Predicting Organic Cation Sorption Coefficients: Accounting for

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Predicting organic cation sorption coefficients: Accounting for competition from sorbed inorganic cations using a simple probe molecule William C. Jolin, Reaha Goyetche, Katherine Carter, John Medina, Dharni Vasudevan, and Allison A MacKay Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 1, 2017

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Predicting organic cation sorption coefficients:

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Accounting for competition from sorbed inorganic

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cations using a simple probe molecule

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William C. Jolin†, Reaha Goyetcheҗ, Katherine Carter җ, John Medina җ, Dharni Vasudevanҗ*,

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and Allison A. MacKay‡*

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Department of Civil and Environmental Engineering, University of Connecticut, Storrs, CT

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06269 җ

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Department of Civil, Environmental and Geodetic Engineering, The Ohio State University,

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11

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Department of Chemistry, Bowdoin College, Brunswick, ME 04011

Columbus, OH 43210

ABSTRACT

With the increasing number of emerging contaminants that are cationic at environmentally

13

relevant pH values, there is a need for robust predictive models of organic cation sorption

14

coefficients (Kd). Current predictive models fail to account for the differences in the identity,

15

abundance, and affinity of surface-associated inorganic exchange ions naturally present at

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negatively charged receptor sites on environmental solids. To better understand how organic

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cation sorption is influenced by surface-associated inorganic exchange ions, sorption coefficients

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of ten organic cations (including eight pharmaceuticals and two simple probe organic amines)

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were determined for six homoionic forms of the aluminosilicate mineral, montmorillonite.

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Organic cation sorption coefficients exhibited consistent trends for all compounds across the

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various homoionic clays with sorption coefficients (Kd) decreasing as follows: KdNa+ > KdNH4+ ≥

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KdK+ > KdCa2+ ≥ KdMg2+ > KdAl3+. This trend for competition between organic cations and

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exchangeable inorganic cations is consistent with the inorganic cation selectivity sequence,

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determined for exchange between inorganic ions. Such consistent trends in competition between

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organic and inorganic cations suggested that a simple probe cation, such as

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phenyltrimethylammonium or benzylamine, could capture soil-to-soil variations in native

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inorganic cation identity and abundance for the prediction of organic cation sorption to soils and

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soil minerals. Indeed, sorption of two pharmaceutical compounds to 30 soils was better described

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by phenyltrimethylammonium sorption than by measures of benzylamine sorption, effective

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cation exchange capacity alone, or a model from the literature (Droge & Goss, Environ. Sci.

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Technol., 2013, 47, 14224). A hybrid approach integrating structural scaling factors derived from

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this literature model of organic cation sorption, along with phenyltrimethylammonium Kd values,

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allowed for estimation of Kd values for more structurally complex organic cations to homoionic

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montmorillonites and to heteroionic soils (mean absolute error = 0.27 log units). Accordingly,

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the use of phenyltrimethylammonium as a probe compound was concluded to be a promising

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means to account for the identity, affinity and abundance of natural exchange ions in the

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prediction of organic cation sorption coefficients for environmental solids.

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INTRODUCTION

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A growing number of environmental contaminants of interest, including pharmaceuticals,

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surfactants, and ionic liquids,1 are positively charged under environmentally-relevant pH values,

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creating the need for accurate sorption models for organic cations.2-5 Traditional sorption

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prediction approaches (e.g. EPISUITE6) accommodate only compounds that are neutral, with no

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ionic moieties. In the case of organic cation sorption, positive charge on the amine moiety

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interacts with negative charge sites on aluminosilicate clay mineral and organic matter

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components of environmental solids.3-5,7-12 Local charge neutrality is maintained because the

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sorbing organic cations displace a naturally-occurring inorganic cation from the charged site;

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however, additional sorption energy contributions from the non-charged portions of the organic

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cation (hydrophobic exclusion from the aqueous phase, polar interactions with the solid phase13-

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15

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counterparts for sorption sites at orders of magnitude lower concentration or abundance.17 As a

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result, accurate predictive models for organic cation sorption must integrate both characteristics

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of the environmental solids (site availability) and structural features of the compound itself.

