Sorption of aromatic organophosphate flame retardants on thermally and hydrothermally produced biochars

Highlights

• TPhP showed faster and higher sorption on biochars than TPPO.

• Pyrochars had higher sorption capacity for TPPO than hydrochar.

• Hydrophobic interactions dominated TPhP sorption by biochars.

• The π-π EDA and electrostatic interactions are involved in sorption.

Abstract

Aromatic organophosphate flame retardant (OPFR) pollutants and biochars are commonly present and continually released into soils due to their increasingly wide applications. In this study, for the first time, the sorption of OPFRs on biochars was investigated. Although triphenyl phosphate (TPhP) and triphenylphosphine oxide (TPPO) have similar molecular structures and sizes, TPhP exhibited much faster and higher sorption than TPPO due to its stronger hydrophobicity, suggesting the dominant role of hydrophobic interactions in TPhP sorption. The π-π electron donor–acceptor (EDA) interactions also contributed to the sorption process, as suggested by the negative correlation between the sorption capacity of the aromatic OPFRs and the aromatic index (H/C atomic ratios) of biochar. Density functional theory calculations further showed that one benzene ring of aromatic OPFRs has no electrons, which may interact with biochar via π-π EDA interactions. The electrostatic attraction between the protonated P = O in OPFRs and the negatively charged biochar was found to occur at pH below 7. This work provides insights into the sorption behaviors and mechanisms of aromatic OPFRs by biochars.

Graphical abstract

Keywords

Organophosphate flame retardants / Hydrochar / Pyrochar / Adsorption / Emerging contaminants / Biochar

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Acknowledgements

This research was supported by the Beijing Natural Science Foundation (No. 8182037), National Natural Science Foundation of China (Grant Nos. 41703097 and 41977317), Fundamental Research Funds for the Central Universities (No. 2018ZY16), and National College Students’ innovation and entrepreneurship training program (No. 201810022077).

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11783-020-1220-6 and is accessible for authorized users.

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