Adsorption of Cr (III) and Cr (VI) ions on muscovite mica: Experimental and molecular modeling studies

Designed synthesis of demethylated-hydroxymethylated lignin-based adsorbent for removal of Cr(VI) ions from wastewater

Lignin-based adsorbents for the removal of Cr(VI) ions have attracted intensive attention due to the advantages of renewability, biodegradablity, low cost and environmental friendliness. However, there are still a lot of challenges such as poor adsorption capacity, low lignin content in adsorbents, and harsh preparation conditions. Here, a tandem hydroxymethylation-demethylation method is proposed for preparing an excellent lignin-based Cr(VI) adsorbent (DHKL), which features with high lignin content (>85 wt%) and high hydroxyl content (up to 6.26 mmol/g) under milder conditions. The prepared DHKL exhibits an adsorption capacity reaching up to 1040.9 mg/g and can maintain this capacity even in the presence of other metal ions in the solution. Model analyses indicate that chemisorption occurring in a monolayer is the main process, which is spontaneous and endothermic. Structural changes of DHKL before and after adsorption indicated that Cr(VI) ions are mainly reduced to Cr(III) ions by hydroxyl groups with some of the absorbed Cr ions dispersed into the inner part of DHKL. Based on these results, the detailed possible adsorption mechanism is deduced, providing guidance for designing, producing and utilizing lignin-based adsorbents.

Formation of cation bridges and its promoting mechanism for sorption of sulfamethoxazole by montmorillonite

The coexistence of metal cations is often accompanied by organic pollution and could affect the environmental fate of organics by mediating the formation of cation bridges. However, the environmental fate and risk of organics in cation co-existing environments are poorly understood due to the lack of accurate identification of cation bridge formation and stability. In this study, the sorption of sulfamethoxazole (SMX) on montmorillonite (MT) with the coexistence of three different valence metal cations (Na+, Ca2+, and Cr3+) was investigated. Ca2+ and Cr3+ can significantly promote the sorption of SMX on MT for about 5∼10 times promotion, respectively, while Na+ bridges displayed little effect on the sorption of SMX. The sorption binding energy of SMX with MT-Ca (-44.01 kcal/mol) and MT-Cr (-64.57 kcal/mol) bridges was significantly lower than that with MT-Na (-38.45 kcal/mol) and MT (-39.39 kcal/mol), indicating that the sorption affinity of SMX on Cr and Ca bridges was much stronger. The higher valence of the cations also resulted in a more stable adsorbed SMX with less desorption fluctuation. In addition, the relatively higher initial concentration of SMX and the valence of cations increased the bonding density of the cation bridges, thus promoting the apparent sorption of SMX on MT to a certain extent. This work reveals the formation and function of cation bridges in the sorption of SMX on MT. It lays a theoretical foundation for further understanding the environmental fate and risk of organics.

Adsorption of Chromium and Nickel Ions on Commercial Activated Carbon-An Analysis of Adsorption Kinetics and Statics

The adsorption of nickel Ni(II) and chromium Cr(III) ions on the commercial activated carbons WG-12, F-300 and ROW 08, which differ in their pore structure and the chemical nature of their surfaces, were analyzed. The nickel ions Ni2+ were best adsorbed on the WG-12 activated carbon, which had the largest number of carboxyl and lactone groups on the surface of the activated carbons, and the largest specific surface area. Chromium, occurring in solutions with pH = 6 in the form of Cr(OH)2+ and Cr(OH)2+ cations, was best adsorbed on the ROW 08 Supra activated carbon, which is characterized by the highest values of water extract. The precipitation of chromium hydroxide in the pores of the activated carbon was the mechanism responsible for the high adsorption of Cr(III) on this carbon. For the other sorbents, the amount of carboxyl and lactone groups determined the amount of Cr(III) and Ni(II) adsorption. The adsorption kinetics results were described with PFO, PSO, Elovich and intraparticle diffusion models. The highest correlation coefficients for both the Cr(III) and Ni(II) ions were obtained using the PSO model. Among the seven adsorption isotherm models, very high R2 values were obtained for the Toth, Temkin, Langmuir and Jovanovic models. The Cr(III) ions were removed in slightly larger quantities than the Ni(II) ions. The capacities of the monolayer qm (calculated from the Langmuir isotherm) ranged from 55.85 to 63.48 mg/g for the Cr(III), and from 40.29 to 51.70 mg/g for the Ni(II) ions (pH = 6). The adsorption efficiency of Cr(III) and Ni(II) cations from natural waters with different degrees of mineralization (spring, weakly and moderately mineralized) was only a few percent lower than that from deionized water.

