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Published: October 14, 2011

r 2011 American Chemical Society 9240 dx.doi.org/10.1021/es201859j| Environ. Sci. Technol. 2011, 45, 92409248

ARTICLE

pubs.acs.org/est

Adsorption of Aromatic Carboxylate Ions to Black Carbon (Biochar)Is Accompanied by Proton Exchange with Water

Jinzhi Ni,Joseph J. Pignatello,*, and Baoshan Xing

College of Geographical Sciences, Fujian Normal University, Fuzhou 350007 ChinaDepartment of Environmental Sciences, Connecticut Agricultural Experiment Station, 123 Huntington Street, P.O. Box 1106,New Haven, Connecticut 06504-1106, United StatesDepartment of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, Massachusetts 01003, United States

bS Supporting Information

INTRODUCTION

The carboxylic acid functional group is abundant in naturalsoil organic matter and is present in the molecular structures ofmany natural and synthetic compounds released to soil,including plant exudates, natural signaling chemicals betweenrhizosphere species, pesticides, and environmental contami-nants. Charcoal black carbon is a component of the soil carbonpool as a result of forest fires and deliberate burning practices.1

In addition, interest has emerged in the application of engineeredcharcoal from biomass waste, known as biochar, to agriculturaland forest lands for its potential benefits to soil quality andfor its carbon sequestration value.2 Contemplated levels of

biochar to croplands and potting soils range from 1 to 10%or more by weight. The effects of natural or added charcoalon chemical and biological processes in the rhizosphere aremostly uncharacterized. A potentially critical property ofcharcoal with respect to these processes is its surface activity asan adsorbent. The adsorbent strength of charcoal toward organiccompounds is a function of the biomass precursor, charringconditions (time and temperature profile, oxygen concentration),degree of postcharring weathering, and other factors that dictatespecific surface area, microporosity, and surface chemistry of thefinal material. Depending on these factors and abundance in soil,charcoal may contribute substantially to sorption, and there-fore reduce the physical mobility and biological availability of

contaminants, as well as the above-mentioned natural com-pound classes.

The factors that govern interactions of neutral organiccompounds with charcoal and soot are well-known andcharacterized.35 By contrast, the interactions of charcoals withweak organic acids that undergo dissociation within the normalpH range of most soilsmost relevantly, carboxylic acids,phenols, and sulfonamideshave received little attention.Sorption of weak acids in soils is a function of pH, ionicstrength, surface charge and charge density, type and concen-tration of metal ions, and in some cases the structural metal ion.Sorption of the neutral molecule is governed by the weak forcesavailable to neutral compounds including van der Waals,hydrogen bonding, and solvophobic effects. Specific interac-tions of organoanions with minerals and whole soils that havebeen identified include (i) anion-exchange at positively chargedsites; (ii) repulsion with the developing negative charge onthe surface as the pH increases above the point of zero netcharge (pzc); (iii) bridging by metal cations; and (iv) whenchelation is possible, inner-sphere coordination to structural

Received: May 31, 2011Accepted: September 22, 2011Revised: September 12, 2011

ABSTRACT: We examined the adsorption of the allelopathic aromatic acids (AA),cinnamic and coumaric, to different charcoals (biochars) as part of a study on bioavail-ability of natural signaling chemicals in soil. Sorption isotherms in pH 7 buffer, where theAAs are >99% dissociated, are highly nonlinear, give distribution ratios as high as 104.8 L/

kg, and are insensitive to Ca2+ or Mg2+. In unbuffered media, sorption becomesprogressively suppressed with loading and is accompanied by release of OH with astoichiometry approaching 1 at low concentrations, declining to about 0.40.5 as the pHrises.Sorptionof cinnamateon graphite as a model forcharcoalwas roughly comparable ona surface area basis, but released negligible OH. A novel scheme is proposed that explainsthepH dependence of adsorption andOH stoichiometry andthe graphite results. In a keystep, AA undergoes proton exchange with water. To overcome the unfavorable protonexchange free energy, we suggest AA engages in a type of hydrogen bond recognized to beof unusual strength with a surface carboxylate or phenolate group having a comparable pKa.This bond is depicted as [RCO2 3 3 3 H 3 3 3 O-surf]

