Chromium contamination and effect on environmental health and its remediation: A sustainable approaches

Advances from conventional to real time detection of heavy metal(loid)s for water monitoring: An overview of biosensing applications

The rapid growth of the industrial sector has expedited the accumulation of heavy metal(loid)s in the environment at hazardous levels. The elements such as arsenic, lead, mercury, cadmium and chromium are lethal in terms of toxicity with severe health impacts. With issues like water scarcity, limitations in wastewater treatment, and costs pertaining to detection in environmental matrices; their rapid and selective detection for reuse of effluents is of the utmost priority. Biosensors are the futuristic tool for the accurate qualitative and quantitative analysis of a specific analyte and integrate biotechnology, microelectronics and nanotechnology to fabricate a miniaturized device without compromising the sensitivity, specificity and accuracy. The characteristic features of supporting matrix largely affect the biosensing ability of the device and incorporation of highly sensitive and durable metal organic frameworks (MOFs) are reported to enhance the efficiency of advanced biosensors. Electrochemical biosensors are among the most widely developed biosensors for the detection of heavy metal(loids), while direct electron transfer approach from the recognition element to the electrode has been found to decrease the chances of interference. This review provides an insight into the recent progress in biosensor technologies for the detection of prevalent heavy metal(loid)s; using advanced support systems such as functional metal-based nanomaterials, carbon nanotubes, quantum dots, screen printed electrodes, glass beads etc. The review also delves critically in comparison of various techno-economic studies and the latest advances in biosensor technology.

Experimental and Modeling Study on Cr(VI) Migration from Slag into Soil and Groundwater

The transport and prediction of hexavalent chromium (Cr(VI)) contamination in “slag–soil–groundwater” is one with many uncertainties. Based on the column experiments, a migration model for Cr(VI) in the slag–soil–groundwater system was investigated. The hydraulic conductivity (Kt), distribution coefficient (Kd), retardation factor (Rd), and other hydraulic parameters were estimated in a laboratory. Combining these hydraulic parameters with available geological and hydrogeological data for the study area, the groundwater flow and Cr(VI) migration model were developed for assessing groundwater contamination. Subsequently, a Cr(VI) migration model was developed to simulate the transport of Cr(VI) in the slag–soil–groundwater system and predict the effect of three different control programs for groundwater contamination. The results showed that the differences in the measured and predicted groundwater head values were all less than 3 m. The maximum and minimum differences in Cr(VI) between the measured and simulated values were 1.158 and 0.001 mg/L, respectively. Moreover, the harmless treatment of Cr(VI) slag considerably improved the quality of groundwater in the surrounding areas. The results of this study provided a reliable mathematical model for transport process analysis and prediction of Cr(VI) contamination in a slag–soil–groundwater system.

Health Risk Assessment during In Situ Remediation of Cr(VI)-Contaminated Groundwater by Permeable Reactive Barriers: A Field-Scale Study

The presence of residual Cr(VI) in soils causes groundwater contamination in aquifers, affecting the health of exposed populations. Initially, permeable reactive barriers(PRB) effectively removed Cr(VI) from groundwater. However, as PRB clogging increased and Cr(VI) was released from upstream soils, the contamination plume continued to spread downstream. By 2020, the level of contamination in the downstream was nearly identical to that in the upstream. The study results show that during normal operation, the PRB can successfully remove Cr(VI) from contaminated groundwater and reduce the carcinogenic and non-carcinogenic risks to humans from the downstream side of groundwater. However, the remediated groundwater still poses an unacceptable risk to human health. The sensitivity analysis revealed that the concentration of the pollutant was the most sensitive parameter and interacted significantly with other factors. Ultimately, it was determined that the residual Cr(VI) in the soil of the study region continues to contaminate the groundwater and constitutes a serious health danger to residents in the vicinity. As remediated groundwater still poses a severe threat to human health, PRB may not be as effective as people believe.

Dynamic Adsorption Characteristics of Cr(VI) in Red-Mud Leachate onto a Red Clay Anti-Seepage Layer

Red-mud leachate from tailings ponds contains Cr(VI), which can pollute groundwater via infiltration through anti-seepage layers. This paper investigates leachate from a red-mud tailings pond in southwest China and the red clay in the surrounding area to simulate the adsorption of Cr(VI) onto clay at different pHs, using geochemical equilibrium software (Visual MINTEQ). We also performed dynamic adsorption testing of Cr(VI) on a clay anti-seepage layer. The dynamic adsorption behaviors and patterns in the dynamic column were predicted using the Thomas and Yoon-Nelson models. Visual MINTEQ predicted that Cr(VI) adsorption in red-mud leachate onto clay was 69.91%, increasing gradually with pH, i.e., adsorption increased under alkaline conditions. Cr(VI) concentration in the effluent was measured using the permeability test through a flexible permeameter when the adsorption saturation time reached 146 days. At a low seepage rate, Cr(VI) adsorption onto the clay anti-seepage layer took longer. Saturation adsorption capacity, q₀, and adsorption rate constant, K_th, were determined using the Thomas model; the Yoon-Nelson model was used to determine when the effluent Cr(VI) concentration reached 50% of the initial concentration. The results provide parameters for the design and pollution prediction of the clay anti-seepage layer of red-mud tailings ponds.