- for more exhaustive listing see ref [16]) allows these compounds to outcompete their inorganic

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To date, the greatest progress toward a quantitative model for organic cation sorption to

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environmental solids has come through the detailed work of Droge and Goss.14,15,18 Their model

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for organic cation sorption to soils integrates environmental solids characteristics through use of

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the solid’s total cation exchange capacity and an attributed cation exchange contribution for the

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organic matter content of the solids. Structural features of the sorbing cations are captured

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through the compound molar volume, amine rank (primary, secondary or tertiary) and corrective

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factors for additional structural moieties, such as multiple rings, -Cl groups, etc. In this model,

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the extent of exchange site to soil aluminosilicate mineral and soil organic matter was separated

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because sorption studies with pure phase minerals and organic matter showed distinct effects of

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compound structure (sign reversal in empirical term) for the amine rank contributions to

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sorption, as well as some differences in correction factors, between the two sorbent types.14,15

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Overall, this approach of separating cation exchange to aluminosilicate minerals and organic

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matter and representing structural feature contributions with relationships developed from pure

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phase sorbents (illite clay and Pahokee peat organic matter) was quite successful in predicting

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organic cation sorption to two representative soils. This success advances prior modeling efforts

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for organic cations that did not include fundamental site density measures5 or appropriate

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structural feature representations.14

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One limitation of the Droge and Goss model is the inability to account for inorganic

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cation identity and abundance at negative charge sites. Earlier, we identified exchangeable

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inorganic cation identity and abundance as a potentially important factor in organic cation

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sorption.16 Droge and Goss’ model development with pure phase sorbents and validation efforts

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with soils were undertaken in the presence of 5 mM CaCl2 that likely yielded Ca2+ to be the

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charge balancing cations in their sorbent systems. Although calcium is an important soil cation,

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the weathering and anthropogenic histories of many soils can yield solids with a mixture of

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inorganic cations. Prior studies of inorganic cation sorption to soils19-21 and organic polymers22

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show clear trends in ‘selectivity’, or relative affinity for the surface, between cations –

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Li+ ≈ Na+ < NH4+ ≈ H+ ≈ K+ < Mg2+ < Ca2+ < Al3+

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that suggest ion identity should play a role in organic cation exchange. From Li+ through to Al3+,

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inorganic cations with increased affinity to soils should impart increased competition with

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sorbing organic cations, thereby yielding reduced sorption coefficients for sorbing organic

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cations. Indeed, this has been demonstrated in the case of Na+ versus Ca2+ as the saturating

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cation: sorption coefficients for Na+-clays and soils are about one order of magnitude greater

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than for Ca2+-clays and soils, a trend that is observed consistently for organic cations,

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independent of sorbate structure.15,22-24 Extension of such comparisons to a broader suite of

(Sequence I)

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inorganic cations has been undertaken only for a small number of cationic pyridine, purine,

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nucleotide and biguanide compounds.22,24 Measured sorption coefficients in these cases

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decreased for homoionic clays with inorganic cations of greater relative affinity for the surface

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according to Sequence I.

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montmorillonite than Ca2+-montmorillonite, indicating that other properties, such as hydration of

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the inorganic cation25-27, may also affect to organic cation sorption. It is unclear from such a

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small dataset whether other inorganic cations impart a consistent influence on organic cation

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sorption for all compound structures. The potential identification of such a consistent

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competition sequence for inorganic cation displacement by organic cations opens the door to

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quantitative integration of inorganic cation identity into organic cation sorption models. To this

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end, we offer that complex variations in inorganic cation identity, affinity and abundance can be

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better accounted for through the use of a cationic probe compound16,28 rather than the

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cumbersome quantitative assessment of concentrations of individual exchange ions on

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environmental solids.

