bark proanthocyanidins with potential for water treatment: Chemical characterization and zinc/bisphenol A removal

This study investigates the chemical structure of proanthocyanidin-rich crude extracts from willow bark and these materials were tested initially as adsorbents for artificial (waste)water treatment. The crude extracts were obtained through mild water extraction and the colorant fractions were further chromatographically fractionated to understand the chemical structure of the willow bark proanthocyanidins. The chemistry of crude extracts and purified fractions were investigated using nuclear magnetic resonance (NMR) and ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Both NMR (liquid and solid-state) and UPLC-MS/ MS suggest that the crude extracts constitute of interflavan linked flavan-3-ols, i


Introduction
The first step in using roundwood trees to produce pulp and timber is to remove the bark, which has been a long tradition in the forest industry for decades.Extensive research has focused on the valorizations of the wood components such as cellulose and lignin, but almost exclusively in those of the matrix of wood.[1] However, wood bark with estimated annual turnover of 359 million m 3 world-wide [2] accounts for about 10-15 % of the volume of a typical wood log material turnover.Often the bark is only used for energy production for the pulp mill or sawmill and thus remains by far the largest underutilized biomass resource on the earth.Unlike the main polymeric components in matrix wood, i.e. cellulose, hemicellulose, and lignin, wood bark mainly consists of tannin-rich extractives, pectin, suberin, and starch.This makes valorization of the wood bark challenging as the bark has also a rather heterogeneous composition morphologically.Because of this complexity, attention is currently paid on solvent-extractable fractions of the bark [3,4].Furthermore, the extractable fractions do not represent the main bulk mass of the bark and thus do not significantly influence the overall energy value of the bark and can be removed without too negatively affecting the energy balance of the mill.
Tannins are complex polyphenolic biomolecules constituted of three major groups: hydrolysable tannins (HTs), proanthocyanidins (PAs) and phlorotannins.[5,6] Of these, phlorotannins are mainly found in marine organisms such as brown algae, while HTs and PAs are widely present in trees and their bark as well.These two tannin groups have significant differences in both their structures and evolutionary history.Gymnosperms can only produce PAs, but no HTs, while many angiosperms can produce both.HTs are esters of a polyol and gallic acid residues with considerable modifications taking place via oxidative and oligomerization processes, reaching all the way to even 8624 Da sizes.PAs are oligomers and polymers of flavan-3-ol units, with as shown e.g. in procyanidins (PCs) and prodelphinidins (PDs) (Fig. 1c).In many cases, plant PAs are a combination of PC and PD units in same molecules, with hundreds of different oligomers and polymers typically present in the same plant species.[5,6] Particularly the tannin fractions from bark of softwood trees, e.g., spruce [7] and pine [8], have been studied.They are reported as adsorbents (or biocoagulant) for removal of metal ions from industrial wastewater due to their non-toxic natural polyphenolic nature with abundant number of adjacent hydroxyl groups.[9,10] There are also applications of utilizing bark tannin as a substitute to phenols for adhesive applications.[11] Short-rotation willow has been studied mainly as an energy resource in the last decades globally.Cultivated willow is a short-rotation woody crop that can germinate and develop effectively for example in discarded peatlands and other low-nutrient soils.Its growth requires only moist soil, and it is insensitive to soil quality (sandy or clay) and pH.[12] Willow trees are harvested in a cycle of two to four years, seeding cuttings are usually planted as 20 cm long cuttings at a density of 10,000 cuts/ha.In southern Sweden, productivity of willow has been reported up to > 30 ton/ha and the willow plantations were reported to be approximately 17,000 ha [13] in the whole of Sweden between 1991 and 1996.This means roughly 0.1 million tons of willow bark available each year.As the willow bark contains high value metabolites (e.g. for pharmaceutical or medical uses) [14,15] and functional fiber bundles (e. g. as antibacterial materials) [16], the value from the bark can be theoretically multi-fold in comparison with the value of wood if the wood and bark be utilized separately [17].Debarking of willow can also substantially enhance the efficiency of acetone-butanol-ethanol (ABE) fermentation from willow.[18] The long-underappreciated willow bark residue may be upgraded from an energy source into high-value fiber and proanthocyanidins-enriched biochemicals.
Increasing industrial activities and technological developments have led to the release of larger number of metals into the environment.Because of the toxic effects of the presence of heavy metals and their accumulation through the food chain, contamination of water environments is a worldwide environmental problem.Although small quantities of heavy metals are essential for the growth of living organisms, excessive levels may lead to damage and their accumulation over time may subsequently pose health threats also for humans.For example, acute toxicity of zinc may lead to throat dryness and cough weakness and may ultimately leads to human epidemics.[19] Many plastics and resin materials contain Bisphenol A (BPA) and it has also been detected in municipal wastewater.BPA enter the environment in a concentration that may pose a risk to human health [20,21].Traditionally, heavy metals and BPA can be reduced by chemical precipitation, ion exchange, or ultrafiltration ect.However, these methods are often either expensive or their manufacturing process is not sustainable.[22] Therefore, the search of more bio-based adsorbents for water treatment have drawn attention.
Preparative chromatography has been demonstrated effective for purifying both high molecular weight colorant fractions and small molecular weight fractions (e.g.picein and triandrin) from willow bark water extracts.[23] As the colorants elute separately from other small Mw aromatic compounds (Fig. S1, Table S1-S2) from the column, we were inspired to elucidate the chemical profile behind the purified colorant fraction.Separation of the conjugated tannin chromophores from aqueous extracts of willow bark could also reveal their full potential as a bioadsorbent for wastewater treatment.A lot of studies have been conducted on the crude extracts of willow bark, especially the small Mw components, [14,24,25] but there is not much scientific data on the chemistry of high Mw proanthocyanidins from willow bark.In this paper we utilize a combination of UPLC-MS/MS, NMR spectroscopy and size exclusion chromatography (SEC) to elucidate the chemical structure of the crude extracts and its purified fractions of polyphenol colorants, and to explore further this fraction of proanthocyanidins as a source for water treatment.The efficiency of water treatment was initially studied through adsorption experiments towards zinc and BPA.

