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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 China Department of Environmental Sciences, Connecticut Agricultural Experiment Station, 123 Huntington Street, P.O. Box 1106, New Haven, Connecticut 06504-1106, United States § Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, Massachusetts 01003, United States ‡
bS Supporting Information ABSTRACT: We examined the adsorption of the allelopathic aromatic acids (AA), cinnamic and coumaric, to different charcoals (biochars) as part of a study on bioavailability of natural signaling chemicals in soil. Sorption isotherms in pH 7 buffer, where the AAs 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 becomes progressively suppressed with loading and is accompanied by release of OH with a stoichiometry approaching 1 at low concentrations, declining to about 0.40.5 as the pH rises. Sorption of cinnamate on graphite as a model for charcoal was roughly comparable on a surface area basis, but released negligible OH. A novel scheme is proposed that explains the pH dependence of adsorption and OH stoichiometry and the graphite results. In a key step, AA undergoes proton exchange with water. To overcome the unfavorable proton exchange free energy, we suggest AA engages in a type of hydrogen bond recognized to be of 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. The proton exchange pathway appears open to other weak acid adsorbates, including humic substances, on carbonaceous materials.
’ INTRODUCTION The carboxylic acid functional group is abundant in natural soil organic matter and is present in the molecular structures of many natural and synthetic compounds released to soil, including plant exudates, natural signaling chemicals between rhizosphere species, pesticides, and environmental contaminants. Charcoal black carbon is a component of the soil carbon pool as a result of forest fires and deliberate burning practices.1 In addition, interest has emerged in the application of engineered charcoal from biomass waste, known as biochar, to agricultural and forest lands for its potential benefits to soil quality and for 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 charcoal on chemical and biological processes in the rhizosphere are mostly uncharacterized. A potentially critical property of charcoal with respect to these processes is its surface activity as an adsorbent. The adsorbent strength of charcoal toward organic compounds is a function of the biomass precursor, charring conditions (time and temperature profile, oxygen concentration), degree of postcharring weathering, and other factors that dictate specific surface area, microporosity, and surface chemistry of the final material. Depending on these factors and abundance in soil, charcoal may contribute substantially to sorption, and therefore reduce the physical mobility and biological availability of r 2011 American Chemical Society
contaminants, as well as the above-mentioned natural compound classes. The factors that govern interactions of neutral organic compounds with charcoal and soot are well-known and characterized.35 By contrast, the interactions of charcoals with weak organic acids that undergo dissociation within the normal pH range of most soils—most relevantly, carboxylic acids, phenols, and sulfonamides—have received little attention. Sorption of weak acids in soils is a function of pH, ionic strength, surface charge and charge density, type and concentration of metal ions, and in some cases the structural metal ion. Sorption of the neutral molecule is governed by the weak forces available to neutral compounds including van der Waals, hydrogen bonding, and solvophobic effects. Specific interactions of organoanions with minerals and whole soils that have been identified include (i) anion-exchange at positively charged sites; (ii) repulsion with the developing negative charge on the surface as the pH increases above the point of zero net charge (pzc); (iii) bridging by metal cations; and (iv) when chelation is possible, inner-sphere coordination to structural Received: May 31, 2011 Accepted: September 22, 2011 Revised: September 12, 2011 Published: October 14, 2011 9240
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Environmental Science & Technology metal ions.610 Sorption of the organoanion may also involve the above-mentioned weak forces and solvophobic effects, depending on the structure of the rest of the molecule, but solvophobic effects are weaker because of the increased water solubility of the anion relative to the neutral molecule. Sorption of the organoanion in some studies is said to be “negligible”, while in others it is found to be appreciable; for example, polychlorophenolate ions sorb significantly to variable-charge soils even at high pH.6 Although soil organic matter (SOM) is known to be important in the binding of weak acids to whole soils, it has been difficult to separate the influence of SOM from the other components. Binding of carboxylic acids and their anions to SOM has also been studied computationally.11,12 The prior literature on adsorption of weak acids to carbonaceous materials is negligible except in regard to activated carbon.1315 It is generally found that adsorption decreases with increasing ionization of the molecule as the pH increases above the pzc of the surface due to