Hexavalent chromium removal by polyacrylic acid-grafted Macadamia nutshell powder through adsorption–reduction mechanism: Adsorption isotherms, kinetics and thermodynamics

Carbide-derived carbon as an extraordinary material for the removal of chromium from an aqueous solution

In the present work, the adsorptive removal of chromium (Cr) from water by carbide-derived carbon (CDC) was investigated. The morphology and structure of the CDC were characterized by using FTIR, SEM, TEM, XRD, and N₂ adsorption-desorption measurements. The effect of adsorption parameters including contact time, initial Cr concentration, temperature, initial solution pH, and CDC dosage was examined on the removal of Cr ions. The kinetic analysis revealed that the experimental data on the removal of Cr ions on CDC is well correlated with the pseudo-second order kinetic model (with R² > 0.999), while the equilibrium data were fitted by the Redlich-Peterson isotherm model (with R² > 0.992). The Langmuir and Sips models were also in good compliance with the equilibrium data, indicating a monolayer coverage of Cr ions onto the CDC surface with some heterogeneous active adsorption sites. The CDC revealed a notable Langmuir adsorption capacity of 159.1 mg/g for Cr ions at pH 6 and room temperature. The thermodynamic analysis illustrated that the Cr ions elimination by CDC is a feasible adsorption process and endothermic in nature. After five adsorption/desorption cycles, less than 18% reduction in the adsorption capacity was obtained indicating the stability and reusability of the CDC. Moreover, the CDC demonstrated an excellent potential in removing the Cr ions from real brackish water. According to the adsorption data, both physical and chemical adsorption processes occurred, and the adsorption was mainly controlled by electrostatic interactions with a possible reduction of hexavalent Cr to trivalent Cr at acidic conditions.

Scavenging of hexavalent chromium from aqueous solution by Macadamia nutshell biomass modified with diethylenetriamine and maleic anhydride

Based on the premise that aqueous anions of hexavalent chromium (Cr(VI)) are capable of electrostatic interaction with cationic and polar active sites, acid-washed Madacamia nutshell biomass was sequentially treated with diethylenetriamine (DETA) and maleic anhydride (MA) to graft poly(diethylenetriamine-co-maleic anhydride). By displaying a new peak at 1685 cm^-1 ascribed to amide CO stretching vibrations, Fourier transform infrared spectroscopy highlighted the formation of amide groups through reaction of DETA with carboxyl groups on the biomass surface. Scanning electron microscopic images of the MA-modified biomass displayed polymeric growths attributed to copolymerization of DETA with MA. The polar and ionizable amide and amine groups of the grafted copolymer endowed the adsorbent with Cr(VI) removal capabilities over a wide pH range demonstrated by removal efficiencies between 70.9% and 81.7% in the pH 1.6 to pH 10.0 range for the treatment of 20 mL solutions containing 100 mg L^-1 Cr(VI) with 200 mg of adsorbent. Conformity of the adsorption isotherm data to the Freundlich model revealed the heterogeneous nature of the adsorbent surface, which comprised a variety of functional groups capable of interaction with Cr(VI) species in solution. The Sips isotherm model provided the best fit to the equilibrium experimental data, and the adsorption capacity was 779.1 mg g^-1 at pH 1.6, room temperature and an adsorbent dosage of 5.0 g L^-1. The findings indicate that Cr(VI) adsorption onto diethylenetriamine and maleic anhydride modified Madacamia nutshell biomass is a promising option for Cr(VI) removal from aqueous solutions.

Biosorption of chromium (VI), iron (II), copper (II), and nickel (II) ions onto alkaline modified Chlorella vulgaris and Spirulina platensis in binary systems

The simultaneous biosorption of chromium (VI), copper (II), iron (II), and nickel (II) was investigated by alkaline-modified Chlorella vulgaris and Spirulina platensis in binary systems. The alkaline modified biosorbents were CV-KCl, SP-KCl, CV-Na2CO3, and SP-Na2CO3. The maximum removal efficiency recorded in this study was 99.7% with a biosorbent dosage of 0.3 g within a pH range of 2 to 6. The highest biosorption capacities obtained were 14.1, 13.5, 21.6, and 15.8 mg/g for Cr (VI), Cu (II), Fe (II), and Ni (II), respectively. The pseudo-second-order best described the biosorption rate, while the Langmuir isotherm model best described the biosorption equilibrium interaction. The values for Gibbs free energy (ΔG°) were in the range of 0.5 to 6.5 kJ/mol (Cr-Fe), 1.3 to 8.4 kJ/mol (Cr-Ni), and 3.9 to 11.3 kJ/mol (Cr-Cu) binary systems. This showed that the biosorption processes were characterized by physisorption reactions. The Temkin constant B values were in the range of 0.339 to 1.485 kcal/mol and the biosorption processes were largely exothermic reactions. The values for the Freundlich constant KF were between 1.4 and 10.4 (L/g), which indicated favourable biosorption. The Temkin isotherm model confirmed a strong binding affinity for Fe (II) and Ni (II). The results suggest that potassium chloride and sodium carbonate modification are very suitable for green algae and cyanobacteria for the efficient removal of heavy metals.

