John D. Wells

The Influence of Temperature on the Adsorption of Cadmium(II) and Cobalt(II) on Kaolinite

The adsorption of Cd(II) and Co(II) onto kaolinite was investigated at five temperatures between 10 and 70 degreesC. Adsorption edges showed that both Cd(II) and Co(II) adsorbed onto kaolinite in two stages, separated by a plateau between pH 4 and 7. Initial adsorption commenced at about the same pH for both cations, but at each temperature the second-stage adsorption occurred at a slightly lower pH for Co(II) than for Cd(II). At higher temperatures adsorption was generally shifted to lower pH. Adsorption isotherms at pH 5.50 for both cations could be fitted closely by a simple Langmuir model at all temperatures. A two-site Langmuir model provided a substantially better fit for isotherms at pH 7.50 for Cd(II) and pH 7.00 for Co(II). At pH 5.50 the maximum adsorption density estimated from Langmuir modelling was approximately the same (1 µmol m-2) for both cations and at all temperatures. A similar value was found for one of the model sites at pH 7.50 for Cd(II) and at pH 7.00 for Co(II). Potentiometric titrations of kaolinite suspensions, in the presence and absence of added Cd(II) or Co(II), could be modeled accurately by a constant-capacitance surface complexation model. The data for adsorption of both cations could be fitted at all temperatures using a model that assumed ion exchange at permanent charge sites on silanol faces and complexation to hydroxyl edge groups. Thermodynamic parameters estimated from both the Langmuir and surface complexation models showed that adsorption of Cd(II) and Co(II) were endothermic. For the surface complexation model, enthalpies of adsorption on exchange sites were about 10 kJ mol-1, but at the variable-charge sites the enthalpy changes were about 70 kJ mol-1. For all these reactions the entropy changes were positive, with values of the order of 100 J K-1 mol-1. Trends for the Langmuir model were qualitatively similar. Copyright 1998 Academic Press.

Characterization of carbonaceous combustion residues. I. Morphological, elemental and spectroscopic features

Scanning electron microscopy, surface area determination, elemental analysis, organic matter extraction and solid-state cross polarization/magic angle spinning and Bloch decay/magic angle spinning 13C nuclear magnetic resonance (NMR) spectroscopy were used to investigate distinctive features among carbonaceous combustion residues. Black carbon (BC) samples included diesel soot, urban dust, carbon black, chimney soot, vegetation fire residues, wood and straw charcoals. Particles varied from small spheres (<50 nm) in fossil BC (>100 m(2)/g), to large layered structures in plant-derived BC (generally <8 m(2)/g). Chimney soot also included large (>1 micrometer) liquid-like structures, while spherules >100 nm were unique to urban dust. The ratios of amorphous to soot carbon (SC) (isolated by thermal degradation) were not necessarily correlated with the degree of aromaticity estimated from H/C ratios. In particular, values of SC in diesel soot were clearly overestimated. Solvent-extractable organic matter (SEOM) was <2% for charcoals and carbon black, but >13% for urban dust, chimney and diesel soot. SEOM is thought to clog pores or to form large waxy globules, hence reducing surface areas. The ratio of polar/nonpolar SEOM was generally <7 for fossil BC, but >30 for plant-derived BC. NMR analysis revealed essentially one chemical shift in the aromatic C region of charcoals, while diesel soot also showed important aliphatic contributions. Aliphatic and oxygenated C predominated over aryl C in urban dust and chimney soot. These morphological and chemical characteristics of the BC samples are discussed in terms of their environmental implications.

