【作者】 乔冬梅；

【导师】 庞鸿宾；

【作者基本信息】 中国农业科学院 ， 农业水土工程， 2010， 博士

【摘要】 水资源紧缺使得污水灌溉面积逐年增加,随之而来的重金属污染问题日益严重。土壤重金属污染具有累积性、滞后性和不可逆性的特点,其治理难度大、成本高、周期长,不仅影响经济社会的可持续发展,而且严重危害人民群众的生命健康,因此重金属污染修复刻不容缓。面对重金属污染修复这项非常艰巨而重要的任务,众多修复方法中植物修复技术是一种可靠的、相对安全的环境友好型的修复技术,但目前对植物修复技术的植物选择、修复机理、应用条件、修复效果等方面的研究还比较薄弱,尤其在根际这个复杂的微生态环境中,土壤重金属修复机理的研究还较薄弱,有必要对土壤重金属的修复机理进行深入研究。植物根系分泌物反映了植物之间、植物与土壤以及植物与微生物之间的物理、化学和生物关系,在土壤重金属污染修复中发挥重要的作用。本文从富集重金属的黑麦草根际环境入手,主要针对基于根系分泌有机酸的植物修复机理进行研究,探索对重金属吸收起关键性促进作用的根系分泌有机酸组成、诱导作用、综合响应特征,进而从理论上阐明植物修复的调控机理。本项研究全文共分九章,除第一章绪论、第九章结论与讨论外,主要包括三部分:（1）重金属污染植物修复的基础研究（二—四章）、（2）黑麦草水培试验（五、六章）、（3）黑麦草土培试验（七、八章）。第一部分采用1次平衡法研究了不同pH（3,5,7,9,11）值和不同有机酸（EDTA、草酸、酒石酸、冰乙酸、丙二酸、苹果酸和柠檬酸）对重金属Pb²⁺吸附-解吸的影响,同时采用Tissier同步提取法研究了不同pH值对重金属Pb²⁺存在形态的影响,以期为重金属修复研究提供理论基础;第二部分通过水培试验,研究了黑麦草对不同浓度（10、25、50、100、200、400、600、800、1000mg/l）重金属Pb²⁺的修复效果,在获得根系分泌有机酸基础上,施加外源有机酸干扰,研究不同浓度（0.1,0.5,1,2,3mmol/l）和不同种类（EDTA,草酸,冰乙酸,丙二酸,酒石酸,苹果酸,柠檬酸）有机酸对黑麦草修复效果及其响应特征;第三部分是在水培试验研究的基础上,为了使研究更具有实用性,采用土培试验,研究黑麦草对不同浓度（0、100、300、500、1000mg/kg）重金属Pb²⁺的修复效果,在分析获得黑麦草根系、根际和非根际土壤有机酸的基础上,施加外源有机酸诱导因素,研究不同浓度（1、3、5、6、7mmol/kg）和不同种类（EDTA、草酸、冰乙酸、丙二酸、酒石酸、苹果酸）有机酸对黑麦草修复效果及其响应特征。取得主要研究结果如下:（1）在pH（3-11）跨度较大范围进行吸附-解吸研究,分析表明:吸附量（率）随初始浓度变化的pH值敏感阈值范围为pH≤7。所引入的关于平衡浓度与吸附量之间的改进模型ln（S）=K1×CK2,较其它模型为最优模型。解吸量随pH值的增大而减小,随吸附量的增加而增大。同时进行了不同有机酸作用下吸附-解吸研究,分析得出,有机酸增加了重金属Pb²⁺的活性,其活化能力顺序为:EDTA>柠檬酸>苹果酸>丙二酸>冰乙酸>酒石酸>草酸。冰乙酸、丙二酸、酒石酸及苹果酸作用下等温吸附可以用Henry模型模拟,柠檬酸和草酸可以用Freundilich模型模拟,而各模型均不适合EDTA条件下的模拟分析。（2）不同pH值条件下重金属Pb²⁺形态分析表明:pH<7时,有利于10-100mg/l重金属Pb²⁺向易吸收的形态转变,而对于200-1000mg/l范围内Pb²⁺的存在形态影响较小;0-100mg/l浓度范围内,铁锰氧化物结合态所占比例最大;200-1000mg/l浓度范围内,Pb²⁺存在形态主要为残渣态。（3）不同重金属Pb²⁺浓度下黑麦草修复的水培试验得出:400mg/l的重金属Pb²⁺浓度为黑麦草分泌有机酸调节pH值的敏感阈值拐点,而影响黑麦草株高、干物质量和根系耐性指数的阈值拐点为200mg/l;随营养液中Pb²⁺浓度的增加植株中Pb²⁺含量增加,但增幅随Pb²⁺浓度的增加而减小;随黑麦草生长时间的推移,根系分泌有机酸由草酸和冰乙酸增加为草酸、酒石酸、苹果酸、冰乙酸及柠檬酸;营养液Pb²⁺浓度大于800mg/l对黑麦草有极大的毒害作用。（4）外源有机酸诱导修复效果的水培试验表明:有机酸促进地上干物质量和根系干重的增加。EDTA有利于重金属Pb²⁺向地上部分转移,但生物量相对较小;1-3mmol/l的丙二酸,酒石酸对地上部分Pb²⁺含量的促进作用较明显;1-3mmol/l的冰乙酸有利于黑麦草生物量的增加,同时也有利于根系重金属Pb²⁺含量的增加;不同浓度有机酸对静水体内重金属Pb²⁺吸收转移量的情况为,有机酸浓度为0.1mmol/l时,酒石酸的吸收累积量最大;有机酸浓度为0.5mmol/l时,丙二酸的吸收累积量最大;有机酸浓度在1-3mmol/l范围内时,冰乙酸的吸收累积量最大。（5）不同重金属Pb²⁺浓度下黑麦草修复的土培试验得出:土壤重金属Pb²⁺诱导黑麦草分泌草酸、苹果酸和冰乙酸;通过生理机制调节,使得根际土壤pH、Eh和Pb²⁺含量均小于非根际;黑麦草植株中吸收积累Pb²⁺量与试验土壤重金属Pb²⁺浓度直接相关,且随生长时间的推移,黑麦草植株中Pb²⁺含量增加,并有向地上部分转移的趋势,生长40-50d的黑麦草对300-500mg/kg重金属Pb²⁺的修复效果最明显;种植黑麦草后,重金属Pb²⁺浓度为200-1000mg/l范围内时,根际土壤的吸附量（率）略高于非根际,且随生长发育时间的推移差距增大;根际环境有利于Pb²⁺向交换态和有机物结合态转变,根际Pb²⁺生物有效性系数高于非根际,其差距随土壤Pb²⁺浓度的增加而减小,随生长时间的推移而增大。（6）外源有机酸诱导修复效果的土培试验分析表明:EDTA修复效果较明显,但由于易造成二次污染,不作为优选材料;黑麦草所分泌的有机酸中,苹果酸可显著增加黑麦草地上部分Pb²⁺含量,且有向地上部分转移的趋势;冰乙酸对土壤重金属Pb²⁺的提取量较大,随冰乙酸浓度的增加,其修复效果增强,3mmol/kg时最明显,其后逐渐减弱,而且黑麦草生物量较大。更多还原

【Abstract】 The use of treated and untreated sewage water for irrigation has increased substantially over the past decades in many countries because of the increased scarcity of fresh water. It is well known that sewage is rich in heavy metals, and irrigation with sewage water could therefore result in metal accumulation in soils. Once ending up in soils, the heavy metals not only contaminate soil and water environments, but also enter the food chain via root uptake by plant roots, thereby endangering people health. As a result, removing the heavy metals from contaminated soils has become an urgent issue in many counties. However, since most heavy metals are adsorptive, remediating soils polluted by heavy metals could be difficult and costly. One efficient technique developed over the past two decades is phytoremediation.The principle of phytoremediation of contaminated soils is to use super-accumulative plants to uptake the pollutants. The