charge repulsion between the anion with the increasingly negatively charged surface, and to the reduced solvophobic effect of the anion relative to the molecule. However, the anion appears to have appreciable affinity for carbons even under strongly alkaline condition. M€uller et al.13,14 modeled adsorption of weak organic electrolytes (benzoic acid and p-nitrophenol) from aqueous solution by combining electrochemical, diffuse-double-layer, and normal adsorption thermodynamic models. Their model assumes that the affinity of the molecular and ionized forms for the surface are identical except for the charge attraction or repulsion term acting on the ionized form. Thus, at pH values where the surface is net negatively charged, the organoanion would be excluded from the surface unless the nonelectrostatic interaction energy outweighed the electrostatic repulsion energy. Our study was undertaken to characterize the adsorption of selected aromatic acid (AA) allelochemicals by black carbon as part of a broader study on the influence of biochar addition to agricultural fields on chemical signaling in the rhizosphere. We studied sorption of cinnamic and coumaric acids to commercial biochar prototypes. Allelochemicals are low molecular weight compounds secreted into soil by plant tissues and/or decay of plant residues that influence the interaction of plants with other individuals of the same species, other plant species, microbes, viruses, or insects. Allelochemicals play an important role in agricultural and ecological dynamics.1620 An important class of allelochemicals is the single-ring “phenolic acids” released by many plants that include coumaric, ferulic, caffeic, p-hydroxybenzoic, phenylacetic, salicylic, trans-cinnamic, vanillic, gallic, and syringic acids, among others.19,20 We have identified an important and heretofore unrecognized mechanism of adsorption of organoanions of weak acids on black carbon—namely, proton exchange with water that results in a speciation change on the surface and concomitant release of hydroxide ion into solution. It should be noted that none of the studies above report any change in pH associated with sorption of organoanions.
’ EXPERIMENTAL SECTION
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CQuest) or gently broken up in a mortar and passed through a sieve to obtain the 18.2 MΩ-cm. Surface/pore analysis was conducted by gas porisimetry on an Autosorb-1 (Quantachrome Instruments., Boynton Beach, FL). The outgas temperature was 200 °C. Gas adsorption isotherms were evaluated with the BrunaurEmmettTeller (N2 isotherm at 77 K; 11 points) or Grand Canonical Monte Carlo Density Functional Theory (CO2 isotherm at 273 K) models using built-in software to calculate surface areas and pore size distribution. Potentiometric Titration of the Biochars. Biochar (0.4 g for Agrichar and 0.5 g for Soil Reef) was prewetted in 5 mL of nanopure water for 48 h at 20 ( 1 °C with end-over-end mixing at 40 rotations per minute (rpm). Then varying amounts of standard HCl or NaOH solution were added to each sample and to a corresponding blank vial containing the water but no biochar. Preboiled water was used for titration in the alkaline region and the vials were degassed with N2 prior to addition of the NaOH through the septum. The pH was measured after 48 h of mixing at 20 ( 1 °C. The nominal initial H+ or OH concentration in the sample was calculated from the pH of its corresponding blank. Sorption Experiments. Sorption isotherms were constructed by placing 40 mg of Agrichar or 100 mg of Soil Reef into a 60-mL polytrifluoroethylene (PTFE)-lined screw cap glass vial, along with 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 sacrificed samples to establish initial pH, and a stock solution of the AA was adjusted to the average pH of the sacrificed samples. This stock solution was used to spike the samples and corresponding controls. The vials were mixed end-over-end at 40 rpm at 20 ( 1 °C for an additional 48 h. The aqueous phase was then sampled and microfiltered (0.45 μm) to remove any biochar. The AA concentration was determined by high-performance liquid chromatography on a C-18 column (S 5 ODS2; phase Sep, Clwyd, U.K.) eluted with 30:70 (v/v) CH3CN/water containing 20 mM acetic acid (pH 3.2) with monitoring at 270 nm for cinnamic acid and 314 nm for coumaric acid. The sorbed concentration was calculated by material balance. In preliminary experiments 48 h appeared sufficient to reach equilibrium. Whereas true equilibrium is difficult to judge, we make the reasonable assumption that trends in sorption observed over the 48-h contact period are representative of trends in any sorption occurring after that time. Isotherms were fit to the Freundlich model (eq 1) and the Langmuir model (eq 2)
Materials. Biochars were generously provided by different manufacturers: Soil Reef by EcoTechnologies Group, LLC, Berwyn, PA; CQuest by Dynamotive Energy Systems Corp., McLean, VA; and Agrichar by BEST Energies Australia, Somersby, Australia. The samples were used either as-received (Agrichar and
ð1Þ
S ¼ K F CN S¼ 9241
Smax L KL C 1 þ KL C
ð2Þ
dx.doi.org/10.1021/es201859j |Environ. Sci. Technol. 2011, 45, 9240–9248
Environmental Science & Technology
Figure 1. Isotherms of (A) cinnamate and (B) coumarate on Agrichar and Soil Reef in phosphate buffer (pH 6.97.0) and fits to two sorption models.
