胶体作用下Zn/Cd在不同质地土壤中的运移实验及其数值模拟

【作者】 王凯丽；

【导师】 徐绍辉；

【作者基本信息】 青岛大学 ， 环境科学， 2010， 硕士

【摘要】 土壤的重金属污染越来越受到人们的关注,重金属能通过多种途径进入土壤,并不断积累,由于土壤对重金属离子的强吸附性,传统上把土壤中的重金属看作是不动的。但是在环境条件变化时,土壤中富集的重金属离子会释放到土壤溶液中,并随之运移到地下水中,对地下环境造成污染。胶体是土壤中最活跃的组分,它在土壤中是普遍存在的,胶体的存在严重影响着重金属在土壤中的积累与迁移。因此,研究重金属在土壤中的吸附、解吸和迁移转化规律及胶体对其运移的影响,对防治环境污染和修复已污染土壤具有重要的理论意义。定量预测胶体在污染物运移中的作用,可以比较准确地掌控污染的趋势,以便做出更加有效的防治措施。本文以较为常见的锌、镉两种重金属和SiO2胶体以及取自辽宁沈阳地区的砂壤土、粉壤土和壤砂土作为研究对象,首先研究了静态情况下,胶体存在与否,这两种重金属在三种不同质地土壤中的吸附、竞争吸附规律,比较了不同重金属、不同土壤的吸附能力及胶体对不同重金属吸附的影响；然后探讨了胶体存在与否条件下两种重金属在不同质地土壤中的迁移、竞争迁移规律,并应用美国盐土实验室开发的CXTFIT 2.1软件分析了Zn、Cd及胶体在土壤中的穿透曲线(Breakthrough curves, BTCs),获得运移参数,进而运用这些模型参数对重金属及胶体在土壤中的迁移行为进行了预测。得出以下结论：1)静态等温吸附实验中,无论胶体存在与否,随着平衡液中Zn、Cd浓度的增加,土壤对Zn、Cd的吸附量逐渐增大；三种质地土壤对Zn、Cd吸附量顺序均为砂壤土>粉壤土>壤砂土；Zn2+、Cd2+共存时,土壤对这两种离子的吸附量比单一离子存在的情况下明显下降；胶体的存在抑制了土壤对Zn的吸附,促进了土壤对Cd的吸附；就本实验来讲,Freundlich方程拟合效果优于Langmuir方程。2)与Br-的BTCs相比,重金属的BTCs左半部分陡峭而右半部分的变化相对平缓且有“拖尾”现象。这表明存在一个持续缓慢的解吸过程,同时也说明离子在迁移过程中存在着速率限制的吸附解吸的化学非平衡现象。Zn、Cd在三种不同质地土壤中运移时,在壤砂土中运移得最快,砂壤土中运移得最慢。这是因为砂壤土pH、有机质含量以及粘粒含量较高,使得Zn、Cd迁移受阻,出流量减少。出流液pH的变化在开始都是下降的(<1 PV(孔隙体积)时),之后上升最后趋于平缓。相同孔隙体积时,pH均表现为砂壤土>粉壤土>壤砂土。3)胶体的存在明显促进了Zn的迁移,这是因为胶体比土壤表面对Zn显示出更高的亲和性,从而减少了土壤对Zn的吸附；而Cd的迁移受到一定程度的抑制,穿透曲线拐点出现时间延后,相对浓度峰值变小。这是因为Cd2+影响了胶体的稳定性,部分胶体及其吸附的Cd从流动相中分离出来聚沉于土壤表面,从而使得Cd出流量减少。在三种土壤中,加胶体时Zn出流液pH高于不加胶体的；加胶体时Cd出流液的pH低于不加胶体的。4)在三种不同质地土壤的淋滤实验中,对比单一重金属离子的出流特点发现,不加胶体时Cd的相对浓度峰值高于Zn的；加胶体时Zn的相对浓度峰值高于Cd的。这主要是由胶体吸附Zn、Cd的能力不同以及Zn、Cd对胶体的稳定性作用不同引起的。在竞争实验中,无论胶体存在与否,Zn、Cd开始出流的时间比非竞争时的提前,峰值也要高。这是因为二者共存时相互竞争结合点位使吸附受到彼此的牵制。其中竞争时Cd的出流量较Zn少,Cd的竞争吸附能力大于Zn,这与其本身的性质有关。不加胶体时,单一Cd出流液pH高于单一Zn的；加胶体时,单一Zn出流液pH’高于单一Cd的；无论胶体存在与否,竞争迁移出流液pH都最低。5)不同重金属对胶体运移的影响有较大差异,Zn单独存在的溶液中胶体运移最快,表现为出流峰值最高,其次是单一Cd溶液,Zn、Cd共存条件下出流液中胶体峰值最低。这主要是由重金属离子类型的不同引起的,不同阳离子由于聚沉能力的差异对胶体的稳定性影响不同。土壤质地不同对胶体的出流量的影响结果是,壤砂土出流液中胶体峰值最高,其次是粉壤,砂壤土出流液中峰值最低,这主要是受土壤粘粒含量不同的影响。6)在将静态吸附的数据用Langmuir、Freundlich方程求得阻滞因子Rd (retardation factor)用于拟合Zn、Cd的过程中,发现除壤砂土不加胶体时Zn的BTC需用Langmuir方法求得的’Rd值拟合外,其余情况均用Freundlich方法拟合得更好。这可能是因为：①各个吸附位点的能量不完全相同；②有可能存在吸附质在表面的反应。而Freundlich考虑了不均匀表面的情况,尤其是在适中浓度时,能够很好的符合。7)采用非平衡两区模型(Two-region Model, TRM)拟合Br-的BTCs得到的可动水体所占的比例接近于1,表明研究重金属在土柱中迁移时可忽略物理非平衡的影响。由非平衡两点模型(Two-site Model, TSM)对Zn、Cd及胶体的迁移行为进行模拟可以看出,拟合BTCs的r2>0.801,MSE<0.0098,这说明应用TSM能较好地描述Zn、Cd及胶体的迁移行为。TSM拟合Zn、Cd及胶体得出的ω值均远小于100,α值变化趋势相同,但f值则无明显的规律性。本文还应用TSM预测了砂壤土不同深度处Zn、Cd及胶体浓度的动态变化,随着深度由5cm增加到15cm,它们的相对浓度对孔隙体积的响应越来越滞后。更多还原

