Experimental evidences for a new model in the description of the adsorption-coupled reduction of Cr(VI) by protonated banana skin

Biosorptive ascendency of plant based biosorbents in removing hexavalent chromium from aqueous solutions - Insights into isotherm and kinetic studies

Chromium is a toxic heavy metal prevalent in higher levels in aqueous matrices owing to industrial applications. Whilst being a key player in industries, the environmental issues caused by Cr(VI) are highly deleterious. Adsorptive remediation is found to be an effective method adopted by researchers in the past decades for Cr(VI) removal from water streams in which variety of naturally available biosorbents have been explored for handling Cr(VI). This review article briefly sketches up the biosorptive potential of plant-based biosorbents used in raw and chemically modified form for the optimum exclusion of Cr(VI) from aqueous sources. Mechanisms and kinetic behavior of the removal process are also discussed. pH of the solution and initial Cr(VI) concentration were found to be the key parameters in Cr removal. The mechanism of Cr removal from aqueous systems was elucidated to be either adsorption or adsorption-coupled-reduction. After precise discussion on various plant-based biosorbents with their maximum adsorption capacities, desorption and regeneration potential, it is perceived that plant-based biosorbents are superior options for Cr(VI) elimination from aqueous streams.

Bioinspired Nanomodification Strategies: Moving from Chemical-Based Agrosystems to Sustainable Agriculture

Agrochemicals have supported the development of the agricultural economy and national population over the past century. However, excessive applications of agrochemicals pose threats to the environment and human health. In the last decades, nanoparticles (NPs) have been a hot topic in many fields, especially in agriculture, because of their physicochemical properties. Nevertheless, the prevalent methods for fabricating NPs are uneconomical and involve toxic reagents, hindering their extensive applications in the agricultural sector. In contrast, inspired by biological exemplifications from microbes and plants, their extract and biomass can act as a reducing and capping agent to form NPs without any toxic reagents. NPs synthesized through these bioinspired routes are cost-effective, ecofriendly, and high performing. With the development of nanotechnology, biosynthetic NPs (bioNPs) have been proven to be a substitute strategy for agrochemicals and traditional NPs in heavy-metal remediation of soil, promotion of plant growth, and management of plant disease with less toxicity and higher performance. Therefore, bioinspired synthesis of NPs will be an inevitable trend for sustainable development in agricultural fields. This critical review will demonstrate the bioinspired synthesis of NPs and discuss the influence of bioNPs on agricultural soil, crop growth, and crop diseases compared to chemical NPs or agrochemicals.

Photochemical reduction of Cr(vi) compounds by amino acid Schiff base copper complexes with a hydroxyl group and titanium oxide composites in aqueous solutions

The photo-driven reduction reaction of Cr(vi) to Cr(iii) by Schiff base Cu(ii) complexes was studied in aqueous and methanol solutions. The Cr(vi) reduction ratio with the CuVDB–TiO₂ system reached 91%.

Cr(VI) removal from soils and groundwater using an integrated adsorption and microbial fuel cell (A-MFC) technology

Remediation of hexavalent chromium [Cr(VI)] has been widely studied for its high mobility and toxicity. As Cr(VI) migrates in natural environment, both soils and groundwater are contaminated simultaneously. In the present study, a novel reactor combining adsorption and microbial fuel cell (A-MFC) using Platanus acerifolia leaves was developed for removing Cr(VI) from groundwater and soils. When initial Cr(VI) concentration was 50 mg/L, the adsorption efficiency of A-MFC achieved 98% after 16 h. Afterwards, the leaves were used for fabricating an MFC-integrated leaching reactor. The A-MFC significantly improved the overall Cr(VI) removal efficiency through leaching and 40% of Cr(VI) in the soil column was removed. The electrical voltage and current of A-MFC reactor achieved averagely 343 mV and 141 μA to maintain the system operation without extra energy supply. This novel A-MFC reactor is an environmentally friendly technology which achieved efficient Cr(VI) removal from groundwater and soils using natural materials, proving the concept that integrated self-remediation of Cr(VI) in contaminated soil and groundwater with natural material and energy.

Removal of Hexavalent Chromium from Aqueous Solution by the Pod of Acacia gerrardii

This study aims at investigating the potential of Acacia gerrardii pod for the removal of Cr(VI) in batch system. Effect of solution pH, biosorbent dosage, initial concentration of Cr(VI), contact time on the removal process was examined. Complete removal of hexavalent chromium was achieved at pH values 1.0 and 2.0 whereas maximum removal of total chromium was obtained at pH of 3.0. The study showed that the biosorption and bioreduction mechanisms were involved in the removal process. The time required for complete removal of Cr(VI) using the pod of Acacia gerrardii was shortened with an increase in biomaterial dosage and decrease in Cr(VI) concentration. Kinetic data was well described using Park kinetic model. Freundlich isotherm model adequately fitted the equilibrium data indication multilayer adsorption of total chromium on the surface of biomaterial. The pod of Acacia gerrardii could be used efficiently for the removal of hexavalent chromium from aqueous solutions.

Microbial synthesis of hollow porous Prussian blue@yeast microspheres and their synergistic enhancement of organic pollutant removal performance

In this work, Prussian blue nanoparticles (PB NPs) were in situ grown on S. cerevisiae cells via one-step hydrothermal synthesis and the as-prepared Prussian blue@yeast (PB@yeast) hybrids exhibited synergistic adsorption and Fenton catalytic activities. FE-SEM, XRD and BET analysis of the prepared samples confirmed the successful formation of hollow porous structured PB@yeast microspheres, while FT-IR and XPS spectra indicated the fine structures were occupied by both functional adsorptive and catalytic sites. The experimental results of adsorption coupled Fenton reaction of PB@yeast hybrid microspheres revealed that the functional groups on the cell wall and the active iron sites in PB framework were fully utilized due to the triple synergistic effects of adsorption-Fenton catalysis-adsorption sites regeneration, thus endowing synergistically enhanced performance in removal of the selected cationic methylene blue (MB), anionic Methyl Orange (MO) and fluorescent brightener 71 (CXT) in aqueous solution. The high Fenton catalytic efficiency was related to the improvement of adsorption, in which the enrichment of contaminant molecules on the outer and inner surface of the hollow porous microspheres could lower mass transfer resistance and shorten charge transport pathways, thereby introducing more efficient Fenton catalytic activity than PB NPs.

