Contemporary and future direction of chromium tanning and management in sub Saharan Africa tanneries

Journal Pre-proof Contemporary and future direction of chromium tanning and management in sub Saharan Africa tanneries R.O. Oruko, R. Selvarajan, H.J.O. Ogola, J.N. Edokpayi, J.O. Odiyo

PII:

S0957-5820(19)30408-2

DOI:

https://doi.org/10.1016/j.psep.2019.11.013

Reference:

PSEP 1987

To appear in:

Process Safety and Environmental Protection

Received Date:

9 March 2019

Revised Date:

13 May 2019

Accepted Date:

14 November 2019

Please cite this article as: Oruko RO, Selvarajan R, Ogola HJO, Edokpayi JN, Odiyo JO, Contemporary and future direction of chromium tanning and management in sub Saharan Africa tanneries, Process Safety and Environmental Protection (2019), doi: https://doi.org/10.1016/j.psep.2019.11.013

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Contemporary and future direction of chromium tanning and management in sub Saharan Africa tanneries *Oruko R.O1., Selvarajan R2, Ogola H.J.O.2, Edokpayi J.N.3 Odiyo J.O3. 1

Department of Ecology and resource management, University of Venda, Private Bag X5050,

Thohoyandou, 0950, South Africa 2

Department of Environmental Sciences, University of South Africa, Florida Campus, South

Africa 3

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Department of Hydrology and Water Resources, University of Venda, Private Bag X5050,

Thohoyandou, 0950, South Africa.

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*Email of corresponding author: [email protected]

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Abstract

In sub-Saharan Africa, chromium tanning during leather processing constitute one of the

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significant sources of large amounts of hazardous solid and liquid waste. The release of chromium with high poisonous quality and portability still remains a big concern for any ecosystems. Poor or improper management of chromium-rich wastes can result in irreversible damage to the

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environment and constitute a public health hazard. With an increased public concern, strict legislative control and ecological awareness, there is increased interest in eco-friendly technologies to minimize the production and management of chromium wastes from leather

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industries. In line with these emerging paradigms of environmental responsibility and sustainable development, the focus of this review is to explore contemporary sustainable alternative tanning techniques available globally that can be applied to partially and/or completely replace traditional chromium tanning commonly used in sub Saharan Africa. In detail, this paper critically highlights on the emerging body of knowledge and research on chromium minimization, recycling and/or re-

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use of chromium waste in the context of applicability and legislative framework to make tanneries in sub Saharan African countries be eco-friendly and competitive in global leather market.

1.0 Introduction

In the last two decades, countries in sub Saharan Africa (SSA) have been experiencing tremendous economic growth, with the GDP projected to rise by an average of 6% a year until

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2023 (ADB, 2018; Silva and Gurría, 2016). Closely related to the overall rate of economic growth, is the increased investments and performance in agro-processing sector. Leather processing is one of the sectors that has benefited to the aforementioned initiatives, partly due to shift from exporting raw unprocessed hides and skins to semi-processed and finished leather goods (Blein et al., 2013; FAO, 2015; Kiraye et al., 2019; Mwinyihija and Killham, 2006; A Teklay et al., 2018). Globally, leather is one of the most widely traded commodities with an estimated global trade value of approximately $100 billion per year; with a demand for leather and leather products projected to continue to grow faster than supply (FAO, 2015). It is estimated that Africa owns a

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fifth of the global livestock population, but only accounts for 4% of world leather production and 3.3% of value addition in leather (FAO, 2015; Silva and Gurría, 2016). However, with increased

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investment and improvement in institutional environment, there has been a sudden growth in SSA leather industry with emergence of many tanneries in the region to gain increased benefit from processed leather commodities. Such trends are expected to be observed in future as many SSA

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economies transforms their agro-processing sectors (Abtew, 2015; Kiraye et al., 2019; Mwinyihija, 2015). Despite the expected economic benefits from leather industry, the nature and the

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magnitude of wastes discharged from leather manufacturing and processing are of great environmental and public health concern in SSA and the whole world.

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Leather processing involves a series of operations ranging from pre-tanning (beamhouse), tanning and finishing steps that transforms raw hides and skins into final leather of desirable properties for different applications (Subramani et al., 2003). Among these operations, tanning

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is the main operating step in leather manufacturing which stabilizes and converts the collagen into leather, making the leather soft, strong, water-resistant, heat-resistant, corrosion-resistant and chemically stable (Yao et al., 2017a). Among the available tanning techniques, chromium tanning is the most preferred process used in almost 80-90% of leather industries (Covington, 2004). However, chromium tanning produces large quantities of chromium-based waste (up to

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3.5-4.5% w/w chromium as Cr2O3) both in liquid and solid form as chromium sludge, chrometanned leather shavings (CTLSs), and chrome leather trimmings into the environment (Mottalib et al., 2015; Nigam et al., 2015; Saha et al., 2011). For example, it is estimated that a mediumsized tannery can discharge daily over 300 million cubic meters of waste liquor and tanning sludge with high levels of chromium (Kanagaraj et al., 2015; Sundar et al., 2011). This has made the leather industry to continue gaining negative image in society with respect to its pollution potential. Chrome wastes has strong impact on the environment due to the negative effects on ecosystems and public health, and the high cost involved in treatment process. There has been enormous

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pressure from the various pollution control bodies to regulate and minimize the amount of chromium pollution generated from the leather processing industries (European Parliament and Council, 2000; USEPA, 2011, 2005; WHO, 2008). Therefore, these wastes should be treated adequately and disposed in a way that comply with all the statutory environment requirements. Due to the acknowledged hazards and pollution potential of leather production, the wet processes such as beamhouse and chromium tanning are currently being discontinued in most European countries and the U.S.A, and the operations being moved to developing countries including Africa, Asia and Latin America where environmental monitoring is not stringent (BREF, 2008; Oruko et

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al., 2014). In addition, initiatives such as End-of-Life Vehicle Directive 2000/53/EC by European Union are being instituted to reduce Cr contamination from motor-vehicle upholstery leather in a

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bid to help protect health and environment (European Parliament and Council, 2000).

In sub Saharan Africa, majority of tanneries are still practicing traditional methods of chrome

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tanning, where the uptake of chrome by pelt is, generally in the range of 50-60% (Covington, 2007). Results of this conventional practice produces large amount of chromium and other

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chemical wastes, which need proper remediation methods to reduce or minimize the level of chromium in the effluent. However, due to low profit margin, most of the tannery industries have not invested in effective treatment system for proper disposal of chromium and other solid wastes.

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Consequently, tannery solid wastes and sludge from wastewater treatment plants are still disposed of in landfills or dumpsites, under the assumption that the dominant species in the tannery waste would be the thermodynamically stable Cr (III) species (Ahamed and Kashif, 2014;

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Belay, 2010). However, this assumption is untenable and not based on scientific findings as wastes still end up polluting the surrounding ecosystems (Belay, 2010; Oruko et al., 2014; A. Teklay et al., 2018). In recent years, many tanneries in sub-Saharan Africa faces economic burden and lack of investment for the efficient waste treatment system. Besides this, stringent enforcement of the environmental regulations are forcing some of the tanneries to shut down

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permanently due to their inefficient or failure to meet the strict guidelines set for chromium and other wastes disposal (Kanagaraj et al., 2015; Minas et al., 2017).

In the last three decades, environmental and public health impacts of chromium wastes from tanneries have been a subject of extensive scientific and technical discourse. There has been increased interest in eco-friendly technologies to minimize the production and management of chromium wastes from leather industries to ensure process safety and environmental protection (Shaikh et al., 2017; Elumalai et al., 2014; Kanth et al., 2009; Kim et al., 2013; Zuriaga-Agustí et

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al., 2015). Research interest has centered largely on the development of alternatives to chromium tanning and management in order to refrain from the pollution of chrome tanning. Attempts on using chrome free tanning agents such as metal salts (Bacardit et al., 2014; Crudu et al., 2010; Yu et al., 2017; Zengin et al., 2012), synthetic tannins (Beghetto et al., 2019; Gao et al., 2019; Li et al., 2009; Sundarapandiyan et al., 2011), natural tanning agents (Marsal et al., 2017; Pinto et al., 2018; Sebestyén et al., 2019) and nano-tanning agents (Liu et al., 2016; Ma et al., 2014; Pan et al., 2017) have revealed various advantages over chromium tanning, which includes reduced environmental impact (low waste) and cheaper cost for white leather production. In addition to

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chrome alternatives, other strategies to manage chromium wastes include chrome recycling methods, recover/reuse methods, utilization of chrome waste and chrome less technologies.

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However, many countries in Sub-Saharan Africa are still facing serious challenges with their current management techniques for minimization, recycling/recovery and removal of chromium wastes and finding alternative sustainable strategies and sustainable development in the leather

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sector.

