Effect of pH on the removal of Cr(III) and Cr(VI) from aqueous solution by modified polyethyleneimine

Applicability of new sustainable and efficient green metal-based nanoparticles for removal of Cr(VI): Adsorption anti-microbial, and DFT studies

Artemisia absinthium leaves were utilized as a reducing agent for green synthesis of Zinc oxide nanoparticles (particle size 17 nm). Synthesized green-ZnO (g-ZnO) were characterized by SEM/EDX, FTIR, XRD, UV, and BET analyses and then further used as an adsorbent to remove Cr(VI) ions from simulated wastewater. Optimal pH, temperature and adsorbent dosage were determined through batch mode studies. High removal efficiency and adsorption capacity were observed at pH 4, 0.25 g L^-1 dosage, and 25 mg L^-1 concentration of Cr(VI). Experimental data were modelled with different adsorption kinetics (Elovich model, PFO, PSO, IDP model) and isotherms (Langmuir, Freundlich, and Temkin), and it was found the adsorption process was well fitted to Langmuir with an R² value greater than>0.99. Computational calculation showed that the g-ZnO nanoparticles became ∼14 times more dynamic with delocalized surface states making them a relevant platform to adsorb Cr with greater work function compatibility supporting the experimental findings. The Q_max adsorption capacity of g-ZnO was 315.46 mg g^-1 from Langmuir calculations. Thermodynamic calculations reveal that the Cr (VI) adsorption process was spontaneous and endothermic, with a positive ΔS value representing the disorder at the solid-solution interface during the adsorption. In addition, the present study has demonstrated that these g-ZnO nanoparticles show strong antibacterial activities against P. aeruginosa (MTCC 1688) and E. coli (MTCC 1687). Also, the novel g-ZnO adsorbent capacity to remove Cr(VI) from simulated water revealed that it could be reused at least six times with higher removal rates during regeneration experiments. The results obtained from adsorption and antimicrobial activities suggest that g-ZnO nanoparticles could be used effectively in real-time wastewater and agricultural safety applications.

Ecological effects, remediation, distribution, and sensing techniques of chromium

Chromium is detected in most ecosystems due to the increased anthropogenic activities in addition to that developed from natural pollution. Chromium contamination in the food chain results due to its persistent and non-degradable nature. The release of chromium in the ecosystem accretes and thereafter impacts different life forms, including humans, aquatic and terrestrial organisms. Leaching of chromium into the ground and surface water triggers several health ailments, such as dermatitis, eczematous skin, allergic reactions, mucous and skin membrane ulcerations, allergic asthmatic reactions, bronchial carcinoma and gastroenteritis. Physiological and biological treatments for the removal of chromium have been discussed in depth in the present communication. Adsorption and biological treatment methods are proven to be alternatives to chemical removal techniques in terms of cost-effectiveness and low sludge formation. Chromium sensing is an alternative approach for regular monitoring of chromium in different water bodies. This review intended to explore different classes of sensors for chromium monitoring. However, the spectrochemical methods are more sensitive in chromium ions sensing than electrochemical methods. Future study should focus on miniaturization for portability and on-site measurements without requiring a large instrument provides a good aspect for future research.

Enhanced removal of Cr(VI) from aqueous solutions by polymer-mediated nitrogen-rich reduced graphene oxide

The efficient removal of heavy metal by rationally designed carbon-based adsorbents is a key challenge in the field of water purification. Herein, we report a nitrogen-enriched lignosulfonate exfoliated graphene oxide (N-LEGO) for hexavalent chromium (Cr(VI)) removal from aqueous solution. The nitrogen content of N-LEGO reached 13.28%, and the ratio of N-bonding configurations (pyri-N:amine-N:pyrro-N:grap-N) was 2.3:1.6:1:2.3. For Cr(VI) with initial concentration of 70 mg L^-1 under pH= 2, the residuary concentration after treated by N-LEGO was close to 0.004 mg L^-1, which meets the industrial wastewater discharge standard. The Cr(VI) adsorption behavior on N-LEGO can be fitted with the pseudo-second-order kinetics and Freundlich isotherm model well. The adsorption mechanism of Cr(VI) on N-LEGO includes anions electrostatic attraction, reduction and surface chelation. Density functional theory (DFT) simulations showed that N atoms doping was feasible and thermodynamically stable, meanwhile the N-doped system was easier to adsorb Cr₂O₇^2- than HCrO₄^-. The findings of this work can provide a new idea for the development of N-doped carbon-based adsorbents for the removal of highly toxic Cr(VI) from aqueous solutions.

