Kinetics of micronutrients and S adsorption onto phyto-biochars: influence of pyrolysis temperatures and properties of phyto-biochars

A laboratory study was conducted to understand the kinetics and mechanism of adsorption of micronutrients and S onto Lantana sp., Pinus sp. and wheat straw biochars pyrolyzed at 300 degrees C and 450 degrees C. Adsorption kinetics of these nutrients onto phyto-biochars conformed well to pseudo-first-order kinetics and intra-particle diffusion model. Boyd's plots revealed the involvement of other adsorption mechanisms besides intra-particle diffusion. Simple correlation analysis revealed that higher contents of N and humin-like C fraction but lower content of alcoholic-OH groups in phyto-biochars increased the equilibrium adsorption coefficient (q(e)) and intra-particle diffusion rate coefficient (k(id)) of Fe while higher contents of total acidity and phenolic-OH groups decreased q(e) and k(id) of Zn, Cu and Mn. Higher S content in phyto-biochars increased the initial adsorption (c) for Zn and Mn adsorption. Higher cation exchange capacity of phyto-biochars increased q(e) and k(id) of B which indicated the cation bridging of B onto phyto-biochars. A significant positive correlation between the pseudo-first-order rate constant (k(01)) of B and the contents of H, total acidity and phenolic-OH group but a negative correlation with pH, P and water-soluble C content in biochars showed the involvement of ligand exchange mechanism for B adsorption. Higher contents of total acidity and phenolic-OH groups in phyto-biochars increased c for S adsorption. Thus, the adsorption kinetics of micronutrients and S onto phyto-biochars followed the pseudo-first-order kinetics and intra-particle diffusion process and the properties of phyto-biochars have an important bearing on the progress of the adsorption reaction of these nutrients.