Reaction sorption

Reardon, E.J., Kjs—Can they be used to describe reversible ion sorption reactions in contaminant migration Groundwater 19, 279-286, 1981. 

P, mendocina, P. aeruginosa, B. subtilis, and B. cereus adsorbed more Cd, Co, and Pb after exposure to acidic solutions than the corresponding unexposed cells. The increase in sorption following acid treatment was attributed to the irreversible displacement of structurally bound Mg and Ca by protons. The protonated sites can participate in reversible metal sorption reactions. 

In order to test the reversibility of metal-bacteria interactions, Fowle and Fein (2000) compared the extent of desorption estimated from surface complexation modeling with that obtained from sorption-desorption experiments. Using B. subtilis these workers found that both sorption and desorption of Cd occurred rapidly, and the desorption kinetics were independent of sorption contact time. Steady-state conditions were attained within 2 h for all sorption reactions, and within 1 h for all desorption reactions. The extent of sorption or desorption remained constant for at least 24 h and up to 80 h for Cd. The observed extent of desorption in the experimental systems was in accordance with the amount estimated from a surface complexation model based on independently conducted adsorption experiments. 

To cast the model in general form, we begin with the basis shown in Equation 9.5 and write each sorption reaction in the form of Equation 9.7. The mass action equation corresponding to the reaction for each sorbed species Aq is... 

When more than one sorption reaction is considered, of course, the summation includes the other sorbed species. 

To be useful in modeling electrolyte sorption, a theory needs to describe hydrolysis and the mineral surface, account for electrical charge there, and provide for mass balance on the sorbing sites. In addition, an internally consistent and sufficiently broad database of sorption reactions should accompany the theory. Of the approaches available, a class known as surface complexation models (e.g., Adamson, 1976 Stumm, 1992) reflect such an ideal most closely. This class includes the double layer model (also known as the diffuse layer model) and the triple layer model (e.g., Westall and Hohl, 1980 Sverjensky, 1993). 

In order for an ion to sorb from solution, it must first move through the electrical potential field and then react chemically at the surface. To write a mass balance equation for sorption reactions (such as Reactions 10.1-10.4), therefore, we must... 

Nearly all of the data are collected at room temperature, and there is no accepted method for correcting them to other temperatures. Far fewer data have been collected for sorption of anions than for cations. The theory does not account for the kinetics of sorption reactions nor the hysteresis commonly observed between the adsorption and desorption of a strongly bound ion. Finally, much work remains to be done before the results of laboratory experiments performed on simple mineral-water systems can be applied to the study of complex soils. 

The iteration step, however, is complicated by the need to account for the electrostatic state of the sorbing surface when setting values for mq. The surface potential T affects the sorption reactions, according to the mass action equation (Eqn. 10.13). In turn, according to Equation 10.5, the concentrations mq of the sorbed species control the surface charge and hence (by Eqn. 10.6) potential. Since the relationships are nonlinear, we must solve numerically (e.g., Westall, 1980) for a consistent set of values for the potential and species concentrations. 

The As(OH)4 and Cr+++ components follow a pattern distinct from the other metals (Fig. 14.11), sorbing at only near-neutral pH. This pattern results from the manner in which the metals speciate in solution. As(III) appears as As(OH)3 when pH is less than 9, and as As(OH)4, or As020H at higher pH. The sorption reactions for these species are,... 

The chromium follows a similar pattern. The component, present as Cr+++ at low pH, reacts successively to form CrOH++, Cr(OH), Cr(OH)3, and CrfOIpJ as pH increases. The sorption reactions are,... 

In simple cases, the mobility in the subsurface of a sorbing contaminant can be described by a retardation factor. Where contaminated water passes into a clean aquifer, a reaction front develops. The front separates clean, or nearly clean water downstream from fully contaminated water upstream. Along the front, the sorption reaction removes the contaminant from solution. The retardation factor describes how rapidly the front moves through the aquifer, relative to the groundwater. A retardation factor of two means the front, and hence the contamination, will take twice as long as the groundwater to traverse a given distance. 

For a species affected only by an equilibrium sorption reaction, if the species sorbed concentration C5. depends directly on its dissolved concentration Q, the reaction rate is,... 

The distribution of metals between solution and the ferric hydroxide surface varies strongly with pH (Fig. 31.5). As discussed in Sections 10.4 and 14.3, pH exerts an important control over the sorption of metal ions for two reasons. First, the electrical charge on the sorbing surface tends to decrease as pH increases, lessening the electrical repulsion between surface and ions. More importantly, because hydrogen ions are involved in the sorption reactions, pH affects ion sorption by mass action. The sorption of bivalent cations such as Cu++,... 