The one exception was lower sorption observed for Mg2+-

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Previously, we have conceptualized that experimentally measured sorption coefficients

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for a cationic probe compound with a simple structure, such as benzylamine or

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phenyltrimethylammonium (PTMA) (structures, Figure S1), could provide a combined measure

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of cation exchange site density and the baseline driving force for sorption to an environmental

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solid while implicitly accounting for competition from naturally-occurring exchangeable

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inorganic cations .16 If a consistent sequence for inorganic cation displacement is noted for a

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wide range of organic cation structures, we propose that the probe compound sorption coefficient

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for a given sorbent should be correlated with sorption coefficients for other organic cations with

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varied structures.

Thus, sorption coefficients for any organic cation, i, (Kid,pred) could be

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predicted from the experimentally-determined sorption coefficient of the probe compound to that

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particular soil (Kd,exptprobe) and a structural scaling factors (Sisoil) that captures the free energy

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differences in sorption arising from structural differences between the organic cation of interest

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and the probe compound:     , = , 

×  

(1)

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Structural differences, manifest in both hydrophobic and electrostatic contributions to sorption

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interactions, could be determined experimentally or from empirical models.14,15,18 While this

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proposed probe approach may imply the assumption of one negative site ‘type’ in Eq. 1, it does

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still facilitate a hybrid approach in which the influence and distribution site characteristic

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including, sorbed inorganic cation identities and abundance could be captured through Kd,exptprobe,

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and the distinct influence of organic cation structural characteristics on sorption to

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aluminosilicate clay minerals and organic matter (Sisoil) could be integrated explicitly through a

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modified form of the Droge and Goss soil sorption model, for example. Without knowledge of

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the exact framework for Sisoil, a simple structure for the probe compound was conceptualized

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because we anticipated that it would be easier to add adjustments for other moieties (-Cl, -OH,

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etc) to build up to more complex structures than to use a structurally-complex probe that could

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be more representative of all of the potential sorption interactions yet require a more complicated

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adjustment for the structural correction. With the limited application to date of probe compounds

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for predicting sorption of other ionic organic compounds,28 there is a need to evaluate whether

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probe compounds could be used to quantitatively account for the identity and abundance of a

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heterogeneous group of exchangeable inorganic cations, using an extensive soil set. Note that

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our emphasis herein are conditions of low sorbate surface coverage (< 2% sites) where linear

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isotherms are applicable.29

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This study is aimed at improving the predictive capability of organic cation sorption to

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soils. To this end, we (i) identify the influence of exchangeable inorganic cation identity on the

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extent of organic cation sorption and quantify this competitive effect for homoionic

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montmorillonite systems; (ii) establish measures of probe cation sorption (Kd,exptprobe) for

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heteroionic systems (soils) that accesses overall organic cation exchange affinity of a soil or a

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soil mineral, while capturing competition effects from naturally-occurring exchangeable

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inorganic cations, and (iii) develop improved predictive models for organic cation sorption to

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aluminosilicate minerals and soils (Eq. 1) that integrate measures of probe sorption (Kd,exptprobe)

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with scaling factors (Si) delineated by leveraging the Droge and Goss structure-based models for

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sorption to aluminosilicate minerals and organic matter.

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METHODS

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Sorbents and chemicals

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Texas Ca-montmorillonite (STx-1, CEC = 0.844 meq kg-1) was obtained from the Clay

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Minerals Society. Silicon carbide (SiC) was from Alfa Aesar. Thirty soil samples were collected

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from 28 sites across the eastern United States as part of an earlier study by Jones et al.30 Soils

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were previously characterized for native soil pH, effective cation exchange capacity (ECEC), and

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concentrations of exchangeable Na+, K+, Ca2+, Mg2+, and Al3+ (Table S1). Sorbate compounds

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phenyltrimethylammonium (PTMA), benzylamine, serotonin, atenolol, metoprolol, tramadol,

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trimethoprim, propranolol, desipramine, and diltiazem (Figure S1) were from Sigma Aldrich. All

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other chemicals and reagents were ACS grade. Solutions were made with high purity 18.2 MΩ

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water (DI water) from a MilliQ system (Waters).

