Improving Cu(II) sorption by biochar via pyrolyzation under CO2: the importance of inherent inorganic species.

Biochar from Spartina alterniflora (SA) and rice straw (RS) under N2/CO2 were evaluated for Cu(II) removal from aqueous solution. The result indicates SA biochar prepared at 700 °C under CO2 can achieve a Cu(II) sorption capacity of 89.12 ± 2.77 mg/g, which is higher than that from N2 by about 50%. CO2 can promote the development of multi-porous structure, enhance specific surface area, and increase the amounts of hydroxyl and carboxyl groups on biochar. In addition, CO2 can inhibit the thermal decomposition of inorganic carbonate, such as MgCO3 and CaCO3 in biochar. These matters facilitate Cu(II) removal via the formation of chemical precipitation of Cu2(OH)2CO3. The dissolution of inherent inorganic matter makes Cu(II) transformed into hydrolyzed species or amorphous precipitation, which contributes to about 75% (w/w) of Cu(II) removal. Metal exchange with complexed cations and the formation of basic cupric carbonate are time-consuming and responsible for about 24% (w/w) of Cu(II) removal. Graphical abstract ᅟ.