【作者】 赵芳；

【导师】 吴晓芙； 胡曰利；

【作者基本信息】 中南林业科技大学 ， 生态学， 2007， 博士

【摘要】 采用人工湿地处理含重金属离子废水是一种新兴工艺,其低能耗、低运行成本的优点吸引着众多研究者的关注。然而,与传统处理方法相比,人工湿地的缺点是处理能力及效率较低、占地面积较大、抗有机、重金属污染物负荷与水力负荷能力有限并受季节、气候的影响。至今为止,人工湿地技术的应用尚无统一的工程规范指南。在人工湿地中引入一个吸附单元、添加具有高效吸附性能的填料是解决上述问题的有效方法之一。吸附单元的设计与填料的选择需要知道污水处理量、起始污质浓度与填料吸附能力及用量的基本关系,其涉及液/固体系吸附机理问题。经典等温吸附理论都将平衡离子吸附密度qe定义为平衡液相离子浓度Ce的一元函数,而所有经典动力学方程也只给出了吸附密度q与吸附接触时间t的关系。后人用热力学原理来解释qe-Ce关系时,认为离子吸附反应存在吸附与解吸的动态平衡,在平衡点上液相离子的化学势μL与固相离子的化学势μs相等,因此qe与Ce应具有一一对应的值,与离子的起始浓度、吸附剂浓度以及吸附过程无关。经典等温吸附与动力学方程在应用中的主要问题是方程参数不稳定。在许多情况下,吸附剂浓度效应（经典等温曲线随吸附剂浓度增大而降低的现象）是导致经典方程参数不稳定的主要原因。受基本函数关系的限制,经典方程也不能直接用于计算已知起始离子浓度C0和吸附剂浓度W0体系的吸附量。因此,改进和完善液/固体系吸附理论、建立科学实用的吸附定量模型,不仅能为人工湿地的工程设计提供基础数据与参数,同时在环境界面化学研究中也具有一定理论意义。针对上述问题,本研究选用天然矿物材料蛭石作为吸附剂,在起始离子浓度20-500 mg/L和吸附剂浓度10-150g/L范围内,布置了蛭石—水溶液体系中Zn²⁺、Cd²⁺吸附的试验。其目的是检测吸附剂浓度效应、分析吸附体系组分因子以及组分因子化学势在平衡点上的基本关系,并在此基础上建立适用的预测模型。试验结果表明：1.蛭石对环境无毒害,廉价易得,阳离子吸附容量大,能迅速、有效地去除水溶液中的Zn²⁺、Cd²⁺,适合作为人工湿地的填料；蛭石—水溶液体系离子吸附的主要机制为交换性吸附；由于存在吸附点竞争效应,共存阳离子能抑制Zn²⁺、Cd²⁺的吸附；在溶液pH 1-3.5区间Zn²⁺、Cd²⁺吸附量随pH值降低显著减小,但在pH 3.5-7区间,溶液pH对Zn²⁺、Cd²⁺的吸附没有显著影响；在15-45℃范围内,升高温度对Zn²⁺、Cd²⁺的吸附有利,但温度影响所导致的差异不显著。2.经典等温吸附曲线存在明显的吸附剂浓度效应,随吸附剂浓度W0增大,传统定义的qe-Ce等温线降低,经典方程的参数也呈现显著差异,说明与经典模型定义的关系不同,平衡吸附密度qe不唯一由液相平衡浓度e所决定,而是Ce和W0两个变量的函数。由于样本系列吸附剂浓度具有显著差异,Langmuir与Freundlich方程均不能用来描述综合样本试验数据。3.平衡离子吸附密度qe为C0/W0（起始点液相离子浓度C0与吸附剂浓度W0的比值）与Ce/W0（平衡液相离子浓度C0与W0的比值）两者之差。重复测试证实qe、Ce/W0与C0/W0三者具有一一对应的关系。观察到的现象表明液/固相离子吸附体系中的强度因子不是qe和Ce而是固相的qe与液相的Ce/W0。支持这一强度因子观点的依据是离子吸附反应的方向与速率取决于系统中离子量与吸附剂量的相对水平。4.提出了液/固离子吸附体系四组分模型理论,该模型认为吸附系统由四个必要并密切相关的因子组成,其分别为：液相离子A、固相离子B、未被占据的吸附点Wu、以及被离子占据的吸附点Wc,离子吸附基本反应式为：A+Wu======B+Wc因此对于理想的单离子吸附系统,吸附反应的平衡常数为：而离子吸附反应达到平衡的条件是离子与吸附剂化学势之和在液固相之间的差异为零,即5.基于四组分模型推导出平衡吸附预测模型：该模型的参数（吸附容量qm与吸附平衡常数k）物理意义明确,试验结果表明新模型在较大检测范围内与实测数据拟合良好。重现性检验证实平衡吸附预测模型具有较高的预测精确度。6.基于四组分离子吸附模型进一步提出了新的动力学方程：试验检测结果表明新方程的参数与起始离子浓度C0和吸附剂浓度W0具有相对稳定的函数关系,可作为给定C0、W0条件下蛭石—水溶液体系中Zn²⁺、Cd²⁺吸附动力学过程的预测模型。本研究提出的平衡体系离子吸附预测模型与离子吸附动态方程在应用吸附技术处理污水的工艺设计中具有一定的理论与实用价值。更多还原