Cleanup of Cr(VI)-polluted groundwater using immobilized bacterial consortia via bioreduction mechanisms

Cr(VI) bioreduction has become a remedial alternative for Cr(VI)-polluted site cleanup. However, lack of appropriate Cr(VI)-bioreducing bacteria limit the field application of the in situ bioremediation process. In this study, two different immobilized Cr(VI)-bioreducing bacterial consortia using novel immobilization agents have been developed for Cr(VI)-polluted groundwater remediation: (1) granular activated carbon (GAC) + silica gel + Cr(VI)-bioreducing bacterial consortia (GSIB), and (2) GAC + sodium alginate (SA) + polyvinyl alcohol (PVA) + Cr(VI)-bioreducing bacterial consortia (GSPB). Moreover, two unique substrates [carbon-based agent (CBA) and emulsified polycolloid substrate (EPS)] were developed and used as the carbon sources for Cr(VI) bioreduction enhancement. The microbial diversity, dominant Cr-bioreducing bacteria, and changes of Cr(VI)-reducing genes (nsfA, yieF, and chrR) were analyzed to assess the effectiveness of Cr(VI) bioreduction. Approximately 99% of Cr(VI) could be bioreduced in microcosms with GSIB and CBA addition after 70 days of operation, which caused increased populations of total bacteria, nsfA, yieF, and chrR from 2.9 × 10⁸ to 2.1 × 10¹², 4.2 × 10⁴ to 6.3 × 10¹¹, 4.8 × 10⁴ to 2 × 10¹¹, and 6.9 × 10⁴ to 3.7 × 10⁷ gene copies/L. In microcosms with CBA and suspended bacteria addition (without bacterial immobilization), the Cr(VI) reduction efficiency dropped to 60.3%, indicating that immobilized Cr-bioreducing bacteria supplement could enhance Cr(VI) bioreduction. Supplement of GSPB led to a declined bacterial growth due to the cracking of the materials. The addition of GSIB and CBA could establish a reduced condition, which favored the growth of Cr(VI)-reducing bacteria. The Cr(VI) bioreduction efficiency could be significantly improved through adsorption and bioreduction mechanisms, and production of Cr(OH)₃ precipitates confirmed the occurrence of Cr(VI) reduction. The main Cr-bioreducing bacteria included Trichococcus, Escherichia-Shigella, and Lactobacillus. Results suggest that the developed GSIB bioremedial system could be applied to cleanup Cr(VI)-polluted groundwater effectively.

Computer chip-inspired design of nanocellulose/carbon dots hydrogel as superior intensifier of nano-sized photocatalyst for effective Cr(VI) removal

Hydrogel, a common carrier of photocatalyst that suffers from compromised catalytic efficiency, is still far from practical application. Herein, based on "computer chip-inspired design", a novel nanocellulose/carbon dots hydrogel (NCH) was fabricated as superior intensifier instead of common carrier of sodium titanate nanofibre (STN), where carbon dots (CDs) enhanced amino group-induced adsorption for Cr(VI), promoted photocatalytic properties of STN via transferring the photogenerated electron-hole pairs and improved amino group-induced desorption for reduced product (Cr(III)) via electrostatic repulsion, showing an efficiency of 1 + 1 > 2. Adsorption and photocatalysis experiments demonstrated superior removal performance of the NCH incorporating STN, as shown by theoretical maximum adsorption capacity of 425.74 mg/g and kinetic constant of 0.0374 min^-1 in the photocatalytic process, which was nearly 6.6 and 7.3 times of STN. A series of experiments was conducted to confirm the novel mechanism of CDs-enhanced adsorption-photocatalysis-desorption synergy. This work not only provides new insights into the fabrication of a superior intensifier for nanosized photocatalyst, but also proposes one new mechanism of CDs-enhanced adsorption-photocatalysis-desorption synergy, which is helpful for designing and optimizing nanosized photocatalyst.

A magnetic MIL-125-NH2@chitosan composite as a separable adsorbent for the removal of Cr(VI) from wastewater

Metal-organic frameworks (MOFs) are gradually used since of their huge specific surface area and superior pore structure. However, there are problems such as easy aggregation and difficult separation in water treatment. In this study, we prepared composite microspheres (FMCS-1) by modifying MIL-125-NH₂ with Fe₃O₄ and chitosan. The structural characterization and performance analysis of the materials showed that the introduction of chitosan effectively prevents the stacking of MOFs. The magnetic test manifested that Fe₃O₄ solved the problem of the difficult separation of MOFs from water. The removal potential of toxic Cr(VI) was tested by adsorption experiments. The isotherm model indicated that FMCS-1 is a single molecular layer adsorbent with a maximum adsorption capacity of 109.46 mg/g at pH = 2. The adsorption kinetics showed that the adsorption of Cr(VI) by FMCS-1 was chemical adsorption. The acid resistance test demonstrated that FMCS-1 can exist stably in acid solutions. The recycling experiments proved that the adsorbent can be reused and the removal percentage still reaches 50 % after 5 cycles. This work expands the application of MOFs in water treatment and also provides an effective adsorbent for Cr(VI) removal.