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Sorption to Homoionic Montmorillonite: Kd Values Obtained by Column Chromatography

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Pulse input column chromatography was used to obtain sorption coefficients for 10 organic

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cations (Figure S1) each with six homoionic (Na+, K+, NH4+, Ca2+, Mg2+, and Al3+)

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montmorillonite systems. Montmorillonite was chosen as a representative aluminosilicate

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mineral because of (i) the pH-independent nature of the cation exchange sites,21(ii) its use in

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many prior studies,4,9,12,31,32 and (iii) similarities in sorbent behavior to other common clay

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minerals.15 Columns were carefully packed with mixtures of montmorillonite (MMT) and inert

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SiC. Following methods detailed and validated previously,33 montmorillonite-to-water ratios of

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19, 5, and 0.75 grams of montmorillonite per liter of column pore water were used in individual

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columns to balance compound retention against peak spreading. A comparative control column

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was packed entirely with SiC to verify that test compounds had no sorptive interactions with this

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inert solid. Column porosity was similar for both the ‘SiC-only’ and ‘SiC-montmorillonite’

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columns because of the small amounts of montmorillonite used.33 Packed columns were loaded

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into a standard HPLC system (Jasco PU-980 pump, AS-950 auto sampler, 40 µL injection loop,

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and MD-1510 multiwavelength detector). Montmorillonite was converted to homoionic Na+,

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NH4+, K+, Ca2+, and Mg2+ forms by flushing the respective column with air-equilibrated (pH 6 ±

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0.3), 15 mM (NaCl, NH4Cl, KCl) or 5 mM (CaCl2, MgCl2) solutions for 24 hrs (2,800 pore

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volumes) before compound injection. Al3+-montmorillonite was created by flushing with 2.5 mM

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AlCl3 adjusted to pH 3 (HCl) to avoid precipitation of Al(OH)3. Flushing solutions were

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designed to have ionic strengths that matched conditions of the Droge and Goss dataset15 and

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they also served as the experimental background eluent solutions for experiments with the

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respective clays. An operating flowrate of 100 µL min-1, previously determined to allow sorptive

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equilibrium to montmorillonite,33 was used for all experiments. All experiments were performed

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at room temperature (25°C).

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The concentrations of test compounds injected into the column were varied from 4.3 × 10-6 to

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2.6 × 10-4 M in a solution that matched the background eluent. Sorption isotherms consisted of

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at least three concentration points that were each measured in triplicate on both the ‘SiC-only’

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and the ‘montmorillonite-SiC’ columns. Absorbance vs. time data collected following each

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injection were exported directly to a MATLAB routine to calculate Kd values using compound

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and tracer retention times as detailed in the Supplemental Information (SI, 3.1). All Kd values

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were assumed to be in the linear range of the isotherm if surface coverage by organic cations was

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less than 2%, two or more initial concentrations had similar Kd values (within 1 standard

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deviation of each other) and skewness values of runs on ‘montmorillonite-SiC’ columns matched

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(within 4%) those on the ‘SiC-only’ column.33 Sorption coefficients for benzylamine, PTMA,

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and desipramine on Ca2+-montmorillonite were re-measured at the end of the study to ensure that

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the columns remained unchanged through the duration of this work.

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confirmed to provide identical retention times and sorption coefficients for these compounds

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indicating that no loss of mass, or irreversible sorption, occurred.