The Adsorption Potential of Cr from Water by ZnO Nanoparticles Synthesized by Azolla pinnata

Aqueous solutions containing toxic elements (TEs) (such as hexavalent chromium (Cr (VI)) can be toxic to humans even at trace levels. Thus, removing TEs from the aqueous environment is essential for the protection of biodiversity, hydrosphere ecosystems, and humans. For plant fabrication of zinc oxide nanoparticles (PF-ZnONPs), Azolla pinnata plants were used, and X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), SEM, and FTIR techniques were used for the identification of PF-ZnONPs and ZnONPs, which were used to remove Cr (VI) from aqueous solution. A number of adsorption parameters were studied, including pH, dose, concentration of metal ions, and contact time. The removal efficiency of PF-ZnONPs for Cr (VI) has been found to be 96% at a time (60 min), 69.02% at pH 4, and 70.43% at a dose (10 mg·L⁻¹). It was found that the pseudo-second-order model best described the adsorption of Cr (VI) onto PF-ZnONPs, indicating a fast initial adsorption via diffusion. The experimental data were also highly consistent with the Langmuir isotherm model calculations.

Exploration on the Cr(VI) resistance mechanism of a novel thermophilic Cr(VI)-reducing bacteria Anoxybacillus flavithermus ABF1 isolated from Tengchong geothermal region, China

Hexavalent chromium resistance and reduction mechanisms of microorganism provide a critical guidance for Cr(VI) bioremediation. However, related researches are limited in mesophiles and deficient for thermophiles. In this work, a novel alkaline Cr(VI)-reducing thermophile Anoxybacillus flavithermus ABF1 was isolated from geothermal region. The mechanisms of Cr(VI) resistance and reduction were investigated. The results demonstrated that A. flavithermus ABF1 could survive in a wide temperature range from 50°C to 70°C and in pH range of 7.0-9.0. Strain ABF1 showed excellent growth activity and Cr(VI) removal performance when initial Cr(VI) concentration was lower than 200 mg L^-1 . 93.71% of Cr(VI) was removed at initial concentration of 20 mg L^-1 after 72 h. The majority of Cr(VI) was found to be reduced extracellularly by enzymes secreted by cells. XPS and Raman analysis further manifested that Cr₂ O₃ was the product of Cr(VI) reduction. Moreover, the Cr(VI) transportation-related gene cysP and Cr(VI) reduction-related gene azoR of A. flavithermus ABF1 played key roles in inhibiting Cr(VI) entering cells and promoting extracellular Cr(VI) reduction respectively. This work provides novel insight into the mechanisms of Cr(VI) resistance and detoxication of thermophiles, which leads to a promising alternative strategy for heavy metal bioremediation in areas with elevated temperature.

Differentiating Wild and Apiary Honey by Elemental Profiling: a Case Study from Mangroves of Indian Sundarban

Honey is a natural substance produced by honeybees from the nectar or secretion of flowering plants. Along with the botanical and geographical origin, several environmental factors also play a major role in determining the characteristics of honey. The aim of this study is to determine and compare the elemental concentration of various macro and trace elements in apiary and wild honeys collected from different parts of Indian Sundarbans. The elemental analysis was performed in inductively coupled plasma optical emission spectroscopy preceded by microwave digestion method. The concentrations of 19 elements (Ag, Al, As, B, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, Pb, Se and Zn) were investigated from thirteen locations of Indian Sundarbans. This comparative study shows in wild honey samples, the concentration of K was highest followed by Ca, Mg and Na and Zn was lowest among all. In contrast, in apiary honey samples, Ca had maximum concentration followed by K, Mg and Na and Ag had minimum among all. The elemental concentration in honey from apiary was either equal or higher than their wild counterpart. The results of the factor analysis of PCA algorithm for wild and apiary honey samples were highly variable which implies that the elements are not coming from the same origin. The concentration of element was found to be highly variable across sites and across sources of honey samples.

Chromium toxicity and its remediation by using endophytic bacteria and nanomaterials: A review

Chromium (Cr) is a crucial element for all life forms. Various anthropogenic activities have been responsible for environmental contamination with Cr (VI) in recent years. For this review, articles were collected using electronic databases such as Web of Science, Pubmed, ProQuest, and Google Scholar as per the guidelines of PRISMA-2015, applying the Boolean search methods. Chromium can cause severe health complications in humans and animals and threatens the surrounding environment, with negative impacts on crop yield, development, and quality. Hence, monitoring Cr contamination is essential, and various remediation technologies have emerged in the past 50 years to reduce the amount of Cr in the environment. This review focuses on chromium exposure and the associated environmental health risks. We also reviewed sustainable remediation processes, with emphasis on nanoparticle and endophytic remediation processes.

Ellagic acid ameliorates hexavalent chromium-induced renal toxicity by attenuating oxidative stress, suppressing TNF-α and protecting mitochondria

Nephrotoxicity is an important adverse effect of oxidative stress induced by hexavalent chromium [Cr(VI)]. The effect of ellagic acid, a dietary polyphenolic compound with potent antioxidant activity, was investigated in Cr(VI)-induced kidney injury. Six groups of male Wistar rats were treated intragastrically with vehicle or ellagic acid (15 and 30 mg/kg) for 10 days. On day 10, rats received saline or Cr(VI) (K₂Cr₂O₇ 15 mg/kg) subcutaneously. Cr(VI) significantly increased kidney weight, affected kidney function assessed by biomarkers in blood and urine (protein, creatinine and urea nitrogen), caused histological changes (tubular injury and glomerular capillary tuft damage), increased markers of oxidative stress and reduced the activity of antioxidant enzymes. In addition, Cr(VI) altered mitochondrial ultrastructure, impaired mitochondrial respiration, increased lipid peroxidation, and inhibited the function of mitochondrial enzymes. Pretreatment with ellagic acid (30 mg/kg) attenuated all the aforementioned alterations. Furthermore, we explored whether ellagic acid might regulate the tumor necrosis factor-alpha (TNF-α)/receptor-interacting protein kinase 3 (RIPK3) pathway, reducing Cr(VI)-induced tubular necrosis. Cr(VI) upregulated both TNF-α and RIPK3, but ellagic acid only decreased TNF-α levels, having no effect on RIPK3 content. Therefore, understanding the mechanisms through which Cr(VI) promotes necroptosis is crucial for future studies, in order to design strategies to mitigate kidney damage. In conclusion, ellagic acid attenuated Cr(VI)-induced renal alterations by preventing oxidative stress, supporting enzymatic activities, suppressing TNF-α, and preserving mitochondrial ultrastructure and function, most likely due to its antioxidant properties.