Experimental flow 2.2.1. Chromatographic purification of the colored fractions
Peeled willow bark was first extracted with water at temperature of 80 • C within 20 mins according to the optimized condition published previously.[14] Then the non-soluble residues were filtered using the qualitative filter paper (12-15 µm, VWR) by vacuum filtration.The crude extracts were then finally freeze dried and stored in the amber glass bottles for further chemical characterization and adsorption experiments.Analysis of the colored fractions of the willow bark crude extracts are essential to provide knowledge for understanding the chemistry of proanthocyanidins.The main recovery steps (Fig. 2) were implemented with preparative-scale chromatography, [23] which separates the targeted compounds depending on their molecular size and hydrophobicity.PF0 (7 w/w, %) and PF1 were separated from crude extracts (100 w/w, %) by CA 10GC, PF 1-1 (3 w/w, %) and PF 1-2 (4 w/ w, %) were further purified from fraction PF1 by Sephadex G-10 as the stationary phase.The estimated mass balance of the purified fraction and fractionation cut points are shown in Fig. 1 and fully summarized in Fig. S1 and Table S1-S2.

Foam and activated carbon preparation
For the adsorption studies the samples were formulated to foams.Briefly, crude extracts were mixed with deionized water, furfural alcohol, and surfactant Tween 85 in a glass beaker, then the mixture was blended mechanically for 5-10 min.Pentane (the blowing agent) and PTSA (65% solution, acid catalyst) were then introduced to the homogenized mixture and blended for another 10 s.Foam formed immediately once the beaker was placed in an oven at 100 • C and hardened when kept at 100 • C for a further 24 h.Finally, the hardened foam was crushed and placed inside a section of stainless-steel pipe and then steam-activated in a tubular fixed-bed reactor.The temperature was increased from 20 • C to 800 • C using a 5 • C/min ramp.At the target temperature, steam activation was carried out by injecting water at velocity of 0.5 mL/min for 3 h to the reactor.Nitrogen was flushed into the reactor at velocity of 10 mL/min during the whole activation to avoid sample's oxidation.The morphological structure of the produced foam and activated foam were imaged using a Zeiss ULTRA plus field emission scanning electron microscopy.The yields after the activations for Klara hybrid was 38.7% and for Karin hybrid it was 36.7% compared to starting mass of crude foam.Detailed amounts of the components to obtain the crude foam were outlined in Table S3.

