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Tannery sludge (TS) contains a number of useful components like Organic carbon, Organic Matter, PO4 3-, Ca2+, Mg2+, and other nutrients essential for plant growth. However, presence of water-soluble toxic metals in TS may cause phytotoxicity. Primary objective of this work was to adopt a bio-chemical strategy for assessing the potential consequences of agricultural application of tannery sludge, via sludge characterization and subsequent bioassay investigations. Heavy metal concentration in the TS was found to be in the order of: Cr (19576±2065.52) > Pb (16.98±2.54) > Cu (1.76±0.20) > Ni (0.98±0.00) > Cd (0.19±0.00) mg kg-1. Further, bioassay tests were performed in terms of germination index (GI), relative seed germination (RSG) and relative root growth (RRG) on Brassica nigra, Vigna radiata, Vigna mungo, Raphanus sativus, and Capsicum frutescens. RSG and RRG were found to be more than 80% whereas, GI was > 66% in all the tested crops exposed to TS extract at 1:5 and 1:10 (w/v) dilutions. Results revealed that the toxicants present in the TS were failed to cause toxicity at early seedling growth and may be used as a soil conditioner after appropriate dilution. However, long term field trails are essential before agricultural utilization. [ABSTRACT FROM AUTHOR]

Digested sludge wasted by tanneries is rich in nutrients and trace elements however, the presence of toxic metals restricts their use in agriculture. The present study explores the possible application of tannery sludge amendment for the cultivation of an energy crop, Hibiscus cannabinus. The toxicity of various sludge amendments (25, 50, 75, and 100%, w/w) was examined during early seedling growth, followed by metal accumulation potential by performing pot experiments. Chemical characterization revealed the presence of Cr (709.6), Cu (366.43), Ni (74.6), Cd (132.71), Pb (454.8) µg.g-1 in tannery sludge beside N (2.1%), P 3.8 & K 316.96 (kg.hec-1.) respectively. Germination of H. cannabinus exposed to sludge extracts ranged between 80 to 95%; Relative seed germination, 81.33 to 84.43%. Relative root growth, 0.9 to 1.16 cm; and germination index, 95 to 110%. It was found that sludge extracts have not caused adverse effects on seed germination and early seedling growth. Heavy metal accumulation was observed as follows: Ni (3.37, 2.38, 1.46 & 0.90 mg.kg-1) > Pb (10.59, 10.15, 5.26, & 2.84 mg.kg-1) > Cu (2.34, 2.24, 0.97 & 0.24 mg.kg-1) > Cd (2.31, 1.19, 1.33 & 1.12 mg.kg-1) > Cr (1458, 1136.12, 601.73 & 211.6 mg.kg-1) in 100, 75, 50, & 25% sludge amended soil, respectively. The bio-concentration pattern of metals was found to be in the order of root > leaf > stem. The findings of the present study give direction for the eco-friendly and cost-effective management of tannery sludge. Further, H. cannabinus can be used for the restoration of metal-contaminated agricultural land, however, results need to be corroborated with field trials. [ABSTRACT FROM AUTHOR]

The present study explores the partial replacement of clay by the addition of tannery sludge (TS) and glass in brick preparation. Bricks were prepared with varying weight percentages of TS (9%, 18%, and 27%) along with glass powder (10%) and fired at different temperatures (900, 950, and 1000 °C). Water absorption, compressive strength, weight loss on ignition, volumetric shrinkage, and bulk density of the bricks were found to be in the range of 12.3%–16.8%, 2.3–4.5 MPa, 4%–13.8%, 6%–11%, and 1600–1850 kg/m3, respectively. Heavy metal leaching was observed to be within the limits of United States Environmental Protection Agency 1311 for the prepared bricks. The addition of TS resulted in a decrease of physical and mechanical properties of the bricks due to the decomposition of organic and inorganic compounds. However, the presence of glass powder compensates for the effect of TS by fusion and densification process. The results indicate that hybrid addition of TS and glass can partially replace clay in brick production and effectively immobilize heavy metals. [ABSTRACT FROM AUTHOR]

The aim of this article is to reduce the environmental harm caused by industrial solid waste, specifically tannery sludge, and enable its reutilization. This study prepared an alkali-activated tannery sludge–slag solidification product (ATSSP) with high mechanical properties using blast furnace slag and tannery sludge as raw materials. The response surface methodology (RSM) was used to optimize the product mix ratio. The hydration mechanism and solidification method of ATSSP for Cr in tannery sludge were studied using X-ray diffraction (XRD), scanning electron microscope-energy dispersive spectrometer (SEM-EDS), and Fourier transform infrared reflection (FT-IR). The results indicate that the compressive strength regression model established through RSM has high accuracy and credibility. When the ratio of activator to binder is 0.2174, the alkali activation modulus is 1.02, and the water-to-cement ratio is 0.37; the 28 d compressive strength of ATSSP can reach 71.3 MPa. The sulfate in tannery sludge can promote the secondary hydration reaction of slag, and the generated hydrated calcium silicate and calcite greatly improve the strength of the ATSSP. The reducing substances contained in slag can reduce Cr (VI) in tannery sludge to Cr (III) in the form of uvarovite. The total Cr and Cr (VI) precipitation concentrations of the product are far less than the specification requirements. [ABSTRACT FROM AUTHOR]

In the present work, a study was carried out on the dosage of wastes from the chemical industry (tannery sludge) and civil construction (concrete and plaster) in mixtures used in concrete blocks' production. The objective was the application of these blocks in paving. The characterization of the materials used was performed employing X-ray diffractometry (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The effect of the different residues on the blocks' properties was evaluated through compressive strength, flexion-traction, water absorption, abrasion resistance, and leaching tests. The results indicated that the concrete paving blocks produced with the addition of residues did not obtain gains in the values of mechanical resistance to compression and traction in bending compared to blocks made with standard raw material. However, the blocks produced with construction waste presented satisfactory results for application in street paving after 7 days of concrete curing, reaching values between 36.54 and 44.6 MPa for the mentioned properties. These values also increased to 21.4% within 28 days of curing. The blocks produced with plaster showed values between 37.03 and 39.85 MPa after 28 days of curing, allowing their use for street paving. On the other hand, the blocks containing residues from the chemical industry had lower strengths, reaching a maximum of 29.36 MPa after 28 days of curing. In addition, it was also noted that the blocks produced with recycled concrete showed an improvement in performance for a composition of 50% recycled material. [ABSTRACT FROM AUTHOR]