Chromium(VI) sorption efficiency of acid-activated banana peel over organo-montmorillonite in aqueous solutions

In the present study, we examined sorption of chromate (Cr(VI)) to acid-activated banana peel (AABP) and organo-montmorillonite (O-mont) as a function of pH, initial Cr(VI) concentration at a sorbent dose of 4 g L^-1 and at 20 ± 1°C in aqueous solutions. In sorption edge experiments, maximum Cr(VI) removal was obtained at pH 3 after 2 hours by AABP and O-mont (88% and 69%). Sorption isotherm data showed that the sorption capacity of AABP was higher than O-mont (15.1 vs. 6.67 mg g^-1, respectively, at pH 4). Freundlich and Langmuir models provided the best fits to describe Cr(VI) sorption onto AABP (R² = 0.97) and O-mont (R² = 0.96). Fourier transform infrared spectroscopy elucidated that for AABP mainly the -OH, -COOH, -NH₂, and for O-mont intercalated amines and -OH surface functional groups were involved in Cr(VI) sorption. The scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX) analyses, although partly, indicate that the (wt. %) proportion of cations (e.g., Ca, Mg) in AABP decreased after Cr(VI) sorption. This may be due to ion exchange of chromite (Cr(III)) (produced from Cr(VI) reduction) with cationic elements in AABP. Also, Cr(VI) desorption (using phosphate solution) from AABP was lower (29%) than that from O-mont (51%) up to the third regeneration cycle. This bench scale comparative study highlights that the utilization of widely available and low-cost acid-activated biomaterials has a greater potential than organo-clays for Cr(VI) removal in aqueous media. However, future studies are warranted to precisely delineate different mechanisms of Cr(VI) sorption/reduction by acid-activated biomaterials and organo-clays.

Removal of anionic pollutants by pine bark is influenced by the mechanism of retention

The use of organic biosorbents for anion removal from water has been less studied than for cationic compounds. In this work, the removal capacity of pine bark for potential anionic pollutants (fluoride, phosphate, arsenate and dichromate) was assessed in column experiments, designed to study the process of transport. The results showed that pine bark has a very low retention capacity for phosphate, arsenate or fluoride, and in turn, very high for dichromate, with retention values close to 100% and less than 2% desorption of the adsorbed dichromate. The large differences observed between anions suggest that differences in the retention mechanism of each anion exist. In the case of phosphate and arsenate, electrostatic interactions with the mostly negatively charged functional groups of the pine bark determine the low retention capacity. Dichromate retention might proceed through reduction of chromium (VI) to chromium (III), what improves the efficiency of the removal.

New approach in modeling Cr(VI) sorption onto biomass from metal binary mixtures solutions

In the last decades Cr(VI) sorption equilibrium and kinetic studies have been carried out using several types of biomasses. However there are few researchers that consider all the simultaneous processes that take place during Cr(VI) sorption (i.e., sorption/reduction of Cr(VI) and simultaneous formation and binding of reduced Cr(III)) when formulating a model that describes the overall sorption process. On the other hand Cr(VI) scarcely exists alone in wastewaters, it is usually found in mixtures with divalent metals. Therefore, the simultaneous removal of Cr(VI) and divalent metals in binary mixtures and the interactive mechanism governing Cr(VI) elimination have gained more and more attention. In the present work, kinetics of Cr(VI) sorption onto exhausted coffee from Cr(VI)-Cu(II) binary mixtures has been studied in a stirred batch reactor. A model including Cr(VI) sorption and reduction, Cr(III) sorption and the effect of the presence of Cu(II) in these processes has been developed and validated. This study constitutes an important advance in modeling Cr(VI) sorption kinetics especially when chromium sorption is in part based on the sorbent capacity of reducing hexavalent chromium and a metal cation is present in the binary mixture.

Novel reduction of Cr(VI) from wastewater using a naturally derived microcapsule loaded with rutin-Cr(III) complex

The harmfulness of carcinogenic hexavalent chromium (Cr(VI)) is dramatically decreased when Cr(VI) is reduced to trivalent chromium (Cr(III)). Rutin, a natural flavonoid, exhibits excellent antioxidant activity by coordinating metal ions. In this study, a complex containing rutin and Cr(III) (rutin-Cr(III)) was synthesized and characterized. The rutin-Cr(III) complex was much easier to reduce than rutin. The reduction of the rutin-Cr(III) complex was highly pH-dependent, with 90% of the Cr(VI) being reduced to Cr(III) in 2h under optimal conditions. A biodegradable, sustained-release system encapsulating the rutin-Cr(III) complex in a alginate-chitosan microcapsule (rutin-Cr(III) ACMS) was also evaluated, and the reduction of Cr(VI) was assessed. This study also demonstrated that low-pH solutions increased the reduction rate of Cr(VI). The environmentally friendly microcapsules can reduce Cr(VI) for prolonged periods of time and can easily biodegrade after releasing the rutin-Cr(III) complex. Given the excellent performance of rutin-Cr(III) ACMS, the microcapsule system represents an effective system for the remediation of Cr(VI) pollution.