Effects of carbonization on the structure and sorption properties of coffee grounds for Pb(II) and Ni(II) in various metal systems

Two biochars (i.e., Mu@CG and Tu@CG) derived from waste coffee grounds (CG) were prepared and muffle furnace (Mu) and tube furnace (Tu) were utilized to inspect the impact of carbonization process. It indicated that the specific surface area of Mu@CG could reach 124.26 m2/g, which was 37 times higher than that of Tu@CG (3.288 m2/g). Sorption experiments revealed that the optimal conditions were 55 °C, pH= 4, time= 10 h, and concentration was 80 mg/L for both combustion furnaces. Specifically, Mu@CG exhibited larger pore size (3.42 nm) than Tu@CG (0.015 nm), suggesting that Mu can increase the pore size of CG. The sorption of Pb(II) and Ni(II) on Mu@CG and Tu@CG followed pseudo-1st-order kinetic model. The Langmuir qm value of Mu@CG for Pb(II) was 67.037 mg/g. While, the cycle times of Mu@CG for Pb(II) and Ni(II) can be arrived at 5 times and HCl is an ideal de-sorbent for the regeneration of Mu@CG for Pb(II) and Ni(II). Further, the negative ∆G and positive ∆H data confirmed that Pb(II) and Ni(II) sorption on Mu@CG and Tu@CG were spontaneous and endothermic processes. Based on these findings, it can be concluded that for CG pyrolysis, muffle furnace will be one of the most favorable manners.