Chromium Concentrate Recovery From Solid Tannery Waste in a Thermal Process

Valorisation of Tannery Waste to Recover Chromium with a View to Reusing It in Industrial Practise

This paper presents the basic assumptions of the concept of a new technology for the valorisation of chromium tannery waste. It assumes the use of an integrated system of the thermal pressure hydrolysis process and membrane filtration techniques for the recovery of chromium compounds and the use of a separated organic matter during anaerobic fermentation. According to the assumptions of the developed technological concept, at the first stage, the crushed mixture of chromium tannery waste is decomposed in the process of thermal pressure hydrolysis using appropriate process conditions in an alkaline environment. Then, the liquid product of this process (the so-called hydrolysate) is processed using centrifugal force separation and ultrafiltration. Such activities enable the recovery of chromium compounds for rawhide currying and concentration of organic matter (fats, proteins) with energy potential. Research carried out under conditions similar to real operating conditions proved that chromium compounds recovered from waste can be successfully used in the processing of cowhides intended for the production of footwear. The industrial implementation of the developed technology for valorising chromium tannery waste would enable the transition from a linear to a circular economy.

Depollution of Polymeric Leather Waste by Applying the Most Current Methods of Chromium Extraction

The leather industry is one of the most polluting industries in the world due to the large amounts of waste following raw hide processing but also due to the high content of chemical substances present in leather waste. The main problem with chromium-tanned leather solid waste is related to the storage, due to the ability of chromium to leach into soil or water, and also owing to the high ability of trivalent chromium to oxidize to its toxic form, hexavalent chromium. The purpose of this article is to present the most current methods of chromium extraction from solid tanned leather waste in order to obtain non-polluting leather, which can constitute secondary raw material in new industrial processes. The extraction methods identified in the present study are based on acid/basic/enzymatic hydrolysis and substitution with the help of organic chelators (organic acids and organic acid salts). In addition, this study includes a comparative analysis of the advantages and disadvantages of each identified extraction method. At the same time, this study also presents alternative chromium extraction methods based on the combination of conventional extraction methods and ultrasound-assisted extraction.

A novel strategy of tannery sludge disposal - converting into biochar and reusing for Cr(VI) removal from tannery wastewater

Tannery sludge with high chromium content has been identified as hazardous solid waste due to its potential toxic effects. The safety disposal and valorization of the tannery sludge remains a challenge. In this study, the chromium stabilization mechanism was systematically investigated during chromium-rich tannery sludge was converted to biochar and the removal performance of the sludge biochar (SBC) for Cr(VI) from tannery wastewater was also investigated. The results showed that increase in pyrolysis temperature was conductive to the stabilization of Cr and significant reduction of the proportion of Cr(VI) in SBC. It was confirmed that the stabilization of chromium mainly was attributed to the embedding of chromium in the C matrix and the transformation of the chromium-containing substances from the amorphous Cr(OH)3 to the crystalline state, such as (FeMg)Cr2O5. The biochar presented high adsorption capacity of Cr(VI) at low pH and the maximal theoretical adsorption capacity of SBC produced at 800°C can reach 352 mg Cr(VI)/g, the process of which can be well expressed by Langmuir adsorption isotherm and pseudo second order model. The electrostatic effect and reduction reaction were dominantly responsible for the Cr(VI) adsorption by SBC800. Overall, this study provided a novel strategy for the harmless disposal and resource utilization for the solid waste containing chromium in leather industry.

Removal of Chromium (III) and Reduction in Toxicity in a Primary Tannery Effluent Using Two Floating Macrophytes

Trivalent chromium (Cr(III)) is a contaminant with toxic activity. Its presence in waters and soils is usually related to industrial activities such as tanneries. The aim of this study was to compare the removal of Cr(III) in hydroponic solutions and tannery effluents using two floating macrophytes: Salvinia auriculata and Eichhornia crassipes. First, to determine the chromium removal capacity in solution and the bioaccumulation factor (BAF) in tissues of each plant, experiments were set up with contaminated solutions with Cr(III) concentrations of 2, 5, 10, 20, and 40 mg/L. Subsequently, both plant species were exposed to a primary tannery effluent contaminated with 12 mg/L of Cr(III) in order to study the removal capacity of organic and inorganic matter, as well as the acute toxicity in the water flea (Daphnia magna) and genotoxicity in zebrafish (Danio rerio). Tests carried out on nutrient solutions revealed that both plants have a high capacity for removing Cr(III) in solution. The BAF in tissues was higher in E. crassipes compared to S. auriculata. In the experiments with a tannery effluent, both species presented low nutrient and organic matter removal efficiency, but they showed good Cr(III) removal capacity, with average reduction values of 57% for S. auriculata and 54% for E. crassipes after 72 h of exposure. E. crassipes contributed most to the reduction in acute toxicity in D. magna, while S. auriculata did not show a similar effect. However, both plant species managed to reduce the genotoxicity marker in D. rerio when compared with the initial effluent and the control.