This paper aims to provide a contribution to understanding the role of sludge macro-components (lipids, proteins and carbohydrates) on the yield/quality of bio-crude obtained via hydrothermal liquefaction (HTL). This was pursued by analysing the HTL process of real sludges and the mixtures thereof at different compositions, a topic that has been explored in a very limited way in the pertinent literature. The HTL experiments were run with municipal sludge, tannery sludge and a mixture of them in 25:75, 50:50 and 75:25 weight ratios in a batch reactor at 350 °C and for different residence times. The outcomes for a single sludge showed a greater bio-crude yield for the municipal one (42.5% at 10 min), which is linked to its significant carbohydrate content. The results obtained from the sludge mixtures suggested that a carbohydrate-to-protein mass ratio of 2:1 would maximise the bio-crude yield (average value of about 38%). Moreover, LC-MS and NMR analyses highlighted that the mixed sludges contributed to the formation of a higher number of compounds after the HTL treatment, with respect to the pure municipal or tannery sludge, with an increase in amine, alcohols and aromatic compounds. [ABSTRACT FROM AUTHOR]

In the context of achieving sustainable development goals, continuously pushing possible extended research and attempting to implement the respective outcomes in expanding a circular economy for a broad range of products are high priorities. In this paper, we considered disposable tannery sludge as a matter of concern and proposed an innovative framework for placing tannery sludge in the economic value chain via an encapsulation techniquebased construction product development. We used polypropylene plastic and a cement-water matrix as encapsulation materials, and the encapsulated tannery sludge bodies were tested for their drop strength, water absorbing behaviour, ability to leach chromium and tendency to lose weight upon ignition. Value of water absorption for the prepared double layered encapsulated tannery sludge bodies was 1.332%, the drop strength performance index arrived was 90% and 0.0001 ìg/g of Chromium leaching was found in Toxicity Characteristic Leaching Procedure (TCLP) and shown 67% weight loss in thermogravimetric (TGA) analysis. The results confirmed the possibility of ecocompatible disposal and recirculation of tannery sludge for the sustainable production of building blocks in the form of encapsulated bodies. The outcomes of our work add upon a new perspective to the existing literature regarding the environmentally positive utilization of tannery sludge in the production of building blocks. [ABSTRACT FROM AUTHOR]

The disposal of tannery sludge (TNS) without treatment causes severe toxic metal contamination in the ecosystem and food chain. Hence, eco-friendly solid waste management is essentially needed for the tanneries. The aim of this study was to degrade the raw TNS into composted sludge using coir pith (CP) as an adsorbent for organic compounds and other pollutants and Nava Rasa Karaisal (NRK) as a consortium of microorganisms. The TNS treated with CP and NRK showed decolorization and a reduction in pH. After the treatment, the electrical conductivity decreased from 24.7 to 6.31 μS/cm; total dissolved solids (28.4 ppm), salinity (21.2 ppm) and dissolved oxygen (7.5 mg/L) in the untreated TNS were reduced to 10.9 ppm, 4.9 ppm, and 4.6 mg/L in the TNS+CP+NRK treatment, respectively. SEM analysis showed small, smooth-walled structures on the surface of composted TNS compared to rough textures on the untreated TNS. Energy-dispersive spectroscopy analysis showed a higher number of elements in NRK (16) than in untreated sludge (8). Four different bacteria and six fungi were dominant in the treated tannery waste. The effect of different dilutions (12.5%, 25%, 50%, 75%, and 100%) of treated TNS on seed germination of Vigna radiata L. was carried out to check the feasibility of using the treated TNS for plant irrigation and growth. The results showed maximum growth at lower concentrations (12.5% and 25%). The results indicated that TNS treatment using CP and NRK is a potential eco-friendly approach to remediating the sludge waste. [ABSTRACT FROM AUTHOR]

Industrial growth plays major role in development of countries. Among many industries leather is one of major industries generating higher income for countries. Waste generated from these industries is also higher and it is considered as a hazardous waste because of its higher concentration of heavy metals. Concentration of Cr(III) gets reacted with oxygen and converts into Cr(VI) which leads to many environmental impacts and health effects. They are disposed as a landfill, composting or anaerobic digestion processes, etc., also pollutes air, water and land. One of the promising budding technologies is reuse of waste material. This study is about the characteristics, effects and treatment of tannery sludge to make it reuse in concrete as replacement material. Hence comparative review is made on tannery sludge which researcher have already done. [ABSTRACT FROM AUTHOR]

The treatment of tannery sludge is an issue that has received considerable attention due to the potential of tannery sludge to cause severe environmental damage. In this study, the solidification process of the waste sludge of the Istanbul Tannery Organize Industry Wastewater Treatment Plant was investigated for safe disposal. The solidification technique was applied using various binders and their different mixtures, such as active carbon, gypsum (CaSO4/2H2O), fly ash, lime (CaO), zeolite, and different cement-sand mixtures on the laboratory scale. The essential parameters such as TOC, fluoride (F- mg/l), chloride (Cl- mg/l), sulfate (SO4= mg/l), chromium (VI) (Cr6+ mg/l), and ammonium (NH4+ mg/l) were determined after solidification process at certain conditions. The analysis results of treated sludge samples were compared to the criteria for storage in the landfills in the Hazardous Waste Control Regulation (HWCR) as well as cost-calculation was done at optimum conditions. After the addition of 10% wt. CaSO4 in the solidification study, the eluate concentration of TOC, Cl- and SO4= was below the limit values. When tannery sludge was solidified using CaO, the necessary criteria could not be obtained. On the other hand, with the addition of 33% CaO and 5% AC mixture TOC and AOX were observed below the limit values. The solidification was occured with mörtel + portland cement and 5% activated carbon additives and the desired limit values in HWCR were succussed for all parameters. Besides, with the addition of 30% zeolite, all parameters declined below the limit values.

