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Application of biochar-based fertilizers is increasingly being considered for its potential agronomic and environmental benefits. However, biochar may contain residues of polycyclic aromatic hydrocarbons (PAHs) as a result of its production by pyrolysis. The strong adsorption of PAHs to biochar makes extraction and analysis of biochar-based fertilizers difficult. This study optimizes the extraction of PAHs in biochar-based fertilizer samples by using an ultrasonic bath for quantification by gas chromatography-mass spectrometry. Among 12 solvents, acetone-cyclohexane (1:1) mixture was selected as the optimum solvent for extraction. Three variables affecting the extraction were studied by Box-Behnken design. The optimum conditions were 57 °C extraction temperature, 81 min extraction time, and two extraction cycles, which were validated by assessing the linearity of analysis, LOD, LOQ, recovery, and levels of PAHs in real biochar-based fertilizer samples. Results revealed that the 16 U.S. EPA PAHs had good linearity, with squared correlation coefficients greater than 0.99. LODs were low, ranging from 2.2 ng g(-1) (acenaphthene) to 23.55 ng g(-1) (indeno[1,2,3-cd]perylene), and LOQs varied from 7.51 ng g(-1) to 78.49 ng g(-1). The recoveries of 16 individual PAHs from the three biochar-based fertilizer samples were 81.8-109.4 %. Graphical Abstract Use of RSM to optimize UAE for extraction of the PAHs in biochar-based fertilizer.

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A method for improving the accuracy in X‐ray fluorescence spectroscopy analysis with pressed pellets was established through matching the calibration standards by the Kβ/Kα ratios. The Kβ/Kα ratios of four major elements (Al, Si, Ca, and Fe) were measured in 14 certified reference materials (CRMs) for geochemistry. The deviation of the biggest Kβ/Kα values relative to the smallest ones for the same element was from 1.52% (Si) to 20.98% (Al) when pressed pellets were used. The relative deviation of Kβ/Kα values for Ca was as high as 3.56% even for two soil CRMs (GBW07404 and GBW07408). X‐ray diffraction analysis confirmed that the Ca elements in the two CRMs were in distinct phases, thereby leading to the different Kβ/Kα values. The Fundamental Parameter Method (FPM) results of two calibration strategies were compared. When one of the 14 CRMs was selected as unknown sample whereas the other 13 CRMs were used for calibration to determine Al2O3, SiO2, CaO, and Fe2O3, the relative errors were 10.80%, 8.77%, 8.33%, and 10.91%, respectively. Another strategy, where only one CRM was used for calibration of samples with large errors, was adopted by matching Kβ/Kα values. The results of calibrating using CRM with similar Kβ/Kα values were more accurate than those with different Kβ/Kα values.