A systematic review on tannery sludge to energy route: Current practices, impacts, strategies, and future directions

The growing amount of tannery sludge (TS) generated from leather processing often undergoes uncontrolled landfilling, or open dumping, releasing a significant volume of harmful pollutants, including carcinogenic chromium (Cr) into the air, water, and soil. Therefore, the sustainable TS management through advanced valorization technologies becomes vital to align with the Sustainable Development Goals (SDGs) and mitigate the adverse environmental, health, and social impacts. Moreover, TS, as biomass, can be considered a renewable energy source for bioenergy generation, which could be a viable solution for meeting contemporary environmental standards and expediting transition towards a circular economy. However, TS valorization is sensitive and critical due to the potential risk of transforming Cr(III) to Cr(VI) during the valorization process. Therefore, there is an urgency to consider efficient and holistic TS valorization technologies in the design, implementation, and operations phases to avoid any environmental and health hazards. In pursuit of this goal, this systematic literature review (SLR) comprehensively and critically analyzes the existing TS valorization processes to develop sustainable energy recovery solutions from TS. This SLR contributes uniquely to the existing literature in different ways. Firstly, it provides a critical evaluation of the current TS valorization technologies identifying the available waste-to-energy recovery options. Secondly, the review encompasses extensive research from three reputed databases such as Scopus, Web of Science, and ScienceDirect, without temporal restrictions to offer a comprehensive understanding of current TS management practices and available valorization techniques. Moreover, the review categorized 124 published papers into distinct groups, revealing promising avenues for future research in this field. The findings indicated that most of the work concentrating on Chrome (Cr) recovery, pyrolysis, anaerobic co-digestion, and solidification while gasification and biodiesel or biofuel production from TS remained largely unexplored. Additionally, vital aspects such as process optimization, life cycle assessment of different valorization techniques, environmental, economy, energy, emergy, and exergy (5E) analysis, life cycle energy balance, and techno-economic analysis including exergoeconomic and exergoenvironmental are completely absent in the literature. Future studies need to concentrate on process optimization, exergy and energy analysis, and techno-economic assessment including exergoeconomic and exergoenvironmental analysis to understand the feasibility and environmental benefits of various TS valorization technologies and to develop industry-scale valorization plants for TS management in an economically and ecologically sustainable manner. Moreover, the review will serve as a comprehensive guide for scholars, authorities, and stakeholders to advance research in this field and formulate policies for the eco-friendly management of TS, paving the way towards clean energy solutions.

Stabilizing Cr(Ⅲ) deriving from tannery sludge with kaolin and organic matter

Stabilizing Cr(III) in tannery sludge (TS) via harmless method has always been the goal of environmental pollution treatment. In this study, a simple method to stabilize Cr(III) in TS is proposed via adding kaolin, based on the fact a large amount of organic matter contained in TS. Comprehensive characterizations confirm that kaolin can stabilize Cr(Ⅲ) via its abundant -OH and lamellar structure. Moreover, there are hydrogen bond interactions and ligand exchange-surface complexation between organic matter and kaolin, which is more conducive to form a stable ternary complex with Cr(III), in a state of organic matter-Cr(III)-kaolin. Simultaneously, the BCR sequential extraction experiment shows that the unstable water and acid soluble state of Cr(III) are reduced (from 0.61% to 0.35%), which further indicates that the stabilization of Cr(III) is successful.

Hexavalent chromium release over time from a pyrolyzed Cr-bearing tannery sludge

Pyrolysis in an inert atmosphere is a widely applied route to convert tannery wastes into reusable materials. In the present study, the Cr(III) conversion into the toxic hexavalent form in the pyrolyzed tannery waste referred to as KEU was investigated. Ageing experiments and leaching tests demonstrated that the Cr(III)-Cr(VI) inter-conversion occurs in the presence of air at ambient temperature, enhanced by wet environmental conditions. Microstructural analysis revealed that the Cr-primary mineral assemblage formed during pyrolysis (Cr-bearing srebrodolskite and Cr-magnetite spinel) destabilized upon spray water cooling in the last stage of the process. In the evolution from the higher to the lower temperature mineralogy, Cr is incorporated into newly formed CrOOH flakes which likely react in air forming extractable Cr(VI) species. This property transforms KEU from an inert waste to a hazardous material when exposed to ordinary ambient conditions.

