Chromium bioavailability in aquatic systems impacted by tannery wastewaters. Part 1: Understanding chromium accumulation by indigenous chironomids

Impact of δ-MnO2 on the chemical speciation and fractionation of Cr(III) in contaminated soils

Oxidation of Cr(III) by birnessite (δ-MnO₂) was an important geochemical reaction determining the toxicity and mobility of dissolved Cr(III) in soils. Herein, changes of Cr speciation and fractionation were systematically studied in Cr(III)-contaminated soils with δ-MnO₂ in soil aging process. The results showed that Cr(III) could be rapidly oxidized to Cr (VI) by δ-MnO₂, and the coating of Fe and Al oxides on δ-MnO₂ had a strong hindering effect on the oxidation of Cr(III). The Cr(III) oxidation process by δ-MnO₂ followed a two-phase model of pseudo first-order kinetics. The rapid decrease of oxidation rate constant in second phase was due to the coverage of adsorbed Cr(III) and newly generated Cr(VI) and Mn(II) on the active sites of δ-MnO₂. X-ray photoelectron spectroscopy analysis further confirmed that the diffusion and adsorption of Cr(III) on the electron-accepting sites were important factors affecting the Cr(III) oxidation by δ-MnO₂. Compared with the soils without δ-MnO₂, high contents of Cr(VI) were generated in silt (22.30 mg/kg) and sandy soil (70.95 mg/kg) with 2 wt% δ-MnO₂ after the addition of Cr(III) wastewater within 2 days, and the Cr(VI) contents were above 1 mg/kg in these two soils during the whole incubation process. Moreover, the total Cr proportion of the exchangeable fraction increased by 12.8% in silt and 5.2% in sandy soil with 2 wt% δ-MnO₂ after soil aging for 120 days. The presence of δ-MnO₂ markedly increased the oxidation potential and mobility of exogenous Cr(III) in soils.

Mobility, ecotoxicity, bioaccumulation and sources of trace elements in the bottom sediments of the Rożnów reservoir

The aim of the study was to use of geochemical, chemical, ecotoxicological and biological indicators for a comprehensive assessment of ecological risks related to the mobility, ecotoxicity and bioavailability of trace elements in the bottom sediment of the Rożnów reservoir. The study found three elements deserving attention in the sediments: cadmium, nickel and chromium. Cadmium proved to be the most mobile and bioavailable, although the total cadmium content and geochemical indicators did not reveal any risk to organisms. Geochemical indicators showed that the sediments are contaminated with nickel and chromium, but both elements had a low bioaccumulation factor. Fractional analysis also revealed relatively low mobility of Cr and Ni and a higher potential risk of bioavailability for nickel. Most of the tested sediment samples had low toxicity in relation to the tested organisms. For H. incongruens, 11% of the samples were non-toxic, 50% of the samples had low toxicity, and 39% of the samples were toxic. For A. fischeri, no toxicity was found in 7% of the samples, low toxicity in 76% of the samples and toxicity in 17% of the sediment samples. The As, Cd, Cu content in the F1 fraction correlated significantly positively with the content of these metals in mussel tissues. Both biotesting and chemical analysis can reveal a potential risk to aquatic organisms. For a real assessment of the ecological risks associated with trace elements, it is necessary to use bioindicators taken from the environment and exposed to trace elements in situ.

Tannery Effluent Treatment by Nanofiltration, Reverse Osmosis and Chitosan Modified Membranes

The objective of this work is to develop an appropriate technology for environmentally sound membrane-based purification of a tannery effluent assuring, simultaneously, the recovery of chromium, considered as the most hazardous inorganic water pollutant extensively used in leather tanning. A comparison between the permeate fluxes obtained during treatment of a synthetic tannery effluent through nanofiltration (NF270 and NF90 membranes) and reverse osmosis (BW30 and SW30) membranes was first performed. Then, a dedicated polymeric membrane was prepared by coating chitosan (cs) on a polyethersulfone (PES) microfiltration membrane (cs-PES MFO22) support. The resulting membrane was characterized by Fourier Transforms Infrared Spectroscopy Attenuated Total Reflectance (FTIR-ATR), Emission Scanning Electronic Microscopy (SEM) to confirm the process of surface modification and cross-linking of chitosan with glutaraldehyde. This membrane was found to be highly effective for chromium removal (>99%), which was more than eight times higher in reference to monovalent cations (e.g., Na⁺ and K⁺) and more than six times higher in reference to the divalent cations (Mg²⁺ and Ca²⁺) studied. The reverse osmosis permeate conforms to local Algerian regulations regarding being discharged directly into the natural environment (in this case, Reghaia Lake) or into urban sewers linked to wastewater biological treatment stations. While the SW30 membrane proved to be the most effective for purification of the tannery effluent, the chitosan modified membrane proved to be appropriate for recovery of chromium from the reverse osmosis concentrate.