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Sorption to Soils: Kd from Batch Sorption Experiments

All columns were

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Sorption of four organic cations (benzylamine, PTMA, tramadol, and desipramine) to 30

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soils was measured using standard batch experiments. Column chromatography was not utilized

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to evaluate sorption to soils because eluting columns with DI water or a background electrolyte

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(NaCl or CaCl2) would modify the distribution of naturally occurring exchangeable inorganic

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cations on the soil. A predetermined mass of soil (see SI, 3.2, for details) was placed in a 15-mL

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centrifuge tube and 10 mL of 5 × 10-5 M solution (pH 6-7) of the test compound prepared in DI

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water was added. Soil-free reactors, prepared in the same manner, provided a measure of initial

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concentration and allowed us to confirm the absence of other loss processes. Soil-free and soil-

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containing reactors were rotated end-over-end for 18-24 hr (based on preliminary kinetic

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experiments) in the dark to attain sorptive equilibrium. Then, solutions from the soil-free reactors

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and supernatant from the soil-containing reactors were filtered with 0.45 µm nylon filers,

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dispensed into clean HPLC vials and centrifuge tubes for analysis of aqueous organic cation

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concentrations (Cw, mol L-1) and pH, respectively. Organic cation aqueous concentrations were

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measured by high performance liquid chromatography with diode array detector (HPLC-DAD)

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using an Agilent 1100 Series system (see SI for HPLC methods). Corresponding sorbed

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compound concentrations (Cs, mol kg-1) were calculated by difference, and Kd values were then

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obtained as Cs/Cw. Based on our earlier studies29, we note that our experimental conditions were

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designed to ensure that Kd values obtained were in the linear range of sorption isotherm. Batch

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experiments with soils were conducted using organic cation dissolved in DI water to preserve the

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native heteroionic composition of the soil; we expected that the use of a buffer or a background

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electrolyte would change native soil pH and the composition of exchangeable inorganic cations

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associated with the soil. All experiments were performed in triplicate at native soil pH.

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Desorption experiments using BaCl2 were used to confirm that loss from solution was due

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primarily to sorption phenomena.

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RESULTS AND DISCUSSION

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Effect of Cation Identity on Organic Cation Sorption to Homoionic Montmorillonite

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We evaluated whether exchangeable inorganic cations showed a consistent sequence of

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competition across several organic cations by measuring the sorption coefficients of ten organic

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compounds (Figure S1), including eight pharmaceuticals, with six homoionic montmorillonite

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clays. Test organic cations were chosen to have a range of factors that could influence cation

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exchange, including hydrophobic exclusion from the bulk aqueous phase, amine rank and the

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presence of polar groups. These structural differences contributed to a wide range in Kd values

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across a given montmorillonite system (e.g., 83 to 29600 L/kg for Na-MMT, Table S3) even

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though all test organic cations had the same charge of +1. Nevertheless, some important trends

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were evident across the test organic cations. For all the organic cations examined, Kd values for

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sorption to homoionic montmorillonite saturated with monovalent cations (Na-MMT, NH4-

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MMT, K-MMT) were greater than Kd values for divalent homoionic montmorillonite (Ca-MMT

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and Mg-MMT) which in turn were greater than values for montmorillonite saturated with

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trivalent cations (Al-MMT) (Figure 1). The resulting Kd sequence for the test organic cations was

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found to be:

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KdNa-MMT > KdNH4-MMT ≥ KdK-MMT > KdCa-MMT ≥ KdMg-MMT > KdAl-MMT (Sequence II)

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Because variations between KdX-MMT values are an implicit measure of the extent to which the

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exchangeable inorganic cation, Xn+, suppressed organic cation sorption, the trend of cation order

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from left to right in Sequence II represents increased competition for cation exchange sites from

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inorganic cations. This inorganic cation ordering showed a similar pattern as Sequence I for

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exchange between inorganic cations. The one exception to ordering between Sequence II and

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Sequence I occurred for the divalent cations: Mg2+ showed stronger, or similar, competition with

