Reducing Hexavalent Chromium to Trivalent Chromium with Zero Chemical Footprint: Borohydride Exchange Resin and a Polymer-Supported Base

A review of the treatment technologies for hexavalent chromium contaminated water

The toxicity of hexavalent chromium (Cr(VI)) present in the environment has exceeded the current limits or standards and thus may lead to biotic and abiotic catastrophes. Accordingly, several treatments, including chemical, biological, and physical approaches, are being used to reduce Cr(VI) waste in the surrounding environment. This study compares the Cr(VI) treatment approaches from several areas of science and their competence in Cr(VI) removal. As an effective combination of physical and chemical approaches, the coagulation-flocculation technique removes more than 98% of Cr(VI) in less than 30 min. Most membrane filtering approaches can remove up to 90% of Cr(VI). Biological approaches that involve the use of plants, fungi, and bacteria also successfully eliminate Cr(VI) but are difficult to scale up. Each of these approaches has its benefits and drawbacks, and their applicability is determined by the research aims. These approaches are also sustainable and environmentally benign, thus limiting their effects on the ecosystem.

Hydrophobicity Tuned Polymeric Redox Materials with Solution-Specific Electroactive Properties for Selective Electrochemical Metal Ion Recovery in Aqueous Environments

Adaptable redox-active materials hold great potential for electrochemically mediated separation processes via targeted molecular recognition and reduced energy requirements. This work presents molecularly tunable vinylferrocene metallopolymers (P(VFc-co-X)) with modifiable operating potentials, charge storage capacities, capacity retentions, and analyte affinities in various electrolyte environments based on the hydrophobicity of X. The styrene (St) co-monomer impedes hydrophobic anions from ferrocene access, providing P(VFc-co-St) with specific response capabilities for and greatly improved cyclabilities in hydrophilic anions. This adjustable electrochemical stability enables preferential chromium and rhenium oxyanion separation from both hydrophobic and hydrophilic electrolytes that significantly surpasses capacitive removal by an order of magnitude, with a robust perrhenate uptake capacity of 329 mg/g VFc competitive with established metal-organic framework physisorbents and 17-fold selectivity over 20-fold excess nitrate. Pairing P(VFc-co-X) with other solution-specific electroactive macromolecules such as the pH-dependent poly(hydroquinone) (PHQ) and the cesium-selective nickel hexacyanoferrate (NiHCF) generates dual-functionalized electrosorption cells. P(VFc-co-X)//PHQ offers optimizable energetics based on X and pH for a substantial 4.6-fold reduction from 0.21 to 0.04 kWh/mol rhenium in acidic versus near-neutral media, and P(VFc-co-St)//NiHCF facilitates simultaneous extraction of rhenium, chromium, and cesium ions. Proof-of-concept reversible perrhenate separation in flow further highlights such frameworks as promising approaches for next-generation water purification technologies.

Redox Polyelectrolytes with pH-Sensitive Electroactive Functionality in Aqueous Media

A framework of ferrocene-containing polymers bearing adjustable pH- and redox-active properties in aqueous electrolyte environments was developed. The electroactive metallopolymers were designed to possess enhanced hydrophilicity compared to the vinylferrocene (VFc) homopolymer, poly(vinylferrocene) (PVFc), by virtue of the comonomer incorporated into the macromolecule, and could also be prepared as conductive nanoporous carbon nanotube (CNT) composites that offered a variety of different redox potentials spanning a ca. 300 mV range. The presence of charged non-redox-active moieties such as methacrylate (MA) in the polymeric structure endowed it with acid dissociation properties that interacted synergistically with the redox activity of the ferrocene moieties to impart pH-dependent electrochemical behavior to the overall polymer, which was subsequently studied and compared to several Nernstian relationships in both homogeneous and heterogeneous configurations. This zwitterionic characteristic was leveraged for the enhanced electrochemical separation of several transition metal oxyanions using a P(VFc_0.63-co-MA_0.37)-CNT polyelectrolyte electrode, which yielded an almost twofold preference for chromium as hydrogen chromate versus its chromate form, and also exemplified the electrochemically mediated and innately reversible nature of the separation process through the capture and release of vanadium oxyanions. These investigations into pH-sensitive redox-active materials provide insight for future developments in stimuli-responsive molecular recognition, with extendibility to areas such as electrochemical sensing and selective separation for water purification.

