Strong chromate-adsorbent based on pyrrolic nitrogen structure: An experimental and theoretical study on the adsorption mechanism

Disclosing the influence mechanism of facet-dependent pyrite photo-activation and photo-dissolution processes on the reduction of Cr(VI)

The photo-activation and photo-dissolution processes of pyrite (FeS2) can affect the environmental behavior of the co-existing hexavalent chromium (Cr(VI)). But the photochemical performance of FeS2 is intimately dependent on its exposed facets. Herein, FeS2 nanosheets (FeS2 NS) and FeS2 nanocubes (FeS2 NC) with the dominant exposed facets of (001) and (210)/(100) respectively are prepared. The more Fe3+, Fe2+, and SO42- are released in the FeS2 NS system than the other system due to its more excellent generation ability of photogenerated electrons and reactive oxygen species. The higher surface energy on (001) facet leads to the faster dissolution rate of FeS2 NS. Due to the optimal production ability of photogenerated electrons and Fe2+ of (001) facet, the much higher Cr(VI) elimination efficiency in the FeS2 NS system is observed than that in the FeS2 NC (72.8%) system within 120 min. This work could help to unveil the influence of FeS2 on the fate of Cr(VI) in surface environment, and offer a theoretical support to clarify the influence of heavy metal ions on the iron sulfide minerals.

The dominant role of extracellular polymeric substances produced by Achromobacter xylosoxidans BP1 in Cr(VI) microbial reduction

Extracellular polymeric substances (EPS) have demonstrated significant benefits for reducing multivalent metal contamination. Using Achromobacter xylosoxidans BP1 isolated from a coal chemical site in China, this study elucidated the contribution of EPS production to Cr (VI) reduction and revealed its biological removal mechanism. BP1 grew at an optimum pH of 8 and the lowest inhibitory concentration of Cr(VI) was 300 mg/L. The spent medium completely removed Cr(VI), whereas resting cells were only able to remove 10.47 % and inactivated cells were nearly incapable of Cr(VI) removal. S-EPS and B-EPS reduced Cr(VI) by 98.59 % and 11.64 %, respectively. SEM-EDS analysis showed that the BP1 cells were stimulated to produce EPS under Cr stress. The XPS results showed that 29.63 % of Cr(VI) was enriched by intracellular bioaccumulation or biosorption and 70.37 % of Cr(VI) was reduced by extracellular enzymes to produce Cr(OH)3 and organic Cr(III) complexes. According to FTIR, EPS with -OH, COO-, and amide groups supplied binding sites and electrons for the reductive adsorption of Cr(VI). Genomic studies showed that BP1 primarily produces extracellular polysaccharides, metabolises sulphur and nitrogen, and reduces reactive oxygen species damage as a result of DNA repair proteases.

Synergistic photocatalytic oxidation and adsorption boost arsenic removal by in-situ carbon-doped TiO2 and nitrogen deficiency C3N4 heterojunction

The efficient removal of arsenic from wastewater is still a challenge. In this paper, a heterojunction consisting of in-situ carbon-doped TiO2 and nitrogen deficiency g-C3N4 (C/TiO2@ND-C3N4) has been constructed, which can completely oxidize As(III) (10,000 μg/L, 40 mL) to As(V) within 12 min under visible light and simultaneously adsorb total As (95.0%) with the pseudo-secondary kinetic equation, superior than in-situ carbon-doped TiO2 (75.0%) and nitrogen deficiency g-C3N4 (50.5%). The good photocatalytic oxidation and adsorption performances of C/TiO2@ND-C3N4 on As(III) removal can be attributed to the successful synthesis of heterojunction. On one hand, the building of C-O-Ti interfacial chemical bonds enable rapid electron transfer and improve the efficiency of photocatalytic oxidation. On the other hand, the decreased As(V) adsorption energy resulted from the synthesized heterojunction boost the adsorption capability of As(V), which was completed by the generation of O-As bonds with oxygen-containing functional groups on the surface of TiO2 and hydrogen bonds with high content pyrrole nitrogen derived from ND-C3N4, respectively. The results manifest that the preparation of bifunctional materials with both photocatalytic oxidation and adsorption properties provides a new strategy to achieve the removal of As.