The aim of this article is to reduce the environmental harm caused by industrial solid waste, specifically tannery sludge, and enable its reutilization. This study prepared an alkali-activated tannery sludge–slag solidification product (ATSSP) with high mechanical properties using blast furnace slag and tannery sludge as raw materials. The response surface methodology (RSM) was used to optimize the product mix ratio. The hydration mechanism and solidification method of ATSSP for Cr in tannery sludge were studied using X-ray diffraction (XRD), scanning electron microscope-energy dispersive spectrometer (SEM-EDS), and Fourier transform infrared reflection (FT-IR). The results indicate that the compressive strength regression model established through RSM has high accuracy and credibility. When the ratio of activator to binder is 0.2174, the alkali activation modulus is 1.02, and the water-to-cement ratio is 0.37; the 28 d compressive strength of ATSSP can reach 71.3 MPa. The sulfate in tannery sludge can promote the secondary hydration reaction of slag, and the generated hydrated calcium silicate and calcite greatly improve the strength of the ATSSP. The reducing substances contained in slag can reduce Cr (VI) in tannery sludge to Cr (III) in the form of uvarovite. The total Cr and Cr (VI) precipitation concentrations of the product are far less than the specification requirements.

Organic matter (OM) has an excellent retention effect on stabilizing chromium (Cr), and functional groups on OM play a predominant role in this process. Based on this result, it is found that a considerable amount of Cr in tannery sludge is immobilized from ion exchangeable species into bound species, benefiting from complexing reaction with functional groups. Especially, the mentioned immobilizing process is enhanced in way of adding with montmorillonite (MMT) which performs adsorption reaction with Cr, as well as plays interaction with functional groups. The result is confirmed by employing density functional theory (DFT) analysis, suggesting the binding ability among Cr, functional groups, and MMT is stronger (− 77.36503 eV) than that of the system of Cr and MMT (− 61.29942 eV), indicating the synergetic roles of OM and MMT. This synergetic role could also be illustrated by a new peak (Cr-OH 20.1%) shown in XPS result. Meanwhile, DFT analysis emphasizes that functional groups on OM give the response for binding with Cr in the order of hydroxyl (-OH) > carboxyl (-COOH) > epoxy (-COC), and all the functional groups tend to donate electron to bind with Cr. In addition, the stabilizing process shows a better fitting effect with pseudo second-order kinetic model (R2 > 0.94), indicating that exchangeable Cr mass transfer and chemical adsorption occur simultaneously. [ABSTRACT FROM AUTHOR]

The treatment of tannery sludge is an issue that has received considerable attention due to the potential of tannery sludge to cause severe environmental damage. In this study, the solidification process of the waste sludge of the Istanbul Tannery Organize Industry Wastewater Treatment Plant was investigated for safe disposal. The solidification technique was applied using various binders and their different mixtures, such as active carbon, gypsum (CaSO4/2H2O), fly ash, lime (CaO), zeolite, and different cement-sand mixtures on the laboratory scale. The essential parameters such as TOC, fluoride (F-mg/l), chloride (Cl-mg/l), sulfate (SO4 = mg/l), chromium (VI) (Cr6+ mg/l), and ammonium (NH4 + mg/l) were determined after solidification process at certain conditions. The analysis results of treated sludge samples were compared to the criteria for storage in the landfills in the Hazardous Waste Control Regulation (HWCR) as well as cost-calculation was done at optimum conditions. After the addition of 10% wt. CaSO4 in the solidification study, the eluate concentration of TOC, Cl- and SO4 = was below the limit values. When tannery sludge was solidified using CaO, the necessary criteria could not be obtained. On the other hand, with the addition of 33% CaO and 5% AC mixture TOC and AOX were observed below the limit values. The solidification was occured with mörtel + portland cement and 5% activated carbon additives and the desired limit values in HWCR were succussed for all parameters. Besides, with the addition of 30% zeolite, all parameters declined below the limit values. [ABSTRACT FROM AUTHOR]

Tannery sludge is disposed of in landfills as it is considered a special residue by the Italian legislation, creating pollution and waste. This paper aims at evaluating the performance of the anaerobic fermentation process to obtain short-chain fatty acids (SCFAs) from this waste. The assessment of the most appropriate conditions, in terms of pH, temperature, initial total solids (TSs) content, and application of oxidizing-thermal pretreatment has been developed. The batch test trials revealed that the combined microwave and hydrogen peroxide (MW-H2O2) pretreatment followed by thermophilic conditions gave the best results, in terms of the acidification yield (0.31 gCODSCFA/gVS0) and maximal SCFA concentration (above 26 g CODSCFA/L). In the tests conducted without pretreatment, the mesophilic temperature should be preferred since the acidification performances were comparable to or even better than their thermophilic counterparts. The SCFA composition analysis showed that in mesophilic fermentation, tannery sludge can generate up to 50% acetic acid (CODAc/CODSCFA), if previously pretreated (MW-H2O2). This research acts as a forerunner for the appropriate handling of this resource, to employ it for the development of a new tannery industry focused on a circular approach, rather than to simply dispose of it in landfills. [ABSTRACT FROM AUTHOR]

Worldwide, tannery sludge management has become a big challenge due to containing heavy metals, especially chromium. In this study, phytoextraction of heavy metals from tannery sludge by Napier grass (Pennisetum purpureum) is presented. Heavy metals‐chromium (Cr), copper (Cu), zinc (Zn), and lead (Pb) in tannery sludge were found at 12686, 371, 265, and 66 mg/kg, respectively. Separately, Napier grass was harvested on raw tannery sludge and garden soil and examined for 16 and 20 weeks after the sowing of seeds. Metal extraction from tannery sludge in Napier grass was found in a series of Cr > Zn > Cu > Pb; the most accumulation was reported in shoot parts of plants in the order of shoots > roots > leaves. Accumulation of Cr in Napier grass at 16 and 20 weeks was 37% and 66%, respectively. An increasing trend of heavy metal gathering in the plant was found significantly by extending the harvesting period from 16 to 20 weeks. Translocation of Cr and Pb was greater than unity (1.623 and 1.339) indicating the suitability of phytoextraction. Further, Napier grass is a rapid‐yielding tropical grass, which showed a high degree of growth tolerance that could be used for treating heavy metals contaminated tannery sludge. [ABSTRACT FROM AUTHOR]

In response to China's rapid urbanization and the increasing accumulation of construction waste, it is essential to rationalize waste treatment without damaging the environment. Besides, natural mineral resources such as stone and sand are incredibly scarce. Therefore, it is urgent to find replaceable materials. The main objective of this study is to find a substitute for construction materials obtained by treating industrial tannery sludge harmlessly and effectively to evaluate its application in concrete. This paper explores the physical and mechanical properties of concrete mixed with sludge, and different pores were characterized by Nuclear Magnetic Resonance（NMR）. The result showed that replacing 4 % fine aggregate by sludge will not have significant adverse effect on the properties concrete under the condition that coarse aggregate is 100 % recycled and 10 % cement is replaced by sludge. Industrial sludge substitution for cementitious materials and fine aggregates can alleviate the problem of industrial sludge disposal to a certain extent, which is conducive to sustainable development.