Immobilization of chromium in real tannery sludge via heat treatment with coal fly ash

The secure and harmless disposal for Cr-bearing tannery sludge (Cr-TS) has attracted an increasing concern, due to potentially adverse effect on ecosystem and human health. A greener alternative method about "waste treatment with waste" for thermally stabilizing real Cr-TS was developed via employing coal fly ash (CA) as dopants in this research. The co-heat treatment of Cr-TS and CA was carried out at the temperature range of 600-1200 °C to investigate the oxidation of Cr(III), immobilization of chromium and leaching risk of the sintered products, and the mechanism of chromium immobilization was further explored. The results indicate that the doping of CA can significantly inhibit the oxidation of Cr(III) and immobilize chromium by incorporating chromium into spinel and uvarovite microcrystal. At the temperature higher than 1000 °C, most of chromium can be converted into stable crystalline phases. Furthermore, a prolonged leaching test was conducted to study the leaching toxicity of chromium in sintered products, indicating that leaching content of chromium is much less than the regulatory limit. This process is a feasible and promising alternative for immobilization of chromium in Cr-TS. The research findings are supposed to offer a theoretical foundation and strategy choice for thermal stabilization of chromium, as well as safety and harmless disposal of Cr-containing hazardous waste.

Enhanced detoxification of Cr6+ by Shewanella oneidensis via adsorption on spherical and flower-like manganese ferrite nanostructures

Maximizing the safe removal of hexavalent chromium (Cr6+) from waste streams is an increasing demand due to the environmental, economic and health benefits. The integrated adsorption and bio-reduction method can be applied for the elimination of the highly toxic Cr6+ and its detoxification. This work describes a synthetic method for achieving the best chemical composition of spherical and flower-like manganese ferrite (MnxFe3-xO4) nanostructures (NS) for Cr6+ adsorption. We selected NS with the highest adsorption performance to study its efficiency in the extracellular reduction of Cr6+ into a trivalent state (Cr3+) by Shewanella oneidensis (S. oneidensis) MR-1. MnxFe3-xO4 NS were prepared by a polyol solvothermal synthesis process. They were characterised by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectrometry (XPS), dynamic light scattering (DLS) and Fourier transform-infrared (FTIR) spectroscopy. The elemental composition of MnxFe3-xO4 was evaluated by inductively coupled plasma atomic emission spectroscopy. Our results reveal that the oxidation state of the manganese precursor significantly affects the Cr6+ adsorption efficiency of MnxFe3-xO4 NS. The best adsorption capacity for Cr6+ is 16.8 ± 1.6 mg Cr6+/g by the spherical Mn0.22+Fe2.83+O4 nanoparticles at pH 7, which is 1.4 times higher than that of Mn0.8Fe2.2O4 nanoflowers. This was attributed to the relative excess of divalent manganese in Mn0.22+Fe2.83+O4 based on our XPS analysis. The lethal concentration of Cr6+ for S. oneidensis MR-1 was 60 mg L-1 (determined by flow cytometry). The addition of Mn0.22+Fe2.83+O4 nanoparticles to S. oneidensis MR-1 enhanced the bio-reduction of Cr6+ 2.66 times compared to the presence of the bacteria alone. This work provides a cost-effective method for the removal of Cr6+ with a minimum amount of sludge production.

Tannery waste as a renewable source of nitrogen for production of multicomponent fertilizers with biostimulating properties

The studies presented in this work show that solid tannery waste-like shavings can be used as high-protein materials for fertilizer production following the concept of the circular economy. To select appropriate process parameters (mass ratio of shavings meal to the hydrolyzing agent (S:L), hydrolysis medium concentration, temperature) and to ensure the highest possible hydrolysis efficiency, it is useful to apply the well-known response surface methodology (RSM). The analyses revealed that chromium shavings (SCr) were most preferably treated with 10% KOH in a ratio of S:L 1:1 with the process being carried out at 160 °C (6.59% N). The optimal hydrolysis conditions for non-chromium (S) shavings were: S:L ratio 1:2, 10% H₂SO₄, and temperature 160 °C (4.08% N). Chromium concentrations in hydrolysates from S and SCr shavings obtained under optimal conditions were 15.2 mg/kg and 9483 mg/kg, respectively. Hydrolysate samples were analyzed by reversed-phase high-pressure liquid chromatography (RP-HPLC) that revealed that the type of hydrolysis (acidic/alkaline) affects the amino acid profile. Approximately 4.5 times more amino acids were extracted in the KOH environment than during acidic treatment. The hydrolysates contained mainly glycine, alanine, and proline, which are primarily responsible for stimulating plant growth by supporting chlorophyll synthesis, chelating micronutrients, improving pollen fertility, or resistance to low temperatures. The conversion of tannery waste into fertilizer requires the control of contaminant levels, especially chromium, which can oxidize to the carcinogenic form Cr(VI) that is hazardous to humans and the environment.