Polydopamine and calcium functionalized fiber carrier for enhancing microbial attachment and Cr(VI) resistance

The formation of biofilm determines the performance and stability of biofilm system. Increasing the hydrophilicity of the carrier surface could efficiently accelerate the attachment and growth of microorganisms. Here, the surface of polypropylene (PP) fiber carrier was modified with polydopamine (PDA) and calcium (Ca(II)) to enhance microbial attachment and toxicity resistance. The results of surface characteristic confirmed the self-polymerization of PDA and the chelation mechanism of Ca(II). Subsequently, the biofilm formation experiments were conducted in sequencing batch biofilm reactors using both normal and chromium-containing wastewater. The biofilm on the surface of the modified carrier exhibited better nitrogen removal and Cr(VI) reduction ability. The biomass of the modified carrier was significantly increased, and the maximum microbial attachment amounts in normal wastewater and chrome-containing wastewater were 1153.34 and 511.78 mg/g carrier, respectively. Furthermore, the confocal laser scanning microscope (CLSM) indicated that the modified carrier coated with PDA and Ca(II) were both biocompatible, and the cell activity was significantly increased. 16S rRNA sequencing results showed that the modified carrier efficiently enriched both denitrification bacteria (Thauera and Flavobacterium) and chrome-reducing bacteria (Simplicispira and Arenimonas) to improve system stability and Cr(VI) resistance. Microbial phenotype prediction based on BugBase analysis further verified the enrichment effect of modified carriers on microorganisms responsible for biofilm formation and oxidative stress resistance. Overall, this work proposed a novel functional carrier that could provide references for advancing the application of biofilm systems in wastewater treatment.

Enhancement of Cr(VI) adsorption on lignin-based carbon materials by a two-step hydrothermal strategy: Performance and mechanism

Cr(VI) is a carcinogenic heavy metal that forms an oxygen-containing anion, which is difficult to remove from water by adsorbents. Here, industrial alkali lignin was transformed into a Cr(VI) adsorbent (N-LC) by using a two-step hydrothermal strategy. The characterization results of the adsorbent showed that O and N were uniformly distributed on the surface of the adsorbent, resulting in a favorable morphology and structure. The Cr(VI) adsorption of N-LC was 13.50 times that of alkali lignin, and the maximum was 326.10 mg g-1, which confirmed the superiority of the two-step hydrothermal strategy. After 7 cycles, the adsorption of N-LC stabilized at approximately 62.18 %. In addition, in the presence of coexisting ions, N-LC showed a selective adsorption efficiency of 85.47 % for Cr(VI), which is sufficient to support its application to actual wastewaters. Model calculations and characterization showed that N and O groups were the main active factors in N-LC, and CO, -OH and pyridinic-N were the main active sites. This study provides a simple and efficient method for the treatment of heavy metals and the utilization of waste lignin, which is expected to be widely applied in the environmental, energy and chemical industries.

Insights into the proton-enhanced mechanism of hexavalent chromium removal by amine polymers in strong acid wastewater: Reduction of hexavalent chromium and sequestration of trivalent chromium

Adsorption is a green technology of treating heavy metal-contaminated strong acid wastewaters for the recycling of heavy metal and reuse of strong acid. Herein, three amine polymers (APs) with different alkalinities and electron donating abilities were prepared to investigate the adsorption-reduction processes of Cr(VI). It was found that the removal of Cr(VI) was controlled by the concentration of -NRH+ on the surface of APs at pH > 2, which relies on the alkalinity of APs. However, the high concentration of NRH+ significantly facilitated the adsorption of Cr(VI) on the surface of APs and accelerated the mass transfer between Cr(VI) and APs at strong acid environment (pH ≤ 2). More importantly, the reduction of Cr(VI) was enhanced at pH ≤ 2, due to the high reduction potential of Cr(VI) (E ≥ 0.437). The ratio of reduction to adsorption (α) of Cr(VI) was above 0.70, and the proportion of Cr(III) bonding on Ph-AP excessed 67.6 %. Finally, a proton-enhanced mechanism of Cr(VI) removal was verified by analyzing FTIR and XPS spectra as well as constructing DFT model. This study provides a theoretical basis for the removal of Cr(VI) in the strong acid wastewater.

