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Simplified modeling of organic contaminant adsorption by activated carbon and biochar in the presence of dissolved organic matter and other competing adsorbates Kyle Koyu Shimabuku, Julian M Paige, Marisol Luna Agüero, and R. Scott Summers Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00758 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017
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Simplified modeling of organic contaminant adsorption by activated carbon
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and biochar in the presence of dissolved organic matter and other competing
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adsorbates
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Kyle K. Shimabuku*, Julian M. Paige, Marisol Luna-Aguero, and R. Scott Summers
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Department of Civil, Environmental and Architectural Engineering, 428 UCB, University of Colorado, Boulder, Boulder, CO 80309, USA * Corresponding author:
[email protected] 8
Abstract
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Cyclohexanol, phenol, benzoic acid, and phenanthrene fractional removal (italicized
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words are defined within the main text) by pulverized granular activated carbon and biochar
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adsorption in deionized water and stormwater was independent of target-adsorbate initial
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concentrations (C0) when C0s were below concentration thresholds. This permits a simple-
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modeling approach. C0-independent removal in deionized water at low-target-adsorbate
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concentrations potentially suggests that DOM in the deionized water induce a competitive effect
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that causes deviations from the Freundlich model. The Ideal Adsorbed Solution Theory-
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Equivalent Background Compound model was used to determine the magnitude of concentration
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thresholds and the competitive effect of stormwater DOM and possibly deionized water DOM.
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These competing substances’ competitive effects were influenced by target-compound
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adsorbability and structure. Concentration thresholds positively correlate with competing
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substances’ competitive effect and negatively correlate with target-adsorbate-Freundlich 1/n
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values. In deionized water, concentration thresholds increase as target-compound adsorbability
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decreases. In stormwater, concentration thresholds do not correlate with adsorbability,
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potentially because stormwater DOM is better suited to compete for aromatic-compound-
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adsorption sites. The extent known-competitor adsorbates decrease target-adsorbate removal in
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the presence of DOM is investigated, which depends on the competing adsorbates’ relative
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adsorbabilities and if they adsorb to a different subpopulation of adsorption sites.
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Introduction
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Organic contaminants (OCs) can have harmful human- and ecological-health effects and
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can be challenging to remove from drinking-water sources, wastewater, and stormwater.1
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Activated carbon is one of the best available technologies to remove many OCs, and biochar can
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be a more sustainable, low-cost alternative.2-4 Due to the heterogeneity of activated carbon- and
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biochar-adsorption sites, as well as the diverse physicochemical properties of OCs, commonly
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used models in some instances cannot adequately model single-solute isotherms.5 For instance,
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under single-solute conditions, adsorption data can deviate from the empirical Freundlich model
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because isotherms can exhibit curvature whereas the Freundlich model falls on a straight line
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when presented on a plot with log-scale coordinates for both axes.5 Also, when background
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dissolved organic matter (DOM), which is ubiquitous in natural and anthropogenic waters, is
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present, different solid-to-liquid-phase-concentration relationships arise depending on the
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experimental approach, i.e., varying the adsorbent dose or target-adsorbate initial concentration
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(C0).6 Moreover, DOM’s competitive effect, which is the extent to which it decreases target-
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compound adsorption by competing for adsorption sites, can vary with different target
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adsorbates, which further complicates adsorption modeling.7,8 By evaluating the adsorption of
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compounds that have some similar attributes (e.g., in size) but differ in other properties (e.g., in
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aromaticity), previous studies have isolated how compound properties influence OC-sorption
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capacities.5,9 Such an approach could also be useful in studying other OC-sorption behaviors,
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such as OC susceptibility to DOM competition. It has also been recognized that DOM can be
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present in deionized water, and that high target-adsorbate concentrations can limit competition
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from DI-water DOM.10,11
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The fractional removal of a target adsorbate (i.e., the initial concentration of the target
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adsorbate minus the final concentration normalized by the initial concentration of the target
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adsorbate) at a given dose is not influenced by the target adsorbate C0 if the competitive-DOM-
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solid-phase concentration is significantly greater than target-adsorbate-solid-phase
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concentration.10-14 (See the section “IAST-EBC model and simplification” for a mathematical
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description of this phenomena.) C0-independent removal (i.e., when the fractional removal of a
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target adsorbate at a given dose remains the same at different target-adsorbate C0s) occurs when
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adsorption isotherms are linear (i.e., the Freundlich 1/n is equal to unity). Sander and Pignatello5
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showed that hydrophobic-normalized isotherms describing the sum of benzene and toluene
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sorption in a bi-solute system are highly nonlinear and are nearly identical to these compounds’
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single-solute isotherms. When they plotted the isotherms of these compounds individually in a
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bi-solute system, the isotherms approached linearity at high competitor concentrations, which
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they suggested occurs as a result of presenting the adsorption of one solute individually, and not
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the sum. They also noted that linear adsorption does not necessarily signify a change in the
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sorption mechanism (e.g., from adsorption to absorption).5
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C0-independent removal provides the basis for a simple modeling approach to predict OC
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removal at different target-adsorbate C0s.10-14 However, there is a maximum C0 below which
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fractional removal remains independent of the target-adsorbate C0, termed herein the
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concentration threshold. The concentration threshold is exceeded when the target-adsorbate C0 is
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great enough that it causes the target adsorbate to impact the competitive-DOM-solid-phase
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concentration.10,14 If the concentration threshold is exceeded, a complex modeling approach is
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required to predict removal at different target-adsorbate C0s.10,14 Thus, there is a great practical
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advantage when C0-independent removal occurs because equilibrium and kinetic modeling is
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greatly simplified.10,14 Many studies assume C0-independent removal without validating that
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target-adsorbate C0s are below concentration thresholds,15-20 and the influence of adsorbate type,
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the DOM concentration, and adsorbent dose on the magnitude of concentration thresholds has
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not been systematically investigated. Also, known-competitor compounds (i.e., compounds
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whose identity is known, unlike DOM which is largely an unknown mixture of compounds) can
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impact the removal of a target adsorbate,5 but at what concentration such compounds impact the
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fractional removal of a target adsorbate in the presence of DOM has not been investigated.
