Cd(II) binding by particulate low-rank coals in aqueous media: sorption characteristics and NICA–Donnan models

Immobilization of Heavy Metal Ions on Coals and Carbons

“…As a type of the electrostatic interactions, mainly cation exchange is mentioned, even for range of pH above the value of the iso-electric point (Radovic et al, 2001). A governing role of the ion exchange was confirmed both for activated carbons (Sekar et al, 2004;El-Shafey et al, 2002) and coals (Murakami et al, 2001;Burns et al, 2004). In addition to cation exchange, other possible mechanisms for metal ion immobilization such as surface precipitation or physical adsorption have been mentioned (Le Cloirec & Faur-Brasquet, 2008;Mohan & Chander, 2006).…”

“…The question thus arises as to the proportion between the cation exchange and the other mechanisms taking part in metal ions immobilization on carbon/coal. The original way to understand the actual role of the cation exchange offers measurement of the change in pH in adsorbate solution during equilibration process (Burns et al, 2004;ElShafey et al, 2002;Klucakova & Pekar, 2006). Namely, in the case of exclusive cation exchange between bivalent metals Me(II) and protons H + , twice the amount of protons should be released from carbon into solution in comparison with the metal uptake.…”

“…Namely, in the case of exclusive cation exchange between bivalent metals Me(II) and protons H + , twice the amount of protons should be released from carbon into solution in comparison with the metal uptake. Indeed, a value of 2 was found for adsorption of cadmium(II) both on activated carbon (El-Shafey et al, 2002) and on low-rank Australian coals at pH 6 (Burns et al, 2004). Mohan & Chander (2006) then showed that during the sorption of Fe(II), Mn(II) or Fe(III) ions on lignite, calcium ions were mainly released to the solution.…”

Donnan Electric Potential Dependence of Intraparticle Diffusion of Malachite Green in Single Cation Exchange Resin Particles: A Laser Trapping-Microspectroscopy Study

“…Since the ion-exchange reactions at the exchange groups in the resin phase are in general very fast, the rate determining step is predicted to be either film diffusion or particle diffusion, depending on various experimental conditions: solution pH, temperature, the molecular size of an ion, the pore-size of the three-dimensional network of a resin, and so forth. Besides these factors governing the ion-exchange processes, the role of the Donnan electric potential in the ion-exchange processes has been sometimes discussed [4][5][6][7][8][9]. In practice, since the Donnan electric potential generated at the resin/solution boundary influences the particle diffusion processes of ions, its effects on ion-exchange reactions have been hitherto studied theoretically and experimentally for both film membrane [4,7] and bead systems [8,9].…”

“…Besides these factors governing the ion-exchange processes, the role of the Donnan electric potential in the ion-exchange processes has been sometimes discussed [4][5][6][7][8][9]. In practice, since the Donnan electric potential generated at the resin/solution boundary influences the particle diffusion processes of ions, its effects on ion-exchange reactions have been hitherto studied theoretically and experimentally for both film membrane [4,7] and bead systems [8,9]. In micrometer-sized spherical ion-exchange resin particles, particular, the electric potential effects on the particle diffusion processes are expected to play a more decisive role compared with those in film membrane systems, since the electric potential is generated three-dimensionally at the spherical particle/solution boundary.…”