Three-dimensional lignin-based polyporous carbon@polypyrrole for efficient removal of reactive blue 19: A synergistic effect of the N and O groups

Reactive blue 19 is one of the abundant carcinogens commonly used in industrial applications. This study transformed industrial lignin into a lignin-based polyporous carbon@polypyrrole (LPC@PPy) by a hydrothermal-activation-in situ polymerization strategy for removal of reactive blue 19. The hydrothermal reaction and polypyrrole polymerization provide abundant O and N groups, and the pore-making process promotes the even distribution of O and N groups in the 3D pore of LPC@PPy, which is favorable for the adsorption of reactive blue 19. The adsorption capacity of LPC@PPy for reactive blue 19 is 537.52 mg g-1, which is 2.04 times the performance of LPC (only hydrothermal and activation process, only have O groups) and 3.36 times that of LC (direct lignin activation, lack of O and N groups). After 8 cycles, LPC@PPy still maintained a high adsorption capacity of 92.14 % for reactive blue 19. In addition, this study found that N and O groups in the material played an important role in adsorption, mainly pyridinic-N, C-OH, -COOR, -C-O- and CC. This work provides a new strategy for the removal of reactive blue 19 and determines the groups that mainly interact with reactive blue 19, which provides a new reference for adsorption, catalysis and related fields.

Internal electric field induced photocarriers separation of nickel-doped pyrite/pyrite homojunction with rich sulfur vacancies for superior Cr(VI) reduction

Improving the separation efficiency and transfer ability of photoinduced electrons/holes in pyrite (FeS2)-based photocatalytic materials is significant for the photoreduction of hexavalent chromium (Cr(VI)) but still remains a challenge. Herein, a novel homojunction was prepared through in-situ growth of nickel (Ni) doped FeS2 nanoparticles on FeS2 nanobelts (denoted as Ni-FeS2/FeS2). Systematical characterizations revealed that Ni doped FeS2 nanoparticles have been successfully in situ grown along the lattice of FeS2 nanobelts. Photoreduction experiments demonstrated that the Ni-FeS2/FeS2 homojunction with 2 mmol Ni doping contents (denoted as 2Ni-FeS2/FeS2) exhibited the optimum Cr(VI) reduction efficiency among the studied catalysts. Density Functional Theory (DFT) calculated results verified that Ni doping could not only be advantageous for the formation of sulfur vacancies but also modify the band gap and band structure of FeS2 nanoparticles. Moreover, several doping energy levels caused by Ni doping have also appeared near the Fermi level of FeS2 nanoparticles. The migration paths of electrons and the existence of internal electric field (IEF) in homojunction were further verified by the calculation of work function. To sum up, the doping energy levels and IEF that produced by homojunction played important roles in accelerating the separation efficiency of its photogenerated carriers.

