Functionalized Magnetic Nanoparticles for Heavy Metal Removal from Aqueous Solutions: Kinetics and Equilibrium Modeling - Advanced Materials for Agriculture, Food and Environmental Safety - page 321

(ΔH), and change in entropy (ΔS). Negative ΔG indicates the spontaneity
of the adsorption process. h e value of ΔH is used to identify the nature
of adsorption [122]. A positive value of ΔH indicates the reaction is endo-
thermic, and the negative value of ΔH shows that the reaction is exother-
mic [122, 123]. A positive value of ΔS indicates increased randomness of
adsorbate molecules on the solid surface than in the solution [124].
h e free energy of adsorption (ΔG) can be related with the Langmuir
equilibrium constant by the following expression [125]:

GTlnK =−

(11.18)

L

Enthalpy and entropy changes are also related to the Langmuir equilib-
rium constant by the following expression:

S H
K R T In =−

(11.19)

L

h us, a plot of ln K L versus 1/T should be a straight line. h e ΔH and ΔS
values could be obtained from the slope and intercept of this plot.

11.9 Metal Recovery and Regeneration of Magnetic

Nanoparticles

Desorption is very much necessary when the material synthesis is costly.
Regenerability of loaded magnetic nanoparticles is a key factor for improv-
ing the economy of the adsorption process. h e economic feasibility of
using adsorbent based on magnetic nanoparticles to remove heavy met-
als from aqueous solution relies on its regeneration ability during multiple
adsorption/desorption cycles. It is possible to decrease the process cost and
also the dependency of the process on a continuous supply of the nanopar-
ticles through desorption. A successful desorption process requires the
proper selection of elutants, which strongly depends on the type of material
and the mechanism of adsorption [126]. Elutant must be (i) non-damaging
to the magnetic nanoparticles, (ii) less costly, (iii) eco-friendly, and (iv)
ef ective. Some workers have conducted exhaustive experiments to identify
appropriate elutants for the recovery of precious metals.
Recently, Ge et al. synthesized novel Fe 3 O 4 magnetic nanoparti-
cles (MNPs) modii ed with 3-aminopropyltriethoxysilane (APS) and