Adsorption processes, surface complexation mechanism

Adsorption is a physicochemical process whereby ionic and nonionic solutes become concentrated from solution at solid-liquid interfaces.3132 Adsorption and desorption are caused by interactions between and among molecules in solution and those in the structure of solid surfaces. Adsorption is a major mechanism affecting the mobility of heavy metals and toxic organic substances and is thus a major consideration when assessing transport. Because adsorption is usually fully or partly reversible (desorption), only rarely can it be considered a detoxification process for fate-assessment purposes. Although adsorption does not directly affect the toxicity of a substance, the substance may be rendered nontoxic by concurrent transformation processes such as hydrolysis and biodegradation. Many chemical and physical properties of both aqueous and solid phases affect adsorption, and the physical chemistry of the process itself is complex. For example, adsorption of one ion may result in desorption of another ion (known as ion exchange). 

Interface and colloid science has a very wide scope and depends on many branches of the physical sciences, including thermodynamics, kinetics, electrolyte and electrochemistry, and solid state chemistry. Throughout, this book explores one fundamental mechanism, the interaction of solutes with solid surfaces (adsorption and desorption). This interaction is characterized in terms of the chemical and physical properties of water, the solute, and the sorbent. Two basic processes in the reaction of solutes with natural surfaces are 1) the formation of coordinative bonds (surface complexation), and 2) hydrophobic adsorption, driven by the incompatibility of the nonpolar compounds with water (and not by the attraction of the compounds to the particulate surface). Both processes need to be understood to explain many processes in natural systems and to derive rate laws for geochemical processes. 

When chemisorption is involved, or when some additional surface chemical reaction occurs, the process is more complicated. The most common combinations of surface mechanisms have been expressed in the Langmuir-Hinshelwood relationships 36). Since the adsorption process results in the net transfer of molecules from the gas to the adsorbed phase, it is accompanied by a bulk flow of fluid which keeps the total pressure constant. The effect is small and usually neglected. As adsorption proceeds, diffusing molecules may be denied access to parts of the internal surface because the pore system becomes blocked at critical points with condensate. Complex as the situation may be in theory,... 

In reality, it is believed that the oxidation of carbonaceous surfaces occurs through adsorption of oxygen, either immediately releasing a carbon monoxide or carbon dioxide molecule or forming a stable surface oxygen complex that may later desorb as CO or C02. Various multi-step reaction schemes have been formulated to describe this process, but the experimental and theoretical information available to-date has been insufficient to specify any surface oxidation mechanism and associated set of rate parameters with any degree of confidence. As an example, Mitchell [50] has proposed the following surface reaction mechanism ... 

Reductive dissolution may be more complex than the two previous mechanisms in that it involves electron transfer processes. Formation of Fe" via reductive dissolution can be effected by adsorption of an electron donor, cathodic polarization of an electrode supporting the iron oxide and by transfer of an electron from within a ternary surface complex to a surface Fe ". Addition of Fe" to a system containing a ligand such as EDTA or oxalate promotes electron transfer via a surface complex and markedly accelerates dissolution. 

The fundamental concept of chemical kinetics is that of reaction mechanism. In the broad sense, the word mechanism ("detailed , "intimate ) is the comprehensive interpretation of all experimental data accumulated on the complex reaction process. In this mechanism, one should discriminate individual stages and reaction steps, give characteristics for intermediates, describe transition states of individual steps, provide energy levels of substances, etc. As far as catalytic reactions are concerned, one should characterize surface properties, examine the adsorption character, etc. "I want to know everything about a complex chemical reaction this is the way one must understand chemists when they speak about their intention to investigate a detailed mechanism. Whether it is possible to realize such good intentions at a modern theoretical and experimental level will be another question. 

This complexity determines that investigations on heterogeneous photo-catalytic processes sometimes report information only on dark adsorption and use this information for discussing the results obtained under irradiation. This extrapolation is not adequate as the characteristics of photocatalyst surface change under irradiation and, moreover, active photoadsorption centers are generated. Nowadays very effective methods allow a soimd characterization of bulk properties of catalysts, and powerful spectroscopies give valuable information on surface properties. Unfortunately information on the photoadsorption extent under real reaction conditions, that is, at the same operative conditions at which the photoreactivity tests are performed, are not available. For the cases in which photoreaction events only occur on the catalyst surface, a critical step to affect the effectiveness of the transformation of a given compound is to understand the adsorption process of that compound on the catalyst surface. The study of the adsorbability of the substrate allows one to predict the mechanism and kinetics that promote its photoreaction and also to correctly compare the performance of different photocatalytic systems. 

Sorption is a surface phenomenon determined by the surface charges and those of the ions surrounding it. One or more of the following mechanisms can be involved in the removal of species by sorption (i) mechanical entrapment, (ii) absorption, (iii) physical sorption, or (iv) chemical sorption on the surface of the solid particle. Physical adsorption (which is weaker than its chemical counterpart) occurs through Van der Waals forces and it is generally reversible and instantaneous. On the other hand, chemical adsorption or chemisorption occurs through the formation of chemical bonds at specific sites. This is closely related to ion exchange processes and complexation. 

The complexation or chelation of trace metals with organic ligands will be discussed separately at the end of this chapter. As we will see, the chemistry and behavior of many trace metals in the water column is dominated by complexation, biological assimilation at uptake sites on cell surfaces, and adsorption on surface sites of suspended particles. All three of these processes, which are particularly important in surface waters where biological activity is most intense, are controlled by similar coordination mechanisms (Hering and Morel, 1990). 

A variety of distinct but interrelated phenomena may be involved in the adsorption process of metal ions onto activated carbons adsorption (physical adsorption or chemisorption), surface precipitation, ion exchange, and surface complexation. The metal sorption is often not the result of one mechanism but of several reactions. The mechanisms involved and their degree of importance seem to depend on the materials and the operating conditions used. 

The variety of mechanisms that may be involved in the sorption process of metal ions onto activated carbon induces a great number of factors that control the adsorption the surface oxygen complex content, the pH of point of zero charge, the pore texture of carbon, the solution pH and its ionic strength, the adsorption temperature, the nature of the metal ion given by its speciation diagram, its solubility, and its size in adsorption conditions. The influence of these various conditions is detailed in Section 24.2.1.4. 

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