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Despite accumulating the knowledge on alternatives to chrome tanning and management practices in the leather sub-sector in the developed countries, there is limited reports on the application of these eco-friendly techniques from sub Saharan African tanneries. Furthermore,

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key informants survey among the tannery managers and owners in Nigeria, Cameroon, Zambia, Tanzania, Botswana, Malawi, Kenya, Ethiopia, Uganda, Zimbabwe, Mozambique and South Africa revealed that there is lack of awareness on the contemporary eco-friendly leather tanning

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techniques and sustainable management of chromium wastes. Therefore, eco-friendly tanning techniques and sustainable management of chromium wastes is already a global concern that African tanneries must address, not only to avoid future challenges in marketing their leather products, but also to ensure process safety and environmental protection. In line with limited knowledge on the emerging paradigms of environmental responsibility and sustainable

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development related to leather processing, main objective of this review paper is to explore available sustainable alternatives to chrome tanning and potential chromium waste management methods to suggest suitable technology for eco-friendly tanning process and management in sub Saharan Africa. In detail, the chemistry of chromium tanning, speciation and effects of Cr (VI) on ecology and human health, partial or complete replacement of chromium with other tanning agents, the minimization of chromium in tanning process, chromium recovery/recycling, and removal of chromium using different techniques are discussed.

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2.0 The chemistry of chrome tanning

Chrome tanning is the most adopted tanning method in leather industries for the conversion of animal hides and skins into useful artefacts. This is accomplished by continuous agitation of the skins/hides in large rotating drums containing chromium sulphate solution, allowing for the tanning agent to be evenly distributed (Figure 1). During chrome tanning process, the positively charged chrome tanning agent i.e. hydrated chromium ions binds with negatively charged collagen carboxyl group through a coordinate bond to fix the structure of collagen fibre, thereby giving

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tanning effects such as thermostability, chemical resistance and flexing endurance (Beghetto et al., 2013; Covington, 2009; Gao et al., 2019). However, this is a complex process involving

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simultaneous reactions dependent on various factors such as soaking time, temperature, pH, and the chemicals used for tanning process, which determines the final quality of leather (Black et al., 2013; Onem et al., 2017). Importantly, two main features of chromium chemistry enables it to act

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as a superior tanning agent. Its ability to form complexes that are of intermediate stability, enabling it to exchange coordinating ligands rather easily; and ability to form polynuclear complexes

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involving Cr-O-Cr bridges between collagen chains in the skin structure, resulting in the tanning

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effects (Bacardit et al., 2014; Covington, 2009).

Figure 1: Large rotating drums used for chrome tanning process in Africa

In detail, the chemistry of chrome tanning process involved hydrolysis, olation (metal ions form polymeric oxides in aqueous solution) and precipitation stages. To achieve this, chromium sulphate is first ionised in water to form Cr (III) ion (equation 1), further, chromium ions interact

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with available water molecules resulting in the formation of hydrated chromium ions [Cr (H2O)6]3+ as shown in equation 2.

2Cr3+ + 3SO42- ……………………………………... (1)

Cr2(SO 4)3

Cr3+ + 6H2O

[Cr(H2O)6]3+

………………….……..……………. (2)

During this process the pH is usually low to allow better penetration of Chromium (III) species into the opened up collagen lattice. However, the subsequent increase in pH makes collagen more

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reactive and makes the chromium species more basic with an increase in molecular weight and reactivity, that facilitates strong complexation between acidic side chains of collagen and

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chromium species via covalent bonding (Menderes, 2002; Onem et al., 2017). Further, the chromium nuclei undergo a self-polymerization process via hydroxyl bridges to form chromium collagen cross-linked complex bridges between the protein chains, which are thermodynamically

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stable (Figure 2).

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Collagen

bassification / +

[Cr(H 2O) 6

hydrolysis

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O-

]3+

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O

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Cr

O

Cr

O

Cr

+

O H S

O O

O O

Chromium–collagen cross-linked complex O

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polymerization

O

O O H O S O

H O Cr

O O-

H O

O

+2

Cr

Cr O O

O Cr

O

Cr O O O

n

Coordinate covalent bonding

Figure 2: Chemistry of chromium tanning explaining the hydrolysis and selfpolymerization process

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When the tanning process is complete, the chromium salts that have not reacted with the skin fibres end up as chromium wastes in the environment. Globally, it is estimated that 30-40% of the chromium salts used in the tanning process do not react with the skin/hides and is thus discharged in the form of spent tanning solution (Liu et al., 2016; Silambarasan et al., 2015; Sundarapandiyan et al., 2011). In addition, 75% of the solid wastes containing chromium (III) species such as chromium sludge, chrome-tanned leather shavings (CTLSs), and chrome leather trimmings are also released during leather-making process and disposed off in landfills (Sundar et al., 2011).

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Despite the chromium (III) species from these wastes showing higher stability in the environment, they may undergo various speciation and transformation in terrestrial ecosystem resulting into

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high chromium pollution.

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3.0 Fate of chromium species in terrestrial ecosystem and its pollution effects

Disposal of chromium containing wastes in terrestrial ecosystem (soil and water) poses a high

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environmental risk due to potential oxidation of chromium (III) to the hexavalent Chromium (VI) species. Generally, Cr (III) species are less toxic and mobile in given ecosystem; however, the conversion of Cr (III) to Cr (VI) becomes more toxic and highly mobile in nature that leads to an

(Kim and Dixon, 2002).

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acute irritation to plant and animal cells, which is also considered as level III toxins for humans

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3.1 Chromium cycle in natural environment

The overall chromium cycle involves three different chemical reactions in the soil and aquatic environments which include hydrolysis, oxidation-reduction, and precipitation (Al-Battashi et al., 2016). Chromium species are redox-reactive material, in general, they may react chemically with the different elemental fractions (oxidants/reductants) and be retained either permanently or

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temporarily in the ecosystem (Armienta et al., 1996). Fate of redox reaction with different elemental fractions may enhance or reduce the mobility and toxicity of chromium species (Kim and Dixon, 2002). However, the speciation and mobility of chromium species is also influenced by various edaphic and ecological factors, such as physico chemical properties, biological properties, and the climatic conditions (Ahemad, 2014; Bolan et al., 2014). The redox reactions associated with Cr speciation can be categorized into assimilatory and dissimilatory reactions. In assimilatory reactions, the metal substrate is involved in the metabolic functioning of the organism by acting as a terminal electron acceptor. In contrast, for dissimilatory reactions, the metal

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substrate has no known role in the metabolic functioning of the species responsible for the

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reaction and indirectly initiates redox reactions (Al-Battashi et al., 2016; Bolan et al., 2014).

Figure 3: Chromium cycle in natural environment

Oxidation of Cr (III) to Cr (VI) is primarily mediated by abiotic oxidizing agents such as Mn (IV),

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and to a lesser extent by Fe (III), while reduction process is mediated through both abiotic and biotic processes. In partial equilibrium with oxygen-soils, the soil sediments that contain Mnoxides and carbon play an important role in redox reactions with chromium species. Such reactions are thermodynamically spontaneous. Mn-oxides have high inner surfaces (e.g., tunnels in structure) and possess an exchange capacity, scavengers for heavy metals including chromium

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under neutral pH condition (Namieśnik and Raba Jczyk, 2012). Cr (III) oxidation in the soil is directly proportional to Mn (IV) oxides in the soil. For instance, soil with higher Mn oxide and lower organic load displays higher Cr oxidation than a soil with higher organic load and lower Mn content (Kim and Dixon, 2002). On the other hand, Chromate (Cr (VI)) can be reduced to Cr (III) in the environment where a ready source of Fe (II) or sulfide is available and this depends on microbial activity. Microbial reduction of Cr (VI) occurs in the presence of high organic matter and reduced sulfur as an electron donor (Ahemad, 2014; Dhal et al., 2013). It also depends on the available sunlight, which contributes to the reduction of iron and further blocks the formation of hydrogen

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peroxide that favors the reduction of Cr (VI) species (Markiewicz et al., 2015). The interesting aspect of the chromium cycle in the soil is that oxidation and reduction can take place simultaneously. For example, in the same ecosystem some of the Cr (III) species will be oxidized while some of the Cr (VI) will be reduced (Dhal et al., 2013).

During the redox reactions, redox potential and pH plays a critical role in the interconversion of chromium species. Cr (VI) exhibits high redox potential during the acidic conditions, which allows the organic matter and other inorganic compounds to reduce the Cr (VI) to Cr (III) by donating

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electrons. Following the conversion, thus Cr (III) usually form complexes or chelates, and may enter minerals, where they substitute iron or aluminum to be immobilized. While at the alkaline

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pH, the oxidation character of Cr (VI) is less effective and has a tendency to continue the same state rather than be reduced (Markiewicz et al., 2015) and it may migrate. Interestingly, the reduction of Cr (VI) by abiotic reductants such as Vitamin C and nano-materials (UV/TiO2) has

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been found to be quite effective in the entire pH range. Besides pH, the availability of other anions in the ecosystem may restrict the potential adsorption, which increases the chromium mobility

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(Namieśnik and Raba Jczyk, 2012). While migrating in the soil, it reaches the rhizosphere and forms a complex with root exudates, which further enhance the mobility of chromium species in the soil. In addition, primary metabolites such as amino acids and carboxylic acids in the roots

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may facilitate the Cr adsorption by plants and retention preferably in the root system. However, there is no conclusive evidence of the role of Cr in plant metabolism (Gomes et al., 2017). The occurrence of microbes in the soil and aquatic systems considerably reduce the chromium levels

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by involving both aerobic and anaerobic reduction process. However, the mechanisms for Cr (Vl) reduction by these microbes is not well known. It may be part of a detoxification mechanism that occurs intracellularly in microbial system. Alternatively, the chromate may be utilized as a terminal electron acceptor as part of the cell's metabolism. A third possibility is that reduction is an extracellular reaction with excreted waste products such as H2S (Nigam et al., 2015; Upadhyay

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et al., 2017).

