Manganese Oxide Modified Biochars: Preparation, Characterization, and Sorption of Arsenate.

Apart from its use for carbon sequestration and soil amendment, biochar as a sorbent shows high potential to remove various contaminants from aqueous solutions. To further improve biochar’s sorption ability, this work used two modification methods, including modify biomass feedstock with MnCl₂·4H₂O  and modifying biochar with synthesized birnessite  to produce two manganese oxide-biochar composites (MPB and BPB, respectively) from pine wood. The resulting biochars were characterized with X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray analysis. The characterization results showed that both methods successfully produced the Mn-biochar composites with MnO and birnessite as the dominant crystalline forms for the MPB and BPB, respectively. SEM imaging showed BPB had rough surface and poor crystalline, and birnessite was evenly dispersed on the biochar surface. Well crystalized structure of Mn oxides particles was found in MPB; however, the MnO particles were not well dispersed on biochar surface. Batch studies were carried out to determine the sorption kinetics and isotherms for arsenate on the modified biochars and both of them demonstrated good sorption ability. However, BPB had better metal sorption than MPB, which can be attributed to greater abundance of hydroxyl (-OH) functional groups. Our results suggested that arsenate sorption on the modified biochars was mainly through complexation with (-OH) groups and formation of arsenic-bearing minerals.