| In recent 20 years, heavy metal contamination in agricultural soils has become more and more serious in China. Studies on the heavy metal fraction transformation, adsorption-desorption mechanism and transport process in soils are very useful to identify the specific chemical behaviors of heavy metals in environment, to predict the transport of heavy metals in soils, and to offer the reference frame for risk assessment of heavy metals. Consequently, these researches have important theoretical and practical significance. In this study, seven typical Chinese soils were collected (including black soil, yellow fluvo-aquatic soil, red soil, grey fluvo-aquatic soil, loessial soil and two paddy soils), which represent the different climatic and zonal characteristics in China. Four kinds of heavy metals, copper (Cu), lead (Pb), cadmium (Cd), and mercury (Hg), were involved. Effect of moisture regime on the fraction transformation of heavy metals in soils, adsorption-desorption characteristics of heavy metals in soils, transport process and accumulation of heavy metals in soils under simulated sewage irrigation were studied by using the methods of sequential selective extraction, adsorption-desorption experiment simulated leaching experiment, respectively. The main conclusions were as follows:1. Effect of moisture regime on the fraction transformation of heavy metals in soilsThe spiked agricultural soils (two metal loading-level) were incubated under three moisture regimes (75% field capacity, wetting-drying cycle and flooding) for 150 days. It was found that, (1) The proportions of exchangeable and carbonate fraction of heavy metals increased at high metal loading-level. The aging process that heavy metals transformed from exchangeable fractions to other fractions was limited at high metal loading-level. The entire transformation process was related to the soil properties. (2) In metal-spiked soils, the proportions of exchangeable fractions gradually decreased as the incubation time growing. The fraction transformation process could be simulated by diffusion equation, and the decrease rate of exchangeable fraction of heavy metal could be used as the transformation rate. Judging from the parameters in the simulated equations, the transformation rates of heavy metals under three moisture regimes were in the order, flooding> wetting-drying cycle> 75% field capacity. (3) Soil moisture could change pH, Eh, organic matter, carbonates and amounts of ferric oxides of soil-water system, and consequently affected the fraction distribution of heavy metal in agricultural soils. 2. The characteristics of adsorption-desorption of heavy metals in typical Chinese agricultural soils.This study deals with the adsorption-desorption behaviors of heavy metals in 7 agricultural soils including influence factors.0.01 mol·L-1 NaNO3 was taken as the background electrolyte. It can be concluded that (1) Black soil had strong adsorption capacity of Pb, Cu, Cd and Hg.. Yellow fluvo-aquatic soil and loessial soil had much larger adsorption capacity of Cu, Pb and Cd, and similar adsorption capacity of Hg compared to variable charge soils (red soil, gray fluvo-aquatic soil and two paddy soils). (2) pH, CEC, organic matter and ferric oxides had great influences on the adsorption capacity of heavy metals in soils. To a large extend, the combination of these factors could control the adsorption capacity of heavy metals in soils. (3) Within the range of 0-1000 mg·kg-1 loading level of heavy metals, the desorption rate of heavy metals in studied soils followed the order, Hg< Pb< Cd< Cu. The relationship between desorption capacity and adsorption capacity of heavy metals could be well expressed by second power function. As the adsorption capacity increased, the desorption rate of heavy metals in agricultural soils raised. The growth of desorption rate had the hysteretic nature, namely, the stronger the adsorption capacity of heavy metals, the smaller the desorption rate. Simultaneous, the stronger the hysteretic nature went on, the slower the growth of desorption rate was bound to be.3. The transport and accumulation of heavy metals in agricultural soils under simulated sewage irrigation.To obtain the knowledge of impact of sewage irrigation on soil, crops and shallow groundwater, soil column experiment were conducted to assess the transport behaviors of Cu, Pb, Cd and Hg in 7 typical Chinese agricultural soils under simulated sewage irrigation. Soil columns were leached with two modes of artificial sewage irrigation water (single and mix) accounted for 20 L. The HYDRUS-1D was used for fitting the Breakthrough Curves of heavy metals. The results manifested that, (1) The studied black soil, yellow fluvo-aquic soil and loessial soil had great adsorption capacity for heavy metals, resulting in that the leachate concentrations of heavy metals did not exceed the value on the Groundwater Quality StandardsⅡ; on the other hand, the studied red soil, grey fluvo-aquic soil, two kinds of paddy soils did not had strong adsorption capacity for heavy metals, making Cu and Cd could rapidly breakthrough the soils, and present potential pollution risk for groundwater or environment. (2) The analysis of HYDRUS-1D indicated that, the transportation of heavy metals in black soil, yellow fluvo-aquic soil and loessial soil under sewage irrigation could be well described by deterministic equilibrium model (DEM), while the transportation of heavy metals in red soil, gray fluvo-aquic soil and two kinds of paddy soils could be well simulated by two site model (TSM). It was found that, the adsorption capacity of heavy metals in soil was the key factor to control the entire process of transportation of heavy metals. The saturated soil conductivity and coefficient of hydrodynamic dispersion also had influences on the transportation of heavy metals in soil. It could affect when breakthrough of heavy metals in soil began, the shape of breakthrough curve of heavy metals in soil, and the lasting time of breakthrough process. Because of competitive adsorption, fitted data of the transport and accumulation of heavy metals in soils under mix irrigation mode were below measured data. (3) According to the prediction modes, long-term exposure to low-levels of heavy metal contamination would make heavy metals migrate downward and accumulate in soils. As the simulated irrigation time last more than 200 years, high concentration of heavy metals distributed in soil profiles below 10 cm, indicating great potential of downward leaching and runoff transport.4. Leaching and release characteristics of heavy metals in contaminated soilsLeaching and release characteristics of Cu, Pb, Cd and Hg in agricultural soils under simulated rainfall for more than 3 years were studied by using the method of soil column leaching experiment. It can be observed that (1) Within 2000 mL amount of leachate, it was the quick-release stage that high concentrations of heavy metals presented in leachate and accumulative release amounts of heavy metals increased with time, accompanying by decreasing release rate. After this process, it followed the stage of quasi-equilibrium. In such stage, the accumulative release amounts of heavy metals kept almost unchanged, and heavy metals in leachate were very low. Among four studied heavy metals, Pb had the lowest experimental risk, while Hg had the highest one for its most stringent environmental standards. (2) Release rate of heavy metal was related to the adsorption capacity of this metal in soil. Generally, the stronger the adsorption capacity was, the smaller the release rate would be. Organic matter, bulk density, soil texture, adsorption capacity and original amounts of heavy metals had great effects on the releases of heavy metals in agricultural soils. (3) The release process of heavy metals under simulated rainfall could be well simulated by corrected Elovich equation and double constant equation. Especially, Elovich equation is the best of various equations to describe the experimental data.
|
PDF Full Text Request