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Article

Eco-Friendly Valorization and Utilization of Plant Waste as a Source of Tannin for Leather Tanning

by
Shahid Rehman Khan
1,*,
Shahzad Maqsood Khan
1 and
Rafi Ullah Khan
2
1
Institute of Polymer and Textile Engineering, University of The Punjab, Lahore 54590, Pakistan
2
Institute of Chemical Engineering and Technology, University of The Punjab, Lahore 54590, Pakistan
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(5), 3884; https://doi.org/10.3390/su15053884
Submission received: 17 December 2022 / Revised: 2 February 2023 / Accepted: 9 February 2023 / Published: 21 February 2023
(This article belongs to the Section Sustainable Chemical Engineering and Technology)
Browse Figures
Versions Notes

Abstract

:
In the present study, bark, which is the waste part of a tree, was utilized as a source of tannin for leather tanning after its value addition. The barks of Acacia nilotica L. and Eucalyptus globulus, which are abundantly available in Pakistan, were selected in this study. Different extraction techniques including mechanical, soxhlet, reflux and ultrasonic extraction were used for the extraction of tannin from the selected barks. The medium of extraction included aqueous as well as different solvent mixtures in different ratios. The solvent mixtures used in this study were methanol-water and acetone–water. The methanol–water ratio was kept (50:50) and the acetone–water ratio was maintained at (50:50 and 70:30). In the first step, the process of extraction was optimized for solvent mixtures and extraction time by taking a fixed dose (5 g) of each bark and a fixed volume of each extracting solvent mixture (150 mL). The total phenolic contents (TPC) of the bark extracts were determined by colorimetric assay (as mg GAE/g bark), taking gallic acid as the reference standard. The tannin contents (TC) of the bark extracts were determined by using the standard hide powder method (SLC 117). It was concluded that among the different extraction techniques, and among the solvent mixtures acetone–water in ratio (70:30), ultrasonic extraction resulted in maximum extraction of tannin (196.1 and 125.2 mg/g), respectively for A. nilotica and E. globulus. FTIR analyses of the dried extracts obtained from both barks were compared with FTIR of the most commonly used commercial vegetable tanning agent in the leather industry, i.e., mimosa. Results were comparable, which confirmed the presence of condensed tannin in both barks. The extracted tannin was applied on a pickle and wet blue leather to evaluate its tanning and re-tanning capability. Mimosa, the most abundantly used vegetable tanning material in the leather industry, was applied as a reference standard on the same pickle and wet blue leather for comparison. Different physical properties, such as leather shrinkage temperature, leather softness, tensile strength, percent elongation and breaking strength, were measured to evaluate the tanning efficacy of the extracted tannins. The results showed that tannin obtained from A. nilotica showed comparable physical properties to those of mimosa. It was concluded from the results that tannin obtained from locally available bark, i.e., A. nilotica, can be utilized as a vegetable tanning agent for the leather industry.

