Macroscopic and molecular investigations of copper sorption by a steam-activated biochar

Excessive Cu concentrations in water systems can negatively affect biological systems. Because Cu can form strong associations with organic functional groups, we examined the ability of biochar (an O-C-enriched organic bioenergy by-product) to sorb Cu from solution. In a batch experiment, KOH steam-activated pecan shell biochar was shaken for 24 h in pH 6, 7, 8, or 9 buffered solutions containing various Cu concentrations to identify the effect of pH on biochar Cu sorption. Afterward, all biochar solids from the 24-h shaking period were air-dried and analyzed using X-ray absorption fine structure (XAFS) spectroscopy to determine solid-phase Cu speciation. In a separate batch experiment, biochar was shaken for 30 d in pH 6 buffered solution containing increasing Cu concentrations; the Cu sorption maximum was calculated based on the exponential rise to a maximum equation. Biochar sorbed increasing amounts of Cu as the solution pH decreased from 9 to 6. The XAFS spectroscopy revealed that Cu was predominantly sorbed onto a biochar organic phase at pH 6 in a molecular structure similar to Cu adsorbed on model humic acid (Cu-humic acid [HA]). The XAFS spectra at pH 7, 8, and 9 suggested that Cu was associated with the biochar as three phases: (i) a complex adsorbed on organic ligands similar to Cu-HA, (ii) carbonate phases similar to azurite (Cu(CO)(OH)), and (iii) a Cu oxide phase like tenorite (CuO). The exponential rise equation fit to the incubated samples predicted a Cu sorption maximum of 42,300 mg Cu kg. The results showed that KOH steam-activated pecan shell biochar could be used as a material for sorbing excess Cu from water systems, potentially reducing the negative effects of Cu in the environment.