# Removal of Volatile Phenols From Wine Using Crosslinked Cyclodextrin Polymers
## 2 Results and Discussion
### Determination of CD Polymer Adsorption Capacity in Model Wine  
The recently developed four-phase HS-SPME GC-MS method showed excellent repeatability for analysis of volatile phenols in model wine at concentrations ranging from 0.5 to 1.0 mg/L (using calibration curves spanning 0 to 2.0 mg/L, at 0.25 mg/L internals) [35]. To evaluate the adsorption capacity of CD polymers, 2% _uv/v_ of \(\beta\)-CD-HDI or \(\gamma\)-CD-HDI polymers were added to model wine samples containing sequential concentrations of volatile phenols (being the same concentrations used for the development of calibration curves). The RPA of treated samples represents the concentration of free volatile phenols, with the corresponding concentrations (\(\mathrm{C_{e}}\)) being calculated from calibration curves. The difference between \(\mathrm{C_{0}}\) and \(\mathrm{C_{e}}\) was considered to be due to volatile phenol adsorption by the polymer, and was used to calculate the adsorption capacity \(\mathrm{q_{e}}\). Plots of \(\mathrm{q_{e}}\) against the concentration of volatile phenols remaining once the adsorption equilibrium was achieved, are shown in Figure 3.  
Figure 1: Preparation of \(\beta\)-cyclodextrinrin (CD)-hexamethylene diisocyanate (HDI) polymer and its hypothetical structure. Figure 3: The adsorption capacity of (**a**) guaiacol, (**b**) 4-methylguaiacol, (**c**) 4-ethylguaiacol and (**d**) 4-ethylphenol by (\(\bullet\)) \(\beta\)-CD-HDI and (\(\Circle\)) \(\gamma\)-CD-HDI polymers in model wine at pH 3.5 and 25 \({}^{\circ}\)C. Values are means of three replicates \(\pm\) standard error (but some standard errors are obscured by symbols). Figure 2: Equilibrium time for binding of volatile phenols by (**a**) \(\beta\)-CD-HDI and (**b**) \(\gamma\)-CD-HDI polymers, expressed as relative peak area. Values are means of three replicates \(\pm\) standard error (but some standard errors are obscured by symbols). Values were not significantly different (one-way ANOVA, \(p=0.05\)). Overall, the adsorption capacity of the \(\beta\)-CD-HDI and \(\gamma\)-CD-HDI polymers was similar, with the \(\gamma\)-CD-HDI polymer showing greater affinity towards guaiacol and 4-methylguaiacol, and the \(\beta\)-CD-HDI polymer showing a higher affinity for 4-ethylguaiacol and 4-ethylphenol. This difference in binding preference by \(\beta\)-CD and \(\gamma\)-CD was consistent with results from other experiments performed in the current study (outlined below). Among the four volatile phenols tested, it was obvious that 4-ethylphenol was by far the most readily removed compound. The adsorption capacity of the CD polymers for each of the volatile phenols (at equilibrium) was around 20 ug/g, and the adsorption capacity increased with increasing volatile phenol concentration. The magnitude of this adsorption is quite low compared with other studies [31; 33; 34; 37]. This was attributed to the range of concentrations used in the current study, which were very low by comparison. Other studies have demonstrated that CD polymers, including \(\beta\)-CD-HDI, could achieve much greater adsorption at higher concentrations of the adsorbate. For example, the citric acid crosslinked \(\beta\)-CD polymer used by [PERSON] and colleagues exhibited a 13.8 mg/g adsorption capacity for phenol, with an equilibrium concentration of around 400 mg/L and a capacity of 3.8 mg/g at an equilibrium concentration of 85.4 mg/L [31]. The epichlorohydrin and chitosan crosslinked \(\beta\)-CD polymers used by [PERSON] and co-workers, showed similar adsorption capacities for phenol, \(p\)-nitrophenol and \(p\)-chlorophenol at equilibrium concentrations above 200 mg/L [34]; while [PERSON] evaluated the \(\beta\)-CD-HDI polymer at higher levels of phenol and reported adsorption capacity of around 15 (molphenol/mol\({}_{\text{CD}}\)) at an equilibrium concentration of 0.94 g/L [33].