Electrolytic recovery of chromium salts from tannery wastewater

Electrochemical treatment of textile wastewater using copper electrodes

The conventional electrode aluminum used in electrocoagulation (EC) for the textile wastewater undergoes pitting type of corrosion, so dissolution of the same is very high during electrolysis. This research focuses on the treatment of real-time textile effluent with copper electrodes that corrode uniformly during electrolysis, with optimizing operating parameters for high color removal efficiency (CRE%). The sludge acquired was analyzed by XPS and XRD to study the mechanism of dye removal. The treated effluent was subjected to phytotoxicity analysis using Vigna radiata to study the toxicity effect of the intermediary products. 98.6% of CRE was attained in treating the effluent with copper electrodes. XPS and XRD results showed that both Cu(OH)2 and CuO served as coagulants in the dye removal. The phytotoxicity results showed that the percentage of germination, shoot and root lengths of Vigna radiata in the treated effluent were similar to the results obtained for the control.

Titanium-Porphyrin Metal-Organic Frameworks as Visible-Light-Driven Catalysts for Highly Efficient Sonophotocatalytic Reduction of Cr(VI)

In this work, we synthesized and characterized four titanium-porphyrin metal-organic frameworks (MOFs) [DGIST-1(M), M = Co(II), Fe(III), Zn(II), and H₂] and used them as visible-light-driven catalysts for sonophotocatalytic Cr(VI) reduction. DGIST-1(M) exhibited open-framework, broad light absorption stemmed from ligand and sensitive photocurrent responses owing to the integration of one-dimensional Ti-oxo chains and 4-connected conjugated TCPP ligand (TCPP = tetrakis(4-carboxyphenyl)-porphyrin). DGIST-1(M) presented efficient reduction of Cr(VI) to Cr(III) in aqueous solution when used as sonophotocatalytic catalysts. The average reduction rates upon Cr(VI) were 0.920, 0.476, 0.377, and 0.194 mg·L^-1·min^-1 for DGIST-1(H₂), DGIST-1(Zn), DGIST-1(Co), and DGIST-1(Fe), which are 1.15-2.45 times higher than those in photocatalysis. Sonophotocatalytic experiments and electron paramagnetic resonance measurement proved that Ti-oxo chain units and porphyrin ligand in the structures of DGIST-1(M) existed as catalytic active centers for sonophotocatalytic reduction of Cr(VI). Photoluminescence and UV absorption spectra revealed that the unity of photocatalysis and sonochemistry strengthened the migration of photogenerated electrons from DGIST-1(M) to Cr(VI), which improved the activities of catalysts. This study suggested that the association of titanium-porphyrin MOFs and sonophotocatalytic technology is an impactful program for enhancing MOF-based photocatalytic systems.

Tannery wastewater treatment: conventional and promising processes, an updated 20-year review

Mismanagement of various wastes especially waste water produced by tanning processes has caused serious environmental problems and ultimately impaired human health. Constant efforts have been making to alleviate the pollution of tannery wastewater (TWW), yet terminal treatment still takes dominance. In this review, research on TWW treatment from 2000 to 2021 was summarized, and main methods such as coagulation and flocculation, adsorption, biological treatment, membrane filtration, advanced oxidation process were briefly discussed. More detailed introduction was given to the method of electrochemical treatment since it has excellent performance such as environmental friendliness and high efficiency, hence attracting more and more research attention in recent years. In view of the harsh physi-chemical conditions of TWW, integrated or combined treatment methods are accordingly recommended with better performance and multi-function, however comprehensive studies on optimization of methods combination and cost-effectiveness are needed. The certain issues that the residue Cr in treatment sludge and high salinity in effluent still remain were put forward in this work and potential solutions were provided. Moreover, this review proposed the perspective that realizing multi-function, recycling, and intensification should be the developing direction for future TWW treatment. This review is expected to provide a general guide for researchers who aspire to ameliorate TWW pollution problems and understand various methods utilized in this field.