Application of an Integrated System of Thermal Pressure Hydrolysis/Membrane Techniques to Recover Chromium from Tannery Waste for Reuse in Hide Tanning Processes

This paper presents the results of research on a new method of chromium recovery from solid waste generated during the tanning of raw hides. In the first stage, the shredded mixture of useless leather scraps is decomposed through thermal pressure hydrolysis (TPH) in nitric acid in appropriate process conditions. Then, the liquid product of this process (hydrolysate) is fractionated using membrane separation techniques. The microfiltration (MF) process enables the initial purification of the hydrolysate by concentrating the organic matter. On the other hand, the nanofiltration (NF) process enables a three-fold concentration of total chromium in the pre-purified hydrolysate. The total chromium concentrate prepared in the above manner was successfully used in the model tanning processes. These processes were carried out on pickled bovine hides, using a mixture of a commercial chromium tanning agent and chromium concentrate after nanofiltration. The reference sample was bovine hide traditionally tanned with a commercial chromium tanning agent. Based on the results of the physical and chemical analyses, it was found that the properties of hides tanned using chromium recovered from waste are similar to those of hides tanned traditionally using a commercially available chromium tanning agent. The industrial implementation of the developed tannery waste valorisation technology would enable transition from a linear economy to circular economy.

Chromium in plant growth and development: Toxicity, tolerance and hormesis

Research over the last three decades showed that chromium, particularly the oxyanion chromate Cr(VI) behaves as a toxic environmental pollutant that strongly damages plants due to oxidative stress, disruption of nutrient uptake, photosynthesis and metabolism, and ultimately, represses growth and development. However, mild Cr(VI) concentrations promote growth, induce adventitious root formation, reinforce the root cap, and produce twin roots from single root meristems under conditions that compromise cell viability, indicating its important role as a driver for root organogenesis. In recent years, considerable advance has been made towards deciphering the molecular mechanisms for root sensing of chromate, including the identification of regulatory proteins such as SOLITARY ROOT and MEDIATOR 18 that orchestrate the multilevel dynamics of the oxyanion. Cr(VI) decreases the expression of several glutamate receptors, whereas amino acids such as glutamate, cysteine and proline confer protection to plants from hexavalent chromium stress. The crosstalk between plant hormones, including auxin, ethylene, and jasmonic acid enables tissues to balance growth and defense under Cr(VI)-induced oxidative damage, which may be useful to better adapt crops to biotic and abiotic challenges. The highly contrasting responses of plants manifested at the transcriptional and translational levels depend on the concentration of chromate in the media, and fit well with the concept of hormesis, an adaptive mechanism that primes plants for resistance to environmental challenges, toxins or pollutants. Here, we review the contrasting facets of Cr(VI) in plants including the cellular, hormonal and molecular aspects that mechanistically separate its toxic effects from biostimulant outputs.

Current trends in solid tannery waste management

The tannery is one of the leading revenue-generating sectors in developing countries. The ever-increasing demand for leather products in the global market requires converting large amounts of rawhide/skins into resilient non-putrescible finished leather. Only 20% of the raw material is converted into a finished product; the rest 80% is discarded as solid and liquid wastes during leather processing. A heavy discharge of improperly treated solid tannery waste (STW) causes a severe impact on the surrounding environment by polluting soil, surface water, and groundwater resources, posing severe hazards to human and animal health. STW comprises proteinaceous untanned and tanned waste, which requires proper treatment for eco-friendly disposal. Several strategies have been developed over the years for the reduction and recycling of STW for producing renewable energy (biogas and biohydrogen), biofuels (biodiesel and briquettes), construction material, fertilizers, commercial products (adsorbents, animal feeds, proteins, fats, and enzymes), and biodegradable packaging and non-packaging materials. In this review, we discuss various strategies adopted for recycling, reutilization, and reduction of STW in an environment-friendly manner. Furthermore, an overview of the current perspectives toward achieving a zero-waste policy is also presented to reduce the environmental burden using green-clean technology to aid the survival of present-day tanneries.

