Exploring the Mechanisms of Selectivity for Environmentally Significant Oxo-Anion Removal during Water Treatment: A Review of Common Competing Oxo-Anions and Tools for Quantifying Selective Adsorption

Abstract

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.

Predicting breakthrough of vanadium in fixed-bed absorbent columns with complex groundwater chemistries: A multi-component granular ferric hydroxide−vanadate−arsenate−phosphate−silicic acid system

“…S8 in the Supporting Information). If to quantify the selectivity for the target oxo-anion vanadate in comparison to the other competitive oxo-anions, a selectivity coefficient β t = K d,t / K d,c can be introduced, where K d,t is the distribution coefficient of the target oxo-anion, and K d,c is the distribution coefficient of the background oxo-anion ( Pincus et al., 2020 ). Selectivity coefficient values of β t = 1.2 for phosphate and 2.5 for arsenate calculated from the batch equilibrium data shown in Fig.…”

“…2 (with equal equilibrium solute concentration of 0.1 mg L −1 ) are all above unity and thus demonstrate high selectivity of the GFH towards the target oxo-anion vanadate. This selectivity can be understood on a theoretical basis if to recall the Pearson Hard Soft Acid Base (HSAB) principle ( Pincus et al., 2020 ). According to this principle, the oxo-anions as hard Lewis bases (electron donors) are adsorbed by the iron hydroxide surface as a hard Lewis acid (electron acceptor).…”

“…Unfortunately, it is not just trivial to predict the lifetime of an FBA with any confidence because of the complex interaction of the many co-factors and competitive co-adsorbates in groundwater ( Pincus et al., 2020 ). For example, a small change in pH can markedly affect the mobility of oxo-anions such as As(III) and As(V) in contaminated groundwater ( Biswas et al., 2014 ).…”

“…Background oxo-anions such as phosphate (PO 4 − 3 ) and silicic acid (H 4 SiO 4 0 ) are known to compete with the trace As for similar adsorption sites, contributing to failure in achieving regulatory limits for the target contaminant within expected lifetime of an FBA ( Nguyen et al., 2011 ). While there is lots of information about adsorption selectivity and competition between the oxo-anions arsenite/arsenate, selenite/selenate, phosphate, and silicic acid ( Pincus et al., 2020 ), there has been less focus on selective and competitive adsorption behavior of the target contaminant vanadate. Elevated vanadium concentrations of up to 0.2 mg L −1 were found in groundwater of volcanic areas throughout the world ( Hamada, 1998 ; Arena et al., 2015 ; Koh et al., 2016 ).…”

Iron oxide coated hollow poly(methylmethacrylate) as an efficient adsorption media for removal of arsenic from water

“…As(V) and As(III) species form a coordinate bond becoming adsorbed on the surface of iron oxide. 2,8,9 Since As(V) or As(III) species possess ligand characteristics 10 (electron-donating ability) and iron oxide has a larger surface area per unit mass and hence more sorption sites, selective separation of As(V) and As(III) oxyanions or oxyacids from drinking water sources is highly facilitated.…”