1. Introduction

Increasing environmentalism has enabled the growth of green sustainability and forced corporate organizations to consider environmental market practices [1]. These days, it is more important than ever to use sustainable ingredients (such as polylactic acid, chitosan, lignin and tannins, among others) to reduce crude oil consumption and the human impact on the environment [2]. Tannins in the form of plant extracts are a suitable choice for green chemistry. They are abundant in certain plant species and are easily harvested [3,4]. Tannins typically have a dry plant material weight between 5% and 10%. They are polyphenolic secondary metabolites widely distributed in vascular plants, particularly in the buds, stems, roots, seeds, bark and leaves. Despite some organisms’ ability to tolerate ingested tannins, they primarily function in several mechanisms that defend plants against diseases, fungi, insects and herbivorous animals.
Leather manufacturing is an ancient technology and involves the conversion of animal hides and skins into consumable material [5,6]. Microorganisms including bacteria and fungi can attack the hides and skin when left without any proper treatment [7]. To prevent the degradation of the collagen structure of hides/skins through bacterial attack, a tanning process is applied to them. In existing tanning processes, chrome tanning is used by approximately 90% of the leather industry worldwide [8,9]. Chromium (III) salt, which is a pollutant, is used during chrome tanning [10]. It also oxidizes to chromium (VI) under various environmental conditions including humidity, temperature, light and ageing. Chromium (III) salts and other auxiliary chemicals replace the molecules of the leather structure and are converted to free radicals due to temperature and light effects. These free radicals are responsible for oxidizing Chromium (III) into Chromium (VI), which is very dangerous for human beings [11].
Keeping in view the hazards of Chromium salt, the use of environmentally friendly natural compounds is gaining importance in leather manufacturing [12,13]. In this context, plant polyphenols are receiving increasing attention for their use in the leather tanning process [14]. Tannins are polyphenolic compounds and are secondary metabolites of larger plants, with a molecular weight between 500 and 20,000 Da. Tannin is soluble in aqueous media as well as in polar organic solvents [15]. It has been used in the leather industry for tanning purposes for centuries [16,17] and as an adhesive for decades [18,19]. The barks of several trees are rich in polyphenolic compounds. Such trees include chestnut and oak, which contain hydrolyzable tannins in their barks, whereas the barks of quebracho and mimosa are rich in condensed tannins. Polyflavonoids and proanthocyanidins are types of tannins that have strong antioxidant and antifungal properties [20,21] in addition to their strong chemical reactivities to protein.
Tannin is present in almost all plants, but only a few species have a significant amount of tannin; such species are located in tropical and sub-tropical climates [22]. Several attempts have been made by different researchers to extract tannin from different parts of trees. Pizzi demonstrated in his work that the barks of chestnut, valonia and myrobalan contain hydrolyzable tannin, whereas wattle, babul and quebracho barks contain condensed tannin [23]. Ersin et al. optimized the isolation of tannin from acorns and its application on leather [24]. Similarly, Patel Preeti et al. optimized the process of extraction to obtain the maximum yield of tannin by varying the process parameters such as mesh size of bark, time and temperature of extraction [25].
Keeping in view the presence of tannin in the bark of different trees, two locally available trees, A. nilotica and E. globulus, were selected for the determination, extraction, quantification, optimization and application of extracts on wet blue and pickle leather as a tanning agent. A. nilotica is a medium-to large-sized, thorny, almost evergreen tree that can grow to a height of 20 to 25 m. It is native to India, Saudi Arabia, Bangladesh, Burma, Sri Lanka, Egypt, Sudan and various other tropical regions of the world. The linear, narrow, flattened fruits of A. nilotica are widely distributed and contain a variety of polyphenolic substances, including flavonoids, alkaloids, tannins, saponins and others [26,27]. In the present work, we first obtained a reasonable amount of tannin from the waste part of two indigenous plant resources of Pakistan through the optimization of the extraction process using different solvents and extraction techniques. The best extraction technique and extraction solvent were proposed in the present study. Sufficient amount of tannin was extracted from selected barks. The suitability of extracted tannin as a tanning/re-tanning agent was also examined in this work. Tannin extracted from barks of selected plants were applied on a pickle and wet blue leather in comparison with commercially available vegetable tanning agent mimosa. The physical properties of tanned/re-tanned leather were evaluated.
The primary aim of this study includes:
  • Optimization of extraction process from barks of selected plants for variables such as solvent, temperature, time and extraction technique to obtain maximum tannin in extract;
  • Application of extract on a pickle and wet blue leather to evaluate its suitability as a tanning/re-tanning agent against mimosa: a most widely used vegetable tanning agent in leather industry;
  • The suitability and efficacy of tannin extract against mimosa, by performing different physical tests on tanned/re-tanned pickle and wet blue leather.

2. Material and Methods

2.1. Materials

Barks of Acacia (A. nilotica) and Eucalyptus (E. globulus) were obtained from Jinnah garden Mall Road, Lahore Pakistan. Mimosa vegetable tanning powder, pickle goatskin (PGS 0.98 mm), neutralizer N and wet blue semi leather of thickness 1.11 mm were obtained from Haji Sardar Ali Akhtar (HAS) tannery Kasur Pakistan. Methanol, acetone and formic acid were purchased from Sigma Aldrich (Steinheim am Albuch, Germany), whereas common salt (NaCl), sodium carbonate (Na2CO3) and sodium bicarbonates (NaHCO3) were purchased from Merck (Darmstadt, Germany).