Zero Liquid Discharge System for the Tannery Industry—An Overview of Sustainable Approaches

The tannery industry is characterized by the consumption of a large quantity of water, around 30–40 m3 for processing 1000 kg of hide or skin. This amount becomes wastewater, containing about 300 kg of different chemicals, mainly refractory organic compounds, with high chemical oxygen demand (COD), total dissolved salts (TDS), chromium, and evolution of toxic gases, such as ammonia and sulfides, etc. The remaining tanning chemicals are released as effluent having high resistance against biological degradation, becoming a serious environmental issue. Usually, end-of-pipe treatment is not sufficient to meet the concerns of environmental issues. In terms of cleaner production options, the redesigning of the existing effluent treatment procedures with alternate or additional treatment techniques, which “supports resource recovery with no added chemicals”, is expected to give a sustainable solution for the management of toxic effluent. The Zero Liquid Discharge (ZLD) system serves to ensure zero water emission, as well as treatment facilities by recycling, recovery, and reuse of the treated wastewater using advanced cleanup technology. The international scenario shows the implementation of ZLD thanks to pressure from regulatory agencies. The ZLD system consists of a pre-treatment system with conventional physicochemical treatment, tertiary treatment, softening of the treated effluent, reverse osmosis (RO) treatment for desalination, and thermal evaporation of the saline reject from RO to separate the salts. By adopting this system, water consumption is reduced. Moreover, ZLD also becomes effective in disaster mitigation in areas where the tannery industry is a strong economic actor. With this review, we aim to give an outlook of the current framework.

Modeling of Non-Ferrous Metallurgy Waste Disposal with the Production of Iron Silicides and Zinc Distillation

This paper presents studies on the possibility of utilization of technogenic waste from the metallurgical industry by the method of complex processing in order to reduce the anthropogenic load on the environment of the region with the example of the zinc silicate-magnetite-carbon system. The selected sample of clinker dump from welting was subjected to chemical and scanning electron microscopic analyses and thermodynamic modeling. Thermodynamic studies were carried out in the temperature range 1600–2200 K and pressure p = 0.1 MPa, modeling the process of electric melting of clinker from welting in an arc furnace using the software application Astra 4 developed at the Bauman Moscow State Technical University (Moscow, Russian Federation). As a result of the thermodynamic modeling, the optimal temperature range was established, which was 1800–1900 K. Thermodynamic studies established that it is possible to drive away zinc from the system under study by 99–100% in the entire temperature range under study. The maximum degree of silicon extraction (α_Si) in the alloy is up to 69.44% at T = 1900 K, and the degree of iron extraction (α_Fe) in the alloy is up to 99.996%. In particular, it was determined and proved that clinker waste from welting can act as a secondary technogenic raw material when it is processed as a mono mixture to produce iron silicides with a silicon content of 18 to 28%.

Synergistic immobilization of chromium in tannery sludge by ZnO and TiO2 and the oxidation mechanism of Cr(III) under alkaline in high temperature

The disposal of the Cr containing tannery sludge has become a serious environmental problem in China, which has attracted increasing attention. However, experimental and theoretical research is still needed. We developed a feasible route to synergistic immobilization the Cr in tannery sludge into the spinel-based phases by addition of ZnO and TiO₂. ZnO and Cr³⁺ can form stable spinel-based phase, and TiO₂ can suppress the formation of Cr⁶⁺. The significantly synergistic effect between ZnO and TiO₂ promoted the immobilization of Cr in tannery sludge and reduce the leaching of the chromium in the annealed sludge. The oxidation of Cr³⁺ to Cr⁶⁺ in the presence of alkaline substances has been elucidated by density functional theory, and it is revealed that the electrons from the Cr-d orbit jump to the Ca-d and directly transform into the O₂.

Composite Polymers from Leather Waste to Produce Smart Fertilizers

The leather industry is facing important environmental issues related to waste disposal. The waste generated during the tanning process is an important resource of protein (mainly collagen) which can be extracted and reused in different applications (e.g., medical, agricultural, leather industry). On the other side, the utilization of chemical fertilizers must be decreased because of the negative effects associated to an extensive use of conventional chemical fertilizers. This review presents current research trends, challenges and future perspectives with respect to the use of hide waste to produce composite polymers that are further transformed in smart fertilizers. Hide waste contains mostly protein (collagen that is a natural polymer), that is extracted to be used in the cross-linking with water soluble copolymers to obtain the hydrogels which are further valorised as smart fertilizers. Smart fertilizers are a new class of fertilizers which allow the controlled release of the nutrients in synchronization with the plant's demands. Characteristics of hide and leather wastes are pointed out. The fabrication methods of smart fertilizers and the mechanisms for the nutrients release are extensively discussed. This novel method is in agreement with the circular economy concepts and solves, on one side, the problem of hide waste disposal, and on the other side produces smart fertilizers that can successfully replace conventional chemical fertilizers.