The combined adsorption and partition effects of biochars with varying fractions of noncarbonized organic matter have not been clearly defined. Biochars, produced by pyrolysis of pine needles at different temperatures (100-700 degrees C, referred as P100-P700), were characterized by elemental analysis, BET-N2 surface areas and FTIR. Sorption isotherms of naphthalene, nitrobenzene, and m-dinitrobenzene from water to the biochars were compared. Sorption parameters (N and logKf) are linearly related to sorbent aromaticities, which increase with the pyrolytic temperature. Sorption mechanisms of biochars are evolved from partitioning-dominant at low pyrolytic temperatures to adsorption-dominant at higher pyrolytic temperatures. The quantitative contributions of adsorption and partition are determined by the relative carbonized and noncarbonized fractions and their surface and bulk properties. The partition of P100-P300 biochars originates from an amorphous aliphatic fraction, which is enhanced with a reduction of the substrate polarity; for P400-P600, the partition occurs with a condensed aromatic core that diminishes with a further reduction of the polarity. Simultaneously, the adsorption component exhibits a transition from a polarity-selective (P200-P400) to a porosity-selective (P500-P600) process, and displays no selectivity with P700 and AC in which the adsorptive saturation capacities are comparable to predicted values based on the monolayer surface coverage of molecule.
The bisolute sorption and thermodynamic behavior of organic pollutants on low temperature biochars (LTB) at 300 °C and high temperature biochars (HTB) at 700 °C were determined to elucidate sorptive properties of biochar changed with pyrolytic temperatures. The structural characteristics and isotherms shape of the biochar were more dependent on the pyrolytic temperature than on the biomass feedstocks, which included orange peel, pine needle, and sugar cane bagasse. For LTB, the thermally altered organic matter colocalized with the carbonized matter, and the visible fine pores of the fixed carbons were plugged by the remaining volatile carbon. For HTB, most of the volatile matter was gone and the fixed matter was composed of fully carbonized adsorptive sites. Monolayer adsorption of 1-naphthol to HTB was dominant but was suppressed by phenol. In comparison, LTB displayed exceptional sorption behavior where partition and adsorption were concurrently promoted by a cosolute and elevated temperature. In addition to monolayer surface coverage, pore-filling mechanisms may contribute to the increase of adsorption fraction. Moreover, the entropy gain was a dominant force driving the partition and adsorption processes in LTB. Thus, the colocalizing partition phase and adsorptive sites in LTB are proposed to be in interencased states rather than in physical separation.
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