Development
of novel adsorbents often neglects the competitive
adsorption between co-occurring oxo-anions, overestimating realistic
pollutant removal potentials, and overlooking the need to improve
selectivity of materials. This critical review focuses on adsorptive
competition between commonly co-occurring oxo-anions in water and
mechanistic approaches for the design and development of selective
adsorbents. Six “target” oxo-anion pollutants (arsenate,
arsenite, selenate, selenite, chromate, and perchlorate) were selected
for study. Five “competing” co-occurring oxo-anions
(phosphate, sulfate, bicarbonate, silicate, and nitrate) were selected
due to their potential to compete with target oxo-anions for sorption
sites resulting in decreased removal of the target oxo-anions. First,
a comprehensive review of competition between target and competitor
oxo-anions to sorb on commonly used, nonselective, metal (hydr)oxide
materials is presented, and the strength of competition between each
target and competitive oxo-anion pair is classified. This is followed
by a critical discussion of the different equations and models used
to quantify selectivity. Next, four mechanisms that have been successfully
utilized in the development of selective adsorbents are reviewed:
variation in surface complexation, Lewis acid/base hardness, steric
hindrance, and electrostatic interactions. For each mechanism, the
oxo-anions, both target and competitors, are ranked in terms of adsorptive
attraction and technologies that exploit this mechanism are reviewed.
Third, given the significant effort to evaluate these systems empirically,
the potential to use computational quantum techniques, such as density
functional theory (DFT), for modeling and prediction is explored.
Finally, areas within the field of selective adsorption requiring
further research are detailed with guidance on priorities for screening
and defining selective adsorbents.