Overall, the oxidation of Cr (III) to Cr (VI) in the environment is quite difficult as compared to the reduction of Cr (VI) to Cr (III). Compared to Cr (III), aqueous hexavalent chromium (Cr (VI) is the most oxidized, mobile, reactive and toxic form of Chromium with no adsorption in most sediment at pH > 7. Environmental conditions such as higher pH value may favour the oxidation while lower pH value favors reduction process (Al-Battashi et al., 2016; Mandal et al., 2011). Chromium waste from leather processing, pose substantial disposition trouble to human wellness and the

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environment (Sarker et al., 2013). The presence of Cr (III) and its salts in the poorly disposed sludge and solid wastes in the sub-Saharan African environments are an inconvenience for their safe reuse. Instead, they are an additional costly factor sustainably dispose of beside being a real threat to environment (Mandal et al., 2011).

3.2 Ecological effects of chromium wastes Chromium enters the environment through both natural processes and human activities. Each ton of processed leather generates more than 0.12kg of Cr pollution to the environment (Fei and Liu,

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2016). Leather tannery effluents and open dumpsites are choked with many process chemicals like chromium, which has direct impact on the upstream (discharge point) and downstream (receiving point) treatment processes (Ganesh and Ramanujam, 2009). The sub-standard quality

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and poor management efficiency of many tanning facilities increase the risk that makes chromium waste reaches the environment. Chromium can affect the air quality through buffing process,

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which eventually can lead to water or soil contamination. These damages both biotic and an abiotic component of an ecosystems. Studies have shown the contamination of terrestrial aquifers

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such as lakes, wetlands, rivers, and streams caused by tannery chromium effluents (Burbridge et al., 2012; Montalvão et al., 2018). Those impacts of tannery chromium effluents on aquatic organisms have shown teratogenicity in Paracentrotus lividus and Sphaerechinus granularis (sea

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urchin species) and reduced growth in green microalgae Selenastrum capricornutum (Oral et al., 2007). Other consequences of tannery chromium effluent pollution effects include genotoxicity in onions (Allium cepa), toxic effects in Daphnia magna, Ceriodaphnia dubia and Hyalella azteca

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and mutagenic activity in Salmonella/microsome (Tagliari et al., 2004). In regards to vertebrates, harmful effects from exposure to water containing tannery chromium effluent have been observed in fish (Aich et al., 2015; Nagpure et al., 2015), birds (de Souza et al., 2017) and mammals (da Silva et al., 2016; Guimarães et al., 2017; Rabelo et al., 2016; Siqueira et al., 2011). A recent study from Montalvão et al., (2018) confirmed that tannery chromium effluents had negative effect

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on amphibians in both pre- or post-metamorphosis life cycle. At this point, hexavalent chromium is considered carcinogenic only to animals in certain circumstances; however, in general chromium is currently not classified as a carcinogen and is fairly unregulated.

According to WHO, the typical chromium levels in most fresh foods are supposed to be low. However, in recent years chromium complexes have been detected in vegetables, fruits, grains, cereals, eggs, cheese, brewer’s yeast, organ meats, molasses, nuts, certain spices and fish between the concentrations of 20 and 520 μg/kg respectively (Edlira et al., 2019). Consequently,

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these levels are above the mean daily dietary intake requirements for human beings. In general, the mean chromium intake of adult men was estimated at 33 μg/day (range 22 to 48 μg/day), and the intake for women was 25 μg/day (range 13 to 36 μg/day) respectively. Mean chromium intake was approximately 15.6 μg/1,000 kcal. The chromium content of daily diets, designed by nutritionists to be well balanced, ranged from 8.4 to 23.7 μg/1,000 kcal with a mean of 13.4 μg/1,000 kcal. The exposure specifically to chromium (VI) compounds has been difficult to quantify, because specific forms of chromium are seldom identified in exposure studies (Edlira et al., 2019). Chromium VI is the most dangerous form of chromium and may cause health problems

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including: allergic reactions, skin rash, nose irritations and nosebleed, ulcers, weakened immune system, genetic material alteration, kidney and liver damage, and may even go as far as death of

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the individual. Thus, hexavalent chromium has been designated as a priority pollutant in human beings by USEPA (Edlira et al., 2019). This is drawing health concerns about food crops and

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animal pastures grown near tannery-based chromium dumpsites in Africa.

In Africa, its common to see food crops such as kales, spinach, banana, sugarcane, maize planted

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next to dumpsites of tanneries. During the dry period, farmers practicing urban dairy farming access such sites to secure pasture for their livestock. A study by Oruko et al., (2014) in one of the dumpsites in African tannery, recorded a total Cr level of 2633.38 mg/kg in the soil far above

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the WHO guideline value of 0.1 mg/kg. This pose serious environmental and health risk, since Cr (VI) occurrence at such sites has not been confirmed. Moreover, it is unknown if biomagnification of the toxic Cr (VI) along the food chain due to tannery wastes in the dumpsites and effluents

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contaminating water sources and soils.

3.3 Potential human health effects of chromium The health effects of chromium tannery waste on the human population is now a global concern (Santa Mitra, 2016). Human populations are exposed to Cr species in the environment through

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two routes; non-occupational exposure via ingestion of chromium-containing food and water, while occupational exposure is via inhalation (Nigam and Shukla, 2015). In general, chromium species enters into the eukaryotic system and induce spontaneous reactions with the intracellular reductants such as ascorbate and glutathione, generating the short-lived intermediates Cr (V) and/or Cr (IV), free radicals and the Cr (III) end-product (Focardi et al., 2013). Within the cytoplasm, the short-lived intermediates is oxidized to Cr (VI) that easily combines with DNA– protein complexes and altering their normal physiological functions (Ateeq et al., 2016; Sun et al., 2015) and damaging their DNA (Reynolds et al., 2009) resulting in genotoxic and mutagenic

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effects. They also block essential functional groups, displacing other metal ions, or modifying the active conformation of biological molecules (Nigam et al., 2015) leads to liver damage and pulmonary congestion and causes skin irritation, gastro intestinal problems resulting in ulcer formation. Furthermore, Cr (VI) species can be able to accumulate in the placenta and damage the development of fetus resulting in birth defects and decrease in reproductive health (Banu et al., 2017).

Several studies have reported on the health effects of tannery workers. For instance, about half

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a million residents of the Bangladesh capital city Dhaka are at risk of contracting serious illness due to chemical pollution from tanneries near their homes (Salam and Billah, 1998). The report

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further identified that l over 10,000 workers at the tanneries suffer from gastrointestinal, dermatological and some other illnesses that could be attributed to the tannery pollution. The most affected area is Hazaribagh, a community in the southeastern part of Dhaka City where 270

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tanneries are located on 25 hectares of land. Similarly, Rastogi et al., (2008) investigated the occupational health risks among the tannery workers at Kanpur, India and revealed that higher

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morbidity (40.1%) among the chromium exposed workers than the control (19.6%) respectively. Further the study showed that the respiratory diseases were predominant than the gastrointestinal, which was observed in control groups. Prolonged exposure with chromium

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species can cause serious allergic reactions and dermatitis in individuals (Nigam et al., 2015). A recent study from tannery workers and residents around the tanneries at Dhaka, Bangladesh revealed that the most prevalent sickness were conjunctivitis (7.7%) and Dermatitis (6.7%) (Uddin

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and Ahmed, 2018). However, with the strong regulatory rules the prevalence of chromium allergy is decreasing in developed countries like Denmark (Alinaghi et al., 2019). Whereas the scenario of Sub-Saharan Africa is different from the other countries. In 2014, according to a senior officer at Leather division in Kenya, two tannery workers died during the cleaning process of chromiumblocked tunnel in one of the tanneries in the country. Such information is rarely documented;

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therefore, there are few documented cases of health effects due to poor management of chromium tanning agent and its waste disposal. Thus, countries like Ethiopia, Kenya, Namibia, Tanzania, Nigeria, Ghana, Zambia, Zimbabwe and others in sub-Saharan Africa are silently experiencing negative risks related to the environmental pollution problems of liquid and solid chromium tannery wastes. This is subject that does not receive adequate attention during the development of tannery industries. Furthermore, very little investment has been put into modern alternative chromium tanning techniques compared to conventional tanning techniques in sub-

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Saharan Africa (Birhanie et al., 2017). Therefore, attention towards alternative chromium tanning in the sub-region needs detailed exploration. 4.0 Strategies to reduce chromium pollution from tanning industry

4.1 Complete or partial replacement of chromium as a tanning agent Although integral to the production of high-quality leather, contribution of chrome tanning as toxic pollutant cannot be overlooked. Environmental concerns associated with the discharge of unused

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chromium and the potential problems associated with the safe disposal of Cr containing solid wastes are becoming important. Management of chromium wastes involves a complicated

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process for waste (sludge) treatment due to their high biochemical oxygen demand (BOD), chemical oxygen demand (COD), total solids (TS), dissolved solids (DS), suspended solids (SS) and chromium salts content in wastewater that is costly to the leather industry (Joseph and Nithya,

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2009; Tariq et al., 2009) that . Furthermore, expensive disposal process of leather scraps such as chrome shavings, splitting, trimming and buffing dust by dumping process are negative

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attributes (Hu et al., 2011). Therefore, the search for eco-friendly greener options to chrome tanning processes has become an issue for discourse and experimental research globally (Bacardit et al., 2014; Beghetto et al., 2019; Cao et al., 2013; Gao et al., 2019; Li et al., 2012; Rao

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et al., 2002; Sundar et al., 2002; Sundarapandiyan et al., 2011; Thanikaivelan et al., 2005; Yao et al., 2018). In last two decades, numerous research studies have been ongoing for to explore strategies to either replace chromium tannin partially or completely (Table 1). Here, the attributes

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of some of the tanning alternatives and the potential application for complete replacement of chromium or in combination tanning in the leather industry is discussed.