Efficient Reduction of Cr(VI) with Carbon Quantum Dots

Hexavalent chromium (Cr(VI)) pollution is a global problem, and the reduction of highly toxic Cr(VI) to less toxic Cr(III) is considered to be an effective method to address Cr(VI) pollution. In this study, low-toxicity carbon quantum dots (CQDs) were used to reduce Cr(VI) in wastewater. The results show that CQDs can directly reduce Cr(VI) at pH 2 and can achieve a reduction efficiency of 94% within 120 min. It is observed that under pH higher than 2, CQDs can activate peroxymonosulfate (PMS) to produce reactive oxygen species (ROS) for the reduction of Cr(VI) and the reduction efficiency can reach 99% within 120 min even under neutral conditions. The investigation of the mechanism shows that the hydroxyl groups on the surface of CQDs can be directly oxidized by Cr(VI) because of the higher redox potential of Cr(VI) at pH 2. As the pH increases, the carbonyl groups on the surface of CQDs can activate PMS to generate ROS, O₂ ^•-, and ¹O₂, which result in Cr(VI) being reduced. To facilitate the practical application of CQDs, the treatment of Cr(VI) in real water samples by CQDs was simulated and the method reduced Cr(VI) from an initial concentration of 5 mg/L to only 8 μg/L in 150 min, which is below the California water quality standard of 10 μg/L. The study provides a new method for the removal of Cr(VI) from wastewater and a theoretical basis for practical application.

Upcycling of blending waste plastics as zwitterionic hydrogel for simultaneous removal of cationic and anionic heavy metals from aqueous system

Upcycling of waste plastics as functional materials is a new approach for synthesizing low-cost and durable adsorbents with zwitterionic property. Herein, a facile process for recycling blending waste plastics to fabricate zwitterionic plastic-g-hydrogel (ZPH) for simultaneous adsorbing cationic and anionic heavy metals was developed. ZPH possessed high affinities for cations and anions in both acid and alkaline conditions owing to its zwitterionic property, and the maximum adsorption capacities of Pb²⁺, Cd²⁺, Ba²⁺, and Cr(VI) (Cr₂O₇^2-) were 132.13, 85.58, 69.92 and 85.15 mg/g, respectively. Mechanism study indicated the incompatibility of blending plastics was skillfully overcome through the crosslinking between sodium alginate (SA)/chitosan (CTS) and plastics. Cations were adsorbed onto ZPH via electrostatic interaction, cation exchange and coordination interactions with Cl/N/O-containing groups. Furthermore, the reduction of Cr(VI) to Cr(III) was another important path for ZPH to capture anionic Cr₂O₇^2-, and subsequently Cr(III) was adsorbed via coordination interaction and cation exchange. Moreover, the regeneration experiment showed ZPH possessed excellent reusability and stable structure. Accordingly, this research provides a profitable approach for recycling blending plastics, and ZPH has potentials for industrial application in wastewater treatment or contaminated site remediation with complex heavy metals pollution.

Adenosine-functionalized UiO-66-NH2 to efficiently remove Pb(II) and Cr(VI) from aqueous solution: Thermodynamics, kinetics and isothermal adsorption

A new zirconium-based adsorption material (UiO-66-AMP) was prepared by modifying UiO-66-NH₂ with 5-adenosine to effectively remove Pb(II) and Cr(VI) from wastewater. The SEM, EDS, XRS and FT-IR characterization confirmed the successful synthesis of UiO-66-AMP. We conducted a sets of experiments to test the adsorption effectiveness of UiO-66-AMP for Pb(II) and Cr(VI). The maximum adsorption capacity of UiO-66-AMP for Cr(VI) (pH=2) and Pb(II) (pH=4) are 196.60 and 189.69 mg/g, respectively. The adsorption process conforms to the pseudo-second-order and Langmuir models, which indicates that the adsorption is a single-layer chemical process. Gibbs free energy (∆G) indicates that the adsorption of Pb(II) is an exothermic reaction, while the adsorption of Cr(VI) is an endothermic reaction. At the same time, the adsorbent maintains excellent adsorption capacity at least after 4 cycles. The good adsorption performance of UiO-66-AMP towards the metal ions was attributed to the surface complexation and electrostatic interactions. Therefore, the new adsorbent has obvious application prospect to remove Pb(II) and Cr(VI) from wastewater.

