Adsorption of mercury from water by modified multi-walled carbon nanotubes: adsorption behaviour and interference resistance by coexisting anions

Abstract

This investigation reports the use of modified multi-walled carbon nanotubes (MWCNTs) with various functional groups for adsorbing inorganic divalent mercury (Hg(II)) from water samples. To elucidate the behaviours and mechanisms of Hg(II) adsorption by modified MWCNTs, their adsorption capacity was studied by considering adsorption isotherms and kinetics. Particular attention was paid to interference of coexisting inorganic ions with Hg(II) adsorption. The results reveal that functionalization with oxygen-containing groups improved the Hg(II) adsorption capacity of the MWCNTs. Kinetic analysis demonstrated that the adsorption of Hg(II) by MWCNTs was closely described by the pseudo-second-order and Elovich models, suggesting that the adsorption of Hg(II) by MWCNTs was significantly affected by chemical adsorption. The kinetic results were also analysed using the intraparticle diffusion model, which revealed that intraparticle diffusion was not the only rate-controlling mechanism. The adsorption of Hg(II) on MWCNTs fell drastically as the ionic strength increased from 0 to 1.0mol/L chloride ions, and declined significantly as the pH increased from 2.2 to 10.5. The elemental maps obtained by energy-dispersive spectrometer (EDS) revealed the formation of surface complexes of chloride ions with functional groups on MWCNTs, which reduced the number of available sites for the adsorption of Hg(II) and strengthened the repulsive forces between Hg(II) and MWCNTs. The EDS results suggest that chloride ions are important in controlling Hg(II) speciation and adsorption on the surfaces of MWCNTs.

Removal of Hg2+ and methylmercury in waters by functionalized multi-walled carbon nanotubes: adsorption behavior and the impacts of some environmentally relevant factors

“…High concentration of Cl − in solution could also promote the formation of very stable MeHgCl complex which showed lower sorption than MeHg to MWCNTs. These results agreed with those of previous studies [20,48]. de Diego et al [48] found that a high concentration NaCl could led to declined adsorption rates of mercury species.…”

“…Figure 3 also shows that for pristine MWCNTs, MWCNTs-OH and MWCNTs-COOH, the removal efficiency of Hg 2+ and MeHg decreased gradually with the increase of pH. This is because with the increase of solution pH, the Hg(OH) + or MeHgOH compounds could be generated, which is stable in the water and reduced the Hg 2+ and MeHg adsorption [17,20]. It should be noted that a slight increase of removal efficiency of Hg 2+ and MeHg were observed for pH > 8.0.…”

“…There are at least The peaks at ~3439 and ~1265 cm −1 were attributed to -OH stretching vibration in the MWCNTs-OH spectrum [20]. Functional groups on the surface of the carbon nanotubes offered a lot of adsorption sites, which may be useful to increase the adsorption ability of carbon nanotubes.…”

“…The MWCNTs have been experimentally proven to possess cylindrical hollow micro-crystals of graphite which have strong adsorption ability, exceptional mechanical properties, unique electrical property, high chemical and thermal stability, a large specific surface area, and high adsorption capacity [13][14][15]. Researches have shown that the surface functional groups such as amino [18,19], carboxyl [20,21], hydroxy [20,21] and thiol [19,[22][23][24] on carbon nanotubes can greatly improve their adsorption capacity for metal ions. Moreover, various aqueous environment relevant factors including pH, ionic strength and natural organic matter (NOM) can affect the adsorption of metal ions onto MWCNTs [12,13,20,[25][26][27].…”

“…Researches have shown that the surface functional groups such as amino [18,19], carboxyl [20,21], hydroxy [20,21] and thiol [19,[22][23][24] on carbon nanotubes can greatly improve their adsorption capacity for metal ions. Moreover, various aqueous environment relevant factors including pH, ionic strength and natural organic matter (NOM) can affect the adsorption of metal ions onto MWCNTs [12,13,20,[25][26][27]. To assess the efficiency of MWCNTs as absorbents to removal of Hg 2+ and MeHg in waters, it is of great importance to understand the effects of NOM and other environmental factors on the sorption of Hg 2+ and MeHg to functionalized MWCNTs.…”