. The same is possible for AA , but results in increased surface charge. Theproton exchange pathway appears open to other weak acid adsorbates, including humic substances, on carbonaceous materials.
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9241 dx.doi.org/10.1021/es201859j |Environ. Sci. Technol. 2011, 45, 92409248

Environmental Science & Technology ARTICLE

metal ions.610 Sorption of the organoanion may also involvethe above-mentioned weak forces and solvophobic effects,depending on the structure of the rest of the molecule, butsolvophobic effects are weaker because of the increased watersolubility of the anion relative to the neutral molecule. Sorptionof the organoanion in some studies is said to be negligible, while in others it is found to be appreciable; for example,

polychlorophenolate ions sorb signifi

cantly to variable-chargesoils even at high pH.6Although soil organic matter (SOM) isknown to be important in the binding of weak acids to wholesoils, it has been difficult to separate the influence of SOM fromthe other components. Binding of carboxylic acids and theiranions to SOM has also been studied computationally.11,12

The prior literature on adsorption of weak acids to carbon-aceous materials is negligible except in regard to activatedcarbon.1315 It is generally found that adsorption decreases withincreasing ionization of the molecule as the pH increases abovethe pzc of the surface due to charge repulsion between the anion with the increasingly negatively charged surface, and to thereduced solvophobic effect of the anion relative to the molecule.However, the anion appears to have appreciable affinity for

carbons even under strongly alkaline condition. Muller et al.13,14

modeled adsorption of weak organic electrolytes (benzoic acidand p-nitrophenol) from aqueous solution by combining elec-trochemical, diffuse-double-layer, and normal adsorption ther-modynamic models. Their model assumes that the affinity of themolecular and ionized forms for the surface are identical exceptfor the charge attraction or repulsion term acting on the ionizedform. Thus, at pH values where the surface is net negativelycharged, the organoanion would be excluded from the surfaceunless the nonelectrostatic interaction energy outweighed theelectrostatic repulsion energy.

Our study was undertaken to characterize the adsorption ofselected aromatic acid (AA) allelochemicals by black carbon aspart of a broader study on the influence of biochar addition to

agricultural fields on chemical signaling in the rhizosphere. Westudied sorption of cinnamic andcoumaric acids to commercialbiochar prototypes. Allelochemicals are low molecular weightcompounds secreted into soil by plant tissues and/or decay ofplant residues that influence the interaction of plants withother individuals of the same species, other plant species,microbes, viruses, or insects. Allelochemicals play an importantrole in agricultural and ecological dynamics.1620 An impor-tant class of allelochemicals is the single-ring phenolic acidsreleased by many plants that include coumaric, ferulic, caffeic,p-hydroxybenzoic, phenylacetic, salicylic, trans-cinnamic,vanillic, gallic, and syringic acids, among others.19,20We haveidentified an important and heretofore unrecognized mecha-nism of adsorption of organoanions of weak acids on black

carbonnamely, proton exchange with water that results in aspeciation change on the surface and concomitant release ofhydroxide ion into solution. It should be noted that none ofthe studies above report any change in pH associated withsorption of organoanions.

EXPERIMENTAL SECTION

Materials. Biochars were generously provided by differentmanufacturers: Soil Reef by EcoTechnologies Group, LLC,Berwyn, PA; CQuest by Dynamotive Energy Systems Corp.,McLean, VA;andAgrichar by BEST Energies Australia, Somersby,Australia. The samples were used either as-received (Agrichar and

CQuest) or gently broken up in a mortar and passed through asieve to obtain the 18.2 M-cm. Surface/pore analysis wasconducted by gas porisimetry on an Autosorb-1 (QuantachromeInstruments., Boynton Beach, FL). The outgas temperature was200 C. Gas adsorption isotherms were evaluated with theBrunaurEmmettTeller (N2 isotherm at 77 K; 11 points) orGrandCanonical Monte Carlo Density Functional Theory (CO2isotherm at 273 K) models using built-in software to calculatesurface areas and pore size distribution.