2.2. Instruments and Apparatus

Different instruments used in this study include an orbital shaker (Digitek Instruments, Hong Kong, China), soxhlet extraction apparatus (Pyrex, Deeside, UK) and ultrasonic extraction apparatus (Elmasonic, Constance, Germany). UV–Visible spectrophotometer (Helios Alpha, Saint Neots, UK) for the quantitative analysis of total phenolic contents in extracts. Rotary evaporator (Buchi, Straubenhardt, Germany) to concentrate the extract by evaporation of organic solvent from the extract. Micro drums (Xinjiang, China) for the application of extracts on pickle goatskin and wet blue leather. Material testing machine (LLOYD, Bognor Regis, UK) for measurement of tear strength, tensile strength and % elongation of tanned and re-tanned leather. Water vapor permeability apparatus (SATRA, Kettering, UK) for measurement of permeable properties of samples after application of tannin extract. Shrinkage temperature apparatus (SATRA, Kettering, UK) to evaluate the shrinkage temperature and leather softness tester (SARTA, Kettering, UK) to measure the softness of tanned and re-tanned leather.

2.3. Treatment of Bark Samples

The barks were washed separately with tap water to remove dust and other unwanted impurities and then dried in sunlight for 48 h. The dried bark samples were cut into small sizes (1–2 cm), chopped well and finally ground, using a mechanical bladder. The ground samples of both barks were passed through a 60 mesh sieve to obtain crude powder. The crude powder was labelled S1 and S2 for A. nilotica and E. globulus, respectively, and stored in air tight polythene bags.

2.4. Premiliinnaory Extraction of Bark in Aqueous Medium

The powdered samples (5 g) of each bark were taken separately in two 250 mL Erlenmeyer flasks, and 100 mL water was added in each flask. Flasks were adjusted on an orbital shaking water bath and temperature of the shaking water bath was adjusted at 80 °C. The contents of each flask were shaken at constant speed (100 rpm) for 2 h at 80 °C. After extraction of 2 h, the crude mixture was filtered. The mixture was filtered, and the filtrate was analyzed for the presence of total phenolic contents (TPC) [28]. Total phenolic contents were identified using ferric chloride, lead acetate and gelatin tests [29].

2.5. Identification Test for Total Phenolic Contents (TPC)

2.5.1. Ferric Chloride Test

To 3 mL of each extract taken in two separate test tubes, 3 mL of 5% w/v ferric chloride solution was added. The appearance of blue-black colour indicated the presence of total phenolic contents in both extracts [30].

2.5.2. Lead Acetate Test

To 3 mL of each extract taken in two separate test tubes, 3 mL lead acetate solution was added. The formation of white precipitates indicated the presence of total phenolic contents in both extracts [30].

2.5.3. Gelatin Test

To 3 mL of each extract taken in two separate test tubes, 1% gelatin solution containing sodium chloride was added. The appearance of white precipitate confirmed the presence of total phenolic contents both extracts [30].

2.6. Optimization of Extraction Process

The extraction process was optimized for variable parameters including extraction technique, extraction solvent mixtures, extraction time and extraction temperature by taking two parameters as constant. Constant parameters were bark dose (5 g) and extraction solvent mixture (150 mL). The extraction process was optimized through the following steps.

2.6.1. Extraction Solvent

The maceration technique was used to optimize the extraction solvent. Three extraction media including aqueous, methanol–water (50:50), and acetone–water (50:50 and 70:30) were used separately, and extraction was carried out at room temperature for 2 h. The doses of barks (5 g) and volume of extraction solvents (150 mL) were kept constant. After 2 h extraction, total phenolics and tannin contents were quantified in all extracts. The results are summarized in Table 1.

2.6.2. Extraction Temperature

After the optimization of the process for best solvent mixture resulted in the maximum yield of tannin (Table 1), the process of extraction was optimized for temperature. Solvent mixture, i.e., acetone–water in ratio (70:30 v/v) finalized in Section 2.6.1 (Table 1), was used to extract tannin from bark powder at 50, 60, 70 and 80 °C using the maceration technique. For this purpose, 150 mL of solvent mixture along with 5 g of powdered bark were taken in separate conical flasks and extraction was performed at 50, 60, 70 and 80 °C using an orbital shaking water bath. Upon completion of the process, the contents of each flask were filtered, and filtrate was quantified for total phenolic and tannin contents using spectrophotometer and hide powder method (Figure 1).