• Microwave pyrolysis is superior to conventional methods in treating tannery sludge. • Microwave pyrolysis facilitates the reduction of Cr(VI) to Cr(III). • Microwave pyrolysis efficiently removes toxic substances from tannery sludge. • Microwave pyrolysis effectively fixes heavy metals in the residues. The efficient and safe treatment of tannery sludge (TS) are causing significant concern. Pyrolysis stands as a pivotal technology in the realm of sludge treatment, offering the benefits of degrading organic matters within TS while simultaneously transforming the sludge into energy and other valuable substances. In order to develop an effective pyrolysis method with good resource recovery performance, this study employs microwave pyrolysis (MP) and conventional pyrolysis (CP) at temperatures ranging from 300 to 900 °C to investigate the transformation of chromium morphology. The results indicate that TS experienced a weight loss of 43 % during the pyrolysis process, with a combined average activation energy measured at 169.58 kJ/mol. MP, due to its more uniform and efficient heating, resulted in pronounced solid degradation, leading to increased pyrolysis gases. This enhanced the removal of organics at lower temperatures and contributed to the ring opening of aromatic hydrocarbons in the high-temperature segment. MP effectively reduced Cr(VI) to Cr(III) in TS, whereas CP showed relatively ineffective reduction of Cr(VI). Furthermore, metal compounds in TS treated by both MP and CP predominantly existed in the residue as FeCr 2 O 4 and Cr 2 O 3 at high temperatures. This suggests that more Cr(III) was immobilized in the pyrolysis residue. This study revalues the significance of MP as a promising treatment for TS, holding the potential for resource utilization. [ABSTRACT FROM AUTHOR]

The sustainability of the tannery sector largely depends on the eco-friendly management of its solid and liquid wastes. Unfortunately, the global tannery sector is strappingly suffering a significant challenge to meet environmental standards towards sustainability due to a lack of tannery sludge (TS) management facilities as well as lacking research on hazardous TS upcycling facilities to value-added products. Hence, TS from leather processing generally undergoes open dumping, responsible for environmental pollution, including water, soil, and air pollution. To ensure the sustainability of tannery sector, pioneeringly, this study attempts to conceptually design gasification based on four novel upcycling potentials of TS-to-value added products such as (i) TS to syngas with steam generation (TS-SS), (ii) TS to hydrogen with power generation (TS-HP), (iii) TS to power generation with carbon capture (TS-PCC), and (iv) TS to methanol production with power production (TS-MPP) using ASPEN Plus® simulator. Further, this research offers a comprehensive techno-economic analysis to evaluate the economic feasibility of the proposed four TS upcycling processes. The TS-PCC and TS-HP processes outperformed for their net energy efficiencies of 72.80 % and 50.22 %, correspondingly, compared to TS-HP (39.13 %) and TS-MPP (40.38 %). Regarding exergy efficiency, the TS upcycling processes coded TS-MPP and TS-HP demonstrated better net exergy efficiencies of 34.34 % and 27.20 %, respectively, compared to TS-SS and TS-PCC upcycling processes. These results confirm that TS-HP is much more efficient in terms of exergy. Furthermore, techno-economic analysis has confirmed that the developed processes, namely TS-SS and TS-PCC, are economically viable at a 70 % plant efficiency, whereas other processes, such as TS-HP and TS-MPP, are financially viable only if a subsidy of $33/ton subsidy is provided. The study findings have a significant impact on the development of circular economy-based industrial-scale TS management facilities in developing countries, which can ensure the sustainability of the leather sector. [Display omitted] • Gasification based four novel hazardous TS upcycling processes are developed. • Process Simulation and 3E Analysis of TS upcycling processes are performed. • TS-PCC and TS-HP processes outperformed for their net energy efficiencies. • TS-MPP demonstrated better net exergy efficiencies of 34.34 %. • TS-SS and TS-PCC are found to be economically viable at a 70 % plant efficiency. [ABSTRACT FROM AUTHOR]

Bioleaching of tannery sludge is an efficient and environmentally friendly way for chromium (Cr) removal, which supports the sustainable development of the leather industry. Acidithiobacillus thiooxidans has been reported effective in Cr bioleaching of tannery sludge. However, little is known about whether the presence of other benefiting species could further improve the Cr leaching efficiency of A. thiooxidans. Here, we studied the enhancing roles of four species namely Acidiphilium cryptum, Sulfobacillus acidophilus, Alicyclobacillus cycloheptanicus, and Rhodotorula mucilaginosa in chromium bioleaching of tannery sludge with A. thiooxidans by batch bioleaching experiments. We found that each of the four species facilitated the quick dominance of A. thiooxidans in the bioleaching process and significantly improved the bioleaching performance including bioleaching rate and efficiency. The bioleaching efficiency of Cr in the tannery sludge could reach 100% on the sixth day by co-inoculating A. thiooxidans and four auxiliary species. The achievements shed a light on the role of the community-level interactions on bioleaching and may also serve as guidance for managing bioleaching consortiums for better outcomes. [ABSTRACT FROM AUTHOR]

The protein hydrolysates were prepared by the thermal-alkaline treatment of tannery sludges, and then the proteins of tannery sludge were successfully extracted by the acidification method. The results showed that the extraction rate of sludge proteins increased and then decreased with the increase of NaOH dosage, time and temperature. The optimum temperature, time and NaOH dosage for extracting sludge proteins were 150 °C, 60 min and 9 g/L respectively, resulting in the highest extraction rate of 71. 2%. The interaction o£ temperature, temperature and time is the most important factor to affect the extraction rate o£ sludge proteins. The purity, total chromium concentration, common amino acid content of extracted tannery sludge proteins is 83. 01%, 1.5 mg/kg and 31. 09%, respectively. In addition, the sludge proteins have a low content of inorganic salts and ashes. The molecular weight of tannery sludge proteins ranges from 5 to 35 kDa. [ABSTRACT FROM AUTHOR]

