Anion adsorption mechanism

The surface proton adsorption which occurs after Step 2, however, complicates the determination of the heat content change resulting from anion adsorption. In order to make this correction, the heat associated with proton adsorption must be determined from the previous potentiometric-calorimetric titrations. Proton adsorption on goethite is exothermic, and Figure 1 provides an average value of -29.6 kj/mol near pH 4. This value, when multiplied by the moles of protons required to return to pH 4 after anion adsorption, allows correction for the heat associated with proton adsorption. This correction, however, is based on the assumption that the proposed two-step anion adsorption mechanism described above represents the only surface reactions which occur during anion adsorption. As such, the results obtained by this procedure are model dependent and are best used for comparative purposes. 

Liu F, De Cristofaro A, Violante A (2001) Effect of pH phosphate and oxalate on the adsorption/desorption of arsenate on/from goethite. Soil Sci 166 197-208 Livesey NT, Huang PM (1981) Adsorption of arsenate by soils and its relation to selected properties and anions. Soil Sci 131 88-94 Manceau A (1995) The mechanism of anion adsorption on iron oxides Evidence for the bonding of arsenate tetrahedra on free Fe(0, OH)6 edges. Geochim Cosmochim Acta 59 3647-3653. 

Figure 6.1 A simple electrostatic adsorption mechanism illustrating the protonation-deprotonation chemistry of surface hydroxyl groups on oxide surfaces (which are neutral at the PZC) and the corresponding uptake of anionic or cationic complexes. Proton transfer to or from the surface can significantly affect the solution pH.

Figure 6.3 Electrostatic adsorption mechanism of Brunelle [1] (a) common peroxide and chloride anionic complexes (b) common cationic ammine complexes (c) table of oxides with PZCs and predicted tendencies to adsorb anions, cations, or both.

The adsorption isotherm of sodium dodecyl sulfate (SDS) on alumina at pH = 6.5 in 0.1 M NaCI (Fig. 4.11a) is characteristic of anionic surfactant adsorption onto a positively charged oxide. As shown by Somasundaran and Fuerstenau (1966) and by Chandar et al. (1987), the isotherm can be divided into four regions. These authors give the following explanation for the adsorption mechanism ... 

Yates, D. E., and T. W. Healy (1975), "Mechanism of Anion Adsorption at the Ferric and Chromic Oxide/ Water Interfaces", J. Colloid Interface Sci. 52, 222-228. 

Evidence from pressure jump kinetic studies (31,32) has suggested that anion adsorption proceeds according to the following mechanism ... 

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... 

Adsorption of anions on oxides is usually accompanied by the uptake of protons (or the release of hydroxyl ions). The ratio between the number of protons that are coadsorbed and the level of anion adsorption is not usually stoichiometric. Studies of adsorption of oxyanions on goethite as a function of pH appear to indicate that, provided only one adsorbed species is present, the proton anion ratio is related to the mode of adsorption (Rietra et al., 1999). It follows that oxyanions which adsorb with a similar proton/anion stoichiometry, have a similar adsorption mechanism. 

Manceau, A. (1995) The mechanism of anion adsorption on iron oxides. Evidence for the bonding of arsenate tetrahedra at free Fe(0, OH) edges. Geochim. Cosmochim. Acta 59 3647-3653... 

Yates, D.E. Healy,T.W. (1975) Mechanism of anion adsorption at the ferric and chromic oxide/water interfaces. J. Colloid Interface Sd. 52 222-228... 

Cao L, Liu Z, Zhu T (2006) Eormation Mechanisms of Non-Spherical Gold Nanoparticles During Seeding Growth Roles of Anion Adsorption and Reduction Rate. J Colloid Interface Sci 293 67-69... 

The UPD of Zn on Pt(lll) electrode in phosphate solutions (pH 4.6 or 3.0) was studied by Aramata and coworkers [194] in the presence of adsorbed anions. In phosphate solutions, the Zn UPD obeyed the Langmuir adsorption mechanism. On the addition of 10 M Br ions, the Zn + ion UPD potential shifts 120 mV to more negative values. The desorption of specifically adsorbed anions is suggested to trigger the UPD of Zn. 

Hydroxotitanate anion, however, has never been detected in the course of hydrolysis of titanium alkoxides. On the basis of electron microscopy data, Diaz-Guemes et al. [477] suggested the two-step adsorption mechanism for the above reaction. According to his assumption hydrolysis of titanium alkox-ide results in a gel of hydrated titanium oxide, which is further diffused by M2+ cations to form crystalline MDTi03 ... 

It has been demonstrated that mixed oxides obtained from calcined LDHs have the ability to act as sorbents for a variety of anionic compounds from aqueous solution. This ability is because of the propensity for the mixed oxide to hydrate and re-form an LDH in such conditions and is of particular interest for the decontamination of waste-water. Hermosin et al. have found, for example, that MgAl-LDHs calcined at 500 °C are potential sorbents for the pollutants trinitrophenol and trichlorophenol from water [208, 209]. The adsorption mechanism was shown, using PXRD, to involve reconstruction of the LDH, with the uptake of the phenolate anions into the interlayers. Similarly, the ability of calcined MgAl-LDHs to remove nitriloacetate anions from solution has been demonstrated [210]. Calcined LDHs have been utilized also for the sorption of radioactive anions, such as 111, from aqueous solution [211]. A particularly attractive feature of the use of calcined LDHs for the remediation of waste-water is that the sorption capacity of the material may be regenerated via calcination of the rehydrated LDH. 

SO4-2 was less adsorbed in Ti02 than NC>3 and Cl-. Nevertheless, SO4-2 inhibits E. coli inactivation to a higher extent than Cl- and N03 . This suggests that other mechanisms about anion adsorption on TiC>2 are involved in the inhibition of the photocatalytic disinfection by certain anions. 

Table VII presents a summary of calorimetric measurements of the differential heat of adsorption of ammonia, water, and carbon dioxide on the sodium form of ZSM-5 zeolite. Ammonia adsorption at 416 K (97.147) shows that NaZSM-5 zeolite is weakly acidic, whereas CO adsorption (147) indicates that in addition there are some weak basic sites. It should be noted that of the two samples studied with ammonia adsorption one was 70% H exchanged and the sodium content of the other was not given. Water adsorption on NaZSM-5 displayed unusual behavior, with a steep increase in the differential heat of adsorption at high surface coverages (166). An adsorption mechanism was proposed to explain these findings in which adsorption occurs first on the hydrophilic sites, consisting of sodium cations and framework anions where water molecules are bound by dipole-field interactions. Further adsorption takes place near these sites through weak interaction with zeolite surfaces, and when the number of water molecules close to these sites exceeds a certain value, they tend to reorient by forming clathrate-like struc-...

More recently, Peraniemi et al. [229] advocated the use of zirconium-loaded activated charcoal as an effective adsorbent for mercury (and especially for arsenic and selenium), the rationale being that the presence of active metal on an impregnated charcoal surface can greatly affect the adsorption affinity. They compared the pH effects on the uptakes by both a loaded and an unloaded commercial charcoal powder and concluded that the adsorption mechanism of mercury differs from that of the anionic arsenic and selenium species. They also noted the highly complicated behavior of mercury in aqueous solutions and did not attempt to explain the apparent absence of pH dependence of the uptakes. 

Here we summarize the key developments with emphasis on the post-1985 period and the role of the carbon surface. Bansal et al. [35] have grouped the postulated mechanisms as follows (1) reduction theory (2) ion-pair adsorption theory (3) aurocyanide anion adsorption theory and (4) cluster compound formation theory. 

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