2.6.3. Extraction Time

The extraction was carried out to optimize the process for time, using maceration as the extraction technique. The extraction was performed by taking 150 mL of acetone–water (70/30 v/v %) and 5 g of each powdered bark, at 70 °C for 2, 4, 5 and 6 h. After extraction, the crude product was filtered, concentrated using rotary evaporator and quantified for total phenolic and tannin contents. Results are summarized in Table 2.

2.6.4. Extraction Technique

After the optimization of solvent, temperature and time of extraction, the process of extraction was optimized for extraction technique. Different techniques include mechanical, soxhlet, ultrasonic and reflux extraction. The selected extraction techniques were applied separately to extract tannin from the individual barks, using solvent mixture, temperature and time selected from aforementioned results. Weight of bark powder (5.0 g) and volume of solvent mixture (150 mL) were kept constant for each technique. Results are summarized in Figure 2.

2.6.5. Ultrasonic Extraction at Variable Watts

The extraction was carried out using ultrasonic technique at variable watts, i.e., 100, 300, 400 and 500 watts. Solvent mixture, temperature and time of extraction were kept fixed for all selected watts. Weight of bark powder (5.0 g) and volume of solvent mixture (150 mL) was taken as fixed each watt. Results are summarized in Table 3.

2.7. Total Phenolic Content (TPC) in Extracts

Total phenolic contents in the separate bark extracts were measured against standard solutions of gallic acid. The calibration curve was drawn with the help of the standard and the unknown concentrations of the total phenolics in all extracts were calculated.
Stock solution of gallic acid was prepared by taking 6.4 mg of gallic acid in the 100 mL volumetric flask and was made up to mark with distilled water. Different standards of gallic acid were prepared by taking 0.1 mL, 0.25 mL, 0.5 mL, 1 mL, 1.5 mL and 2 mL in separate 10 mL volumetric flasks. We added 0.6 mL of Folin–Ciocalteu (diluted 1:3) reagent and 1.5 mL of 20% Na2CO3 solution into each flask and volume was made up to mark with distilled water. A bluish color complex formed in each flask. The flasks were placed in dark for 90 min and then absorbance of each standard was observed at 760 nm using a UV–Visible spectrophotometer (Helios Alpha). A calibration curve was drawn as reference by taking concentrations along x-axis and absorbance values along y-axis. The calibration curve is shown in Figure 3.
To measure the total phenolic contents in the separate extracts, 5 mL of each extract was taken in separate measuring flasks and diluted to 100 mL with distilled water. We took 1 mL of each diluted sample in separate volumetric flasks of 10 mL and 1.5 mL Na2CO3 solution (20%) and 0.6 mL Folin–Ciocalteu reagent was added into each flask and diluted to mark with distilled water. Bluish color complexes developed in all flasks. All the flasks were kept in a dark area for 90 min and absorbance was observed at 760 nm using a UV–Visible spectrophotometer (Helios Alpha). Results were expressed as milligram of Gallic acid equivalent per g of dry matter (mg of GAE/g bark). Calibration curve obtained for gallic acid was taken as refence to calculate total phenolic contents in each extract.

2.8. Tannin Contents (Hide Powder Method)

Tannin contents in separate bark extracts were determined using standard testing methods published by the society of leather technologists and chemists. Hide powder was used for the determination of tannin contents in all extracts. The total solids in the extracts were determined using the standard method. The filtered extract (100 mL) of each bark was taken in a separate flask and 7 g hide powder was added into each flask. The contents of each flask were stirred for half an hour using an orbital shaker at 50 rpm. After stirring, the contents of flasks were left for thirty minutes and filtered through a sintered glass filter. We took 50 mL of each filtrate to determine the non-tannins using the standard method [31,32,33]. The Tannin contents (mg/g) in the individual extracts were calculated as the difference between the amounts of total soluble and non-tannins using the following formula:
Tannin (mg/g) = Soluble Solids (mg/g) − Non-tannins (mg/g)

2.9. FTIR of Dried Extracts

The extracts obtained for both barks after ultrasonic extraction were decanted, centrifuged, filtered and then dried in an air oven at 70 °C. The FTIR spectra of both dried powders were taken using a Thermo Nicolet spectrophotometer. An FTIR spectrum of mimosa, commercial vegetable tannin, was also taken for comparison. Results are shown in Figure 4.

