Chromium Shaving Upcycling: Sludge Valorization Research

A Further Valorization of Chromium Shavings and Upcycling of Sludge Thereof
Md. Nymul Islam 1, Al Mizan1,2,3, Tülin Deniz Çiftçi4, Gülşah Türkmen1
Bahri Basaran1,2
1
Department of Leather Engineering, Faculty of Engineering, Ege University, Turkiye
2
Graduate School of Natural and Applied Sciences, Ege University, Turkiye
3
Department of Leather Engineering, Khulna University of Engineering & Technology (KUET),
Khulna-9203, Bangladesh
4
Department of Chemistry, Faculty of Science, Ege University, Turkiye
Abstract
Leather tanning is an ancient protocol to convert animal skins into leather. Throughout the processes, a
considerable amount of solid and liquid waste is produced. The disposal of these wastes poses many
challenges and is a major threat to the environment in many countries across the world. The majority of
the solid wastes including shavings, trimmings, and sludge, are dumped into the environment and bring
about major concerns to the environment if properly not treated. Commercially, many producers are
extracting amino acids from Cr shavings to mitigate this bulk amount of waste and contribute to a
circular economy. As the Cr shavings generated after the mechanical adjustment of thickness in leather
manufacturing contain a high amount of protein, it could be properly utilized in different sectors when
obtained. Currently, Cr shaving is being hydrolyzed with alkali process to produce collagen hydrolysates
as a value-added product namely amino acid to use for sustainable agriculture as a biostimulant.
However, after the process, it produces 15-25 % of Cr containing sludges that need to be managed after
recovery. Having separated the inorganic parts in the given percentage, which is composed of chromium
and other neutral salts, disposals will possibly be reduced and thus lower the waste processing costs
which is the best practice ever known. This research aims to optimize the pH conditions for optimal
chromium recovery and to search the viable options to reuse the use of chromium (Cr) from Crcontaining sludge deposited after amino acid extraction. In the study, the collected sludge sample was
oven-dried at 105°C, ground, and adjusted the pH with a range between 3-12, filtered subsequently
determined the Cr concentration in both filtrate and residue with atomic absorption spectroscopy (AAS).
Results proved that after separation and redissolving of chromium from the sludge masses would
possibly be reutilized in different processes as a whole or a part of the combination, that is upcycling of
Cr shaving waste. This inter-valorization of Cr sludge could pave the way for resource recovery and
sustainable waste management. It is a waste-to-wealth economy model including green manufacturing
which would help stop or slow down man-made disasters.
Keywords: Leather industries solid wastes, Chrome-containing sludge, Amino acid extracted sludge,
Chromium recovery, Upcycling.
Introduction
Tanning is a significantly important process of converting putrescible hides and skins to
upgrade the raw hides/ skins into non-putrescible leather. Animal skins are the main raw
material for leather processing. The raw skins are collected after slaughtering of the animals for
meat production. Hence, leather is a revalorized product that is upcycled from the by-product
of meat industries to protect the valuable environment as well as contribute to a huge economic
circulation. According to statistics, the global leather goods market was estimated at USD 419.0
billion in 2022 and is anticipated to grow at a CAGR of 6.5% from 2022 to 2030. The global
leather goods market is expected to reach USD 693.4 billion by 2030
(https://www.thebrainyinsights.com). However, leather processing includes several processes
and steps such as pre-tanning, tanning, and post-tanning that engage a tanner to use different
types of chemicals to convert the rawhide/ skins into usable leather. In leather processing, 1
metric ton of raw hides/ skins produces 200- 250 Kg of finished leather; the rest is waste
accounting for 750 – 800 Kg (Muralidharan, et al., 2022). The waste mainly includes raw
trimmings 10- 15 %, limed fleshings 20 – 35 %, Cr shavings 15 – 25 %, and others on a weight
basis (Muralidharan, et al., 2022). Among them, Cr shaving leather waste is known as tanned
waste and contains Cr inside it, when the tanning is carried out by basic chromium sulfate
(BCS). Commonly, more than 90 % of leather is produced by using the BCS for its unparalleled
strength properties which generate a huge amount of Cr-containing shaving wastes during
production (Covington, AD 2009). In the United States, the leather industry generates nearly
60,000 metric tons of chromium-containing waste annually, and globally, this figure is
approximately 10 times higher. For most of the twentieth century, the conventional practice
was to either apply this waste to land or dispose of it, despite being costly and environmentally
unfriendly. This approach is illogical because this waste material could be repurposed.
Furthermore, as suitable landfill sites become scarcer, and transportation costs rise, disposal
expenses are increasing. In the past, chrome shavings were utilized as fertilizers, with the
fertilizer producers covering the waste and transportation costs. Nowadays, tanners often bear
the costs of transport and disposal (Cabeza et al., 1998).
Hence, it is noteworthy to consider the profit potential from these solid chromium-containing
tannery wastes. Over past decades, leather researchers have dedicated considerable effort to
exploring the reuse of leather waste. Before 1970, reports predominantly focused on
applications not requiring extensive pretreatment of the tanned wastes, such as the production
of insulators, building materials, fibrous sheets, and shoe soles. Additionally, a method for
making paper was introduced to create substitutes for both leather and paper (Mu et al., 2003).
