It is to be noted that the particle size influences the hydrodynamics in fixed bed reactors. Very small particle size results in
higher hydrostatic resistance and slower flow velocity. The optimum particle size is thus a trade-off between the adsorption kinetics and the hydrodynamics (Chen et al., 2006). In contrast,
stirred tank reactors require subsequent separation of hydrogel
from the solution, after the completion of adsorption or desorption
stage. If the hydrogel particle size is too small (e.g., in micro or nano
range), separation from the aqueous solution becomes a serious
challenge (Tang and Lo, 2013). The separation of tiny hydrogel
particles through conventional techniques such as sedimentation is
ineffective, while other methods such as centrifugation are energyintensive (Tang et al., 2010). Recently, it has been demonstrated that
if the hydrogels are embedded with magnetic nanoparticles, they
can be separated from the solution magnetically (discussed in more
detail in section 5).