• 关键词：吸附  解吸  热力学  重金属  磷  天然水  颗粒物  吸附可逆性  分子界面化学

• 基金项目：面上基金(20073060);中科院“百人计划”的资助

• 潘纲
• 中国科学院生态环境研究中心,环境水质学国家重点实验室,北京 100085

• 摘要：传统吸附热力学是建立在吸附密度(吸附量,molm²)为热力学状态函数的基础之上的.MEA理论指出,实际反应的吸附密度具有非状态函数的本质.在这个新的基础之上推导所得的MEA不等式指出:传统定义的吸附反应平衡常数具有热力学非常数性,它从根本上受反应过程(如可逆性、动力学)的影响,因此无法用来准确描述实际反应的平衡特征.只有建立可以描述亚稳平衡态吸附的理论体系,才有可能准确表征实际吸附反应的平衡限度.本文综述了MEA理论的宏观热力学原理以及应用该理论解决实际环境科学问题的3个范例.一是用MEA理论解释了国际上界面吸附领域中长期悬而未解的“固体浓度效应”问题.二是用MEA理论发现了环境生物地球化学中磷循环所遵循的一个基本原理和规律.三是用MEA原理改造地球化学与土壤化学中常用的固液分配系数(K_d)的定义(即重新定义K_d).这3个范例分别代表了由MEA概念所引发的新的研究方向.最后介绍了从分子水平实验验证和发展MEA理论的最新结果以及用XAFS(X光精细结构吸收光谱)所发现的金属离子在水合金属氧化物表面上吸附的微观机制.这一发现可以统一解释金属离子在水合金属氧化物表面上静电吸附、离子交换吸附、非离子交换吸附、外层表面络合物、内层表面络合物、表面沉淀等作用.

• Abstract：Over the last century, thermodynamics played a fundamental role in the understanding of adsorption, which affected a broad range of physical, chemical and environmental processes. It has been a basic concept in thermodynamic adsorption theories that adsorption density Г(mol/m²) is a state variable (a given Г corresponds to a unique value of chemical potential) so that equilibrium adsorption constant, in which Г is used, can represent the equilibrium characteristic of a reaction. The metastable-equilibrium adsorption (MEA) theory (Pan and Liss 1998) proposed that experimentally measured Г was not a thermodynamic state variable. Equilibrium adsorption constant may therefore be fundamentally affected by the kinetics of adsorption. This implies that previously measured equilibrium adsorption data may show a lack of consistency, thereby making the use and comparison of the data problematic. When Г is not treated as a thermodynamic state variable, the basic thermodynamic surface adsorption equations will have to be re-formulated. In this review, I will first introduce the new principles/equations of the MEA theory. I will then explain how the theory can be used to solve important environmental problems, which would be otherwise impossible in the past. At last I will show how the technique of X-ray absorption fine structure (XAFS) can be used to confirm, at molecular level, the basic hypothesis of the MEA theory and to experimentally measure the MEA state of adsorbed molecules at solid-liquid interfaces. The first application of the MEA theory was to explain the anomalous phenomenon of particle concentration effect (i.e. adsorption isotherms change with C_p), which has puzzled chemists and environmental scientists over the last two decades since it cannot be explained by classical thermodynamic adsorption theories (Pan et al., 1998, 1999). According to MEA theory, particle concentration can fundamentally influence adsorption isotherms by affecting MEA state or adsorption reversibility. After the kinetic process of adsorption is finished, or a certain MEA state achieved, changes in C_p (e.g. centrifugation)will have no physicochemical effect on the adsorption isotherm. One of the inferences of MEA theory is to predict a general rule for the existence of the C_p effect. If an increase in particle concentration causes a decrease in adsorption reversibility (which is often a result of increased adsorption rate), then a C_p effect should exist. However, if a change in C_p causes no changes in adsorption reversibility or MEA states, then a C_p effect should not physicochemically exist in such a system. The second application of the MEA theory was to find a new rule in the biogeochemical cycling of phosphorus during the land-ocean-air interaction in the Eastern Mediterranean (Pan et al., 2002). A new crossover-type adsorption-desorption model was developed based on the MEA theory, which explained the contrasting adsorption-desorption behavior between Saharan dust (a source of P) and Nile particulate matter (dual roles of P in surface and deep waters). The model indicates that when natural particles are transported between different waters, they can be a sink (adsorption) or a source (desorption) of phosphorus depending on the ‘specific concentration (λ)', which is the ratio between the aqueous P concentration and the zero equilibrium P concentration (EPC₀). EPC₀ refers to the solute concentration value where the adsorption isotherm crosses over the aqueous concentration axis. When λ>1, adsorption occurs, whereas when λ<1, desorption occurs. The model added a general development to the methodology of adsorption isotherm, where, for the first time, effects of solute concentration, solid concentration and aqueous medium (EPC₀) on the adsorption and desorption of P in natural waters were simultaneously described by a single equation.

• Key words：adsorption  desorption  thermodynamics  metals  phosphorous  particles  adsorption hysteresis

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