Adsorption of Cu and Cd by biochars with various contents of endogenous heavy metals and their responses to aging

Abstract: Biochar has gained increasing attention in recent years due to its potential use in environmental remediation. The application of biochar may adsorb heavy metals from wastewater and decrease the bioavailability of heavy metals in soil. The concentrations of endogenous heavy metals in biochar are significant higher than in its feedstock after pyrolysis treatment. However, limits for heavy metals in biochars are lacking in some countries, which may lead to potential environmental risk resulting from the large-scale application of biochars rich in heavy metals. Therefore, three kinds of biochars named RB, SB, and JB with various contents of Cu and Cd were prepared from the straws of Pennisetum sinese grew in clean soil, moderately-polluted, and highly-polluted soils by heavy metals, respectively. The physicochemical properties of three biochars were investigated by Scanning Electron Microscopy (SEM), Adsorption capacities of Cu2+ and Cd2+ for three biochars were evaluated by batch experiments. Finally, the effects of Dry-Wet (DW) and Freeze-Thaw (FT) aging on the stability of heavy metals adsorbed by three biochars with different contents of endogenous heavy metals were investigated. Results showed that large amount of micro-pores were distributed on the surface of biochars, and RB contained the highest pH value and ash content. The XPS analysis indicated that biochars had a lot of organic functional groups, such as C?C/C-H, C-OH, C=O and O=C-OH. The adsorption data were better fitted by Langmuir isotherm model (R2=0.951-0.998) for three biochars. Adsorption capacities of Cu2+ followed the order of RB>SB>JB, and the order of RB>JB>SB for adsorption capacities of Cd2+. RB had the highest adsorption amounts of Cu2+ and Cd2+ with 54.3 and 37.3 mg/g among three biochars, respectively. Similarly, the highest concentrations of total Cu and Cd after saturated adsorption were found in RB with 21.4 and 4.78 mg/g, respectively. DW and FT aging significantly changed the bioavailability of Cu and Cd in three biochars after saturated adsorption. DW aging significantly reduced the TCLP-extractable Cu in biochar after saturated adsorption compared with that of FT aging. For instance, concentrations of TCLP-extractable Cu in SB-DW and SB-FT were decreased by 1.57 and 0.39 times than that of SB. Moreover, DW aging promoted the transformation of Cu from acid-soluble and residual fractions to reducible and oxidizable fractions, and reduced the environmental risk of Cu. However, DW and FT aging significantly increased the TCLP-extractable Cd in biochars after saturated adsorption, promoted the transformation of Cd from residual fraction to acid-soluble, reducible and oxidizable fractions, and increased the environmental risk of Cd. Especially for RB-DW and RB-FT, contents of acid-soluble Cd were increased by 6.55 and 7.99 times than that of RB. It may be due to the surface complexation and chemical precipitation played key roles for the adsorption of Cu and Cd, respectively. In short, RB retained the highest amount of Cu and Cd, but DW and FT aging increased the environmental risk of Cd in three biochars after saturated adsorption. The study is of great significance for evaluating the long-term remediation stabilization of biochar.