Specific surface area, pore volume, and pore size distribution
Nitrogen adsorption-desorption isotherms were obtained to characterize the specific surface area and pore size distribution using Micromeritics ASAP 2020 instrument (Micromeritics Instrument, Norcross, USA).Roughly 150 mg sample was degassed under low pressure (2 mmHg) at 140 • C for two hours.Adsorption isotherms were obtained by submerging the sample tubes under liquid nitrogen to achieve isothermal state followed by purging with gaseous nitrogen.BET (Brunauer-Emmett-Teller) method [26] was used to calculate the specific surface areas.Assuming the pore size is slit-formed, the calculation of pore size distribution (PSD) of pore volumes (vol %) followed the individual pore size (micro-, meso-, and macro-) volumes with NLDFT model [27,28].Micro-pores as small as 1.5 nm in diameter could be identified.The specific surface area can be measured with a precision of 5 % based on a previous study.[29]

Batch adsorption experiments
The adsorption capacity towards BPA and zinc was studied by batch adsorption.Raw materials (Karin and Klara willow bark), foam samples (Ka-F and Kl-F) and activated foams (Ka-A-F and Kl-A-F) were investigated.Samples were sieved to under 150 µm particle size.Experiments were made at pH 6 using initial adsorbate concentration of 50 mg/L, h adsorption time and adsorbent dosage of 5 g/L.Volume of the batch was 25 mL.Laboratory shaker with 300 rpm was implemented to mix adsorbent and adsorbate.All adsorption experiments were conducted in duplicate.Samples were filtered through 0.45 µm filter paper (Sartorium Stedim Biotech) after 24-hour adsorption.

Characterization 2.3.1. SEC of proanthocyanidin
SEC experiments for both the crude extracts and the purified colorant fractions were carried out with an Agilent 1260 Infinity II Multi-Detector GPC/SEC System including a UV detector.Three Waters 7.8 mm × mm Ultrahydrogel columns (500 Å, 250 Å, and 120 Å) with a 6 mm × mm Ultrahydrogel guard column were implemented with the flow rate of 0.5 mL/min using 0.1 M NaNO 3 as the eluent.The injection volume was 100 µl.For molar mass determination, the columns were calibrated using narrow-polystyrene sulfonate (PSS) standards under the 280 nm UV absorbance.

Liquid-state ( 13 C and HSQC) NMR of proanthocyanidin
Liquid-state NMR spectroscopy was applied for analyses of the chemical structures of crude extracts and purified fractions.Measurements were performed using a 400 MHz Bruker Avance III spectrometer.Acetone-d 6 : D 2 O (v:v; 1:1) (δC/δH, 29.92/2.05ppm) was used not only as the internal reference for the chemical shift calibration but also as the deuterated solvent to dissolve the sample.[30] 13 C spectra were measured at room temperature with a relaxation delay of 2 s with 18 K scans and 2D HSQC was performed to correlate the proton and carbon shifts.

Solid-state NMR of proanthocyanidin
Solid-state 13 C CPMAS NMR spectra were acquired by employing a double resonance CPMAS probehead.The ground samples were packed into 4 mm outer diameter ZrO2 rotors, sealed with KEL-F endcaps and spun at spinning frequency of 8 kHz. 13C CPMAS spectra were obtained at room temperature with at least 3000 scans using a 5 s relaxation delay and 1 ms contact time for cross polarization.The spectra were externally referenced to adamantane [31].

UPLC-MS/MS of proanthocyanidin
Dried samples were dissolved in 10% aq.acetonitrile (final concentration 2 mg/mL), filtered with 0.20 µm polytetrafluoroethylene (PTFE) filters, and analysed with UHPLC-MS/MS as outlined previously [32,33].Procyanidin (PC) and prodelphinidin (PD) subunits (both terminal and extension units) of oligomeric and polymeric proanthocyanidins (PA) were detected by the PC-and PD-specific group-specific MS/ MS tools to allow the calculation of total PA content of the analysed sample in mg/g dry weight, the PC and PD share, and the mean degree of polymerization (mDP) of PAs present in samples analysed based on two or three independent measurements.Previously purified and well characterized PC-rich and PD-rich PA-fractions [32] were used as external standards for both PC and PD quantitation and for the calibration of the mDP.