2.10. Application of Extract on Wet Blue and Pickle

2.10.1. Re-Tanning of Wet Blue Leather

The tanning micro drums were washed and cleaned thoroughly with plenty of water before proceeding for re-tanning operation. Two pieces of wet blue (50 g each) were weighed with the help of electric balance and placed in separate micro drums, which were already marked with S1 and S2. The wet blue samples placed in the separate micro drums were washed with distilled water to remove impurities and other undesirable substances. About 2% by weight of wet blue Neutralizer N (1 g) and 150% by weight of wet blue water was added to each micro drum and their rotation was started. The pH of the contents of each micro drum was checked after every half an hour during rotation. When the pH reached between 5.5–6.0, micro drums were rotated for a further 45 min. After the completion of specified rotation, liquid contents of each drum were drained off, keeping behind the neutralized wet blue pieces. Aqueous extract (375 mL) of each bark was added separately into individual drums and drums were rotated for further 4-5 h. After the Rotation was completed, drums were stopped, and wet blue pieces were kept soaked in the same extracts overnight. The next day, 15 g (30%) by weight of wet blue fatliquor was added into each drum and the pH of the contents of each drum was adjusted to 3.5 with the help of formic acid solution (1:10). The micro drums were rotated for 45 min and then the re-tanned wet blue samples were removed from each drum. The re-tanned wet blue leather samples were left for drying on a wooden pelt in a clean area. Similarly, the extract obtained from each technique was applied for re-tanning of wet blue, after the removal of organic solvent, using rotary evaporator (Buchi, Germany).

2.10.2. Tanning of Pickle Hide

To tan the pickle hide, a solution of sodium chloride was prepared and its Baume was adjusted at 6 with the help of a Baume-meter. We added 250% by weight of pickle hide salt solution (Baume 6), 1.5% by weight of pickle hide sodium formate, 4% by weight of pickle hide de-greasing agent and 0.5% by weight of pickle hide sodium carbonate into separate micro drums. Two pieces of pickle hide (50 g each) were weighed and added into separate micro drums which were already marked with S1 and S2. Pickle hides were soaked into liquid contents, already added into each drum, and drums were rotated for 4 h. After 4 h rotation, the pH of the contents of the drums was adjusted between 2.5-3.0 with the help of formic acid. The micro drums were rotated for further 15–30 min and then the liquid contents of the drums were drained off, leaving behind de-pickled hides. Aqueous extract of each bark (375 mL) was added separately into each micro drum and micro drums were rotated for 5 h. After completing the rotation time, material was soaked in the extract overnight. The next day, the pH of the contents of the drums was adjusted to pH 4.0 and the drums were rotated for a further 45 min.

2.10.3. Re-Tanning of Tanned Pickle

The re-tanning of tanned pickle obtained in the previous step was proceeded in the same way as was applied for the re-tanning of wet blue. The tanning and re-tanning process is summarized in Table 4.

2.11. Physical Properties of Re-Tanned Wet Blue and Tanned Pickle Samples

The physical properties of tanned and re-tanned leather samples were measured to assess the quality of samples. Quality tests performed were leather shrinkage temperature (ISO 3380, IULTCS/IUP 6) [34] using leather shrinkage temperature apparatus (SATRA, UK); breaking force (ISO 3376, IULTCS/IUP 6), tensile strength and % elongation (ISO 3376, IULTCS/IUP 6) [35], using universal testing machine (LLOYD, UK); water vapor permeability (ISO 21420) [36], using water vapor permeability apparatus (SATRA, UK); and leather softness (ISO 17235) [37] using a leather softness tester (SDL Atlas, UK). The comparative study was performed for reference samples, tanned/re-tanned with commercially available vegetable tannin, i.e., mimosa. Results are shown in Table 5 and Figure 5.

3. Results and Discussions

In the present study, the first step was the confirmation of the presence of total phenolic contents in extracts of selected barks. For this purpose, a preliminary examination of both barks was performed using different identification tests for the bark extracts. The different identification tests performed on each extract were described in Section 2.5. The results of the identification tests showed that both barks were rich in total phenolic contents. Results are summarized in Table 6.