Potentiometric Titration of the Biochars. Biochar (0.4 g forAgrichar and 0.5 g for Soil Reef) was prewetted in 5 mL of

nanopure water for 48 h at 20 ( 1C with end-over-end mixingat 40 rotations per minute (rpm). Then varying amounts of

standard HCl or NaOH solution were added to each sample andto a corresponding blank vial containing the water but nobiochar. Preboiled water was used for titration in the alkalineregion and the vials were degassed with N2 prior to addition ofthe NaOH through the septum. The pH was measured after 48 hof mixing at 20 ( 1 C. The nominal initial H+ or OH

concentration in the sample was calculated from the pH of itscorresponding blank.

Sorption Experiments. Sorption isotherms were constructedby placing 40 mg of Agrichar or 100 mg of Soil Reef into a 60-mLpolytrifluoroethylene (PTFE)-lined screw cap glass vial, alongwith 50 mL of nanopure water or 0.05 M phosphate buffer (pH

7.0). A parallel set of controls without biochar was set up.Samples and controls without buffer were degassed with N2.After 48 h prewetting, the pH was measured in three sacrificedsamples to establish initial pH, and a stock solution of the AA wasadjusted to the average pH of the sacrificed samples. This stocksolution was used to spike the samples and correspondingcontrols. The vials were mixed end-over-end at 40 rpm at20 ( 1 C for an additional 48 h. The aqueous phase was thensampled and microfiltered (0.45m) to removeany biochar. TheAA concentration was determined by high-performance liquidchromatography on a C-18 column (S 5 ODS2; phase Sep,Clwyd, U.K.) eluted with 30:70 (v/v) CH3CN/water containing20 mM acetic acid (pH 3.2) with monitoring at 270 nm forcinnamic acid and 314 nm for coumaric acid. The sorbed

concentration was calculated by material balance. In preliminaryexperiments 48 h appeared sufficient to reach equilibrium. Whereas true equilibrium is difficult to judge, we make thereasonable assumption that trends in sorption observed over the48-h contact period are representative of trends in any sorptionoccurring after that time.

Isotherms were fit to the Freundlich model (eq 1) and theLangmuir model (eq 2)

S KFCN 1

S SmaxL KLC

1 KLC2


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where S and C are the sorbed (mg/kg) and solution (mg/L)concentrations, respectively, N is the Freundlich exponent,KF is the Freundlich affinity-capacity parameter, SL

max is theLangmuir capacity parameter, and KL is the Langmuir affinityparameter. The Freundlich parameters were determined bylinear regression of log-transformed data, while the Langmuirparameters were determined by nonlinear regression of un-transformed data. In both cases the data were weighted bythe dependent variable. The distribution ratio,Kd , isdefinedasS/Cat a specified concentration.

Sorptionexperiments to determine stoichiometrywere carriedout in the same way except using a higher biochar/water ratio(0.4 g for Agrichar, 1.0 g for Soil Reef, and 3.0 g for graphite per10 mL).

Experiments to determine the influence of metal ions onsorption of AA by Agrichar (40 mg of solids and 50 mL of liquidphase)were conducedin a similar manner exceptfor theadditionat the prewetting step of CaCl2 or MgCl2 and NaCl to keep ionicstrength equal in all vials. A constant mass of AA was added toeach vial.

RESULTS AND DISCUSSION

In screening tests we measured the reduction in solution-phase concentration of AAs after equilibration with increasing

biochar concentrations in water initially adjusted to pH 5 or 7with HCl/NaOH (Figure S1, SI). At pH 5, the fraction ofcinnamic and coumaric acids in dissociated form is 78.5% and80.3%, respectively. At pH 7, cinnamic and coumaric acids are>99.7% dissociated. We found that sorption is greater at pH 5than 7, follows the order Agrichar > Soil Reef . CQuest, and isslightly greater for cinnamic than coumaric acids in allcasesat the

tested concentrations. Undoubtedly the weak sorbent propertyof CQuest in comparison to the others is due to the fast pyrolysismethod of production, which leaves the material with significantincompletely charred biopolymer and permeated with a greateramount of tarry residue.

