Transitional Adsorption and Partition of Nonpolar and Polar Aromatic Contaminants by Biochars of Pine Needles with Different Pyrolytic Temperatures

TLDR

The interaction between adsorption and partitioning in biochars with differing noncarbonized organic matter fractions remains poorly understood. The study produced pine‑needle biochars at 100–700 °C, characterized them by elemental, BET, and FTIR analyses, and compared their sorption isotherms for naphthalene, nitrobenzene, and m‑dinitrobenzene. Sorption parameters increase with biochar aromaticity, which rises with pyrolysis temperature, and the sorption mechanism shifts from partition‑dominated at low temperatures to adsorption‑dominated at high temperatures; partitioning originates from amorphous aliphatic fractions at low temperatures and from condensed aromatic cores at intermediate temperatures, while adsorption changes from polarity‑selective to porosity‑selective, becoming non‑selective at the highest temperatures where capacities match monolayer predictions.

Abstract

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 °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.