mechanisms of phytoremediation are complex, involving a number of physical and biochemical processes occurring in both soil and plant. Therefore, it is not surprising that our understanding of the phytoremediation mechanisms is still limited. It has been conjectured that the physical and biochemical processes induced by root exudates in rhizosphere could mobilize the immobile heavy metals, thereby enhancing uptakes by plant roots. But the mechanism associated with this is not well understood. We hypothesized that the root exudates released by super-accumulative plants into rhizosphere decrease the pH value of soil water; a decrease in pH value could enhance metal desorption, thereby mobilizing the adsorbed metals for plant roots to uptake. To test this hypothesis, we took the phytoremediation of lead by ryegrass roots as an example. A series of experiments were carried out in attempts to understand 1) if a change in pH value increases lead desorption, and 2) if the root exudates of ryegrass results in a change in pH and by how much, 3) the effect of lead concentration on the amount of root exudates released by roots and the associated root uptake.This thesis consists of nine chapters and is organized as follows. We first measured the adsorption and desorption of the Pb2 + under different pH value （3, 5, 7, 9, 11） and the various organic acids （EDTA, oxalic acid, tartaric acid, acetic acid, malonic acid, malic acid and citric acid）; the Tissier method was used to study the effect of various pH values on the form of Pb²⁺ in soils. We then experimentally investigated the efficiency of uptake of Pb²⁺ by the roots of ryegrass under Pb²⁺ concentration ranging from 10 mg/l to 1000mg/l; in the meantime, we also analyzed the organic acids exudated by ryegrass roots under various Pb²⁺ concentrations. To see if the root uptake of Pb²⁺ can be engineered, we externally added organic acids （EDTA, oxalic acid, acetic acid, malonic acid, tartaric acid, malic acid, citric acid） at various concentrations （0.1,0.5,1,2,3 mmol/l） into the solution where the ryegrass were grown. Finally, the uptake of Pb²⁺ by ryegrass roots from soil was studied. The seedling of ryegrass were grown in pots packed with soils under various Pb²⁺ concentrations （0,100,300,500,1000mg/kg） and externally added organic acids （EDTA, oxalic acid, acetic acid, malonic acid, tartaric acid, malic acid） at concentration from 1to 7mmol/kg was investigated. Mesh was used to separate the rhizopshere from the bulk soils. Results obtained from the experiments are summarized as follows.（1） The adsorption-desorption of Pb²⁺ under various pH values indicated that pH value have a significant impact on adsorption-desorption. The increasing amplitude of adsorption capacity is much larger when the pH value less than 7. The adsorption isotherm can be described by a nonlinear model ln（S）=K1×CK2, where S is the adsorbed Pb²⁺ and C is Pb²⁺ concentration in water, K1 and K2 are fitting parameters. The desorption quantity decreased with pH value, and increased with absorption capacity. And it is evident that the added organic acid improved the mobility of Pb²⁺. The order of the acid follows EDTA>citric acid> malic acid> malonic acid> acetic acid>tartaric acid>oxalic acid in terms of their impact on Pb²⁺ desorption.（2） The results on the form of Pb²⁺ under different pH value showed that when the pH value is less 7, Pb²⁺ is more bio-available if its concentration is in the range 10-100mg/l. The bio-availability decreases when concentration increases from 200 to1000 mg/l. The proportion of Fe-Mn oxides fraction is the maximum when Pb²⁺ concentration was in the range of 0-100mg/l. Residual fraction is much larger than other forms in the range of 200-1000mg/l.