Kinetics and Mechanisms of Cr(VI) Removal by nZVI: Influencing Parameters and Modification

In this study, single-spherical nanoscale zero valent iron (nZVI) particles with large specific sur-face area were successfully synthesized by a simple and rapid chemical reduction method. The XRD spectra and SEM–EDS images showed that the synthesized nZVI had excellent crystal struc-ture, but oxidation products, such as γ-Fe2O3 and Fe3O4, were formed on the surface of the parti-cles. The effect of different factors on the removal of Cr(VI) by nZVI were studied, and the opti-mum experimental conditions were found. Kinetic and thermodynamic equations at different temperatures showed that the removal of Cr(VI) by nZVI was a single-layer chemical adsorption, conforming to pseudo-second-order kinetics. By applying the intraparticle diffusion model, the ad-sorption process was composed of three stages, namely rapid diffusion, chemical reduction, and in-ternal saturation. Mechanism analysis demonstrated that the removal of Cr(VI) by nZVI in-volved adsorption, reduction, precipitation and coprecipitation. Meanwhile, Cr(VI) was reduced to Cr(III) by nZVI, while FeCr2O4, CrxFe1−xOOH, and CrxFe1−x(OH)3 were formed as end products. In addition, the study found that ascorbic acid, starch, and Cu modified nZVI can promote the removal efficiency of Cr(VI) in varying degrees due to the enhanced mobility of the particles. These results can provide new insights into the removal mechanisms of Cr(VI) by nZVI.

Biosorption and adsorption isotherm of chromium (VI) ions in aqueous solution using soil bacteria Bacillus amyloliquefaciens

This study looked at the development of effective biosorbents to recover the most toxic elements from industrial water. B. amyloliquefaciens was isolated from marine soils showing extreme resistance to Chromium (Cr(VI)) ions. During the 60 min of contact time, 79.90% Cr(VI) was adsorbed from the aqueous solution. The impact of important factors such as biomass concentration, pH of the medium, and initial metal ions concentration on biosorption rate was also examined. The desorption study indicated that 1 M HCl (91.24%) was superior to 0.5 M HCl (74.81%), 1 M NaOH (64.96%), and distilled water (3.66%). Based on the Langmuir model, the maximum adsorption capacity of the bio-absorbent was determined to be 48.44 mg/g. The absorption mechanism was identified as monolayer, and 1/n from the Freundlich model falls within 1, thus indicating favorable adsorption. Based on the findings of the present study, the soil bacterium B. amyloliquefaciens was found to be the best alternative and could be used to develop strategies for managing existing environmental pollution through biosorption.

Phytoextraction by Moso Bamboo under high level chromium stress in mediterranean conditions

In this study a bamboo species, Moso Bamboo (MB) - Phyllostachys pubescens - has been selected for its heavy metal accumulation capacity and translocation potential to restore Cr-contaminated soil. Experiments have been conducted so to evaluate the capability of MB to remove Cr from soil, growing under Mediterranean conditions, irrigated with water containing 180 mgCr/L, at flow rate of 600 mm/year. The soil has been contaminated by the irrigation water. When the concentration of Cr in soil reached 300 mgCr/kg, Cr phytoextraction by MB from soil at the same irrigation rate of 600 mm/year with uncontaminated water has been evaluated. Cr removal from soil was approx. 42% after 6 weeks and 60.7% after 12 weeks, starting from a Cr content in soil of approximately 300 mg/kg. MB growing in Cr contaminated soil has shown Cr concentration per gram of dry biomass in aerial parts greater than the underground parts of the plants. After 12 weeks of cultivation, the quantity of Cr in roots and rhizome was measured as 1.79 mg/g, while in stems and leaves as 2.49 mg/g. Results shown a bioconcentration factor of 0.77, 0.65, 0.18, 0.08, after 6 weeks and 0.64, 0.98, 0.53, 0.26 after 12 weeks for roots, rhizomes, stems and leaves, respectively and a translocation factor equal to 0.23 and 0.11 after 6 weeks and 0.83 and 0.40 after 12 weeks, for stems and leaves, respectively.

Humic Acids Affect the Detection of Metal Ions by Cyanobacteria Carbon Quantum Dots Differently

A "top-down" synthesis of carbon quantum dots (CQDs), novel fluorescent C materials from waste biomass, is both cost-effective and environmentally friendly. N-rich cyanobacteria are promising precursors to produce CQDs with high fluorescence (FL) intensity for the detection of metal ions. Herein, we synthesized cyanobacteria-based CQDs using a hydrothermal process and evidenced their high FL intensity and stability. The cyanobacteria-based CQDs showed powerful sensitivity for the specific detection of Fe³⁺ and Cr⁶⁺, which could be ascribed to (i) static FL quenching as a result of the interaction between -OH, -NH₂, and -COOH groups with the metal ions, (ii) internal filtering effects between the CQDs and Fe³⁺ or Cr⁶⁺, and (iii) fluorescence resonance energy transfer between CQDs and Cr⁶⁺. Humic acids (HAs) coexisting led to an underestimation of Fe³⁺ but an overestimation of Cr⁶⁺ by the CQDs due to the different FL quenching mechanisms of the CQDs. HAs sorbed Fe³⁺ and wrapped the CQDs to form a barrier between them, inhibiting FL quenching of CQDs by Fe³⁺. As for Cr⁶⁺, HAs reduced Cr⁶⁺ and also led to FL quenching; the sorbed HAs on the CQDs acted as a carrier of electrons between Cr⁶⁺ and the CQDs, enhancing FL quenching of the CQDs. This study is the first work to evidence the interference of HAs in the detection of metal ions by CQDs derived from cyanobacteria, which would enlighten the application of CQDs in a natural aqueous environment.

