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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. Box1106, New Haven, Connecticut 06504-1106, United States § Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, Massachusetts 01003, United States
S b 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 signalingchemicals 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 OHÀ with a stoichiometry approaching 1 at low concentrations, declining to about 0.4À0.5 as thepH rises. Sorption of cinnamate on graphite as a model for charcoal was roughly comparable on a surface area basis, but released negligible OHÀ. A novel scheme is proposed that explains the pH dependence of adsorption and OHÀ stoichiometry and the graphite results. In a key step, AAÀ undergoes proton exchange with water. To overcome the unfavorable proton exchange free energy, we suggest AAengages 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]À. The same is possible for AAÀ, 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 forestfires 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 addedcharcoal 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 ofpostcharring 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 naturalcompound classes. The factors that govern interactions of neutral organic compounds with charcoal and soot are well-known and characterized.3À5 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...