Regulating Effects Of Humic Acids On The Speciation And Bioavailability Of Mercury In Soils And Its Mechanisms

As a kind of global trace amount toxic heavy metal contaminant, mercury (Hg) can be absorbed by the plant in the soil, and cause damage through food chain in animal and human hazard. The damage of Hg is not only related with total mercury, but also depends on morphology distribution of Hg in a larger extent. Soil organic matter is one of the most important factors which affect the morphology distribution of Hg. Humic acid which is a kind of natural organic matter is widely distributed in environmental medias such as rivers, lakes, oceans, groundwater, soil and sediment. There is exchange adsorption and chelation between Hg and humic acid and in this way, morphology distribution and biological activity of Hg can be changed, because there are many active perssads in humic acid such as hydroxyl (-OH), carboxyl (-COOH), carbonyl (C=O), amino (-NH2) and sulphur (-SH). The researches on the interaction of humic acid and Hg are mainly focused on the mechanisms such as chelation and adsorption constant, but the other subjects such as cooccurrence relation between humic acid and Hg, the morphology distribution of Hg under the effect of external humic acid, the transformation between the forms of Hg and the biological activity of Hg need to be deeply studied. Therefore, this paper designed Simulative Experiments and pot experiments to study the morphology distribution of Hg under the effect of humic acid and the effect of humic acid on biological activity of Hg. The results are as follows:Humic acid has a significant (P<0.05) impact on distribution of Hg speciation in soil. Adding humic acid could promote the transformtion of water soluble Hg, acid soluble Hg and residual Hg to alkali soluble Hg and Hg combined with organic matter in soil. Hg specation is affected by many factors, such as Hg concentration, humic acid concentration and the type of soil.Different concentrations of humic acid result in different effects on Hg speciation. The soils are cultivated for15days with humic acid of different concentrations after adding10mg/kg HgCl2and ageing2months, and the result indicates:The soil with humic acid of1%C is the one whose acid soluble Hg has the greatest decrease compared with the control, and the decrease amplitude is6.56%; The soil with humic acid of1.5%C is the one whose water soluble Hg and residual Hg have the greatest decrease compared with the control, and their decrease amplitudes are0.32%and12.74%respectively; The soil with humic acid of1.5%C is the one whose exchangeable Hg, alkali soluble Hg and Hg combined with organic matter have the greatest increase compared with the control, and their increase amplitudes are1.87%,6.99%and10.20%respectively.The content of Hg added has a significant effect on Hg species distribution in soil. The soils are cultivated for15days with HgCl2of different concentrations after adding humic acid of1%C and ageing2months, and the result indicates:the propotion of acid soluble Hg and exchangeable Hg increase with the increase of Hg content, but in reverse with the propotion of residual Hg. when Hg concentration increases from0to50mg/kg, the propotion of acid soluble Hg and exchangeable Hg increases by8.15%and9.42%respectively, and the propotion of residual Hg decreases by11.38%.Humic acid has different effects on Hg speciation in different soils. water soluble Hg, exchangeable Hg and residual Hg change more in purple alluvial than in mineral mineral yellow soil and in grey brown purple alluvial, acid soluble Hg, alkali soluble Hg and Hg combined with organic matter change more in mineral mineral yellow soil than grey brown purple alluvial and in purple alluvial. The transformation of the speed and the degree between different species are different, which result from composition and physicochemical properties of soil.The kinetic process of transformation between different Hg species in soil can be well simulated by elovich equation and parabola equation. The elovich equation is the best for water soluble Hg, acid soluble Hg, alkali soluble Hg, Hg combined with organic matter and residual Hg, which reach significant level (R2≥0.8) except water soluble Hg, but the exchangeable Hg can be best simulated by parabola equation which also reach significant level (R2≥0.8). The average rates of transformation vary with different Hg species, the rate order to increase is water soluble Hg<exchangeable Hg<acid soluble Hg<alkali soluble Hg<Hg combined with organic matter<residual Hg, but the average rates of all Hg species are drease with time.Humic acid can not only regulate the content of Hg in soil solution, but also can regulate the Hg species distribution in soil solution. The soils are cultivated for60days with humic acid of different concentrations after adding10mg/kg HgCl2and ageing2months, and the result indicates:total Hg content in soil solution decreases with the increase of humic acid content; the regulations on total Hg content by humic acid in different soil solutions are different, when humic acid concentration increases from0%C to2%C, the total Hg in solution of purple alluvial has the greatest decrease (27.56μg/kg), followed by mineral yellow soil (25.74μg/kg) and grey brown purple alluvial (21.04μg/kg). The main Hg species are different in different soil solutions. Grey brown purple alluvial takes Hg combined with organic matter, which account for56.6%, as the main Hg species, while purple alluvial and mineral yellow soil take the ionic species, which account for51.6%and64.0%respectively, as the main Hg species. Adding humic acid can increase the content of Hg combined with organic matter and decrease the content of ionic Hg. when humic acid concentration increases from0%C to2%C, the main Hg species of the three soil solution tested are Hg combined with organic matter, which has the the greastest increase of33.4%in mineral yellow soil solution.Humic acid can inhabit the bioavailability of soil Hg and enhance the ability of stabilizing Hg of soil. The result of pot experiment indicates:the Hg content of all plants in soils with humic acid decrease compared with the control, the Hg content of all soils incresae compared with the control, and the average Hg content of plant in purple alluvial takes the greatest decrease (the decrease amplitude is9.83μg/kg), and the average Hg content of grey brown purple alluvial takes the greatest increase (the increase amplitude is7.90μg/kg). The biological activity coefficients of Hg is the ratio of Hg content of plant in soil treated with humic acid and Hg content of plant in soil treated with no humic acid. The biological activity coefficients of Hg are different in different soils, in which mineral yellow soil has the largest coefficient (0.74), follwed by grey brown purple alluvial (0.69) and purple alluvial (0.42). The results show that the passivations to active Hg by humic acid in soils are significantly different; purple alluvial has the largest passivation, followed by grey brown purple alluvial and mineral yellow soil. The effects on active Hg in different plants by humic acid are different, the total Hg content in lettuce decreases by9.83μg/kg compared with the control, far outweigh the decrease of1.5μg/kg in cabbage, which shows a larger passivation of humic acid to lettuce than to cabbage. The content of humic acid and Hg has a significant effect on bioactivity. When soil in which7mg/kg HgCl2is added is placed two weeks, bioactivity of Hg decreases with the increase of humic acid; when soil in which1%humic acid is added is placed two weeks, bioactivity of Hg increases with the content of mercury in a certain range.The stem and leaf are the main parts of soil Hg accumulation. Adding humic acid can affect Hg accumulation in plant stem, leaf and root, but Hg content is more sensitive to humic acid concentration in stem and leaf than in root. The enrichment ability and capacity are larger in stem and leaf than in root.