where S and C are the sorbed (mg/kg) and solution (mg/L) concentrations, respectively, N is the Freundlich exponent, is the KF is the Freundlich affinity-capacity parameter, Smax L Langmuir capacity parameter, and KL is the Langmuir affinity parameter. The Freundlich parameters were determined by linear regression of log-transformed data, while the Langmuir parameters were determined by nonlinear regression of untransformed data. In both cases the data were weighted by the dependent variable. The distribution ratio, Kd, is defined as S/C at a specified concentration. Sorption experiments to determine stoichiometry were carried out 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 per 10 mL). Experiments to determine the influence of metal ions on sorption of AA by Agrichar (40 mg of solids and 50 mL of liquid phase) were conduced in a similar manner except for the addition at the prewetting step of CaCl2 or MgCl2 and NaCl to keep ionic strength equal in all vials. A constant mass of AA was added to each vial.
’ RESULTS AND DISCUSSION In screening tests we measured the reduction in solutionphase concentration of AAs after equilibration with increasing
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biochar concentrations in water initially adjusted to pH 5 or 7 with HCl/NaOH (Figure S1, SI). At pH 5, the fraction of cinnamic and coumaric acids in dissociated form is 78.5% and 80.3%, respectively. At pH 7, cinnamic and coumaric acids are >99.7% dissociated. We found that sorption is greater at pH 5 than 7, follows the order Agrichar > Soil Reef . CQuest, and is slightly greater for cinnamic than coumaric acids in all cases at the tested concentrations. Undoubtedly the weak sorbent property of CQuest in comparison to the others is due to the fast pyrolysis method of production, which leaves the material with significant incompletely charred biopolymer and permeated with a greater amount of tarry residue. Sorption isotherms of cinnamic acid and coumaric acid for Agrichar and Soil Reef in phosphate buffer at pH 6.9 are shown in Figure 1 and the model parameters are listed in Table S2. Isotherms on CQuest were not constructed in view of its poor sorbent ability in the screening tests. The isotherms are highly nonlinear even on log scale. Neither the Freundlich nor the Langmuir models proved universally suitable. The order in sorption intensity regardless of liquid phase concentration is Agrichar > Soil Reef. Sorption intensity follows the order cinnamate > coumarate over most of the tested concentration range; the difference is more pronounced for Soil Reef than Agrichar. The trends displayed in the screening tests and the isotherms have conventional explanations. Sorption is greater at pH 5 due to the greater abundance of the molecular form and the lower negative charge of the surface (see below) compared to pH 7.14 Sorption trends qualitatively with the N2 BET of the biochars listed in Table S1: namely, Agrichar (427 m2/g) > Soil Reef (338 m2/g) . CQuest (0.1 m2/g). Sorption also trends with the CO2 GCMC surface area. The order in sorption intensity between the two AAs is plausibly related to solvophobic effects. The octanol water partition coefficient (Kow) is a commonly used index of solvophobicity. According to SPARC calculator (http://sparc. chem.uga.edu/sparc/; accessed November 17, 2010) the log Kow of the molecular and anionic forms of cinnamic acid are 2.50 and 0.42, respectively, and those of coumaric acid are 1.78 and ∼ 1, respectively, consistent with this conclusion. Sorption of the organoanions, reflected in the Kd at pH 6.9, is remarkably strong, however, a fact that is not well-explained by solvophobic effects alone. 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 and on Soil Reef ranges 2.63.9. The Kd values are thus many orders of magnitude greater than the estimated Kow value of the respective organoanion. This finding seems inconsistent with the sorbed species being the free organoanion. Rather, it implicates either a speciation change or a strong specific interaction of the organoanion on the surface. We next determined the effects of up to 0.1 M Ca2+ and Mg2+ on sorption of the AAs at constant mass of AA added and ionic strength (Figure S2, SI). We expected that if the anionic form were sorbing, these metal ions would enhance sorption by serving as a cation bridge between the carboxylate group and a negatively charged surface group, such as a carboxylate or phenolate group (e.g., RCO2 3 3 3 M2+ 3 3 3 O2C-BC). The metal may interact with these anions either by contact or solvent-separated ion pairing.23 Cation bridging is an important mechanism triggering the aggregation of humic molecules into larger colloidal structures (NOM) according to molecular dynamics computations.23 Cation bridging also has been 9242
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Figure 2. Sorption isotherms of cinnamate for biochars comparing buffered (phosphate pH 6.9) and nonbuffered conditions and the accompanying evolution of hydroxide ion concentration. The initial solution composition was 0.005 M CaCl2. The initial nonbuffered pH averaged 7.38 for Agrichar and 7.95 for Soil Reef.