Between 1970 and 1993, numerous publications and patents concentrated on hydrolyzing
CCLW to recycle amino acids and peptides for use in feeds and fertilizers (Alves Dos Reis and
Beleza, 1991; Ohtsuka, 1973; Taylor et al., 1992, 1993). Commercially, many producers are
extracting amino acids from Cr shavings to address this substantial amount of waste and
contribute to a circular economy. However, this process results in 5 - 15% of Cr-containing
sludges that need management after recovery. According to the authors knowledge, there is no
research work has been published regarding the utilization of the sludge produced after amino
acid production practically. This research aims to recover Cr from the sludge and propose an
approach for viable options to reuse chromium (Cr) from Cr-containing sludge deposited after
amino acid extraction. Therefore, implementing a secondary valorization process to further
maximize the utilization of chrome shaving wastes is essential.
Materials and Methods
Sample Preparation
The chromium (Cr) sludge produced after the alkali treatment of chromium shaving leather
wastes in amino acid production plants was collected for the experiment. The collected sludge
sample was dried at 105°C until there was a weight difference of ≤0.02 to remove all of the
moisture from the sample followed by precise weighing and subsequent grinding to achieve a
powdered form.
Optimization of pH and Cr content analysis
The pH meter was calibrated using precision buffer solutions. To adjust the desired pH,
solutions of 0.1M NaOH (Merck) and 0.1M H2SO4 (Merck) were prepared. Samples, each
weighing 1 g and spanning a range of pH values from 3 to 12. Subsequently, these samples
were shaken for 24 h to achieve an equilibrium pH. The resulting mixture was then filtered to
separate the solid residue from the filtrate. Each of the sample were replicated twice in this
study. The obtained solution was taken for Cr determination via Atomic Absorption
Spectroscopy (Varian 220 FS model).
The residues obtained after filtration were dried at 105°C in an oven. 0.1000 g of each sample
was taken for digestion. Placing it into a 100 ml beaker, 10 ml of HNO3 and 3 ml of H2O2 were
added directly. A watch glass was placed upon the beaker. The beaker was then set on a hot
plate and the sample was allowed to boil until dryness. 0.5 ml of HNO3 and distilled water were
added as needed, maintaining a lower pH to prevent precipitation of chromium as hydroxide.
Additionally, the solution was diluted to 25 ml with distilled water and filtered. Subsequently,
AAS analysis was conducted to determine the chromium content in the solid.
Findings and Discussion
Quantification and Analysis of Sludge Surface pH
To determine the surface pH of the sludge, the identification of equilibrium pH plays a
substantial role. With the determination of equilibrium pH (Figure 1), the surface pH was
specified as 9.2 shown in Figure 2. Surface pH facilitates characterization of the sludge.
Δ pH = Equilibrium pH - Initial pH
F gure 1. Equ l br um pH vs n t al pH
(Eq.1.)
F gure 2. Surface pH of the sludge
Cr recovery from the sludge
Since the experiment aimed to optimize the pH conditions for the recovery of chromium (Cr)
from sludge, AAS analysis of the filtrate revealed a substantial increase in chrome recovery at
pH 3, as indicated by a pronounced upward trend in the graph at Figure 3. The maximum
amount of chromium recovery was noticed at pH 3, reaching 3200 mg/kg which suggests that
at lower pH levels, chromium is more likely to be in its ionized form, indicating the highest
recovery efficiency. In the pH range of 5 – 7, a stable Cr content was found. However, in pH 8
to 12, fluctuations in Cr content were noted, ranging between 1200 to 1400 mg/kg. Notably, a
distinct downward trend was observed specifically between pH 4 to 7.
F gure 3. Concentrat on of Cr n the f ltrate between 3-12 pH range
Cr content of the digested residue
Fig. 4 shows the Cr concentration of the digested residue samples after the pH adjustments. The
residue at pH 4 exhibited the highest concentration of Cr (37082 mg/kg), while the residue at
pH 3 displayed the lowest concentration (29859 mg/kg), likely attributed to the notable
recovery of chromium in its filtrate which is in congruence with the revealed result as shown
in Figure 3. Cr concentration of all the other samples was found to be nearly identical that
confirms that pH 3 could be viable pH for the Cr recovery.
By adding the Cr concentration in the filtrate and the digested sludge, total Cr concentration of
the sludge was determined as 35869 ± 1846 mg/kg (Relative standard deviation was 5.1%,
n=20).
Chromium Conc. (mg/kg)
40000
35000
30000
25000
20000
15000
10000
5000
0
3
4
5
6
7
8
9
10
11
12
pH
F gure 4. Concentrat on of Cr n d gested sludge
Conclusion and Recommendations
This research seeks to optimize the pH conditions to achieve optimal chromium recovery and
explore viable alternatives for reusing chromium (Cr) extracted from Cr-containing sludge
generated after amino acid extraction from Cr shaving waste. The highest recovery of Cr in
both filtrate and residue was observed at distinct pH levels. This experiment not only establishes
a foundation for future experiments on the reuse of Cr as a tanning agent but also implements
a secondary valorization process, thereby maximizing the utilization of chrome shaving waste.
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