For a first assessment of the performance of the different materials, batch experiments were carried out. The kinetics of the sorption processes of arsenic onto the different materials should give an indication of their efficiency. Figure 1 shows the results for the measured As(V) concentrations in dependence on time. The activated carbon gives poor results, as expected. However, the Zr loaded activated carbon shows a rapid reaction. The zirconyl ions at the surface of the activated carbon are a highly efficient phase for the sorption of arsenate. The half-life of this sorption reaction was < 10 min. 

The surface complexation approach is distinct from the Stern model in the primacy given the specific chemical interaction at the surface over electrostatic effects, and the assignment of the surface reaction to the sorption reactions themselves (Dzombak and Morel, 1990). 

Reaction kinetics. The time-development of sorption processes often has been studied in connection with models of adsorption despite the well-known injunction that kinetics data, like thermodynamic data, cannot be used to infer molecular mechanisms (19). Experience with both cationic and anionic adsorptives has shown that sorption reactions typically are rapid initially, operating on time scales of minutes or hours, then diminish in rate gradually, on time scales of days or weeks (16,20-25). This decline in rate usually is not interpreted to be homogeneous The rapid stage of sorption kinetics is described by one rate law (e.g., the Elovich equation), whereas the slow stage is described by another (e.g., an expression of first order in the adsorptive concentration). There is, however, no profound significance to be attached to this observation, since a consensus does not exist as to which rate laws should be used to model either fast or slow sorption processes (16,21,22,24). If a sorption process is initiated from a state of supersaturation with respect to one or more possible solid phases involving an adsorptive, or if the... 

It is for this reason that spectroscopy offers the only experimental method for characterizing the interfacial region that is not automatically destined to run into basic conceptual difficulties. This is not to say that difficulties of a technical nature will not arise (40-48), nor that the conceptual difficulty of differing time scales among spectroscopic techniques will cause no problems (50). Nonetheless, it is to be hoped that future investigations of sorption reactions will focus more on probing the molecular structure of the mineral/water interface than on attempting simply to divine what the structure may be. 

G. Barkhordarian, T. Klassen, R. Bormann, Kinetic investigation of the effect of milling time on the hydrogen sorption reaction of magnesium catalyzed with different Nb205 contents, J. Alloys Compd. 407 (2006) 249-255. 

Most of these sensors fit the general configuration shown in Fig. 5.1.C, i.e. the simultaneous sorption, reaction and detection involved cannot be resolved in space or time. However, separation occasionally takes place in the same zone, but sequentially, before (type A) or after (type B) the (bio)chemical reaction, whereas detection is simultaneous with the reaction or separation, respectively. 

The descriptions of sensors integrating sorption, reaction and detection provided below are classified according to the type of immobilization... 

In this case study, the selected phases are pyrite, amorphous FeS, calcite (present in limestones in the roof strata Fig. 5), dolomite (possibly also present in the limestones), siderite (which occurs as nodules in roof-strata mudstones), ankerite (present on coal cleats in the Shilbottle Seam), melanterite and potassium-jarosite (representing the hydroxysulphate minerals see Table 3), amorphous ferric hydroxide (i.e., the ochre commonly observed in these workings, forming by precipitation from ferruginous mine waters), and gypsum (a mineral known to precipitate subaqueously from mine waters with SO4 contents in excess of about 2500 mg/L at ambient groundwater temperatures in this region, and with which most of the mine waters in the district are known to be in equilibrium). In addition, sorption reactions were included in some of the simulations, to contribute to the mole transfer balances for Ca, Na, and Fe. 

They used the constant capacitance model (surface capacitance of 18F/m2) to fit the following three sorption reactions to observed absorption edge data ... 

This example illustrates the qualitative nature of information that can be gleaned from macroscopic uptake studies. Consideration of adsorption isotherms alone cannot provide mechanistic information about sorption reactions because such isotherms can be fit equally well with a variety of surface complexation models assuming different reaction stoichiometries. More quantitative, molecular-scale information about such reactions is needed if we are to develop a fundamental understanding of molecular processes at environmental interfaces. Over the past 20 years in situ XAFS spectroscopy studies have provided quantitative information on the products of sorption reactions at metal oxide-aqueous solution interfaces (e.g., [39,40,129-138]. One... 

In the column infiltration experiments with strontium, the model predictions closely resemble the experimental curves for the four flow rates compared. The input parameters to the ARDISC model were derived from experimental data obtained in infiltration experiments. The model predictions were based on the assumptions that the rate for adsorption and the rate for desorption were equal and that the sorption reactions were both first order. 

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