HPLC/UV-vis of foam and activated carbon
Zinc concentration was determined using atomic absorption spectroscopy (PerkinElmer A Analyst 200, Waltham, MA, USA).BPA concentration was quantified using high-pressure liquid chromatography (HPLC) at 226 nm wavelength with ultraviolet-visible (UV-vis) detectors (Shimadzu SPD-10 A).

FESEM of foam and activated carbon
The microstructure shown in FESEM images were obtained using a Zeiss Sigma field emission scanning electron microscope (FESEM, Carl Zeiss Microscopy GmbH, Jena, Germany) at an accelerating voltage of 5 kV.

Calculation
The adsorption capacity, q: where C 0 refers to the initial concentration of adsorbate (mg/L), C e refers to the equilibrium concentration of adsorbate (mg/L), V is the volume of solution (L) and m is the adsorbent mass (g).

Mass balance and chromatographic purification
Crude water extracts represent approximately 20 (w/w, %) of the bark from Salix (Klara and Karin) hybrids.[34] Table 1 displays the mass balance of the crude extracts from bark of both Klara and Karin willow hybrids.The main identified phenolic compounds are salicin, catechin, epicatechin, and triandrin.[34] The Stiasny number reaction was adopted to determine the tannin reactivity of the crude extracts towards formaldehyde.[35] The determined reactive tannin was 56 (w/w, %) in Karin bark.The identified phenol represents only 6.6 (w/w, %) and 15.5 (w/w, %) from crude extracts of bark from Karin and Klara, respectively, indicating the rest of formaldehyde-reacting phenol could possibly originate from the unreacted proanthocyanidins (approximately 49.5 (w/w, %), Table 1) that are present largely at the crude extracts of the willow hybrid Karin bark.
To improve the understanding of the composition of the high Mw proanthocyanidin, the crude extract was fractionated by stationary phases of CA-10GC and Sephadex G10.The resulting multiple colorant tannin fractions were purified from the crude extracts using the column chromatography.[23] The Mw of each purified fraction was measured by SEC (Fig. S2 and Table S4).The average degree of oligomerization of the crude extracts was 5-6 flavan-3-ol units.It has been reported that 70% ethanol extract of Salix purpurea L. bark contains up to 3-unit flavan-3-ol oligomers [36], including derivatives formed from reactions with other phenolic compounds.The flavan-3-ols obtained were catechin, epicatechin, gallocatechin, and catechin-3-O-(1-hydroxy-6oxo-2-cyclohexene-1-carboxylic acid)-ester.
The liquid-state 13 C NMR spectra are shown in Fig. 4. Inspection of the liquid state NMR spectra and comparison with the published data [30,39,40] show typical signals attributed to procyanidin (PC) and prodelphinidin (PD) units.The signal centered at 145.0 ppm and 145.1 ppm (Fig. 4) can be identified to C3 ′ and C4 ′ in the B-ring of the flavan-3ol units in PC form, respectively.Typical chemical characteristic signals for C3 ′ and C5 ′ (centered 145.9 ppm) of the PD units are also seen in 13 C NMR, Fig. 4. The remaining specific aromatic resonances show the 116.2 ppm and 119.8 ppm for the C2 ′ (or C5 ′ ) and C6 ′ , respectively.The signals at 131.7 (C-1), 77.2 (C-2, cis form), 70.7 (C-3, extender part), and 67.5 (C-3, terminal part) ppm can be assigned to the attached flavan-3-ol units (Fig. 1).The peaks that are shown between the region 90-110 ppm

Table 1
Mass balance of the water extracts (mg/g crude extracts) [34] from willow bark Karin and Klara.Overall determined phenolics refer to the sum of catechin, epicatechin, and triandrin."a" denotes that the proanthocyanidins (w/w %) = overall reactive tannin (w/w %) -overall determined phenolics (w/w %).All the measurements were performed two times and the standard deviation are included inside the parenthesis.was sensitive to the units (C2 ′ , C6 ′ , C6, C8, and C10) in both form of PC and PD.The HSQC spectrum at Fig. 5, confirms the structure characteristics of PC and PD form based on literature [37,41,42]    prodelphinidin (PD) units and they were both identified from the crude extracts. 13C NMR spectrum of the fraction PF 1-2 contained several overlapped signals at chemical shifts of polymeric tannins from leaves of Leucaena leucocephala hybrid Rendang in D 2 O/acetone-d 6 (v:v; 1:1) (Fig. S3) [30].Only procyanidin (PC) form was identified from its purified polyflavonoid tannins (i.e.PF0, PF 1-1, and PF 1-2).This is possibly because 13 C measurement parameters (such as relaxation delay) were not optimized for the purified fractions so that prodelphinidin (PD) units may be filtered out.