Iron-sulphur transformation control for enhancing Cr(VI) removal in flake and nanoscale porous pyrrhotite (Fe7S8) added wastewater

Hexavalent chromium (Cr(VI)) contaminated wastewater should be addressed efficiently in the environmental field. In previous applications, nano iron sulfides amendment has not been well controlled for iron-sulfur transformation. In this study, the novel flake and nanoscale porous pyrrhotite (Fe₇S₈) (FNPP) amendment was synthesized. The iron-sulphur transformation of FNPP was controlled and optimized for enhancing Cr(VI) removal. The specific surface area and average pore diameter of the FNPP amendment reached 115.7 m²/g and 2.1 nm. The maximum adsorption capacity of total chromium reached 66.3 mg/g. The optimized iron-sulphur transformation condition was an initial FNPP and Cr(VI) molar ratio of 8, pH at 5.6, in which the Cr(VI) removal reached 96.5% and all producing S^2- was utterly consumed. It is confirmed that S^2- fast induced Fe³⁺/Fe²⁺ circulation and FNPP has a speedier adsorption rate for Cr(III) than Cr(VI). Fe²⁺ and S^2- mediated the Cr(VI) reduction to Cr(III), thus, much faster Cr(VI) removal was achieved. High efficiency removal mechanism of Cr(VI) was combined with surface adsorption/reduction and solution reduction/precipitation. The research demonstrated that controlling and optimizing the iron-sulphur transformation of Fe₇S₈ amendment can significantly enhance Cr(VI) removal.

Unveiling the Potential of Novel Struvite–Humic Acid Composite Extracted from Anaerobic Digestate for Adsorption and Reduction of Chromium

A novel struvite–humic acid composite (S–HA) was derived from an anaerobic digestate and evaluated for the adsorption and reduction of chromium [Cr (VI)] in this study. The results indicated that the struvite–humic acid composite (S–HA) contains higher contents of oxygen-containing and aromatic functional groups (47.05% and 34.13%, respectively) and a higher specific surface area (19.3 m2 g−1). These special characteristics of S–HA contributed to its higher adsorption capacity (207.69 mg g−1 and 254.47 mg g−1 for pseudo-first and second-order kinetic models, respectively) for chromium. Furthermore, XPS analysis showed that a portion of the bonded Cr (VI) was reduced to Cr (III) by carboxyl and hydroxyl functional groups, which oxidized and changed into ketone and phenol functional groups. Based on the findings, it was concluded that the phosphate–humic acid composite has an outstanding chromium adsorptive and reduction capacity. However, more research is needed to fully understand the potential of the struvite–humic acid composite for chromium adsorption and reduction.

Accumulation of chromium in plants and its repercussion in animals and humans

The untreated effluents released from industrial operations have adverse impacts on human health, environment and socio-economic aspects. Environmental pollution due to chromium is adversely affecting our natural resources and ecosystem. Chromium is hazardous carcinogenic element released from spontaneous activities and industrial procedures. Chromium toxicity, mobility and bioavailability depend mainly on its speciation. Chromium mainly exists in two forms, first as an immobile, less soluble trivalent chromium [Cr(III)] species under reducing conditions whereas hexavalent chromium [Cr(VI)] as a mobile, toxic and bioavailable species under oxidizing conditions. Hexavalent chromium is more pernicious in comparison to trivalent form. Chromium negatively affects crop growth, total yield and grain quality. Exposure of chromium even at low concentration enhances its accretion in cells of human-beings and animals which may show detrimental health effects. Many techniques have been utilized for the elimination of chromium. The selection of the green and cost-efficient technology for treatment of industrial effluent is an arduous task. The present review highlights the problems associated with chromium pollution and need of its immediate elimination by suitable remediation strategies. Further, investigations are required to fill the gaps to overcome the problem of chromium contamination and implementation of sustainable remediation strategies with their real-time applicability on the contaminated sites.

Effect of low permeability zone location on remediation of Cr(VI)-contaminated media by electrokinetics combined with a modified-zeolite barrier

Research on electrokinetics-permeable reactive barrier (EK-PRB) remediation to date has mainly focused on homogeneous soils or soils with micro-scale heterogeneities. The potential impact of macro-scale physical heterogeneities, such as stratified layers or lenses, on EK-PRB remediation has not received much attention. This study investigates the effect of a low permeability stratum on EK-PRB remediation of hexavalent chromium (Cr(VI)). Sandbox experiments were conducted to treat Cr(VI)-contaminated kaolinite/sand media, consisting of vertically-layered high permeability (HPZ) and low permeability zones (LPZ), where distance between LPZ and anode (DLA) was 3, 9, or 15 cm. Parameters including current, moisture content (MC), pH, and removal of Cr(VI) were evaluated. With 72 h of EK-PRB treatment, tests with larger DLA (15 cm) had greater Cr(VI) migration from contaminated area to modified-zeolite PRB. Cr(VI), Cr(III), and Cr(Total) removal and energy utilization efficiency followed the trend as: DLA-15 > DLA-9 > DLA-3. MC generally decreased from anode towards cathode and pH was alkaline in all the zones for DLA-3 and DLA-15. In DLA-9 (LPZ in the middle), MC increased and pH was alkaline in HPZs near cathode whereas HPZs near anode were very dry (MC < 1%) and acidic (pH < 5.5). Our results show that the location of LPZ relative to electrode locations has a significant influence on Cr(VI) removal efficiency and macro-scale physical heterogeneity is an important factor to be considered during EK-PRB remediation.

