Sorption of ionizable organic compounds to a range of biochars

Biochar is a product of biomass pyrolysis considered as an environmentally friendly carbon sink as well as a valuable soil additive. Production conditions (e.g. feedstock, temperature) influence biochar characteristics (e.g. specific surface area, ash content, polarity), which can lead to a high sorption potential for a variety of inorganic and organic compounds. The use of biochar has therefore been proposed as a possible remediation strategy for both inorganic and organic contaminants in sediment, soil and water. While sorption of metals and hydrophobic organic compounds to biochar has received great attention, little data are available for ionizable organic compounds. Many pollutants including pesticides and pharmaceuticals, as well as hormones and infochemicals are ionizable organic compounds. Due to the influence of pH on the protonation of these compounds, conventional approaches to predict their sorption are not always suitable.To gain a better understanding of sorption behavior and support the development of more reliable prediction approaches, we performed sorption batch experiments with a series of biochars and a series of organic acids with similar structure and covering a range of dissociation constant: 2,4-D (pKa = 2.8), MCPA (pKa = 3.7), 2,4-DB (pKa = 4.1) and triclosan (pKa = 8.1). Despite the sorbates structural similarity, sorption affinity covered a range of up to 3 orders of magnitude and generally followed the order 2,4-D < MCPA < 2,4-DB < triclosan. Sorption of 2,4-DB and triclosan decreased with increasing pH (p < 0.05). Differences in sorption behavior are discussed mechanistically, based on differences in the molecular structure of the sorbates, the surface properties of the series of biochars studied, and aspects specific to the sorption behaviour of ionizable compounds.