Redox-Active Magnetic Composites for Anionic Contaminant Removal from Water

Global water security is jeopardized by the presence of anthropogenic contaminants, which can persist resiliently in the environment and adversely affect human health. Surface adsorption of polluting species is an effective technique for water purification. In this work, redox-active magnetic compounds were designed for the targeted removal of inorganic and organic anions in water via polymeric redox-active vinylferrocene (VFc) and pyrrole (Py) moieties. An Fe₃O₄@SiO₂@PPy@P(VFc-co-HEMA) composite was prepared in a four-step process, with the outermost layer possessing heightened hydrophilicity as a result of the optimized incorporation of 2-hydroxyethylmethacrylate (HEMA) monomers into the backbone of the ferrocene macromolecule. The synthesized materials are able to separate carcinogenic hexavalent chromium oxyanions and other charged micropollutants, and exhibit a 2-fold or greater enhancement in adsorption uptake once the redox-active ferrocene groups are oxidized to ferrocenium cations, with capacities of 23, 49, 66, and 95 mg/g VFc for maleic acid, 2-(6-methoxy-2-naphthyl)propionic acid (Naproxen), (2,4-dichlorophenoxy)acetic acid (2,4-D), and (2-dodecylbenzene)sulfonic acid (DBS), respectively, and a > 99% extractability of chromium in the 1 ppm range. The application of redox-active components to a magnetic particulate scaffold improves maneuverability and phase contact, giving rise to new potential aqueous separation process frameworks for water or product purification.

A novel lignin-based hierarchical porous carbon for efficient and selective removal of Cr(VI) from wastewater

A novel lignin-based hierarchical porous carbon (L-HPC) was prepared to remove Cr(VI) from water by using industrial alkali lignin through simple hydrothermal-induced assembly and alkali activation strategy. The adsorbent were characterized by SEM-EDS mapping, TEM, BET, XPS, FTIR, Raman spectroscopy and zeta potential. The characterization results indicated that L-HPC contained three-dimensional connected channels and many adsorbing N, O and other adsorption groups, which is very beneficial for Cr(VI) adsorption. The kinetics showed that the L-HPC adsorption of Cr(VI) was chemical adsorption and mainly controlled by intraparticle diffusion. The isotherm and thermodynamics indicated that L-HPC adsorption of Cr(VI) conforms to the Freundlich model, L-HPC is a kind of multimolecular layer adsorbent, and the adsorption capacity of Cr(VI) by L-HPC was 887.8 mg/g, which was significantly higher than values for other adsorbents. Ion competition simulation and actual water body tests showed that L-HPC exhibits high selectivity for Cr(VI) adsorption, adsorption cycle experiments show that L-HPC maintains over 83% performance after 12 cycles. Cost analysis shows that L-HPC is suitable for mass production. Therefore, L-HPC is a Cr(VI) adsorbent with high efficiency, high selectivity, and high reusability, which is broadly applicable and shows favorable prospects.

High exposure effect of the adsorption site significantly enhanced the adsorption capacity and removal rate: A case of adsorption of hexavalent chromium by quaternary ammonium polymers (QAPs)

Enhancing the performance of adsorbents to the utmost extent is an objective but challenging in applying adsorption technology to wastewater treatment. In this work, novel quaternary ammonium polymers (QAPs) with high density adsorption site (i.e., quaternized N, confirmed by FT-IR results) were designed and prepared for rapid selective removal of Cr(VI) from water. The results of EDS analysis indicated the maximum exposure rate of N on the surface of QAPs was as high as 86.1%, which almost doubled comparing to that of Cr(VI) ions imprinted polymers (Cr(VI)-IIP) (46.2%). Interestingly, the maximum adsorption capacity (211.8 mg/g) and initial adsorption rate (h0, 66.6 mg/ (g·min)) of QAPs (i.e., 5:1(TRIM)) for Cr(VI) are about 3.6 times and 4.9 times those of Cr(VI)-IIP (63.0 mg/g and 13.5 mg/(g·min)), respectively. Impressively, flow-through adsorption experiments demonstrated 5:1(TRIM) can completely remove 5 mg/L of Cr(VI) within five seconds. Additionally, 5:1(TRIM) exhibited a remarkable selectivity for Cr(VI) adsorption, and high purity (100%) of chromium can be readily obtained. The proposed idea of high exposure effect of the adsorption site can provide a valuable guidance for designing rapid selective adsorbents to remove and reclaim Cr(VI) from wastewater.