Owing to containing high fraction of organic matter, the tannery sludge seemed to be fit for composting. Actually, it was intensively harmful to the environment, due to containing chromium (Cr). So it might undergo a long time of storage until finding a proper way to dispose it. In the storage period, it would expose the surrounding environment a risk via releasing Cr. In this study, an approach was proposed to minimize the amount of released Cr, and reveal the mechanism on immobilizing Cr. Collagen protein waste (CPW) was adopted to immobilize Cr, and it was evaluated via leaching experiment. The lowest leaching concentration of Cr was 12 mg/L, meeting the limits of related standard in China (GB 5085.3–2007, Tcr < 15 mg/L). Moreover, the compositions and functional groups of the optimum sample (12 mg/L) were also characterized, confirming that the dominant functional groups cross-linking with Cr were hydroxyl (-OH), carboxyl (-COOH), and epoxy (-COC). Importantly, density functional theory (DFT) calculation was also employed, suggesting that Cr was restrained by accepting electrons from O atoms donating by functional groups. [ABSTRACT FROM AUTHOR]

Anthropogenic pollution due to inappropriate treatment of agricultural and industrial waste has been posing an ecological threats worldwide. However, an increasing pressure and demand for sustainability in farming, brought about the use of many hazardous chemicals and industrial wastes, such as the use of tannery sludge as an effective organic manure. This study was initiated as a follow up investigation to assess the potential ecological risk; the level of metal pollution in soil and Lycopersicon esculentum (L. esculentum) and its potentiality for in-situ phytoextraction in Hausawan - Kaba, Kano State, Nigeria. The suspected sludge, soil and plants grown in three different farms; farmland-1, farmland-2, and farmland-3 (control) in the study area were collected. The concentration of Chromium (Cr), Cadmium (Cd) and Lead (Pb) were spectrophotometrically detected. In addition, Bioaccumulation Factors (B.F), Pollution Index (P.I), Ecological Risk Index (ERI) and Potential Ecological Risk (PER) index of metals in three different sampling sites were calculated. The results indicated that PER value and concentration of metals in soil from farmland-1 is higher than that of farmland-2 and control site. Cd was observed to be the major contributor of ERI and had the highest average concentration in soil and plant samples. The results also showed that properties of soil such as pH, Cation Exchange Capacity (CEC), and organic matter content influenced the metal concentrations. Bioaccumulation Factor (B.F) > 1 observed for the plants manifested their phytoremediation potential. Therefore, the study concluded that based on the PER analysis, the study area is a potentially “low ecological risk” environment and the L. esculentum has a great potential to uptake large quantity of Cd, Pb and Cr in its root system; translocate to shoots and can be evidently used as hyperaccumulator for in-situ phytoextraction when cultivated in a soil contaminated with either Cr, Cd and/or Pb.

The current treatment (landfill, composting, and incineration) of chromium-containing sludge can contaminate soil, surface water and groundwater. In this study, tannery sludge and granulated blast furnace slag was chosen to make a geopolymer and its ability to resist acid and alkali was investigated by immersing them in different solutions for 28 d. The experimental results showed that when pH value increased from 1.0 to 13.0, the compressive strength linearly increased from 36.7 MPa to 68.4 MPa while the porosity decreased from 4.37 % to 3.4 %. The results of NMR, SEM and FIR showed that the H+ in HCl acid solution depolymerized the three-dimensional network structure of the geopolymer and acicular product (CaCl 2) was generated, which leaded to the increase in small pores and large pores. However, when immersed in alkaline solution the secondary polymerization occurred and more C-S-H appeared, making the geopolymer denser. In addition, the environmental benefits of the geopolymer were investigated and the results showed that the leaching concentration ranges of total chromium and Cr(VI) were 1.21–0.97 mg/L and 0.04–0.06 mg/L, both of which were far below the requirement of standards. The CO 2 emission was 8.06 kg/(m3 Mpa), which was roughly half of the ordinary Portland cement (18 kg/(m³ MPa)). Therefore, the geopolymer could be a way to treat tannery sludge and the geopolymer made of tannery sludge and slag had the potential to be environmentally friendly building materials. • STSG is granulated blast furnace slag and chrome-containing tannery sludge based geopolymer. • It showed significant acid resistance and alkaline environment was beneficial to its performance. • Its CO 2 emission was half of the OPC and leaching concentration of total chromium and Cr(Ⅵ) were far lower than standard. • STSG is an effective way to transform wastes into resources and environment-friendly building material. [ABSTRACT FROM AUTHOR]

Abstract Leather making is the process of converting raw hides into leather. Amounts of solid waste containing hazardous and high value components are generated during this process. Therefore, the elimination of the potential pollution and resource utilization of leather solid waste are the primary research hotspots. Herein, we comprehensively review the recent advances in the resource utilization of leather solid waste generated from chrome tanning including the utilization of tannery hair waste, untanned solid waste, chrome-containing leather solid waste, tannery sludge and finished leather waste, emphasizing on the general and novel utilization approaches. Finally, the remaining challenges and perspectives were summarized, in order to accelerate the development of resource utilization of leather solid waste. Graphical abstract

The sludge discharged from the preliminary treatment of a tannery effluent plant is considered for the development of activated carbon. The carbon is developed by impregnating the precursor in an acid activating agent (H2SO4). The activation of the preliminary sludge is carried under different operating conditions such as varying temperature (600-800°C) and acid impregnation ratios (1 and 2) for two different particle sizes such as 600 µm and 300 µm. A carbon content of 74% was obtained from the sludge activated at a temperature of 650°C for 3 hrs with particle size 300 µm (3S-52). The characterization of developed tannery sludge carbon 3S-52 was carried out using Scanning Electron Microscopy (SEM) and Brunauer--Emmett--Teller (BET) isotherm plots to analyze the surface morphology and pore size distribution, pore volume and pore diameter. The nitrogen physisorption for surface analysis and pore size distribution was also done at -196°C (77K). The pore volume, pore size and the surface area of 3S-52 were determined as 0.133 cm³/g, 5.15 A° and 212.32 m²/g respectively. These results demonstrated feasibility to treat the tannery sludge as a precursor to produce microporous activated carbons. [ABSTRACT FROM AUTHOR]