Adsorption of cadmium(II) on kaolinite

Abstract Three types of experiment were used to study the adsorption of Cd(II) onto two kaolinite samples at 25°C. (1) Adsorption edges were characterised by a plateau around pH 5–6 separating an initial adsorption stage beginning about pH 4, and a second stage in the pH range about 7–9. The plateau was higher for the sample with greater face area. (2) Adsorption isotherms at constant pH could be fitted closely by a simple Langmuir model at pH 5.50, but a two-site Langmuir model was better for the data at pH 7.50. One of the model sites at pH 7.50 had a similar maximum adsorption as the single site at pH 5.50, but the equilibrium constant was greater. At pH 5.50 one proton was released into the solution for the adsorption of about five cadmium ions, but at pH 7.50 the ratio was about 1:1. (3) Potentiometric titrations of kaolinite suspensions in the presence and absence of Cd(II) could be modeled very closely by a surface complexation model assuming constant capacitance. Parameters from this model were used in turn to predict the adsorption edges with remarkable precision. The results from all the experiments are consistent with the view that Cd(II) adsorbs to kaolinite by two distinct processes: ion-exchange at the permanently-charged sites on the silanol faces, and complexation to aluminol and perhaps silanol groups, which occur in particular at the crystal edges.

Selective adsorption of dyes and other organic molecules to kaolinite and oxide surfaces

Abstract Adsorption of 23 organic molecules, including a number of polyaromatic dyes, from aqueous solution to kaolinite and amorphous alumina was measured at pH 9. For 22 of the 23 solutes, adsorption was greater to kaolinite than to alumina, and more than seven times as much was adsorbed to kaolinite for eight of the 23 adsorbates. Four dyes, 3,6-diaminoacridine, 9-aminoacridine, azure-A and safranin-O, were selected for further study, including an investigation of the effect of pH on adsorption, and measurement of adsorption to gibbsite (α-Al(OH) 3 ) and silica at pH 9. Strong preferential adsorption to kaolinite was observed over the whole pH range, and at pH 9 the dyes also adsorbed much more to kaolinite than to gibbsite or silica. The basal spacing of kaolinite crystals, measured by X-ray diffraction, did not change when 3,6-diaminoacridine was adsorbed, indicating that the dye molecules were not intercalated between the crystalline layers. It is suggested that the negatively charged, flat silica faces of kaolinite crystals may serve as templates for pi-stacking association of the positively-charged polyaromatic molecules.

Adsorption of cadmium(II) onto goethite and kaolinite in the presence of benzene carboxylic acids

Abstract The effect of benzene carboxylic acids on the adsorption of Cd(II) (5×10 −5 M) by goethite and kaolinite has been studied in 0.005 M NaNO 3 at 25°C. The concentrations of phthalic (benzene-1,2-dicarboxylic acid), hemimellitic (1,2,3), trimellitic (1,2,4), trimesic (1,3,5), pyromellitic (1,2,4,5) and mellitic (1,2,3,4,5,6) acids varied from 2.5×10 −5 to 1×10 −3 M. Mellitic acid complexes Cd(II) strongly above about pH 3, but the other acids only at higher pH, phthalic acid forming the weakest complexes. Phthalic, trimesic and mellitic acids adsorbed strongly to goethite at pH 3, but adsorption decreased at higher pH; however, mellitic acid was still about 50% adsorbed at pH 9, by which the other two were almost entirely in solution. At 10 −3 M all the acids enhanced the adsorption of Cd(II) to goethite, the higher members of the series being the most effective. The higher members of the series suppressed Cd(II) adsorption onto kaolinite, but phthalic and trimesic acids caused slight enhancement. The effects of mellitic acid on Cd(II) adsorption depended strongly on its concentration. The maximum enhancement of Cd(II) adsorption onto goethite was at 10 −4 M. The greatest suppression of Cd(II) adsorption onto kaolinite was at 10 −3 M, and at 2.5×10 −5 M mellitic acid enhanced Cd(II) adsorption onto kaolinite at intermediate pH. The results are interpreted in terms of complexation between metal and ligand (acid), metal and substrate, ligand and substrate, and the formation of ternary surface complexes in which the ligand acts as a bridge between the metal and the surface.

Studies on the adsorption of dyes to kaolinite.

