Characterization and diagenesis of strong-acid carboxyl groups in humic substances J.A. Leenheer*, R.L. Wershaw, G.K. Brown, M.M. Reddy US Geological Survey, PO Box 25046, MS 408, Bldg 95, Denver Federal Center, Denver, CO 80225-0046, USA Received 22 November 2001; accepted 8 April 2002 Editorial handlingby R. Jaffe´ Abstract AsmallfractionofcarboxylicacidfunctionalgroupsinhumicsubstancesareexceptionallyacidicwithpK a valuesas low as 0.5. A review of acid-group theory eliminated most models and explanations for these exceptionally acidic car- boxyl groups. These acidic carboxyl groups in Suwannee River fulvic acid were enriched by a 2-stage fractionation process and the fractions were characterized by elemental, molecular-weight, and titrimetric analyses, and by infrared and 13 C- and 1 H-nuclear magnetic resonance spectrometry. An average structural model of the most acidic fraction derived from the characterization data indicated a high density of carboxyl groups clustered on oxygen-heterocycle alicyclic rings. Intramolecular H-bonding between adjacent carboxyl groups in these ring structures enhanced stabili- zation of the carboxylate anion which results in low pK a1 values. The standard, tetrahydrofuran tetracarboxylic acid, was shown to have similar acidity characteristics to the highly acidic fulvic acid fraction. The end products of 3 known diagenetic pathways for the formation of humic substances were shown to result in carboxyl groups clustered on oxy- gen-heterocycle alicyclic rings. Published by Elsevier Science Ltd. 1. Introduction Fully ionized acid groups in humic substances (so called strong acid groups) are important with regards to mineral weathering reactions, nutrient availability, and buffering characteristics of humic and fulvic acids (Ste- venson, 1994). Carboxyl groups clustered close to each otherandtootherpolargroupsmaybeindirectindicators of the ampliphilic character that gives humic substances surfactant properties. Hydrophobic domains in humic surfactants facilitate nonpolar organic contaminant inter- actionssuchasDDTbindingtodissolvedhumicmaterials (Carter and Suffet, 1982). Carboxyl-group clustering shouldalsoenhancethecomplexationofvariousmetalsby theformationofchelatestructures(Leenheeretal.,1998). Recent studies of acid-group heterogeneity in a num- ber of fulvic acids from soil and aqueous environments estimated pK a1 values of 2.0  0.3 for 4- and 5-site modelsofacidgroups(Ephraimetal.,1991;Leenheeret al., 1995a,b). Furthermore, strong acid sites with pK a1 values in this range in an aquatic fulvic acid were not primarily caused by sulfonic or amino acid structures, but were due to carboxyl groups clustered in aliphatic, alicyclic structures with a-ether or a-ester groups in the ring (Leenheer et al., 1995a). The purpose of this report is to examine the theore- tical and practical limits on strongly acidic carboxyl groups in humic substance structures. These limits will be identified by measuring the most acidic sites in a fractionated fulvic acid. A structural model of the most acidic fulvic-acid fraction is presented, and diagenetic reaction sequences are proposed to account for devel- opment of strongly-acid carboxylic acid sites in the environment. 2. Background 2.1. Acid-group theory Acid-group theory has been developed for simple organic compounds and for synthetic polymers con- taining well-defined acid groups. Application of acid- 0883-2927/03/$ - see front matter Published by Elsevier Science Ltd. PII: S0883-2927(02)00100-2 Applied Geochemistry 18 (2003) 471–482 www.elsevier.com/locate/apgeochem * Corresponding author. E-mail address: leenheer@usgs.gov (J.A. Leenheer).