Rapid removal of Cr(III) from high-salinity wastewater by cellulose-g-poly-(acrylamide-co-sulfonic acid) polymeric bio-adsorbent

A cellulose-g-poly-(acrylamide-co-sulfonic acid) polymeric bio-adsorbent (CASA) was prepared by grafting copolymerization, and used to adsorb Cr(III) from leather wastewater. The SEM, XRD, FTIR, and XPS results showed that CASA contains many spherical particles and functional groups such as NH₂, CO, and HSO₃. The adsorption experiments revealed that CASA presented excellent adsorption performance for Cr(III) (274.69 mg/g of max adsorption capacity) from high-salinity wastewater, which was much better than other reported adsorbents with different structures. Meanwhile, adsorption equilibrium could be reached within 10 min due to the introduction of abundant sulfonic acid groups on its surface. In addition, the adsorption process followed the Langmuir adsorption isotherm, and the experimental data conformed to the pseudo-second-order kinetics model. Moreover, the main adsorption mechanisms include chelation, electrostatic interactions, and cation exchange, which provide an important theoretical basis for the removal of toxic inorganic pollutants from leather wastewater.

Highly efficient removal of Cr(III)-poly(acrylic acid) complex by coprecipitation with polyvalent metal ions: Performance, mechanism, and validation

The Cr(III)-organic complexes formed between Cr(III) and multifunctional group ligands, such as polyacrylate, are highly water soluble and difficult to be removed from wastewater by common treatments. A novel strategy for efficiently removing Cr(III)-poly (acrylic acid) complex (Cr(III)-PAA) from wastewater without introducing secondary pollution is proposed using a coprecipitation method with polyvalent metal ions. Al(III), Fe(III), Zr(IV), and Ti(IV) were combined with the carboxyl of Cr(III)-PAA to decrease hydrophilia and achieve fast and efficient coprecipitation. Cr(III)-PAA was efficiently removed from wastewater by using these polyvalent metal ions, especially at low pH, where the ions exist as monomer. The residual concentration of Cr(III) in treated wastewater under the optimized experimental condition was less than 1.0 mg/L. No Cr(VI) and negligible amount of polyvalent metal ions were detected in the treated wastewater, indicating that almost all of the ions coprecipitated with Cr(III)-PAA. No secondary pollution also occurred. The high reactivity between the polyvalent metal ions and Cr(III)-PAA and the sharp decrease in the hydrophilia of the complex contributed to its highly efficient removal from wastewater. Actual tannery wastewater containing Cr(III)-organic complexes with high solubility and stability was treated through coprecipitation with Al(III). A high Cr(III) removal efficiency of 99.0% was obtained. This work provides new insights into the removal of soluble Cr(III)-organic complexes from wastewater by using an environment-friendly and cost-effective method.

Fabrication of core@shell structural Fe-Fe2O3@PHCP nanochains with high saturation magnetization and abundant amino groups for hexavalent chromium adsorption and reduction

The rational design of novel adsorption materials is imperative to remove toxic metal species from the polluted water. Herein, a core@shell structural Fe-Fe₂O₃@poly (hexachlorocyclotriphosphazene-co-polyethylenimine) (Fe-Fe₂O₃@PHCP) magnetic nanochain with high saturation magnetization was fabricated and used for effective adsorption and reduction of hexavalent chromium. The morphology and microstructure of Fe-Fe₂O₃@PHCP were characterized by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The effects of concentration, pH, contact time, temperature and coexisting ions on Cr (VI) removal were studied. Four kinetic models (pseudo-first-order, pseudo-second-order, Bangham and intraparticle diffusion models) and two isotherm models (Freundlich and Langmuir) were used to fit experimental data. Results show the adsorption capacity of Fe-Fe₂O₃@PHCP for Cr (VI) is up to 229.0 mg g^-1. The excellent performance was ascribed to the favorable reduction of Cr (VI) to Cr (III), followed by the chelation of Cr (III) with imino groups. Meanwhile, the residual Cr (VI) were adsorbed on protonated amino and imino groups. The adsorption process is exothermic and spontaneous and nicely follows pseudo-second-order kinetics, intraparticle diffusion model and Langmuir isotherm model. These results indicated that easily separable Fe-Fe₂O₃@PHCP magnetic nanochains could be a promising adsorbent to remediate chromate wastewater.

Biochar from extracted marine Chlorella sp. residue for high efficiency adsorption with ultrasonication to remove Cr(VI), Zn(II) and Ni(II)

The biochar BC-450 derived from the extracted marine Chlorella sp. residue (EMCR) had high surface area (266 m²/g) and was rich in ash and O-functional groups. Its characteristics are suitable for heavy metal adsorption. The adsorption parameters were investigated to optimize the removal efficiency of Cr(VI), Zn(II) and Ni(II) from aqueous solution by conventional adsorption (CA) and by ultrasonication adsorption (UA). The adsorption was fit by Langmuir isotherm and by pseudo-second-order model. The equilibrium times were 10, 8, 15 min and 40, 60, 80 min for removal of Cr(VI), Zn(II) and Ni(II) with UA and CA, respectively. The maximum adsorption capacities of Cr(VI), Zn(II) and Ni(II) for CA and UA were 15.94, 17.62 and 24.76 mg/g and 18.86, 21.31 and 27.45 mg/g, respectively. UA presented 1.1-1.3 times greater removal efficiencies than CA in much shorter time. The EMCR is a promising feedstock for producing low cost and high efficiency adsorbents.

