Birnessite-Induced Binding of Phenolic Monomers to Soil Humic Substances and Nature of the Bound Residues Chengliang Li, †,‡ Bin Zhang, § Tanya Ertunc, †,▽ Andreas Schaeffer, † and Rong Ji* ,†,⊥,⊗ † Biology 5, Environmental Biology and Chemodynamics, RWTH Aachen University, D-52056 Aachen, Germany ‡ College of Resources and Environment, Shandong Agricultural University, Daizong Road 61, 271018 Tai’An, China § Key Laboratory of Crop Nutrition and Fertilization, Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Zhongguancun 12 South Main Street, 100081 Beijing, China ⊥ State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, 163 Xianlin Avenue, 210046 Nanjing, China ⊗ Institute for Climate and Global Change Research, Nanjing University, 22 Hankou Road, 210093 Nanjing, China ABSTRACT: The nature of the abiotic birnessite (δ-MnO 2 )-catalyzed trans- formation products of phenolic compounds in the presence of soil organic matter is crucial for understanding the fate and stability of ubiquitous phenolic carbon in the environment. 14 C-radioactive and 13 C-stable-isotope tracers were used to study the mineralization and transformation by δ-MnO 2 of two typical humus and lignin phenolic monomers―catechol and p-coumaric acid―in the presence and absence of agricultural and forest soil humic acids (HAs) at pH 5-8. Mineralization decreased with increasing solution pH, and catechol was markedly more mineralized than p-coumaric acid. In the presence of HAs, the mineralization was strongly reduced, and considerable amounts of phenolic residues were bound to the HAs, independent of the solution pH. The HA-bound residues were homogeneously distributed within the humic molecules, and most still contained the unchanged aromatic ring as revealed by 13 C NMR analysis, indicating that the residues were probably bound via ester or ether bonds. The study provides important information on δ-MnO 2 stimulation of phenolic carbon binding to humic substances and the molecular distribution and chemical structure of the bound residues, which is essential for understanding the environmental fates of both naturally occurring and anthropogenic phenolic compounds. ■ INTRODUCTION Monomeric phenolic compounds are ubiquitous in the environment, 1,2 accounting for up to 10% of the dissolved organic carbon in soil. 3 Phenolic compounds of natural origins are derived from microbial degradation of biomass (especially plant residues), microbial synthesis, and plant root exudation. 2 Moreover, wastewater irrigation and solid-compost application of industrial waste also introduce phenolic compounds (including chlorinated and nitrified phenols) into the environ- ment, as phenolic compounds are widely used as intermediates in production processes. 4,5 Phenolic compounds are subjected to biotic and abiotic humification processes in the environment and are therefore regarded as important precursors of both soil and aquatic humic substances. 2,6-9 Moreover, phenolic compounds are antioxidants, which may be key to the stability of organic matter in the soil environment. 10-12 In the transformation of phenolic compounds in the environment, extracellular phenol oxidase and soil minerals (e.g., birnessite) play a catalytic role. 13-16 Phenol oxidase and birnessite (δ-MnO 2 ) are very common in the environ- ment, 16-18 and their contributions to the abiotic and enzyme- mediated formation of humic substances in soil have been extensively discussed in the literature. 7,13,16,19,20 It has been shown that phenol oxidase can initiate oxidative binding of both natural phenolic compounds and phenolic xenobiotics (e.g., chlorophenol) to humic substances, via formation of free phenoxyl radicals (e.g., in the case of laccase and peroxidase) or quinones (e.g., in the case of tyrosinase) that undergo coupling reactions with the existing humic substances, 16,21 even though it was believed that the participation of phenol oxidases in the formation of humic substances would be of minor significance in the soil. 16 Owing to its high oxidation potential (E 0 = 1.2 V), 22 δ-MnO 2 can catalytically oxidize both phenolic and nonphenolic pollutants. 7,23-34 Most of the studies on oxidative trans- formation of both naturally occurring and anthropogenic phenolic compounds by δ-MnO 2 have been conducted in systems without humic substances (see, e.g., refs 9, 15, 20, 23, 26-30, 32, 33, and 35), even though humic substances are the major component of organic matter in the environment. Both polymerization and strong mineralization of phenolic com- Received: May 15, 2012 Revised: July 25, 2012 Accepted: July 27, 2012 Article pubs.acs.org/est © XXXX American Chemical Society A dx.doi.org/10.1021/es3018732 | Environ. Sci. Technol. XXXX, XXX, XXX-XXX