Performance of treatment schemes comprising chromium-hydrogen peroxide-based advanced oxidation process for textile wastewater

The present study investigated the performance of a chromium-based advanced oxidation process using chromium (as Cr³⁺ or Cr⁶⁺) and H₂O₂ for the treatment of synthetic and simulated textile wastewaters. With the Cr³⁺/H₂O₂ system, the maximum total organic carbon (TOC) and color removals from the synthetic dye wastewater (Remazol Brilliant Violet 5R dye concentration = 100 mg/L) were 75% and 99%, respectively, within 30 min duration ([Cr³⁺]:[H₂O₂] = 1:30, stoichiometric H₂O₂ dose = 2.01 ml/L and pH = 7). Whereas the same catalyst and oxidant combination resulted in chemical oxygen demand (COD) and color removals of ~ 46%, and 84%, respectively, after 3 h of reaction at the optimized reaction conditions (i.e., [Cr³⁺]:[H₂O₂] = 1:50, stoichiometric H₂O₂ dose = 11.6 ml/L and pH = 7) from the simulated textile wastewater (initial pH = 10.2, and COD = 1820 mg/L). Further, the addition of stoichiometric H₂O₂ dose to the pretreated wastewater and pH adjustment increased the overall COD removal to 77%. Both oxidation and precipitation reactions were found responsible for organics removal from the wastewater. The other alternative involving activated carbon adsorption as second step, was not found as effective as the above scheme. The data on COD removal from simulated textile wastewater could be fit adequately in the retarded first-order kinetic model. Based on the COD and color removal results and preliminary cost analysis, this can be suggested that the Cr³⁺/H₂O₂ oxidation process followed by pH adjustment and further H₂O₂ treatment was the best option for the removal of COD and color from the simulated combined textile wastewater.

Adsorption behavior of chromium in an aqueous suspension of δ-alumina in absence and in presence of humic substances

The radioisotope Cr-51 was exploited for studying the chromium adsorption behavior in aqueous media of alumina in aqueous media. Where, it represents 1.8% by weight and exists in earth’s crust in different forms. Factors affecting this adsorption behavior are pH, amount of alumina and humic acid presence. In case of pH adsorption curves, three different areas under peak can be described based on pH changes which lead to the formation of different species too. The first area is the maximum constant adsorption at pH, range 1–3, the second one is adsorption decreasing with increasing pH through pH range 4–7 and the third one is step-down adsorption at higher pH range. The increasing amount of alumina leads to increase in the percent adsorption, where 10 and 2 g/l alumina were found to have 100% while in case of 0.2 g/l it is 80%. The presence of humic acid decreases the adsorption of chromate with increasing pH to be 30% comparing to 80% in case of 0.2 g/l alumna at pH 2. This can be also indicated by adsorption capacity which is found to be 436.8 μg/g in case of 0.2 g alumina; and it decreases in presence of Humic Acid (HA) to 145.8 μg/g at same weight of alumina. Also, the equilibrium capacities are found as 54.6 μg/g for 2 g/l and 1.2 μg/g for 10 g/l. Triple layer model (TLM) was used for simulation of chromium adsorption behavior in presence of alumina with the applied conditions of study. The results showed high coincidence with the practically found data.

A high-performance polymer hydrogel derived from konjac flying powder for removal of heavy metals

Agricultural byproducts have excellent potential for pollutant remediation due to the low-cost and environmental sustainability.