Assessment of the variation of heavy metal pollutants in soil and crop plants through field and laboratory tests

Heavy metal contamination in cultivated land is turning out to be a major problem these days. This paper presents the experimental results of the variation of heavy metal concentration in the cultivated land before and after planting different crops and accumulation of heavy metal concentration in the crops. Six crops including corn, potato, broad beans, oats, beans and soybeans were planted in 13 test fields (15,686.75 m²) in Wushan County, Gansu, China in 2020. In total, 26 subsurface soil specimens and 47 crops samples were collected for which the concentration of heavy metals such as arsenic (As), lead (Pb), cadmium (Cd), chromium (Cr) and mercury (Hg) and pH were analyzed. The results showed that the average concentration of As, Cd, Cr, Hg and Pb in 26 subsurface soil samples were 58.9, 0.251, 72.4, 0.0342 and 32.2 mg/kg, respectively. The concentration of As in the study area was even higher than the average concentration of A_s in Gansu Province. It was also detected from the test results of crops that Hg was mostly accumulated in corn, broad beans, soybeans and oats, while Cd was most likely accumulated in potato. The ecological risk of heavy metal contamination was assessed using index of geoaccumulation (I_geo), pollution index (P_i), potential ecological risk index (RI) and bioconcentration factor (BCF). The reason of high heavy metal concentration in the study area was also explored and suitable crops and planting strategies were recommended. This paper provided a comprehensive approach to investigate the heavy metal contamination in cultivated land soil and crops and offered a reasonable method to mitigate heavy metal contamination.

Kinetics of Chromium Reduction Associated with Varying Characteristics of Agricultural Soils

Chromium (Cr)(VI) is carcinogenic; thus, the excessive presence of Cr(VI) in soils can pose potential risks to water quality, food safety, and human health. The kinetics of Cr(VI) reduction in soils are important for assessing the fate of Cr in the environment. The present study tested physio-chemical and microbial properties in twenty-eight agricultural soils collected in Taiwan to evaluate the relationship between the reduction rate of Cr(VI) and soil properties, using 49-day incubation at 25 °C. At the beginning of incubation, 100 mg Cr(VI) kg−1 was spiked into the soils. The reduction of Cr(VI) was described by first-order kinetics at a significant level (p < 0.05) for the tested soils. The rate constant (k) of Cr(VI) reduction ranged from 0.01 to 4.21 day−1. In addition, the k value significantly increased with organic carbon (OC) and cation exchange capacity, but significantly decreased with increasing pH and dithionite-citrate-bicarbonate extractable Mn (Mnd). However, a predictive model using stepwise regression analysis indicated that the k value of the kinetics was controlled by OC, dissolved organic carbon (DOC), and Mnd, thereby identifying the complex interactions between Cr(VI) reduction and soil factors in the humid tropics.

Mechanistic insights of hexavalent chromium remediation by halloysite-supported copper nanoclusters

Chromium (Cr) pollution is a significant environmental concern with remediation challenge. Hexavalent chromium (Cr(VI)) is more toxic than trivalent chromium (Cr(III)) due to its mutagenicity and oncogenicity. In this investigation, a multi-functional material, copper nanoclusters (CuNCs)-halloysite nanotubes (HNT) composite (CuNCs@HNT), has been synthesised in an eco-friendly manner and utilised for Cr(VI) remediation. Advanced analytical tools confirmed the seeding of ultra-fine CuNCs onto HNT surfaces. The maximum adsorption capacity of CuNCs@HNT is 79.14 ± 6.99 mg/g at pH 5 ± 0.1 with an increment at lower pHs. This performance was comparable for real surface stream water as well as other reported materials. The pseudo-second-order kinetic-, intra-particle diffusion- and Freundlich isotherm models well fit the experimental data implying that the chemisorption, multiphase diffusion and multi-molecular layer distribution occurred during adsorption. The Fourier-transform infrared and the x-ray photoelectron spectra also ensured the transformation of Cr(VI) to Cr(III) indicating the material's suitability for concurrent adsorption and reduction of Cr(VI). While coexisting cations and anions did not overwhelm this adsorption, CuNCs@HNT was regenerated and reused five successive times in adsorption-desorption cycles without significant loss of adsorption capacity and material's integrity. Therefore, this multi-functional, biocompatible, low-cost and stable CuNCs@HNT composite may have practical application for similar toxic metals remediation.

Influence of aquifer heterogeneity on Cr(VI) diffusion and removal from groundwater

Previous studies have indicated aquifer heterogeneity has an important influence on the removal of Cr(VI) in groundwater, but little attention is paid to the effects of aquifer heterogeneity during the process especially under conditions like actual groundwater temperature and hydraulic gradient in the field. Thus, in this study, in situ remediation of Cr(VI)-contaminated shallow groundwater in a sandbox was conducted, and the influences of the heterogeneous aquifer composed of coarse, medium, and fine sand on Cr(VI) diffusion and removal before and after emulsified vegetable oil (EVO) injection were investigated, under the conditions of 19±0.5 °C and hydraulic gradient 3‰. The results show that Cr(VI) diffused consistently with groundwater from top left to bottom right; Cr(VI) spread faster in the horizontal direction than in vertical direction, and the horizontal diffusion of Cr(VI) in coarse, medium, and fine sand was 0.054 m/day, 0.036 m/day, and 0.018 m/day, respectively; a high performance of EVO toward Cr(VI) removal by over 95% was mainly because different concentrations of microorganisms migrated among heterogeneous aquifers vertically and horizontally; compared with coarse and medium sand, fine sand, with a better adsorption capacity and a lower permeability, retained relatively more microorganisms, providing favorable conditions during the remediation; a stable and unified effective removal zone, similar to the shape of Ʃ (approximately 1357.87 cm2), was ultimately formed downstream of the injection well.

