Biochar from Grapevine Pruning Residues as an Efficient Adsorbent of Polyphenolic Compounds

Abstract

Agricultural waste, which is produced in large quantities annually, can be a threat to the environment. Biochar (BC) production represents a potential solution for reducing the amount of grapevine pruning residues and, accordingly, the impact on the environment and climate change. Biochar produced by the process of pyrolysis from grapevine pruning residues was investigated and characterized to be applied as an adsorbent of polyphenolic compounds with the aim of using the waste from viticultural production to obtain a quality product with adsorption and recovery potential. Standards of caffeic acid (CA), gallic acid (GA), and oleuropein (OLP) were used as polyphenolic representatives. The obtained data were fitted with the Langmuir and Freundlich isotherms models to describe the adsorption process. The best K_L (0.39) and R² (0.9934) were found for OLP using the Langmuir model. Furthermore, the adsorption dynamics and recovery potential of BC were investigated using an adapted BC column and performed on an HPLC instrument. The adsorption dynamics of biochar resulted in the adsorption of 5.73 mg CA g^-1 of BC, 3.90 mg GA g^-1 of BC, and 3.17 mg OLP g^-1 of BC in a 24 h contact. The online solid phase extraction of the compounds performed on an HPLC instrument yielded a recovery of 41.5 ± 1.71% for CA, 61.8 ± 1.16% for GA, and 91.4 ± 2.10% for OLP. The investigated biochar has shown a higher affinity for low-polar compound adsorption and, consequently, a higher polar compound recovery suggesting its potential as an efficient polyphenolic compound adsorbent.

Keywords: adsorption; biomass valorization; phytochemicals; polarity; pyrolysis.

Figure 1

Scanning electron microscope (SEM) pictures of GPRs and biochar obtained from GPRs: (a) SEM magnification 600×, visible GPRs particles size of 100 µm; (b) SEM magnification at 800×, visible BC particles size of 100 µm; (c) SEM magnification at 3000×, visible BC particles size of 30 µm; (d) SEM magnification at 3000×, visible BC pores size.

Figure 2

Fourier transform infrared spectra (FTIR) of GPRs biochar obtained by pyrolysis.

Figure 3

Langmuir isotherms of adsorption of CA, GA, and OLP by GPRs biochar (CA—caffeic acid, GA—gallic acid, OLP—oleuropein, R²—coefficient of determination, q_e—amount of adsorbate concentration in the solid phase at equilibrium (mg g⁻¹), γ_e—amount of adsorbate concentration in the liquid phase at equilibrium (mg L⁻¹)).

Figure 4

Freundlich isotherms of adsorption of CA, GA, and OLP by GPRs biochar (CA—caffeic acid, GA—gallic acid, OLP—oleuropein, R²—coefficient of determination, q_e—amount of adsorbate concentration in the solid phase at equilibrium (mg g⁻¹), γ_e—amount of adsorbate concentration in the liquid phase at equilibrium (mg L⁻¹)).

Figure 5

The influence of different amounts of biochar (BC) on different polyphenol adsorption: (a) different biochar dosages using 30 mg of caffeic acid L⁻¹; (b) different biochar dosages using 30 mg of gallic acid L⁻¹; (c) different biochar dosages using 30 mg of oleuropein L⁻¹.

Figure 6

Adsorption dynamics of polyphenolic compounds on GPRs biochar (BC) (a) caffeic acid adsorption on BC in 24 h; (b) gallic acid adsorption on BC in 24 h; (c) oleuropein adsorption on BC in 24 h.

Figure 6

Adsorption dynamics of polyphenolic compounds on GPRs biochar (BC) (a) caffeic acid adsorption on BC in 24 h; (b) gallic acid adsorption on BC in 24 h; (c) oleuropein adsorption on BC in 24 h.

Figure 7

Standards recovery from GPRs biochar column.