Polar and aliphatic domains regulate sorption of phthalic acid esters (PAEs) to biochars

Molecular variations among different biochar categories translate into differences in their ability to function as sorbents to three phthalic acid esters (PAEs) representing a gradient in hydrophobicity. The sorption capacity (KOC) for all three PAEs was the greatest for amorphous biochars (heat treatment temperature HTT = 400 °C), followed by biochars produced at 300 °C, and was best explained by the hydrophobicity of the sorbate. Greater alkyl C content and higher polarity of grass chars versus wood chars prepared at similar temperatures explained both (a) the difference in sorbent strength between feedstocks and (b) the maximum in sorbent strength at relatively low HTTs (300–400 °C). Hydrophobic partitioning into ‘soft’ alkyl carbon and specific H-bonding involving char-bound O and N groups jointly account for high affinities of PAEs for low-HTT biochars. The results highlight the influence of feedstocks and HTTs on PAEs sorption strength and mechanism.