Sorption of monoaromatic compounds to heated and unheated coals, humic acid, and biochar: Implication for using combustion method to quantify sorption contribution of carbonaceous geosorbents in soil

Batch sorption isotherms of two nonpolar compounds (1,3-dichlorobenzene and 1,3,5-trichlorobenzene) and two polar compounds (1,3-dinitrobenzene and 1,3,5-trinitrobenzene) to heated (at 375 °C for 24 h) and unheated coals (lignite and anthracite) were compared with those to a soil humic acid and a maize stalk derived biochar. For all test compounds, unheated lignite and anthracite exhibited much stronger sorption than humic substances (the organic carbon-normalized distribution coefficient was up to 2-3 orders of magnitude larger), but lower sorption than biochar. This sorption trend is consistent with the degree of sorbent condensation (biochar > coal > humic acid). The results indicate that sorption of the test sorbates (regardless of the difference in polarity) to soils would be dominated by carbonaceous geosorbents. Notably, the organic carbon contents of the coals were pronouncedly lowered by the heat treatment, from 47.4% to 7.3% for lignite, and from 80.1% to 58.1% for anthracite. Moreover, the heat treatment markedly decreased organic carbon-normalized distribution coefficient to coals (up to one order of magnitude), attributable to the decreased hydrophobicity of sorbents due to increased O-containing groups from oxidation. An important implication is that heat treatment, which is commonly used to quantify the content of carbonaceous geosorbents in soil and sediment, may cause significant underestimation of sorption contribution of carbonaceous geosorbents due to the combined effect of reduced organic carbon content and decreased hydrophobicity of less graphitized carbonaceous geosorbents (coals). This was illustrated using a widely adopted dual-component model that combines linear partition to humic substances (represented by humic acid) and nonlinear adsorption on condensed geosorbents (represented by biochar and coal).