1 Generalized Two-Dimensional Perturbation Correlation Infrared 2 Spectroscopy Reveals Mechanisms for the Development of Surface 3 Charge and Recalcitrance in Plant-Derived Biochars 4 Omar R. Harvey,* ,†,‡ Bruce E. Herbert, § Li-Jung Kuo, ∥ and Patrick Louchouarn ⊥ 5 † Water Management and Hydrologic Sciences, Texas A & M University, College Station, Texas 77843, United States 6 ‡ Department of Geography and Geology, The University of Southern Mississippi, Hattiesburg, Mississippi 39406, United States 7 § Geology and Geophysics, Texas A & M University, College Station, Texas 77843, United States 8 ∥ Marine Science Laboratory, Pacific Northwest National Laboratory, Sequim, Washington 98382, United States 9 ⊥ Department of Marine Science, Texas A&M University at Galveston, Galveston, Texas 77553, United States 10 * S Supporting Information 11 ABSTRACT: Fundamental knowledge of how biochars develop surface-charge and 12 resistance to environmental degradation is crucial to their production for customized 13 applications or understanding their functions in the environment. Two-dimensional 14 perturbation-based correlation infrared spectroscopy (2D-PCIS) was used to study the 15 biochar formation process in three taxonomically different plant biomass, under oxygen- 16 limited conditions along a heat-treatment-temperature gradient (HTT; 200−650 °C). 17 Results from 2D-PCIS pointed to the systematic, HTT-induced defragmenting of 18 lignocellulose H-bonding network and demethylenation/demethylation, oxidation, or 19 dehydroxylation/dehydrogenation of lignocellulose fragments as the primary reactions 20 controlling biochar properties along the HTT gradient. The cleavage of OH ... O-type H- 21 bonds, oxidation of free primary hydroxyls to carboxyls (carboxylation; HTT ≤ 500 °C), 22 and their subsequent dehydrogenation/dehydroxylation (HTT > 500 °C) controlled 23 surface charge on the biochars; while the dehydrogenation of methylene groups, which 24 yielded increasingly condensed structures (R−CH 2 −R →RCH−R →RCR), controlled biochar recalcitrance. Variations 25 in biochar properties across plant biomass type were attributable to taxa-specific transformations. For example, apparent 26 inefficiencies in the cleavage of wood-specific H-bonds, and their subsequent oxidation to carboxyls, lead to lower surface charge 27 in wood biochars (compared to grass biochars). Both nontaxa and taxa-specific transformations highlighted by 2D-PCIS could 28 have significant implications for biochar functioning in fire-impacted or biochar-amended systems. 29 ■ INTRODUCTION 30 Char/charcoal black carbon produced during natural pyrogenic 31 events (e.g., vegetation fires) or under simulated conditions 32 (hereon referred to as biochars) is receiving significant 33 attention due to increased recognition of their dynamic role 34 in the biogeochemical cycling of carbon, contaminants, and 35 nutrients. 1−5 The dynamic nature of biochars is largely 36 attributable to physical and chemical heterogeneity stemming 37 from di fferences in feedstock chemistry and pyrolysis 38 conditions. 4−7 The development of structure−reactivity 39 relationships and a fundamental understanding of how biochars 40 develop their functionalities are therefore crucial to 1) 41 predicting/explaining the behavior of biochars in fire-impacted 42 or biochar-amended systems and 2) designing and selecting 43 optimal biochars for specific environmental applications. 44 Significant progress has been made in elucidating pertinent 45 structure−reactivity relationships. 4,8−10 However, a fundamen- 46 tal understanding of the mechanisms controlling the develop- 47 ment of biochar functionalities and subsequent structure− 48 reactivity relationships is still lacking. Functional group 49 chemistry of biochars is commonly studied using Fourier 50 transform infrared (FTIR) spectroscopy. However, significant 51 peak overlaps in key areas of the biochar infrared spectra and 52 subsequent inability to effectively decipher perturbation- 53 induced (e.g., heat-treatment-temperature or HTT) changes 54 in specific bonds limits the suitability of conventional FTIR 55 analysis for mechanistic assessments. One alternative to 56 conventional FTIR analysis, for mechanistic assessments, is 57 generalized two-dimensional perturbation correlation infrared 58 spectroscopy (2D-PCIS). 11,12 In addition to providing a better 59 resolution of significant peaks, 2D-PCIS allows for the 60 elucidation of simultaneously- and sequentially occurring 61 processes. 12 62 Despite the widespread use of 2D-PCIS in mechanistic 63 studies on colloids and polymeric materials, 13−15 no previous Received: July 24, 2012 Revised: September 2, 2012 Accepted: September 5, 2012 Article pubs.acs.org/est © XXXX American Chemical Society A dx.doi.org/10.1021/es302971d | Environ. Sci. Technol. XXXX, XXX, XXX−XXX sls00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.3.i2:3867 | 2.0 alpha 39) 2012/08/14 13:46:13 | PROD-JCA1 | rq_549576 | 9/13/2012 16:04:30 | 10