Benzene polycarboxylic acid — A useful marker for condensed organic matter, but not for only pyrogenic black carbon

Highlights: • Sequential removal of soil fractions leaves 2.4–10.1% BPCAs in black carbon. • Soils of different depth show similar distribution patterns of BPCAs. • BC-like structure of organic matter may be referred to BPCA-probed OM content. • BPCA biomarkers are useful fingerprints to probe OM properties and sources. Benzene polycarboxylic acid (BPCA) molecules are a widely used marker method for the qualitative and quantitative analysis of pyrogenic black carbons (BC). Based on an overview of the development and chemical reaction mechanism of the BPCA method, we propose that the commonly used BPCA markers may not be solely indicative of BC but more generally of condensed organic matter in soils and aquatic systems. First, we sequentially removed the soil fractions and observed that the BPCA contents were abundant in humic acids (HAs). After sequential treatment, the residual particles were supposed to contain BC and minerals; however, the BPCAs in the residue accounted for only 2.4–10.1% of that detected in the entire soil. In addition, substantial quantities of BPCAs were detected in both thermally treated samples and composted biomass. Furthermore, humic acids extracted from all the samples showed that obvious BPCA contents in the samples accounted for 0.1–121.7 mg/g. Therefore, soil fractionation may also partly extract BCs as suggested by BPCAs in the HAs of the biochars. However, organic matter without any thermal treatment may contain BPCAs. A series of standard substances without any BC showed high BPCA content in the samples from 5.9–124.5 mg/g. These observations create a serious concern for the proper application of BPCAs as a marker for BCs. Combining a systematic literature review of BPCA that deviates from BC content, we suggest that the BC-like structure of organic matter may be referred to as BPCA-probed organic matter content, which could be a more useful term for studies on the multimedia environmental behaviors of contaminants.