Multi-functional metal-organic frameworks for detection and removal of water pollutions

Water pollutions have caused serious threats to the aquatic environment and human health, it is of great significance to monitor and control their contents in water. Compared with the traditional...

Preparation and applications of chitosan and cellulose composite materials

Chitosan is a natural fiber, chemically cellulose-like biopolymer, which is processed from chitin. Its use as a natural polymer is getting more attention because it is non-toxic, renewable, and biocompatible. However, its poor mechanical and thermal strength, particle size, and surface area restrict its industrial use. Consequently, to improve these properties, cellulose and/or inorganic nanoparticles have been used. This review discusses the recent progress of chitosan and cellulose composite materials, their preparation, and their applications in different industrial sectors. It also discusses the modification of chitosan and cellulose composite materials to allow their use on a large scale. Finally, the recent development of chitosan composite materials for drug delivery, food packaging, protective coatings, and wastewater treatment are discussed. The challenges and perspectives for future research are also considered. This review suggests that chitosan and cellulose nano-composite are promising, low-cost products for environmental remediation involving a simple production process.

Review on the Use of Heavy Metal Deposits from Water Treatment Waste towards Catalytic Chemical Syntheses

The toxicity and persistence of heavy metals has become a serious problem for humans. These heavy metals accumulate mainly in wastewater from various industries' discharged effluents. The recent trends in research are now focused not only on the removal efficiency of toxic metal particles, but also on their effective reuse as catalysts. This review discusses the types of heavy metals obtained from wastewater and their recovery through commonly practiced physico-chemical pathways. In addition, it covers the advantages of the new system for capturing heavy metals from wastewater, as compared to older conventional technologies. The discussion also includes the various structural aspects of trapping systems and their hypothesized mechanistic approaches to immobilization and further rejuvenation of catalysts. Finally, it concludes with the challenges and future prospects of this research to help protect the ecosystem.

Novel recycling of phosphorus-recovered incinerated sewage sludge ash residues by co-pyrolysis with lignin for reductive/sorptive removal of hexavalent chromium from aqueous solutions

Incinerated sewage sludge ash (ISSA), a by-product generated from the combustion of dewatered sewage sludge, has been extensively studied as a secondary resource for phosphorus recovery by acid extraction methods. Recycling of the P-recovered ISSA residues is crucial to complete and sustain the whole process. In this study, the ISSA residue rich in iron was reused and co-pyrolyzed with lignin at 650, 850 and 1050 °C under N₂ atmosphere for the synthesis of a composite material to remove hexavalent chromium (Cr(VI)) from aqueous solutions. Characterization analysis including XRD, XPS, and FTIR showed that iron oxides in the residue were reduced to zero valent iron at 1050 °C that exhibits the optimal Cr(VI) removal performance. The Cr(VI) removal process was rapid and reached a plateau at around 30 min. The maximum removal rate was obtained at pH 2.0, which was conducive for the treatment of a synthetic Cr(VI)-containing wastewater in fix-bed column experiments, whereby Cr(VI) as well as total Cr were continuously removed. Overall, this study proposed a new routine for the recycling of ISSA residue after phosphorus recovery by the acid extraction method and provided a value-added product for Cr(VI) removal from wastewaters.

Myco-remediation: A mechanistic understanding of contaminants alleviation from natural environment and future prospect

Industrialization and modernization of agricultural systems contaminated lithosphere, hydrosphere, and biosphere of the Earth. Sustainable remediation of contamination is essential for environmental sustainability. Myco-remediation is proposed to be a green, economical, and efficient technology over conventional remediation technologies to combat escalating pollution problems at a global scale. Fungi can perform remediation of pollutants through several mechanisms like biosorption, precipitation, biotransformation, and sequestration. Myco-remediation significantly removes or degrades metal metals, persistent organic pollutants, and other emerging pollutants. The current review highlights the species-specific remediation potential, influencing factors, genetic and molecular control mechanism, applicability merits to enhance the bioremediation efficiency. Structure and composition of fungal cell wall is crucial for immobilization of toxic pollutants and a subtle change on fungal cell wall structure may significantly affect the immobilization efficiency. The utilization protocol and applicability of enzyme engineering and myco-nanotechnology to enhance the bioremediation efficiency of any potential fungus was proposed. It is advocated that the association of hyper-accumulator plants with plant growth-promoting fungi could help in an effective cleanup strategy for the alleviation of persistent soil pollutants. The functions, activity, and regulation of fungal enzymes in myco-remediation practices required further research to enhance the myco-remediation potential. Study of the biotransformation mechanisms and risk assessment of the products formed are required to minimize environmental pollution. Recent advancements in molecular "Omic techniques"and biotechnological tools can further upgrade myco-remediation efficiency in polluted soils and water.