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organic cations than Ca2+ (Sequence II) compared to inorganic cation affinities to cation

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exchange sites (Sequence I). This trend of KdCa-MMT ≥ KdMg-MMT for organic cations has been

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observed previously by others.22,24

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Figure 1. Sorption coefficients (Log Kd) of organic cations to Na+ (grey), NH4+ (black), K+ (red),

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Ca2+ (green), Mg2+ (yellow), and Al3+ (blue) exchanged montmorillonite followed a general

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pattern of decreased sorption with increased inorganic cation charge. Compound numbers refer

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to structures found in Figure S1.

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The experimentally-determined Kd values from our set of test organic compounds were

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used to quantify relative ion affinity constants for each inorganic cation. The ion affinity

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constant, Exion-X, was defined as:

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  = log   log 

(2)

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where KdX-MMT (L/kg) is the experimentally determined Kd value for montmorillonite homoionic

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in cation X. Ca-montmorillonite was chosen as a reference homoionic clay because a detailed

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structural model for organic cation sorption to Ca2+-saturated clay was developed by Droge and

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Goss;15 by definition, Exion-Ca = 0. For the other five cations, Exion-X values were remarkably

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similar across the ten test organic cations. The average values were determined to be Exion-Na =

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0.85 ± 0.06; Exion-NH4 = 0.65 ± 0.12; Exion-K = 0.58 ± 0.13; Exion-Mg = -0.15 ± 0.09 and Exion-Al = -

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0.90 ± 0.13. First, we note that our Exion-Na for Na+ matches well with the +1 log unit offset in

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cation exchange capacity-normalized Kd values recommended by Droge and Goss to translate

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their structure-based organic cation sorption model from Ca2+-saturated to Na+-saturated clays.

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Second, the coefficients of variation for Exion-X ranged from 7% (Na+) to 22% (K+), quantitatively

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supporting our prior hypothesis that sorption of each organic cation was influenced similarly by a

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particular inorganic cation. The only exception was Exion-Mg that varied by 60% across the

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compounds because of the differential effect of Mg2+ on Kd values between compounds 1-7

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(Exion-Mg = -0.17 ± 0.06) and compounds 8-10 (Exion-Mg = -0.017 ± 0.006). Our experimentally-

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derived Exion-X values allow for simple adjustments to the Droge and Goss model to predict

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organic cation sorption to other homoionic clays by adding the value of Exion-X to log Kd values

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calculated from the Ca2+-clay model. (Note that Droge and Goss define sorption coefficients as

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normalized to solid cation exchange capacity instead of mass as is our definition here; however,

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application of a constant CEC value as a divisor to the two Kd terms in Eq. 2 yields the same

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values for the two approaches.) The close similarity in Exion-X values across our test organic

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cations shows that each inorganic cation affects the sorption of organic cations in a consistent

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and predictable manner, regardless of organic cation structure. Importantly, this observation

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supports the promise of using a probe cation to account for inorganic exchange ion identity,

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affinity and abundance in the prediction of organic cation sorption coefficients.

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Delineation of Scaling Factors (Si) and Incorporation of Kd,exptprobe into a Predictive Model for

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Cation Sorption in Homoionic Systems

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Using our data for organic cation sorption to homoionic montmorillonites (Table S3),we

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first validated the concept of using probe compound sorption (Kd,exptprobe), along with a scaling

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factor (Si), to predict Kd values for other compounds. PTMA was chosen to be the probe

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compound because structural corrections (Si, Eq. 1) would largely be additive to extrapolate to

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other compounds in our test set. Although benzylamine also meets this criteria, the low Kd

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values for this compound resulted in greater relative error (e.g., 30% for Al3+-MMT, Table S3),

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thus favoring PTMA. Here, we implemented a hybrid approach by using Droge and Goss’

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detailed structural model of organic cation sorption15 to calculate structural scaling factors by

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utilizing differences between molar volume and amine rank for PTMA and organic cation i: log ( ! ) = 1.22(&  &'( )  0.22*+, ± ./ !