Graphical abstract

Treatment of tannery effluent by chemical coagulation combined with batch-recirculated electro-oxidation at different anode materials

The aim of this work was to evaluate the pollutant load from tannery effluents treated by chemical coagulation (CC) followed by electro-oxidation (EO), performed in two different experimental batch-recirculated setups, one with a BDD anode and the other with Ti/Pt/PbO₂ and Ti/Pt/SnO₂-Sb₂O₄ anodes (PS). Results were compared with those obtained from EO of the raw sample. CC was performed with a Fe³⁺ concentration of 0.25 g L^-1, and the applied current densities for EO in each setup were 60 mA cm^-2 for BDD and, in the PS setup, 20 and 40 mA cm^-2 for Ti/Pt/SnO₂-Sb₂O₄ and Ti/Pt/PbO₂, respectively. During CC, removals of 27% in chemical oxygen demand (COD), 14% in total nitrogen, 100% in sulfide, and 73% in Cr(VI) were observed. COD removal in the EO of the raw sample was higher than that obtained for the combined CC + EO, for both setups, showing that the organic compounds removed by CC are mainly those that would be more easily removed by EO. For most of the other parameters related with carbon and nitrogen, the removals for CC + EO were higher than for EO alone. During EO, sulfide is converted to sulfate, especially with BDD. Concerning Cr(VI) concentration, it increases during EO, in particular for PS setup. Combined treatment, with both setups, proved to be an effective choice to treat tannery effluents.

Comparison between sinusoidal AC coagulation and conventional DC coagulation in removing Cu2+ from printed circuit board wastewater

A new Electrocoagulation (EC) technique, sinusoidal AC coagulation (SACC), is creatively put forward for Cu²⁺ removal in the wastewater from the printed circuit board (PCB) production in this paper. The removal efficiency of Cu²⁺ from PCB wastewater and energy consumption are compared by SACC and conventional direct current coagulation (DCC). The optimal process parameters were established through analysis of response surface methodology (RSM). The coagulations containing Cu²⁺ was characterized by SEM, EDS, TEM，BET, XRD and FTIR. The nano-ferrum collosol, mainly composed of goethite (α-FeOOH) and magnetite (γ-Fe₂O₃), absorbs the Cu²⁺ and coagulates to remove Cu²⁺. The results show that the removal rates of Cu²⁺ by SACC and DCC are 99.86％ and 98.21％, respectively, and the energy consumption is 2.76 × 10^-2 kWh⋅m^-3 for SACC and 4.42 × 10^-2 kWh⋅m^-3 for DCC under the optimal process conditions of c₀ (Cu²⁺) = 41.99 mg⋅dm^-3, pH = 7.14, j = 0.293 A⋅m^-2, t = 16.7 min. The pilot tests indicate that the SACC technique is feasible in industrial application. Cu²⁺ removal were completed through electrodeposition of Cu²⁺ on iron electrode, the deposition of Cu(OH)₂ and the adsorption of Cu²⁺ by ferrum collosol. The adsorption follows the pseudo-second order kinetics model well. The maximum saturated adsorption capacity (q_max) of Cu²⁺ on ferrum collosol produced by SACC is larger than that by DCC. The adsorption of Cu²⁺ on the ferrum collosol prepared by SACC and DCC are in accordance with Langmuir's adsorption isotherms. The novel SACC technique is a promising technique for the highly-efficient treatment of Cu²⁺ from PCB wastewater.

Simultaneous removal of cadmium, zinc and manganese using electrocoagulation: Influence of operating parameters and electrolyte nature

In the present study, the influence of operating parameters and electrolyte nature on the simultaneous removal of toxic metals (cadmium, zinc and manganese) from synthetic smelting wastewater by batch electrocoagulation was investigated. This wastewater contained high concentrations of anion-cation electrolytes. Results indicated that the efficiency of heavy metals removal can be enhanced by increasing the solution pH and current density. The Fe-Fe electrode combination is more effective than the other combinations (Al-Al, Al-Fe and Fe-Al). The interaction of heavy metal ions showed that the increase of initial Zn²⁺ concentration adversely affects on Cd²⁺ removal. In addition, the single chloride system exhibits the optimum removal efficiency on Mn²⁺. Single sulfate and binary anion systems exert a more positive effect on Cd²⁺ and Zn²⁺ removal because of the stronger charge neutralization and destabilization of iron hydroxide flocs. Increases of Ca²⁺ and Mg²⁺ ions exert a significant negative effect on metal removal. However, the addition of a small amount of sodium chloride into a high sulfate and hardness solution can accelerate the removal of heavy metals. Finally, the sludge samples generated from electrocoagulation were characterized by XRD and SEM-EDS analyses.