Phytohormones Producing Acinetobacter bouvetii P1 Mitigates Chromate Stress in Sunflower by Provoking Host Antioxidant Response

Different physical and chemical techniques are used for the decontamination of Cr+6 contaminated sites. The techniques are expensive, laborious, and time-consuming. However, remediation of Cr+6 by microbes is viable, efficient, and cost-effective. In this context, plant growth-promoting rhizobacteria Acinetobacter bouvetii P1 isolated from the industrial zone was tested for its role in relieving Cr+6 induced oxidative stress in sunflower. At the elevated Cr+6 levels and in the absence of P1, the growth of the sunflower plants was inhibited. In contrast, the selected strain P1 restored the sunflower growth under Cr+6 through plant growth-promoting interactions. Specifically, P1 biotransformed the Cr+6 into a stable and less toxic Cr+3 form, thus avoiding the possibility of phytotoxicity. On the one hand, the P1 strengthened the host antioxidant system by triggering higher production of enzymatic antioxidants, including catalases, ascorbate peroxidase, superoxide dismutase, and peroxidase. Similarly, P1 also promoted higher production of nonenzymatic antioxidants, such as flavonoids, phenolics, proline, and glutathione. Apart from the bioremediation, P1 solubilized phosphate and produced indole acetic acid, gibberellic acid, and salicylic acid. The production of phytohormones not only helped the host plant growth but also mitigated the harsh condition posed by the elevated levels of Cr+6. The findings mentioned above suggest that P1 may serve as an excellent phyto-stimulant and bio-remediator in a heavy metal-contaminated environment.

Biochars produced from various agro-industrial by-products applied in Cr(VI) adsorption-reduction processes

This study aimed to reuse different agro-industrial by-products (poultry litter, pig manure, sewage sludge and coffee husk) for biochar production and to evaluate their Cr(VI) removal capacities in aqueous medium. The biochars showed different morphologies with porous structures. The percentages of Cr(VI) removal from solution were higher in acid medium (pH = 2), reaching values up to 87%. For all biochars, Cr(VI) removal occurs via both adsorption and reduction, being a rapid (30 min) process, which fits best to the pseudo-second order kinetic model. The biochars, especially from coffee husk, were able to reduce up to 20% of Cr(VI) to Cr(III). The maximum Cr(VI) removal capacities ranged from 10.86 mg g^-1 (sewage sludge biochar) to 18.52 mg g^-1 (coffee husk biochar). Therefore, the production of biochars from the agro-industrial by-products using the same experimental conditions in one single study is important to compare the Cr(VI) removal capacities from different biomasses. Thus, this study explored the corresponding raw material without the need of further treatment. Biochars showed potential for environmental applications considering Cr(VI)-polluted environments. It is hoped to provide basis to future studies using real wastewater samples.

Understanding the impacts of the COVID-19 pandemic on sustainable agri-food system and agroecosystem decarbonization nexus: A review

The existing finite natural resources have witnessed unsustainable usage in the past few years, especially for food production, with accompanying environmental devastation and ecosystem damage. Regrettably, the global population and consumption demands are increasing ceaselessly, leading to the need for more resources for food production, which could potentially aggravate the sustainability and ecosystem degradation issues, while stimulating drastic climate change. Meanwhile, the unexpected emergence of the COVID-19 pandemic and some implemented measures to combat its spread disrupted agricultural activities and the food supply chain, which also led to a reduction in ecosystem carbonization. This study sets out to explore policy framework and selected feasible actions that are being adopted during the COVID-19 pandemic, which could potentially reduce the emissions even after the pandemic to promote a resilient and sustainable agri-food system. In this study, we reviewed 27 articles that focus on the current state of the agri-food system in light of the COVID-19 pandemic and its impact on the decarbonization of the agroecosystem. This review has taken the form of a systematic methodology in analyzing the adoption and implementation of various measures to mitigate the spread of COVID-19 on the impact of the agri-food system and reduction in ecosystem degradation. Up to 0.3 Mt of CO2 reduction from the agri-food system alone was reportedly achieved during the first 6 months of the pandemic in 23 European countries. The various adopted measures indicate that the circular economy approach is a panacea to achieve the needed sustainability in the agri-food system. Also, it dictates a need for a paradigm change towards improvement on localized food production that promotes sustainable production and consumption.

Amberlite IRC-718 ion chelating resin extraction of hazardous metal Cr (VI) from aqueous solutions: equilibrium and theoretical modeling

Under varied conditions, the IRC 718 ion-exchange resin is used to extract chromium (VI) ions from aqueous solutions. On chromium (VI) removal effectiveness, the effects of adsorption dosage, contact time, beginning metal concentration, and pH were examined. The batch ion exchange process reached equilibrium after around 90 minutes of interaction. With an initial chromium (VI) concentration of 0.5 mg/dm³, the pH-dependent ion-exchange mechanism revealed maximal removal in the pH 2.0-10 range. The adsorption mechanism occurs between Cr (VI) determined as the electron acceptor, and IRC 718 determined as the electron donor. The equilibrium ion-exchange potential and ion transfer quantities for Amberlite IRC 718 were calculated using the Langmuir adsorption isotherm model. The overall ion exchange capacity of the resin was determined to be 187.72 mg of chromium (VI)/g of resin at an ideal pH of 6.0.

Stress amelioration response of glycine betaine and Arbuscular mycorrhizal fungi in sorghum under Cr toxicity

Chromium toxicity is a major problem in agricultural soils that negatively affects a plant’s metabolic activities. It reduces biochemical and antioxidant defence system’s activities. In search of the solution to this problem a two-year pot experiment (completely randomized design with three replications), in three genetically different varieties of sorghum (SSG 59–3, HJ 513 and HJ 541) under Cr toxicity (2 and 4 ppm) was conducted to determine the effect of glycine betaine (50 and 100mM) and Arbuscular mycorrhizal fungi (AMF) on the antioxidant system (enzymes viz. superoxide dismutase, ascorbate peroxidase, catalase, glutathione reductase, peroxidase and metabolites viz. glutathione, ascorbate, proline, β-carotene) along with Cr accumulation and indices of oxidative stress parameters (polyphenol oxidase, hydrogen peroxide and malondialdehyde) at two growth stages (vegetative and grain filling). According to results; Cr stress (2 & 4 ppm) increased its accumulation and indices of oxidative stresses significantly (p≤0.05) in all varieties of sorghum at both growth stages. However, soil application of glycine betaine (GB) and AMF decreased Cr accumulation and indices of oxidative stress by increasing antioxidant enzymes and metabolites activities at both growth stages in all varieties. The combination of 100mM GB with AMF was observed most significant (p≤0.05) in decreasing oxidative stress and improved the antioxidant system’s activities. The SSG 59–3 cultivar showed the lowest Cr accumulation (1.60 and 8.61 ppm), indices of oxidative stress and highest antioxidant system’s activity among these three cultivars at both growth stages. Thus, SSG 59–3 was found most tolerant cultivars followed by HJ 513 and then HJ 541. These findings suggest that both GB and AMF, either individually or combined can play a positive role to reduce oxidative stress and increased antioxidant attributes under Cr toxicity in sorghum.

