A comprehensive review of barley crop-based materials as adsorbents for the removal of aquatic pollutants

Grain crops represent one of the most significant plants that have nutritional and industrial values. Barley (Hordeum vulgare L.) is among the oldest cultivated cereal crops in the world, following wheat, rice, and maize. It is primarily utilized for malt production, animal feeding, and human consumption. Large amounts of wastes, including straw, husk, bran, etc., are released after the harvesting and processing of barley crops. Such wastes have been efficiently applied as precursors for adsorbents because of their abundance, renewable nature, lignocellulosic structure, and active group's content. This review article addresses the use of different barley wastes as feedstocks for raw, chemically modified, carbonaceous, composite, and other adsorbents against different aquatic environmental contaminants. The performance of these adsorbents against pollutants under the influence of common adsorption variables was considered. The highest uptakes of most studied contaminants were 74.2, 344.83, 143.9, and 87.35 mg/g for cadmium, lead, copper, and chromium, 531.4 and 121.95 mg/g for methylene blue and malachite green, and 448.0, 398.0, 111.1, and 613.3 mg/g for levofloxacin, norfloxacin, phenol, and oil, respectively. Most of the equilibrium, kinetic, and thermodynamic data followed the Langmuir isotherm, pseudo-second order kinetics, and endothermic nature, and the regeneration results indicated the best use of HCl eluent. Future directions and significant conclusions are also given.