Biosorption efficacy of Bacillus strain DPAML065, isolated from the tannery sludge, was appraised for the removal of toxic hexavalent chromium (VI) ions from synthetic wastewater. Effects of the process variable on biosorbent surface by variation in pH, metal Cr(VI) concentration and retention time were examined using batch experiments. The isolated Bacillus strain biosorbent was studied for its morphology and surface chemistry through FE-SEM, EDX and FTIR. It discloses that, the reduction mechanism of Cr(VI) during the process is mainly attributed to precipitation in addition to the functional groups (such as –COOH, –OH, C–O, P=O) present on the cellular matrix of Bacillus. Biochemical tests and 16s rRNA sequencing were also performed to identify the biosorbent at the genus level. A 95% Cr(VI) removal efficiency was procured by Bacillus strain DPAML065 biosorbent at pH 6, incubation period 24 h, 80 mg/L initial feed concentration and operational temperature 35 °C. Equilibrium behaviour of chromium binding follows the Langmuir isotherm model (R2 = 0.968) with an adsorption capacity of 106.38 mg/g. Kinetic modelling disseminates that biosorption of Cr(VI) ions by Bacillus strain DPAML065 obeyed pseudo-second-order model (R2 = 0.984) rather than the pseudo-first-order model. Concisely, the results indicate that the Bacillus strain DPAML065 is a potential, economically feasible and eco-friendly biosorbent which can be effectively used for removal of chromium (VI) from wastewater. [ABSTRACT FROM AUTHOR]

The paper analyses the possibilities of recycling tannery sludge (TS) in clay brick manufacture. The physico-mechanical properties and microstructure of clay brick and cement mortars vary depending on TS content in the mix. 2.5%-15.0 TS (by weight) was added to clay bricks fired at 1000 °C temperature and maintained at maximum heat for 1 hour. The tests revealed that 2.5 TS addition had a positive effect on physical and mechanical properties of clay brick: reduce drying and firing shrinkage, reduce water absorption, and increase compressive strength. Chromium leaching values in clay brick (2.5% TS) were found not to exceed the limit values set forth in 2003/33/EC. [ABSTRACT FROM AUTHOR]

Tannery sludge, a challenging waste, was utilized as a substrate for the production of Short-Chain Fatty Acids (SCFAs) through a series of six thermophilic Continuous Stirred-Tank Reactor runs. The sludge was subjected to a mild thermal pre-treatment and incorporated zeolites (chabazite in run II, and clinoptilolite in run III) in the acidification process. Results highlighted zeolites' impact on chromium concentration and the SCFAs/COD SOL ratio. Ammonia release remained consistent at around 47 % and 51 % for run I and II, respectively, but surpassed 60% in run III, suggesting limited zeolite effectiveness in NH 4 absorption. Chromium release in the liquid fraction, due to thermal pretreatment, reached 335 mg/L. While in tests without zeolite, complete removal proved challenging, in zeolite-amended runs, complete removal was achieved, showcasing the materials' heavy metal absorption capacity. SCFA concentrations reached 20260 mgCOD/L, with acidification efficiency varying; runs I and III had ratios around 0.70 COD/COD, while run II showed substantial improvement (0.92) with chabazite. Anaerobic fermentation-digestion mass balance indicated a 41% reduction in landfill sludge mass, reducing its environmental footprint while yielding valuable byproducts like biogas and SCFAs. These findings underscore zeolites' potential in heavy metal absorption and acidification process enhancement, paving the way for applications with tannery sludge. • Up to 20260 mgCOD/L SCFAs with thermally pretreated tannery sludge with HRT 8d. • Chabazite improved sCOD removal by around 20% compared to the blank. • Chabazite with HRT8d allowed for 92% of the sCOD to be converted into SCFAs. • Only runs with Zeolites had a complete removal of Cr from the fermentation broth. [ABSTRACT FROM AUTHOR]

Heavy metal stabilization is an effective method to treat chromium in tannery sludge. Here we show that mainly investigated NaH 2 PO 4 (MSP) and organic matter (OM) to stabilize chromium in tannery sludge. The experimental investigation revealed that the addition of montmorillonite (MMT) and MSP samples showed a significant increase in the percentage of reducible and oxidizable Cr in the former compared to the samples with the addition of MMT. This is attributed to the formation of Cr–O bond, which allows the MSP to undergo an inner-sphere complexation reaction with the metal oxide of Cr via ligand exchange. Significantly, the MSP moiety adsorbs on the surface of OM through monodentate, which increases the adsorption sites of OM for Cr6+ and promotes the reduction of Cr6+ to Cr3+. Moreover, PO 4 3− reacts with Cr3+ to produce CrPO 4 precipitation, thus reducing the free Cr3+ content. Finally, DFT calculations confirmed that a ternary system is formed between PO 4 3−, OM, and Cr, and the binding energy is negative, which indicated that PO 4 3− could co-stabilize Cr with OM. [Display omitted] • Stabilization of Cr by MSP is achieved through ligand exchange. • MSP amplifies the adsorption sites of OM and promotes the reduction of Cr6+. • PO 4 3− from the dissociation of MSP reacts with Cr3+ to form CrPO 4 precipitate. • Results from DFT indicate the presence of PO 4 3−, OM, and Cr in a ternary system. [ABSTRACT FROM AUTHOR]

The recovery of chromium from tannery sludge and from its respective ash constitutes an alternative process of tannery sludge treatment in order to ensure safe disposal. Chromium recovery was investigated based on simple hydrometallurgical processes, that is, leaching using aqueous solutions of acids. Leaching was applied either directly to air-dried sludge or to its respective ash, which results from thermal treatment of the air-dried sludge under anoxic conditions. The major parameters that influence the effectiveness of the leaching process were investigated to optimize chromium leaching. Specifically, contact time, pH value, temperature, liquid per solid ratio, and leaching agent (H2SO4 or HCl) were tested. Leached chromium, obtained directly from the initial tannery waste after 100 min leaching with H2SO4 at pH 1 and at 60°C, was used for the trivalent chromium precipitation by adding magnesia, calcium hydroxide, and sodium hydroxide as precipitation agents. The results revealed satisfactory leaching of trivalent chromium directly from the air-dried sludge. The chromium content in the solid precipitated using sodium hydroxide was about 59 wt %. In contrast, chromium was difficult to leach from the respective ash. [ABSTRACT FROM AUTHOR]

