Predictive model for Pb(II) adsorption on soil minerals (oxides and low-crystalline aluminum silicate) consistent with spectroscopic evidence

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

“…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]. …”

“…These values are in the range of N2-BET values previously reported for natural allophanes (~200-400 m²/g) and synthetic allophanes (~250-900 m²/g) [10,29,33]. Noteworthy, the BET values obtained before and after the adsorption experiments were identical to within the analytical precision of the BET analyses, which indicates that changes in the surface area and surface properties of the adsorbents are negligible throughout the experiments.…”

“…SynAllo-1 revealed a better adsorption for Ba , Zn and Sr than HCoO ions, while SynAllo-2 showed a preferable uptake of Zn over Ba , HCoO and Sr ions and NatAllo displayed a better adsorption for Zn relative to HCoO , Ba and Sr ions. The obtained results likely depend on the individual surface charge properties of the adsorbents (although not confirmed by our data) and in particular on the aqueous speciation of the adsorbate [16,33,35,37]. A similar selectivity of metal ion uptake from solution by allophanes grafted with phosphate was observed by Okada et al [35], which increased in the order Ni ≤ Mg < Co < Ca < Zn < Cu at pH 5.1-6.2.…”

“…The affinity between a chemical component and the adsorbent controls the extent at which an aqueous metal ion is adsorbed onto a mineral surface. Indeed, the aqueous speciation of a metal ion, pH, contact time, temperature and the metal ion concentration are important factors that determine the kinetics and the efficiency of a given adsorption process [8,12,13,33]. In these experiments, the aqueous speciation of Ba and Sr was predominated by "free" Ba and Sr ions at pH 8.5 to 4.0.…”

Removal of Barium, Cobalt, Strontium, and Zinc from Solution by Natural and Synthetic Allophane Adsorbents

“…The specific surface areas are 294 m 2 /g for NatAllo, 358 m 2 /g for SynAllo-1, and 370 m 2 /g for SynAllo-2, respectively. These values are in the range of N 2 -BET values previously reported for natural allophanes (~200-400 m 2 /g) and synthetic allophanes (~250-900 m 2 /g) [10,29,33]. It is noteworthy that the BET values obtained before and after the adsorption experiments were identical to within the analytical precision of the BET analyses, which indicates that changes in the surface area and surface properties of the adsorbents are negligible throughout the experiments.…”