Enhanced Cd adsorption by red mud modified bean-worm skin biochars in weakly alkali environment

Abstract

Two common industrial wastes, red mud (RM) and waste bean-worm skin (BWS), were co-pyrolyzed at 400–800 °C to develop novel modified biochars (RMBWSs) for Cd adsorption in weakly alkali environment. Modification process facilitated the conversion of Fe in RM to Fe3O4 and Fe0, and the development of porous structures with higher pyrolysis temperature increased more of the BET surface areas (from 23.7 to 31.5 m2/g and from 57.1 to 425 m2/g at 600 and 800 °C, respectively) and micropore volume (from 0.016 to 0.044 cm3/g and from 0.039 to 0.5 cm3/g at 600 and 800 °C, respectively), both of which were helpful for Cd2+ adsorption. The adsorption capacity of RMBWSs was significantly increased by 43.2–118.7 times as compared to virgin biochars. The highest quantity of Cd2+ adsorbed on RMBWSs could reach 73.52 mg/g based on Langmuir model, which was higher than most modified biochars reported previously. The results of adsorption kinetics and isotherms demonstrated that all three RMBWSs exhibited favorable adsorption of Cd2+, with a more rapid initial adsorption rate, higher adsorption capacity and stronger sorption affinity at higher modification temperatures. The results of sorption thermodynamics indicated that Cd2+ adsorption on RMBWSs were spontaneous and this was aided by higher pyrolysis temperatures. Mechanism analysis suggested that Cd2+ adsorption on RMBWSs was caused by both chemisorption (e.g., creation of new FeOCd bonds, surface complexation, cation-interaction, precipitation) and physisorption (e.g., pore filling, electrostatic interaction), with chemisorption being the more prominent. The results of the soil study showed that using RMBWSs in weakly alkali soil may decrease readily available Cd by 44.81–70.47% while increasing less available Cd by 36.02–54.14%, proving the practicality and usefulness of RMBWSs in practice.