重金属污染土壤动电修复过程试验研究

【作者】 林君锋；

【导师】 李玉林；

【作者基本信息】 福州大学 ， 环境工程， 2005， 硕士

【摘要】 重金属土壤污染是国内亟待解决的环境问题,土壤污染具有累积性、滞后性、隐蔽性和不可逆性等使其治理难度加大,因此重金属污染土壤修复技术是保证重金属污染耕地持续利用的一个重要环节。本课题分别以模拟重金属污染土壤和实际重金属污染土壤为研究对象,以动电修复技术为研究手段,就固定电压条件下的动电过程、固定电流密度条件下的动电过程、具有较强pH缓冲能力土壤的动电改良措施以及实际污染土壤的处理效果进行了研究分析,得出以下结论:1.在固定电压条件下的动电试验结果表明:不控制阴阳极液的pH值,阴极附近土壤层出现pH突跃导致重金属Cd和Pb在此处沉淀,不利于重金属Cd和Pb迁出土壤层;阳极附近的土壤层过度酸化导致其pH值低于土壤的零电荷点PZC,出现不利于重金属去除的反渗流现象。通过采取控制阴阳极液pH值的改良措施,可有效防止或消除上述现象的发生,从而提高整个土壤层的重金属去除率。其中全态Cd和Pb平均去除率分别由改良前的68%和37%提高到改良后87%和51%,相应地有效态Cd和Pb的平均去除率分别由81%和50%提高到97%和73%。2.在固定电流密度条件下的动电试验结果表明:电流密度的大小直接影响土壤pH值的变化程度,但其与土壤pH值突跃点的关系不大;土壤温度随电流密度的增加而升高,这对降低孔隙液粘度,提高电渗流流速有积极作用,但土壤温度过高会引起水分蒸发而降低土壤的含水率,影响电渗流的流动并最终导致重金属去除效率的降低;而且电流密度越高总电压也越高甚至有可能超过电源额定电压,因此确定合适的电流密度是重要的。通过对三种电流密度条件下的土壤pH、总电压以及温度的变化进行综合考虑,确定较合适的电流密度为1.06mA/cm2;在此电流密度条件下,通过采取控制阴阳极液pH值的改良措施,可有效防止或消除上述现象的发生,但总电压随运行时间的延长而降低,这会减缓重金属离子向阴极方向的迁移速率,此时整体土壤中有效态Cd的平均去除率约为95.6%,有效态Pb的平均去除率约为48%。3.以模拟具有较强pH缓冲能力的Cd污染土壤为研究对象,研究采用阴极液不加EDTA和加EDTA两种试验手段对比Cd污染物的去除情况,结果表明:控制阳极液为中性,阴极液为弱碱性可以有效抑制氢离子迁入土壤中,为Cd金属络合物的形成创造条件;阴极液不加EDTA时,Cd污染物较难去除,当阴极液 1<WP=3>中加入EDTA时,其络合阴离子在电场作用下向阳极方向迁移并浸透土壤,同时与重金属Cd络合形成螯合络合阴离子并迁向阳极,达到去除金属Cd的目的;但由于形成的螯合络合阴离子的迁移方向与电渗流流向相反,使得其迁移速度降低,因而要获得较好的去除效果需延长试验时间;通过对电流密度变化的分析进一步证明了在阴极液中加EDTA络合剂能够有效去除具有较强pH缓冲能力的土壤中的Cd污染物。4. 动电修复技术对实际土壤中不同重金属的去除效果与土壤中重金属的化学形态、移动性以及其与土壤的结合力有着密切关系。其中,对于同一种重金属,其各形态的去除率大小顺序为交换态>碳酸盐结合态> Fe-Mn氧化结合态>有机结合态>残留态;对于不同重金属,各形态Pb的去除率均较相应形态的Cd、Cu和Zn低,说明具有高移动性和弱吸附性的重金属较具有弱移动性和强吸附性的重金属去除效果好;由能耗分析可知,试验时间以144小时计,单位能耗为153kWh/m3,试验时间以96小时计,单位能耗为109kWh/m3,以Cd为例,从第96小时至144小时,其全量去除率仅上升了4.1%,而能耗却增加了40%,可见试验时间的控制应综合考虑去除效果和能耗因素。更多还原

【Abstract】 Soil contaminated by heavy metals is one of the environmental problems thatshould be resolved. Soil pollution is accumulative, lagging, dormant, andirretrievable and so on, which causes its treatment to be more difficult. Therefore,remeditiation technology used in heavy-metal contaminated soil is an importantlink to guaranteeing continual farm. These topic, which adopted the simulatedheay metal contaminated soil and the acutal heavy soil respectively, studied theelectrokinetic processes in condition of both the fixed voltage and the fixedelectric’current, the enchanced measure applied to remediation of the strong pHbuffering capacity soil and the the processing effictiveness of the actualheavy-metal contaminated soil by using electrokinetic technology as the researchmethod. The tests drew the conclusions below:(1) Conditoioned on the fixed voltage, the electrokinetic test indicated that improvement of pH in the soil near the cathode without controlling the pH of catholyte, caused heavy metal Cd and Pb to precipitate in the area and didn’t favor removal of Cd and Pb; the soil near the anode was acidified excessively without controlling the pH of anolyte,which caused the soil’s pH to be lower than its PZC leading to counter-electroosmosis that didn’t favor the removal of Cd and Pb. The enchanced measure to control the pH of eltrolyte effectively prevented above phenomenon from occuring, then improved the removal efficiencies of heavy metal Cd and Pb in entire soil. Conditioned on the enchanced condition, average removal efficiencies of Cadmium and Lead in soils improve from 68% and 37% to 87% and 51% in the total digestion, respectively; correspondingly from 81% and 50% to 97% and 73% in theexchangeable fraction, respectively.(2) Conditioned on the fixed current density, the electrokinetic test indicated that size of current density directly influenced the change extent to pH in soil, but didn’t correlate with the jumping point of pH in soil: the temperature in soil increases along with the improvement of current density, which enchanced the rate of electroosmosis flow due to reducing the hole fluid viscosity, but the excessive temperature caused moisture evaporation lead to reducing the moisture content in soil which affected electroosmosis flow 3<WP=5>causing the removal efficiency to reduce; moreover, the total voltage rose with the current density, and it was possbile that the total voltage surpassed the rated voltage in power source; therefore, it was important to determine the appropriate current density. In terms of the change of pH of soil, total voltage and temperture in soil by three kind of current density experience, 1.06mA/cm2 was the more appropriate current density. Under this current density condition, the enchanced measure to control the pH of eltrolyte effectively prevented above phenomenon from occuring, but the total voltage reduced with running time, which could reduce the velocity of migration of the heavy metal Cd2+ and Pb2+ ions to cathode. Under the enchanced condition, average removal efficiencies of Cadmium and Lead in soils was 95.6% and 48% in the exchangeable fraction, respectively.(3) The test, which adopted the simulated Cd-contaminated soil having strong pH buffering capacity, studied the processing effictiveness of the Cd pollutant by contasting to two experiments by not adding EDTA to the catholyte and adding EDTA to the catholyte. The test indicated that keeping neutrality in anolyte and weak basicity in catholyte suppressed the hydrogen ions to move into the soil, which made the conditon for forming Cd complex compoud. It was difficult to remove heavy metal Cd without EDTA in catholyte; however, EDTA complexing anions moved toward the anode and soak the soil under the electric field by adding EDTA to the catholyte, and formed the che更多还原