A novel biosorbent functionalized pillar[5]arene: Synthesis, characterization and effective biosorption of Cr(VI)

Among toxic chemicals, hexavalent chromium (Cr(VI)) is one of the most carcinogenic and toxic pollutants that hostiles to the health of both humans and other living things. Therefore, the removal of Cr(VI) is of great importance to keep our environment clean and tidy. In this study, an easy-make, inexpensive, and natural biosorbent material (Sp-P[5]) was prepared to preserve our environment using a pillar[5]arene based-on sporopollenin microcapsule. The prepared biosorbent was successfully characterized by some techniques such as FTIR, XRD, and SEM. The biosorbent, Sp-P[5], exhibited an open mesoporous structure richly decorated with multi-amine-containing moieties resulting in enhanced Cr(VI) sorption. The sorption behavior of Cr(VI) ions is satisfactorily adapted from the sorption kinetics pseudo-second-order law and the isotherm models to the Langmuir model at different temperatures. The Langmuir model fits at different temperatures (298-328 K) and the maximum sorption capacities of the Cr(VI) ion ranged from 106.38 to 117.26 mg/g. The thermodynamic calculations reveal that the sorption of Cr(VI) ions on the Sp-P[5] is entropy-driven, endothermic, and spontaneous. The prepared biosorbent was also applied to the natural wastewater samples and different ions (chromate and dichromate). The sorption and desorption experiments showed that the sorption efficiency for Cr(VI) ions of the Sp-P[5] decreased to 70.88 % after 8 cycles. As result, the synthesized biosorbent, Sp-P[5], has outstanding potential in the removal of Cr(VI) ions from water bodies and natural wastewater systems.

Cr(VI) removal from contaminated waters using ultra-thin layered meixnerite

Cr(VI) is of great concern to public health and environmental safety due to its high toxicity. Here, we report a low-cost yet highly efficient method to prepare a novel LDH, ultra-thin layered meixnerite, which performed superiorly in treatment of aqueous Cr(VI) with little secondary pollution being induced. The produced ultra-thin layered meixnerite was composed of nanoparticles with a thickness of around 7 nm, less than 9 times the thickness of a single LDH layer. The XRD patterns of the ultra-thin layered meixnerite, in which the characteristic diffraction peaks of a typical LDH were weakened or even disappeared, confirmed the successful delamination. This special morphology of the ultra-thin layered meixnerite was not only helpful to its full dispersion in the Cr(VI)-bearing solutions but also facilitated the formation of more active sorption sites on its external surface. As a result, the maximum sorption capacity of UTLM for Cr(VI) removal was 480.9 mg g^-1, far higher than that of OM (196.9 mg g^-1). In addition to electrostatic attraction and anion exchange, the ultra-thin layered meixnerite could also become restacked during removal of aqueous Cr(VI) to generate inner-sphere complexation, finally inducing an enhanced Cr(VI) uptake. Furthermore, XPS analysis characterized the promotion of the break of Al-OH bond with the increase in temperature, and the Cr-O peak increased correspondingly from 29.69% at 25 °C to 48.77% at 85 °C, resulting that the ultra-thin layered meixnerite could remove Cr(VI) more effectively at higher reaction temperatures. Therefore, ultra-thin layered meixnerite is very suitable for future application in treatment of industrial wastewaters with elevated temperatures.

Polyamide microplastics as better environmental vectors of Cr(VI) in comparison to polyethylene and polypropylene microplastics

The adsorption, desorption, and their influence factors of Cr(VI) by microplastics (MPs) in the solution was investigated in this study. The results demonstrated that UV aging promoted adsorption, while the increase of salinity and natural organic matter (NOM) inhibited adsorption. The particle size affected the total Cr(VI) active adsorption sites on MPs, while the pH changed the electrostatic force. The Cr(VI) adsorption by MPs conformed to the pseudo-first and pseudo-second order kinetic models, and was monolayer and inhomogeneous. SEM-EDS, XPS and ATR-FTIR analyses demonstrated that the physical adsorption dominated the adsorption process. Especially, the highest adsorption capacity of Cr(VI) by polyamide (PA) MPs was attributed to their hydrophilic amide groups. However, Cr(VI) was only released from PA MPs, of which desorption rate followed the sequence of seawater (79.5 %) > fresh water (66.6 %) > deionized water (34.8 %). Thus, PA MPs might be non-negligible environmental vectors for the transport of Cr(VI).