4.1.1 Use of alternatives to chrome tanning In the last two decades, several compounds such as aluminum (III) salts, iron (III) salts, sodium,

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silicates, titanium (III) sulfates, potassium titanyl oxalate, zirconium (IV) salts, bi-functional aldehydes (such as glutaraldehyde), synthetic resins, polymers and nanocomposites as potential alternatives for chromium tanning have been reported. The positive attributes and shortcomings hindering wider applicability of some of the alternatives are summarized in Table 1. Inorganic metal tannages such as aluminum, zirconium, titanium and silicates has shown promise as alternatives for complete replacement of chrome in tanning. For example, aluminum salts that are abundant in nature, easily available, non-toxic and gives a better performance than other metal tanning agents in terms of physicochemical stability, organoleptic properties, and resistance to

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the proteolytic activity (Covington, 2006; Gaidau, 2013; Xiao et al., 2012). Attempts to replace chromium salts in the tanning process of hides with titanium are presented in the literature (Crudu et al., 2014; Zuriaga-Agustí et al., 2015). However, most the metal tannage systems results in either inferior processed leather quality related to undesirable shrinkage, lightfastness, discoloration and process limitations in terms instability, low solubility and in addition to being expensive or toxic in nature (Covington, 2006; Crudu et al., 2010; Gaidau, 2013; Onem et al., 2017; Pomelli et al., 2017; Thanikaivelan et al., 2005).

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Similarly, to achieve chrome free tanning various small molecules have been experimented including urea, melamine, phenol, formaldehyde, and its condensed products on the stability of

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the collagen in leather to reduce pollution level (Chen et al., 2011). Further, the effects of urea, surfactant and salt additive environments for micelle of synthetic tanning materials in the stability, conformation, and geometry of the collagen have been studied using hydrodynamic and

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thermodynamic techniques (Krishnamoorthy et al., 2013). These molecules attribute high tensile strength, elongation modulus, low water absorption, mild shrinkage, high surface hardness,

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elongation at break and volume resistance which could be considered for replacing chromium (Tillet et al., 2011). At the same time, these molecules and their derivatives pose occupational hazard and harm to human health (Cao et al., 2009). For instance, (Zengin et al., 2012)

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synthesized organic based titanium tanning agents using melamine as an alternative to chrome tanning, which was found to produce light colored and chromium-free leather with the closest properties to chromium leathers. However, the prolonged exposure in humans may cause

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irritation and pain in chest, coughing and difficulty in breathing. Polyaldehyde tannage system (wet-white tannage) is yet another tannage that can produce chrome-free leather by cross-linking the NH2 groups of collagens with glutaraldehyde. However, this tanning process is unable to bring about a full tannage with a broad spectrum of properties of leather sellable on the market (Bacardit

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et al., 2014).

An effective alternative to chromium must be naturally abundant, easily obtainable, low priced, eco-friendly and offering competitive tanned leather performances, in addition to being safe. However, most of the suggested alternatives still produce leather of comparable inferior quality in terms of shrinkage temperature, elasticity, softness, durability, heat resistance and other properties obtained through chromium tanning. In addition, some of the alternative tanning agents are not economically viable (Zhang et al., 2016; Zouboulis et al., 2012). Emerging insights into the molecular basis of chrome tanning reveals that different factors may be involved in the

14

stabilization of collagen producing changes in secondary, tertiary and quaternary structures of collagen conferring the desirable tanned leather properties (Covington, 2009). With expanding knowledge of tanning chemistry, innovative tanning techniques to completely or partially replace chromium needs to emerge to move the leather industry towards a green economy (Covington, 2007; Crow, 2019). Currently, combination tanning methods are also showing promise to overcome the problems associated a single tanning process using chromium alternatives comparable to chrome tanning process.

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4.1.2 Combination tanning process In searching for greener and commercially viable chrome-free tannage, combination tanning

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based organic substances such as vegetable-syntans, D-amino acids, aldehydes, biocatalysts and other syntans with inorganic metals, nanocomposites and biopolymers with promising results have become promising options (Beghetto et al., 2019, 2017; Cao et al., 2015, 2013; Gao et al.,

-p

2019; Li et al., 2009, 2012; Qiang et al., 2016; Saravanabhavan et al., 2004; Shi et al., 2019; Xiao et al., 2012). For instance, oxazolidines are saturated heterocyclic compounds of primary amino

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alcohols and formaldehyde with wide variety of applications including as a tanning agent. Results of tanning with oxazolidine, in combination with synthetic or vegetable re-tanning showed good physical strength and adequate smoothness, softness, fullness and flexibility, and moreover, no

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significant difference were identified between the two combinations (Li et al., 2009; Roig et al., 2011). A novel pickle less combination tannage method based on nano-SiO2 and oxazolidine have also been reported (Yan et al., 2008). This tanning process exhibited significant reduction

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in total solids (TS) and better biodegradability of organic compounds in the effluent. Leather tanned with oxazolidine-nano-SiO2 demonstrated good tanning properties in addition to a higher resistance to mold growth than conventional wet blue. Such matrix helped in eliminating the conventional pickling process and the related problem of total dissolved solids (TDS) associated with the effluent. Further, the matrix was established to have potential application in leather

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industry as an eco-friendly approach to chrome tanning.

Fathima et al., (2011) also reported a new combination tanning systems based on aldehydic derivative tetrakis (hydroxymethyl) phosphonium sulphate (THPS) with aluminium and silica. In the study, silica-THPS-aluminium tanning gave leather with high shrinkage temperature (Ts), good organoleptic and strength properties comparable to that of conventional chrome-tanned leather. In contrast to conventional tanning, THPS has very low toxicity, low recommended treatment level, rapid breakdown in the environment, no bioaccumulation and reduced risk to both

15

human health and environment (Fathima et al., 2011; Fathima et al., 2006; Shi et al., 2019). Similarly, to improve the shrinkage temperature Haroun et al., (2008) evaluated the vegetable pre-tannage followed by aluminum retannage and found a better result than the aluminum pretanning followed by vegetable tannage. A combination tannage of vegetable tannin and nanosilicate (Shi et al., 2013) or laponite clay (Shi et al., 2019) for wet-white leather manufacture has been considered as a promising alternatives to conventional chrome tannage due to natural resources and appropriate tanning properties. In contrast to THPS, these wet-white tanning systems are completely free of formaldehyde, a common problem associated with aldehydes

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based syntans. However, these wet-white tanning processes did not show absolute advantages

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in all the environmental impacts from LCA perspective (Shi et al., 2016).

Chrome free tanning using unnatural D-amino acids (L-Lysine) along with aldehydes has also been demonstrated (Krishnamoorthy et al., 2013). The process improved tanned leather

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properties such as physicochemical stability, organoleptic properties, and resistance to the proteolytic activity. The authors The incorporation of unnatural amino acids into proteins was

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found to be an important tool for understanding protein function such as structural support, engineering proteins and introducing useful building blocks and molecular scaffold for proteinbased heterochiral biomaterials in the future application (Annavarapu and Nanda, 2009). Although

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the D-amino acids (D-AA) - aldehydes processes result in significant reduction in total solids content and better biodegradability of organic compound present in the effluent compared to chrome tanning, these systems are hazardous to human health (Bacardit et al., 2014). In general,

1.

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the combination of tanning agents has its own advantages and disadvantages presented in Table

Various commercial combination tanning process have been patented for complete replacement of chromium in leather industry around the world.

Examples of such solutions include

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polyaldehydes, secondary amines and an aluminium salt based tanning agents in USA (Siegler, 1987), wet–white aluminium based product called RHODITAN and ecological dry–white leather in France (Gavend et al., 1991), organic-based tanning agents X-white by Lanxess and Swiss firm Clariant, wet–white procedure based on aluminium salts or vegetable and synthetic tanning agents and aluminium in Germany (Tonigold et al., 1990), synthetic organic tannage based on melamine formaldehyde resins supplementary crosslinked in situ with oxazolidinones in UK (Covington et al., 1997). However, there was no solid evidence and reports about the use of chrome-less technologies sub-Saharan African tanneries. Key informants interviews and

16

observation made in most tanneries indicate that most of them are practicing traditional chromium tanning. In these tanneries, total wet white tanning include combination tannage is yet to be embraced by the sector. This implies that there is still a lack of knowledge and research gap in this area and the attention of scientist and tanners in the region needs to be drawn to the more sustainable and greener options available.