Sorption isotherms of cinnamic acid and coumaric acid forAgrichar and Soil Reef in phosphate buffer at pH 6.9 are shownin Figure 1 and the model parameters are listed in Table S2.Isotherms on CQuest were not constructed in view of its poorsorbent ability in the screening tests. The isotherms are highlynonlinear even on log scale. Neither the Freundlich nor theLangmuir models proved universally suitable. The order insorption intensity regardless of liquid phase concentration is Agrichar > Soil Reef. Sorption intensity follows the order

cinnamate > coumarate over most of the tested concentrationrange; the difference is more pronounced for Soil Reef thanAgrichar.

The trends displayed in the screening tests and the isothermshave conventional explanations. Sorption is greater at pH 5 dueto the greater abundance of the molecular form and the lowernegative charge of the surface (see below) compared to pH 7.14

Sorption trends qualitatively with the N2 BET of the biocharslisted in Table S1: namely, Agrichar (427 m2/g) > Soil Reef (338m2/g) . CQuest (0.1 m2/g). Sorption also trends with the CO2GCMC surface area. The order in sorption intensity between thetwoAAs is plausibly related to solvophobic effects. The octanolwater partition coefficient (Kow) is a commonly used index ofsolvophobicity. According to SPARC calculator (http://sparc.

chem.uga.edu/sparc/; accessed November 17, 2010) the logKowof the molecular and anionic forms of cinnamic acid are 2.50 and0.42, respectively, and those of coumaric acid are 1.78 and 1, respectively, consistent with this conclusion. Sorption of theorganoanions, reflected in the Kd at pH 6.9, is remarkably strong,however, a fact that is not well-explained by solvophobic effectsalone. Depending on concentration, the log Kd for cinnamate on Agrichar ranges 3.74.2 and on Soil Reef ranges 3.13.8.Likewise, log Kd of coumarate on Agrichar ranges 3.54.8 andon Soil Reef ranges 2.63.9. The Kd values are thus many ordersof magnitude greater than the estimated Kow value of therespective organoanion. This finding seems inconsistent withthe sorbed species being the free organoanion. Rather, itimplicates either a speciation change or a strong specific inter-

action of the organoanion on the surface.We next determined theeffectsofupto0.1MCa2+ andMg2+

on sorptionof the AAs at constant massof AA added and ionicstrength (Figure S2, SI). We expected that if the anionic formwere sorbing, these metal ions would enhance sorption byserving as a cation bridge between the carboxylate group and anegatively charged surface group, such as a carboxylate orphenolate group (e.g., RCO2

3 3 3 M2+

3 3 3O2C-BC). The

metal may interact with these anions either by contact orsolvent-separated ion pairing.23 Cation bridging is an importantmechanism triggering the aggregation of humic molecules intolarger colloidal structures (NOM) according to moleculardynamics computations.23 Cation bridging also has been

Figure 1. Isotherms of (A) cinnamate and (B) coumarate on Agricharand Soil Reef in phosphate buffer (pH 6.97.0) and fits to two sorption

models.


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proposed as a mechanism for sorption of carboxylate andphenolate compounds to whole soils,10,6 model soil minerals,9,24

and soil organic matter25 on the basis of physical experiments, as well as to model humic structures on the basis of compu-tations.11,12 Figure S2, however, reveals little, if any, systematicchange in sorption induced by Ca2+ and Mg2+. This findingimplies that sorption of the AAs is not greatly affected by chargedsites under the influence of these metal cations.

Figure 2A shows linear-scale plots of the isotherms of cinnamateon Agrichar in phosphate-buffered vs nonbuffered suspensions. Atzero concentration of cinnamate the buffered and nonbufferedsuspensionshadequilibrated during theprewetting stage to a similarpH (6.9 and 7.2, respectively). The isotherms are seen to deviatefrom one another as AA concentration increasesthe nonbufferedsamples giving reduced sorption relative to the buffered samples.Moreover, the OH concentration of the nonbuffered solutionsincreases relative to the buffered solution as loading increases.Because the AA stock solution was adjusted to the approximateinitial concentration of the biochar suspension, vials containing justthe aqueous phase showed no significant increase in hydroxide ionconcentration with increasing cinnamate concentration up to thesame levels added (data not shown). Soil Reef showed resultsqualitatively similar to those for Agrichar, except the isotherm and[OH] data are more scattered (Figure 2B). Taken together, theresults show that sorption of AAby biochar is accompaniedby therelease of hydroxide ion into solution (eq 3), which presumably isthe cause of progressive sorption suppression.

