Green Extraction of Polyphenols from Waste Bentonite to Produce Functional Antioxidant Excipients for Cosmetic and Pharmaceutical Purposes: A Waste-to-Market Approach

Abstract

In an ever-growing perspective of circular economy, the development of conscious, sustainable and environmental-friendly strategies to recycle the waste products is the key point. The scope of this work was to validate the waste bentonite from the grape processing industries as a precious matrix to extract polyphenols by applying a waste-to-market approach aimed at producing novel functional excipients. The waste bentonite was recovered after the fining process and opportunely pre-treated. Subsequently, both the freeze dried and the so-called "wet" bentonites were subjected to maceration. PEG200 and Propylene Glycol were selected as solvents due to their ability to dissolve polyphenols and their wide use in the cosmetic/pharmaceutical field. The extracts were evaluated in terms of yield, density, pH after water-dilution, total phenolic (Folin-Ciocalteu) and protein (Bradford) contents, antioxidant power (DPPH), amount of some representative polyphenols (HPLC-DAD), cytocompatibility and stability. Both solvents validated the bentonite as a valuable source of polyphenols and led to colored fluids characterized by an acidic pH after water-dilution. The best extract was obtained from the wet bentonite with PEG200 and highlighted the highest phenolic content and consequently the strongest antioxidant activity. Additionally, it displayed proliferative properties and resulted almost stable over time. Hence, it might be directly used as polyphenols-enriched functional novel raw material for cosmetic and pharmaceutics purposes.

Keywords: PEG200; bentonite; cosmeceutical excipients; extraction; grape processing industry; pharmaceutical excipients; polyphenols; propylene glycol; waste product; waste-to-market approach.

Figure 1

(A) Residual DPPH (%) as a function of incubation time when evaluating the GW (black—solid symbol), GD (black—open symbol), PW (red—solid symbol) and PD (red—open symbol) extracts. (B) Data in the semi-logarithmic scale and linear curve fitting (linearity range: 20–60 min) (n = 9).

Figure 2

Concentration (μg mL⁻¹) ± SE of: Resveratrol (RSV—black), Gallic Acid (GA—red) and Quercetin (QRC—blue) detected in each extract by HPLC-DAD analyses (n = 9).

Figure 3

Chromatograms at (A) 271 nm; (B) 305 nm and (C) 370 nm of the PW extract (as a representative sample) employed to evaluate the amount of gallic acid, resveratrol and quercetin respectively. The recorded UV-Vis spectra of the quantified polyphenols are also reported.

Figure 4

Folin–Ciocalteu data (blue) and Bradford assay results (orange) for all the extracts reported in terms of equivalent mg ± SE of gallic acid and bovine serum albumin used respectively as standard molecules per 1 g of extract (n = 9).

Figure 5

Cell viability assay on BALB/3-T3 cells. Cells were treated for 24 h in the absence (control) or the presence of (A) PEG200 and PEG-based extracts, (B) Propylene Glycol and Glycol-based extracts at different concentrations (2.5–20 µL/mL). Values are means ±SD of three separate experiments conducted in triplicate. With respect to control, * p < 0.05; ** p < 0.01; *** p < 0.001 (ANOVA associated with Tukey’s test).

Figure 6

Stability evaluation by comparation of the quantitative analyses conducted on the freshly prepared extract and 6-months-old extracts: (A) HPLC-DAD data reported as concentration (μg mL⁻¹) ± SE of RSV, QRC and GA; (B) Folin–Ciocalteu data reported as equivalent mg ± SE of gallic acid per 1 g of extract; (C) Bradford assay results reported as equivalent mg ± SE of BSA per 1 g of extract (n = 6).