Trophic transfer and biotransformation of selenium in the mosquito (Aedes albopictus) and interactive effects with hexavalent chromium

As an essential micronutrient for animals with a narrow range between essentiality and toxicity, selenium (Se) usually coexists with chromium (Cr) in contaminated aquatic environments. This study investigated effects of three diets (Microcystis aeruginosa, Chlorella vulgaris and biofilms) exposed to Se or/and Cr on Aedes albopictus as a vector for the aquatic-terrestrial transfer of Se and Cr. Se(IV)-exposed mosquitoes concentrated Se up to 66-fold faster than Se(VI)-exposed ones, corresponding to the greater Se enrichment in Se(IV)-treated diets. Analysis using synchrotron-based X-ray absorption spectroscopy (XAS) showed that Se(0) (61.9-74.6%) dominated Se(VI)-exposed mosquitoes except for the C. vulgaris-fed larvae (organo-Se, 94.0%), while organo-Se accounted for 93.3-100.0% in Se(IV)-exposed mosquitoes. Cr accumulation in larvae (56.40-87.24 μg Cr/g DW) or adults (19.41-50.77 μg Cr/g DW) was not significantly different among all Cr(VI) treatments, despite varying diet Cr levels. With Cr(0) being dominant (57.7-94.0%), Cr(VI)-exposed mosquitoes posed little threat to predators. Although mosquitoes exposed to Se or Cr had shorter wings, adults supplied with C. vulgaris or biofilms co-exposed to Se(VI) and Cr(VI) had wings significantly (1.1-1.2 fold) longer than Se(VI) only exposed ones. Overall, our study reveals the role of Ae. albopictus in transferring waterborne Se and Cr from the contaminated aquatic ecosystem to the terrestrial ecosystem with the resulting eco-risks to wildlife in both ecosystems.

Modified heavy metal Pollution index (m-HPI) for surface water Quality in river basins, India

India is bestowed with huge amount of surface water resources. However, India lacks the quality monitoring of surface water and comprehensive management for sustainable surface water development. A new approach for indexing has been proposed to represent pollution due to heavy metals in surface water. Heavy metal pollution indices (m-HPI) for 60 surface water samples in the peninsular stretch were evaluated during pre-drought, drought and post-drought condition. The Index will be represented by a Positive Index (PI) and a Negative Index (NI), where PI represents the level pollution exceeding the maximum desirable limit and NI reflects the index within the required limit. The PI is assigned as 0 when indicators are present below the detection limit or equal to the maximum required limit. However, the value calculated for NI could be 0 to -1 when the indicators are equal to or less than the suggested maximum desirable limit, and the value could be -1 when the indicators are present below the suggested detection limit. The spatiotemporal variation of water quality pattern was studied by the interpolation maps extracted from ArcGIS. The results are compared with WHO standard to validate the drinking water quality. The calculated indices indicated the suitability of water for domestic and irrigation purposes. The developed indexing system is user friendly, robust, flexible and may evaluate the index considering any water quality standard.

Hydrometallurgical Recovery of Cr(III) from Tannery Waste: Optimization and Selectivity Investigation

Tanneries produce significant quantities of hazardous waste/waters and, according to international environmental organizations, their further recycling/reuse/exploitation in the context of circular economy is considered mandatory. A relevant case study is the recovery of Cr(III), used in large amounts during the tanning process, which can be recovered from the corresponding industrial waste/sludge. The aim of this work is to optimize the Cr(III) hydrometallurgical recovery from the tannery sludge by examining the major experimental conditions that affect the efficiency of its different sequential steps, namely extraction/leaching and precipitation. The chemical characterization revealed that tannery sludge contains high amounts of Cr(III) (14.1%), Ca (14.8%), and organic matter (22%). The extraction of Cr(III) was examined by applying various common acids (H2SO4, HNO3, HCl) in the concentration range of 0.02–2 N. The highest selectivity and efficiency (93%) were obtained by the addition of H2SO4 as the insoluble CaSO4 and soluble CrSO4+ species were formed. Regarding the experimental conditions, only the increase in temperature showed a positive effect on Cr recovery. The Cr(III) can be further precipitated from the resulting leaching solution by increasing the equilibrium pH; the precipitate with the higher purity in Cr(OH)3 (70%) was obtained by the application of NaOH and could possibly be reused by tanneries.