3.1. Optimization of Extraction Parameters

After the confirmation of phenolic contents in the extracts of both barks, optimization of the extraction process was carried for the selection of the most suitable extraction solvent, temperature, time and extraction technique. Four different solvents were selected, keeping in view the work performed by different researchers on different barks. [38,39]. Maceration (mechanical extraction) was used in the whole optimization process. Among four different solvent mixtures used for extraction, it was observed that acetone–water in the ratio (70:30) was the best solvent which resulted in the maximum yield of total phenols and tannin contents. Results are summered in Table 1.
After finalizing the extraction process for solvent, optimization of temperature was carried out by keeping solvent mixture as constant for each experiment. It was observed that maximum yield of total phenols and tannin contents was obtained at 70 °C for both barks, as shown in Figure 1. The extraction temperature of (70 °C) and extraction solvent (acetone–water, ratio 70:30) were finalized for further evaluations.
To optimize the extraction time, extraction temperature and extraction solvent were kept constant. It was observed that maximum yield of total phenols and tannin contents were obtained after 5 h extraction (results are shown in Table 2). A small but almost negligible increase in both total phenolic and tannin contents was observed after 6 h extraction. Hence, an extraction time of 5 h was fixed for further study.
The study was further extended for different extraction techniques at constant extraction time, extraction solvent and extraction temperature; it was observed that ultrasonic extraction resulted in maximum yield of tannin and total phenolic contents. Results are shown in Figure 2.
After the optimization of extraction parameters (solvent, temperature, time and technique), extraction was performed at variable watts (100, 300, 400 and 500) through power ultrasound. It was observed that extraction carried out at 400 W resulted in maximum yield of total phenolic and tannin contents. Further increase in watts reduced the extraction yield of tannins. Hence, ultrasonic extraction at 400 W was selected for obtaining maximum yield of the total phenolic and tannin contents (Results are reported in Table 3).

3.2. Study of FT-IR Spectra and Comparison with Mimosa

Mimosa is considered an excellent vegetable tanning agent which is used by most of the leather industry for vegetable tanning; so, mimosa was selected as standard to evaluate the properties of tannin extracted from both barks. The comparison of FTIR spectra of mimosa and dried extracts of both S1 and S2 in fingerprint region (600–2000) cm−1 showed that both samples gave comparable peaks to that of mimosa. Peaks at 1030 cm−1(vs), 1200 cm−1 (ms), 1452 cm−1 (ms), 1507 cm−1(m) and 1612 cm−1 (s) for the dried extracts of both samples S1 and S2 were comparable to that of mimosa. This confirmed the presence of condensed tannin in both samples (results are shown in Figure 4).

3.3. Application of Tannin Extract

The extracts obtained after ultrasonic extraction were applied on pickle and wet blue leather to evaluate their tanning and re-tanning capability. Mimosa was taken as reference material to evaluate the effectiveness of tannin extracts obtained from both barks. Mimosa was applied in the same way and under same conditions as used for tannin extracts to tan pickle and wet blue leathers. The tanning and re-tanning process was explained in Table 4.

3.4. Evaluation of Properties of Tanned Leather

Physical properties, including tensile strength, breaking load, % elongation, shrinkage temperature and softness of tanned and re-tanned samples, were examined. Same properties of samples obtained after the application of mimosa were determined to obtain standard values of all physical properties, mentioned above. Results showed that samples tanned/re-tanned with A. nilotica extract have comparable properties to that of the control sample, i.e., mimosa, as shown in Table 5 and Figure 5. Results showed that tannin extracted from bark of A. nilotica can be used for the tanning and re-tanning process in leather industry. Although E. globulus produced good results, better results were obtained for the extract of A. nilotica. The results obtained for A. nilotica. were almost equivalent to the results obtained from mimosa tannin.

3.5. Environmental Advantages of Work

In a nut shell, the work explored for leather tanning has been proved ecofriendly as tannin based plants have been employed under suitable conditions. Moreover, the effluent shed after utilization of plant waste as tanning of leather did not pose any threat. Additionally, the inclusion of ultrasound rays for the extraction of tannin and selection of mild extraction conditions has made the process greener and more sustainable.

