Adsorption Characteristics of Biochar on Heavy Metals (Pb and Zn) in Soil

Abstract

Three types of biochars, poplar branch biochar (PBC), water hyacinth biochar (WHC), and corn straw biochar (CSC), were prepared in a fixed-bed pyrolyzer at different pyrolysis temperatures (300-700℃). The effects of biochar species, pyrolysis temperature, and biochar addition on adsorption characteristics of typical heavy metals (HMs) such as Pb and Zn in vegetable soil (collected from a lead-zinc-silver mining area, Nanjing, China) were investigated. The adsorption mechanism of biochar on HMs was discussed based on the analyses of pore structure, XRD, and FTIR of biochars. WHC biochar showed the best adsorption ability at the same experimental conditions with adsorption efficiencies on Zn and Pb of 21.83% and 44.57%, respectively. The relative adsorption capacities of Zn and Pb were 227.65 μg·g-1 and 363.76 μg·g-1 at the pyrolysis temperature of 500℃ and biochar addition of 5%. The adsorption efficiency of biochar on HMs in soil increased gradually with increasing pyrolysis temperature. WHC biochars prepared at 500℃ and 700℃ had similar adsorption capacities on Zn and Pb in soil indicating that the moderate pyrolysis may be a good choice for WHC with better physicochemical properties. Increasing the amount of WHC addition benefits the adsorption efficiency of HMs in soil, but does not increase the adsorption capacity. The adsorption efficiency of Pb in soil reaches 93.93% by adding 10% of WHC into the soil sample. The combined analyses based on the physicochemical properties of biochar and the results of soil HMs adsorption experiments suggest that ion exchange and complexation are prevailing mechanisms of the remediation of HM-contaminated soil by WHC biochars.