Adsorption of Cr (VI) on lignocellulosic wastes adsorbents: an overview and further perspective

Hexavalent chromium (Cr VI) has mutagenic and carcinogenic properties and causes severe health and environmental effects. The chromium’s adsorption equilibrium in aqueous media, its mechanisms, and different lignocellulosic wastes have been studied by many researchers. In this way, this paper provides a systematic, updated, and critical overview of Cr (IVI) adsorption by lignocellulosic waste adsorbent. Besides, analyzing the Cr (VI) problems in the aquatic environment addressed to lignocellulosic waste and its properties as adsorbent. Also, it was made a deep revision about important topics such as chemical regeneration and Cr (VI) recovery, characterization techniques, and interfering factors effects (e.g., pH, dosage). In addition, it was studied carefully the adsorption phenomena through several mathematical models of kinetics and isotherms and analyzed the thermodynamics results reported by an important number of research papers. Through this review, it was observed that the lignocellulosic wastes adsorption capacities usually range from 0.32 to 403.23 mg g−1, the adsorbent percentage removal after regenerating cycle ranged between 26.5 and 98.69%, and a greater performance of removing Cr (VI) occurs at pH range 1–6. The pseudo-second-order and Langmuir models were the models that presented the best adjustment of the experimental data for Cr (VI) adsorption, and the thermodynamic nature was spontaneous and endothermic for most cases. Further studies about lignocellulosic waste adsorbents lifecycle, stability, large-scale production, and production cost analysis are still necessary to determine the stability and efficacy of lignocellulosic wastes adsorbents in real effluent treatment.