RCO2 BC h RCO2

3 3 3 BC OH 3

To determine the magnitude of OH release the bufferingcapacity of the biochar must be taken into account

OH BC h BC H2O 4

At a given pH, the amount ofOH released by AA sorption is theobserved amount appearing in solution plus the amount consumedby the biochar at the final pH as determined in an independenttitration experiment using the same equilibration period (48 h) andtemperature (20 C) as the sorption experiment. The raw titrationcurves and the curves representing specific uptake of H+ or OH

versus pH calculated from the raw titration data are provided inFigures S3 andS4, respectively. The crossoverpHwherethepHofthe sample is equal to the pH of the blank (see Figure S3)is 8.07for Agrichar, 7.96 for Soil Reef, and 6.6 for CQuest. Consumption ofOH at any pH above the crossover pH, which represents thebiochars buffering capacity, follows the order CQuest > Soil Reef >Agrichar. Consumption of H+ at any pH below the crossover pHfollows the reverse order. The pH at the pzc is best determined byelectrophoretic mobility. The pHpzc for Agrichar is 3.94.3 (TableS1),indicating that thenet chargeonthesurface is negative under theconditions ofallsorptionexperimentsof this study.This is likely tobetrue also for Soil Reef because of the similarity in the crossover pH.

Quantification of OH released as a function of AA sorbed(the stoichiometry) required separate experiments using higherbiochar/water ratios than used for constructing the isotherms in

Figure 2 in order to obtain greater accuracy in the pH change.Figure 3 shows the results of these experiments. Total molesOH generated is the observed moles OH in solution in thesesorption experiments plus the moles OH consumed by thebiochar at the same pH in the titration experiments, both after48 h. Moles OH consumed by the biochar at each pH wasestimated by curve fitting the titration curve in the alkalineregion, shown as the curves in Figure S4, and using the fit forinterpolation purposes in the sorption experiment.

Figure 3 shows that the stoichiometry between OH andcinnamate sorbed is not constant but decreases with increasingcinnamate loading and/or pH accompanying loading. At thelowest sorbed concentration the OH/cinnamate molar ratio is

Figure 2. Sorption isotherms of cinnamate for biochars comparing buffered (phosphate pH 6.9) and nonbuffered conditions and the accompanyingevolution of hydroxide ion concentration. The initial solution composition was 0.005 M CaCl2. The initial nonbuffered pH averaged 7.38 for Agricharand 7.95 for Soil Reef.


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approximately 1, while this ratio decreases to about 0.4(Agrichar) or about 0.5 (Soil Reef).

We also tested whether hydroxide is released on adsorption ofAA to nonporous powdered graphite, which we found pre-viously to be a good model for black carbon with respect toadsorption of nonionic compounds.26 Sorption and titrationexperiments were conducted for cinnamate on graphite in thesame way as for the biochars. Not surprisingly, sorption ofcinnamate was much weaker on graphite than on Agrichar andSoil Reef on a sorbent mass basis (Figure S3, Table S2), the Kd(L/kg) being >300 times smaller than on Agrichar and >70 timessmaller than on Soil Reef. However, on a N2BET surface area

basis, adsorption of cinnamate was


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following proposal does not require a distinction. Potential sourcesof hydroxide release include (i) a spike artifact; (ii) displacement ofadsorbed hydroxide ion by AA; and (iii) proton exchange of AA

withwater.Anartifactofthe spikeprocedureis ruled out because thepH of the controls without biochar changed insignificantly withloading (Figure 3), since the pH of the stock solution was adjustedto the same pH as the prewetted biochar. Moreover, the change in

ionic strength accompanying spiking (


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where the brackets represent concentration in appropriate units,andKeq 7 andKeq 8 are the respective equilibrium constants. Notethat eq 7 incorporates the proton exchange in eq 5.

