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(max)(o)) of both pristine and modified biosorbents toward three metal ions (Pb2+, Cu2+, andCd(2+)) 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(max)(o) 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.