Environmental surface chemistries and adsorption behaviors of metal cations (Fe 3+ , Fe 2+ , Ca 2+ and Zn 2+ ) on manganese dioxide-modified green biochar.

The facile preparation and modification of low-cost/efficient adsorbents or biochar (CP) derived from the carbonization of palm kernel cake (lignocellulosic residue) has been studied for the selective adsorption of various metal cations, such as Fe 3+ , Fe 2+ , Ca 2+ and Zn 2+ , from aqueous solution. The CP surface was modified with KMnO 4 (CPMn) and HNO 3 (CPHNO 3 ) in order to improve the adsorption efficiency. The physicochemical properties of the as-prepared adsorbents were investigated via BET, pH pzc , FT-IR, Boehm titration, TG-DTG, XRD and SEM-EDS techniques. The surfaces of all adsorbents clearly demonstrated negative charge (pH pzc > pH of the mixture solution), resulting in a high adsorption capacity for each metal cation. Fe 2+ was found to be more easily adsorbed on modified CP than the other kinds of metal cations. Synergistic effects between the carboxylic groups and MnO 2 on the surface of CPMn resulted in better performance for metal cation adsorption than was shown by CPHNO 3 . The maximum adsorption capacities for Fe 3+ , Fe 2+ , Ca 2+ and Zn 2+ using CPMn, which were obtained from a monolayer adsorption process via Langmuir isotherms ( R 2 > 0.99), were 70.67, 68.60, 5.06 and 22.38 mg g -1 , respectively. The adsorption behavior and monolayer-physisorption behavior, via a rapid adsorption process as well as single-step intra-particle diffusion, were also verified and supported using Dubinin-Radushkevich, Redlich-Peterson and Toth isotherms, a pseudo-second-order kinetic model and the Weber-Morris model. Moreover, the thermodynamic results indicated that the adsorption process of metal cations onto the CPMn surface was endothermic and spontaneous in nature. This research is expected to provide a green way for the production of low-cost/efficient adsorbents and to help gain an understanding of the adsorption behavior/process for the selective removal of metal ions from wastewater pollution.