4. Conclusions

Different extraction techniques including maceration, reflux, soxhlet and ultrasonic extraction have been applied in this study. It was concluded that ultrasonic extraction using 70% acetone was the best extraction technique for maximum extraction of tannin, which can further be utilized for application as tanning/re-tanning agents. Results of tensile strength, breaking force, percentage elongation, leather shrinkage temperature, water vapor transmission and leather softness tests for A. nilotica extract showed comparable properties to that of mimosa. It was concluded that the extract obtained from the barks of A. nilotica was rich in tannin contents and can be used commercially for tanning and re-tanning processes in the leather industry. Although these barks have been used by different scientist in numerous studies, this work is important in the sense that optimization has been undertaken to extract the maximum quantity of tannin from the waste bark. The recovery of the organic solvent, i.e., acetone using a rotary evaporator, makes the process cheaper, and utilization of concentrated aqueous solution makes the process ecofriendly as well.

Author Contributions

S.R.K. conducted the experiments; S.M.K. supervised the work; R.U.K. has technically guide the student to complete the task. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

It is part of a PhD study.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Effect of extraction temperature on yield of total phenolic contents (TPC) and tannin contents (TC). S1: Sample 1 (A. nilotica), S2: Sample 2 (E. globulus).
Figure 1. Effect of extraction temperature on yield of total phenolic contents (TPC) and tannin contents (TC). S1: Sample 1 (A. nilotica), S2: Sample 2 (E. globulus).
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Figure 2. Effect of extraction technique on concentration of total phenolic content (TPC) and tannin contents (TC). S1: Sample 1 (A. nilotica), S2: Sample 2 (E. globulus).
Figure 2. Effect of extraction technique on concentration of total phenolic content (TPC) and tannin contents (TC). S1: Sample 1 (A. nilotica), S2: Sample 2 (E. globulus).
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Figure 3. Calibration Curve for Gallic Acid Standard.
Figure 3. Calibration Curve for Gallic Acid Standard.
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Figure 4. FTIR Spectra of mimosa (Upper): A. nilotica (Middle) and E. globulus (Lower).
Figure 4. FTIR Spectra of mimosa (Upper): A. nilotica (Middle) and E. globulus (Lower).
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Figure 5. Physical properties of extracted tannins in comparison with commercial tanning agent (mimosa).
Figure 5. Physical properties of extracted tannins in comparison with commercial tanning agent (mimosa).
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Table
Table 1. Concentration of total phenolic and tannin contents using different solvent mixtures.
Table 1. Concentration of total phenolic and tannin contents using different solvent mixtures.
Extraction SolventExtraction ConditionsTotal Phenolic Contents
(mg GAE/g Bark) S1 (A. nilotica)		Total Phenolic Contents
(mg GAE/g Bark) S2 (E. globulus)		Tannin Contents (Hide Powder Method mg/g)
S1 (A. nilotica)		Tannin Contents (Hide Powder Method mg/g)
S2 (E. globulus)
Water (150 mL)Maceration (150 rpm)-5 g 150 mL solvent-2 h-RT91.758.966.440.6
MoH:Water (50:50)100.262.371.746.2
Ac:Water (50:50)107.568.485.751.6
Ac:Water (70:30)116.671.696.354.2
RT: Room Temperature, Ac: Acetone, MoH: Methanol S1: Sample 1 (A. nilotica), S2: Sample 2 (E. globulus).
Table
Table 2. Effect of extraction time on yield of total phenolic and tannin contents.
Table 2. Effect of extraction time on yield of total phenolic and tannin contents.
Extraction TimeExtraction ConditionsTotal Phenolic Contents
(mg GAE/g Bark) S1 (A. nilotica)		Total Phenolic Contents