Agricultural waste materials for adsorptive removal of phenols, chromium (VI) and cadmium (II) from wastewater: A review

Management of basic natural resources and the spent industrial and domestic streams to provide a sustainable safe environment for healthy living is a magnum challenge to scientists and environmentalists. The present remedial approach to the wastewater focuses on recovering pure water for reuse and converting the contaminants into a solid matrix for permanent land disposal. However, the ground water aquifers, over a long period slowly leach the contaminants consequently polluting the ground water. Synthetic adsorbents, mainly consisting of polymeric resins, chelating agents, etc. are efficient and have high specificity, but ultimate disposal is a challenge as most of these materials are non-biodegradable. In this context, it is felt appropriate to review the utility of adsorbents based on natural green materials such as agricultural waste and restricted to few model contaminants: phenols, and heavy metals chromium(VI), and cadmium(II) in view of the vast amount of literature available. The article discusses the features of the agricultural waste material-based adsorbents including the mechanism. It is inferred that agricultural waste materials are some of the common renewable sources available across the globe and can be used as sustainable adsorbents. A discussion on challenges for industrial scale implementation and integration with advanced technologies like magnetic-based approaches and nanotechnology to improve the removal efficiency is included for future prospects.

Aging shapes Cr(VI) speciation in five different soils

To make sound decisions regarding management of heavy metal contamination in soils, it is necessary to understand contaminant transformations over extended periods. In this study, sequential extraction methods were applied to quantify the changes of Cr fractions [available Cr(VI), immobile Cr(VI) and immobile Cr(III)] in five contrasting soils spiked with Cr(VI) over a 240-day incubation. Results showed that available Cr(VI) in soils continually decreased during aging, with a sharp decline occurring in the first 30 days. The best fit of available Cr(VI) data was obtained using an Elovich model for Brunisol and Anthrosol-1, a fractional power model for Anthrosol-2, and a pseudo first-order kinetic model for Luvisol-1 and Luvisol-2. After aging for 240 days, immobile Cr(VI) increased by 4.5-31% and immobile Cr(III) increased by 68-95% of total spiked Cr(VI) in Brunisol, Anthrosol-1 and Anthrosol-2. The two Luvisol soils had relatively high reduction rates with no Cr(VI) immobilized. A multireaction model was developed in MATLAB Simulink toolbox to describe transformation flow rates among soluble Cr(VI), adsorbed Cr(VI), immobilized Cr(VI) and immobilized Cr(III) in soils with aging. We conclude that (i) Cr(VI) reduction and immobilization were occurring concurrently in soils and competing for available Cr(VI) species; (ii) Cr(VI) reduction is favored by low soil pH and high organic carbon, while Cr(VI) immobilization occurs with cations (such as Ca²⁺) and Fe oxides.

Heavy metal removal from aqueous solutions by chitosan-based magnetic composite flocculants

In this study, three magnetic flocculants with different chelating groups, namely, carboxymethyl chitosan-modified Fe₃O₄ flocculant (MC), acrylamide-grafted magnetic carboxymethyl chitosan flocculant (MCM), and 2-acrylamide-2-methylpropanesulfonic acid copolyacrylamide-grafted magnetic carboxymethyl chitosan flocculant (MCAA) were prepared, synthesized, and characterized by photopolymerization technology. They were applied to the flocculation removal of Cr(III), Co(II), and Pb(II). The effect of flocculation condition on the removal performance of Cr(III), Co(II), and Pb(II) was studied. Characterization results show that the three magnetic carboxymethyl chitosan-based flocculants have been successfully prepared with good magnetic induction properties. Flocculation results show that the removal rates of MC, MCM, and MCAA on Cr(III) are 51.79%, 82.33%, and 91.42%, respectively, under the conditions of 80 mg/L flocculant, pH value of 6, reaction time of 1.5 hr, G value of 200 s^-1, and precipitation magnetic field strength of 120 mT. The removal rates of Co(II) by MC, MCM, and MCAA are 54.33%, 84.99%, and 90.49%, respectively. The removal rates of Pb(II) by MC, MCM, and MCAA are 61.54%, 91.32%, and 95.74%, respectively. MCAA shows good flocculation performance in composite heavy metal-simulated wastewater. The magnetic carboxymethyl chitosan-based flocculant shows excellent flocculation performance in removing soluble heavy metals. This research provides guidance and ideas for the development of efficient and low-cost flocculation technology to remove heavy metals in wastewater.