4.1.3 Use of chrome-less (minimal chromium use) technologies In practise, if the alternative to chromium salts tanning for chrome-less tanning is not feasible,

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then technical advancements allowing for improved efficiency of “wet-blue” technology is the best option to make tanning process move closer to a sustainable and greener process. It is estimated

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that the uptake of chromium during conventional tanning process is about 50-70%; the low uptake generally attributed to nature of the collagen matrix and environmental conditions employed during tanning processes. In literature, there are several debates on chromium management and

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control measures that can potentially be employed individually or in combination to minimise chromium during tanning cycle (Dixit et al., 2015; Kanagaraj et al., 2015; Rao et al., 2002; Sundar

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et al., 2002). Therefore reassessment on some of available cleaner technologies targeting minimisation of chromium use in the tanning process has been attempted.

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Over the years, tanning process has evolved and >85% uptake of chromium salts can now be achieved through the use of additives that acts as high chromium exhaustion auxiliaries or aids. These chrome tanning auxiliary or additive are based on either long chain dicarboxylic acids,

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aromatic polycarboxylates, silicates of Mg or Al, polyacrylic acids, polyamides, mannich bases, protein hydrolysates, polyelectrolytes, polyester carbonyls, oxazolidines, polyhydroxyaluminium gels, heavy metal oxides and phosphate (Kanagaraj et al., 2015; Rao et al., 2002; Covington et al., 2015; Sundar et al., 2002). These compounds generally complexes with collagen matrix creating additional sites for the interaction of chromium (Covington, 2006). They also promote

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formation of chromium-chromium bridges that facilitates stronger collagen-chromium binding thereby improving the exhaustion level of chromium compared to conventional methods (Nashy et al., 2011; Thanikaivelan et al., 2005). Several tanning auxiliaries are currently available on the market such as oxazolidines (Sundarapandiyan et al., 2011),

hydroxymethyl phosphonium

sulphate (THPS) (Fathima et al., 2011), gallic acid (Silambarasan et al., 2015) and glyoxylic acid (Fuchs et al., 1993), fleshing-acrylate composite (FH) (Kanagaraj et al., 2015), fibrin hydrolysate nanocomposites (Kanagaraj et al., 2007), styrene/butyl acrylate co-polymers nanocomposites

17

(Nashy et al., 2011) and hydroxyl- or carboxyl-terminated hyper branched polymer (C-HBP) (Qiang et al., 2016; Yao et al., 2019, 2018).

Recently, Kanagaraj et al., (2015) reported that using 4% of (FH) used in the tanning bath resulted in chrome exhaustion of 91.2% in contrast to 65% achieved with conventional chrome tanning. High exhaust chrome tanning systems using biotannin, protein-based tanning agent prepared from fleshing waste, has also been demonstrated (Kanagaraj et al., 2015). Further, the process improved the exhaustion of chrome in pelt from 67 to 92% when used at 2% level during chrome

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tanning. Equally, Yao et al., (2019) also demonstrated that using of C-HBP dendrimer or hyper‐ branched polymer (HBP) with characteristic high solubility, reactivity, and chelation ability, can

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also improve up to 95.2% chrome uptake much higher than gallic acid (89.5%) and citric acid (79.3%) and control trial (69.3%). Further, the same study group demonstrated that the leather tanned with 3% chrome and 1.5% HBP possessed similar properties to the traditional tanning

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process using 6-8% chrome, proving the utility of hyper-branched polymer (HBP) for high chrome exhaustion during tanning (Yao et al., 2017b). Due to high chromium exhaustion levels achieved,

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the use of these additive or auxiliaries lead to reduced pollution loads of chromium in the effluent and less water consumption. For example, use of FH resulted in the reduction of BOD, COD, TDS, TSS to the level of 60, 43, 1 and 3%, respectively, with the method saving up to 90% of

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water in contrast to conventional process. These few examples illustrates the potential application of additives to improve chromium uptake and reduce pollution during tanning. However, In addition to exhaust aids, other strategies involving in-plant controls and process changes such as

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waterless tanning (Silambarasan et al., 2015), closed loop aluminium–chrome combination tanning (Rao et al., 2002; Sundar et al., 2002), two-stage tanning (Ramasami, 2001; Wu et al., 2014) and modifying tanning process environmental conditions such as pH, temperature, float, basicity, neutral salt concentration, drum geometry and speed (Covington, 2006; Onem et al., 2017; Sundar et al., 2002) have also been reported to improve absorption levels of chrome. A

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study by Silambarasan et al. (2015) reported that chrome tanning in the ethanol medium leads to a higher exhaustion (87% for pickle-based and 95% for pickle-less), with additional benefits of better chromium content, distribution and shrinkage temperature, and low chromium leaching in tanned leathers compared to conventional chrome tanning. The process also resulted in the reduction of COD, BOD and TS loads in the composite liquor by 14–26, 21–28 and 42–46%, respectively.

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Closed loop combination and two- or multi-stages tanning processes are examples of chrome liquor recycling strategy. In closed-loop aluminium and chromium combination tanning process, delimed skins are initially pickled with sodium sulphate and sodium formate at pH 3.0–3.5 prior to tanning using 0.5% aluminium and 6% basic chromium sulphate (Sundar et al., 2002). The spent chrome tan liquor is again pre-acidified with sulphuric acid to pH 3.0–3.5 and reused for pickling as a float before tanning using 0.5% aluminium and 6% basic chromium sulphate. During the process, spent tan liquor is quantitatively checked for the chromium and aluminium levels, and the cycle can be repeated several times until near total elimination of the discharge of chromium

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in the effluent is achieved. In contrast to other strategies, this method is a much greener and cleaner sustainable alternative to conventional tanning due to ability to reduce pollution load by

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90%, material economy in terms less water consumption (water saving from pickling and tanning operations by about 90%) and reduction in wet finishing chemicals up to 20% (J. R. Rao et al., 2002; Sundar et al., 2002; Thanikaivelan et al., 2005). Similar to closed loop combination tanning,

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two- or multi-stages tanning systems involve direct reuse of a spent tan liquor for the subsequent batch pickling and tanning to achieve up to 95% chromium exhaustion levels and 80-85%

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reduction of chromium in the waste effluents (Ramasami, 2001; Rao et al., 2002; Wu et al., 2014). In addition to material economy, these two processes can be implemented in existing systems without any major modifications in the production infrastructure. However, a viable eco-friendly

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tanning involving the discussed strategies will require a careful process audit, technical knowhow, personnel capacity and in-plant controls to ensure the desired material economy and environment

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management of chromium is effectively be achieved.

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Table 1: Advantageous and Disadvantageous of alternative chrome tannings and its combinations.

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Disadvantages 

References

Crudu et al., (2014), Peng et al., (2007), Roşca et al., (2008), Pomelli et al., (2017) Leather plasticity still Gaidau, (2013) lower than chrome tanned Covington, (2006) leathers. Avoids chrome contamination in tannery effluents and in wastewater treatment sludge







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Al (III) and Ti (IV) complexes







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iii)

Result in wet-white leather with microbial steadfastness, good elastoplastic properties. Chemical characteristics of the leather can be made similar to the wet-blue if subjected chrome salt tannage. Leather retain the desired features of the individual metal tannages, making an adequately tanned full, soft leather.

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Al-Zr-Mg heterocomplexes

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ii)

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Advantages 1. Metal salts and their heterocomplexes i) Titanium  Non-toxic, inert and non-allergenic  Gives leather with average Ts = 76.1oC

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Al(III) is not fixed to the Gaidau, (2013) collagen and is easily Covington, (2006) removed by water from Xiao et al., (2012) the product ( can be corrected by masking the metal molecular ions with a polyhydroxymonocarboxyl ligand.) Use of titanium (IV) salts in leather tanning require high levels of auxiliaries and the resulting plumpness of the leather limits its applications.



 v)

Fe salts / Fehydroxysuccinimi de complex

 



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Titanium (IV), zirconium (IV) and hydroxysuccinimi de

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vi)

vii)

Zirconium (IV) and Aluminium (III) salts and



  

Zirconium salts are costly Requires a large amount of alkali for subsequent neutralization. Highly unstable and have low solubility.

Onem et al., (2017) Roşca et al., (2008) Crudu et al., (2010) Fathima et al., (2006)

Iron-tanned leather loses strength and darkens on ageing Exhibits lower hydrothermal stability (these undesirable attributed can be overcomes using Fe in complex with synthetic tanning agents) Expensive due to high cost of Ti and Zr

Wenzel et al., (2010) Fathima et al., (2006) Tavani and Lacour, (2001) Crudu et al., (2015)







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 

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Does not pose any health or ecology hazards and its exhaustion rates are exceedingly high. Zirconium-tanned leather is usually fuller and firmer with pleasing white colour, good lightfastness, has a higher Ts (83 - 90°C) shrinkage temperature. Ts above 80°C. Able to produce an intermediate leather suitable for automotive upholstery. High exhaustion of iron salts can be achieved to <500ppm in the spent float at pH of 4.0.

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

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Zirconium / Zirconium oxychloride

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iv)

Less toxic to the environment (reduce Cr(VI) risk, chromium-free sludge and waste water and very low formaldehyde content) Increased assortment and colour diversity of leather Performance is comparable to chromium tanned leather Performance is comparable to vegetable tannins tanned leather.