Comparative analysis of biological versus chemical synthesis of palladium nanoparticles for catalysis of chromium (VI) reduction

The discharge of hexavalent chromium [Cr(VI)] from several anthropogenic activities leads to environmental pollution. In this study, we explore a simple yet cost effective method for the synthesis of palladium (Pd) nanoparticles for the treatment of Cr(VI). The presence of elemental Pd [Pd(0)] was confirmed by scanning electron microscope (SEM), electron dispersive spectroscopy and X-ray diffraction (XRD). We show here that the biologically synthesized nanoparticles (Bio-PdNPs) exhibit improved catalytic reduction of Cr(VI) due to their size being smaller and also being highly dispersed as compared to chemically synthesized nanoparticles (Chem-PdNPs). The Langmuir–Hinshelwood mechanism was successfully used to model the kinetics. Using this model, the Bio-PdNPs were shown to perform better than Chem-PdNPs due to the rate constant (k_bio = 6.37 mmol s⁻¹ m⁻²) and Cr(VI) adsorption constant (K_Cr(VI),bio = 3.11 × 10⁻² L mmol⁻¹) of Bio-PdNPs being higher than the rate constant (k_chem = 3.83 mmol s⁻¹ m⁻²) and Cr(VI) adsorption constant (K_Cr(VI),chem = 1.14 × 10⁻² L mmol⁻¹) of Chem-PdNPs. In addition, product inhibition by trivalent chromium [Cr(III)] was high in Chem-PdNPs as indicated by the high adsorption constant of Cr(III) in Chem-PdNPs of K_Cr(III),chem = 52.9 L mmol⁻¹ as compared to the one for Bio-PdNPs of K_Cr(III),bio = 2.76 L mmol⁻¹.

An overview of palladium supported on carbon-based materials: Synthesis, characterization, and its catalytic activity for reduction of hexavalent chromium

Palladium plays a pivotal role in most of the industrial heterogeneous catalysts, because of its unique properties such as well-defined structure, great intrinsic carrier, outstanding electronic, mechanical and thermal stability. The combination of palladium and various porous carbons (PCs) can widen the use of heterogeneous catalysts. This review highlights the advantages and limitations of carbon supported palladium-based heterogeneous catalyst in reduction of toxic hexavalent chromium (Cr^(VI)). In addition, we address recent progress on synthesis routes for mono and bimetallic palladium nanoparticles supported by various carbon composites including graphene-based materials, carbon nanotubes, mesoporous carbons, and activated carbons. The related reaction mechanisms for the Cr^(VI) reduction are also suggested. Finally, the challenge and perspective are proposed.

Research on green technologies for immobilizing mercury in waste to minimize chemical footprint

This paper is devoted to the use of the principles of green chemistry in the search for technologies to reduce the chemical footprints of areas. The chemical footprint for mercury and its compounds was taken as an example to study. These chemicals belong to priority pollutants and their ever-increasing amounts in the environment have caused concern around the world, which is reflected in the adoption of the Minamata Convention. The Minamata Convention aims to protect human health and the environment from anthropogenic releases of mercury and mercury compounds. This Convention is an important component of efforts to achieve sustainable, inclusive and resilient human development through SDGs, which were adopted in September 2015 and especially SDG Goal 12: Ensure sustainable consumption and production patterns. Relevancy of this work is due to the need for the adopting of a series of measures to withdraw some mercury-containing goods from the production cycle. Also, one of the most important statements of the Convention is in reference to the issue of mercury contamination when recycling mercury. An important aspect of the work described in this paper is the reduction of mercury pollution from mercury-containing waste products by the development of technology in accordance with the principles of green chemistry. These are energy-efficient and without waste -water discharge technology. The main result of this work is the fundamental research for a transformation of elemental mercury and its compounds into less dangerous forms for the human body and the environment, providing a guaranteed absence of mercury-containing waste in the atmosphere and water systems. Various conditions for reaction of the immobilization of metallic mercury in mercury-containing wastes were investigated and it was established that it proceeded best under the following conditions: