Chemisorption of Cu(II) and Co(II) on Allophane and Imogolite

Abstract

Abstract--Adsorption of Cu 2+ and Co 2+ by synthetic imogolite, synthetic allophanes with a range of SiO2/ A1203 ratios, and allophanic clay fractions from volcanic ash soils was measured in an ionic medium of 0.05 M Ca(NO3) 2. The effect ofpH (and metal concentration) on adsorption was qualitatively similar for the synthetic and natural allophanes with relatively minor changes in behavior caused by variable SLOE/ A1203 ratios. Cu and Co were chemisorbed by aUophane at pH 5.0-5.5 and 6.9-7.2 (pH values for 50% adsorption level), respectively, with concomitant release of 1.6-1.9 protons/metal ion adsorbed. Quantitatively, adsorption by imogolite was less than that by the allophanes, presumably because of fewer sites available for chemisorption on the tubular structure of imogolite. Electron spin resonance studies of the imogolite and allophanes revealed that Cu 2 § was adsorbed as a monomer on two types of surface sites. The preferred sites were likely adjacent A1OH groups binding Cu 2+ by a binuclear mechanism; weaker bonding occurred at isolated A1OH or SiOH groups. These chemisorbed forms of Cu E+ were readily extracted by EDTA, CHaCOOH, and metals capable of specific adsorption, but were not exchangeable. In addition, the H20 and/or OH-ligands of chemisorbed Cu E+ were readily displaced by NH3, with the formation of ternary Cu-ammonia-surface complexes.

Removal of barium, cobalt, strontium and zinc from solution by natural and synthetic allophane adsorbents

“…This indicates that the sorption capacity does not solely depend on the purity of an absorbent and that the specific surface area plays only a minor role. This finding is consistent with previous results [18,22,35], where the sorption efficiency was directly related to the amount of ≡Al-OH° groups in allophane. Herein, it was impossible to quantify the ≡Al-OH° groups due to the presence of halloysite in NatAllo and varying amounts of polymeric SiO attached on the octahedral sheet of the synthetic allophanes.…”

“…Furthermore, one can see that 5-10 min is enough to achieve adsorption equilibrium under the experimental conditions used in this study, which indicates that physical adsorption rather than ion exchange contributes mainly to the removal of metal ions by natural and synthetic allophane adsorbents. This finding is in line with previous experimental results, where allophane has been successfully used in the remediation of aqueous solutions that were contaminated with, for example, heavy metals cations [15,22,33,35], metal (oxy)anions [18,23,30], heterocyclic organic components [10] and anionic surfactants and organic acids [17,23]. …”

“…5). The obtained results reflect the affinity of the aqueous metal ions to bind to charged allophane surfaces and the tendency to hydrolyze [22]. Si-rich allophane (like SynAllo-1 and SynAllo-2) has a permanent (structural) negative charge, while Al-rich allophane (NatAllo) has a higher amount of variable (pH-dependent) charges [36], which could explain of the pH-depended metal ion uptake of NatAllo, relative to SynAllo-1 and SynAllo-2 (Fig.…”

“…Allophane "nanoballs" are characterized by a high surface area (> 300 m²/g). They are well-known to adsorb appreciable amounts of ionic or polar pollutants due to an amphoteric ligand exchange capacity [17,22,23]. To date, however, the sorption properties of allophane have not been effectively utilized in the remediation of wastewater.…”

Influence of Glycine on Cu2+ Adsorption by Microcrystalline Gibbsite and Boehmite

“…Water washing evidently removed the second glycine as H20 molecules competed for ligand positions, resulting in a conversion of this complex to the one proposed to form on gibbsite (complex 1). Similar conversion of one Cu-ligand species to another with changing solution conditions has been noted for Cu-ammonia complexes bound to allophane (Clark and McBride, 1984). The orientation-dependence of the ESR spectra of bound Cu-glycine complexes on gibbsite (Figure 4) is consistent with a scheme of bonding in which Cu(gly) § Figure 7.…”