*2.7. Static Adsorption/Desorption Evaluation*

To define the adsorption capacity of the resins for polyphenolic compounds, five amounts of activated macroporous resin (1, 2, 3, 4, and 5 g) were placed in 100 mL flasks and 50 mL of hazelnut skin extract was added. The sealed flasks were shaken at 22 ◦C for 24 h in the dark using a VDRL 711 orbital shaker under constant rotatory agitation at 120 rpm. TPC was evaluated before and after adsorption, and the adsorption capacity and adsorption ratio were calculated using the following equations:

> Ce) × Vi)/M

qa = ((C0 −

Adsorption capacity:

Adsorption ratio:

$$\text{A } \text{(\%)}=\text{((C}\_0-\text{C}\_e)/\text{C}\_0)\times 100\tag{4}$$

 (3)

where qa is the adsorption capacity (mg/g dry resin), C0 is the TPC value of the extract before the adsorption phase (mg GAE/mL), Ce is the TPC value of the extract after the adsorption phase (mg GAE/mL), Vi is the volume of the extract (mL), and M is the weight of the resin (g).

After adsorption, the resins were placed in a 100 mL flask and treated with 50 mL of three ethanol solutions (50%, 70%, and 99.9% *v*/*v*). The sealed flasks were shaken at 22 ◦C for 24 h in the dark using a VDRL 711 orbital shaker under constant rotatory agitation at 120 rpm. The desorption ratio (D%) was calculated using the following equation:

$$\rm D\ (\%) = (C\_{\rm d} \times V\_{\rm d}) / ((C\_0 - C\_{\rm e}) \times V\_{\rm i}) \times 100\tag{5}$$

where Cd is the TPC value of the ethanol solution after desorption (mg GAE/mL), Vd is the volume of the ethanol solution used for the desorption phase (mL), Vi is the volume of the extract (mL), C0 is the TPC value of the extract before the adsorption phase (mg GAE/mL), and Ce is the TPC value of the extract after the adsorption phase (mg GAE/mL).

To evaluate the adsorption kinetics, 1 g of each resin was mixed in a flask containing 50 mL of extract with a TPC value of 5 mg GAE/mL and shaken at 22 ◦C in the dark on a VDRL 711 orbital shaker under constant rotatory agitation at 120 rpm. The TPC of the solution was analyzed every 15 min for the first 2 h and then every 30 min for 6 h. To evaluate the adsorption kinetics, the obtained results were fitted using two widely used kinetic models, the pseudo-first-order [6] and pseudo-second-order models [7]:

$$\text{Ln} \left( \mathbf{q}\_{\text{f}} - \mathbf{q}\_{\text{f}} \right) = -\mathbf{k}\_{\text{l}} \text{ t} + \text{ln } \mathbf{q}\_{\text{e}} \tag{6}$$

$$\mathbf{t}/\mathbf{q}\_{\mathbf{t}} = \mathbf{1}/(\mathbf{k}\_2 \ \mathbf{q}^2 \ \mathbf{e}) + \mathbf{t}/\mathbf{q}\_{\mathbf{e}} \tag{7}$$

where qe (mg/g) and qt (mg/g) are the adsorption capacities at equilibrium and at time t (min), respectively, and k1 (min−1) and k2 (g/mg min) are the rate constants of the pseudo-first and pseudo-second order models, respectively [15].

The fit of each model to the experimental data was estimated using the linear regression correlation coefficient (R2).