【Abstract】 As a new technology developed in recent years with characters of low energy consumption and low operation and maintenance cost, constructed wetlands （CW） used for treatment of wastewaters containing heavy metal pollutants have received great attention in fields of environmental science and ecology. Compared with that of conventional treatment processes, however, the application of CW techniques has been limited to certain areas mainly due to their relatively low treatment efficiency, large land use area, low capacity to resist hydraulic, organic and heavy metal pollutant loads and particularly instability to seasonal changes. There are thus so far no standardized guidelines and handbooks for design of CW processes that can be used for treatment of different types of wastewaters. As an effective solution to above mentioned problems, introduction of an adsorption buffer unit using materials with high adsorption capacity into a CW system can improve its treatment efficiency as well as enhance its sustainability. For selection of proper adsorbents and design of the buffer unit for removal of metal ions it needs to know the quantitative relationship between the amount of wastewater to be treated, the metal ion concentration in the wastewater, the adsorption capacity of the adsorbent and the adsorbent quantity to be used for reducing the metal ion concentration to a stipulated discharge standard. This involves mechanisms of ion adsorption in liquid/solid systems.All traditional adsorption isotherms, when being applied to describe the ion adsorption in liquid/solid systems, define the equilibrium ion adsorption density qe as a single function of the ion concentration in bulk solution Ce while all classical kinetic adsorption models deal with only the relationship between adsorption density q and contacting time t. The fundamental ground of classical models is that it implies a "dynamic equilibrium" between liquid and solid phases and therefore the equilibrium adsorption density qe only depends on the equilibrium concentration in bulk solution Ce, irrespective of the adsorption process history. The theory of thermodynamics applied to support the qe-Ce relation is that at the equilibrium state the chemical potential of the ions in the solid phase should be equal to that in the liquid phase. The main problem associated with classical adsorption isotherms and kinetic models is the instability of their constant parameters. The adsorbent concentration effect （a phenomenon of decline of traditionally defined adsorption isotherms with increasing adsorbent concentration） has been found to be in most cases responsible for the parameter inconstancy problem. Limited by their defined functions, classical models cannot be directly used for prediction of ion adsorption for a given adsorption system with known initial ion concentration Co and adsorbent concentration Wo. Improvement of existing liquid/solid adsorption theories and establishment of quantitative relationships with adsorption as functions of Co and W0 are therefore of both theoretical and practical significances not only for scientific research in the field of environmental interface chemistry but also for use of CW technology in wastewater treatment engineering practices.Designed experiments were thus carried out to investigate the adsorption characteristic of Zn²⁺and Cd²⁺in vermiculite-aqueous solution systems in the range of initial ion concentration 25-500 mg/L and adsorbent concentration 10-150 g/L under different conditions. The main objective was to test the adsorbent effect, analyze the basic relationship among adsorption components as well as their chemical potentials, and establish proper prediction models for practical use. Main results obtained from this study are summarized as follows:1. Accounted for by its nontoxic nature, low cost and high cation adsorption capacity, the natural vermiculite was proved to be a fine wetland filler for removal of Zn²⁺and Cd²⁺from wastewaters. Ion exchange was found to be the main mechanism for Zn²⁺and Cd²⁺adsorption in the tested system. Adsorption competition between K+ Zn²⁺and Cd²⁺was observed in mixed metal adsorption systems. The negative effect of decrease in solution pH on Zn²⁺and Cd²⁺adsorptions was found to be significant only in the pH range below 3.5, showing that Zn²⁺and Cd²⁺adsorption could be significantly depressed when high amounts of H+ ions were present in the sample solution. Temperature had a positive effect on Zn²⁺and Cd²⁺adsorption but its influence was statistically insignificant in the tested range between 15 and 45℃.2. There were obviously effects of adsorbent concentration （Wo） on the traditional adsorption isotherms （i. e., qe-Ce curves）. In both Zn²⁺and Cd²⁺sample series the qe-Ce curves declined apparently with increasing Wo and the traditionally defined equilibrium constants also varied significantly at different W0 levels, showing clearly that, unlike that defined by classical models, qe is not a single function of Ce but a function of Ce and Wo. Due to presence of significant variation in adsorbent concentrations in tested samples, both Langmuir and Freundlich equations cannot be used to describe the combined data obtained from the present sample series.3. The equilibrium adsorption density qe is the difference between C0/W0 （the ratio of initial ion concentration Co to adsorbent concentration W0） and Ce/W0 （the ratio of equilibrium ion concentration in liquid phase to adsorbent concentration）. Repeated tests indicate that these three ion/adsorbent ratios are closely related with unique values in the tested range. The observed phenomenon indicates that the intensity factor in liquid/solid ion adsorption systems is not Ce but Ce/W0 in the liquid phase and Q/Wo in the solid phase. The argument to support this intensity factor concept is that it is the relative level of ion quantity to adsorbent quantity that determines the direction and the rate of ion adsorption reactions.4. Based on an assumption that the equilibrium state of a liquid/solid ion adsorption system is determined by four mutually related essential components:ions in liquid phase A, ions in solid phase B, uncovered adsorption sites Wu and covered adsorption sites Wc, an ion adsorption reaction model for an ideal system containing a single ionic species was proposed as: A+Wu======B+Wc which defines a new equilibrium coefficient as In accordance with the reaction model, the condition for ion adsorption to reach its equilibrium defined by the chemical potentials of the components will be5. Based on the four adsorption components model, the following model is established for prediction of equilibrium adsorption, The proposed model fit well the experimental data obtained from the examined samples with satisfactory prediction accuracy.6. Based on the four adsorption components model, a new kinetic equation is further presented as Results from the kinetic experiment indicate that the above defined parameters remain nearly constant in the tested range, showing that given Co and Wo, the presented equation can be used to describe the kinetic ion adsorption process for Zn²⁺and Cd²⁺adsorptions in vermiculite-aqueous solutions.The proposed equilibrium and kinetic adsorption models are of high values both in theory and practice for design of wastewater treatment processes using adsorption techniques.更多还原