Cd²⁺ Adsorption Characteristics of Animal-Waste-and-Plant-Derived Biochars in Water

Four types of biochars were prepared out of chicken droppings or wheat straw through pyrolysis at 350 or 650℃ separately and analyzed for physic-chemical properties. Batch sorption tests were conducted to explore Cd²⁺ adsorption characteristics of the biochars relative to duration of the adsorption, pH of the solution, and Cd²⁺ concentration in the solution. Results show that the higher the temperature of pyrolysis, the higher the pH, ash content, aromaticity and hydrophobicity and the lower the polarity of the biochar. Under the same temperature in pyrolysis, the chicken manure derived biochar was higher in pH and ash contents than the plant-derived one, and more complete in aromatic structure. The Cd²⁺ sorption kinetics of the biochars well fitted the pseudo second order kinetic model (R²>0.99), and the adsorption reached equilibrium in 12 h. And the sorption processes were controlled jointly by external liquid film diffusion, surface adsorption and intraparticle diffusion, and the latter was found to be the main rate-controlling step. With pH in the solution rising from 2.0 to 6.0, Cd²⁺ adsorption rate of the biochars gradually increased, and peaked at pH 6.0. The Langmuir model can be used to describe Cd²⁺ adsorption isotherm of the plant-derived biochars (R², 0.970 3~0.981 5), whereas the Freundlich model was better at describing the chicken-manue-derived biochars (R², 0.971 7~0.976 9). The chicken manure-derived biochars are superior to the plant-derived biochars in Cd²⁺ adsorption. Cation-π action and precipitation were the two main mechanisms for biochars pyrolyzed at 650℃ adsorbing Cd²⁺.