Mistakes and inconsistencies regarding adsorption of contaminants from aqueous solutions: A critical review

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

“…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.…”

“…Noteworthy, the plot of Eq. 3 is linear and the calculated and measured values match perfectly (slope: 0.998; n = 24; R² = 0.987), which proves the ability of the PFO model to fit the experimental data [16]. The calculated values for k and are reported in Table 1.…”

“…The kinetics of the metal ion uptake by allophane was described by a pseudo-first-order (PFO) model [16,34], according to Eq. 3:…”

“…Over the last decades, a huge variety of models have been developed to describe adsorption isotherms, but the most commonly used models are the Langmuir, Freundlich, Dubinin-Radushkevich (D-R model) and Redlich-Peterson models. The advantages, disadvantages and limitations of these models are discussed in detail in the review paper by Tran et al [16]. In this study, the Langmuir equation and the D-R model are used to describe the isotherms.…”

“…≡Al-OH° and ≡Si-OH°) in adsorbents are favorable to ensure a desired adsorption rate and appropriate metal ion removal efficiency [10,11]. Recently, agricultural and industrial waste products, activated carbon nanotubes, cellulose, carbonates, Feand Al-oxyhydroxides, zeolites, phosphates and clay minerals are used in the remediation of water that is severely polluted with persistent and hazardous components such as heavy metal ions, dyes, antibiotics, biocide compounds and other organic chemicals [12][13][14][15][16]. The use of clay minerals for wastewater treatment can be advantageous over other sorbents due to their worldwide occurrence, low mining and processing costs and outstanding physicochemical and surface properties, making clayey materials suitable for selective ion exchange, adsorption and catalyst uses [1,17,18].…”