Optimization of copper removal from aqueous solutions in a continuous electrochemical cell divided by cellulosic separator

Copper, as an inseparable part of many industrial discharges, threatens both public and environmental health. In this work, an electrochemical cell utilizing a cellulosic separator was used to evaluate Cu removal using graphite anodes and stainless steel cathodes in a continuous-flow mode reactor. In the experimental matrix, Cu concentration (1-5 mg L^-1), electrolysis time (20-90 min), and current intensity (0.1-0.4 A) were employed. Results showed that the maximum removal efficiency of copper was obtained as 99%. The removal efficiency was independent of initial copper concentration and directly related to electrolysis time and current intensity. Energy consumption was more dependent on current intensity than electrolysis time. Under optimal conditions (75.8 min electrolysis time, 0.18 A current intensity, and 3 mg L^-1 copper concentration), the removal efficiency was obtained as 91% while 7.05 kWh m^-3 electrical energy was consumed. The differences between the actual and predicted data under optimal conditions were 0.42% for copper removal and 0.23% for energy consumption, which signify the performance and reliability of the developed models. The results exhibited the suitability of the electrochemical reduction for copper removal from aqueous solutions, which was facilitated under alkaline conditions prevailing in the cathodic compartment due to applying a cell divided by a cellulosic separator.

Treatment of highly concentrated tannery wastewater using electrocoagulation: Influence of the quality of aluminium used for the electrode

This paper deals with the ability of electrocoagulation (EC) to remove simultaneously COD and chromium from a real chrome tanning wastewater in a batch stirred electro-coagulation cell provided with two aluminium-based electrodes (aluminium/copper/magnesium alloy and pure aluminium). Effects of operating time, current density and initial concentration of Cr(III) and COD have been investigated. The concentrations of pollutants have been successfully reduced to environmentally acceptable levels even if the concentrated effluent requires a long time of treatment of around 6h with a 400A/m(2) current density. The aluminium alloy was found to be more efficient than pure aluminium for removal of COD and chromium. Dilution of the waste has been tested for treatment: high abatement levels could be obtained with shorter time of treatment and lower current densities. Energy consumption of the electrocoagulation process was also discussed. The dilution by half of the concentrated waste leads to a higher abatement performance of both COD and chromium with the best energy efficiency.

Ecotoxicological risks associated with tannery effluent wastewater

The problem of water pollution acquires greater relevance in the context of a developing agrarian economy like Pakistan. Even though, the leather industry is a leading economic sector in Pakistan, there is an increasing environmental concern regarding tanneries because they produce large amounts of potentially toxic wastewater containing both trivalent and hexavalent chromium, which are equally hazardous for human population, aquaculture and agricultural activities in the area. Therefore, we defined the scope of the present study as to employ different bioassays to determine the eco-toxic potential of tannery effluent wastewater (TW) and its chromium based components, i.e., potassium dichromate (K(2)Cr(2)O(7)) and chromium sulfate Cr(2)(SO(4))(3). Particle-induced X-ray emission (PIXE) analysis of TW was carried out to determine the concentration of chromium in TW and then equal concentrations of hexavalent (K(2)Cr(2)O(7)) and trivalent chromium Cr(2)(SO(4))(3) were obtained for this study. Cytotoxicity assay, artemia bioassay and phytotoxicity assay was utilized to investigate the eco-toxicological potential of different concentrations of TW, K(2)Cr(2)O(7) and Cr(2)(SO(4))(3). All the dilutions of TW, K(2)Cr(2)O(7) and Cr(2)(SO(4))(3) presented concentration dependent cytotoxic effects in these assays. The data clearly represents that among all three tested materials, different dilutions of K(2)Cr(2)O(7) caused significantly more damage (P<0.001) to vero cell, brine shrimp and germination of maize seeds. Interestingly, the overall toxicity effects of TW treated groups were subsequent to K(2)Cr(2)O(7) treated group. Based on biological evidences presented in this article, it is concluded that hexavalent chromium (K(2)Cr(2)O(7)) and TW has got significant eco-damaging potential clearly elaborating that environmental burden in district Kasur is numerous and high levels of chromium is posing a considerable risk to the human population, aquaculture and agricultural industry that can obliterate ecosystem surrounding the tanneries.

Characterization of tannery effluent wastewater by proton-induced X-ray emission (PIXE) analysis to investigate their role in water pollution

INTRODUCTION

Over the last few decades, the chromium-based tanning industry has shown rapid growth in Pakistan. However, the rules and regulations promulgated by the government are not strictly followed for processing the effluent discharge from the tanneries. Consequently, tannery effluents have become a great source of water pollution in surrounding areas.

MATERIALS AND METHODS

In this case study, characterization of tannery effluent wastewater (TW), shallow groundwater (SW), and deep groundwater (DW) samples was carried out to determine the source of water pollution in the district of Kasur, Pakistan.

