【作者】 张洪；

【导师】 魏世强；

【作者基本信息】 西南大学 ， 农业资源利用， 2008， 博士

【摘要】 土壤重金属污染潜伏期长,危害呈慢性积累,不易被人们察觉,而且一旦污染环境,就不易从环境中去除,因此重金属常被列为人们优先考虑去除的污染物之一。土壤重金属污染的植物修复技术是广谱的原位处理技术,以其廉价、绿色、清洁等特点而备受人们的青睐。为了有效地利用植物修复技术,可以通过两条途径保证植物将重金属超量富集在地上部位。一条是广泛从自然界现有资源中筛选野生超积累植物,但超积累植物区域性分布较强、生长缓慢、周年生物量小、且移植本地时,其生态位低于本土植物,处于竞争劣势。另一条就是利用物理、化学、生物学、农艺学等技术诱导超积累植物和本地优势植物,其中化学诱导植物修复中最活跃、最有效的技术,因而越来越受到人们的广泛重视。目前,很多研究着重于重金属在土壤-植物体系中的归趋,而对根际微生态圈中重金属的化学行为和植物修复机理尚显薄弱。与此同时,大多数研究表明,十字花科植物对重金属有较强的富集吸收作用,且周年生物收获量大,甘蓝型油菜（Brassica napus L.）是我国主要农作物之一,其吸收重金属能力强,绝大部分分布在植物易收割移走的部位,因此具有较强的修复重金属污染能力。针对重庆市紫色土农地重金属污染的特点,本研究选取污染严重的Cd为代表重金属,以甘蓝型油菜为修复植物材料,通过土壤根箱试验、吸附—解吸试验、土壤盆栽试验和溶液培试验,研究了油菜根际土壤Cd的吸附—解吸过程以及根际土壤Cd的分布与形态转化特征,探讨了外源氮、螯合剂、有机酸等几类化学物质对油菜吸收Cd的诱导,为深入了解紫色土Cd的根际环境化学过程及其化学诱导修复技术提供科学依据。吸附—解吸试验表明,油菜根际土壤与非根际土壤之间存在着明显差异,虽然两者对Cd²⁺的吸附量均随着加入Cd²⁺浓度的增加而增加,但根际土对Cd²⁺的吸附能力高于非根际土。根际和非根际土壤对Cd²⁺的吸附等温线与Langmuir和Freundlich方程都有较好的拟合性。根际和非根际土壤Cd²⁺最大吸附量分别为112.3 mg kg^-1和105.4 mg kg^-1,相比于非根际土,根际土吸附的Cd²⁺更难解吸,解吸率更低。吸附—解吸试验还表明,pH和有机酸影响土壤Cd²⁺的吸附—解吸过程。Cd²⁺的解吸曲线随介质pH的增加呈“S”型曲线变化,分为最大解吸阶段（pH≤2.0）、解吸率快速降低阶段（pH2.0～5.0）以及沉淀和最小解吸阶段（pH5.0～9.0）。在pH≤2.0时解吸率最大,解吸量超过80%;在pH2.0～5.0时,随着pH增加,Cd²⁺的解吸率从85%降到18.9%左右;在pH＞5.0时,Cd²⁺的解吸率基本平稳,为沉淀和最小解吸阶段。根系分泌物有机酸种类和浓度也显著影响着土壤中Cd²⁺的解吸行为。随着柠檬酸、醋酸、苹果酸和酒石酸的浓度从10^-4增加到0.1 mol L^-1,土壤解吸的Cd²⁺有先降低后增加的趋势。低浓度(≤10^-3 mol L^-1)的有机酸抑制Cd²⁺的解吸,高浓度柠檬酸、醋酸、苹果酸(≥10^-3 mol L^-1)和酒石酸(≥10^-2 mol L^-1)等有机酸促进Cd²⁺的解吸。当解吸液中存在0.1 mol L^-1的柠檬酸、醋酸、苹果酸、水杨酸的时候,土壤吸附的Cd²⁺大部分被解吸了。在本研究中,柠檬酸或醋酸在高浓度时显著提高了Cd²⁺的解吸,其次为水杨酸、苹果酸和酒石酸。根箱培养试验表明,油菜根际Cd的形态可能是因为根系分泌作用导致的DOC、Eh和pH变化而发生显著的变化。油菜生长大约40 d左右,根际交换态Cd呈现先增加后下降的趋势,碳酸盐结合态Cd呈现了相似的变化趋势,只是碳酸盐结合态Cd的变化速度较交换态Cd慢。碳酸盐结合态Cd在油菜生长35 d达到最大,55 d后开始净损失。氧化物结合态Cd在开始增加。而有机物结合态在生长初期下降。油菜富集的Cd与生物量呈正相关关系。油菜体内累积的Cd超过了土壤交换态Cd的量,显示出低生物有效性Cd向高生物有效性Cd的转变。土壤盆栽试验表明,不同形态的氮对油菜地上部Cd的积累有显著的影响,施用铵态氮肥可以显著提高油菜地上部Cd含量,而施用硝态氮肥则对地上部Cd含量没有明显的影响。铵态氮肥能显著降低油菜根际pH,使得油菜根际残渣态、碳酸盐态和铁锰结合态Cd强烈活化,促使这三种形态Cd向交换态转换,从而增加了Cd的生物有效性。溶液培养试验表明,添加EDDS和EDTA后,油菜对培养液中重金属Cd的吸收和转运的影响。EDDS和EDTA存在下油菜根系Cd含量减少,地上部Cd含量与对照相比显著增加。EDDS和EDTA能增强油菜Cd从根系到地上部的转运,EDTA对Cd向油菜地上部转运的能力显著大于EDDS。这与EDTA可显著增强木质部液Cd浓度有关。外源有机酸可活化土壤Cd,强化植物修复效果,活化能力与有机酸类型有关。对Cd活化能力的强弱顺序为:柠檬酸＞胡敏酸＞草酸＞苹果酸＞富里酸＞酒石酸＞水杨酸＞水。与对照相比,柠檬酸和胡敏酸对油菜地上部Cd含量有一定的促进作用,也增加了油菜地上部Cd含量。本文仅从根际过程及其化学诱导等方面探索了土壤重金属植物修复技术,但应用过程的实际修复效率受到诸多因素的影响,如以重金属为代表的有机—无机复合污染、根际有机污染物的非生物氧化过程、根际微生物群落结构变化、化学诱导的土壤和地下水的环境风险等,有待今后进一步的研究。更多还原

【Abstract】 Soil heavy metal contamination impacts have long incubation period and hardly were discerned at the early stage due to slow toxic accumulation process.It is however difficult to be removed once environment receives contamination.How to remove pollutants from the habitat or render them harmless hence has been the priority in environment contamination clean-up research field. Phytoremediation,which acts as a potential strategy to remediate contaminated soils in the use of plants to remove pollutants,is now becoming an established technology for environmental clean up and protection,and increasing acquires concern due to its cheap,green and clean.In general,there are currently two means to employ Phytoremediation.One is to screen wild hyperaccumulation plants from available natural resources,which rather often gets some problems like the hyperaccumulation plants slow growth,small harvests and weak competition to native plants. Another is to utilize physical,chemical,biological,agronomical technologies to induce hyperaccumulation and