21





Crudu et al., (2010)

It has non- Redwood et al., biodegradability, if the (2015) mixing is not properly Fathima et al., (2006)

Form colloidal aggregation state that is relevant for mineral tanning at optimum pH.

  

Reduce the COD and TDS loads high exhaustion of zinc is about 90%. Organoleptic and the strength properties of the garment leathers are generally comparable to those of conventional chrome tanned leather.

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iii)

Aluminium-silicon compounds (Tanfor T™ system Zinc, tannic acid and silica combination

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ii)

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achieved at preparation stage









the Fathima et al., (2011)

Drawbacks include lack of desired shrinkage temperature and strength properties in the leathers. Mineral salts precipitate at pH above their stability and does not produce good tanning. Zinc and tannic acid do not give the required fullness to the leathers Silica “tanning” is known to produce fluffy and soft leathers.

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phosphonium salts 2. Other inorganic compounds i) Silicon dioxide  Produces leathers with softness and and sodium fluffiness because of the gelling nature silicate of silica.

Fathima et al., (2006)

Bacardit et al., (2014)

Saravanabhavan et al., (2004)

3. Aldehydes and other compounds Tetrakis(hydroxy methyl) Phosphonium sulphate (THPS) and Al, Siica complexes

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i)

  



Produce white colour and fastness to light leathers. Shrinkage temperature of the leathers obtained is 86°C. Organoleptic and the strength properties of the leathers comparable to chrome tanned leather. Pickle less combination tanning system results in the reduction of COD and TS by 41 and 67%, respectively.

22

 

Increased cost of leather Fathima et al., produced. THPS is teratogenic, very (2011) toxic to aquatic organisms Li et al., (2012)

  

has low toxicity due to low recommended treatment level, rapid breakdown in the environment, No bioaccumulation and reduced risk to both human health and environment. Combination tannage give leather comparable to quality chrome-tanned leather. Leather tanned good softness and fullness, good physical/mechanical properties, sweat resistance and wash ability. Oxazolidine is bio-degradable. Environment friendly. Results in leather with good fullness, softness, smoothness, colour and general appearance comparable to chrome tanned leather Ts is more or less equal to chrome tanned leather

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iv) Oxazolidine I and II and synthetic or vegetable tanning agents

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

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v) Unnatural amino acids (D- amino acids and aldehydes)

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



not documented

Shi et al., (2019)

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iii) Fe-THPS combination

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 

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ii) Tetrakis(hydroxy methyl) phosphonium sulfate (THPS) and synthetic laponite clay nanoparticles

Presence of THPS in the combination increases the strength properties. leather with a Ts above 85 °C improves yellowing resistance and lightfastness but also enhances strength properties of the wet-white leather synergistic effects between THPS and Laponite to reduce HCHO contents

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

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

TPHS is expensive. Fathima teratogenic, very toxic to (2006) aquatic organisms



Single tannage has low Li et al., (2009) quality compared to Sundarapandiyan et al., (2011) chromium tannage.



Uses harmful aldehyde- Krishnamoorthy et al., (2012) based process

et

al.,

Krishnamoorthy et al., (2013)



Beghetto et al., (2016) Beghetto et al., (2017)

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Significant reduction in total solids contents (TSC) in effluents and organics improves biodegradability of wastes  Cheaper than chromium tanning (US$ 0.2207 vs 0.4074 per kg of goat skin) 4. Syntans and other synthetic compounds i) Active cross ACL is give non-toxic final products of  ACL technology is not yet Linking agents comparable quality to chrome tanned readily available and it is (ACL) leather. expensive initially. (Carbodiimides)  Significant reduction in waste disposal costs.  Possibility for recovery of leather scraps as secondary raw material for high value added product. ii) Di-(4,6 Produce leather with shrinkage DMTMM technology is not dimethoxy-1,3,5readily available. temperatures as high as Ts=870C. triazin-2-yl)-4 Chemicals consumption is reduced by methylmorpholini over 100 kg for 1000 kg of processed um chloride raw bovine hides with a saving of about (DMTMM) 23 euros.  Reduces processing time of up to 20 h.  Savings raise up to 96.00€ per ton of raw hides on power consumption. iii) Polyhydral  Treated leather show better oligomeric hydrothermal stability, fullness and Silsesquioxane physical strength (POSS) Improves chromium exhaustion in the containing spent liquor polymer  reduced the chromium content, nanocomposites COD, BOD in the effluent  POSS are non-toxic to environment

24

Beghetto (2019) Beghetto (2017)

et

al.,

et

al.,

Jia et al., (2019)

CHBP endow the leather up to 95% chrome uptake and higher thermal stability

v) Cage-like octa(aminosilsesquioxane) and tetrakis(hydroxy methyl) phosphonium sulfate (THPS)



THPS and POSS nanoparticles have synergistic effect in the combination tanning process. Process is optimized as 6.0% POSSNH2 combination with 2.5% THPS, results in tanned leather with Ts above 830C and thickness rate 44.6%.

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

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

Chen et al., (2007) Ibrahim et al., (2013) Yao et al., (2018) Qiang et al., (2015) Shrinkage temperature Gao et al., (2019) never reached 100%

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iv) Hyper-branched polymer (HBP) nanocomposites

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

4.2 Recycling and recovery of tannery chromium wastes Due to stricter environmental controls, chrome recycling/recovery and reuse have become compulsory in the leather industry towards management of unspent chromium present in the effluent from tanning. In practise, direct and indirect recycling of spent chrome liquor is widely practiced chromium recycling. As discussed previously, direct recycling methods entails the spent float being reused for the pickling or chromium tanning process. In contrast, indirect methods involves recovery of chromium from spent tanning and retanning baths for reconstitution for use in tanning process. Towards this, several techniques have been proposed and implemented

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ranging from chemical precipitation, coagulation, solvent extraction, membrane process, ion exchange and adsorption methods (Bhaumik et al., 2011; Cassano et al., 2007; Kanagaraj et al.,

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2008; Lee et al., 2003; Liu et al., 2002; Sun et al., 2014; Wionczyk et al., 2006). In general, both direct and indirect methods can achieve up to >90% removal of chromium from the liquor. However, the effectiveness of each method depend on the float collection or recycling/reuse

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technique and process conditions such temperature, pH and time, that need to be controlled in

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order to achieve chromium efficient recovery (Belay, 2010; Sarker et al., 2013).

In addition to recovery of chromium from spent tan liquors and wastewater, various treatment methods have been developed to recover chromium from chrome shavings, splits, trimmings and

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buffing’s of tanned leather. For example, basic hydrolysis, such as using Ca(OH) 2 with steam or NaOH at elevated temperature and/or pressure acid hydrolysis, and enzymatic hydrolysis have been in use for chrome recovery and the isolation of protein fractions in the last three decades

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(Fathima et al., 2014; Mu et al., 2003; Ramamurthy et al., 2015; Rao et al., 2002; Sundar et al., 2011). Ramamurthy et al. (2015) also reported that keratin hydrolysate (KH) from poultry feathers can be utilized with chrome shavings (CS) for total elimination of the polluting solid and liquid wastes in a tannery. However, methods mentioned above entails additional chemicals or agents that increase the cost of effluents treatment. Incineration under restricted aqueous environment

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to induce wet air oxidation and separation of the chromium from the oxidized liquor has been reported as a potential chromium recovery strategy from chrome shavings and trimmings (Sundar et al., 2011). In this method, the respective ash rich in chromium (III) and some in hexavalent form is recovered in the form of soluble chromate by oxidation and re-used in tanning by reduction with Na2SO3. These methods illustrates some of the strategies that can potentially be used in sub Saharan African tanneries for better utilization of spent chrome liquor and chrome shaving wastes, for effective chrome recovery that may be environmentally friendly with economical benefit. However, application of each method is dependent on the capital outlay cost and infrastructure

26

needed and its efficacy in the removal of chromium and other auxiliary chemicals in the effluent taking into account the advantages and disadvantages of each method outlined by Fathima et al. (2014). 5.0 Management and treatment of chromium wastes in sub Saharan Africa

Globally chromium wastes treatment involves physical, chemical and biological techniques. Among these, landfill, open dumping, incineration, composting and occasional recycling are

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traditional methods, while the others are comparatively new options that are yet to be applied in a number of developing countries due to the cost and technical skills (Dutta and Das, 2010; Minas

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et al., 2017). To understand the chromium removal strategies in sub-Saharan African countries, author conducted the questionnaire survey in 2018 among selected key informants and concluded that 70% of these countries dominantly use physical and chemical treatment methods

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(Unpublished data). However, the treatment outcomes are not encouraging because they are applied rudimentarily resulting in the pollution of the ecosystems (Oruko et al., 2014). According

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to Tadesse and Seyoum, (2015), chemical precipitation is the common method for treating chromium wastes in Africa. For instance, primary treatment of chrome liquors (Figure 4a) is treated with lime to raise the pH of the effluent from acidic to alkaline that precipitate Cr (III)

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suspended in the liquor and settles as sludge. The resultant chromium sludge is pumped into drying beds to dry and later form chromium oxide (cake), which often results in minimized chromium effluent (Figure 4b). This process of chemical precipitation has been used successfully

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in reducing chrome content in tannery effluent before discharge into other ponds, sewer systems or directly to the environments (Mottalib et al., 2015; Tadesse and Seyoum, 2015). The management of the resultant sludge after airing in the drying beds generates additional management difficult, i.e. the disposal of chromium cake. In a number of African tanneries, they are kept inside the tannery as a heap of waste (Figure 4c), sometimes they were illegally

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transported with other wastes to the common municipal dumping sites. Majority of the tanneries in Africa release their effluent after neutralization into municipal authorities’ sewer lines for further treatment (Oruko et al., 2014), while others discharge them into water bodies like rivers and wetlands (Mwinyihija and Mwinyihija, 2010).