The strong adsorption to kaolinite of four polyaromatic, cationic dyes (9-aminoacridine, 3,6-diaminoacridine, azure A and safranin O), which adsorb much less to alumina or silica, was investigated by means of acid-base titrations, measurements of adsorption at varying pH and dye concentration, and by ATR-FTIR spectroscopy. The four dyes adsorb to kaolinite to similar extents, with little change over the pH range 3–10, but at higher pH (above the pKas of the dyes) the adsorption of 9-aminoacridine and 3,6-diaminoacridine decreases, that of azure A increases, and that of safranin O stays approximately constant. Although the dyes adsorb to kaolinite much more strongly than metal ions do, titration and spectroscopic data show that there is only limited chemical interaction between the adsorbed dyes and the kaolinite surface. The results indicate that electrostatic interaction between the dye molecules and the kaolinite surface is necessary for adsorption, but that hydrophobic interactions also contribute. It is proposed that the relatively hydrophobic silica faces of kaolinite, which carry low-density permanent negative charge, facilitate aggregation and adsorption of the positively charged, flat, aromatic dye molecules.

MODELING THE ADSORPTION OF ORGANIC DYE MOLECULES TO KAOLINITE

Simple extended constant capacitance surface complexation models have been developed to represent the adsorption of polyaromatic dyes (9-aminoacridine, 3,6-diaminoacridine, azure A and safranin O) to kaolinite, and the competitive adsorption of the dyes with Cd. The formulation of the models was based on data from recent publications, including quantitative adsorption measurements over a range of conditions (varying pH and concentration), acid-base titrations and attenuated total reflectance-Fourier transform infrared spectroscopic data. In the models the dye molecules adsorb as aggregates of three or four, forming outer-sphere complexes with sites on the silica face of kaolinite. Both electrostatic and hydrophobic interactions are implicated in the adsorption processes. Despite their simplicity, the models fit a wide range of experimental data, thereby supporting the underlying hypothesis that the flat, hydrophobic, but slightly charged silica faces of kaolinite facilitate the aggregation and adsorption of the flat, aromatic, cationic dye molecules.

SORPTION OF 3-AMINO-1,2,4-TRIAZOLE AND Zn(II) ONTO MONTMORILLONITE

Acid-base titrations and attenuated total reflectance-infrared (ATR-IR) spectroscopy of solutions containing Zn(NO3)2 and the herbicide 3-amino-1,2,4-triazole suggested that soluble complexes ZnL2+ and Zn(OH)L+ form, where L represents aminotriazole. Sorption experiments and modeling in systems containing K-saturated Wyoming (SWy-K) montmorillonite suggest that at low concentrations the aminotriazole sorbs primarily in cationic form via an ion-exchange mechanism. Sorption isotherms for aminotriazole are ‘s’-shaped, indicating a co-operative sorption mechanism as the concentration of the molecule increases. At higher concentrations, ATR-IR spectroscopy indicated the presence of cationic and neutral triazole molecules on the surface, while X-ray diffraction data suggest interaction with interlayer regions of the clay. When the concentration of the herbicide was high, initial sorption of aminotriazole cations modified the clay to make the partitioning of neutral molecules to the surface more favorable. Experiments conducted in the presence of Zn(II) indicated that below pH 7, Zn(II) and aminotriazole compete for sorption sites, while above pH 7 the presence of Zn(II) enhances the uptake of aminotriazole. The enhancement was attributed to the formation of an inner-sphere ternary surface complex at hydroxyl sites (SOH) on crystal edges, having the form [(SOZn(OH)L)]0.

COMPETITIVE ADSORPTION OF Cd AND DYES TO KAOLINITE

The competitive adsorption to kaolinite between Cd(II) and four polyaromatic dyes (9-aminoacridine, 3,6-diaminoacridine, azure A and safranin O) was studied in 5 mM KNO3 at 25°C. Under these conditions, Cd adsorbs to the silica face of kaolinite between about pH 4 and 6.5, but at higher pH, adsorbed Cd is progressively relocated to the crystal edges. In the presence of dye, less Cd adsorbed to kaolinite below pH 7. If sufficient dye was added to saturate the kaolinite surface, Cd adsorption was totally suppressed up to ∼pH 6. At higher pH, Cd followed the characteristic pattern for edge adsorption. In separate experiments 9-aminoacridine and azure A displaced pre-adsorbed Cd from kaolinite. The displacement curves were initially linear, with one Cd ion being displaced for every 13 dye molecules adsorbed at pH 5.5, and one Cd ion for every 35 dye molecules at pH 7.5. The interpretation of these results is that the dyes bind to kaolinite much more strongly than Cd(II) does, but only to the silica face.