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The application or disposal of char derived from tannery sludge is directly influenced by the mobility and bioavailability of Cr during pyrolysis process. This study focused on the changes of Cr speciation and organic matter in tannery sludge during low-temperature pyrolysis (100-400 °C) to evaluate the toxicity of char in terms of the leaching possibility of Cr. The results showed that (1) lower char yield and more porous structure were observed after pyrolysis. (2) Higher pyrolysis temperature increased Cr content in the char; however, Cr in this case was converted into the residual fraction which minimized its bioavailability therefore lowers its potential risk to the environment. (3) Organic matters in the acid and alkali leachates were mainly humic acid-like substance, and condensed organic matter might appear at 200 °C and then destruct. (4) Despite the comparatively high content of Cr in the char, the leaching toxicity of char was within the security range according to the national standard of China. The Cr content in the acid and alkali leachates decreased to the range of 16.5-35.3 and 0.2-6.8 mg/L, respectively. It was suggested that the potential toxicity of tannery sludge from Cr could be reduced before utilization or disposal by pyrolysis, especially under 400 °C. [ABSTRACT FROM AUTHOR]

Bioleaching has been demonstrated as a feasible and effective technology for removing Cr(III) from tannery sludge, but the recovery of bioleachate derived from tannery sludge by bioleaching is rarely reported. The purpose of this study was therefore to figure out the components of bioleachate and develop a chromium-iron tanning agent using the bioleachate and reuse it in tanning process. Component analysis indicated that the bioleachate contained high concentrations of Cr and Fe, as well as small quantities of other elements. The dissolved organic matter in the bioleachate was detected to be less than 1000 mg/L, which was mainly composed of polysaccharide, aliphatic or aromatic carboxylic acid compounds. A chromium-iron tanning agent was then prepared using the bioleachate, iron and chromium salts in a suitable ligand environment, and reused for tanning. In the tanning experiments, the pickled pigskins were tanned with this prepared tanning agent and the results indicated that chromium-iron tanned crust leather presented comparable physical and mechanical properties as well as air permeability when compared with conventional chromium tanned crust leather, but the thermal shrinkage temperature was about 4 °C lower than that of the chromium tanned crust leather. Therefore, this study provides not only a reuse route for tannery sludge bioleachate but also a clean tanning agent for leather processing. [ABSTRACT FROM AUTHOR]

With the rapid development of tanning industry, a noticeable number of chromium-containing tannery sludge (Cr-ITS) has been produced, which poses an impending threat to ecosystem and human health, and needs special disposal ahead of landfilling. In this study, an alternative method for thermally stabilizing Cr-ITS via employing ZnO and TiO 2 as dopants was developed. Oxidation of Cr(III), competitive formation of Cr-spinel, immobilization of Cr, and leaching toxicity of sintered products were investigated during Cr-ITS thermal treatment at the temperature range of 800–1200 ℃. Cr in Cr-ITS could be immobilized effectively through transforming Cr into spinel structure, as demonstrated by XRD analysis. Thus, the toxic Cr-ITS would be remedied and detoxified. Co-doping of ZnO and TiO 2 not only noticeable inhibited oxidation of Cr(III), but also accelerated conversion of Cr(III) to spinel dominated by ZnCr 2 O 4. Increasing temperature was conducive to the formation of spinel. Most of Cr were incorporated into spinel phase when thermal treatment temperature was over 1000 ℃, where Cr(VI) leached was not detected, and leaching concentrations of Cr and Zn after prolonged leaching time of 25 d were 13.32 mg/L and 23.07 mg/L respectively, which all meet regulatory limits. When increasing temperature to 1100 ℃, their leaching risk was further reduced to 4.39 mg/L and 5.24 mg/L respectively, successfully achieving simultaneous stabilization of heavy metals. The overall results indicate that Cr-ITS immobilized with ZnO and TiO 2 is a promising and feasible strategy by incorporating Cr into spinel phases using sintering temperature reached in ceramic industry. • Real chromium-containing tannery sludge was thermally treated under air atmosphere. • No Cr(VI) was detected in samples sintered above 1000℃. • Co-doping of ZnO and TiO 2 can inhibit oxidation of Cr(III) and accelerate formation of spinel dominated by ZnCr 2 O 4. • Leaching risks of Cr and Zn satisfy the regulatory limits by a prolonged leaching experiment. • Tannery sludge can be effectively detoxified and stabilized via synergic action of metals. [ABSTRACT FROM AUTHOR]

Tannery sludge contained plenty of organic matter, and the organic substance stability had direct impact on its derived chars' utilization. In this paper, the stabilization of tannery sludge and the variation of humic acid-like (HAL) extracted by different methods were investigated in a magnetic stirring reactor under low temperature pyrolysis of 100-400 °C. Results showed that the aromatic structure of pyrolysis chars increased with the increase of temperature and time. The char contained highly aromatic structure and relatively small dissolved organic matters (DOM) at 300 °C. The similar behaviors appeared in two HAL series by different extraction methods. The N content, H/C value, and aliphatic structures of HAL decreased with the increase of pyrolysis temperature, while the C/N value and aromatic structures increased with the rise of pyrolysis temperature. The composition and functional groups of HAL were similar with the purchased humic acid (HA). The fluorescence spectra revealed that two main peaks were found at Ex/Em = 239/363-368 nm and 283/359-368 nm in each HAL series from raw and 100 °C pyrolysis tannery sludge, representing a protein-like matter. The new peak appeared at Ex/Em = 263-283/388 nm in each HAL series from 200 °C pyrolysis tannery sludge-represented humic acid-like matter. The fluorescence intensity increased strongly compared to the other two peak intensity. Therefore, the humification of organic matter was increased by pyrolyzing. Notably, the HAL from 200 °C pyrolysis tannery sludge contained simple molecular structure, and the polycondensation increased but with a relative lower humification degree compared to soil HAL and purchased HA. Therefore, the sludge needs further oxidation. The humic substance was negligible by direct extraction when the temperature was 300 and 400 °C. [ABSTRACT FROM AUTHOR]

