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Effective treatment of water contaminated with hexavalent chromium is vital due to its genotoxic and carcinogenic nature. Considering the new stringent regulatory limits being set up for Cr(VI) in ground and surface water, it is imperative to develop efficient treatment technologies/materials for the remediation of Cr(VI)-laden impaired waters. The most common issue is the lack of selectivity of sorbents towards chromate anion and their performance in real systems. Of late, polyethylenimine (PEI) polymer-based adsorbents have gained escalating attention of researchers, owing to their redox properties and biocompatible nature. Here we review various PEI-based adsorbents developed so far to remove Cr(VI) from contaminated waters. Based on the relevant peer-reviewed articles available in the open literature, the adsorbents can be divided into six broad categories, depending on the substrate used for PEI immobilization: magnetic materials, carbon-based, silica-based, clay minerals and geomaterials, biologically-derived materials, and other hybrid composites. This review discusses the efficiency of various PEI-based adsorbents for Cr(VI) removal; facilitates a better understanding of the removal mechanism and the regeneration process of the spent adsorbents. A critical evaluation of the existing literature, research shortcomings, and future research directions required to devise improved methodologies for practical applications have also been provided. Researchers who desire to work on Cr(VI) removal, with an inclination towards developing sophisticated adsorbents, can benefit from this article. © 2023 Elsevier Ltd

Department of Civil & Infrastructure Engineering

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Journal of Environmental Chemical Engineering

Removal of hexavalent chromium from impaired water: Polyethylenimine-based sorbents − A review

Prolonged exposure to trace Cr(VI) concentrations in potable water can cause serious health problems in living beings. A weak base anion exchange resin, Duolite A7, showed a large capacity for trace Cr(VI) removal from a background of competing anions present at much higher concentrations, while conventional ion exchangers and adsorbents become ineffective due to lack of selectivity under similar conditions. The objective of this study was to find out the mechanism behind such significant capacity shown by the resin. Fixed-bed column studies showed that the resin was fully exhausted below 4,000 bed volumes (BVs) for pH 7, whereas no breakthrough was observed for 25,000 and 131,000 BVs at pH 5 and 3, respectively. Extensive characterization studies revealed that redox reactions were also occurring inside the resin in addition to ion exchange, where Cr(VI) oxidized the amine functional groups and phenol–formaldehyde matrix while itself getting reduced to Cr(III). Cr(III) formed was either precipitated inside the resin as Cr(OH)3 or bound with the oxidation products such as carboxylic acid groups. Analysis of treated water showed that formaldehyde, a carcinogen, was formed due to oxidative attack of Cr(VI) on the resin and was released in the effluent at trace concentrations. A detailed understanding of the mechanism would motivate the development of such redox-active sorbents for selective trace Cr(VI) removal from contaminated drinking water. © 2022 The Korean Society of Industrial and Engineering Chemistry

Journal of Industrial and Engineering Chemistry

Detailed investigation of effective trace Cr(VI) removal mechanism by anion exchange resin with phenol–formaldehyde matrix

With increasing groundwater and surface water contamination due to improper discharge of chromium-laden wastewaters, it is compelling to synthesize a material that can selectively remove Cr(VI) from the impaired water at neutral or near-neutral pH. In this study, various composites were synthesized through chemical grafting of polyethylenimine (PEI) polymer on the surface of INDION PA 800 polystyrene beads by varying the PEI loading and glutaraldehyde (GA) crosslinking for Cr(VI) removal. The adsorbent called PS-7 with 16% PEI and 2.5% GA crosslinking performed best, demonstrating Cr(VI) removal greater than 99% even at pH 9. The adsorbent showed significant trace Cr(VI) removal from the impaired solution containing competing anions at 500 times the concentration of Cr(VI) at neutral pH. Further, 1.0 g/L adsorbent showed 100% Cr(VI) removal from a solution containing 5 mg/L Cr(VI) at pH 7 and 9. PS-7 showed significant adsorption capacity of 355.26 and 143.6 mg/g for fixed-bed column runs at influent pH 3 and 5, respectively. An insightful investigation revealed that the use of hydrophobic matrix for PEI immobilization played an important role in Cr(VI) removal at neutral pH, as opposed to the other reported PEI-based adsorbents with hydrophilic matrix which were capable of removing chromate anions at acidic pH only. The phenomenon of redox reaction occurring in the adsorbent was validated by X-ray photoelectron spectroscopy, where Cr(VI) was reduced to Cr(III) with the help of electron-donating NH2 groups and the adsorbent got oxidized. Thus, PS-7 adsorbent holds immense potential for treating real-life Cr(VI)-contaminated waters. © 2022 Elsevier Ltd

