Experimental methods Gas chromatographic headspace measurements

The modification of HF and HS was performed according to Sajab et al. [37]. HF and HS were mercerized by soaking in 5 wt.% aqueous NaOH solution and stirring at room temperature for 1 h. The concentration in the mixture of both HF and HS was fixed to be 50 g/L. For the preparation of treated adsorbents (HFT and HST), the materials were rinsed with deionized water until the pH of the filtrate reached 7 and then they were dried under vacuum at 60 °C. Citric VX689 modification was conducted by soaking HF and HS in an aqueous solution of citric acid (12.5 wt.%) followed by stirring at room temperature for 30 min. The concentration of the adsorbents in the citric acid solution was fixed at 100 g/L. The liquid of the mixture was removed and the adsorbents were first dried under vacuum at 60 °C and then cross-linked at 120 °C for 90 min, while the excess of citric acid was rinsed out with warm deionized water. Finally, the treated adsorbents were dried under vacuum at 60 °C. The preparation procedure is schematically presented in Fig. 1.