UPLC-MS/MS characterization
UPLC-MS/MS analyses were performed to further support compound identifications.The PCs and PDs can be detected and quantified using group-specific 2D fingerprint method.[33] This method relies on the fragmentation of oligomeric and polymeric PAs in the MS ion source by using a cone voltage much higher than used for the analysis of individual flavan-3-ols such as catechin and gallocatechin.The fragments thus represent the PC and PD units originally bound to the oligomeric and polymeric PAs and are detected by MS/MS methods specific to both terminal and extension units of the PCs and PDs.The use of a small cone voltage revealed sharp peaks presenting gallocatechin and epigallocatechin (Fig. 6e) together with catechin and epicatechin (Fig. 6i) monomers.The PD-containing PA oligomers showed as peaks with higher cone voltages in Fig. 6 b->d and the PC-containing PA oligomers in Fig. 6 h->g.These same chromatograms also displayed the smaller polymeric PD-and PC-containing PAs as the chromatographic humps eluting after the oligomeric peaks.Finally, the largest PD-containing polymeric PAs showed with the highest cone voltage used in the chromatogram 6b, and the largest PD-containing polymeric PAs in the chromatogram 6f.The gradual detection of larger and larger PAs with increasing cone voltage was evident also from the chromatograms as the PC and PD hump shifted to the right as the method fragments and thus detects bigger and bigger PAs from Fig. 6 d->b and Fig. 6h->f.The detection of PD was very low at samples PF 1-1 and PF 1-2, i.e. close to the baseline noise, as the hump was barely visible even with this sensitive detection method (Fig. 6b).
According to the identification protocol [33], the total content of proanthocyanidins, PC and PD, and mDP were reported in Table 2. Unlike the NMR results, prodelphinidins (PD) -units were also detected from PF0 although their presence was almost negligible at fractions of PF 1-1 and PF 1-2 (Table 2).The instrumentation of UPLC-MS/MS is more sensitive to the qualitative determination of oligomers from both prodelphinidin (PD) and procyanidin (PC) type tannins.Overall prodelphinidin (PD) and procyanidin (PC) form of tannin were both identified for its presence at willow bark crude extracts by NMR and UPLC-MS/MS.The relative ratio of PC/PD was roughly 2.5 for crude extracts and PF0 at Table 2, and it was supported by the integral ratio (i.e.2.3) between PC form and PD form from Fig. 4 and Fig. 5.The mDP was the highest from the crude extracts and dropped roughly 1.5 times to the purified fractions, indicating that the purified fraction was also highly representative of the proanthocyanidins.Both the UPLC-MS/MS (Table 2) and size exclusion chromatography (SEC) (Table S4) suggest that the crude extracts have an average oligomerization degree of roughly 5-6 flavan-3-ol units.

