Adsorption and desorption of potentially toxic metals on modified biosorbents through new green grafting process

Six lignocellulosic waste-derived biosorbents [cantaloupe peel (CAN), pine cone (PC), litchi fruit peel (LP), annona squamosal (AS), bamboo shoot (BS), and sugarcane bagasse (SB)] were selected as low-cost and renewable materials to prepare chemically modified biosorbent. The modified biosorbent was prepared through a newer carboxyl groups-grafting process onto the biosorbent's surface using acrylic acid. The results showed that the cation exchange capacity (CEC) of biosorbents increased by approximately 66.3-104% after modified. The modified biosorbent exhibited significantly higher adsorption capacity of Pb²⁺, Cu²⁺, and Cd²⁺ ions than the pristine biosorbent. The maximum Langmuir adsorption capacity (Q^o_max) of both pristine and modified biosorbents toward three metal ions (Pb²⁺, Cu²⁺, and Cd²⁺) followed the decreasing order: CAN > PC > LP > AS > BS > SB. The preference ranking of three metal ions on the pristine and modified biosorbents (mmol/kg) was generally in the order: Pb²⁺ > Cu²⁺ > Cd²⁺. Among these biosorbents, cantaloupe peel exhibited an excellent adsorption affinity to metal cations compared to the five others. The Q^o_max values of modified and pristine cantaloupe peels were ordered as follows: 143.2 and 81.1 mg/g for Pb²⁺ adsorption, > 45.4 and 30.4 mg/g for Cd²⁺ adsorption, > 33.1 and 23.5 mg/g for Cu²⁺ adsorption. After five adsorption-desorption cycles, the removal efficiency of Pb²⁺ by modified CAN was maintained at around 70%. The ion exchange played a determining role in adsorption mechanism. It can be concluded that modified cantaloupe peel can serve as a newer and promising biosorbent with a high adsorption capacity to various potentially toxic metals.