An investigation was carried out into the crop uptake of Cr(III) and its accumulation in two alkaline soils that have received tannery sludge as organic fertilizer for many years. The Cr(III) concentrations in the soils did not exceed the safe levels. However, Cr content in wheat and rice straw was up to 17 mg kg-1 in recently sludge-applied soils, and Cr content in wheat grain exceeded the hygienic standard of China for food. After the sludge application, Cr(III) remained in top soil in the first few years and its transfer to a depth of 60 cm was discovered in the soils with application of the sludge for over 15 years. A sequential extraction procedure and seedling culture experiment were used to assess the Cr(III) chemical forms and their phytoavailability. The results showed that Cr in straw and wheat grain was significantly correlated with total Cr(III), H2O-NH4OAc and DPTA extractable Cr(III). 2 mol L-1 HCl extractable Cr(III) was an important source of soil Cr available to the crops. In the seedling culture experiment, it was discovered that 1 mol L-1 NH4OAc and 0.05 mol L-1 DPTA exhibited higher extraction capacities for Cr(III) than plant uptake at lower soil Cr(III) concentration, while they were coincident at higher Cr(III) concentration. Cr content in wheat seedlings reached a maximum of 23.5 mg kg-1, which was hardly attainable under field conditions. [ABSTRACT FROM AUTHOR]

Efficient and sustainable treatment of real chromium-containing tannery sludge is attracting an increasing concern due to its potentially severe impacts on eco-system and human-beings. In this study, a greener alternative was studied for the removal and recovery of chromium from tannery sludge by ultrasound-assisted supercritical water oxidation approach. Supercritical water oxidation or ultrasonic treatment was respectively conducted to explore the detailed experiment parameters on organic destruction and chromium liberation. Ultrasound-assisted supercritical water oxidation treatment was then investigated to further explore the mechanism for Cr liberation. Experimental results indicate that supercritical water oxidation or ultrasonic treatment cannot effectively remove Cr from tannery sludge, while almost all Cr (99.6%) will be liberated to supernatant at optimized experimental conditions by ultrasound-assisted supercritical water oxidation treatment, with TOC removal efficiency over 99.9%. This indicates that toxic substances in sludge will be completely removed after ultrasound-assisted supercritical water oxidation treatment, and liberated Cr in the supernatant can be re-utilized after purification. This process is an efficient alternative for toxicity removal and chromium recovery from real tannery sludge contaminated by high concentration of Cr and refractory organics. • Ultrasonic assisted supercritical water oxidation approach is used for tannery sludge treatment. • About 99.9% COD and 99.6% Cr can be simultaneously removed or recovered. • UL pretreatment can facilitate TOC removal and Cr recovery during SCWO treatment. • Cr in sludge can be completely liberated and recovered as Cr 2 O 7 2− after SCWO treatment. [ABSTRACT FROM AUTHOR]

As a hazardous waste, the disposal of chromium enriched tannery sludge has attracted increasing public concern due to its potential adverse risks towards the environment. And incineration is considered to be an effective method to stabilize heavy metals, like Cr, in solid phase during tannery sludge treatment. In this study, real chromium enriched tannery sludge without pre-treatment was incinerated at 300°C–1200 °C under air atmosphere to investigate the transformation of chromium speciation. Here detailed thermal behavior, phase transformation and chromium speciation were characterized by TG-DSC, XRD and XPS, respectively. Experimental results show that content of Cr(VI) increases gradually with the increase of temperature from 300 °C to 500 °C and reaches a maximal level of 46% total Cr at 500 °C, with different Cr(VI) species of CaCrO 4 , MgCrO 4 and Cr 5 O 12. However, the content of Cr(VI) decreases gradually with the further increase of temperature, with only about 5% Cr(VI) at high temperature of 1200 °C, due to formation of Cr(III) species of Cr 2 O 3 crystallite and MgCr 2 O 4 spinel. Besides, a growing number of hexagonally shaped flake-like crystallite Cr 2 O 3 can be discovered from characterization results of XRD and SEM. Finally, the reduction of CaCrO 4 to Cr 2 O 3 in the presence of SiO 2 is thermodynamically feasible over 700 °C, indicating possible transformation of Cr(VI) to Cr(III) through controlled incineration temperature. • Chromium-rich tannery sludge was incinerated under air atmosphere without any pretreatment. • Cr(VI) content reaches a maximal level of 46% of total Cr at 500 °C. • Most of Cr(VI) is gradually reduced to Cr(III) with the temperature increasing over 500 °C. • SiO 2 presented in tannery sludge can induce the reduction of Cr(VI) to Cr(III) above 700 °C. • Main Cr-containing crystalline phases are CaCrO 4 , MgCrO 4 , MgCr 2 O 4 , Cr 5 O 12 and Cr 2 O 3. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • Real Cr-bearing tannery sludge and coal fly ash were thermally treated. • Doping coal fly ash can inhibit Cr(III) oxidation and promote Cr-crystal formation. • Cr can be securely immobilized in Cr-spinel and Uvarovite crystal above 1000 °C. • Leaching risk of Cr satisfies regulatory limit by a prolonged leaching experiment. • Tannery sludge can be detoxified and stabilized via "waste treatment with waste". [ABSTRACT FROM AUTHOR]