【Abstract】 Heavy metal ions are the most toxic inorganic pollutants which occur in soils. The heavy metals which are introduced into the soils either accumulate in the surface layer or transport into the groundwater. Colloid is the most active component of soils which is ubiquitous in soils, and it has a critical impact on the accumulation and migration of heavy metals in soils. Therefore the study of transport processes is of utmost importance for the evaluation of environmental risks. First of all, the present study evaluates the adsorption and competitive adsorption of two heavy metals (Zn, Cd) in three types of soils in Shenyang, Liaoning Province by means of sorption isotherms, evaluates the capacity of these soils to sorb these metals, establishs the selectivity sequences of the metals (Zn, Cd) in these soils, and investigates the effect of colloid on the adsorption of these metals by soils. And then, we are to evaluate the transport and competitive transport of the metals (Zn, Cd) in these soils based on miscible displacement experiments when colloid is present or absent and CXTFIT2.1 package is used to determine model parameters and predict the outflow dynamics. The conclusions have been drawn as followed:The adsorption amounts of zinc and cadmium in soils are increased with its increasing equilibrium concentration; the sequence of their adsorption amounts by three soils is sandy loam> silty loam> loamy sand; the adsorption amounts of two ions in the competitive experiment are less than that in single system; and the adsorption amount of zinc is decreased because of the colloid, on the contrary, the adsorption amount of cadmium is increased. In this paper, the Freundlich equation is more accurately than the Langmuir equqtion to describe adsorption behavior of two heavy metals.The BTCs (Breakthrough curves) of heavy metals from the transport experiment are exceedingly asymmetric. That indicates that there is a continuous slowly desorption process, and shows the existence of chemical non-equilibrium phenomena in the migration of ions. The soil textures can obviously influence the shape and peak values of the BTCs of heavy metals in soil column because of the difference of soil pH, organic matter content and clay content. The change of the leachate pH is degressive, then leveling off. When the pore volume is same, the sequence of pH is sandy loam> silty loam> loamy sand.In this paper, the colloid can advance the transport of zinc and restrain the transport of cadmium. Because the colloid has higher affinity than the soil for zinc, and cadmium affects the stability of colloid. The pH of zinc adding colloid is higher than that without colloid in three soils, whereas that of cadmium is the opposite.In the single system, when there is no colloid, the peak value of cadmium is higher than that of zinc, the pH of cadmium is higher than that of zinc; when colloid is present, the peak value of cadmium is lower than that of zinc, the pH of zinc is higher than that of cadmium. In the competitive experiment, the resident concentration of heavy metals is higher than that in the single system regardless of the presence or absence of colloid, the competitive capacity of cadmium is higher than that of zinc, and the pH is the lowest.The BTCs of colloid from the transport experiment are affected by the different types of heavy metals. The sequence of the migration velocity of colloid of different solutions is the single Zn solution> the single Cd solution> their competitive solution. And the sequence of its migration velocity in three soils is loamy sand> silty loam> sandy loam.When we use the retardation factor to fit the BTCs, we find the retardation factors from Freundlich are suitable in most cases. Because the energy of each adsorption site is not exactly the same, and there may be the reaction of adsorbate on the surface. The Freundlich considers the surface irregularity, and it can be a very good fit especially in moderate concentrations.The proportions of the movable water are close to 1 which are gotten through fitting the BTCs of Br- by TRM (Two-region Model). It suggests that we can neglect the effect of the physical non-equilibrium in the migration of heavy metals. The BTCs of heavy metals and colliod in soil column can all be fitted by model TSM (Two-site Model) with high goodness of fit (r2>0.801, MSE<0.0098). Through these model parameters acquired by the model TSM, we can predict the BTCs of heavy metals and colliod at different depth of soil columns. As the depth of soil column increased from 5cm to 15cm, the hysterestic effect of response of relative concentration of heavy metals and colliod to the pore volumes will be more and more obvious.更多还原