native dominant plants.Among of these technologies,chemical induction as its distinctive advantages has got wide application.Currently,most researches focus on the fate of heavy metal under soil-plant ecology system, and heavy metal chemical behavior of Rhizosphere eco-system as well as Phytoremediation process are nonetheless rarely covered.Meanwhile many researches have showed that cruciferous vegetables are of strong heavy metal accumulation sorption and also liable for good annual harvest.Brassica napus L.as one of chinese main crops and cruciferous vegetables,mostly distributed in plants cutting parts can provide strong pollutants remediation.Chongqing agricultural land purple soils for many years have got heavy metal contamination and the cadmium（Cd） contamination is typically serious.In this study,Cd was chosen as heavy metal representative and Brassica napus L.was adopted as Phytoremediation material.Rhizobox cultivation,sorption-desorption behavior examination,soil plot planting and nutrient solution experiments were employed to investigate the sorption-desorption process of Cd in the Rhizosphere soil of rape and its distribution as well as forms transformation.The inductions of elevated nitrogen, chelating agents,organic acid and other chemical constitutes on rape sorption of Cd were also given detailed discussion.The main results were described as follows:Sorption-desorption behavior of Cd²⁺ in the Rhizosphere and bulk soil of rape was studied by the batch method.The results indicated that there were obvious difference between sorption capacity of Rhizosphere soil and that of bulk soil although both sorption capacities increased with the Cd²⁺ concentration growth.The sorption capacity of Rhizosphere soil to Cd²⁺ was higher than that of bulk soil.Isothermal curves of Cd²⁺ sorption by Rhizosphere and bulk soil fitted Langmuir,Freundlich equations well.The biggest sorption capacity of Rhizosphere soil and bulk soil to Cd²⁺ was 112.3 mg kg^-1,105.4 mg kg^-1 respectively.Compared with bulk soil,the desorption of Rhizosphere soil to Cd²⁺ was more difficult.The results also showed that pH and organic acid prohibited sorption-desorption of soil Cd²⁺.With the growth of pH,the desorption of Cd²⁺ exhibited S curve,which consisted of biggest desorption phase（pH≤2.0）,desorption rapid decrease phase（pH2.0～5.0） and precipitation & least desorption phase（pH5.0～9.0）.Desorption of Cd²⁺ reached peak at pH≤2.0（Desorption capacity surpassed 80%） and it decreased sharply from nearly 90.0%to 20.0%within pH 2.0～5.0 in acidic purple soils,and then decrease diminished at pH＞5.0.Meanwhile the results found that the types and concentration of root exudates organic acids also affected the sorption-desorption behavior of soil Cd²⁺.With the concentrations of citric acid,salicylic acid,acetic acid increased from 10^-4 to 0.1 mol L^-1,desorption of soil Cd²⁺ showed U curve trend.Organic acid at relatively low concentrations (≤10^-3 mol L^-1) slightly inhibited Cd²⁺ desorption,but enhanced Cd²⁺ desorption at higher concentrations with citric acid,salicylic acid,acetic acid(≥10^-3 mol L^-1) and tartaric acid(≥10^-2 mol L^-1).Citric acid,salicylic at higher concentrations(≥10^-1 mol L^-1 had the greatest improvement of Cd²⁺ desorption,followed by malic acid,acetic acid and the smallest was tartaric