A comparative study of acid-treated, base-treated, and Fenton-like reagent-treated biomass for Cr(VI) sequestration from aqueous solutions

A wide variety of biomass materials have been used for the removal of toxic chromium(VI) by biosorption. The current study investigated the efficacy of Macadamia nutshells treated with sodium hydroxide, nitric acid, and the Fenton-like reagent in the removal of Cr(VI). The adsorbents were characterized by FTIR, SEM, TGA, and elemental analysis. Effects of functional parameters influencing the adsorption of Cr(VI), solution pH (pH 1-11), contact time (5-250 min), concentration of adsorbent (1-10 g/L), and adsorbate concentration (10-200 mg/L) were investigated. The optimum conditions for biosorption were pH 1.4, adsorbent dose of 5 g/L, and 160 min of contact time. In all cases, the base-treated adsorbent displayed superior performance compared to others, with highest percent removal of 98%. The adsorbate-adsorbent interactions were better explained by the Freundlich isotherm and the pseudo-first-order rate model. The Macadamia-based adsorbents are potentially useful for Cr(VI) removal from aqueous solutions. PRACTITIONER POINTS: Three different chemical activators were investigated for the modification of Macadamia surface. The base-treated material exhibited the highest specific surface area of 12.1 m² /g. The Cr(VI) adsorption performance for the base-treated material dwarfed the other materials. Excellent Cr(VI) removal efficiency in the presence of competitors was achieved.

Reduction and Biosorption of Cr(VI) from Aqueous Solutions by Acid-Modified Guava Seeds: Kinetic and Equilibrium Studies

The use of guava seeds (GS) and acid-modified guava seeds (MGS) for the removal of Cr(VI) from aqueous solutions was investigated. Batch-type experiments were performed with Cr(VI) aqueous solutions and biosorbents to determine the kinetic and equilibrium sorption parameters. Results indicated that GS and MGS were capable of reducing and remove Cr(VI) from solutions, but the reduction was only observed at some experimental conditions. Infrared analysis showed that several functional groups were involved in the reduction, and biosorption of Cr(VI), particularly alcohol, phenolic, carboxylic, and methoxymethyl structures. The mechanisms of reduction and biosorption depended upon the type of biosorbent, pH, and temperature of the system. The pseudo-second-order kinetic model describes the kinetic sorption data, and the Langmuir-Freundlich (L-F) model describes the isotherm data in most cases. Significantly high total chromium biosorption capacities were obtained. Acid modification of guava seeds improves chromium biosorption performance.

Efficient Removal of Cr(VI) Ions by a Novel Magnetic 4-Vinyl Pyridine Grafted Ni3Si2O5(OH)4 Multiwalled Nanotube

The contamination of water systems by heavy metals greatly threatens human health and ecological safety. An efficient adsorbent is critical for the removal of these contaminants. In this work, magnetic Ni3Si2O5(OH)4 nanotubes (NTs) have been synthesized via in situ hydrothermal reduction and further functionalized by grafting poly(4-vinyl pyridine) (P4VP) brushes on its surface via atom transfer radical polymerization. Characterizations by Fourier transform infrared, X-ray diffraction, thermogravimetric analysis, transmission electron microscopy, and X-ray photoelectron spectroscopy proved that P4VP was successfully grafted on the surface of magnetic Ni3Si2O5(OH)4 NTs. The resultant Ni3Si2O5(OH)4-g-P4VP NTs are efficient nanosorbents for removing Cr(VI) anions from water. The Cr(VI) adsorption capacity of Ni3Si2O5(OH)4-g-P4VP NTs reaches 1.49 mmol/g at a pH of 3. The pseudo-second-order kinetic model and the Freundlich isothermal model are suitable to describe the adsorption process. The analysis using Weber-Morris and Boyd models indicates that both intraparticle diffusion and external film diffusion affect the Cr(VI) adsorption process. The adsorption enthalpy is estimated to be 18.37 kJ/mol. More than 90% of the Cr(VI) adsorption capacity of the Ni3Si2O5(OH)4-g-P4VP NTs remains after eight adsorption and desorption cycles.

Recent advances in hexavalent chromium removal from aqueous solutions by adsorptive methods

Chromium exists mainly in two forms in environmental matrices, namely, the hexavalent (Cr(vi)) and trivalent (Cr(iii)) chromium. While Cr(iii) is a micronutrient, Cr(vi) is a known carcinogen, and that warrants removal from environmental samples. Amongst the removal techniques reported in the literature, adsorption methods are viewed as superior to other methods because they use less chemicals; consequently, they are less toxic and easy to handle. Mitigation of chromium using adsorption methods has been achieved by exploiting the physical, chemical, and biological properties of Cr(vi) due to its dissolution tendencies in aqueous solutions. Many adsorbents, including synthetic polymers, activated carbons, biomass, graphene oxide, and nanoparticles as well as bioremediation, have been successfully applied in Cr(vi) remediation. Initially, adsorbents were used singly in their natural form, but recent literature shows that more composite materials are generated and applied. This review focused on the recent advances, insights, and project future directions for these adsorbents as well as compare and contrast the performances achieved by the mentioned adsorbents and their variants.