proposed as a mechanism for sorption of carboxylate and phenolate 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 computations.11,12 Figure S2, however, reveals little, if any, systematic change in sorption induced by Ca2+ and Mg2+. This finding implies that sorption of the AAs is not greatly affected by charged sites under the influence of these metal cations. Figure 2A shows linear-scale plots of the isotherms of cinnamate on Agrichar in phosphate-buffered vs nonbuffered suspensions. At zero concentration of cinnamate the buffered and nonbuffered suspensions had equilibrated during the prewetting stage to a similar pH (6.9 and 7.2, respectively). The isotherms are seen to deviate from one another as AA concentration increases—the nonbuffered samples giving reduced sorption relative to the buffered samples. Moreover, the OH concentration of the nonbuffered solutions increases relative to the buffered solution as loading increases. Because the AA stock solution was adjusted to the approximate initial concentration of the biochar suspension, vials containing just the aqueous phase showed no significant increase in hydroxide ion concentration with increasing cinnamate concentration up to the same levels added (data not shown). Soil Reef showed results qualitatively similar to those for Agrichar, except the isotherm and [OH] data are more scattered (Figure 2B). Taken together, the results show that sorption of AA by biochar is accompanied by the release of hydroxide ion into solution (eq 3), which presumably is the cause of progressive sorption suppression. RCO2 þ BC h ðRCO2 Þ 3 3 3 BC þ OH
ð3Þ
To determine the magnitude of OH release the buffering capacity of the biochar must be taken into account OH þ BC h BC þ H2 O
ð4Þ
At a given pH, the amount of OH released by AA sorption is the observed amount appearing in solution plus the amount consumed by the biochar at the final pH as determined in an independent titration experiment using the same equilibration period (48 h) and temperature (20 °C) as the sorption experiment. The raw titration curves and the curves representing specific uptake of H+ or OH versus pH calculated from the raw titration data are provided in Figures S3 and S4, respectively. The crossover pH—where the pH of the sample is equal to the pH of the blank (see Figure S3)—is 8.07 for Agrichar, 7.96 for Soil Reef, and 6.6 for CQuest. Consumption of OH at any pH above the crossover pH, which represents the biochar’s buffering capacity, follows the order CQuest > Soil Reef > Agrichar. Consumption of H+ at any pH below the crossover pH follows the reverse order. The pH at the pzc is best determined by electrophoretic mobility. The pHpzc for Agrichar is 3.94.3 (Table S1), indicating that the net charge on the surface is negative under the conditions of all sorption experiments of this study. This is likely to be true 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 higher biochar/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 moles OH generated is the observed moles OH in solution in these sorption experiments plus the moles OH consumed by the biochar at the same pH in the titration experiments, both after 48 h. Moles OH consumed by the biochar at each pH was estimated by curve fitting the titration curve in the alkaline region, shown as the curves in Figure S4, and using the fit for interpolation purposes in the sorption experiment. Figure 3 shows that the stoichiometry between OH and cinnamate sorbed is not constant but decreases with increasing cinnamate loading and/or pH accompanying loading. At the lowest sorbed concentration the OH/cinnamate molar ratio is 9243
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Figure 3. Stoichiometry of hydroxide ion release versus moles cinnamic acid adsorbed for (A) Agrichar (per 0.4 g), (B) Soil Reef (per 1.0 g), and (C) graphite (per 3.0 g). The pH record of the blanks (without biochar) shows that adding AA does not contribute appreciably to the increase in [OH] in solution in the absence of biochar.
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 of AA to nonporous powdered graphite, which we found previously to be a good model for black carbon with respect to adsorption of nonionic compounds.26 Sorption and titration experiments were conducted for cinnamate on graphite in the same way as for the biochars. Not surprisingly, sorption of cinnamate was much weaker on graphite than on Agrichar and Soil Reef on a sorbent mass basis (Figure S3, Table S2), the Kd (L/kg) being >300 times smaller than on Agrichar and >70 times smaller than on Soil Reef. However, on a N2BET surface area
basis, adsorption of cinnamate was