Synthesis and validation of polystyrene-based polyethylenimine composite for Cr(VI) removal from aqueous solution: Performance and mechanism

It is known that some organic materials show a good capacity for selective removal of Cr(VI) from contaminated water; however, the exact mechanism of such removal has yet to be investigated and established. In this study, coconut husk, a waste material with no or low cost, has been used as a representative material to investigate the exact mechanism of Cr(VI) removal by organic matter from synthetic wastewater. The investigation has been carried out in two significant stages. First, the adsorption phenomenon was studied using batch kinetic and column studies to determine the isotherms and kinetic models and effects of process parameters like solution pH and contact time. 100% Cr(VI) removal was achieved at pH 2 within 30 min at a stirring rate of 300 rpm. Although an impressive adsorption capacity of 222.22 mg/g was calculated using the Langmuir model, there was a poor fit of the observed data with the known adsorption models, leading to the inference that the mechanism of removal involved processes other than simple adsorption. The virgin and exhausted material were further characterized using field emission-scanning electron microscope (FE-SEM), thermal analysis, Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). From the characterization results, it was found that Cr(VI) was first adsorbed at the functional groups followed by oxidation of major components like hemicelluloses, cellulose, and lignin, present in the adsorbent, resulting in the formation of ethers, carboxylic groups, and other carbonyl functionalities. Cr(VI) itself got reduced to Cr(III) and precipitated as Cr(OH)3 inside the pores of the adsorbent. The study conclusively established that Cr(VI) removal by organic matters such as coconut husk is essentially a sorption coupled redox process. © 2021

Journal of Cleaner Production

Comprehensive investigation of the mechanism for Cr(VI) removal from contaminated water using coconut husk as a biosorbent

The presence of Cr(VI) in water, even in trace concentrations, is toxic to numerous animals, plants, and microorganisms. Remediation efforts require selective removal of trace concentrations of Cr(VI) anions from the background of other competing ions present in concentrations at least 2 orders of magnitude greater than Cr(VI). The objective of this study was to evaluate the trace Cr(VI) removal capacities of commonly available anion exchangers. Batch and column studies revealed that Duolite A7 showed extraordinarily high Cr(VI) removal capacity compared to other anion exchangers. For a synthetic influent solution containing trace Cr(VI) at pH 5, Duolite A7 removed Cr(VI) for 30 000 bed volumes (BVs), while the columns having other anion exchangers reached breakthrough almost instantaneously less than 1000 BVs. Equilibrium and kinetic studies were performed on Duolite A7 at varying pH and contact time, coupled with characterization studies on the raw and exhausted ion exchanger to elucidate the mechanism behind such a significantly higher Cr(VI) removal. Based on the results, it was concluded that there was an occurrence of redox reaction within the resin in addition to ion exchange because of the unique resin composition. Duolite A7 is a weak-base anion exchange resin with a phenol formaldehyde matrix, and both secondary and tertiary amine functional groups. The redox reactions resulted in the perpetual regeneration of ion exchange sites and 30× greater Cr(VI) removal capacity of Duolite A7. This finding may pave the way to design a new class of redox-active ion exchangers for various water and wastewater treatment applications. ©