In early 1990s, Leather Industry Research Institute (LIRI) developed biological filtration process for chromium waste, however, it has not been practiced in any African local tanneries, the reason being that for biological filtration to be effective, pre-treatment (screening) and primary treatment

27

processes (settling of lime/chrome sludge and fat oil/grease removal) need to be very effective. This helps to remove problematic solids and fats which can block the filter bed when released into the equalization ponds. One of the pilot study indicated that primary treatment should be upgraded for the potential use of biological technique in the secondary treatment process. So far, none has been reported in recent years in the continent. Similarly, some research has been directed towards the use of microfiltration to establish the feasibility of its use together with the reverse osmosis technique. At the end of experimental trial, it was concluded that tannery effluent required a very effective secondary biological treatment or further tertiary treatment to eliminate the

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suspended solids and soluble fats, oils and grease for microfiltration technique to be economically viable (Ganesh and Ramanujam, 2009; Itankar and Patil, 2014; Patil et al., 2016; Sundar et al.,

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2011). Until now, there are no reports of secondary and tertiary treatment stages in most African

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tanneries; therefore, microfiltration has not been practiced in Africa.

Figure 4: Scenario of chromium dumping and removal in Africa. (a) Chromium waste oxidation pond (b) dried sludge (c) dried chromium cakes packed in sacks (d-g) chromium waste dumped inside the tannery (h) chromium waste burnt in the open environment (i) kiln in a tannery

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Figure 4 (d-g) shows the current situation of crude dumping of chromium wastes as practiced in most African countries. This is because it is the simplest solution of disposing solid wastes from the leather processing industry. However, it is also widely used in developing countries and even in some developed countries (Kolomaznik et al., 2008). The problem experienced with these tannery-based solid waste dump sites is the accumulation and later leaching of chromium salts by rain into the drinking water sources and subsequent oxidation of Cr (III) to Cr (VI) during disinfection of water for human consumption (Sallam et al., 2015). Such a solution is not environmentally suitable and proper disposal of chromium-contaminated hazardous sludge has

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also become a major environmental problem to many tanning industries (Gowd et al., 2010; Tariq et al., 2010). Large areas around such dumpsites are rendered unsuitable for living and other

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human activities. Therefore, the present system of crude dumping of chromium wastes, which is common in the African continent is unacceptable and needs to be addressed.

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In some instances, crude combustion is also commonly used to disposal strategy for tannery wastes in Africa (Figure 4 h-i). This method completely removes the organic content of chrome

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waste, however, during the combustion process the possibility of total oxidation of trivalent chromium into hexavalent chromium is very high, which is considered a carcinogenic compound (Wells et al., 2014). Therefore, perfect separation of combustion gases and ash is necessary as

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well as safe disposal of the ashes. Unfortunately, the current practices, do not address these shortcomings. More attention should also be given to increased content of nitrogen oxides which is produced during the combustion of collagen proteins (Kolomaznik et al., 2008). The improved

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incineration method using wet air oxidation and separation of the hexavalent chromium from the oxidized liquor should be considered instead of crude combustion (Sundar et al., 2011). This will also help the management of gases and the collection of raw materials for re-use in making other useful products as opposed to crude combustion currently being practiced.

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Compositing waste, as a low cost and less sophisticated technology, has been found feasible in managing organic wastes, but it has not found favor in most tanning industries in Africa and other parts of the world (Buljan, 2005). Compost made out of thermal fleshing’s are reported to show poor plant tolerance or even plant intolerance. However, the fertilizing properties of the latter can be improved by adding animal manure. Moreover, hair residues can be composed together with thermal fleshing’s, which can result into a compost product of acceptable quality (Kolomaznik et al., 2008; Zuriaga-Agustí et al., 2015). The disadvantage of the compost method is the associated obnoxious odors and the presence of insects in the area of compost. The other drawback is the

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requirement of substantial space which is lacking in most tanneries built in the congested neighborhood in Africa, and disposal of an increased volume of wastes due to the addition of bulking agents. Therefore, the technology should be applied in a vessel where compositing with a proper waste gas treatment (bio-filter) could help to avert these problems. Regrettably, in many countries in Africa, properly designed, constructed and/or maintained landfills are not available, instead tannery wastes are regularly crudely dumped without any control in illegal dumpsites ending up in the pollution of the ecosystems (soils, water, and air). Thus endangering human health by becoming bioavailable along the food chains (Ahamed and Kashif, 2014). The above

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problem is compounded by lack of properly installed effluent treatment plants in a number of tanneries in sub-Saharan Africa. This has led to their closure in some countries due to the public

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complaint and environmental concerns from authorities.

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6.0 Potential bioremediation technologies for tannery wastes in sub-Saharan Africa

Bioremediation is a process in which microorganisms are used to reduce or eliminate an

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undesirable chemical contaminant. Currently, in many countries both physicochemical and biological wastewater treatment plants are operated for the purpose of rendering the effluent from the tanning industries safe. The known and unknown applied bioremediation techniques in sub-

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Saharan Africa include monitored natural attenuation (MNA) also known as natural attenuation (NA), it relies on natural physical, chemical and biological processes to reduce or attenuate contaminant concentrations (De Voogt, 2015; Kuppusamy et al., 2016). MNA requires active

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monitoring, which is lacking in African tanneries and should be included as part of the design plan for a site. In some cases, such long-term monitoring may be more expensive than active remediation. Therefore, it is only applicable to carefully controlled and monitored sites and must reduce contaminant concentrations to levels that are protective of human health and the environment in reasonable timeframes (Abatenh et al., 2017). However, this technique is

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unconsciously being implemented in sub-Saharan Africa tanneries, without monitoring at crude dumpsites.

Bioaugmentation is a process that involves the addition of microbial cultures, it is used to enhance the bio-treatment of various wastes including contaminants like chromium that are not degraded by the indigenous organisms (Kim et al., 2013). Bioaugmentation is always performed in conjunction with biostimulation techniques which includes bioventing, land farming or land treatment, biopiles, composting, and sometimes anaerobic reduction (Pradhan et al., 2017). This

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treatment process can be applied in both in situ or ex-situ conditions, however, very few have been tried and implemented due to the fact, that they require technical knowledge and skills in their application which is still lacking in most African tanneries. On the other hand, bioventing is the stimulation of the natural in situ biodegradation of aerobically degradable chromium compounds in soil by providing oxygen to existing soil microorganisms (Basu et al., 2015). In African tanneries, this method can be used for both long and short term cleanup of chromiumcontaminated soils, as it ranges from a few months to several years. However, this technique may not perform well in low moisture soil, low temperature, and high water table areas, as these

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properties limit the biodegradation process. Co-metabolism is another bioremediation processes where microbes do not gain energy or carbon from degrading a contaminant, instead, the contaminant is degraded via a side reaction. Technologies based on co-metabolism are more

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difficult to implement globally and even in the sub-Saharan Africa since the microbes do not benefit from the desired reactions (Hazen, 2009). Other limitations include lack of skilled workers,

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removal rate of contaminants are unknown and not predictable and establishing biological processes as the primary mechanism for contaminant removal is also more difficult. However,

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researchers should improve this remediation technique to understand the removal rate in chromium wastes polluted sites, which further ease the limitations and accepted globally.

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Land farming or land treatment is a bioremediation technology that generally treats the top 30 cm layer of the soil and involves the addition soil bacteria along with indigenous microbes to degrade the contaminants. This process requires excavation and placement of contaminated sludges, soils

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and sediments into lined beds which are periodically tilled to aerate the soil and it can be applied at a large scale (Antizar-Ladislao et al., 2008). This treatment process is cost effective, however this treatment is applied in a few places in Africa due to requirement of heavy machinery which is lacking in many tanneries. On the other hand, biopile technology, is an engineered composting system where aeration is provided through a network of sparger pipes and a leachate collection

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system is used for water-soluble contaminant (Mohee and Mudhoo, 2012). This technology can provide both more rapid degradation rates and improve the economics of treatments. Compared to land treatment units, the biopiles which has not been experimented in African tanneries though requires less space. This technique is well suited for the treatment of wastes generated from crust and finishing stages like chromium buffing dust of the tanning process. This is because they mostly contain a combination of volatile organic compounds and chromium wastes which requires a small space. Nevertheless, the cost of setting up the composting system is expensive that a number of upcoming tanners will hardly go for it in sub-Saharan Africa.

