【作者】 郑顺安；

【导师】 章明奎；

【作者基本信息】 浙江大学 ， 土壤学， 2010， 博士

【摘要】 近二十多年来,我国农田土壤重金属污染问题日益突出,部分地区的土壤重金属污染已危及粮食安全生产。研究我国典型农田土壤重金属积累过程中重金属化学形态转化、吸附与解吸及运移的规律,有利于全面弄清不同土壤条件下重金属的环境化学行为,预测重金属在土壤-植物、土壤-水体中的迁移,为重金属的风险评价提供参考依据,对评价重金属在实际环境中的影响和预测农田土壤重金属的污染趋势有着重要理论和实际的意义。本研究选择了我国不同气候区的7种典型农田土壤(包括黑土、黄潮土、红壤、灰潮土、黄绵土和两种水稻土),以目前我国土壤重金属污染较为突出的铅(Pb)、镉(Cd)、铜(Cu)、汞(Hg)等4种重金属元素作为研究对象,采用室内培养、重金属形态分级、吸附解吸实验、室内淋溶实验及模拟模型等综合研究方法,探讨了我国典型农田土壤中重金属环境化学行为过程及其风险性。研究内容包括水分条件对典型重金属在农田土壤中形态转化动态过程的影响,农田土壤对Cu、Pb、Cd和Hg的吸附与解吸特性,污水灌溉下典型重金属在农田土壤中的运移和积累及其模拟,以及污染土壤中重金属的淋溶特性及其释放动力学。获得的主要结论如下：1.不同水分条件下重金属在农田土壤中形态转化特点通过连续检测2个污染水平的4种重金属元素(Cu、Cd、Pb和Hg)在7种农田土壤中的3种水分条件(水分适度、干湿交替及长期淹水)下的培养试验过程(150 d)中重金属化学形态的变化,发现：(1)随着外源重金属添加量的增加,土壤中交换态和碳酸盐结合态重金属的比例增加,残渣态重金属的比例则随之降低；高浓度外源重金属进入土壤后,转化为紧结合态重金属的能力有所减弱；土壤性质在一定程度上决定了外源重金属在土壤中各形态转化的方向。(2)外源重金属进入土壤后,活性重金属形态的比例随老化时间的增加逐渐下降；扩散方程可较好地拟合交换态重金属在土壤中的转化动态过程；交换态重金属在土壤中的下降速率可以作为重金属形态转变率的表征参数,用于比较与评价不同水分条件下重金属在农田土壤中形态转变速率的快慢。三种水分管理模式下,Cu、Pb、Cd和Hg在7种农田土壤中形态转变速率快慢的顺序是：淹水>于湿交替>75%田间持水量。(3)水分条件可改变土-水系统中pH、Eh、有机质、碳酸盐及氧化铁形态,从而影响外源重金属在农田土壤中的形态转化及分布。2.我国典型农田土壤中重金属Cu、Pb、Cd、Hg的吸附解吸特征以0.01 mol·L-1 NaNO3作为背景电解质溶液,对7种农田土壤中重金属Pb、Cu、Cd和Hg的吸附-解吸行为及其影响因素进行了研究,结果表明：(1)黑土对4种重金属都有极强的吸附能力,黄潮土、黄绵土等恒电荷土壤对Cu、Pb、Cd的吸附能力远大于可变电荷土壤(红壤、灰潮土和两种水稻土),但对Hg的吸附能力与可变电荷土壤相当。(2)pH、CEC、有机质、氧化铁含量对于土壤对重金属的吸附有较大影响,这几种因子间的组合很大程度上决定了对土壤对重金属的吸附能力,蒙脱石、伊利石和高岭石在土壤粘土矿物中所占比例也可影响土壤对重金属的吸附性能。(3)在重金属污染水平0～1000 mg·kg-1范围内,农田土壤对重金属的解吸率为Hg<Pb<Cd<Cu,二次幂函数最适合拟合土壤中重金属的解吸量与吸附量之间的关系。随着吸附量的增加,农田土壤对重金属的解吸率增大,但解吸率增长存在一定的滞后性,对重金属吸附能力越强,则解吸率越小,解吸率增长的滞后性越强,解吸率增长越慢。3.污水灌溉下重金属在农田土壤中的运移、积累规律通过室内土柱淋溶实验并结合HYDRUS-1D软件,模拟单一和复合两种污水灌溉条件,对重金属在土壤垂直方向的迁移及积累行为进行了研究,结果表明：(1)土壤对重金属的吸附性能是污水灌溉条件下淋出液中重金属浓度及土柱剖面中重金属分布的决定性因素。恒电荷土壤对Cu、Cd、Pb和Hg的吸附性能很强,进入土壤的外源重金属主要被吸附在土壤0-5 cm表层,淋出液中重金属浓度极低,符合地下水水质Ⅱ类标准。可变电荷土壤对重金属的吸附能力较低,进入土壤的外源重金属含量超过了土壤吸持固定能力,在淋溶过程中会出现穿透。在穿透过程中,土壤饱和导水率和弥散度可在微观上影响重金属在土壤中开始穿透的时间、穿透曲线的形状及穿透持续的时间。(2)确定性平衡模型(DEM)和化学非平衡两点模型(TSM)分别对单一淋溶模式下重金属在恒电荷土壤和可变电荷土壤中的迁移及积累过程有较好的拟合效果,对复合淋溶模式下拟合的相关性虽然很好,但返回的吸附参数低于实测值。(3)根据模型预测,长期低浓度的污染仍然会使重金属在土壤中向下迁移和积累。当模拟的灌溉时间超过200年时,10cm以下的土壤剖面中也可有较高浓度的重金属积累,可对环境及地下水产生一定的污染风险。4.污染土壤中重金属的淋溶特性及其释放动力学采用室内土柱淋溶实验,模拟自然雨水的性质和成分,研究了3年以上降雨条件下污染农田土壤中重金属Cu、Pb、Cd、Hg的淋溶及释放特征,发现：(1)模拟降雨作用下,淋溶量2000 mL内是重金属快速释放时期,在这个期间,淋溶液内重金属含量较高,累积释放量随时间推移而迅速增加,但释放速度不断减小,至2000 mL后,淋溶液内重金属含量达到很低水平,累积释放量几乎不变,是准平衡阶段。在研究的4种重金属中,Pb的环境风险最小,Hg在农田土壤中的迁移能力虽然较弱,但Hg的环境质量标准最为严格,因此具有较高的环境污染风险。(2)重金属的释放率与土壤对重金属的吸附性能具有一致性,土壤对重金属的吸附能力越强,则该重金属释放率越低。有机质、容重、土壤质地、吸附性能、土壤中重金属的初始含量及CEC对于土壤重金属的释放均有较大影响。(3)模拟降雨作用下污染农田土壤重金属的释放过程可以使用修正的Elovich方程和双常数方程进行拟合,且以Elovich方程拟合的效果最好。更多还原

【Abstract】 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.更多还原