Oxygen-functionalized Typha angustifolia biochars derived from various pyrolysis temperatures: Physicochemical properties, heavy metal capture behaviors and mechanism

Biochars treated with acids always presented enhanced sorption capacities for heavy metals. Biochars pyrolyzed at various temperatures have different compositions and present different oxidation states for the acid treatment. To optimize the surface oxidation of biochars, an appropriate pyrolysis temperature of biomass is of great importance. In this work, raw biochars prepared from Typha angustifolia at 300, 400, 450, 500 and 600 degrees C were oxidized by HNO3. The function of HNO3 mainly focused on the surface oxidation of biochars rather than the creation of micropores in matrixes. Biochars pyrolyzed at the higher temperature were more difficult to be oxidized by the acid. The HNO3-activated biochars (NABCs) enriched Pb(II) and Cd(II) under various geochemical conditions including contact time, pH, ionic strengths, coexisting HA, initial heavy metal concentrations and temperatures. NABCs exhibited excellent sorption capacities (i.e., 232.6 mg/g for Pb(II), 88.9 mg/g for Cd(II) at pH 5.0 and 298 K) which were 6.9 and 5.5 times higher than these of untreated biochars. The maximum uptake capacities toward Pb(II) and Cd(II) showed an excellent linearity with the values of [O-C--O content x Surface area], demonstrating the essential role of the surface O-C--O species to coordinate heavy metals. NABCs could be well reused for six cycles through desorption using the acid pickling.