Amino-functionalized sewage sludge-derived biochar as sustainable efficient adsorbent for Cu(II) removal

Sludge biochar as promising low-cost adsorbent has increasingly gained interests, but its poor surface functionality severely undermines its adsorption capacity and selectivity. Here a facile amino functionalization strategy was first proposed to enhance the surface functionality of sewage sludge derived biochar (SSDB) via the combination of sol-gel process for mesoporous silica coating and silylation for highly selective removal of Cu(II). The prepared amino-functionalized SSDB showed excellent adsorption capacity of 74.51 mg/g at room temperature, increasing by nearly 118% with regard to the unfunctionalized SSDB, and prominent selectivity (minute separation factor SF_{Co, Ni, Zn/Cu}) toward Cu(II) uptake. Characterizations demonstrated amine groups on the SSDB, the surface density of which reached 1.34 mg/m². The adsorption kinetics of Cu(II) on amino-functionalized SSDB was well described by a pseudo-second order kinetic model while the adsorption isotherm data was well fitted by Sips model. The pH range in which the adsorption preferentially occurred was 3-5. The occurrence of amine group protonation undermined adsorption performance at a pH < 3. These effects of amino-functionalized SSDB toward Cu(II) uptake were a result of the grafted amine groups specifically complexing with Cu(II) in the tetrahedron. Hence, prominent adsorption performance and low-cost feedstock make amino-functionalized SSDB a sustainable adsorbent for Cu(II) removal in water resulting in a cleaner utilization of sewage sludge.