Adsorption of Zn(Ⅱ) on Pristine and SPLP/TCLP Leached Rice Straw Biochar: an Interplay of Precipitation and Ion Exchange

摘要

The inorganic mineral content in biochar influences the adsorption of Zn(Ⅱ) metal ions. Metal ion adsorption on mineral rich rice straw biochar is influenced upon washing. Rice straw slow pyrolysis biochar BC1-3, respectively, prepared at 400, 500, and 600 ℃, were leached under Toxicity Characteristic Leaching Procedure (TCLP) and Synthetic Precipitation Leaching Procedure (SPLP) conditions to furnish BT1-3 and BS1-3, respectively. The Zn(Ⅱ) adsorption studies were carried out for pH and dose optimization, initial concentration, isotherm fit, and kinetic studies. The Zn(Ⅱ) adsorption by B(C/ S/T)1-3 showed Langmuir and Freundlich isotherm, with pseudo-second-order kinetics at optimum pH 5 and dose 1 g/L. The adsorption of Zn(Ⅱ) followed the trend BC3(q_m 47 mg/g)＞BC2＞BC1 ＞BS2＞BS1 ＞BS3＞BT2＞BT1＞BT3 (q_m 3.5 mg/g), i.e., metal ion adsorption decreased with extent of leaching. The Zn(Ⅱ) adsorption on biochar involved precipitation as dominant factor for metal ion adsorption on the bio-chars followed by ion exchange and proton exchange. The precipitation of Zn(Ⅱ) ions in case of BC1-3 is attributed to the pH of biochar, which increases with proportion of minerals to organic content in biochar. In case of biochar BS1-3 and BT1-3, ion exchange and proton exchange mechanisms driven by demin-eralization are responsible for Zn(Ⅱ) adsorption. The adsorption mechanism for Zn(Ⅱ) on biochar is supported by XPS, solid state NMR studies.