（3） The results measured under water culture condition and various Pb²⁺ concentrations indicated that, at concentration of 400mg/l is the inflection point to the sensitivity of organic acids to pH values, and 200mg/l is the inflection point of plant height, dry matter quantity and root tolerance index. Pb²⁺ content measured in ryegrass increased with the Pb²⁺ concentration in the nutrient solution, but the rate decreased as Pb²⁺ concentration increased. Organic acid exudation by the ryegrass roots is mainly oxalic acid and acetic acid at initial stage, but as the plants grow, they became dominated by oxalic acid, tartaric acid, malic acid, acetic acid and citric acid. The solution becomes toxic when Pb²⁺ concentration in nutrient solution was over than 800mg/l.（4） The effect of adding exogenous organic acids on the uptake of Pb²⁺ by plant roots grown in solution culture indicated that, organic acid increased the dry matter in both shoot and root. Adding EDTA enhanced the transfer of Pb²⁺ from soil to shoot, but the increase in biomass is limited. Malonic acid （from 1 to 3mmol/l）, tartaric acid can increased the content of Pb²⁺ in shoot, and Acetic acid （from 1 to 3mmol/l） increased the biomass of and the content of Pb²⁺ in roots. In terms of the effect of different organic acids on absorption and transfer of Pb²⁺ in static water, tartaric acid gave the maximum when at concentration of 0.1mmol/l, malonic acid gave the maximum when at concentration of 0.5mmol/l, and acetic acid gave the maximum when at the concentration in the range of 1-3mmol/l.（5） When grown in soil pots under different Pb²⁺ concentration, the main organic acids exudated by the ryegrass roots were found to be oxalic acid, malic acid and acetic acid. The pH value, Eh and Pb²⁺ content measured from soil taken from rhizosphere were less than that in bulk soil. The Pb²⁺ content measured in plant was directly related to Pb²⁺ concentration in soil, and increased as plan grows. The uptake of Pb²⁺ by ryegrass is the highest 40-50 days after the seedling and when Pb²⁺ concentration is in the range from 300 mg/kg to 500mg/kg. The adsorption capacity （rate） of soil in rhizosphere is slightly larger than bulk soil when the Pb²⁺ concentration is in the range of 200-1000mg/l, and the difference increases with growth stage. The rhizosphere of ryegrass can improve the proportion of exchangeable fraction and organic fraction of Pb²⁺. The bioavailability rate of Pb²⁺ in rhizosphere was higher than that in non-rhizosphere, and the difference decreased as the Pb²⁺ concentration in soil decreased. The bioavailability rate of Pb²⁺ in both rhizosphere and non-rhizosphere increased as plant grow.（6） The measurements by adding exogenous organic acids on the uptake of Pb²⁺ by ryegrass roots in soil indicated that, the remediation effect of EDTA was significant but has to be used with car as it could lead to a secondary pollution. Malic acid as one of organic acids exudated by ryegrass root could also increase the content of Pb²⁺ in the shoots. The Pb²⁺extraction amount of acetic acid treatment is more than other organic acid. And acetic acid can enhance phytoremediation effect. And it reached the maximum when concentration was at 3mmol/kg, then decreased gradually. And acetic acid could enhance the biomass.更多还原