The relationship between surface H-acceptor and H-donorsites is given by

9OH aKaS

9O H;

KaS 9O 3Kaw

OH9OH KintraS e

zF=RT 9

where KaS is the acid dissociation constant of9OH at existingcharge density, KaS

intr is the intrinsic acid dissociation constant of9OH groups, Kaw is the acid dissociation constant of water

22

(1014), z is the proton valence (+1), Fis the Faraday constant, is the surface potential with respect to bulk solution, Ris thegas constant, and Tis temperature.13,14,31

Both AA and AA may undergo interactions also with sitesother than those represented by reactions7 and8. We mayreferto these interactions collectively as solvophobic adsorp-tion sites. Solvophobic adsorption of AA on the charcoalsurface is demonstrated by the significant adsorption of

cinnamate ion by graphite, and further supported by the factthat the OH/adsorbate stoichiometry decreases with pHwhile adsorption remains strong (Kd > 1000 L/kg), even fourpH units above the pKa. Solvophobic adsorption may bewritten as

RCO2H 9 aKd,AAsolvo

9 3 3 3 RCO2H

Kd,AA solvo 9 3 3 3 RCO2H

RCO2H10

RCO2 9 a

Kd,AAsolvo9 3 3 3 RCO2

Kd,AA

solvo K

non-elec

d,AA

solvoe

zF=RT

9 3 3 3 RCO2

RCO2 11

whereKd, AA solvononelec is thenonelectrostaticcomponent and ezF/RTis

the electrostatic component of the surface potential operative forAA.

The observed, concentration-dependent adsorption distribu-tion ratio may be written

Kd S

C 9OHO2CR

reaction7

9OHO2CRreaction8 RCO2HKd, AA solvo

RCO2Kd,AAsolvo=C 12

where S and C refer to the total adsorbed and dissolvedconcentrations,, respectively, including all species.

Recognizing that C [RCO2] above pH 7, and combiningeqs 79 and 12 gives

Kd 9O

OHKeq7 Keq8 3

1

1014 3KintraS e

zF=RT

!

1

1 OH 3 1014 pKa 3

Kd, AA solvo

Knon-elecd,AAsolvoezF=RT 13

Equation 13 predicts that overall adsorption will decline withrising pH, consistent with observation. The ratio [9-O]/[OH]

in the first term decreases with increasing pH because 9OHgroups are incompletely deprotonated by released (or added)hydroxide due to charge buildup on the surface. The second termincludes aqueous speciation of the neutral molecule and isinversely related to pH. The thirdterm reflects the growingchargerepulsion of AA on the surface ( is negative) as pH rises.In addition, eq 13 predicts the decline in reaction 7 relative to

reaction 8 with pH owing to the term 1/1014

3KaSintr

ezF/RT

, whichincreases with pH becausebecomes progressively more nega-tive. The decline in the contribution of reaction 7 relative toreaction 8, in turn, predicts the observed decline in OH/adsorbate stoichiometry with loading. Another way to look at itis that, with increasing pH,protonexchange becomes energeticallymore costly at a faster pace than placing a charge on the surface.

Interestingly, the OH/adsorbed-cinnamate stoichiometryfor graphite increases with loading (Figure 3C). This result mayrepresent a cooperative effect in which adsorbed AA ions act asH-bond acceptor sites for AA; that is, AA and AA form aconjugate pair (RCO2 3 3 3 H 3 3 3 O2CR)

on the graphite surfacelinked by a ()CAHB

RCO2

H2O RCO2

3 3 39 h

RCO2HO2CR

3 3 39

OH

14

However, the cooperative effect, if it occurs, is quite small and isunlikely to contribute much to sorption of the aromatic acids onthe biochars.