acid.Chemical forms of cadmium in the Rhizosphere and bulk soil of Brassica napus L.were investigated using rhizobox cultivation.The results showed that the continuous changes in cadmium fraction within the Brassica napus L.Rhizosphere might derive from DOC,Eh and pH variance.The amount of exchangeable cadmium increased before dropping below the initial level after 40 days or so.Carbonate associated cadmium followed a similar trend of change,but with a slower pace than the exchangeable cadmium.The increase in carbonate-associated cadmium reached the top,but with the net loss occurring after 55 days.There were also initial increase in oxide bound cadmium as well as decrease in the organic matter associated cadmium.The accumulation of cadmium in the Brassica napus L.was found to be positive correlation with biomass.The amount of accumulated cadmium absorbed in the plant material exceeded the initial quantity of the exchangeable cadmium in the soil, revealing a transformation from less bioavailable to more bioavailable fractions.During cultivation, decreases in redox potential and increase in pH,dissolved organic carbon（DOC）,in Brassica napus L.Rhizosphere were observed.The change in cadmium speciation may result from root-induced change in DOC,redox potential in the Rhizosphere.Soil plot planting results showed that the N form has a deep impact on the Cd in shoots of Brassica napus L..The ammonium-fed oilseed Brassica napus L.could significantly increase the content of Cd in shoots.However,the nitrate-fed oilseed Brassica napus L.could not increase the content of Cd in shoots of oilseed Brassica napus L.The rhizophere pH was significantly decreased with ammonium-based fertilizers.This changed the form of Cadmium bound to iron and manganese, cadmium bound to carbonates and Cd residual to exchange Cd.It enhanced the bioavaility of Cadmium.Nutrient solution experiments investigated the influence of the EDDS and EDTA on uptake and transport of Cd by Brassica napus L..The results showed that Cd were decreased in roots and increased significantly in shoots in the presence of EDDS and EDTA.The transportation of Cd from roots to shoots was enhanced in the presence of EDDS and EDTA and the results of Cd transportation indicated that EDTA was more effective than EDDS.It might be related to the fact that the concentration of Cd can be enhanced by EDTA via the xylem.Nutrient solution experiments also found that organic acids could mobilize soil Cd,the sequences of mobilizing ability of organic acids were followed,citric acid＞humic acid＞oxalic acid＞malic acid＞fulvic acid＞tartaric acid＞salicylic acid＞water.Compared with control,citric acid and humic acid both contributed to Cd content increase in shoot.In this dissertation,the author investigated Rhizosphere processes and mechanism involved in hytoremedition of cadmium contaminated purple soil.In addition,the results can provide scientific support for better understanding the chemical behavior of heavy metals in the hytoremedition. However,Soil contamination remediations involve many other complex factors,like organic and inorganic compound contamination,Rhizosphere organic pollutants abiotic oxidation,Rhizosphere microbial communities change,chemical induced soil and underwater risk,still further work should be employed in future research.更多还原