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The objectives of this study are to (i) evaluate how competing substances influence
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isotherms in deionized water and stormwater, (ii) quantify the DOM-competitive effect with
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different adsorbates, (iii) model the impact of adsorbate type, adsorbent dose, and DOM
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concentration on the magnitude of the concentration threshold with biochar and pulverized
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granular activated carbon (pGAC), and (iv) evaluate at what concentration known-competitor
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compounds impact the fractional removal of a target adsorbate in the presence of DOM. The
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approach was to adsorb cyclohexanol, phenol, benzoic acid, and phenanthrene with pGAC and
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biochar in phosphate-buffered deionized water and stormwater with initial dissolved organic
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carbon concentrations (DOC0) of 1.25, 2.5, and 5 mg/L. Adsorption data was collected by
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varying the target-adsorbate C0 and adsorbent dose, and modeled using the Ideal Adsorbed
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Solution Theory-Equivalent Background Compound model (IAST-EBC), which was originally
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developed by Najm et al.21 The IAST-EBC model has been used to mathematically explain why
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C0-independent removal arises.10,14 It has also been used to predict whether or not C0-
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independent removal occurs using solid-phase concentrations and single-solute Freundlich 1/n
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values of target adsorbates and the EBC.10,14 The IAST-EBC was used here to determine the
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magnitude of concentration thresholds and background-competing substances’ competitive
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effect.
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Materials and Methods
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Target adsorbates. Cyclohexanol, phenanthrene, benzoic acid, and phenol were selected
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as target adsorbates because they (i) have environmental relevance or share chemical structures
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with compounds that have environmental relevance,22 (ii) exhibit a wide range of
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hydrophobicities (see Supporting Information (SI) Table S1) while having an experimentally
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accessible range of adsorbabilities (see the section “DOM competition” for a definition) in batch
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tests, (iii) have limited volatilities that allow for open-air batch studies, and (iv) are structurally
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different while also being roughly similar in size (except phenanthrene), which allows the
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influence of different properties (e.g., aromaticity) on adsorption to be isolated. The SI discusses
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reasons for adsorbate selection further.
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phenanthrene were obtained from American Radiolabeled Chemicals, Inc. (St. Louis, MO) and
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combined with respective unlabeled compounds [Sigmga Alrich, (St. Louis, MO)] to target
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different specific activities between 20 and 0.0014 mCi/mmol. Detection limits ranged between
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10 and 100,000 ng/L using a Packard Tri Carb 2300 liquid-scintillation analyzer (Downers
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Grove, IL) with a 30-min counting time. C0s were targeted at 100 ng/L when adsorption data was
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collected by varying the adsorbent dose. For data collected when the adsorbent dose was held
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constant, target-adsorbate C0s were varied between 165 and 12,000,000 ng/L.
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C cyclohexanol, 3H benzoic acid, 3H phenol, and 3H
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Adsorbents. Biochar was produced by heating pelletized-pine-forestry waste (Black
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Hills GOLD, Spearfish Pellet Company LLC, Spearfish, SD) at 850 °C for two hours in a
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covered ceramic crucible under oxygen-limited conditions by filling the crucible to the top with
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pine-pellets to minimize headspace. The heating rate was 10°C/min. The biochar and a
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bituminous-coal-based GAC (Norit 1240) (Atlanta, GA) were ground to pass a 325 U.S.
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standard-mesh sieve. They were dried at least overnight at 105 °C prior to their use in batch
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experiments. The pGAC- and biochar-BET surface areas are 1062 and 346 m2/g, respectively,
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and their average pore widths are 42.9 and 39.4 Å, respectively. These properties were measured
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with a Micrometrics Gemini VII Series Surface Area Analyzer (Norcross, GA). A few
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experiments were also conducted with these adsorbents that underwent a cleaning procedure as
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described in the SI.
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Waters. Experiments were conducted in deionized water produced with a Sartorius
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Arium® pro UV-system (DOC