The results of this study point to a novel pathway foradsorption of carboxylic acid anions from water to black carbonthat involves proton exchange with water and speciation changeon thesurface accompanied by release of hydroxide into solution.Adsorption through this pathway appears to constitute from40% to 100% of total adsorption in the pH range of ourexperiments. The reaction appears driven by the formation of anespecially strong H-bond on the surface, supplemented by the

increase in solvophobicity of the adsorbate. Although accuratenumbers are unavailable, it is estimated that these processestogether contribute 70 kJ/mol to offset the 55 kJ/molstandard free energy of proton exchange. It is possible that protonexchange-assisted adsorption on natural particles rich in car-boxylic and phenolic groups is characteristic of any weak acidthat is capable of forming a very strong H-bond by virtue of pKarough equivalency. Other examples include various kinds ofamines and heterocyclic amines (e.g., anilines, azines, azoles),sulfonamides, and phenols with electronegative substituents.28

We have now demonstrated proton exchange-assisted adsorptionon Agrichar of the weak acid sulfamethazine, a veterinary anti-biotic having a pKa2 of 7.42 for the sulfonamide (SO2NH)group.37 The stroichiometry of OH release ranged from about 1

at pH 9.3 to about 0.4 at pH 10.5. Just as with the aromaticcarboxylates, we have proposed that adsorption of sulfamethazineis augmented by a ()CAHB between the sulfonamide moietyand a surface oxyl group: [(R)(R0SO2)N 3 3 3 H 3 3 3 O-9]

.Extension of the sorption edge of weak acids well above the

pKa has been noted many times previously for soils6 and activated

carbons.1315 We think our proposed mechanism can partlyaccount for this observation because it hasthe effectof raising thepKa of the acid on the surface. We are aware of no study linkingorganoanion sorption to protonation, nor of any mention offormation of a ()CAHB with the surface. The closest to comewas a study32 finding that aromatic sulfonic acids do not fit aPolanyi-linear solvation free energy model for adsorption of


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neutral compounds by activated carbon because they are fullyionized (pKa < 1). The authors proposed that sulfonate ions takeup protons from solution and adsorb onto the surface. However,they did not provide direct evidence for proton exchange, norsuggest a driving force that could overcome the free energy ofproton exchange, which is even more unfavorable than forthe AAs.

Proton exchange during weak acid sorption has gone unrec-ognized until now (apparently) for two main reasons. First, mostexperiments are performed using buffered systems. Second, thesorbent itself serves as a buffer, masking the release of hydroxideto some degree. Indeed, the observed changes in solution pHwere smaller for Soil Reef than for Agrichar because Soil Reefcontains more O (inferred from the C, H, and ash composition;Table S1) and consumes more OH at any given pH (FigureS4). An important question is whether weak acids undergoproton exchange-assisted sorption to soil organic matter, whichis rich in carboxylic and phenolic functionality. Proton exchangein this case may be difficult to observe, however, because of thehigh buffering capacity of soil organic matter. In a computationalstudy,12various H-bonds were observed between the carboxylate

group of the antimicrobial Ciprofloxacin, and a fully protonatedmodel humic molecule and were concluded to be important tothe stability of the complex. Finally, the results of this study haveimplications for adsorption of dissolvednatural organic matter bycarbonaceous materials including black carbon,33,34 activatedcarbon,35 and engineered carbon nanoparticles,36 in whichinteractions of dissociable groups are likely to be important.

ASSOCIATED CONTENT

bS Supporting Information. Tables and figures on sourcesand properties of the biochars; sorption model parameters; theresults of sorption screening experiments; effect of metal ionconcentration, titration curves of the biochars and graphite;

sorption isotherm of cinnamate on graphite; and estimation ofconjugate bond formation free energy in water. This informationis available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected]; phone: 203-974-8518.

ACKNOWLEDGMENT

The bulk of this work was performed while J.Z. Ni was a Visiting Scholar at the Connecticut Agricultural ExperimentStation through an arrangement with the University of

Massachusetts, Amherst, and with funding provided by FujianNormal University, China. J.Z. Ni thanks these institutionsfor the opportunity. The authors also thank the Departmentof Analytical Chemistry at CAES for the emergency use ofinstrumentation.

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