(3)

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where Vxi (L mol-1) is the molar volume, NAii is amine rank or number of hydrogen atoms

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bonded to the cationic amine group, and CFclay represents corrective factors for compound

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structural moieties (Table S5). By definition NAi = 0 for PTMA. Sorption coefficients of the

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other nine test compounds (all except PTMA) for the six homoionic clays were predicted with

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Eq. 1 using Kd,exptPTMA for each clay type, along with scaling factors, Siclay, obtained using Eq. 3.

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The resultant Kd,predX-MMT values were within 0.3 log units of the experimental values (Figure 2).

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We note that predictions for each individual homoionic clay (Xn+-MMT) are uniformly

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distributed around the 1:1 line, even for Mg2+-MMT that had the most variation in Exion-Mg values

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across the compound set. These observations suggest that there was no bias in the application of

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the probe concept (Eq. 1 with Eq. 3) for a particular homoionic clay.

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Figure 2. Application of PTMA as a probe compound to predict the sorption of other organic

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cations onto MMT showed a strong correlation between predicted and experimental Kd values

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(R2 = 0.97, p < 0.01), indicating that Kd,exptPTMA and Siclay were able to account for competition

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from different inorganic exchange ions in the cases of homoionic Na+ (grey circles), NH4+ (black

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circles), K+ (red circles), Ca2+ (green circles), Mg2+ (yellow circles) and Al3+ (blue circles)

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montmorillonite. Solid line is 1:1 and dashed lines represent ± 0.3 log units.

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A potential source of bias in application of PTMA as a probe to predict organic

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compound sorption to the homoionic montmorillonite surface is the choice of CFclay factors in

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the calculation of Si (Eq. 3). Our test compounds possessed six structural moieties for which

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CFclay as defined by Droge and Goss (Table S5) were required. Poorly ascribed correction

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factors for a given compound would shift predicted Kd,predX-MMT values systematically above or

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below the 1:1 line (Figure 2). Of all the CFclay factors used, we observed that only the CFclay for

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ether moieties (-COC-) were ineffective: Predicted Kd values were 2 to 10 times lower than

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measured values, possibly resulting from overcorrection by assuming additivity when more than

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one ether moiety was present. Therefore, values in Figure 2 were obtained by setting CFclay = 0

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for -COC- (Table S5). All other CFclay values assigned by Droge and Goss were effective in

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arriving at our scaling factors. Together, these results indicated that Kd,exptPTMA could be used to

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account for exchangeable inorganic cation identity in pure phase aluminosilicates without

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explicit measures of exchangeable cation identities or surface abundances.

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Establishing a Hybrid Approach for Using a Probe Compound to Predict Sorption to

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Heteroionic and Heterogeneous Soil Sites

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Soils are heteroionic systems with a variety of inorganic cations balancing negative

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charge at surface sites. Quantitative analysis of inorganic cations displaced in the measurement

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of CEC does reveal the distinct composition of inorganic cations associated with the soil (e.g.,

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f(Exi), Table S5); however, it is unclear how empirical Exion-X values might be coupled with such

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analyses to apportion the relative competition effects of inorganic soil cations on sorbing organic

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cations. Therefore, we evaluated the potential for using sorption coefficients from a probe

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compound to establish an aggregate measure of inorganic cation competition on aluminosilicate

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mineral-containing soils.

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Evidence that inorganic cation identity and abundance influences organic cation sorption

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to soils was obtained through several regression analyses. First, we performed linear regressions

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of experimental Kd,expt values with soil cation exchange capacity (Fig. 3A, S2A). Experimental

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log Kd,expt values for PTMA, a compound with a simple structure, and tramadol, a

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pharmaceutical, showed correlations with log ECECsoil that were statistically significant (p