This investigation focused on the simultaneous decrease of tannery sludge and the reduction of its high chromium (Cr(VI)) content. This was accomplished through the addition of mixed bacterial strains that were cultured in the laboratory, subsequent to their isolation from tannery sludge. The results indicated that under anaerobic conditions, the amount of the tannery sludge was decreased by 27% with these mixed bacteria. The impacts of various parameters were explored, such as pH, processing duration, strain inoculation, and temperature. Along with the decreased volume of sludge, the Cr(VI) concentration was lowered as well. Among the isolated bacterial strains, WY601 (belonging to Stenotrophomonas sp.) demonstrated the highest Cr(VI) resistance; from an initial concentration of 300 mg L−1, the Cr(VI) level was decreased by 90% within 65 h. Hexavalent chromate reductase was found to be localized primarily within the extracellular membrane or adsorbed to its surface, and a mechanism was proposed for the removal of Cr(VI) via WY601. Further, the WY601 isolate was found to be tolerant to other toxic heavy metals. In summary, the isolated mixed bacterial strains in our study demonstrated a strong potential for the treatment of tannery sludge, as they could simultaneously decrease its volume while lowering high Cr(VI) levels. Unlabelled Image • We treated high Cr containing tannery sludge with mixed bacteria, which were isolated from tannery sludge. • The amount of the sludge decreased by 27% with these mixed bacteria and the concentration of Cr(VI) reduced simultaneously. • A new strain of WY601 (belongings to Stenotrophomonas sp.) showed very high resistance and reduction ability to Cr (VI). • It's potential for the application in tannery industries as an environmentally compatible technology. [ABSTRACT FROM AUTHOR]

Bioleaching is a successful and cost-effective strategy with which to remove heavy metals from contaminated sludge. Most of the current studies on bioleaching are concentrated on municipal sludge, and bioleaching of tannery sludge has not been studied extensively. The present study investigated the bioleaching of chromium (Cr) from tannery sludge using the mixture of iron-oxidizing bacteria (Acidithiobacillus ferrooxidans) and sulfur-oxidizing bacteria (Acidithiobacillus thiooxidans) in batch flasks. Changes in the pH, ORP, Fe2+ and SO4 2- concentrations and the efficiency of bioleaching of Cr from tannery sludge were analyzed. The results showed that the bioleaching efficiency of co-inoculation of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans was better than any of them. The co-inoculation with a mixture of iron-oxidizing and sulfur-oxidizing bacteria together with the addition of Fe2+ and elemental sulfur accelerated acid production and an increase in the oxidation-reduction potential originating from the bio-oxidation of Fe2+ and elemental sulfur. The bioleaching efficiency of co-inoculation of Cr increased as the pH of sludge decreased, reaching a maximum removal of 97.1%. The residual Cr concentration which is 531 mg/kg in bioleached sludge is below the thresholds for agriculture, according to China National Standard GB4284-2018 (<1000 mg/kg dry weight) for pollutants in sludges from agricultural use. The bioleaching efficiency was distinctly correlated with pH or ORP. The data presented showed that the bioleaching efficiency was negatively correlated (R² = 0.9864) with pH and positively (R² = 0.9821) with ORP, which implied the existence of a substantial indirect leaching mechanism responsible for Cr solubilization. [ABSTRACT FROM AUTHOR]

Chromium laden waste produced from tannery industry was immobilized in ceramic matrix for fabrication of the tubular single channel microfiltration membranes by extrusion. The presence of chromia resulted in substitutional solid solution formation with alumina and catalyzed mullite phase growth, hence increasing the mechanical and chemical stability of the membranes. The structural, morphological and water permeation characteristics of the membranes were studied to analyze their formation mechanism and effect of different parameters, viz. the sintering temperature, amount of waste added, presence of organics and extent of chromium immobilization. The surface of the macroporous membrane was hydrophobically modified, by polydimethylsiloxane (PDMS), producing contact angle of 141°. The process efficiency of the hydrophobic membrane was assessed in terms of the removal of atrazine, a contaminant of emerging concern, following the principle of hydrophobic interaction. Effect of different operating parameters affecting atrazine removal, viz. transmembrane pressure, cross flow velocity and filtration time was studied in cross flow filtration mode. High atrazine removal of >95% was obtained along with the maintenance of high flux during the filtration operation. The prepared cost-effective microfiltration membranes can thus be further modified for efficient water treatment applications. [ABSTRACT FROM AUTHOR]

• Repeated-batch process was applicable for chromium bioleaching from tannery sludge. • Two-stage process reduced sludge retention time and achieved high treatment rate. • High abundance of Acidithiobacillus was critical for effective chromium bioleaching. By using an enriched microbial community, repeated-batch processes for bioleaching of chromium from tannery sludge was set up. The effects of batch-feeding ratio (FR) and sludge total solid (TS) on sludge retention time and treatment rate were investigated and a two-stage repeated-batch process was developed. The results of flask-scale repeated-batch bioleaching suggested that the sludge TS and FR impacted the bioleaching performance and a maximum sludge treatment rate of 6.82 g/L/d was achieved at a TS of 6% and FR of 0.5. Repeated-batch bioleaching using single continuously stirred tank reactor (CSTR) did not yield a satisfactory treatment rate. However, a two-stage process (two CSTRs in series) successfully reduced the sludge retention time and a maximum sludge treatment rate of 4.60 g/L/d with over 90% chromium removal ratio was achieved. The high relative abundance of Acidithiobacillus was found critical for effective bioleaching; however, other genera including Sulfobacillus, Alicyclobacillus, Mycobacterium , and Acidiphilium might also play important supplementary roles. [ABSTRACT FROM AUTHOR]

A field experiment was conducted to observe the effect of TS amendments on soil enzymes and phytoremediation potential of two economically important cultivars of geranium. Different doses of TS were applied in soil to examine threshold limit of HMs where geranium cultivars can be grown successfully in contaminated sites. Treatment variation significantly affected pH, EC, OC, N, P, K and HM content in soil after 50 days of incubation. After harvest, both cultivars were examined to assess the impact of various treatments on their fresh herb, dry matter, essential oil yield and HM accumulation. C/G ratio close to 1 was observed at 50 tha−1 sludge treatment in both cultivars. Urease and β-glucosidase activities in soil were maximum at 50 tha−1 whereas dehydrogenase and phosphatase activities were maximum at 100 tha−1 in both cultivars. β-glucosidase, acid and alkaline phosphatase, urease and dehydrogenase activities were relatively high after 85 days over 45 days in both cultivars. Maximum metal uptake was found in roots of cv. Bourbon followed by leaves. Geranium was observed to be a good candidate for phytoremediation as it mitigates metal toxicity by root absorption and cv. Bourbon is better candidate for the same. [ABSTRACT FROM AUTHOR]

