Abstract
Anthropogenic activities are principally responsible for the manifestation of toxic and carcinogenic hexavalent chromium (Cr(VI)) triggering water pollution that threatens the environment and human health. The World Health Organisation (WHO) restricts Cr(VI) ion concentration to 0.1 and 0.05 mg/L in inland surface water and drinking water, respectively. The available technologies for Cr(VI) ion removal from water were highlighted with an emphasis on the adsorption technology. Furthermore, the characteristics of several polypyrrole-based adsorbents were scrutinized including amino-containing compounds, biosorbents, graphene/graphene oxide, clay materials and many other additives with reported effective Cr(VI) ion uptake. This efficiency in Cr(VI) ions adsorption is attributed to enhanced redox properties, increased number of functional groups as well as the synergistic behaviour of the materials making up the composites. The Langmuir isotherm best described the adsorption processes with maximum adsorption capacities ranging from 3.40-961.50 mg/g. The regeneration of Cr(VI) ion-laden adsorbents was studied. Ion exchange, electrostatic attractions, complexation, chelation reactions with protonated sites and reduction were the mechanisms of adsorption. Nevertheless, there are limited details on comprehensive adsorbent regeneration studies to prolong robustness in adsorption-desorption cycles and utilization of the Cr(VI) ion-laden adsorbent in other areas of research to limit the threat of secondary pollution.
Highlights
Heavy industrialisation and technological advancement has culminated in the discharge of large volumes of heavy metals including hexavalent chromium (Cr(VI)) (Rajeev et al 2019; Ahmad et al 2021)
This review focuses on the main sources of Cr(VI) ions and their toxic effects, as well as remediation using adsorption performance of hybrid polypyrrole-based adsorbents
The Cr(VI) adsorption by bamboo-like polypyrrole nanofibrous mats was dictated by electrostatic attractions, anionic exchange and reduction mechanisms owing to the special porous structure, high surface area and abundant adsorption active sites (Zhan et al 2018a, 2018b)
Summary
Heavy industrialisation and technological advancement has culminated in the discharge of large volumes of heavy metals including hexavalent chromium (Cr(VI)) (Rajeev et al 2019; Ahmad et al 2021). Some challenges have been encountered including, tedious and expensive methods of preparation, generation of additional solid wastes, low adsorption capacities owing to the low specific surface areas and unavailability of active functional groups (Dinker and Kulkarni 2015; Liu et al 2017; Hato et al 2019; Du et al 2020; Li et al 2020) It is worthy reviewing polypyrrole (PPy)-based adsorbents because of their unique and enhanced properties PPy has shown great prospects in adsorption application due to its reputable environmental stability, extraordinary conductance, desirable redox properties and easiness of processing (Das and Prusty 2012; Maity et al 2019). A review aimed at critically examining the extent to which PPy-based adsorbents for Cr (VI) ions removal is important if the Cr(VI) ions environmental mitigation is to be clearly understood
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