Arsenic adsorption by different Fe-enriched biochars conditioned with sulfuric acid.

In this study, ferric chloride and sulfuric acid were used to increase the Fe-containing minerals on the biochar surface before a pyrolysis at 600 °C. The pristine and Fe-modified biochars prepared at different concentrations of sulfuric acid (50FBC and 72FBC) were characterized and analyzed, and their capacity of As(V) adsorption under various pH and ionic strength were evaluated. The results showed that the maximum adsorption capacities of As(V) calculated by the Langmuir model for 50FBC and 72FBC are 10.33 and 15.61 mg g-1, respectively, which are enhanced by 5.0 and 7.8 times compared with the pristine biochar. The higher dosage of H2SO4 (72%) used in the modification leads to a better adsorption capacity of As, especially under neutral to alkaline conditions (7.0 < pH < 10.0). It might result from the increased amounts of Fe-containing minerals formed on the biochar surface, and the enriched functional groups such as phenolic hydroxyl and carboxyl, resulting in the resistance to alkaline conditions. Overall, the Fe-modified biochar, especially 72FBC, had good potential as an environmentally friendly adsorbent for removing As from contaminated water under a wider pH range.

Phosphate Removal Mechanisms in Aqueous Solutions by Three Different Fe-Modified Biochars.

Iron-modified biochar can be used as an environmentally friendly adsorbent to remove the phosphate in wastewater because of its low cost. In this study, Fe-containing materials, such as zero-valent iron (ZVI), goethite, and magnetite, were successfully loaded on biochar. The phosphate adsorption mechanisms of the three Fe-modified biochars were studied and compared. Different characterization methods, including scanning electron microscopy/energy-dispersive spectrometry (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS), were used to study the physicochemical properties of the biochars. The dosage, adsorption time, pH, ionic strength, solution concentration of phosphate, and regeneration evaluations were carried out. Among the three Fe-modified biochars, biochar modified by goethite (GBC) is more suitable for phosphate removal in acidic conditions, especially when the pH = 2, while biochar modified by ZVI (ZBC) exhibits the fastest adsorption rate. The maximum phosphate adsorption capacities, calculated by the Langmuir-Freundlich isothermal model, are 19.66 mg g-1, 12.33 mg g-1, and 2.88 mg g-1 for ZBC, GBC, and CSBC (biochar modified by magnetite), respectively. However, ZBC has a poor capacity for reuse. The dominant mechanism for ZBC is surface precipitation, while for GBC and CSBC, the major mechanisms are ligand exchange and electrostatic attraction. The results of our study can enhance the understanding of phosphate removal mechanisms by Fe-modified biochar and can contribute to the application of Fe-modified biochar for phosphate removal in water.