(mg GAE/g Bark) S2 (E. globulus)		Tannin Contents (Hide Powder Method mg/g)
S1 (A. nilotica)		Tannin Contents (Hide Powder Method mg/g)
S2 (E. globulus)
2 hMaceration (shaking water bath
Ac:Water(70:30)-5 g bark-150 mL solvent-70 °C		10653.969.652.4
3 h12964.784.360.9
5 h19588.9107.681.4
6 h19989.4110.480.3
Table
Table 3. Concentration of total phenolic and tannin contents using Ultrasonic Extraction at Variable Watts.
Table 3. Concentration of total phenolic and tannin contents using Ultrasonic Extraction at Variable Watts.
Extraction TechniqueExtraction ConditionsTotal Phenolic Contents
(mg GAE/g Bark)
S1 (A. nilotica)		Total Phenolic Contents
(mg GAE/g Bark)
S2 (E. globulus)		Tannin Contents
(Hide Powder Method mg/g)
S1 (A. nilotica)		Tannin Contents
(Hide Powder Method mg/g)
S2 (E. globulus)
Ultrasonic Extraction-100 WAc: Water (70:30)-70 °C
5 g bark-150 mL solvent-5 h		228.6106.4131.4102.3
Ultrasonic Extraction-300 W276.7124.9169.4111.9
Ultrasonic Extraction-400 W318.7199.1196.1125.2
Ultrasonic Extraction-500 W298.6170.5181.4102.3
Table
Table 4. Tanning process for pickle and re-tanning process for wet blue.
Table 4. Tanning process for pickle and re-tanning process for wet blue.
ProcessAmount (o.w.m)
Leather		ChemicalsTemperature (°C)Time (min)Remarks
Re-tanning of Wet Blue (50 g)
WashingPlenty of waterWaterRoom Temperature20 min-
Neutralization2
50		Neutralizer N
Water		Room Temperature2.45 h RotationpH 5.5–6.0 drain
Re-Tanning350 mLBark ExtractRT4–5 h Rotation-
Soaking-Bark ExtractRTOvernight (Without Rotation)-
Re-Tanning30Fatliquor, Formic Acid to adjust pH at 3.5RT45 min, RotationDrain
WashingPlenty of waterWaterRT20 min, RotationRe-tanned Wet Blue
Tanning of Pickle (50 g)
De-pickle250NaCl Solution Baume 6–7RT4 h RotationAdjust pH 2.5–3 using Formic Acid
1.5Sodium Formate to adjust pH at 22.5–3.0
4Degreasing Agent
0.5Sodium Carbonate
WashingPlenty of waterWaterRT30 min, RotationDrain
Tanning350 mLBark ExtractRT4–5 h, Rotation-
Soaking-Bark ExtractRTOvernight (Without Rotation)-
Tanning-Adjust pH 4 with Formic AcidRT45 min, RotationDrain
Re-TanningUsing Same Steps as used for Re-Tanning of Wet Blue
Table
Table 5. Physical properties of extracted tannins in comparison with commercial tanning agent (mimosa).
Table 5. Physical properties of extracted tannins in comparison with commercial tanning agent (mimosa).
Bark with Article DescriptionTensile Strength
IUP 6 (N/mm2) 		Breaking Force IUP 6 (N) % Elongation IUP 6 (%Age)Shrinkage Temperature (°C)
A. nilotica pickle381488580 °C
A. nilotica wet blue4312352>100 °C
E. globulus pickle351387475 °C
E. globulus wet blue3611162>100 °C
Wet blue reference228435>100 °C
Mimosa pickle421545876 °C
Mimosa wet blue3813254>100 °C
Table
Table 6. Identification Tests for the presence of total phenolic contents (TPC) in bark extracts.
Table 6. Identification Tests for the presence of total phenolic contents (TPC) in bark extracts.
No.TestA. nilotica
(S1)		E. globulus
(S2)		Total Phenolic Content (TPC) Presence
1Ferric Chloride test++++++Positive
2Lead Acetate test++++++Positive
3Gelatin test++++++Positive
+++ (identified).
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Khan, S.R.; Khan, S.M.; Khan, R.U. Eco-Friendly Valorization and Utilization of Plant Waste as a Source of Tannin for Leather Tanning. Sustainability 2023, 15, 3884. https://doi.org/10.3390/su15053884

AMA Style

Khan SR, Khan SM, Khan RU. Eco-Friendly Valorization and Utilization of Plant Waste as a Source of Tannin for Leather Tanning. Sustainability. 2023; 15(5):3884. https://doi.org/10.3390/su15053884

Chicago/Turabian Style

Khan, Shahid Rehman, Shahzad Maqsood Khan, and Rafi Ullah Khan. 2023. "Eco-Friendly Valorization and Utilization of Plant Waste as a Source of Tannin for Leather Tanning" Sustainability 15, no. 5: 3884. https://doi.org/10.3390/su15053884

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