【作者】 芦鹏；

【导师】 吴晓芙； 胡曰利；

【作者基本信息】 中南林业科技大学 ， 环境科学， 2007， 硕士

【摘要】 随着电镀、印染等工业的快速发展,含Cr废水的污染形势日益严峻,严重影响人类的健康和生存。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。人工湿地中吸附剂对污染物的吸附是处理工艺中重要环节之一。因此填料的选择、定量及其对重金属离子吸附行为的研究在应用人工湿地处理重金属废水中具有重要意义。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C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。在实际应用中经典吸附方程存在着吸附剂浓度效应和方程中参数不稳定等问题。受基本函数关系的限制,经典方程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/固吸附理论,建立更为科学实用的离子吸附定量模型成为当前吸附领域的研究重点之一本研究的理论基础是离子吸附四组分模型。模型认为,吸附系统应由四个必要因子组成,其分别为：液相离子C、固相离子Q、未被占据的吸附点Wu、以及被离子占据的吸附点Wc。和经典等温吸附理论不同,四组分理论认为平衡离子吸附密度qe(离子吸附量Qe与吸附剂量Wo的比值)是C0/W0(起始液相离子浓度C0与吸附剂浓度W0的比值)与Ce/W0(平衡液相离子浓度Ce与W0的比值)的一元函数。基于这一原理,可得出qe与ce的基本关系：k=Ce(qm-qe)/qe2 Ce=Ce/W0及qe与c0的基本关系(即平衡吸附预测模型)：qe={co+qm-[[(c0+qm)2-4c0qm(I-k]1/2}/[2(1-k)] co=C0/W0本研究的目的是通过探讨三个离子/吸附剂比值,qe,ce和c0之间的关系来检验四组分模型是否适合于描述固/液吸附体系中阴离子的吸附过程；并在四组分理论的基础上建立新吸附动力学方程,用来预测不同时间t点上离子的吸附量。针对以上问题,本研究选择红壤作为吸附剂,在起始离子浓度5-375 mg/L和吸附剂浓度50-250 g/L范围内进行了Cr(Ⅵ)的等温吸附试验和动态吸附试验。主要研究结果如下：(1)红壤来源广泛、廉价易得,含有大量的铁、铝氧化物凝胶,具有较强吸附Cr(Ⅵ)的能力。作为人工湿地填料,可以高效地去除废水中的Cr(Ⅵ)。(2)Cr(Ⅵ)吸附速率在初始2小时内很高,吸附在10小时左右趋向平衡。吸附达到平衡所需的时间与起始离子浓度C0和吸附剂浓度Wo相关,基本趋势是随C0增大而延长,随Wo增大而减短。(3)经典方程的应用主要存在两大问题：一是吸附剂浓度效应,表现为等温吸附曲线(即qe-Ce曲线)随吸附剂浓度增大而降低；二是方程参数不恒定为常数,随吸附剂浓度变化而变化,说明吸附密度qe不是平衡液相离子浓度Ce的一元函数。(4)遵循系统的状态函数唯一由状态间的差异来决定的热力学原理,平衡点固相离子/吸附剂比(Qe/W0)唯一由起始点液相离子/吸附剂比(C0/W0)与平衡点液相离子/吸附剂比(Ce/W0)的差异来决定,与离子、吸附剂的体积浓度及吸附反应过程无关。这意味着液/固离子吸附体系中的强度因子不是离子的体积浓度,而是离子量与吸附剂量的比值,其决定吸附反应的方向与速率。离子吸附达到平衡的条件是离子与吸附剂化学势之和在固液相之间的差异为零。(5)离子/吸附剂比试验与体积影响试验结果均表明,给定c0,qe与ce值基本保持为常数,说明qe是ce或者c0的一元函数。qe与ce而不是与Ce具有一一对应函数关系,因此经典等温曲线(即q,-Ce曲线)存在吸附剂浓度效应是一种必然现象。(6)反复测试的结果显示,平衡吸附模型预测值与试验数据吻合很好,具有较高的预测精度。新定义的qe-ce和qe-c0等温曲线基本消除了吸附剂浓度效应。(7)基于qe-c0关系,建立了新的离子吸附动力学方程：q=qe{1-[b/(b+t)]a},a=1/2,b=qe/(c0qm)1/2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、Wo条件下红壤-水溶液体系中Cr(Ⅵ)吸附动力学过程的预测模型。本试验结果证明四组分吸附模型适用于描述阴离子吸附过程。更多还原

