Abstract

The contamination due to heavy metals poses a significant threat to both the environment and human health. Among the heavy metal pollutants, hexavalent chromium [Cr(VI)] presents a significant risk to human health due to its high toxicity and carcinogenic nature. Due to its high solubility and carcinogenicity, the regulatory limit for chromium in drinking water has been decreased to ≤25 μg/L. This review primarily focuses on recent developments in chromium removal from aquatic systems using various straw biochar materials. It discusses the modulation of the physicochemical properties of the straw biochar materials under different pyrolysis conditions and surface modification techniques. Consequently, it outlines the various surface modification strategies adopted by various researchers to acquire optimum chromium removal from the water systems. The adsorption capacities of the straw biochar systems range from ~20 to 450 mg/g, with the feedstock pyrolysis range from 400°C to 700°C. The modified biochar outperforms the pristine materials under acidic conditions, with the adsorption following monolayer Langmuir adsorption and pseudo-second-order kinetics. These modified biochar materials also promote the conversion of the Cr(VI) to the less toxic and more stable trivalent chromium at the redox-active sites on the biochar surface. In addition, the straw biochar materials can retain ~70–90% of adsorption capacity even after 3–5 regeneration cycles. Overall, this review discusses straw-derived biochar as an effective and economically viable material for chromium detoxification. Despite this, there is a need for further research on realistic wastewater matrices to translate the laboratory results into real field applications.

Key words: Biochar; Chromium; Cr(VI); Adsorption; Straw; Mechanism; Regeneration