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Controlled bioreactor systems is an alternate technique that can overcome the deficiencies of land farming, composting or biopiling methods (Varjani et al., 2018). The choice of the appropriate bioreactor should be associated with the different operational conditions required for Cr removal such as hydrodynamics, mass transfer and growth conditions (Fernández et al., 2018). In 2014, first up-scaled demonstration of an effective biological Cr (VI) bioremediation system was erected in South Africa (Williams et al., 2014). This system is reported to have successfully resulted in 99% reduction of contaminants in the effluent. However, this has not been replicated across other

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African countries due to high costs involved and lack of skills in remediation technology. In future, these technologies offer potential for substantial advances in African and other global tannery

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sectors; but yet a wide-ranging research have to be made in this field to solve the problems associated with the large-scale remediation process.

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The removal mechanism of chromium using constructed wetlands is a complex combination of physicochemical and biological processes (Sultana et al., 2014). Biological processes associated

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with wetlands include bioremediation (microbial-based remediation) and phytoremediation (plantbased remediation). Wetlands inherently have a higher rate of biological productivity/activity than many other natural ecosystems and are thus capable of efficiently and economically transforming

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many common contaminants to harmless by-products (Kadlec et al., 2012). However, wetlands are sensitive to high ammonia levels, herbicides, and contaminants that are toxic to the plants or microbes (USEPA, 2005). This technique can be experimented in Africa tanneries but the

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challenge has been in the selection of right plant species to remediate heavy metals in the tannery wastewater and presence of a high concentration of ammonia and hydrogen sulphide gases.

Phytoremediation is also an emerging technology for cleaning up contaminated sites, which is cost-effective and has aesthetic advantages and long-term applicability. It is best applied at sites

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with shallow contamination of organic, nutrient or metal pollutants that are amenable to one of the five

applications;

phytotransformation,

rhizosphere

bioremediation,

phytostabilization,

phytoextraction, and rhizofiltration (Ahemad, 2015; Sampanpanish et al., 2006). The technology involves efficient use of plants to remove, detoxify or immobilize environmental contaminants in a growth matrix (soil, water or sediments) through the natural, biological, chemical or physical activities or processes of the plants (Ullah et al., 2015). Remediation using this technology is most appropriate for large areas of low and moderately contaminated soils where the application of conventional remediation technologies would be prohibitively expensive (Antoniadis et al., 2017).

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The most important parameter for selection of suitable plants in Africa is not the tolerance of the plant to heavy metals, but effectiveness in the accumulation of heavy metals. In general, the contaminant must be in a biologically accessible form, and root absorption must take place. Translocation of the contaminant from root to shoot makes harvesting easier (Antoniadis et al., 2017; Kuppusamy et al., 2016). In African tanneries, phytoremediation is still not yet fully exploited for remediation of chromium wastes.

Overall, the conventional technologies of physicochemical methods such as precipitation,

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combustion, crude landfills, and combustion are generally costly and partially incomplete due to the conversion of parent compounds into transformation products which are more persistent and

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equally hazardous to non-target organisms (Kuppusamy et al., 2016). Then biological remediation processes like phytoremediation, bioaugmentation, biopiles, land farming, natural attenuation among others can present environmentally friendly and economically possible options to remove

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hazardous chromium wastes from the environment. Since the natural ability of plants and microbes to decontaminate pollutant are being exploited, the inorganic pollutants like chromium

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could be completely reduced into harmless compounds without any residual effects. Thus, the intended reduction and less bioavailability of the target contaminant will be cost-effective. Currently, bioremediation in the tanning industries in sub-Saharan Africa has not focused on its

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developments like physico-chemical technologies (Megharaj et al., 2011; Rayu et al., 2012). Therefore, the integrated use of bioremediation with advance physicochemical techniques could lead to improvements in the reduction of chromium wastes existing risk-based environmental

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remedial systems performance. The exploitation of these advance novel remediation approaches will eventually bring about a rapid, reliable, low cost and risky contaminant clean-up strategy for sub-Saharan African tanning industries. Nevertheless, they can only be implemented with properly instituted policies and regulations.

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7.0 Challenge of environmental laws for tanneries in sub-Saharan Africa

In most sub-Saharan countries, tannery effluents are subjected to overall legislation of industrial waste discharge rather than specific limits. Environmental Management and Coordination Act (no. 8 of 1999), Kenya (GOK, 1999), National Environment Management Act (Act 107/1998) of South Africa (Republic of South Africa, 2013), Environmental Pollution Control Proclamation No. 3002/2002 of Ethiopia (Federal Democratic Republic of Ethiopia, 2002), Loi 96-03 du 26 février 1996 Portant Code de l' Environnement of Senegal (République du Sénégal, 1996), and

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Environment protection and pollution control Act number 12 of 1990 of Zambia (Republic of Zambia, 1990), are examples of the these legislations, regulations and policies that have been enacted to control the disposal and/or recycling of industrial wastes for total protection of the environment, local communities, and promotion of business ethics.

Although some of these laws appear to offer adequate guarantees for environmental protection, there is growing pressure from effluent-quality regulations that tend to be more stringent than in many non-African countries (Mwinyihija, 2015). The technology and economics of the tanneries

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could be sustainable if there is introduction of realistic national regulations set limits. Many countries have standards/regulatory limits in tandem with WHO and EPA standards that are

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based on completely different climatic and environmental conditions. The definition of these set limits does not consider the real level of technology and skill in each country making their implementation and enforcement challenging. In some countries, due to strict legal systems on

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compliance with environmental regulations, many industries including tanneries are facing challenges in cost-effectiveness and have to shut down their operations (Kiruthu, 2007;

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Mwinyihija, 2015; Mwinyihija and Killham, 2006).

Many countries in Africa still lacks the capability to manage and implement the norms as adapted

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in their environmental regulations (Akele et al., 2016; Bertinelli et al., 2012; Ndimele et al., 2017; Sikder et al., 2013; Zinabu et al., 2018). Governments in the region have established environmental protection offices in different departments and bureaus with insufficient

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coordination between them, leading to conflicts and inefficient implementation. In the last two decades, pressures to attract investors for industrial activities is also reducing regards to pollution controls (Akele et al., 2016; Ndimele et al., 2017; Sikder et al., 2013; Zinabu et al., 2018). The apparent mismatch between environmental enforcement and investment policies leads to no clear measures taken to control industrial discharges. Many tannery technologies used in Africa are

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also quite old as there is a tendency to import cheaper technologies to cope with environmental requirements under increasing pressure of economical returns in some countries (Kiraye et al., 2019; Kiruthu, 2007; Mwinyihija, 2015).

Despite, many difficulties faced in implementing tannery pollution control in Africa, some current and important global regulations on pollution prevention measures have been incorporated into regulations of countries like Ethiopia, Namibia, Tanzania, Tunisia, Zambia, Zimbabwe and others. For examples, tanneries having a treatment capacity higher than 12 tonnes of finished products

34

per day are subject to an obligation to get an authorization certificate (Mwinyihija, 2015). This obligation arises from the European Council Directive 96/61/EC on integrated prevention and reduction of pollution (IPPC) (European Council, 1996). This environmental compatibility of the products and the production processes are becoming a factor of company competitiveness globally. Therefore, adoption of progressive sustainable development such as process innovations, effluent management and treatment technologies in tannery sector coupled with operational capacity of environmental institutions, environmental economics will be important for

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the future of leather industry in Africa.

8.0 Conclusion

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In conclusion, this comprehensive review has established that chromium tanning is still extensively employed to convert hides and skins into leather products which are non-putrescible. In the process, a substantial amount of chromium wastes are generated which are hazardous

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after undergoing redox reactions in the ecosystems. This can affect tannery workers and people living close to tanning industries, and the need to manage chromium in leather industries. This

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study has also highlighted the advantages and disadvantages of alternative tanning techniques which are readily available to partially or completely replace chromium tanning, where gaps in the knowledge exist globally and in the sub-Saharan Africa region. But then there is still an argument

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from leather experts that some of them are not able to produce quality leathers with fine properties unless they are used in appropriate combination tannage. This implies that more research is still needed on alternative tannage within the sub-Saharan Africa where conventional chrome tanning

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practice is still common. Chromium wastes minimization techniques along with recycling/recovery techniques are yet to be embraced and practiced in the Sub-Saharan tanneries. Therefore, leather industries in the region should consider modifying and applying some of the discussed minimization, recycling and recovery methods with a view of reducing chromium wastes impacts on the environment. Strategies for removing chromium contaminants as currently being practiced

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or proposed in the region are also described comprehensively. However, the physicochemical techniques as practiced currently are not advantageous as they are energy intensive and transforms chromium wastes into mobile and persistence compounds. On the other hand, bioremediation techniques are cost-effective and eco-friendly but they suffer from limitations due to the fact that they require a long time to work, the poor performance of microbial groups in real contaminated sites, and lack of technical knowledge and resources to undertake some of them in the region. In spite of that, bioremediation remains the best option to use in the future to remove chromium wastes pollutants in the region. Lastly chromium content in the leather and leather

35

goods is a major policy priority as per the European Council Directive 96/61/EC on integrated prevention and reduction of pollution (IPPC). This is coupled with consumer’s consciousness about environmental protection. But strict compliance with environmental norms has cost effects on tanning activity. Therefore, there is a need to develop process innovations in the tannery sector that will cope with new environmental regulations and consumers demand that may disfavor the production and marketing of quality leather in the future, especially in sub-Saharan Africa.

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