【Abstract】 Following the rapid development of galvanization, printing and dyeing industries, Chromium-containing wastewater pollution has become a severe problem that gives potential hazard to ecosystems as well as the public health. Development of new techniques for removal of Cr (VI) from wastewaters has thus become an important topic in water environmental protection. As a new technology developed in recent years with characters of low energy consumption low operation and maintenance cost, constructed wetlands (CW) used for treatment of wastewaters containing heavy metal pollutants have received great attention in the field of environmental sciences. Removal of harmful pollutants using adsorbent as CW fillers plays an important role in the CW process for wastewater treatment. For selection of proper adsorbents with low cost and high adsorption capacity 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 concentration1 to a stipulated discharge standard. This involves mechanisms of ion adsorption in liquid/solid systems.All classical 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 existing kinetic adsorption models deal with only the relationship between adsorption density q and contacting time t. The main problem associated with classical adsorption isotherms and kinetic models is the instability of their constant parameters. Furthermore, 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. Establishment of a quantitative relationship with adsorption as a function of Co and Wois thus of high values for use of CW technology in wastewater treatment engineering practices.The theoretical ground of this study is the recently developed ion adsorption component model. According to this model, the equilibrium state of a liquid/solid ion adsorption system should be determined by four mutually related components:ions in liquid phase C, ions in solid phase Q, uncovered adsorption sites Wu and covered adsorption sites Wc. Different from that defined by classical adsorption isotherms, the equilibrium ion adsorption density qe (the ratio of the equilibrium ion adsorption Qe to adsorbent concentration Wo) in the adsorption component model is defined as a single function of Ce/Wo (the ratio of the equilibrium ion concentration in liquid phase Ce to Wo) or Co/Wo (the ratio of initial ion concentration Co to Wo) in the following forms, k=ce(qm-qe)/qe2 ce=CeWo qe={co+qm-[(co+qm)2-4coqm(1-k)]1/2}/[2(1-k)] Co=rCo/WoThe main objective of this study is to test the fitness of the developed model to anion adsorptions in liquid/solid systems with focus on examining the basic relationship among the ion/adsorbent ratios, qe, ce and co. In addition a kinetic adsorption model was further developed for prediction of anion adsorption as a function of contacting time t.Using red soil as the adsorbent, both equilibrium and kinetic adsorption experiments were therefore conducted to investigate the adsorption characteristic of Cr (Ⅵ) in red soil-aqueous solution systems in the range of initial ion concentration 5-375 mg/L and adsorbent concentration 50-250 g/L under different conditions. The experimental results are summarized as follows:(1) Red soil contains high amount of iron and aluminum oxides and has thus relatively high capacity to adsorb Cr (Ⅵ). As a natural resource widely distributed in south China, red soil can be easily obtained with very low cost and has thus also economic values when being used as a CW filler for treatment of Cr(Ⅵ)-containing wastewater.(2) The rate of Cr (Ⅵ) adsorption on red soil was found to be very high in the first 2h. After that period the adsorption rate gradually descended and finally approached to zero at approximately 10 h. The time needed for adsorption to reach its equilibrium was related to both the initial Cr (Ⅵ) concentration (Co) and the red soil concentration (Wo) in the adsorption system. The general trend was that the time needed was longer at higher initial Cr (Ⅵ) concentration levels but shorter at higher adsorbent concentration levels. (3) Two major problems were observed when applying the classical adsorption model to describe the Cr (VI) adsorption on red soil:one was a decline of the adsorption isotherm with increasing adsorbent concentration, interpreted as adsorbent effect, and the other was the inconstancy of the equation parameters, namely, the defined constant parameters varied significantly in the sample series with great variation in adsorbent concentration. The observed results suggests that unlike that defined by classical adsorption isotherms, the equilibrium Cr (VI) adsorption density qe is not a single function of its equilibrium concentration in bulk solution Ce.(4) In agreement with the theory of thermodynamics that the sate function of a system is only determined by its difference between states, the obtained results showed that the equilibrium Cr (VI) adsorption density qe was uniquely determined by the difference between Co (the ratio of initial Cr (VI) concentration Co to adsorbent concentration Wo) and ce (the ratio of the equilibrium Cr (VI) concentration in liquid phase Ce to Wo) independent of Co, Wo and the process history. This is an indication that the intensity factor in a liquid/solid ion’adsorption system is not the volume-based ion concentration but rather the ratio of ion quantity to adsorbent quantity as it is the relative level of ion quantity to adsorbent quantity that determines both the direction and rate of the ion adsorption. Ion adsorption arrives at equilibrium only when the difference in sum of ion and adsorbent chemical potentials between liquid and solid phases is zero.(5) Both the concentration and volume tests conducted at different Co and Wo levels proved that giving the Co/Wo ratio co, the CJWo ratio ce and QJWo ratio qe remained nearly unchanged, confirming that qe can be expressed as a single function of either ce or Co. As qe corresponds to a unique ce rather than to a unique Ce, the widely observed adsorbent concentration effect on traditionally defined adsorption isotherm is an expected result.(6) Results from repeated Cr (VI) adsorption tests indicated that the proposed model fit well the combined experimental data with satisfactory prediction accuracy. The adsorbent concentration effect was eliminated in both the qe-ce and qe-co plots.(7) Subject to the qe-co relation, the following kinetic equation was further proposed q=qe{1-[bl(b+t)a), a=1/2, b= qe/(coqm)1/2 Results from the kinetic experiment indicated that the above defined parameters remain nearly constant in the tested range, showing that given C0 and W0, the proposed model can be used to describe the kinetic Cr (VI) adsorption process in red soil-aqueous solutions.The result from the present study confirms that the adsorption component model can be applied to describe the anion adsorption in aqueous solution.更多还原