Ethanol and sludge were added into a high-pressure reactor to realize the liquefaction of sludge to oil by using the supercritical characteristics and hydrogen supply of ethanol. Then the residue and oil products were extracted by vacuum filter and rotary distillation, respectively. The effects of reaction temperature, residence time, sludge addition and catalyst type on the oil production rate have been investigated. It was found that when the reaction temperature is290 ℃, the residence time is 60 min and the solid-liquid ratio of sludge/ethanol is 1/20, the oil yield is the highest and can be increased to 45. 5% when sodium hydroxide is added. According to the analysis of the liquefied oil, the content of C element is increased by 105. 8%, the content of H element is increased by 71. 8%, the content of O element is greatly reduced, and the calorific value of the liquefied oil is 20. 12 MJ/kg higher than that of the sludge.The carbon chain of each component in liquefied oil is concentrated in C16-C20, mainly esters, accounting for56. 36%, while the content of alkanes is low, only 6. 32%. The esters should be deoxidized to further improve oil quality. [ABSTRACT FROM AUTHOR]

To reduce heavy metals (HMs) contamination from tannery sludge, this study investigated the migration behaviour of arsenic (As) and chromium (Cr) at 700–900 °C using CO 2 gasification. The HMs enrichment results showed that As contents of ash decreased (6.42→1.87 mg/kg) while Cr contents increased (41.40→78.24 mg/kg) over 700–900 °C. More Si-O bonds and fewer Ca-O bonds with increasing temperature in ash primarily determined this migration behaviour of HMs. Meanwhile, the proportions of toxic As(III) and Cr(VI) declined from 96.02% and 64.26–76.96% and 21.24%, forming As(0) and Cr(III) with less toxicity. This reduction was conducted via two pathways: (i) carbon reduced As(III)/Cr(VI) and (ii) carbon reduced Fe(II)/Fe(III) to Fe(0), then Fe(0) reduced As(III)/Cr(VI) assisted with carbon via Fe(0)→Fe(II)→Fe(III). However, free calcium ions oxidized As(0)/Cr(III) to As(III)/Cr(VI) at 700 ○C. At higher temperatures, silicate glass conversion of ash immobilized free calcium ions and barely oxidized HMs. Furthermore, this study identified the positive effect of increasing temperature on enhancing the stability of HMs in ash by transforming bioavailable HMs into non-bioavailable HMs, which decreased the leaching toxicity and environmental risk. Regarding HMs emissions control and cold gas efficiency, CO 2 gasification treatment of tannery sludge is most effective at 800 °C. [Display omitted] • Silicate glass transformation of ash occurred above 800 °C under CO 2 gasification. • Temperatures exceed 800 °C greatly reduced As(III)/Cr(VI) to As(0)/Cr(III). • Over 88.16% and 87.63% of As and Cr in ash were non-bioavailable above 800 °C. • Environmental risk of ash was at low risk at 700–900 ○C under CO 2 gasification. • 800 °C was more suitable for tannery sludge disposal under CO 2 gasification. [ABSTRACT FROM AUTHOR]

Exploiting tannery sludge as potential source of short-chain fatty acids (SCFAs) and biogas offers a promising solution for a new management of this waste, which is currently landfilled due to chromium presence. This study assessed the optimal hydrogen peroxide (H 2 O 2) dosage, temperature (T) and hydraulic retention time (HRT) through hydrolysis and acidification tests to maximize SCFAs production and to recover biogas from the solid-rich fermentation residue. A dosage of 0.4 g H 2 O 2 /g TS and mesophilic T (40 °C) were the most suitable conditions to promote sludge biodegradability and SCFAs production in batch tests. Afterwards, among the two investigated HRT (4 and 8 days) in the semi-continuous processes, longer HRT increased SCFAs concentration and acidification yield up to 16.0 g COD SCFAs /L and 0.32 g COD SCFA /g VS 0 , respectively. Chromium was initially released in the liquid phase, but completely removed in less than 5 HRTs, allowing the utilization of fermentation liquid without environmental-safety issues. Finally, the solid-rich residue showed its potential as biogas source, having a specific gas production (SGP) of 0.48 m3/kg VS. According to the mass balance assessment, more than 50% of sludge can be saved from landfill, with potential economic benefit close to 2,700,000 €/y compared to the current management practice. [Display omitted] • An increase of 11–13% of tannery sludge biodegradability (k h) with 0.4 g H 2 O 2 /g TS. • Higher HRT (8 d) boosted the mesophilic SCFAs yield compared to shorter one (4 d). • Acetic acid accounted for more than 60% of the total SCFAs (COD basis). • Fermentation broth had no environmental issues due to the Cr precipitation. • The mass balance assessment showed a reduction of > 50% of this sludge to landfill. [ABSTRACT FROM AUTHOR]

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. • A simple method is proposed to stabilize Cr(Ⅲ) in tannery sludge via adding kaolin. • The strongest binding capacity of Cr(Ⅲ) to OH is confirmed via DFT theoretical calculations. • The addition of kaolin can synergize with the organic matter in tannery sludge to stabilize Cr(Ⅲ) via forming an organic matter-Cr(Ⅲ)-kaolin ternary complex. [ABSTRACT FROM AUTHOR]

Chromium (Cr) accumulation in soil and plants has been reported after successive applications of tannery sludge, which is a matter of concern because Cr can promote environmental contamination and affect the food chain. In this study, we evaluated the growth of and Cr accumulation in maize and cowpea after seven years of consecutive applications of composted tannery sludge (CTS) under field conditions. The experiment consisted of application of CTS at five levels: 0 (control), 2.5, 5, 10, and 20 Mg ha-1 (dry basis). The growth of and Cr accumulation in maize and cowpea were evaluated at 75 and 65 days after plant emergence, respectively. CTS application increased the shoot biomass of maize and cowpea. Accumulation of Cr was similar for both plant species, with higher accumulation in roots. However, Cr accumulation in grains differed as the CTS doses increased; cowpea accumulated more Cr while maize did not accumulate more Cr in grains with higher CTS doses. In conclusion, application of CTS led to higher growth of maize and cowpea plants, and both species exhibited similar Cr accumulation in roots and shoots.