Adsorption performance of activated foams (or foams) from crude extracts
The structural foundation (or characterization) of crude extracts provides a more rational design to apply this activated foam made of willow bark-derived proanthocyanidins as promising bio-based adsorbents for possible water treatment.Considering the potential applicability for the industrial wastewater treatment, crude extracts from two willow hybrids (Karin and Klara) were included for the water treatment experiments by using model solutions.Willow bark crude extracts (including hybrids Karin and Klara) were chemically foamed and further activated with steam treatment.They were then used as adsorbents in initial experiments for removing zinc (Zn 2+ ) and Bisphenol A (BPA).The specific surface area and porous morphology of the activated foam was achieved using the nitrogen sorption, as shown at Fig. 7. Interestingly, the mesoporous structure appears after the activation (with the yield of roughly 38%, Table 3), as reflected by the surface characteristics like specific surface area and pore volumes (micropores, mesopores, and macropores).
The surface acidity and porosity structure of the adsorbents are important aspects regarding the adsorption on the removal of zinc and BPA, respectively.As indicated in Table 3, the raw materials, Klara and Karin bark, did not remove BPA or zinc at all.The foams and especially the activated foams have a sponge-like structure with a large surface area.Foams (Ka-F and Kl-F) removed considerable amount of BPA but very low amount of zinc.Interestingly, activated foam samples (Ka-A-F and Kl-A-F) removed significantly both zinc and BPA for both willow hybrids.In specific, adsorption capacities of 9.19 mg/g (91 % removal efficiency) and 7.96 mg/g (79 % removal efficiency) were reported towards zinc removal for Ka-A-F and Kl-A-F, respectively.Interestingly, 100 % of initial BPA from the solution could be removed using activated foams (Ka-A-F and Kl-A-F) and can also be reported as having an adsorption capacity of 9.18 mg/g.In addition, the willow bark was carbonized previously as electrode materials.The adsorption capacities shown here from the crude extracts, representing 20 (w/w %) of Karin bark, maybe partly explain the hierarchically organized pore structure of the obtained electrode from willow bark as an energy storage system.[43] Overall there is clear established correlation between the surface area and the adsorption capacities in this study, as reported also previously.[44] Biomass-derived carbon materials have been used as an adsorbent earlier and results presented in literature showed huge variety towards the removal of BPA and zinc as seen in Table 4. Results obtained in this study was compared to the studies in which quite simple production step was used in the context of lignocellulosic biomass-derived material.Even though the screening adsorption experiments are currently only conducted by screening rather than under optimal conditions, the adsorption capacity has been proven to be in the same order of magnitude than the results presented in literature.Therefore, the willow bark crude extracts are of high importance comparing to the  other bio-based materials.

Conclusion
The chemical composition of the proanthocyanidins fractions from crude extracts of willow bark has been determined using a novel analytical approach combining the use of both liquid ( 13 C and twodimensional HSQC) and solid-state NMR spectroscopy.This NMR approaches offered information regarding the chemical structure of the elusive proanthocyanidins from crude extracts and its purified colorant fractions.To be specific, solid-state NMR provided generalized characteristic resonances of the overlapped procyanidins (PCs) and prodelphinidins (PDs) units of proanthocyanidin (PA) fractions from purified colorants and crude extracts.The flavan-3-ol unit showed clear and wellresolved absorbance regions in both 13 C and 2D HSQC NMR.Both the NMR and UPLC-MS/MS recognized both PCs and PDs units of proanthocyanidin from crude extracts and procyanidins units were identified as the main units present in the proanthocyanidins.The oligomeric nature of the crude extracts was also revealed by both the UPLC-MS/MS and SEC.Overall, the sensitivity of the applied technique towards the proanthocyanidin chemistry elucidation has been studied and this demonstrates the power of the concerted use of multiple techniques for the structural elucidation of bark-derived crude extracts.This study showed that the activated foam from willow bark crude extracts has the potential to be successfully applied to the industrial wastewater treatment.
In this study, only initial adsorption studies were implemented to test is the developed material showing adsorption capacity towards organics (BPA as model) or metals (Zn as model).Adsorption-regeneration cycles were not implemented and therefore, the detailed plan to optimize various parameters of adsorption and carry out ''long-term performance study'' using the willow bark-derived crude extracts (or its purified fractions) as adsorbent is out of scope of this present work and this will be investigated more carefully at another study.

Fig. 3 .
Fig. 3. Solid state CPMAS NMR spectrum of the purified polyflavonoid tannin fractions in comparison with the original Karin bark and its crude extracts.a Karin bark.b Crude extracts.c PF0. d PF 1-1.e PF 1-2.

Fig. 7 .
Fig. 7. Morphology of the prepared foam and activated foam from Karin bark crude extracts by FESEM imaging analysis.a Karin bark crude extracts.b Foam of crude extracts (Ka-F).c Activated foam of crude extracts (Ka-A-F).For Klara bark, see Fig. S7.

Table 3
Surface characteristics and adsorption capacities towards zinc and Bisphenol A (BPA) for the studied adsorbents."a" denotes that the adsorption capacities for crude extracts was zero.Standard deviations are included inside the parenthesis.
UnitKa-F Ka-A-F Kl-F Kl-A-F Specific surface area, pore volume, and pore size distribution SSA (specific surface area) D. Jinze et al.

Table 4
Comparison of adsorption capacity towards removal of organics (i.e.BPA) and heavy metals (i.e.zinc) between willow bark crude extracts (our study) and other lignocellulosic biomass-derived carbons.