Physicochemical And Microbial Characterization Of Mercury Contamination In Soils

This study was a part of the research project "the mechanisms of movement,transform, and accumulation of typical heavy metals in agricultural soil and theirrelationships to plant ecosystem and farm produce safety", a sub-project of the NationalKey Basic Research Program of China (973)"the temporal and spatial variation andadjusting and controlling principles of the environmental quality of soil and atmosphere inthe areas of Yangtse Rive and Pearl Rive Deltas"(No. 2002CB410804)". According to thedemands of the specific research content and aims of the national program, Three paddysoils with contrasting properties were studied. A silty loam soil (Paddy soil 1) wascollected from Huajiachi campus, Zhejiang University, Hangzhou, a yellowish red soil(Paddy soil 2) from Deqing County, and a purplish clayey soil (Paddy soil 3) from JiaxingCounty in Zhejiang Province, China. All of them belong to Gleyi-Stagnic Anthrosols inFAO/UNESCO nomenclature.Both laboratory and greenhouse pot experiments using rice, Chinese cabbage andradish as indicator plants were conducted to investigate the adsorption-desorptioncharacteristics and availability of Hg, and the ecological effects of increasing Hg loadingson microbial biomass, microbial activity, microbial community structure, and soil enzymeactivities. The major objectives of this study were to understand biogeochemical behaviorof Hg in three tested soils and to compare the potential of chemical, biochemical, andmicrobiological parameters for indicating and predicting Hg contamination. The resultswere summarized as follows:1. Adsorption-desorption of Hg at contaminated levels in three tested soils wereinvestigated. In all cases, Hg²⁺ adsorption decreased in the order: YRS soil＞PCS soil＞SLSsoil, with YRS having the highest sorption capacity for Hg. The maximum adsorption (K_L)value was 111.11 mg kg^-1 for SLS soil, 212.77 mg kg^-1 for YRS soil and 188.68 mg kg^-1for PCS soil. The isotherms of Hg²⁺ adsorption in the soils can be well described by thesimple Langmuir and Freudlich model. The simple Langrnuir model gave a relativelybetter representation of the experimental data based on the fitting correlation coefficients(r²).Adsorption of Hg²⁺ decreased soil pH by 0.75 unit for the SLS soil, 0.91 unit for theYRS soil and 0.55 unit for the PCS at the highest loading. The distribution coefficient (k_d)of Hg in the soils decreased exponentially with increasing Hg²⁺ loading. The adsorptionequilibrium pH of YRS was higher than that of SLS at the same Hg²⁺ concentration.Most of the adsorbed Hg²⁺ in the soils was not desorbed in the 0.01 M KCl solution. Afterfive successive desorptions, the accumulative amounts of Hg²⁺ desorbed accounted foronly 24.40ï¼…of the adsorbed Hg²⁺ for SLS soil, 14.37ï¼…for YRS soil and 18.35ï¼…for PCS soil, indicating that the YRS soil had the greatest affinity for Hg²⁺ at the same Hg²⁺loading. Different mechanisms may be involved in Hg²⁺ adsorption/desorption atdifferent levels of Hg²⁺ loading and among three testedsoils.Desorption characteristics as affected by desorption solution pH, organic acids, andcompetitive ions were also studied, pH was the most important factor controlling Hg²⁺desorption. Mercury desorption with increasing solution pH was characteristic of a "U"pattern and consisted of a desorption decreasing stage at pH 3.0-5.0, a precipitation andminimal desorption stage at pH 5.0-7.0; and a desorption increasing stage at pH 7.0-9.0.Organic acids remarkably influenced the desorption behavior of Hg²⁺ in the paddysoils. The presence of organic acids at low concentrations (＜10^-4 M) tended to inhibit Hg²⁺desorption, but enhanced Hg²⁺ desorption at higher concentrations (＞10^-4 M). Citric acid athigh concentrations (＞10^-3 M) was the most effective in increasing Hg²⁺ desorption,followed by tartaric acid and malic acid, and oxalic acid was the least effective. Thedesorption of Hg²⁺ was also affected by the presence of competitive cations (Cu²⁺ or Zn²⁺).The desorption of adsorbed Hg²⁺ increased with increasing concentrations of added Cu²⁺or Zn²⁺, and Cu²⁺ was more effective than Zn²⁺ in competing with Hg²⁺ for adsorbing sitesat the same concentration levels.2. Four extractants, including 0.1 M HCl, 1 M NH₄OAc(pH7.0), 0.005 M DTPA and 0.1M CaCl₂ (pH5.0) were evaluated for their extraction of available Hg²⁺ in the three testedsoils at different equilibrium times and different soil: solution ration.The amount of Hg²⁺ extracted increased with the increasing of equilibrium time. Theequilibrium time required for maximum extraction of the added Hg²⁺ varied among thefour extractants: 10 min for the four extractants in the SLS soil; 10 min for the HCl, 30min for the other extractants for the YRS and the PCS soil. Overall, 30 min is the bestequilibrium time. The amount of Hg²⁺ extracted decreased with increasing soil: solutionratio. Our study shows that the soil:solution ratio of 1:5 is adequate. The amount of Hg²⁺extracted increased in the order of NH₄OAc＜CaCl₂＜DTPA＜HCl for the SLS soil,NH₄OAc＜CaCl₂≈DTPA＜HCl for the YRS soil, and DTPA＜NH₄OAc＜HCl＜CaCl₂ for thePCS soil.There were significant correlations between concentrations of Hg in edible tissue andthe amounts of Hg extracted with the four extractants for soil-rice system and soil-radishsystem, but not for soil-Chinese cabbage system. According to the capability of differentextractants and the correlation of the extractable Hg²⁺ and Hg contents in edible planttissues, the 0.1M HCl was the best extractant among the four extractants and couldindicate