Nitrogen Effect on Growth-Related Parameters and Evaluation of Portulaca oleracea as a Phytoremediation Species in a Cr(VI)-Spiked Soil

In a pot experiment, we assessed the potential of purslane (Portulaca oleracea) as a phytoremediation species in Cr(VI)-contaminated soils. We focused on the evaluation of phytotoxic Cr(VI) effects at concentrations reaching 150 mg Cr(VI) kg−1 and the possible stress amelioration effect of nitrogen on Cr(VI)-stressed plants. Treatments were T-0 (control), T-1 (25 mg Cr(VI) kg−1), T-2 = 50 mg kg−1, T-3 = 100 mg kg−1, and T-4 = 150 mg kg−1. We measured Cr(VI) concentration in aerial and root tissues, a series of parameters related to photosynthesis and plant growth, phosphorus aerial plant tissue content, and we also calculated indices (ratios) related to leaf growth and above ground tissue water content. Cr(VI) almost exclusively was found in root tissues; all physiological and growth parameters studied were severely affected and plants selectively accumulated phosphorus in aerial plant tissues with increasing Cr(VI) soil concentrations. On the other hand, N amendment resulted in improved plant features in some of the measured parameters: chlorophyll index was improved with added N at T-2, plant height was significantly higher at T-0, T-1, and T-2, and aerial dry weight and leaf area was higher at T-0; these effects indicate that added N did increase P. oleracea potential to ameliorate Cr(VI) toxic effects. We conclude that purslane showed a potential as a possible species to be successfully introduced to Cr(VI)-laden soils, but more research is certainly necessary.

Recent Advances on Properties and Utility of Nanomaterials Generated from Industrial and Biological Activities

Today is the era of nanoscience and nanotechnology, which find applications in the field of medicine, electronics, and environmental remediation. Even though nanotechnology is in its emerging phase, it continues to provide solutions to numerous challenges. Nanotechnology and nanoparticles are found to be very effective because of their unique chemical and physical properties and high surface area, but their high cost is one of the major hurdles to its wider application. So, the synthesis of nanomaterials, especially 2D nanomaterials from industrial, agricultural, and other biological activities, could provide a cost-effective technique. The nanomaterials synthesized from such waste not only minimize pollution, but also provide an eco-friendly approach towards the utilization of the waste. In the present review work, emphasis has been given to the types of nanomaterials, different methods for the synthesis of 2D nanomaterials from the waste generated from industries, agriculture, and their application in electronics, medicine, and catalysis.

Fabrication of different SnO2 nanorods for enhanced photocatalytic degradation and antibacterial activity

The acid-mediated (oxalic acid [OXA], cinnamic acid [CA], and itaconic acid [IA]) SnO₂ nanorods were synthesized by the hydrothermal method. The synthesized SnO₂ nanorods, in turn, were analyzed with various physico-chemical techniques such as the X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), and Raman spectroscopy. Furthermore, the photocatalytic activity of the different SnO₂ nanorods was investigated with the malachite green (MG) dye under visible light illumination. The OXA-SnO₂ nanorods displayed an excellent degradation performance with observed value at 91% and it was compared to CA and IA-SnO₂ nanomaterials. This tetragonal phase was identified and confirmed by XRD studies. In this regards, obtained band gap energy is low then optimally performed to the photocatalytic evolution. The OXA-SnO₂ materials were tested for antibacterial and antifungal studies; this was as shown in good biological activities with admire to the different bacterial strains. The Candida albicans (antifungal) and Enterococcus faecalis (Gram-positive) bacteria were not affected in the microbial studies.

Game Theory-Based Analysis of Local Governments’ Behavioral Dissimilation in the Third-Party Soil Pollution Control under Chinese-Style Fiscal Decentralization

The participation of a third party of the environmental service enterprise theoretically increases the level and efficiency of soil pollution control in China. However, Chinese-style fiscal decentralization may have a negative impact on the behaviors of participants, especially the local government. First, this paper conducts a positioning analysis on participants of the third-party soil pollution control in China and discusses the behavioral dissimilation of the local government under fiscal decentralization. Second, taking the government’s third-party soil pollution control as a case, a two-party game model of the central government and the local government is established around the principal-agent relationship, and a tripartite game model of the central government, the local government, and the third-party enterprise is designed around the collusion between the local government and the third-party enterprise. The results show that Chinese-style fiscal decentralization may lead to the behavioral dissimilation of local governments, that is, they may choose not to implement or passively implement the third-party control, and choose to conspire with third-party enterprises. Improving the benefits from implementing the third-party control of local governments and third-party enterprises, enhancing the central government’s supervision probability and capacity, and strengthening the central government’s punishment for behavioral dissimilation are conducive to the implementation of the third-party soil pollution control. Finally, this study puts forward policy suggestions on dividing the administrative powers between the central and local government in third-party control, building appraisal systems for the local government’s environmental protection performance, constructing environmental regulation mechanisms involving the government, market and society, and formulating the incentive and restraint policies for the participants in the third-party soil pollution control.