Selective Adsorption of CR (VI) onto Amine-Modified Passion Fruit Peel Biosorbent

This study aimed to prepare surface amino-riched passion fruit peel (DAPFP) by amination reaction with low-cost biomaterials and use it as a biosorbent to adsorb Cr (VI). The specific physicochemical and structural properties of DAPFP were characterized by SEM, EDS, XRD, TG, Zeta, XPS, and FT-IR. The effects of pH value, initial concentration, adsorption time, coexisting ions, and temperature on the adsorption of Cr (VI) were systematically investigated. The results showed that within 90 min, DAPFP could reduce the concentration of Cr (VI) solution (1 mg/L−1) to an allowable safe level of drinking water (0.05 mg/L−1) specified by the World Health Organization. The adsorption process complies with pseudo-second-order kinetics and the Langmuir isotherm model. The adsorption capacity of the prepared biosorbent could reach 675.65 mg/g−1. The results of thermodynamic studies confirmed that the adsorption process was a self-discharging heat process. DAPFP also showed good reusability; even after being used repeatedly five times, it still showed excellent adsorption performance. FT-IR and XPS analyses showed that electrostatic attraction and reduction were the main reasons for the adsorption. By virtue of its low cost and excellent adsorption performance, DAPFP has a potential practical application as an adsorbent in treating Cr (VI) containing wastewater.

Efficient toxicity elimination of aqueous Cr(VI) by positively-charged BiOClxI1-x, BiOBrxI1-x and BiOClxBr1-x solid solution with internal hole-scavenging capacity via the synergy of adsorption and photocatalytic reduction

The ecological toxicity of photocatalysts and toxicity reduction of Cr(VI) have attracted much attention. The development of environmentally-friendly photocatalysts with adsorption and internal hole-scavenging capacity for toxicity reduction of aqueous Cr(VI) via facile preparation method is desirable. In this study, visible-light-active BiOCl_xBr_1-x, BiOCl_xI_1-x and BiOBr_xI_1-x solid solutions were prepared by simple solvothermal strategy. The BiOCl_0.3Br_0.7, BiOCl_0.7I_0.3 and BiOBr_0.7I_0.3 solid solutions exhibited excellent adsorption capacity and photoreduction ability. The removal efficiency of Cr(VI) by BiOCl_0.3Br_0.7 was 97.7% within 60 min. BiOCl_0.7I_0.3 and BiOBr_0.7I_0.3 can remove Cr(VI) almost completely within less than 30 min, which were much higher than those by BiOCl (81.6%) and BiOBr (67.4%) due to joint effect of adsorption and photoreduction. More importantly, the toxicity evaluation confirmed nontoxicity of BiOCl_xBr_1-x, BiOCl_xI_1-x and BiOBr_xI_1-x, and rapid toxicity elimination process of Cr(VI).

Exploration of the reduction mechanism of Cr(VI) in anaerobic hydrogen fermenter

The bio-reduction of hexavalent chromium (Cr(VI)) by anaerobic fermentation is considered as a promising, low-cost and environment-friendly way. However, it is unclear for the reduction mechanisms of Cr(VI) in an anaerobic hydrogen fermenter, such as reduction kinetics, related electron donors, migration and transformation, reduction site and key components, and related microorganisms. To clarify these issues, a hydrogen fermenter was designed to reduce Cr(VI) at 55 °C with glucose as initial substrate. Results show that 100 mg/L Cr(VI) can be completely reduced (99.5%) to trivalent chromium (Cr(III) through chemical and biological reactions. Bio-reduction dominates Cr(VI) removal in a first-order exponential decay mode with both glucose and its metabolites (volatile fatty acids) as electron donors. Moreover, volatile fatty acids are more suitable as electron donors for Cr(VI) bio-reduction than glucose. Bacilli, Clostridia and Thermotogae in the fermenter dominated the reduction of Cr(VI) by regulating the production and composition of extracellular polymers (EPSs), in which carboxyl and hydroxyl groups play an important role for Cr(VI) reduction by coordination. The results can guide us to regulate the bio-reduction of Cr(VI), and provide reference for the development of bio-reduction technology of Cr(VI).