RESULTS

The concentrations of calcium (Ca), chlorine (Cl), chromium (Cr), iron (Fe), potassium (K), Mg, sulfur (S), silicon (Si), and Sr in TW were significantly higher than SW and DW, which also exceeded the international limits. In addition, increased concentrations of major toxic elements (Cl, Cr, Fe, K, Ni, and Si) were also observed in SW, which were higher in comparison to DW. Strikingly, the concentrations of Cr and Si in various DW samples were also beyond World Health Organization (WHO) safe limit, which reinforced the trend that water pollution in the area is directly linked to the distance from the source (TW). The particle-induced X-ray emission (PIXE) indices also suggested that TW is a main contributory source of water-based pollution in the area, which is imposing great threat to local inhabitants due to known hazardous and carcinogenic potential of these elements.

CONCLUSION

Protecting the water resources will be a formidable challenge in the study area, which requires modernization of tannery industry, thereby improving the recovery and recycling of TW. Moreover, PIXE analysis presented here as a successful tool, could serve as landmark for the contemporary research in environmental toxicology.

Studies on the kinetics and equilibrium of the solvent extraction of chromium(III) from alkaline aqueous solutions of different composition in the system with Aliquat 336

The kinetics and equilibrium of Cr(III) extraction with Aliquat 336 from the alkaline aqueous solutions containing anions of various complexing abilities and solvating properties, as: nitrates, or perchlorates, or sulphates, were studied and interpreted. Stoichiometry of Cr(III) complexes formed in the organic phases was also discussed. It was found that under the same conditions the yield and the rate of Cr(III) extraction as well as values of mass transfer coefficient increase in the following order of anions: perchlorates<nitrates<sulphates. The equilibrium of extraction of Cr(III) in the systems with nitrates and/or perchlorates differs significantly from that one established for the extraction of Cr(III) from the alkaline aqueous solutions of sulphates. Experiments with the modified Lewis cell and analysis of the experimental data by mathematic models appeared that the studied extraction of Cr(III) occurs in the mixed diffusion-kinetic regime, irrespective of the kind of anions present in the aqueous phase.

Enhancement of chromium uptake in tanning using oxazolidine

Monocyclic and bicyclic oxazolidines were offered at three different junctures of chrome tanning process viz. prior to BCS offer, along with BCS and after basification. It was found that oxazolidine when offered after basification brought about better chromium uptake and reduction of chromium load in the wastewater. Offer of oxazolidine was also varied. Increase in offer of oxazolidine from 0.25% to 1% was found to enhance the chromium uptake and decrease the chromium load in wastewater. But the increase in uptake was not proportionate to the increase in oxazolidine offer more than 0.75%. Offer of 1% Zoldine ZA 78 (monocyclic oxazolidine) and Zoldine ZE (bicyclic oxazolidine) after basification brought about 63.4% and 73.1% enhancement in chrome content in leather compared to control where oxazolidine was not offered. The tone of the wetblue was found to be altered moderately. However this did not call for any process adjustments in wet-finishing. The oxazolidine treated leathers were found to be immensely fuller and tighter. It was found experimentally that offer of 1% of oxazolidine facilitated reduction in the offer of syntans administered for filling and grain tightening by around 46%. Oxazolidine could bring about significant reduction in cost of chemicals apart from resulting environmental benefits due to enhancement of chromium uptake during tanning.

Bacterial diversity in Fez tanneries and Morocco's Binlamdoune River, using 16S RNA gene based fingerprinting

Tannery wastewater causes serious ecological and sanitary damage. Chemical analysis of water from Binlamdoune River of the medina of Fez was conducted and the results revealed the presence of toxic elements from tanneries and other industrial activities, which strongly affected water quality. To determine the effectiveness of bioremediation for depollution, we studied the abundance and diversity of bacteria residing in these polluted environments. Conducting denaturing gradient gel electrophoresis (PCR-DGGE) of the 16S rDNA area using primers related to bacteria showed a bacterial community belonging to eubacterial groups, that is, Epsilonproteobacteria, Clostridia, Lactobacillales, Bacteroidetes, Gammaproteobacteria, and Alphaproteobacteria. In addition, cloning displayed the presence of clones belonging to the Firmicutes group. Moreover, scanning electron microscopy revealed a significant heterogeneity of microorganism forms and structures. These endogenous microbes could have a significant role in the purification of Binlamdoune River and Fez tannery wastewater.