble-Hg, and the effects of humic acid-hematite complex, goethite were best, their percentages were 36.03%,32.89% smaller than that of the control group respectively; the increase amplitudes of water soluble-Hg were 16.09%,8.15% and 8.06% smaller than that of the control group under the influence of humic acid-goethite complex, goethite and hematite respectively, which were beneficial to lower the danger of Hg to stem and leave. In the first months, the percentage of residual-Hg rose 36.59%(humic acid)-18.05%(goethite) under the influence of all the additives, but all the additives activated residual-Hg and its decrease amplitude was 39.92%(hematite)-11.76% (humic acid+goethite mixture) larger than that of the control group in the second month. As for roots, humic acid, goethite were helpful for the transformation from ethanol soluble-Hg to residual-Hg; humic acid-goethite complex, hematite and humic acid+goethite mixture promoted ethanol soluble-Hg (the decreasing amplitude respectively were 9.22%,6.77% and 2.70% larger than that of the control group) transform to hydrochloric acid soluble-Hg; The proportion of water soluble-Hg fell by 7.22% under the influence of humic acid+hematite mixture. These all may improve the ability of root to resist to Hg pollution.(5) All additives can reduce the uptake of Hg in plants effectively. For the root of lettuce, the effect intensity:humic acid+goethite mixture (decreased by 79.43%)>goethite>humic acid> humic acid-goethite complex>hematite>humic acid-hematite complex>humic acid+hematite mixture; For the leaf and stem of lettuce, the effect intensity:humic acid-goethite complex (decreased by 79.41%)>hematite>goethite>humic acid>humic acid+goethite mixture>humic acid+hematite mixture>humic acid-hematite complex. Humic acid and humic acid-goethite complex (decreased by 78.40% and 61.72% respectively) were most effective for cabbage leaf and stem, and root respectively. The most proper additive can be choose to restrain the accumulation of Hg in edible part of plants to prevent more soil mercury from transferring into the food chain.(6) The absorption of Hg by plants decreased with increasing humic acid contents in a certain range (<2%). When the amount of humic acid was 1.5%, the humic acid restrained 88% and 73% of Hg uptook by leaf and stem, and root respectively. The absorption of Hg by plants decreased with increasing goethite contents.