plant availability of Hg in soil-plant systems.3. The changes of soil microbial activities and the functional diversity of microbialcommunity can be used to indicate the dose effect of Hg²⁺ on soil ecological system. Somesoil microbial parameters such as microbial biomass carbon, microbial quotient, AWCD, and urease activity were sensitive to Hg pollution.Mercury ecological dose responses of basal respiration, microbial biomass carbon,metabolism quotient, microbial quotient, and microbial community diversity wereinvestigated in the SLS, YRS, and PCS soils at Hg²⁺ loadings of 0, 0.25, 0.5, 1, 1.5, 2, 3,6mg kg^-1. The results showed that mercury (Hg²⁺) pollution had significant effects on themicrobial and enzymatic indices. There were several ecological effect characteristics ofHg in the soil-rice system. After harvesting rice plant, except for basal respiration andmetabolism quotient, which increased with increasing level of Hg treatments, all measuredmicrobial and enzymatic indices including microbial biomass carbon, microbial quotientincreased with the increasing level of Hg treatments at low concentrations (＜2mg kg^-1) anddecreased at the 0.5, 1, 1.5, 2mg kg^-1 Hg treatments. The results also indicate that soilmicrobial quotient is more sensitive to Hg contamination than the other microbial indices.Some important soil enzymes including urease, dehydrogenase, and acid phosphatase,which are related to cycling of soil C, N and P, were investigated. Mercury input had verydifferent effects on the enzyme activities. Enzymatic activity of the YRS soil was highest,followed by the PCS soil, and the SLS soil was the least.. The analysis of ED50 indicatedthat ecological toxicity of urease was the strongest for the SLS soil; ecological toxicity ofacid phosphatase was the strongest for the YRS soil; and ecological toxicity ofdehydrogenase was the strongest for the PCS soil.After harvesting rice plants, soil microbial function and community structurediversity were analyzed using BIOLOG, soil microbial functional diversities of microbialcommunity were changed to varying extent under the stress of Hg²⁺ pollution. The averagewell color development (AWCD) on the ECO plate reduced with increasing level of Hg.There was a nonlinearly relationship between AWCD and incubation time, and therelationship followed a growth dynamic model of microbial population (Logistic curve).The microbial community richness and McIntosh index decreased in polluted soil than thecontrol soil. The results suggest that the structure of microbial community has beenchanged under Hg²⁺ stress. Mercury pollution decreased the functional diversity ofmicrobial community, and reduced the microbial utilization of different carbon resources.4. The ecological effect of Hg in soil-vegetable (Chinese cabbage and radish) systemwas similar to those in soil—rice system. Namely, small addition of Hg (＜2mg kg^-1) to soilcould promot above soil microbial and enzymatic indices, but excessive addition of Hg(＞2mg kg^-1) to soil declined the soil microbial and enzymatic indices.In the soil-Chinese cabbage system, the results indicate that soil microbial quotient ismore sensitive to Hg contamination than the other microbial indices. In addition,enzymatic activities were higher in the YRS and PCS soil than the SLS soil. Afterharvesting Chinese cabbage, except for acid phosphatase whose activity increased withincreasing level of Hg treatments, most of Hg treatments had inhibitory effects on enzymatic activity in contrast with soil-rice system. Mercury treatments had the mostinhibitory effects on urease activity in SLS soil, less in PCS soil, and the least in YRS soil.There were no significant relationships between dehydrogenase and soil Hg loadings, norbetween urease activity and soil Hg²⁺ loadings except for acid phosphatase anddehydrogenase in SLS soil. The analysis of ED50 indicated that ecological toxicity ofurease and dehydrogenase was the strongest in the SLS soil; ecological toxicity of acidphosphatase was the strongest in the YRS soil after harvesting Chinese cabbage.After harvesting radish, microbial biomass carbon in SLS soil was the highest, butbasal respiration of SLS soil was the smallest. In soil-radish system, the peak values ofsoil enzymatic activity varied with soils, 1 mg kg^-1 for SLS soil, 1.5 mg kg^-1 for YRS andPCS soils. The soil-radish system differed from the soil-Chinese cabbage system in thatthe microbial quotient of three tested soils in the radish system decreased in the order ofPCS soil＞SLS soil＞YRS soil.The analysis of BIOLOG after harvesting radish also indicated that the average wellcolor development (AWCD) on the ECO plate was reduced with increasing level of Hg.There was a nonlinear relationship between AWCD and incubation time, and therelationship followed a growth dynamic model of microbial population (Logistic curve),similar to other cropping system.The study illuminated physicochemical and biological characterization of mercurycontamination in representative paddy soils in the areas of Yangtse Rive and Pearl RiveDeltas, revealed transplant, accumulation,and equilibium law of mercury and someimportant factors controlling mercury contamination in soil-crop system, and highlightedecological toxicity, prediction method, and regulation and control mechanism of mercurycontamination in soil—crop system. It also made major breakthrough in fundamentalresearch aspect consisting of transplant, transformation, and accumulation law of mercuryand healthy effect, which would supply scientific theoretics gist of control, restore projectof soil mercury pollution for our country.