Adsorption and reduction of chromium(VI) from aqueous solution using polypyrrole/calcium rectorite composite adsorbent

Chromate is considered to be a toxic contaminant because of its potential to harm animal and human health. In this study, polypyrrole/calcium rectorite clay composites (PPy/Ca-REC composites) were prepared as a potential adsorbent, via in situ polymerization of pyrrole monomer for adsorption of Cr(VI) from aqueous solution. The XRD results indicated that the clay sheets were exfoliated in the prepared composites. SEM results showed good dispersion of the PPy on the clay sheets. The adsorption of Cr(VI) onto the PPy/Ca-REC adsorbent was highly pH-dependent, and the removal efficiency by PPy/Ca-REC composites was much higher than the PPy homopolymer. Adsorption kinetics followed a pseudo-second-order kinetic model with an equilibrium reached within 30-180 min. The adsorption isotherm data were fitted well by the Langmuir isotherm model with the maximum adsorption capacity of 714.29-833.33 mg/g at 25-45 °C. The PPy/Ca-REC composites could be regenerated and reused for three consecutive adsorption-desorption cycles without loss of the original removal efficiency for Cr(VI) removal. Furthermore, the selective adsorption of Cr(VI) was demonstrated in binary adsorption systems with coexisting ions. The mechanisms of Cr(VI) removal containing electrostatic interactions, ionic interaction as well as reduction of Cr(VI) to Cr(III), which could be observed by the XPS results.

Sustaining the efficiency of the Fe(0)/H2O system for Cr(VI) removal by MnO2 amendment

This study aims to provide new knowledge regarding the effect of MnO₂ co-presence on efficiency of Cr(VI) removal with Fe(0). Non-disturbed batch experiments (≤40 days) were conducted using two types of Fe(0) (milli- and micro-sized), two Cr(VI) concentrations (5 and 100 mg/L), in three different systems ("Fe(0) only", "MnO₂ only", and "Fe(0) + MnO₂"), at an initial pH value of 6.9. Compared to "Fe(0) only" system, the efficiency and rate of Cr(VI) removal were highly promoted in "Fe(0) + MnO₂" system; moreover, while for the "Fe(0) only" system removal of Cr(VI) was severely hindered by increasing Cr(VI) concentration, in "Fe(0) + MnO₂" system comparable high efficacies were noticed both at low and high concentration. Recycling experiments indicated that total Cr(VI) removal capacity of "Fe(0) + MnO₂" system was up to 48.1 times greater than of the "Fe(0) only" system. Enhanced removal of Cr(VI) with Fe(0) was achieved at low doses of MnO₂, with an optimal mass ratio Fe(0):MnO₂ of 4:1. The favorable synergistic effect observed in "Fe(0) + MnO₂" system was ascribed to capacity of MnO₂ to accelerate Fe(0) oxidative dissolution, and to generate supplementary amounts of secondary adsorbents/reductants with removal ability towards Cr(VI). This study provides compelling evidence that "Fe(0) + MnO₂" system could represent a highly efficient and cost-effective alternative for the abatement of Cr(VI) aqueous pollution.

Light-induced desorption of trivalent chromium from adsorbents: one step closer to sustainability

Trivalent chromium can be repelled from surface-bound merocyanine upon visible light irradiation, avoiding any chemical desorbent, and thus secondary contamination, to restore the adsorption capacity of the exhausted adsorbents.