Adsorption of Cd(II) From Aqueous Solutions by Modified Biochars: Comparison of Modification Methods

In this study, adsorption experiments on Cd(II) ions in aqueous solution by biochars pyrolyzed from millet bran (MBBC) at 400~800 °C were carried out and MBBC had superior adsorption performance at pyrolysis temperature of 600~800 °C. Then, biochars were modified by potassium permanganate (MBBC-PP), potassium ferrate (MBBC-PF), and citric acid (MBBC-CA). The results showed that the adsorption capacities of these adsorbents were in the order: MBBC-PP > MBBC-PF > MBBC-CA > MBBC. FT-IR, XRD, and Raman were used to determine the characteristics of biochars and explore the main adsorption mechanism causing by modification. These characterizations confirmed that manganese oxide and iron oxide particles were loaded on the surface of MBBC-PP and MBBC-PF, respectively. For MBBC-PP and MBBC-PF, the adsorption sites generated by the loaded metal oxides play more important role than other factors in Cd(II) adsorption. For MBBC-CA, surface functional groups were the main contributor during Cd(II) adsorption. Batch adsorption experiments demonstrate a Langmuir model fit for Cd(II). The sorption kinetics of Cd(II) on adsorbents follows pseudo-second-order kinetics. Modified biochars, especially the one modified by KMnO4, could act as effective alternatives to enhance heavy metals removal from aqueous solutions.