Adsorption of Cd2+ and Cr3+ ions from aqueous solutions by using residue of Padina gymnospora waste as promising low-cost adsorbent

Recently, a great attention has been given for applying a low-cost and effective adsorbents instead of expensive and dangerous chemical materials as a promising approach to treat wastewater. In this work, residue powder of brown macroalga Padina gymnospora (RPG), after extracting most of its active components by 70% methanol, was used as an adsorbent material for wastewater treatment. This work also reduces the costs of residue disposal. The adsorption ability of RPG is studied for removing Cd²⁺ and Cr³⁺from wastewater. We investigated metal adsorption isotherms and kinetics, the effect of initial metal concentration, contact time, adsorbent dosage, temperature, pH and the RPG reusability on metal ions removal. The results showed that the removal % generally increases with decreasing concentration of metal ions. RPG has higher metal removal percentages reaching 96.2% and 78.8% for Cd²⁺ and Cr³⁺, respectively, with a maxiumum adsorption capacity of 96.46 and 31.52 mg/g for Cd²⁺and Cr³⁺,respectively at pH 6.2, 50 mg, 25 °C and initial metal concentration of 100 mg/L. The metal ions removal % increased by increasing the dosage of adsorbent and it decreased after a certain limit. The metal removal % slightly changes with increasing temperature for Cd²⁺ and decreased at high-temperature for Cr³⁺. The adsorption increased with increasing pH value from 3 to 5, and decreases at pH value of 6.2 then it increased again at pH 8. The removal % and adsorption capacity at pH 8 reaches 99.58%, 99.65%, 99.85 mg/g and 39.86 mg/g for Cd²⁺ and Cr³⁺, respectively. The results also showed that RPG can be reused several times for metal ions removal. In addition, Tempkin isotherms and pseudo-second-order kinetic fit the adsorption of Cd²⁺ and Cr³⁺ well.

A novel method to remove chromium, vanadium and ammonium from vanadium industrial wastewater using a byproduct of magnesium-based wet flue gas desulfurization

A novel treatment for chromium, vanadium and ammonium from vanadium industrial wastewater using a byproduct of magnesium-based wet flue gas desulfurization is investigated. In the present study, the byproduct is used as a reductant for chromium and vanadium removal by chemical precipitation, and the residual magnesium ion can also be used to remove ammonium in the present of phosphate by struvite crystallization. Besides, the effects of main operational parameters (reaction pH, byproduct dosage and reaction time) on the heavy metal removal and ammonium removal (reaction pH, Mg²⁺:NH₄⁺:PO₄^3- molar ratio and reaction time) are investigated, and the reaction mechanism for this treatment technology is also proposed. Under the optimal conditions, the residual concentrations of chromium(IV), total chromium and vanadium are 0.046mg/L, 0.468mg/L and 0.06mg/L, respectively. The removal efficiency of ammonium is 95.72% and the residual concentrations of ammonium and phosphorus are 137.12mg/L and 5.49mg/L, respectively. Additionally, the precipitations are characterized using X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), scanning electron microscope-energy dispersive spectrometer (SEM-EDS) and thermogravimetry differential scanning calorimetry (TG-DSC), respectively. Finally, a resource utilization method of the precipitation sludge from this technology is also presented.

Highly stable and covalently functionalized magnetic nanoparticles by polyethyleneimine for Cr(vi) adsorption in aqueous solution

Covalently functionalized magnetic nanocomposite by polyethyleneimine (PEI) acted as superior adsorbent for removal of Cr(vi) form water.

Controllable Synthesis of Zn2GeO4Nanorods for Photocatalytic Reduction of Aqueous Cr(VI) and Oxidation of Organic Pollutants

Zn2GeO4nanorods were successfully synthesized by a simple hydrothermal method. The composition, morphology, and optical properties of as-synthesized Zn2GeO4samples were characterized by X-ray diffraction, scan electron microscopy, and UV-vis diffuse reflectance spectra. The photocatalytic properties of Zn2GeO4nanorods were evaluated by the reduction of Cr(VI) and oxidation of organic pollutants in aqueous solution. The effects of solution pH on Cr(VI) reduction by Zn2GeO4nanorods were studied in detail. The results indicated that the efficiency of Cr(VI) reduction was highest at pH 5.96. Moreover, Zn2GeO4nanorods also showed excellent photocatalytic ability for the oxidation of organic pollutants such as rhodamine B and 4-nitrophenol.